In vitro assessment of connection strength and stability of internal implant-abutment connections

Open AccessPublished:March 12, 2019DOI:https://doi.org/10.1016/j.clinbiomech.2019.03.007

      Highlights

      • In-vitro biomechanical studies measured abutment stability and fit on the implant.
      • Abutment micromovements were quantified following force applications.
      • Internal conical connections have microgaps that reduce abutment fit.
      • A tight connection provides abutment and crown stabilization.
      • A tight connection minimizes the risk of bacterial colonization.

      Abstract

      Background

      Various connections have been machined to improve the fit between the dental abutment and implant. In vivo, the instability created by imprecisely fitting components can cause soft tissue irritation and bacterial colonization of the implant system. The aim of this study was to quantify abutment stability under in vitro force applications.

      Methods

      Abutment stability and fit were quantitatively measured after application of rotational, vertical, and horizontal forces.

      Findings

      The abutment connection held by friction (Friction-Fit) was the only group to have 0° angular rotation. A significantly greater vertical force was required to pull the abutment from the implant for the Friction-Fit connection as compared to all other experimental groups. The abutment connection held by a mechanically locking friction-fit with four grooves (CrossFit) and Friction-Fit demonstrated significantly lower lateral movement as compared to all other connections. The remaining connections evaluated included two hexagon connections that rely on screw placement for abutment fit (Conical + Hex #1 and Conical + Hex #2), one connection with protruding slots to align with recessed channels inside the implant (Conical + 6 Indexing Slots), and an internal connection that allows for abutment indexing every 120° (Internal Tri-Channel).

      Interpretation

      Internal connection geometry influenced the degree of abutment movement. Friction-Fit and CrossFit connections exhibited the lowest rotational and horizontal motions. Significant differences were found between Friction-Fit and CrossFit following the application of a vertical force, with the Friction-Fit requiring a significantly greater pull force to separate the abutment from the implant.

      Keywords

      1. Introduction

      Two-piece implant systems, consisting of the implant and abutment, are widely used in dental restorations. The advantage over a one-piece implant system is that it allows the implant to be unloaded during the bone healing phase and provides the benefit to adjust the prosthetic angle depending on the abutment selected (angled or straight) for placement on the implant. A disadvantage to a two-piece implant system is the resultant microgaps or spaces that exist along the implant-abutment interface when the abutment is seated on the implant and connected via the abutment screw. Under loading conditions, these spaces permit rotation and micromotion of the abutment and can lead to screw preload reduction, screw loosening, bending, or fracture (
      • Kano S.C.
      • Binon P.P.
      • Curtis D.A.
      A classification system to measure the implant-abutment microgap.
      ;
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      Mechanical performance of conical implant-abutment connections under different cyclic loading conditions.
      ). The existence of microgap has been demonstrated using scanning electron microscopy and microleakage studies (
      • Alves D.C.C.
      • de Carvalho P.S.P.
      • Elias C.N.
      • Vedovatto E.
      • Martinez E.F.
      In vitro analysis of the microbiological sealing of tapered implants after mechanical cycling.
      ;
      • Cavusoglu Y.
      • Akça K.
      • Gürbüz R.
      • Cehreli M.C.
      A pilot study of joint stability at the zirconium or titanium abutment/titanium implant interface.
      ;
      • do Nascimento C.
      • Miani P.K.
      • Pedrazzi V.
      • Gonçalves R.B.
      • Ribeiro R.F.
      • Faria A.C.
      • Macedo A.P.
      • de Albuquerque Jr., R.F.
      Leakage of saliva through the implant-abutment interface: in vitro evaluation of three different implant connections under unloaded and loaded conditions.
      ;
      • El Haddad E.
      • Gianni A.B.
      • Mancini G.E.
      • Cura F.
      • Carinci F.
      Implant-abutment leaking of replace conical connection Nobel Biocare® implant system. an in vitro study of the microbiological penetration from external environment to implant-abutment space.
      ;
      • Koutouzis T.
      • Mesia R.
      • Calderon N.
      • Wong F.
      • Wallet S.
      The effect of dynamic loading on bacterial colonization of the dental implant fixture-abutment interface: an in vitro study.
      ;
      • Quirynen M.
      • van Steenberghe D.
      Bacterial colonization of the internal part of two-stage implants, an in vivo study.
      ;
      • Steinebrunner L.
      • Wolfart S.
      • Bössmann K.
      • Kern M.
      In vitro evaluation of bacterial leakage along the implant-abutment interface of different implant systems.
      ;
      • Tripodi D.
      • D'Ercole S.
      • Iaculli F.
      • Piattelli A.
      • Perrotti V.
      • Iezzi G.
      Degree of bacterial microleakage at the implant-abutment junction in Cone Morse tapered implants under loaded and unloaded conditions.
      ;
      • Verdugo C.L.
      • Núñez G.J.
      • Avila A.A.
      • San Martín C.L.
      Microleakage of the prosthetic abutment/implant interface with internal and external connection: in vitro study.
      ;
      • Zipprich H.
      • Miatke S.
      • Hmaidouch R.
      • Lauer H.C.
      A new experimental design for bacterial microleakage investigation at the implant-abutment interface: an in vitro study.
      ). A correlation between microleakage and screw loosening has been demonstrated, as screw loosening enlarges the microgap (
      • Sahin C.
      • Ayyildiz S.
      Correlation between microleakage and screw loosening at implant-abutment connection.
      ). Thus, microgaps contribute to mechanical failure.
      Microgap also provides space as a potential site for bacteria, fluid and macromolecules from the crevicular fluid and saliva. Abutment micromovements, while seated on the implant, have been reported to create a pumping effect that may allow the bacteria to migrate and macromolecules to flow through the implant-abutment junction to the internal aspect of the implant system (
      • Alves D.C.C.
      • de Carvalho P.S.P.
      • Elias C.N.
      • Vedovatto E.
      • Martinez E.F.
      In vitro analysis of the microbiological sealing of tapered implants after mechanical cycling.
      ;
      • do Nascimento C.
      • Barbosa R.E.S.
      • Issa J.P.M.
      • Watanabe E.
      • Ito I.Y.
      • Albuquerque Jr., R.F.
      Use of checkerboard DNA-DNA hybridization to evaluate the internal contamination of dental implants and comparison of bacterial leakage with cast or pre-machined abutments.
      ;
      • Quirynen M.
      • van Steenberghe D.
      Bacterial colonization of the internal part of two-stage implants, an in vivo study.
      ;
      • Scarano A.
      • Perrotti V.
      • Piattelli A.
      • Iaculli F.
      • Iezzi G.
      Sealing capability of implant-abutment junction under cyclic loading: a toluidine blue in vitro study.
      ). The average microgap of the implant abutment junction has been reported to be from 1 to 49 μm (
      • Mishra S.K.
      • Chowdhary R.
      • Kumari S.
      Microleakage at the different implant abutment interface: a systematic review.
      ;
      • Quirynen M.
      • van Steenberghe D.
      Bacterial colonization of the internal part of two-stage implants, an in vivo study.
      ), while the average size of bacteria is 0.2 to 1.5 μm in width and 1 to 10 μm in length depending on shape (
      • El Haddad E.
      • Gianni A.B.
      • Mancini G.E.
      • Cura F.
      • Carinci F.
      Implant-abutment leaking of replace conical connection Nobel Biocare® implant system. an in vitro study of the microbiological penetration from external environment to implant-abutment space.
      ;
      • Nascimento C.d.
      • Pita M.S.
      • Santos Ede S.
      • Monesi N.
      • Pedrazzi V.
      • Albuquerque Junior R.F.
      • Ribeiro R.F.
      Microbiome of titanium and zirconia dental implants abutments.
      ). The localized bacteria colonization initiates recruitment of inflammatory cells. The subsequent inflammatory reaction affects the maintenance of soft and hard tissue formation around the implant complex (
      • Jones A.A.
      • Cochran D.L.
      Consequences of implant design.
      ). In extreme cases, bacterial colonization can lead to peri-implantitis, which is characterized by severe soft tissue inflammation, bone resorption, and loss of implant integration (
      • El Haddad E.
      • Gianni A.B.
      • Mancini G.E.
      • Cura F.
      • Carinci F.
      Implant-abutment leaking of replace conical connection Nobel Biocare® implant system. an in vitro study of the microbiological penetration from external environment to implant-abutment space.
      ;
      • Pokrowiecki R.
      • Mielczarek A.
      • Zaręba T.
      • Tyski S.
      Oral microbiome and peri-implant diseases: where are we now?.
      ;
      • Raab P.
      • Alamanos C.
      • Hahnel S.
      • Papavasileiou D.
      • Behr M.
      • Rosentritt M.
      Dental materials and their performance for the management of screw access channels in implant-supported restorations.
      ;
      • Sahin C.
      • Ayyildiz S.
      Correlation between microleakage and screw loosening at implant-abutment connection.
      ;
      • Salvi G.E.
      • Cosgarea R.
      • Sculean A.
      Prevalence and mechanisms of peri-implant diseases.
      ;
      • Siadat H.
      • Beyabanaki E.
      • Mousavi N.
      • Alikhasi M.
      Comparison of fit accuracy and torque maintenance of zirconia and titanium abutments for internal tri-channel and external-hex implant connections.
      ;
      • Tallarico M.
      • Canullo L.
      • Caneva M.
      • Özcan M.
      Microbial colonization at the implant-abutment interface and its possible influence on peri-implantitis: a systematic review and meta-analysis.
      ;
      • Tsuruta K.
      • Ayukawa Y.
      • Matsuzaki T.
      • Kihara M.
      • Koyano K.
      The influence of implant-abutment connection on the screw loosening and microleakage.
      ;
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). Because of this, connections are designed and machined with the goal to minimize areas of microgap as these contribute to biological and mechanical failure.
      In an attempt to limit microgap and increase abutment stability, a variety of connection designs have been manufactured. The first was the Brånemark external connection. In this design the connection feature extends superior to the coronal portion of the implant. Because the abutment sits on the external connection feature, it has a high center of rotation relative to the implant and the screw is the only component securing the abutment. Common external connection shapes include hexagon, octagon, and spline, with external hexagon being the most common. Advantages to this design are that long-term follow-up data is available and compatibility exists among multiple implant systems. External hexagon connections have been shown to have rotational misfit in the range of 3 to 10°, while a rotational misfit of less than 2° is required to provide a stable screw joint that limits screw loosening (
      • Binion P.P.
      The spline implant: design, engineering, and evaluation.
      ). Thus a disadvantage to the external hexagon connection includes a higher prevalence of screw loosening, which ranges from 6% to 48% (
      • Levine R.A.
      • Clem III, D.S.
      • Wilson Jr., T.G.
      • Higginbottom F.
      • Saunders S.L.
      A multicenter retrospective analysis of the ITI implant system used for single-tooth replacements: preliminary results at 6 or more months of loading.
      ;
      • Pardal-Peláez B.
      • Montero J.
      Preload loss of abutment screws after dynamic fatigue in single implant-supported restorations. A systemic review.
      ), a higher prevalence of rotational misfit, a less esthetic result, and often, an inadequate microbial seal. Design modifications to improve abutment stability and fit and to limit rotational misfit have been applied to the height and width of the external hexagon. The external hexagon currently ranges from 0.7 to 1.2 mm in height and from 2.0 to 3.4 mm in width. Deepening the abutment screw engagement limits the tipping forces on the abutment and reduces the incidence of screw loosening. Other modifications include machining a 1.5° taper to the hex flats, which creates a friction fit between the abutment and implant. Micro-stops have also been added in the corners of the abutment hexagon to engage the implant hexagon and limit misfit. Modifications have also been made to the abutment screw material and design that cover the shank, number of threads, diameter, length, thread design and torque applications (
      • Binion P.P.
      The external hexagonal interface and screw-joint stability: a primer on threaded fasteners in implant dentistry.
      ). Studies have shown the mean preload using a gold alloy screw to be greater than that of a titanium alloy screw; a greater preload will minimize the incidence of screw loosening (
      • Binion P.P.
      The external hexagonal interface and screw-joint stability: a primer on threaded fasteners in implant dentistry.
      ).
      Internal connections have the connection feature inferior to the coronal portion of the implant, which is inside the implant body. The connection may be a butt joint (90°, flat-on-flat), a slip-fit or passive joint, where a gap exists along the implant-abutment junction, or a friction-fit or active joint, where there is physical contact between the components (
      • Sarfaraz H.
      • Paulose A.
      • Shenoy K.K.
      • Hussain A.
      A three-dimensional finite element analysis of a passive and friction fit implant abutment interface and the influence of occlusal table dimension on the stress distribution pattern on the implant and surrounding bone.
      ). The internal connections exist in a wide variety of shapes including, but not limited to, hexagon, octagon, spline, cone screw, cylinder hex, tri-channel, and cam tube. Internal connections have several advantages over external hexagon connections. Placing the connection internal in the implant lowers the center of rotation and provides greater abutment stability by resisting lateral loads. Overall, internal connections have less screw loosening, provide better esthetics, provide an improved microbial seal, provide better joint strength and provide more platform switching options as compared to external connections. Porter, Lazzara, and Gardner introduced the concept of platform switching, by which a larger-diameter implant is combined with a narrower abutment, resulting in movement of the implant-abutment gap away from the implant shoulder (
      • Salamanca E.
      • Lin J.C.
      • Tsai C.Y.
      • Hsu Y.S.
      • Huang H.M.
      • Teng N.C.
      • Wang P.D.
      • Feng S.W.
      • Chen M.S.
      • Chang W.J.
      Dental implant surrounding marginal bone level evaluation: platform switching versus platform matching-one-year retrospective study.
      ). As a result, there is a shift in the loading forces from the implant neck to the center of the abutment interface and blocking of inflammatory cell infiltration (
      • Hsu P.F.
      • Yao K.T.
      • Kao H.C.
      • Hsu M.L.
      Effects of axial loading on the pull-out force of conical connection abutments in ankylose implant.
      ). The clinical benefits of platform switching include less marginal bone resorption in platform-switched implants than in platform-matched implants. As the force distribution occurs deep within the implant wall and distributes itself outwards towards the bone, rather than on the abutment screw, the incidence of screw loosening has been reduced to 3.7% (
      • Levine R.A.
      • Clem III, D.S.
      • Wilson Jr., T.G.
      • Higginbottom F.
      • Saunders S.L.
      A multicenter retrospective analysis of the ITI implant system used for single-tooth replacements: preliminary results at 6 or more months of loading.
      ). A disadvantage to internal connections is the thinner, and thereby weaker, implant wall at the collar to allow space for the connection. As a more recent design concept, there is also less historical data on internal connections as compared to external connections.
      Conical connections are a specific type of internal connection based on the machine taper of the two structures. A true conical connection is a Morse Taper connection that is press-fit together with significant friction existing between the two components that result in cold-welding. The Morse Taper angle is determined according to the mechanical properties of each material (
      • Macedo J.P.
      • Pereira J.
      • Vahey B.R.
      • Henriques B.
      • Benfatti C.A.
      • Magini R.S.
      • López-López J.
      • Souza J.C.
      Morse taper dental implants and platform switching: the new paradigm in oral implantology.
      ). Compressive forces may cause deeper settling of the abutment into the implant body of conical connections, minimizing the microgap and allowing the 2-piece system to behave as one piece (
      • Tsuruta K.
      • Ayukawa Y.
      • Matsuzaki T.
      • Kihara M.
      • Koyano K.
      The influence of implant-abutment connection on the screw loosening and microleakage.
      ;
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      Mechanical performance of conical implant-abutment connections under different cyclic loading conditions.
      ). The connection may form a hermetic seal that prevents microbial invasion and anti-rotation through the cold-welded friction fit. It has a high resistance to bending and rotational torque, which reduces the possibilities of screw loosening or screw fracture as compared to other connections. This is because the surface area of the implant-abutment interface is greater in conical connections as compared to parallel walled connections, contributing to reduced measurements of microleakage and screw loosening (
      • Tsuruta K.
      • Ayukawa Y.
      • Matsuzaki T.
      • Kihara M.
      • Koyano K.
      The influence of implant-abutment connection on the screw loosening and microleakage.
      ). As compared to external and other internal connections, microleakage has been shown to be less in Morse Taper (conical connection) geometries, providing better stabilization, marginal bone loss, maintenance of soft tissue anatomical dimensions, and esthetics (
      • Macedo J.P.
      • Pereira J.
      • Vahey B.R.
      • Henriques B.
      • Benfatti C.A.
      • Magini R.S.
      • López-López J.
      • Souza J.C.
      Morse taper dental implants and platform switching: the new paradigm in oral implantology.
      ). It can also support platform switching.
      In a true conical connection, the fit relates to the taper degree and connection area. The anti-torsional ability is dependent on the frictional resistance in the cone being greater than the torsion movement (
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      The potential risk of conical implant-abutment connections: the antirotational ability of cowell implant system.
      ;
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      Mechanical performance of conical implant-abutment connections under different cyclic loading conditions.
      ). Given high occlusal forces, this is often difficult to maintain clinically, so a hybrid connection is often designed to incorporate self-locking mechanisms that can mitigate abutment rotation and micromovements that can compromise restorative success. A disadvantage of conical connections is the potential for vertical discrepancy, which ranges from 22.6 to 62.2 μm, and results when the abutment is not fully seated on the implant (
      • Tsuge T.
      • Hagiwara Y.
      • Matsumura H.
      Marginal fit and microgaps of implant-abutment interface with internal anti-rotation configuration.
      ). This mismatch will contribute to reduced engagement of the anti-rotational feature, reduced engagement of the anti-bending feature and loss of abutment stability.
      Masticatory forces when chewing cause micromotion between the abutment and implant because the microgaps exist. The size range of micromotion is from 1.52 μm to 94.00 μm (
      • Karl M.
      • Taylor T.D.
      Parameters determining micromotion at the implant-abutment interface.
      ). This movement causes the material to undergo fretting or material wear due to small amplitude oscillatory motion, which has been shown to enlarge the microgap and subsequently increase micromotion (
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      Mechanical performance of conical implant-abutment connections under different cyclic loading conditions.
      ). Thus, the implant-abutment systems require adequate mechanical characteristics to resist functional occlusal loads. The present study is the first to systematically evaluate abutment micromotion in six commercially available internal connections. The dynamics of chewing can cause abutment micromovements in vertical, horizontal, or rotational directions. Testing methods applied forces to the implant-assembly in vertical, horizontal, or rotational directions and quantified the amount of misfit. The null hypothesis was that the various internal connections would show no significant difference with regards to measurements in response to the application of the vertical, horizontal, and rotational forces.

      2. Methods

      2.1 Implants and abutments

      Table 1 lists the implant/abutment combinations evaluated in this study and includes generic connection terms, key features, trade names, and manufacturers. Each force application test (detailed below) involved the use of five independent implant and abutment replicates (n = 5).
      Table 1Experimental groups.
      ConnectionKey featuresDescriptionManufacturer, location
      Friction-Fit
      • Slip-fit conical portion
      • 1° taper in the hex portion provides the friction-fit
      4.1mmD Tapered Screw-Vent® implant + Straight abutmentZimmer Biomet, United States of America
      CrossFit®
      • An internal cone of 15° provides flexibility in prosthetic treatments
      • The 15° conical-cylindrical connection incorporates a mechanically locking friction fit with four internal grooves to provide stability under loading
      4.1mmD Roxolid® SLActive® implant + RC Anatomic abutmentStraumann, Switzerland
      Conical + Hex #1
      • 1.5mm hex flats provide anti-rotation
      • Connection of the implant and abutment relies on the screw, which utilizes Spiralock® technology
      3.8mmD Tapered Internal Laser Lok implant + Straight Esthetic abutmentBiohorizons®, United States of America
      Conical + 6 Indexing Slots
      • 5 symmetrically placed slots and one additional slot located towards the highest point on the implant collar allows for one-position-only placement of all indexed components and index-free placement of one-piece abutments
      • Slip-fit conical portion
      • Connection of the implant and abutment relies on the screw
      4.2mmD OsseoSpeed® EV implant + TiDesign™ EV abutmentAstra Tech, Sweden
      Conical + Hex #2
      • The hexagon provides anti-rotation and allows for prosthetic indexing every 60°
      • The hex flats provide anti-rotation
      4.3mmD NobelActive® Internal RP implant + Esthetic Abutment Conical Connection RPNobel Biocare®, Switzerland
      Internal Tri-Channel
      • Limited paths of insertion make it easy to restore
      • Internal connection with indexing every 120°
      • Connection of the implant and abutment relies on the screw
      4.0mmD NobelReplace® Straight Groovy implant + Snappy abutmentNobel Biocare®, Switzerland
      Note: All trademarks and product names are the property of their respective owners.

      2.2 Internal connection images

      The internal connection of each implant was imaged using scanning electron microscopy (Model: 6440; JOEL USA, Inc., Pleasanton, California, USA) at 20 keV to view the characteristic internal features of each connection (Fig. 1a–f ). The Friction-Fit internal connection is machined to provide a slip-fit in the conical portion of the coupling and a 1° of taper in the hex portion to provide a friction-fit (Fig. 1a). The lead-in bevel connection reduces horizontal stresses better than flat “butt-joint” connections (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Karl M.
      • Taylor T.D.
      Parameters determining micromotion at the implant-abutment interface.
      ). The CrossFit connection has a 15° conical-cylindrical connection that incorporates a mechanically locking friction fit with four internal grooves to provide stability under loading (Fig. 1b). The Conical + Hex #1 has 1.5 mm hex flats (Fig. 1c). The connection relies on standard bolt technology, in which the implant and abutment are connected via a screw. Referred to as Spiralock® technology, the abutment screw must be tightened to the appropriate torque to increase the strength of the connection and reduce the potential for abutment screw loosening. The Conical + 6 Indexing Slots has five symmetrically placed slots and one additional slot located towards the highest point on the implant collar (Fig. 1d). This connection allows for one-position-only placement of all indexed components and index-free placement of one-piece abutments. The Conical + Hex #2 is an internal conical connection with a hexagon portion that allows for prosthetic indexing every 60° (Fig. 1e). The Internal Tri-Channel is an internal connection that allows for prosthetic indexing every 120° (Fig. 1f).
      Fig. 1
      Fig. 1Internal conical connections evaluation. Scanning electron micrographis were obtained of each internal conical connection. [a] The Friction-Fit internal connection is machined to provide a friction-fit in the hexagon portion and a slip fit in the conical portion of the coupling. The abutment hex flats are machined with 1° of taper to create the friction-fit to the walls of the implant. [b] The CrossFit connection has four internal grooves to position the prosthetic and a slip fit conical portion that has an internal cone of 15°. [c] The Conical + Hex #1 has 1.5 mm hex flats. [d] The Conical + 6 Indexing Slots has a slip fit conical portion. [e] The Conical + Hex #2 has an internal conical connection with a hexagon portion that allows for prosthetic indexing every 60°. [f] The Internal Tri-Channel is an internal connection that allows for prosthetic indexing every 120°.

      2.3 Abutment Angular Rotation (AAR)

      AAR (n = 5) was measured as the absolute difference between induced clockwise and counterclockwise abutment movements (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Nicholls J.I.
      • Basten C.H.
      A comparison of three mechanical properties of four implant designs.
      ). Briefly, the abutment was placed on the implant and the screw was torqued to the manufacturer's recommended value using a digital torque gauge (Model BGI Mark-10; Wagner Instruments, Copiague, New York, USA). The abutment screw was reverse torqued and removed. Next, the implant system was inverted and secured in a collet. The abutment was lowered into a previously heated low melting point metal reservoir. A horizontal degree scale was fabricated based on Pythagorean Theorem and the equipment being placed at a fixed distance from the scale. Once cured, a laser pointer positioned under the construct allowed the assembly to be centered at 0° (Fig. 2a ). To measure relative angular movement, the abutment was gently rotated in the clockwise direction and released. The resultant angle was recorded. The process was repeated by centering the assembly at 0° and gently rotating the abutment in the counter clockwise direction. The resultant angular measurement was recorded.
      Fig. 2
      Fig. 2Equipment descriptions. [a] Abutment Angular Rotation (AAR) equipment was used to measure the degrees of clockwise and counterclockwise rotation the abutment can withstand when seated on the implant. An Instron E3000 materials testing system was used to measure [b] Abutment Pull Force (APF) and [c] Abutment Micromotion (AMM). [d] For AMM measurements, the force was applied to an abutment at a 30° angle to the implant axial axis, while the laser detected abutment motion.

      2.4 Abutment Pull Force (APF)

      APF (n = 5) measured the force required to separate the abutment and implant (
      • Hsu P.F.
      • Yao K.T.
      • Kao H.C.
      • Hsu M.L.
      Effects of axial loading on the pull-out force of conical connection abutments in ankylose implant.
      ). Briefly, the abutment was placed on the implant and the screw was torqued to the manufacturer's recommended value using a digital torque gauge. The abutment screw was reverse torqued and removed. Next, the implant was inverted. Abutment-implant connections with no retention features between the two components had immediate separation. These abutments completely rely on the previously removed screw for abutment retention and the pull force was recorded as zero Newtons. Abutment-implant constructs that remained connected had the implant secured upside down in a collet. The retained abutment was lowered into a previously heated low melting point metal reservoir. Once cured, the force required to separate the implant and abutment was recorded using an Instron materials testing system (Model: E3000; Instron, Norwood, Massachusetts, USA) (Fig. 2b) at a crosshead speed of 5.0 mm/min.

      2.5 Abutment MicroMotion (AMM)

      While under cyclic loading, AMM (n = 5) measured the length of abutment micromotion, at the base of the abutment, relative to the stationary implant (
      • Karl M.
      • Taylor T.D.
      Parameters determining micromotion at the implant-abutment interface.
      ). Briefly, implants were compression mounted in phenolic resin (Buehler, Lake Bluff, Illinois, USA) at bone level simulating 0 mm of clinical crestal bone resorption. Each abutment was milled to 4 mm height using a Bridgeport Milling machine (Model: F537A; Hardinge Inc., Elmira, New York, USA). This was the minimal cutting height that prevented interference from the abutment screw during testing. Standardization of the abutment height normalized the abutment bending moment under load and minimized deflection of the post. The corresponding abutment was placed on the implant and the screw was torqued to the manufacturer's recommended value using a digital torque gauge. Implant assemblies were subjected to a 200 N force at a 30° angle with respect to the implant axial axis for 1000 cycles using the Instron E3000 materials testing system (
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). These testing parameters were selected as posterior occlusal forces range up to 200 N with peak forces acting at an angle of 30° to the implant axis [ISO 14801] (
      • Yao K.T.
      • Kao H.C.
      • Cheng C.K.
      • Fang H.W.
      • Huang C.H.
      • Hsu M.L.
      Mechanical performance of conical implant-abutment connections under different cyclic loading conditions.
      ). The micromotion at the base of the abutment was detected using a Laser Doppler Vibrometer connected to a data acquisition system (Model: OMS LaserPoint LP01; Optical Measurement Systems, Laguna Hills, California, USA) (Fig. 2c and d).

      2.6 Statistical analysis

      Data are presented as mean (SD). Statistical analysis used a one-way Analysis of Variance to determine the presence of statistical significance (α = 0.05) and a Tukey's post-hoc test to determine which groups were significantly different.

      3. Results

      Quantitative testing of the various internal connections provides an indication of the specific design features that may contribute to a successful outcome following clinical implant placement.

      3.1 Abutment Angular Rotation

      AAR measurements are used to show the degrees of abutment rotational freedom. A lower measurement indicates a resistance to rotation and greater abutment stability. AAR measurements (Fig. 3) demonstrated the presence of a friction-fit provided an angular rotational freedom of 0°. Hybrid conical-hexagonal connections with a slip fit (Conical + Hex #1 and Conical + Hex #2) had a gap through the implant-abutment junction that allowed for a significantly greater degree of rotation as compared to a similar connection that incorporated a friction-fit in the hex portion of the connection. Without the abutment screw, the Spiralock Technology of Conical + Hex #1 was unable to engage. Slip fit and non-indexed connections demonstrated the greatest angular rotational freedom, ranging from 2° to 5°. The square CrossFit connection demonstrated no significant difference as compared to the Friction-Fit connection.
      Fig. 3
      Fig. 3Abutment Angular Rotation (ARR) measurements. Lower rotational measurements indicate a tighter fit between the abutment and implant. Each Friction-Fit connection had an angular rotational measurement of 0°. * indicates statistical significance as compared to Internal Tri-Channel. † indicates statistical significance as compared to Conical + Hex #2. γ indicates statistical significance as compared to Conical + 6 Indexing Slots, and ψ indicates statistical significance as compared to Conical + Hex #1. (α = 0.05).

      3.2 Abutment Pull Force

      The tighter the fit between the abutment and implant, the more force is needed to pull the abutment off of the implant. APF (Fig. 4) demonstrated the more static friction created in the connection during the application of abutment screw torque necessitated the strongest forces to separate the abutment from the implant. The force required to separate the abutment from the implant was significantly greater for the Friction-Fit connection as compared to all other connection designs. When inverted, slip fit abutment connections (Conical + Hex #1, Conical + 6 Indexing Slots, and Internal Tri-Channel) separated from the implant unaided, demonstrating the abutment screw is the only feature that connects the abutment to the implant in these systems.
      Fig. 4
      Fig. 4Abutment Pull Force (APF) measurements. A tighter fit between the abutment and implant are demonstrated by use of a higher force to detach the abutment from the implant. Statistical analysis demonstrated a significantly greater abutment pull force was required to separate the abutment from the implant for the Friction-Fit connection as compared to all other experimental groups. (α = 0.05).

      3.3 Abutment Micromotion

      AMM measured lateral abutment movement under dynamic loading. The Friction-Fit and CrossFit connections demonstrated significantly less horizontal micromotion as compared to the Conical + 6 Indexing Slots and Conical + Hex #2 connections (Fig. 5). The Internal Tri-Channel connection demonstrated significantly less micromotion as compared to the Conical + 6 Indexing Slots connection.
      Fig. 5
      Fig. 5Abutment Micromotion (AMM) measurements. Displacement between the implant and abutment in vivo creates microgaps that enable bacterial invasion. Measurements quantified abutment micromotion under cyclic loading. * indicates statistical significance as compared to Friction-Fit; † indicates statistical significance as compared to CrossFit; γ indicates significance as compared to Internal Tri-Channel. (α = 0.05).

      4. Discussion

      Prior work has demonstrated that greater dimensional variation in the machining tolerances during manufacturing produces a more asymmetric connection (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Nicoll R.J.
      • Sun A.
      • Haney S.
      • Turkyilmaz I.
      Precision of fit between implant impression coping and implant replica pairs for three implant systems.
      ). Machining tolerances in implant dentistry have been reported in the literature to range from 1 to 120 μm (
      • do Nascimento C.
      • Miani P.K.
      • Pedrazzi V.
      • Gonçalves R.B.
      • Ribeiro R.F.
      • Faria A.C.
      • Macedo A.P.
      • de Albuquerque Jr., R.F.
      Leakage of saliva through the implant-abutment interface: in vitro evaluation of three different implant connections under unloaded and loaded conditions.
      ;
      • Ma T.
      • Nicholls J.I.
      • Rubenstein J.E.
      Tolerance measurements of various implant components.
      ;
      • Nicoll R.J.
      • Sun A.
      • Haney S.
      • Turkyilmaz I.
      Precision of fit between implant impression coping and implant replica pairs for three implant systems.
      ), representing a wide variability that influences the fit of prosthetic components and the mechanical stability of the entire implant system (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Nicoll R.J.
      • Sun A.
      • Haney S.
      • Turkyilmaz I.
      Precision of fit between implant impression coping and implant replica pairs for three implant systems.
      ). To understand the effect of loading on implant systems, internal connection implant systems were exposed to rotational, vertical, and horizontal forces.
      Internal conical connections have improved dissipation of loads and reduced risk of unscrewing and/or fracture of the abutment screw as compared to external or other internal connections (
      • Caricasulo R.
      • Malchiodi L.
      • Ghensi P.
      • Fantozzi G.
      • Cucchi A.
      The influence of implant-abutment connection to peri-implant bone loss: a systematic review and meta-analysis.
      ). Clinically during occlusal loading, abutment rotation creates tension in the screw connection, which contributes to screw loosening and screw joint failure (
      • Binion P.P.
      The effect of implant/abutment hexagonal misfit on screw joint stability.
      ;
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). Internal connections have less rotational freedom than external connections (
      • Siadat H.
      • Beyabanaki E.
      • Mousavi N.
      • Alikhasi M.
      Comparison of fit accuracy and torque maintenance of zirconia and titanium abutments for internal tri-channel and external-hex implant connections.
      ). The CrossFit connection demonstrated rotational freedom ranging from 0° to 1.25°, which corroborates previously reported rotational measurements, acquired without the abutment screw, of mean 1.21° (SD 0.236°) (
      • Gigandet M.
      • Bigolin G.
      • Faoro F.
      • Bürgin W.
      • Brägger U.
      Implants with original and non-original abutment connections.
      ). Gigandet et al. reported the greatest rotational misfit by the Internal Tri-Channel connection at mean 3.50° (SD 0.285°) (
      • Gigandet M.
      • Bigolin G.
      • Faoro F.
      • Bürgin W.
      • Brägger U.
      Implants with original and non-original abutment connections.
      ). The results from the current study also demonstrated the greatest rotational freedom from the Internal Tri-Channel connection at mean 5.10° (SD 0.29°). Implant-abutment connections, fastened with the abutment screw, and having greater than 5° of rotational freedom, have been reported as extremely prone to screw loosening, which occurred between 1.1 and 2.5 million cycles of dynamic loading (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ). Implant-abutment connections, joined with the abutment screw, with less than 2° of rotational freedom have the greatest resistance to screw loosening, completing an average of 6.7 million cycles of dynamic loading (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Binion P.P.
      The effect of implant/abutment hexagonal misfit on screw joint stability.
      ). Connections with rotational freedom greater than 2° can result in vibration and micromovement between the components during functional loading, which subsequently decrease the clamping force until screw-joint failure occurs (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ;
      • Binion P.P.
      The effect of implant/abutment hexagonal misfit on screw joint stability.
      ). Self-locking is the term used to define resistance to rotation; the self-locking effect is caused by static friction created between the abutment and implant surfaces or machined rotational locks (
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). In the results presented here, the Friction-Fit was the only connection that provided complete resistance to angular rotation. This is attributed to the proprietary abutment hex flat that is machined at a 1° taper to provide a tight friction-fit between the implant and abutment. Binion previously reported the degree of rotational misfit can be directly correlated to size discrepancies between the flat-to-flat dimensions of the implant hexagonal and its corresponding abutment hexagonal recess (
      • Binion P.P.
      The evolution and evaluation of two interference-fit implant interfaces.
      ), which are related to machining tolerances and quality control at the time of manufacture.
      APF testing measured the vertical force required to overcome the static frictional force established in the connection once the abutment screw has been fully seated on the implant by torquing the screw to the manufacturer's recommended value. The abutment screw is removed prior to testing. APF results demonstrated the Friction-Fit connection required a significantly greater force to separate the abutment from the implant as compared to all other connections tested. The CrossFit and Conical + Hex #2 measurements ranged from 0 to 66.6 N and from 0 to 68.6N, respectively, which indicate an absence of a frictional force in some, but not all, specimens. Karl et al. previously noted the occasional failure for stabilizing features to engage (
      • Karl M.
      • Taylor T.D.
      Parameters determining micromotion at the implant-abutment interface.
      ). The Conical + Hex #1, Conical + 6 Indexing Slots, and Internal Tri-Channel connections demonstrated 0 N was required to separate the implant and abutment for each specimen tested. These particular connections, in mechanical terms, are considered non-self-locking (
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). The Friction-Fit connection APF ranged from 81.8 to 150.6 N. Thus, Friction-Fit was the only connection to consistently engage and contribute to the abutment stability.
      AMM testing measured the horizontal abutment displacement under cyclic loading. The current results were in alignment with prior micromotion testing that demonstrated the internal geometry of the connection influences the degree of micromotion (
      • Karl M.
      • Taylor T.D.
      Parameters determining micromotion at the implant-abutment interface.
      ). The least amount of micromovement was measured in the Friction Fit and CrossFit connections. Zipprich et al. measured 4–8 μm micromovement between the abutment and implant during dynamic loading of the Internal Tri-Channel connection (
      • Zipprich H.
      • Weigl P.
      • Lange B.
      • Lauer H.C.
      Micromovements at the implant-abutment interface: measurement, causes, and consequences.
      ). These results are in agreement with the micromotion measurement for the Internal Tri-Channel connection at mean 6.9 μm (SD 3.7 μm).
      Measurements of micromotion indicate the presence of a microgap. Other studies evaluate the presence of microgaps at the implant-abutment junction using leakage tests. Leakage at the implant-abutment junction is dependent on the fit between the abutment and implant, the degree of micromovement between the components, and the torque force used to connect them. Leak teating reported in the literature indicates that 2-piece implant systems can contain a gap between the implant and abutment (
      • Abdelhamed M.I.
      • Galley J.D.
      • Bailey M.T.
      • Johnston W.M.
      • Holloway J.
      • McGlumphy E.
      • Leblebicioglu B.
      A comparison of zirconia and titanium abutments for microleakage.
      ;
      • Alves D.C.C.
      • de Carvalho P.S.P.
      • Elias C.N.
      • Vedovatto E.
      • Martinez E.F.
      In vitro analysis of the microbiological sealing of tapered implants after mechanical cycling.
      ;
      • Cavusoglu Y.
      • Akça K.
      • Gürbüz R.
      • Cehreli M.C.
      A pilot study of joint stability at the zirconium or titanium abutment/titanium implant interface.
      ;
      • Dellow A.G.
      • Driessen C.H.
      • Nel H.J.
      Scanning electron microscopy evaluation of the interfacial fit of interchanged components of four dental implant systems.
      ;
      • D'Ercole S.
      • Tripodi D.
      • Marzo G.
      • Bernardi S.
      • Continenza M.A.
      • Piattelli A.
      • Iaculli F.
      • Mummolo S.
      Microleakage of bacteria in different implant-abutment assemblies: an in vitro study.
      ;
      • Fernández M.
      • Delgado L.
      • Molmeneu M.
      • García D.
      • Rodriguez D.
      Analysis of the misfit of dental implant-supported prostheses made with three manufacturing processes.
      ;
      • Quirynen M.
      • van Steenberghe D.
      Bacterial colonization of the internal part of two-stage implants, an in vivo study.
      ;
      • Scarano A.
      • Valbonetti L.
      • Degidi M.
      • Pecci R.
      • Piattelli A.
      • de Oliveira P.S.
      • Perrotti V.
      Implant-abutment contact surfaces and microgap measurements of different implant connections under 3-dimensional X-ray microtomography.
      ;
      • Zanardi P.R.
      • Costa B.
      • Stegun R.C.
      • Sesma N.
      • Mori M.
      • Laganá D.C.
      Connecting accuracy of interchanged prosthetic abutments to different dental implants using scanning electron microscopy.
      ). The microgap provides a potential site for the accumulation of bacteria and their metabolites and allows for the micropumping effect that allows for bacteria to colonize the internal aspect of the implant. This may lead to higher levels of inflammatory cells around the soft tissue surrounding the implant-abutment interface, tissue redness, swelling, malodor, and bleeding (
      • Alonso-Pérez R.
      • Bartolomé J.F.
      • Ferreiroa A.
      • Salido M.P.
      • Pradíes G.
      Evaluation of the mechanical behavior and marginal accuracy of stock and laser-sintered implant abutments.
      ;
      • Gross M.
      • Abramovich I.
      • Weiss E.I.
      Microleakage at the abutment-implant interface of osseointegrated implants: a comparative study.
      ). Previous studies demonstrated bacterial leakage can occur at the implant-abutment junction and along the abutment screw channel, leading to malodor or peri-implant disease (
      • Raab P.
      • Alamanos C.
      • Hahnel S.
      • Papavasileiou D.
      • Behr M.
      • Rosentritt M.
      Dental materials and their performance for the management of screw access channels in implant-supported restorations.
      ). do Nascimento et al. incubated specimens, of internal-hex, external-hex, and Morse cone connections, for 7 days in human saliva (
      • do Nascimento C.
      • Miani P.K.
      • Pedrazzi V.
      • Gonçalves R.B.
      • Ribeiro R.F.
      • Faria A.C.
      • Macedo A.P.
      • de Albuquerque Jr., R.F.
      Leakage of saliva through the implant-abutment interface: in vitro evaluation of three different implant connections under unloaded and loaded conditions.
      ). The saliva samples contained a variety of bacteria and leakage was dependent on the size and shape of bacteria. For internal-hex and external-hex connections, loaded implants showed higher bacterial counts and percentages of species occupying the internal aspect of the implant as compared to unloaded implants. The differences in connection geometry also influenced leakage. Under loaded and unloaded conditions, the Morse cone connection demonstrated lower bacterial counts than the internal-hex and external-hex connections (
      • Caricasulo R.
      • Malchiodi L.
      • Ghensi P.
      • Fantozzi G.
      • Cucchi A.
      The influence of implant-abutment connection to peri-implant bone loss: a systematic review and meta-analysis.
      ).
      Microgaps can cause prosthetic instability and increase the stress states in the surrounding bone and implant components (
      • Coelho P.G.
      • Sudack P.
      • Suzuki M.
      • Kurtz K.S.
      • Romanos G.E.
      • Silva N.R.
      In vitro evaluation of the implant abutment connection sealing capability of different implant systems.
      ). Increased stress and strain levels on the endosseous implant can overload the peri-implant bone and accelerate marginal bone loss. Multiple clinical studies have demonstrated that greater levels clinical peri-implant bone loss have been measured with external connections, while internal connections and conical connections have the lowest amounts of marginal bone loss (
      • Caricasulo R.
      • Malchiodi L.
      • Ghensi P.
      • Fantozzi G.
      • Cucchi A.
      The influence of implant-abutment connection to peri-implant bone loss: a systematic review and meta-analysis.
      ).
      Finite element analysis has supported the clinical observations of crestal bone loss (
      • Chun H.J.
      • Shin H.S.
      • Han C.H.
      • Lee S.H.
      Influence of implant abutment type on stress distribution in bone under various loading conditions using finite element analysis.
      ;
      • Sarfaraz H.
      • Paulose A.
      • Shenoy K.K.
      • Hussain A.
      A three-dimensional finite element analysis of a passive and friction fit implant abutment interface and the influence of occlusal table dimension on the stress distribution pattern on the implant and surrounding bone.
      ). Newer implant abutment connection designs have been commercialized to reduce stress concentrations to the marginal bone. The external connection implant-abutment interface is coronal to the implant and marginal bone. Thus, stress generated is transferred and localized to the marginal bone, inducing crestal bone loss. Internal connections have a more apically oriented implant abutment interface, so stresses are more broadly distributed and extend deeper into the implant and further away from the marginal bone, thereby reducing the quantity of crestal bone loss as compared to external connections (
      • Chun H.J.
      • Shin H.S.
      • Han C.H.
      • Lee S.H.
      Influence of implant abutment type on stress distribution in bone under various loading conditions using finite element analysis.
      ). Conical connections with an active, friction-fit under loading conditions had greater stress distributed on the implant and less stress distributed on the surrounding bone as compared to conical connections with a passive, slip-fit (
      • Sarfaraz H.
      • Paulose A.
      • Shenoy K.K.
      • Hussain A.
      A three-dimensional finite element analysis of a passive and friction fit implant abutment interface and the influence of occlusal table dimension on the stress distribution pattern on the implant and surrounding bone.
      ). Stresses on a friction-fit connection are localized at the implant-abutment interface and abutment neck. The friction-fit creates a wedging effect that improves the implant-abutment joint stability against lateral forces and distributes stresses primarily on the implant. The friction-fit also provides a greater resistance to deformation and fracture under oblique compressive loading as compared to passive-fit conical connections. Ultimately, these features reduce biological and mechanical complications when using conical friction-fit connections. As cold-welding does not occur in passive-fit connections, there are more likely gaps at the implant-abutment interface which contribute to micromotions and interfere with adequate stress distributions. Load transfer at the bone-implant interface is a complex phenomenon. The implant geometry and design of the implant-abutment connection affect load transfer, which is also affected by the loading protocol and the type of occlusion, the number of implants and position, and the quality and quantity of the surrounding bone (
      • Sarfaraz H.
      • Paulose A.
      • Shenoy K.K.
      • Hussain A.
      A three-dimensional finite element analysis of a passive and friction fit implant abutment interface and the influence of occlusal table dimension on the stress distribution pattern on the implant and surrounding bone.
      ).
      AMM tested components were assembled according to the manufacturer's instructions for use and subjected to cyclic loading to imitate in vivo use. During study implementation, abutment screw removal prior to AAR and APF testing means the interpretation of pre-clinical test results is limited and may not be indicative of clinical performance. Abutment screw removal allowed for a more direct physical assessment of the tightness and stability of the fit between the abutment and implant. The angular rotation was indirectly applied to the abutment via a manually operated lever. The lever was moved until a reasonable force could no longer advance the lever; however, despite all attempts to prevent this, the force applied to each abutment could have been variable. Incorporating a mechanistic method of control for component rotation would be an improvement over the current system. Additional AAR testing with the abutment screw torqued and retained in position would likely reduce the amount of motion and thereby potentially eliminate significant differences between the experimental groups. Lastly, limitations exist because universally accepted methods for testing abutment stability and fit do not exist. This makes it difficult to draw direct comparison between results achieved following different test methodologies throughout the published literature.

      5. Conclusions

      Reliable and stable implant-abutment connections are required for the clinical success of a dental implant system. Selection of an implant connection with self-locking features that will resist micromovement due to applied forces is likely to provide better performance under biomechanical loading in the oral environment. Our results demonstrated that connections incorporating anti-rotational features, such as a friction-fit surface or machined micro-stops, performed the best in preventing abutment rotation, resisting separation, and resisting micromotion. Based on the results presented here, the Friction-Fit and CrossFit connections performed best overall in limiting motion in the rotational, vertical, and horizontal, directions examined.

      Acknowledgements

      The authors would like to thank Jim Reams for technical assistance. The authors would also like to thank Elnaz Ajami, PhD and Prabhu Gubbi, PhD for providing review of the manuscript.
      Role of the funding source
      This research was supported by Biomet 3i , LLC, a Zimmer Biomet company.

      Conflict of interest

      All authors were employees of Biomet 3i, LLC, a Zimmer Biomet company at the time this research was conducted and have received remuneration and other compensation.

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