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Ceramics in total disc replacements: A scoping review

Open AccessPublished:October 12, 2022DOI:https://doi.org/10.1016/j.clinbiomech.2022.105796

      Highlights

      • Clinical results of ceramic and other cervical Total Disc Replacements comparable.
      • In vivo studies show osseointegration comparable to non-ceramic devices.
      • Tribological studies suggest appropriate wear properties.
      • We found no indications of systematic problems of ceramic Total Disc Replacements.

      Abstract

      Background

      Ceramics are used in Total Disc Replacements (1) in articulating surfaces for their wear resistance and biocompatibility and (2) on endplates to promote osseointegration. They furthermore exhibit MRI and CT compatibility. These properties address main challenges associated with non-ceramic Total Disc Replacements i.e. wear, migration and postoperative imaging. While brittleness of ceramics caused fear of fracture in the past, improvements of ceramic materials were made and considerable clinical experience with ceramic Total Disc Replacements was gained. This review aims to assess the evidence on the use of ceramics in Total Disc Replacements and compare safety and effectiveness of ceramic Total Disc Replacements to spinal fusion and Total Disc Replacements in general.

      Methods

      We conducted a scoping review on the use of ceramics in Total Disc Replacements using Scopus, Web of Science and PubMed. The review includes 36 clinical, ex vivo and nonhuman in vivo, tribological and mechanical studies and case reports.

      Findings

      Ceramics are used in cervical Total Disc Replacements, with safety and efficacy confirmed in clinical studies, with up to 10 and 3.3 years follow-up, for articulation and osseointegration applications, respectively. Clinical evidence shows that ceramic Total Disc Replacements (alike non-ceramic ones) restore segmental motion and result in non-inferior and possibly superior outcomes to spinal fusion. In vivo studies show osseointegration comparable to non-ceramic devices. Tribological studies suggest appropriate wear properties.

      Interpretation

      We found no indications of systematic problems with the use of ceramics in Total Disc Replacements. Ceramics are suitable materials for Total Disc Replacements.

      Keywords

      1. Introduction

      Neck and low back pain are the main cause of disability in many countries (
      • Vos T.
      • Allen C.
      • Arora M.
      • et al.
      Global, regional, and national incidence, prevalence, and years lived with disability for 310 diseases and injuries, 1990–2015: a systematic analysis for the global burden of disease study 2015.
      ). With lifetime prevalences of approximately 40% for low back pain (
      • Manchikanti L.
      • Singh V.
      • Falco F.J.E.
      • et al.
      Epidemiology of low back pain in adults.
      ) and 14–71% for neck pain (
      • Wright A.R.
      • Shi X.A.
      • Busby-Whitehead J.
      • et al.
      The prevalence of neck and shoulder symptoms and associations with comorbidities and disability: the Johnston county osteoarthritis project.
      ), they constitute a serious socioeconomic challenge. A common source of pain is intervertebral disc (IVD) herniation, due to degenerative disc disease (
      • Büttner-Janz K.
      • Guyer R.D.
      • Ohnmeiss D.D.
      Indications for lumbar total disc replacement: selecting the right patient with the right indication for the right total disc.
      ) or trauma (
      • Chang H.-K.
      • Huang W.-C.
      • Wu J.-C.
      • et al.
      Cervical arthroplasty for traumatic disc herniation: an age- and sex-matched comparison with anterior cervical discectomy and fusion.
      ). If conservative treatments prove ineffective for 6 months in case of lumbar pain (
      • Büttner-Janz K.
      • Guyer R.D.
      • Ohnmeiss D.D.
      Indications for lumbar total disc replacement: selecting the right patient with the right indication for the right total disc.
      ) or 6 weeks in case of cervical pain (
      • Auerbach J.D.
      • Jones K.J.
      • Fras C.I.
      • et al.
      The prevalence of indications and contraindications to cervical total disc replacement.
      ), invasive treatment is considered.
      Two main surgical treatment options are: (1) spinal fusion, in which the vertebrae adjacent to the IVD are permanently connected eliminating motion, and (2) arthroplasty, in which the motion of the spinal segment is preserved through a total disc replacement (TDR). Both treatments are effective, but stricter contraindications make TDRs suitable for fewer patients (43% of cervical (
      • Auerbach J.D.
      • Jones K.J.
      • Fras C.I.
      • et al.
      The prevalence of indications and contraindications to cervical total disc replacement.
      ) and 5% of lumbar spine surgery patients (
      • Huang R.C.
      • Lim M.R.
      • Girardi F.P.
      • Cammisa F.P.
      The prevalence of contraindications to total disc replacement in a cohort of lumbar surgical patients.
      )). Although TDRs are related to such challenges as heterotopic ossification and wear debris, they allow faster return to work and less need for postoperative bracing in the cervical spine (
      • Pj Tortolani
      • Moatz B.
      Cervical disc arthroplasty: pros and cons.
      ) compared to spinal fusion.
      Motion preservation by TDR is typically achieved through articulating surfaces, which are prone to wear under repetitive motion and loading. TDRs are particularly attractive for younger patients. However, young patients long remaining lifetime and high physical activity are expected to cause more wear (
      • Pj Tortolani
      • Moatz B.
      Cervical disc arthroplasty: pros and cons.
      ) which brings high demands on implant materials. Polymeric debris released from metal-on-polymer bearing couples can lead to osteolysis and implant-loosening (
      • Gornet M.F.
      • Singh V.
      • Schranck F.W.
      • et al.
      Serum metal concentrations in patients with titanium ceramic composite cervical disc replacements.
      ). Metal-on-metal couples have superior wear properties, but elevated metal concentrations and nanodebris can cause local and systemic effects (
      • Gornet M.F.
      • Singh V.
      • Schranck F.W.
      • et al.
      Serum metal concentrations in patients with titanium ceramic composite cervical disc replacements.
      ). Ceramic-on-ceramic bearing couples produce less wear debris with lower biological reactivity (
      • Hamadouche M.
      • Sedel L.
      Ceramics in orthopaedics.
      ). Their high strength, excellent biocompatibility and tribological properties, as well as degradation-resistance, make bioinert ceramics an attractive material for load bearing articulating applications.
      Beside wear, implant migration and dislocation are major concerns related to TDRs as this type of complication often requires reoperation (
      • Lou J.
      • Wang B.
      • Wu T.
      • et al.
      In-vivo study of osseointegration in prestige LP cervical disc prosthesis.
      ;
      • Ozbek Z.
      • Ozkara E.
      • Arslantaş A.
      Implant migration in cervical disk arthroplasty.
      ). Achieving primary (initial) and secondary (long-term) stability is of critical importance in preventing such complications. Bioactive ceramics can promote bone ingrowth into TDR endplates and thus secondary stability. For example, osseointegration of silicon nitride is superior to standard biomaterials, such as titanium and PEEK (
      • Webster T.J.
      • Patel A.A.
      • Rahaman M.N.
      • Sonny Bal B.
      Anti-infective and osteointegration properties of silicon nitride, poly(ether ether ketone), and titanium implants.
      ), as is silicon nitrides biofilm inhibition (
      • Bal S.
      • Gorth D.
      • Puckett S.
      • et al.
      Decreased bacteria activity on Si3N4 surfaces compared with PEEK or titanium.
      ;
      • Webster T.J.
      • Patel A.A.
      • Rahaman M.N.
      • Sonny Bal B.
      Anti-infective and osteointegration properties of silicon nitride, poly(ether ether ketone), and titanium implants.
      ) that might lower the risk of implant-related infections.
      Wettability of ceramics makes them an attractive material for articulating surfaces or outer endplates of TDRs, as their self-lubricating properties are expected to reduce adhesive wear (
      • Bierbaum B.E.
      • Nairus J.
      • Kuesis D.
      • et al.
      Ceramic-on-ceramic bearings in total hip arthroplasty.
      ) and wettability enhances osseointegration (
      • Khaskhoussi A.
      • Calabrese L.
      • Currò M.
      • et al.
      Effect of the compositions on the biocompatibility of new alumina–zirconia–Titania dental ceramic composites.
      ;
      • Surmeneva M.A.
      • Kleinhans C.
      • Vacun G.
      • et al.
      Nano-hydroxyapatite-coated metal-ceramic composite of iron-tricalcium phosphate: improving the surface wettability, adhesion and proliferation of mesenchymal stem cells in vitro.
      ;
      • Thian E.S.
      • Ahmad Z.
      • Huang J.
      • et al.
      The role of surface wettability and surface charge of electrosprayed nanoapatites on the behaviour of osteoblasts.
      ). Furthermore, ceramics are more compatible with MRI and CT imaging than metals, which can cause problematic artifacts or commonly used polymers that are radiolucent.
      Despite these promising properties, application of ceramics in TDRs remains limited. One significant concern related to use of ceramics in TDRs is implant fracture due to the brittleness of ceramics. Historically, this concern stems from experience with ceramic hip replacements, which are subject to different loading scenarios than spinal implants, especially in the cervical spine. Furthermore, it has been addressed to some degree by improved manufacturing techniques and improved properties, such as purity, density, grain size and grain distribution (
      • Hamadouche M.
      • Sedel L.
      Ceramics in orthopaedics.
      ). The current state of knowledge on ceramic TDRs performace and complications remains unclear, as a comprehensive overview of studies performed with ceramic TDRs appears to be missing in the literature.
      The main aim of this scoping review is to assess the evidence on the use of ceramics in TDRs. With a scoping review, the scope of a body of literature is accessed. To this end, we compiled clinical, tribological, mechanical, biomechanical and osseointegrative evidence available in scientific literature to compare safety and effectiveness of ceramic TDRs to alternative treatment options, namely spinal fusion and TDRs in general.

      2. Methods

      The following search string on Scopus (Elsevier) was used to identify literature relevant for the review: TITLE((replacement* OR nonfusion OR non-fusion OR movement-preserv* OR arthroplasty* OR artificial OR motion-preserv* OR motionpreserv* OR prosthe*) AND (spine OR spinal OR vertebral OR cervical OR lumbar OR intervertebral OR disc OR disk OR IVD)) AND ALL(Ceramic OR Silicon Nitride OR Si3N4 OR silicon-nitride OR Zirconia OR Zirconium dioxide OR zro2 OR calcium phosphate OR bioactive glasses OR Al2O3 OR alumina OR Bioglass OR Hydroxyapatite OR Hydroxylapatite OR aluminumoxide OR Aluminum Oxide OR aluminum-oxide OR aluminiumoxide OR Aluminium Oxide OR aluminium-oxide OR CoC OR ZTA) AND (LIMIT-TO (LANGUAGE, ”English”).
      This resulted in 282 publications (status: September 2021). PubMed and Web of Science were searched with equivalent search strings resulting in additional 38 and 62 publications respectively. All publications were held up against the following exclusion criteria:
      • Publications that did not perform a study on ceramic based TDRs intended for humans.
      • Study scope and results unrelated to and/or not influenced by ceramics.
      • Preliminary results if newer publications of the same study were available.
      • Studies that investigated tribology of material pairings without a specific TDR design (such as pin on disc or ball on disc studies).
      • Implants that replace not only intervertebral discs, but also vertebrae
      • Not peer-reviewed scientific literature
      Additional five relevant papers that were not identified by the search strings but were known to the authors, were included. Thus, a total of 36 articles were included in the review. These studies were categorized as clinical (17), ex vivo and nonhuman in vivo (12), tribological (6) and mechanical (1), see (Fig. 1).

      3. Results

      The studies included in this scoping review report on 6 TDR devices that use ceramics for articulation and 10 TDRs that use ceramics to promote osseointegration (Table 1). More devices are intended for use in the cervical region (9) than for the lumbar region (7), especially for implants that use ceramics for articulation (4 cervical vs 2 lumbar, the latter not reported in clinical studies). In devices with articulating ceramic surfaces, they were made from alumina (3), zirconia (2), silicon-nitride (1), titanium alloy/titanium carbide composite (1) or zirconia toughened alumina (1). These devices were used in the following bearing couples: self-mating ceramics (4), ceramic-on-other-ceramic-material (1), ceramic-on-polymer (1). For osseointegration, devices used TiCaPHA (3) or HA (5) coatings, a coating containing HA or appatite-wollastonite granules (1 TDR in multiple design variations) or HA composite endplates (1) and in some cases, additional HA-composite pins (2).
      Table 1Ceramic TDRs. PEEK = Polyether ether ketone; ZTA = Zirconia Toughened Alumina; HA = Hydroxyapatite; PE = Polyethylene; PET = Polyethylene terephthalate; UHMWPE = Ultra-high-molecular-weight polyethylene; LDPE = Low-density polyethylene; CoCrMo = cobalt‑chromium‑molybdenum; HA/PLLA = hydroxyapatite/poly-l-lactide composite; TiCaPHA = titanium/calcium phosphate hydroxyapatite coating.
      TDRMaterialsRegionInitial FixationSecondary Fixation
      ↓ Ceramics for articulation ↓
      Altia TDI™ (Amedica, USA)silicon-nitridecervicalkeels
      Discocerv® Cervidisc Evolution

      (Scient‘X, France)
      alumina, zirconia, titaniumcervicalgrooves, teeth
      Prestige LP (Medtronic, USA)titanium alloy/titanium carbide compositecervicalrailstitanium coating
      Simplify® Disc (Simplify Medical,

      Inc., USA)
      PEEK, ZTAcervicalserrated teeth, inline finstitanium coating
      TDR described in (Shaheen and Shepherd, 2007) (Rotaru and Olaru, 2015) (Shankar and Kesavan, 2015) (Shankar and Kesavan, 2016)alumina; in (Shankar and Kesavan, 2016) also: zirconialumbar
      Sofamor Danek (Sofamor Danek, USA)alumina, titaniumlumbargroovesbeaded titanium
      ↓ Ceramics for osseointegration ↓
      3DFUHMWPE coated with LDPEcervicalpins (HA/PLLA)unsintered HA coating
      Discover (DePuy Spine, USA)titanium, PEcervicalspikesHA coating
      NuNec® (Pioneer Surgical, USA)PEEK, titanium, tantalum markerscervicalcam bladesHA coating
      Porous Coated Motion (PCM) (Cervitech, USA / NuVasive, USA)titanium alloy, UHMWPEcervicalteethTiCaPHA coating
      Pretic-Ititanium alloy, UHMWPEcervicalserrationsTiCaPHA coating
      Charité (DePuy Spine, USA)CoCrMo, UHMWPElumbarteethTiCaPHA coating
      fabric TDR described in (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      ); 3-DF disc (Takiron, Japan)
      UHMWPE coated with LDPElumbarpins (HA-PLLA) ((
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      ) (
      • Takahata M.
      • Kotani Y.
      • Abumi K.
      • et al.
      Bone ingrowth fixation of artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric.
      ) without pins)
      unsintered HA coating ((
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      ): either sintered HA or apatite-wollastonite)
      Maverick (Medtronic Inc., USA)CoCrMolumbarkeelsHA coating
      SB Charité (DePuy-Acromed, USA)CoCrMo, UHMWPElumbarteethHA coating
      TDR described in (
      • Gloria A.
      • De Santis R.
      • Ambrosio L.
      • et al.
      A multi-component fiber-reinforced PHEMA-based hydrogel/HAPEXTM device for customized intervertebral disc prosthesis.
      )
      HA-reinforced PE, hydrogel reinforced with PET fiberslumbarpegsHA composite endplates
      The 36 articles included in this review report on clinical studies (15) and case reports (2) (Table 2), in vitro (2) and in silico (4) tribological studies (Table 3), 1 mechanical study (Table 3), and in vivo and ex vivo nonhuman (8) and human cadaveric studies (4) (Table 4). If controls were used, they were usually fusion (7 clinical and 5 cadveric studies). In five studies another TDR was compared with: 2 nonhuman studies (same coating in different regions or different coatings in same region) and 3 in silico tribological studies (same design, different materials).
      Table 2Clinical studies and case reports on ceramic TDRs. p.o. = postoperatively, pre.o. = preoperatively; n = # patients; nTDR = #TDRs; y. = years, m. = months, w. = weeks; Age in years, values are mean, mean (range) or mean ± SD, % of n (if % of nTDR it is indicated) HO = Heterotropic ossification; SAE = Serious adverse events; related SAE = serious adverse events related to implant/implantation; VAS = visual analogue scale (converted to 0-100 mm scale when reported differently);, RoM = range of motion, NDI = Neck disability index (converted to 0–50 scale when reported differently); MMRM = Mixed Model for Repeated Measures.
      ReferenceStudy TypeTDRStudy DescriptionFollow-upRegionPatientsCompared withResults
      RoMClinical OutcomeRelated SAE, secondary surgeries, etc.
      ↓ Ceramics for articulation ↓
      (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      )
      prospective,

      controlled,

      randomized,

      clinical trial
      Prestige LPdual level TDR (adjacent

      levels)
      10 y.cervicaln = 209dual level fusion, n = 188superior level:

      6.6°, inferior

      level: 5.9°
      overall success: 80.4%, fusion:

      62.2%; NDI: 7.0, fusion: 11.5; neck

      pain: 3.8, fusion: 6.5; arm pain: 3.0,

      fusion: 4.5
      related SAE: 3.8%, fusion: 8.1%;

      secondary surgeries (either operated

      level): 4.7%, fusion: 17.6%; revision

      surgeries (either operated level): 0.0%, fusion: 1.4%
      (
      • Gornet M.F.
      • Burkus J.K.
      • Shaffrey M.E.
      • et al.
      Cervical disc arthroplasty: 10-year outcomes of the prestige LP cervical disc at a single level.
      )
      nonrandomizedPrestige LPsingle level TDR10 y.cervicaln = 280fusion, n = 265

      (historical),

      follow-up: 7 y.
      6.85° ± 4.96,

      fusion: 0.48° ± 0.46
      overall success: 74.3%, fusion:

      63.2%; NDI: 7.5, fusion: 11.9;

      neck pain: 11.2, fusion: 19.4; arm

      pain: 8.5, fusion: 15.0
      related SAE: 7.8%, fusion: 5.6%;

      secondary surgeries (operated level):

      10.3%, fusion: 13.6%; revision surgeries

      (operated level): 0.4%, fusion:

      2.1%
      (
      • Gornet M.F.
      • Singh V.
      • Schranck F.W.
      • et al.
      Serum metal concentrations in patients with titanium ceramic composite cervical disc replacements.
      )
      prospective,

      nonrandomized,

      study
      Prestige LPsingle level TDR7 y.cervicaln = 30Ti concentrations: significantly higher at every time point p.o. (0.25, 0.5, 1, 2,

      3, 5, 7 y. p.o.) than pre.o.
      (
      • Liu Z.
      • Rong X.
      • Liu H.
      • et al.
      Effect of facet tropism on postoperative cervical range of motion after single-level cervical disc arthroplasty.
      )
      retrospective studyPrestige LPsingle level TDR2 y.cervicaln = 90facet tropism <7° (symmetry) group: 7.36° ± 3.37 facet tropism >7° (asymmetry) group: 6.39° ± 3.34Symmetry group: Arm pain (VAS): 18.0 ± 11.2 mm, neck pain (VAS): 23.2 ± 15.4 mm, NDI: 14.20 ± 4.60; asymmetry group: Arm pain (VAS): 17.6 ± 14.8 mm, neck pain (VAS): 21.2 ± 15.1 mm, NDI: 14.80 ± 4.65effect of facet tropism on RoM
      (
      • Enan A.
      • Abu-Hegazy M.
      • Abo-Hegy M.
      • Al-Kerdany A.
      Single level cervical arthroplasty with the discocerv® prosthesis : a preliminary report.
      )
      prospective

      nonrandomized

      study
      Discocerv® Cervidisc Evolutionsingle or dual level TDR1.1 y.cervicaln = 1412.9° ± 2.9Odom‘s criteria: excellent or good in

      81.6%; NDI: 12.5; neck pain (VAS):

      31 mm; radicular pain (VAS): 22 mm
      (
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      )
      prospective

      noncomparative

      study
      Discocerv® Cervidisc Evolutionsingle or dual level TDR0.4 y.cervicaln = 174.9° (0–19)Odom‘s criteria: good or excellent

      in 100% of the cases, NDI: 11(0–24),

      neck pain (VAS): 13 mm (0–60), radicular

      pain (VAS): 5 mm (0−20)
      (
      • Nguyen N.Q.
      • Kafle D.
      • Buchowski J.M.
      • et al.
      Ceramic fracture following cervical disc arthroplasty: a case report.
      )
      case reportDiscocerv® Cervidisc Evolutiondual level TDR, report on ceramic fracturecervicaln = 1convex cranial ceramic insert of TDR in

      inferior level fractured (ca. 10 pieces),

      revision: fusion of both levels, 0.5 y.p.o.: neck and arm pain (VAS): 1/10
      (
      • Swamy G.N.
      • Hopwood S.
      • Nanjayan S.K.
      • et al.
      Clinical and radiological review of a semi-constrained cervical disc replacement with a ceramic-ceramic articulation with a minimum seven years follow-up.
      )
      Prospective studyDiscocervSingle and dual level TDR7 y.cervicalnTDR = 60; single level: n = 44, dual level: n = 812.9° ± 2.9NDI: 10subsidence (<2 mm): n = 3
      (
      • Geisler F.H.
      • Maislin D.G.
      • Keenan B.T.
      • Maislin G.
      One-year {NDI} and {VAS} outcomes from the single-level {PEEK}-on-ceramic {SimplifyTM} Disc {FDA} {IDE} trial.
      )
      prospective

      clinical trial
      Simplify® Discsingle level TDR1 y.cervicaln = 148fusion: n = 119

      (historical)
      NDI (MMRM): 8.25, fusion:

      12.55, VAS neck and arm

      pain (MMRM): 17.5, fusion: 24.8
      secondary surgeries: n = 1, fusion: n = 1
      (
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      )
      prospective, nonrandomized trialSimplify® DiscSingle level TDR2 y.cervicaln = 150fusion: n = 117 (historical)9.6°NDI: 6.8, fusion: 11.5; neck and arm pain (VAS): 15.6 mm, fusion: 23.3 mm, overall success: 93.0%, fusion: 73.6%secondary surgery (operated level): 2.7%, fusion: 5.1%; related SAE: 0.7%, fusion: 0.9%
      ↓ ceramics for osseointegration ↓
      (
      • Shi S.
      • Zheng S.
      • Li X.-F.
      • et al.
      Comparison of 2 zero-profile implants in the treatment of single-level cervical spondylotic myelopathy: a preliminary clinical study of cervical disc arthroplasty versus fusion.
      )
      retrospective

      clinical study
      Discoversingle level TDR2 y.cervicaln = 60fusion, n = 687.91° ± 1.86,

      fusion: 1.03° ± 0.32
      NDI: 5.77 ± 1.24, fusion: 5.66 ± 1.30
      (
      • Skeppholm M.
      • Svedmark P.
      • Noz M.E.
      • et al.
      Evaluation of mobility and stability in the discover artificial disc: an in vivo motion study using high-accuracy 3D CT data.
      )
      clinical studyDiscoversingle or dual level TDR3.33 y.cervicaln = 285.1° (0.2-

      15.8)
      NDI: 2 y. p.o.: 20ankylosis: 5%;

      motion between TDR and vertebra:

      8%
      (
      • Choi D.
      • Petrik V.
      • Fox S.
      • et al.
      Motion preservation and clinical outcome of porous coated motion cervical disk arthroplasty.
      )
      retrospective

      review
      Porous Coated

      Motion (PCM)
      single or dual level TDR1.25 y.

      (median)
      cervicalnTDR = 80, n = 53>5°: 21% of

      nTDR
      NDI: 23.8; VAS: 3.1revision surgery: n = 1
      (
      • Pimenta L.
      • McAfee P.C.
      • Cappuccino A.
      • et al.
      Superiority of multilevel cervical arthroplasty outcomes versus single-level outcomes: 229 consecutive PCM prostheses.
      )
      prospective

      nonrandomized

      study
      Porous Coated

      Motion (PCM)
      single and multiple level TDR2.2 y.cervicalnTDR = 229, single

      level: n = 71,

      multiple levels:

      n = 69
      Odom‘s criteria: excellent or good:

      single level: 76%, multiple levels:

      85%; NDI improvement: single level:

      37.6%, multiple levels: 52.6%, VAS

      improvement: single level: 58.4%,

      multiple levels: 65.9%
      reoperation rates and SAE: single

      level: nTDR = 3, multiple levels: nTDR = 2
      (
      • McAfee P.C.
      • Cappuccino A.
      • Cunningham B.W.
      • et al.
      Lower incidence of dysphagia with cervical arthroplasty compared with ACDF in a prospective randomized clinical trial.
      )
      prospective,

      randomized,

      clinical trial
      Porous Coated

      Motion (PCM)
      single level TDR2 y.cervicaln = 151fusion, n = 151Bazaz Dysphagia Score (none, mild,

      moderate, severe): 85%, 11.9%, 2.9%,

      0, Fusion: 72.4%, 13.8%, 13.8%, 0; incidence

      of Dysphonia: all visits: 9.03

      ±15.4, fusion: 13.1 ± 18.8
      (
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      )
      randomized,

      prospective,

      controlled,

      clinical trial
      Porous Coated

      Motion (PCM)
      single level TDR2 y.cervicaln = 218fusion, n = 1855.7° (0–17.2),

      fusion: 0.8°

      (0–6.3)
      Odoms criteria: excellent or good:

      91.5%, fusion: 86.3%; Overall success:

      TDR: 75.1%, fusion: 64.9%;

      NDI 10.9, fusion: 12.75, ≥ 20 mm

      improvement of VAS: neck pain: 74.3%, fusion: 75.3%; worst arm

      pain: 79.1%, fusion: 75.3%
      related SAE: 5.6%, fusion: 7.4%; Secondary

      surgeries: 5.2%, fusion: 5.4%
      (
      • Gragnaniello C.
      • Seex K.A.
      • Eisermann L.G.
      • et al.
      Early postoperative dislocation of the anterior maverick lumbar disc prosthesis: report of 2 cases.
      )
      case reportMavericksingle level TDR: n = 1, hybrid: n = 1,

      report on

      dislocations
      lumbarn = 2dislocation 2 w. p.o., TDR removal and

      fusion, 1 y. p.o.: recovered well
      Table 3Studies that evaluate the tribology or mechanical properties of TDRs.
      ReferenceStudy TypeTDRStudy DescriptionRegionCompared withResults
      Volumetric WearOther results
      ↓ tribology: ceramics for articulation ↓
      (
      • Siskey R.
      • Ciccarelli L.
      • Lui M.K.C.
      • Kurtz S.M.
      Are PEEK-on-ceramic bearings an option for total disc arthroplasty? An in vitro tribology study.
      )
      In vitroSimplify® Disccervicalidealized: 0.7 ± 0.1, impingement:

      1.5 ± 0.4, abrasive: 2.1 ± 0.5
      (
      • Shaheen A.
      • Shepherd D.E.T.
      Lubrication regimes in lumbar total disc arthroplasty.
      )
      In silicoball-and-socket;

      alumina-on-alumina
      elastohydrodynamic lubrication

      theory; ball radii: 14, 21, 28 mm
      lumbarsame design made from: Co-Cr-Mo on

      Co-Cr-Mo, Co-Cr-Mo on UHMWPE
      MOM and MOP: boundary lubrication

      regimes CoC: for ball radius: 14 mm and velocity > 0.9 rad/s:

      mixed lubrication regime, for 21 mm

      & 28 mm: potentially fluid-film lubrication
      (
      • Rotaru I.
      • Olaru D.
      Theoretical pressure and friction in total disc prosthesis for lumbar spine. Influence of ball radius and biomaterial combination.
      )
      In silicoball-and-socket;

      alumina-on-alumina
      ball radii(mm):

      8, 10, 12, 14, 16, 18, radial clearance: 0.05 mm
      lumbarsame design made from: Co-Cr-Mo on

      Co-Cr-Mo, Co-Cr-Mo on UHMWPE
      increasing ball radius increased frictional

      torque and decreased maximum

      pressure; MoM and CoC: higher maximal

      pressures than MoP MoM: higher

      frictional torque than MoP and CoC
      (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      )
      In silicoball-and-socket;

      alumina-on-alumina
      radial clearances: 0.05, 0.1, 0.2 mmlumbarradial clearances: 0.05, 0.1, 0.2 mm:

      0.01, 0.01, 0.01
      increase of radial clearance increases

      contact pressure; low radial clearance

      lead to lowest wear
      (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      )
      In silicoball-and-socket;

      alumina-on-alumina,

      zirconia-on-zirconia
      lumbarsame design made from: CoCrMo-

      UHMWPE, 316 SS-UHMWPE,

      Ti6Al4V-UHMWPE, CoCrMo-

      CoCrMo, Ti6Al4V-Ti6Al4V
      CoCrMo-UHMWPE: 6.34, 316 SS-UHMWPE: 11.18,

      Ti6Al4V-UHMWPE: 16.72, CoCrMo-CoCrMo: 0.06, Ti6Al4V-Ti6Al4V: 1.98, zirconia-zirconia:

      1.29, alumina-alumina: 0.01
      ↓ tribology: ceramics for osseointegration ↓
      (
      • Brown T.
      • Bao Q.-B.
      The use of self-mating PEEK as an alternative bearing material for cervical disc arthroplasty: a comparison of different simulator inputs and tribological environments.
      )
      In vitroNuNeccervicalsame TDR coating removed1.23 ± 0.07, control: 0.89 ± 0.08
      ↓ mechanical properties: ceramics for osseointegration ↓
      (
      • Gloria A.
      • De Santis R.
      • Ambrosio L.
      • et al.
      A multi-component fiber-reinforced PHEMA-based hydrogel/HAPEXTM device for customized intervertebral disc prosthesis.
      )
      In vitrofiber reinforced hydrogel

      with HAPEX™

      endplates
      static and dynamic

      testing; static:

      compression,

      compression-shear,

      torsion
      lumbardynamic properties: lumbar porcine IVDsappropriate mechanical behaviour
      All values are mean (range) or mean ± SD, MC = million cycles, MOM = metal-on-Metal, MOP = metal-on-polymer, COC = ceramic-on-ceramic, volumetric wear in mm3/MC.
      Table 4in vivo nonhuman and cadaveric studies investigating biomechanical effects and osseointegration.
      ReferenceStudy TypeModelTDRStudy descriptionRegionSubjectsCompared withResults
      RoMIngrowthOther results
      ↓ Ceramics for articulation ↓
      (
      • Finn M.A.
      • Brodke D.S.
      • Daubs M.
      • et al.
      Local and global subaxial cervical spine biomechanics after single-level fusion or cervical arthroplasty.
      )
      ex vivohumanAltia TDI™single level TDRcervicalcadaveric specimens:

      C2/C7; n = 6
      intact, fusionintact: 7.1° ± 2.4,

      TDR: 5.9° ± 3.1,

      fusion: 1.5° ± 1.2
      (
      • Dooris A.P.
      • Goel V.K.
      • Grosland N.M.
      • et al.
      Load-sharing between anterior and posterior elements in a lumbar motion segment implanted with an artificial disc.
      )
      ex vivo, in silicohumanSofamor Daneksingle level TDR; FEMlumbarcadaveric specimens: L1/S1; n = 7intactload-displacement behaviour of cadaveric specimens and FEM FSU, facet loads of FEM FSU
      ↓ Ceramics for osseointegration ↓
      (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of cervical artificial disc reconstruction: in vitro human cadaveric spine model.
      )
      ex vivohuman3-DF discsingle level TDRcervicalcadaveric

      specimens:

      Occipital-T2; n = 7
      intact, fusioncompared to intact: TDR: 145.2% ± 41.6, autograft: 46.6% ± 32.4, plate: 16.8% ± 10.9no loosening or dislodgement
      (
      • Lou J.
      • Wu W.
      • Li H.
      • et al.
      Analysis of bony ingrowth in novel cervical disc prosthesis.
      )
      in vivogoatPretic-Isingle level TDR; killing: 6 m. p.o.cervicaln = 842.5% (32.5–54.6)no subsidence, migration or

      loosening
      (
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      )
      in vivo, ex vivogoatPCMsingle level TDR; killing: n = 6: 6 m. p.o., n = 6: 12 m. p.o.cervicaln = 12intactintact: 15.96° ± 0.15, TDR:

      significantly lower than intact
      40.51% ± 24.35 6 m. p.o. and 58.65% ± 28.04

      12 m. p.o.
      no loosening, no migration, no subsidence, no endplate radiolucencies
      (
      • Cunningham B.W.
      • Hu N.
      • Zorn C.M.
      • McAfee P.C.
      Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation.
      )
      in vivoPCM: goat,

      Charite: baboon
      Charité; PCMsingle level TDR; goats: 6 m. and 12 m. n = 6 killed p.o.; baboons: killed 6 m. p.o.PCM: cervical,

      Charité: lumbar
      goats: n = 12;

      baboons: n = 17
      cervical: 39% (1–78); lumbar: 46% (2–78); ingrowth

      depending on TDR

      placement: cervical: ideal placement: 44% ± 23, suboptimal: 26% ± 33,

      poor: 21% ± 30; lumbar: ideal:

      51% ± 13, suboptimal: 49% ± 19, poor: 34% ± 29
      (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      )
      ex vivohumanfabric TDRsingle level TDR (L4/L5)lumbarcadaveric specimens:

      L1/S1; n = 7
      L4/L5: fusion; L2/L3:

      fabric subtotal disc replacement;

      L2/L3: fusion
      L2/L3: intact: 6.0° ± 1.7; fabric subtotal disc replacement:

      8.2° ± 1.4; fusion:1.2° ± 1.0; L4/L5: intact: 7.7° ± 3.2; fabric TDR: 8.7° ± 4.3; cage: 3.4° ± 3.4; cage

      and pedicle screw instrumentation:

      0.7° ± 0.6
      no loosening, no dislodgement
      (
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      )
      in vivo, ex vivosheepfabric TDRdual level TDR; Group 1: no

      fixation, Group 2: rod (temporary),

      Group 3: HA/PLLA rod

      (bioresorbable); killing 6, 15,

      24 m. p.o.
      lumbarGroup 1: n = 13,

      Group 2: n = 13,

      Group 3: n = 10
      intact: n = 10intact: 11.4°, Group 1: 28%

      of intact; Group 2: 65%; Group

      3: 15 and 24 m. p.o.: 49% and

      40%
      trabeculae inserting

      into fabrics: Group 1:36%,

      Group 2 (6 m.p.o.): 63%, Group

      3 (24 m.p.o.): 80%
      Group 1: 6 months p.o.:

      some displacements without

      dislodgement; Group 2: 6 m.

      p.o.: implants in place, Group

      3: 6 m. p.o.: all rods broke
      (
      • Takahata M.
      • Kotani Y.
      • Abumi K.
      • et al.
      Bone ingrowth fixation of artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric.
      )
      in vivosheep3-DF:

      FABRICUBE
      dual level TDR; additonal: rod

      (Group 1a), 4 and 6 m. p.o.: n = 4

      killed, HA/PLLA rod (bioresorbable)

      (Group 1b), 15 and 24 m. p.o.: n = 4 killed
      lumbarGroup 1a: n = 8,

      Group 1b: n = 8
      dual level fusion: bioceramic

      spacers, additonal

      rod system, n = 4,

      (Group 2), killing 6 m.

      p.o.
      Group 1a: 4 m. p.o.: 2/4 cases, at 6 m. p.o.: 3/4

      cases, Group 1b: all cases, Group 2: 3/4 cases
      no loosening
      (
      • Shikinami Y.
      • Kawabe Y.
      • Yasukawa K.
      • et al.
      A biomimetic artificial intervertebral disc system composed of a cubic three-dimensional fabric.
      )
      in vivo, ex vivobaboons3-DF discsingle level TDR; killing: 6 m. p.o.lumbarn = 8intactintact: 6.9° ± 2.9; TDR: 4.9° ± 1.9
      (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      )
      in vivo, ex vivobaboonsSB Charitésingle level TDR; killing: 6 m. p.o.lumbarn = 7AcroFlex (DePuy-

      Acromed, USA), n = 10; intact n = 10
      Intact: 6.93° ± 2.90; SB

      Charite: 7.71° ± 3.30;

      AcroFlex: 3.86° ± 1.51
      SB Charite: 47.9% ± 8.12, AcroFlex: 54.59% ± 13.24no loosening, no migration,

      no lucencies
      (
      • McAfee P.C.
      • Cunningham B.W.
      • Orbegoso C.M.
      • et al.
      Analysis of porous ingrowth in intervertebral disc prostheses: a nonhuman primate model.
      )
      in vivo, ex vivobaboonsSB Charitésingle level TDR; killing: 6 m. p.o.lumbarn = 7fusion: n = 10 (historical);

      intact: n = 10
      intact: 5.9° ± 2.9; TDR: 7.7° ± 3.3; Fusion: 1.69° ± 1.2047.9% (35.5–58.8)no loosening or radiolucency,

      no migration
      Age in years, all values are mean (range) or mean ± SD, n = # subjects; nTDR = #TDRs, y. = years, m. = months, RoM = range of motion, FSU = functional spinal unit.

      3.1 Clinical studies

      Most clinical studies in this review focused on cervical TDRs with only one case report dedicated to a lumbar device (Table 2). The investigated devices use ceramics for two applications: articulating surfaces and promotion of osseointegration.

      3.1.1 Patient outcomes

      In the two clinical trials with the longest follow-up time, overall success was found in 74.3% of patients treated at a single level (
      • Gornet M.F.
      • Burkus J.K.
      • Shaffrey M.E.
      • et al.
      Cervical disc arthroplasty: 10-year outcomes of the prestige LP cervical disc at a single level.
      ) and 80.4% when treated at two levels (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ) with the same TDR ten years postoperatively (y. p.o.). Here, overall success was defined as improvement of Neck disability index (NDI) ≥ 7.5 points, no neurological worsening, no serious adverse events related to implant or implantation (related SAE), no secondary surgery due to treatment failure. With a similar definition, (
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ) found success in 93% of TDR and 73.6% of fusion patients 2 y. p.o. (NDI improvement value converted to 50 point scale). The percentage of patients who received a ceramic TDR with clinical outcome rated as “excellent” or “good”, according to Odom's criteria, ranged from 76% (single level group) (
      • Pimenta L.
      • McAfee P.C.
      • Cappuccino A.
      • et al.
      Superiority of multilevel cervical arthroplasty outcomes versus single-level outcomes: 229 consecutive PCM prostheses.
      ) to 100% (
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      ). Statistically significant superiority of overall success compared to fusion was reported in the few studies that made this comparison (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ;
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ).
      Mean reported NDIs range from 6.8 (
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ) to 14.80 (facet tropism group) (
      • Liu Z.
      • Rong X.
      • Liu H.
      • et al.
      Effect of facet tropism on postoperative cervical range of motion after single-level cervical disc arthroplasty.
      ) for TDRs that use ceramics for articulation, and from 5.77 (
      • Shi S.
      • Zheng S.
      • Li X.-F.
      • et al.
      Comparison of 2 zero-profile implants in the treatment of single-level cervical spondylotic myelopathy: a preliminary clinical study of cervical disc arthroplasty versus fusion.
      ) to 23.8 (
      • Choi D.
      • Petrik V.
      • Fox S.
      • et al.
      Motion preservation and clinical outcome of porous coated motion cervical disk arthroplasty.
      ) for TDRs that use ceramics for osseointegration (0–50 scale, converted when reported differently). NDI scores of patients who received TDR were significantly lower, i.e. superior (
      • Geisler F.H.
      • Maislin D.G.
      • Keenan B.T.
      • Maislin G.
      One-year {NDI} and {VAS} outcomes from the single-level {PEEK}-on-ceramic {SimplifyTM} Disc {FDA} {IDE} trial.
      ;
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ;
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ) or not statistically different (
      • Shi S.
      • Zheng S.
      • Li X.-F.
      • et al.
      Comparison of 2 zero-profile implants in the treatment of single-level cervical spondylotic myelopathy: a preliminary clinical study of cervical disc arthroplasty versus fusion.
      ), when compared to fusion outcomes.
      Mean postoperative neck pain in patients treated with ceramic TDRs, ranged from 13 mm (
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      ) to 31 mm (
      • Enan A.
      • Abu-Hegazy M.
      • Abo-Hegy M.
      • Al-Kerdany A.
      Single level cervical arthroplasty with the discocerv® prosthesis : a preliminary report.
      ) on a 0 mm–100 mm Visual Analog Scale (VAS). Mean reported radicular/arm pain ranged from 5 (
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      ) to 22 (
      • Enan A.
      • Abu-Hegazy M.
      • Abo-Hegy M.
      • Al-Kerdany A.
      Single level cervical arthroplasty with the discocerv® prosthesis : a preliminary report.
      ). Neck pain scores in TDR groups were significantly lower than in patients treated with fusion (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ), however, radicular/arm pain scores were not (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ). There are inter-study methodological variations in the reporting of pain scores: (
      • Enan A.
      • Abu-Hegazy M.
      • Abo-Hegy M.
      • Al-Kerdany A.
      Single level cervical arthroplasty with the discocerv® prosthesis : a preliminary report.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ;
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      ) used a simple VAS scale from 0 mm–100 mm, (
      • Gornet M.F.
      • Burkus J.K.
      • Shaffrey M.E.
      • et al.
      Cervical disc arthroplasty: 10-year outcomes of the prestige LP cervical disc at a single level.
      ) used a numeric scale and multiplied duration (0−10) with intensity (0–10) for neck and arm pain, whereas (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ) used numeric ratings based on frequency and intensity. Others did not report arm and neck pain separately but used a single value for both (
      • Choi D.
      • Petrik V.
      • Fox S.
      • et al.
      Motion preservation and clinical outcome of porous coated motion cervical disk arthroplasty.
      ;
      • Geisler F.H.
      • Maislin D.G.
      • Keenan B.T.
      • Maislin G.
      One-year {NDI} and {VAS} outcomes from the single-level {PEEK}-on-ceramic {SimplifyTM} Disc {FDA} {IDE} trial.
      ;
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ) or only improvement (
      • Pimenta L.
      • McAfee P.C.
      • Cappuccino A.
      • et al.
      Superiority of multilevel cervical arthroplasty outcomes versus single-level outcomes: 229 consecutive PCM prostheses.
      ).

      3.1.2 Complications

      The rates of serious adverse events related to implant or implantation (related SAE) reported for ceramic TDRs varied from 0.7% (
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ) to 7.8% (
      • Gornet M.F.
      • Burkus J.K.
      • Shaffrey M.E.
      • et al.
      Cervical disc arthroplasty: 10-year outcomes of the prestige LP cervical disc at a single level.
      ). One study found significantly less related SAE in the ceramic TDR group compared to the fusion group (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ), but another study found no statistical difference (
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ). Reported rates of secondary surgeries following treatment with ceramic TDR were in the range from <1% (1/148) at 1 y. p.o. (
      • Geisler F.H.
      • Maislin D.G.
      • Keenan B.T.
      • Maislin G.
      One-year {NDI} and {VAS} outcomes from the single-level {PEEK}-on-ceramic {SimplifyTM} Disc {FDA} {IDE} trial.
      ) to 10.3% at 10 y. p.o. (
      • Gornet M.F.
      • Burkus J.K.
      • Shaffrey M.E.
      • et al.
      Cervical disc arthroplasty: 10-year outcomes of the prestige LP cervical disc at a single level.
      ). Compared to patients treated with fusion, patients who received ceramic TDR required significantly fewer secondary surgeries (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ) or there was no statistically significant difference (
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ).
      Case reports present 2 early dislocations of a device using ceramics for osseointegration (
      • Gragnaniello C.
      • Seex K.A.
      • Eisermann L.G.
      • et al.
      Early postoperative dislocation of the anterior maverick lumbar disc prosthesis: report of 2 cases.
      ) and one ceramic fracture of a TDR using ceramics for articulation (
      • Nguyen N.Q.
      • Kafle D.
      • Buchowski J.M.
      • et al.
      Ceramic fracture following cervical disc arthroplasty: a case report.
      ). Implant instability (detectable motion between TDR and bone) was reported in 8% of cases in one study (
      • Skeppholm M.
      • Svedmark P.
      • Noz M.E.
      • et al.
      Evaluation of mobility and stability in the discover artificial disc: an in vivo motion study using high-accuracy 3D CT data.
      ), anterior migration in 11.7% and migration in 16.7% (
      • Shi S.
      • Zheng S.
      • Li X.-F.
      • et al.
      Comparison of 2 zero-profile implants in the treatment of single-level cervical spondylotic myelopathy: a preliminary clinical study of cervical disc arthroplasty versus fusion.
      ) for a TDR using ceramics to encourage osseointegration. They point to (
      • Thaler M.
      • Hartmann S.
      • Gstöttner M.
      • et al.
      Footprint mismatch in total cervical disc arthroplasty.
      ) that showed that about 60% of this TDRs footprints do not match anatomical size which could lead to migration – explaining this with a design issue, not a material issue. The percentage of patients with grade 3 or 4 heterotopic ossification was reported in a range from 4.4% treated with a TDR using ceramics to enhance osseointegration to 39.0% at either/both of the two levels treated with a TDR using ceramics for articulation (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ). Physiological motion, defined as >5° flexion/extension, was found in only 21% of patients (
      • Choi D.
      • Petrik V.
      • Fox S.
      • et al.
      Motion preservation and clinical outcome of porous coated motion cervical disk arthroplasty.
      ); ankylosis (no detectable motion) was reported in 5% of cases (
      • Skeppholm M.
      • Svedmark P.
      • Noz M.E.
      • et al.
      Evaluation of mobility and stability in the discover artificial disc: an in vivo motion study using high-accuracy 3D CT data.
      ) both for TDRs using cermics to improve osseointegration. Incidence of dysphagia was significantly lower in a group treated with a TDR using ceramics to enhance osseointegration than in the fusion control group 2 y. p.o. (
      • McAfee P.C.
      • Cappuccino A.
      • Cunningham B.W.
      • et al.
      Lower incidence of dysphagia with cervical arthroplasty compared with ACDF in a prospective randomized clinical trial.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ) but incidence of dysphonia was similar (
      • McAfee P.C.
      • Cappuccino A.
      • Cunningham B.W.
      • et al.
      Lower incidence of dysphagia with cervical arthroplasty compared with ACDF in a prospective randomized clinical trial.
      ). (
      • Gornet M.F.
      • Singh V.
      • Schranck F.W.
      • et al.
      Serum metal concentrations in patients with titanium ceramic composite cervical disc replacements.
      ) found titanium concentrations in blood serum to be significantly higher at every time point p.o. than pre.o. in patients treated with a titanium alloy/titanium carbide composite TDR.

      3.1.3 Motion

      Mean range of motion (RoM) of the operated levels in flexion/extension ranged from 4.9° (3 months p.o.) (
      • Ramadan A.S.
      • Mitulescu A.
      • Schmitt P.
      Total cervical disc replacement with the Discocerv® (Cervidisc evolution) cervical prosthesis: early results of a second generation.
      ) to 12.9° (1 and 7 y. p.o.) (
      • Enan A.
      • Abu-Hegazy M.
      • Abo-Hegy M.
      • Al-Kerdany A.
      Single level cervical arthroplasty with the discocerv® prosthesis : a preliminary report.
      ; (
      • Swamy G.N.
      • Hopwood S.
      • Nanjayan S.K.
      • et al.
      Clinical and radiological review of a semi-constrained cervical disc replacement with a ceramic-ceramic articulation with a minimum seven years follow-up.
      ), with both values reported for the same TDR device.

      3.2 Tribological and mechanical studies (studies without biological specimens)

      3.2.1 Tribology

      Wear of cervical ceramic TDRs (Table 3) was investigated in vitro using spine wear simulators. For a ceramic-on-polymer TDR, idealized, impingement and abrasive wear modes caused mean volumetric wear rates of 0.7 mm3/MC, 1.5 mm3/MC, and 2.1 mm3/MC, respectively with most wear particles originating from PEEK endplates (MC = million cycles) (
      • Siskey R.
      • Ciccarelli L.
      • Lui M.K.C.
      • Kurtz S.M.
      Are PEEK-on-ceramic bearings an option for total disc arthroplasty? An in vitro tribology study.
      ). Third body wear of a ceramic coating used in TDRs for osseointegration was investigated and while coated devices produced more wear (1.23 mm3/MC) than similar devices in which the coating was removed (0.89 mm3/MC), no third body wear was found in microscopic inspection (
      • Brown T.
      • Bao Q.-B.
      The use of self-mating PEEK as an alternative bearing material for cervical disc arthroplasty: a comparison of different simulator inputs and tribological environments.
      ). The authors speculated that the difference in wear rates could have been due to specimen handling.
      Computational tribological studies were performed for lumbar TDRs using ceramics for articulation, in a generic ball-and-socket design, rather than true to detail of a specific product. Wear of lumbar ceramic TDRs was studied in silico, using the Finite Element Method assuming linear wear based on Archard's wear theory, volumetric wear calculated from linear wear and geometry adjusted due to linear wear (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      ;
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ). These studies reported volumetric wear rates ranging from 0.0113 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      ) to 1.293 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ).
      In silico tribological studies furthermore investigated factors that influence wear: lubrication regimes (
      • Shaheen A.
      • Shepherd D.E.T.
      Lubrication regimes in lumbar total disc arthroplasty.
      ), design optimization for reduced friction (pressure & torque) (
      • Rotaru I.
      • Olaru D.
      Theoretical pressure and friction in total disc prosthesis for lumbar spine. Influence of ball radius and biomaterial combination.
      ), effect of radial clearance on wear and contact pressure (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      ) and wear in different bearing couples (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ). Low radial clearances (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      ), small ball radii (
      • Rotaru I.
      • Olaru D.
      Theoretical pressure and friction in total disc prosthesis for lumbar spine. Influence of ball radius and biomaterial combination.
      ) and ceramic-on-ceramic bearing couples (
      • Shaheen A.
      • Shepherd D.E.T.
      Lubrication regimes in lumbar total disc arthroplasty.
      ;
      • Rotaru I.
      • Olaru D.
      Theoretical pressure and friction in total disc prosthesis for lumbar spine. Influence of ball radius and biomaterial combination.
      ) seem advantageous for lumbar TDRs with low wear.

      3.2.2 Mechanical properties

      Mechanical testing of a TDR with endplates made of a hydroxyapatite composite with a hydrogel center reinforced with PET fibers lead the authors to conclude that the mechanical behaviour of the device was appropriate (
      • Gloria A.
      • De Santis R.
      • Ambrosio L.
      • et al.
      A multi-component fiber-reinforced PHEMA-based hydrogel/HAPEXTM device for customized intervertebral disc prosthesis.
      ) (Table 3). Mechanical testing conducted in another study leads the authors of this study to judge the evaluated TDR as appropriate (
      • Shikinami Y.
      • Kawabe Y.
      • Yasukawa K.
      • et al.
      A biomimetic artificial intervertebral disc system composed of a cubic three-dimensional fabric.
      ) (Table 4).

      3.3 Ex vivo and nonhuman in vivo studies

      In vivo and ex vivo studies of ceramic TDRs (Table 4) investigated mainly segmental kinematics and/or osseointegration and were based on human cadaveric models with devices implanted postmortem, or nonhuman models (baboon, goats and sheep) with devices implanted in vivo and investigated ex vivo.

      3.3.1 Motion

      RoM for flexion/extension in the operated levels ranged from 5.9° (cervical) (
      • Finn M.A.
      • Brodke D.S.
      • Daubs M.
      • et al.
      Local and global subaxial cervical spine biomechanics after single-level fusion or cervical arthroplasty.
      ) to 8.7° (lumbar) (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      ) in cadaveric studies using human models. In nonhuman models (sheep, goat, baboon), reported RoM ranged from 3.2° (lumbar) (Kotani et al., 2004) to 7.7° (lumbar) (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ;
      • McAfee P.C.
      • Cunningham B.W.
      • Orbegoso C.M.
      • et al.
      Analysis of porous ingrowth in intervertebral disc prostheses: a nonhuman primate model.
      ). Compared to the intact condition, RoM in flexion/extension was significantly reduced (
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      )), in 2 of 3 investigated groups of (
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      ), significantly increased (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of cervical artificial disc reconstruction: in vitro human cadaveric spine model.
      ) or not significantly different (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ;
      • McAfee P.C.
      • Cunningham B.W.
      • Orbegoso C.M.
      • et al.
      Analysis of porous ingrowth in intervertebral disc prostheses: a nonhuman primate model.
      ;
      • Shikinami Y.
      • Kawabe Y.
      • Yasukawa K.
      • et al.
      A biomimetic artificial intervertebral disc system composed of a cubic three-dimensional fabric.
      ; (
      • Finn M.A.
      • Brodke D.S.
      • Daubs M.
      • et al.
      Local and global subaxial cervical spine biomechanics after single-level fusion or cervical arthroplasty.
      ; (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      ).

      3.3.2 Osseointegration

      Studies defining ingrowth [%] as:”apparent bone contact area/gross total endplate area”, reported means in the range from 39% (
      • Cunningham B.W.
      • Hu N.
      • Zorn C.M.
      • McAfee P.C.
      Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation.
      ) to 58.65% (
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      ). In a study comparing two TDRs with ceramic coating vs. sintered titanium beading, mean ingrowth was 47.9% and 54.59%, respectively (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ). The authors of this study judge, that both devices achieved complete osseointegration. Ingrowth over time was also investigated, with reported mean values of 40.51% at 6 m. p.o. and 58.65% 12 m. p.o. (
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      ). TDR positioning was reported to affect osseointegration with a reported mean ingrowth values of 44% for optimal vs 21% for poor placement, for a cervical device, and 51% for ideal vs. 34% for poor placement, in case of a lumbar device, with both TDRs having the same ceramic coating (
      • Cunningham B.W.
      • Hu N.
      • Zorn C.M.
      • McAfee P.C.
      Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation.
      ). The tensile failure strength of the implant-bone connection 6 m. p.o. was reported to be significantly lower for the fusion group (0.15 MPa, bioceramic spacer) than for the ceramic-coated TDR group (1.79 MPa) (
      • Takahata M.
      • Kotani Y.
      • Abumi K.
      • et al.
      Bone ingrowth fixation of artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric.
      ).
      No device loosening was observed in postmortem implantations in human spines (
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of cervical artificial disc reconstruction: in vitro human cadaveric spine model.
      ;
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      ) or in vivo nonhuman studies (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ;
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      ;
      • Lou J.
      • Wu W.
      • Li H.
      • et al.
      Analysis of bony ingrowth in novel cervical disc prosthesis.
      ;
      • Takahata M.
      • Kotani Y.
      • Abumi K.
      • et al.
      Bone ingrowth fixation of artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric.
      ). Consequently, no migration (
      • Lou J.
      • Wu W.
      • Li H.
      • et al.
      Analysis of bony ingrowth in novel cervical disc prosthesis.
      ;
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ;
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model.
      ;
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      ;
      • Kotani Y.
      • Cunningham B.W.
      • Abumi K.
      • et al.
      Multidirectional flexibility analysis of cervical artificial disc reconstruction: in vitro human cadaveric spine model.
      ) was found. In one study though (
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      ), some implant displacements were reported in the group without additional fixation, however none in the group with temporary fixation.

      4. Discussion

      The aim of this review was to assess the evidence on the use of ceramics in TDRs. To this end, we compiled clinical, tribological, biomechanical, osseointegrative and mechanical evidence reported in 36 scientific publications identified through the systematic literature search. TDRs described in peer-reviewed literature use ceramics for articulation or for osseointegration (a TDR using ceramics to reinforce a polymer can be found in non-peer reviewed literature (
      • Boughton P.
      • Ferris D.
      • Ruys A.J.
      A ceramic-polymer functionally graded material: A novel disk prosthesis.
      )). Clinical evidence was found to support application of ceramics in TDRs for both articulation and osseointegration, but was limited to largely cervical devices. Tribological and mechanical evidence are promising but sparse. Ex vivo and nonhuman in vivo studies indicate appropriateness of ceramics for TDRs in the investigated aspects.

      4.1 Clinical outcomes

      The clinical studies identified and included in this review regard almost exclusively cervical devices; beside a single case report, no clinical studies were found on ceramic TDRs for lumbar spine application. Clinical evidence suggest that ceramic TDRs (alike non-ceramic ones) restore segmental motion and result in non-inferior and possibly superior outcomes to spinal fusion. The use of ceramics does seem to fullfill the basic function of TDRs, to preserve motion of the treated segment. Clinical studies reviewed in this work reported mean RoMs in flexion/extension in a range from 4.9° to 12.9°, which is substantially more than 0.48° to 1.03° reported for fusion controls. Generally, cervical TDRs (ceramic and non-ceramic) provide RoM of 8° on average, which is materially greater than about 1° for RoM after cervical fusion (
      • Findlay C.
      • Ayis S.
      • Demetriades A.K.
      Total disc replacement versus anterior cervical discectomy and fusion.
      ).
      Reported patient outcomes following treatment with cervical ceramic TDRs were found to be significantly better than after fusion when evaluated as the overall treatment success (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ;
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ), and comparable (
      • Shi S.
      • Zheng S.
      • Li X.-F.
      • et al.
      Comparison of 2 zero-profile implants in the treatment of single-level cervical spondylotic myelopathy: a preliminary clinical study of cervical disc arthroplasty versus fusion.
      ) or better (
      • Geisler F.H.
      • Maislin D.G.
      • Keenan B.T.
      • Maislin G.
      One-year {NDI} and {VAS} outcomes from the single-level {PEEK}-on-ceramic {SimplifyTM} Disc {FDA} {IDE} trial.
      ;
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ;
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ;
      • Guyer R.D.
      • Coric D.
      • Nunley P.D.
      • et al.
      Single-level cervical disc replacement using a PEEK-on-ceramic implant: results of a multicenter FDA IDE trial with 24-month follow-up.
      ) when evaluated using NDI. This is in line with the literature, as significantly more favorable treatment outcomes (
      • Findlay C.
      • Ayis S.
      • Demetriades A.K.
      Total disc replacement versus anterior cervical discectomy and fusion.
      ;
      • Hu Y.
      • Lv G.
      • Ren S.
      • Johansen D.
      Mid- to long-term outcomes of cervical disc arthroplasty versus anterior cervical discectomy and fusion for treatment of symptomatic cervical disc disease: a systematic review and meta-analysis of eight prospective randomized controlled trials.
      ;
      • Cai S.
      • Tian Y.
      • Zhang J.
      • et al.
      Efficacy and safety of total disc replacement compared with anterior cervical discectomy and fusion in the treatment of cervical disease.
      ) and lower NDI values (
      • Zhang Y.
      • Liang C.
      • Tao Y.
      • et al.
      Cervical total disc replacement is superior to anterior cervical decompression and fusion: a Meta-analysis of prospective randomized controlled trials.
      ) were reported for cervical TDRs when compared to fusion in previous meta-analyses of randomized controlled trials.
      The rates of serious adverse events related to implant or implantation and secondary surgeries were similar (
      • Phillips F.M.
      • Lee J.Y.B.
      • Geisler F.H.
      • et al.
      A prospective, randomized, controlled clinical investigation comparing PCM cervical disc arthroplasty with anterior cervical discectomy and fusion: 2-year results from the US FDA IDE clinical trial.
      ) or lower (
      • Gornet M.F.
      • Lanman T.H.
      • Kenneth Burkus J.
      • et al.
      Two-level cervical disc arthroplasty versus anterior cervical discectomy and fusion: 10-year outcomes of a prospective, randomized investigational device exemption clinical trial.
      ) in patients that received ceramic TDRs compared to patients treated with fusion. Cervical TDRs in general have been previously reported to be associated with significantly lower rates of related SAE (
      • Hu Y.
      • Lv G.
      • Ren S.
      • Johansen D.
      Mid- to long-term outcomes of cervical disc arthroplasty versus anterior cervical discectomy and fusion for treatment of symptomatic cervical disc disease: a systematic review and meta-analysis of eight prospective randomized controlled trials.
      ) and secondary surgeries (
      • Hu Y.
      • Lv G.
      • Ren S.
      • Johansen D.
      Mid- to long-term outcomes of cervical disc arthroplasty versus anterior cervical discectomy and fusion for treatment of symptomatic cervical disc disease: a systematic review and meta-analysis of eight prospective randomized controlled trials.
      ;
      • Cai S.
      • Tian Y.
      • Zhang J.
      • et al.
      Efficacy and safety of total disc replacement compared with anterior cervical discectomy and fusion in the treatment of cervical disease.
      ) than fusion. In this context, the results of our review indirectly indicate that complication rates related to ceramic TDRs might be similar or slightly higher than for TDRs in general, even if still comparable or superior to fusion.
      In all of the reviewed publications, only one case of ceramic fracture was reported (
      • Nguyen N.Q.
      • Kafle D.
      • Buchowski J.M.
      • et al.
      Ceramic fracture following cervical disc arthroplasty: a case report.
      ), and may be connected to questionable patient selection (paracentral spur and foraminal stenosis). Although other clinical studies did not explicitly mention ceramic fractures, it is possible that fractures occurred but were counted as adverse events and included in the overall complication rates. However, failures of non-ceramic materials have been reported in articulating and non-articulating TDRs: fractures of polyethylene cores (
      • Kurtz S.M.
      • van Ooij A.
      • Ross R.
      • et al.
      Polyethylene wear and rim fracture in total disc arthroplasty.
      ) and rubber tears (
      • Fraser R.D.
      • Ross E.R.
      • Lowery G.L.
      • et al.
      AcroFlex design and results.
      ) in lumbar TDRs, cracking of a polyurethane sheath (
      • Fan H.
      • Wu S.
      • Wu Z.
      • et al.
      Implant failure of Bryan cervical disc due to broken polyurethane sheath.
      ) tear of a sheath (
      • Clark N.J.
      • Francois E.L.
      • Freedman B.A.
      • Currier B.
      Early implant failure of a 2-level M6-cervical total disc replacement.
      ), disintegration of sheath and artificial annulus (
      • Xia M.-A.M.
      • Winder M.J.
      M6-C cervical disc replacement failure associated with late onset infection.
      ) and defect of the artificial annulus-fibers with migration of the artificial core (
      • Brenke C.
      • Schmieder K.
      • Barth M.
      Core herniation after implantation of a cervical artificial disc: case report.
      ) for cervical TDRs.

      4.2 Tribological and mechanical studies

      The results of this review suggest that wear rates of TDRs with ceramic articulating surfaces are within appropriate range, but evidence in literature is sparse. Mean volumetric wear rates for ceramic-on-ceramic TDRs were reported between 0.0113 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear in ceramic on ceramic type lumbar total disc replacement: effect of radial clearance.
      ) and 1.293 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ) both values reported for lumbar devices investigated with in silico models. For a ceramics-on-polymer TDR, up to 2.1 mm3/MC (
      • Siskey R.
      • Ciccarelli L.
      • Lui M.K.C.
      • Kurtz S.M.
      Are PEEK-on-ceramic bearings an option for total disc arthroplasty? An in vitro tribology study.
      ) were reported in the abrasive wear mode in vitro. While the range reported for these devices overlaps with the wear rates of TDRs that do not use ceramics for articulation: 0.26 (
      • Brown T.
      • Bao Q.-B.
      The use of self-mating PEEK as an alternative bearing material for cervical disc arthroplasty: a comparison of different simulator inputs and tribological environments.
      ) - 16.715 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ), and ceramic total hip replacements: 0.014–1.015 mm3/MC (
      • Uddin M.S.
      • Zhang L.C.
      Predicting the wear of hard-on-hard hip joint prostheses.
      ), the range for non-ceramic TDRs is scattered much wider with wear rates reported as high as 16.715 mm3/MC (
      • Shankar S.
      • Kesavan D.
      Wear prediction of the lumbar total disc replacement using finite element method.
      ) which is about eigth times higher than the highest rate for a ceramic TDR (ceramic-on-polymer, 2.1 mm3/MC). Even though the reported works addressed all three aspects of tribology: wear, friction and lubrication, the current evidence on tribology of ceramic TDRs is limited by the small number of studies published. Furthermore, only one study evaluated wear of ceramic bearing couples experimentally (
      • Siskey R.
      • Ciccarelli L.
      • Lui M.K.C.
      • Kurtz S.M.
      Are PEEK-on-ceramic bearings an option for total disc arthroplasty? An in vitro tribology study.
      ), while other works were performed in silico. Computational models of complex multi-factorial phenomena, such as wear, might suffer from assumptions introducing limitations to their predictions. The current evidence indicates that the tribology of ceramic TDRs is suitable, but future work is needed to investigate not only wear rates but also the effects of wear debris from different materials/material pairings.
      We judge the level of evidence on ceramic TDRs mechanical properties in scientific literature as very sparse but refer interested readers to “FDA Summary of Safety and Effectiveness Data” documents () that complement this by offering information on mechanical testing. These documents were not included in this review, as they are not peer-reviewed scientific literature, which is one of the exclusion criteria.

      4.3 Ex vivo and non-human in vivo studies

      Reported mean RoM in flexion/extension compared to intact showed no clear tendency for hypermobility or loss of mobility of patients receiving ceramic TDRs.
      Some TDRs use ceramics to enhance osseointegration. The reviewed in vivo nonhuman studies reported mean ingrowth of these devices between 39% (
      • Cunningham B.W.
      • Hu N.
      • Zorn C.M.
      • McAfee P.C.
      Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation.
      ) and 58.65% (
      • Hu N.
      • Cunningham B.W.
      • McAfee P.C.
      • et al.
      Porous coated motion cervical disc replacement: a biomechanical, histomorphometric, and biologic wear analysis in a caprine model.
      ), whereas for non-ceramic TDRs ingrowth rates were reported between 30.1% (
      • Jensen W.K.
      • Anderson P.A.
      • Nel L.
      • Rouleau J.P.
      Bone ingrowth in retrieved Bryan cervical disc prostheses.
      ) and 54.59% (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ). Only one of the reviewed studies compared ingrowth of a ceramic to a non-ceramic TDR, which was in favour of the non-ceramic TDR (54.59% ingrowth vs. 47.9%) but the statistical significance of the difference was not reported (
      • Cunningham B.W.
      • Dmitriev A.E.
      • Hu N.
      • McAfee P.C.
      General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.
      ). Most of the reviewed nonhuman and cadaveric studies reported that no device loosening or dislodgement occurred, except for (
      • Kotani Y.
      • Abumi K.
      • Shikinami Y.
      • et al.
      Two-year observation of artificial intervertebral disc replacement: results after supplemental ultra-high strength bioresorbable spinal stabilization.
      ) who reported some implant displacements. These results indicate use of ceramics is rather similar in terms of promoting bone ingrowth, compared to non-ceramic materials used for this purpose. Studies evaluating osseointegration of non-ceramic TDRs appear to be not more abundant than those dedicated to the osseointegration achieved by ceramics.

      4.4 Limitations

      It is possible that some relevant publications were not identified by our search criteria, e.g. if the materials of the investigated TDR were not explicitly reported and thus not found by our search string. While patient selection criteria were stricter in some studies, others included TDRs implanted in complex revision surgeries (
      • Pimenta L.
      • McAfee P.C.
      • Cappuccino A.
      • et al.
      Superiority of multilevel cervical arthroplasty outcomes versus single-level outcomes: 229 consecutive PCM prostheses.
      ) or patients that had prior surgeries (
      • McAfee P.C.
      • Cappuccino A.
      • Cunningham B.W.
      • et al.
      Lower incidence of dysphagia with cervical arthroplasty compared with ACDF in a prospective randomized clinical trial.
      ) – no distinction was made between these studies in this review. Generally, varying choice and definitions of outcome measures and greatly varying follow-up time, limits our ability to do a direct inter-study comparison. It was not possible to make distinctions between the single ceramic materials` outcomes as the data was scarce, and only few studies allowed direct comparison between ceramic and non-ceramic TDRs. Studies may have reported on the same cohorts which may create a false impression of greater evidence collected on a certain device where in fact the same samples were analyzed for multiple aspects.

      5. Conclusion and future outlook

      Ceramics are used for articulation and osseointegration in TDRs for cervical applications, with safety and efficacy confirmed in clinical studies, with up to 10 and 3.3 years follow-up, for articulation and osseointegration applications, respectively.
      Tribological and in vivo studies suggest promising wear properties but not advantageous osseointegration properties over non-ceramic designs. Although stronger clinical evidence exists for cervical devices, the number of nonhuman and in silico studies on lumbar ceramic TDRs indicates development efforts of TDRs using ceramics also for lumbar applications (that have been generally limited due to past challenges). In conclusion, the current state of knowledge is that the use of ceramics in TDR design does not compromise device properties or performance. Results of pre-clinical studies encourage further research and development of ceramic TDRs for possible improvements in TDR properties and expanding their applications, which would have a great potential in the growing market of motion-preserving spinal treatments. Future studies should focus on addressing the following knowledge gaps: the influence of implant malpositioning on fracture risk (similar to the effect described in ceramic Total Hip Replacements (
      • Goretti C.
      • Polidoro F.
      • Paderni S.
      • Belluati A.
      Ceramic on ceramic total hip arthroplasty and liner fracture. Two case reports and review of literature.
      )), in vivo and in vitro characterization of polymer-on-ceramic or different ceramic material pairings, and developing new implant testing methodology, in particular in vitro testing capturing adverse scenarios and methods for more accurate assessment of in vivo wear.

      Funding

      This work was supported by the European Union's Horizon 2020 research and innovation programme (Nu-Spine grant No. 812765).

      Declaration of Competing Interest

      Dominika Ignasiak used to be a consultant for NuVasive in the field of computational biomechanics and related clinical studies. The other authors declare no possible conflict of interest.
      The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
      Stephen Ferguson reports financial support was provided by Horizon 2020. Dominika Ignasiak reports a relationship with NuVasive Inc. that includes: consulting or advisory.

      Acknowledgements

      The authors would like to thank Prof. Richard Hall (University of Leeds, UK) for his comments on the manuscript. This work was supported by the European Union's Horizon 2020 research and innovation programme (Nu-Spine grant No. 812765).

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