- Email: [email protected]
Strain analysis of 9 different abutments for cement-retained crowns on an internal hexagonal implant
RESEARCH AND EDUCATION
Strain analysis of 9 different abutments for cement-retained crowns on an internal hexagonal implant Louai G. Salaita, DDS, MS,a Burak Yilmaz, DDS, PhD,b Jeremy D. Seidt, PhD,c Nancy L. Clelland, DMD, MS,d Hua-Hong Chien, DDS, PhD,e and Edwin A. McGlumphy, DDS, MSf Although the predictability of implant treatment has increased significantly, loosening of the abutment screw still occurs more frequently than other prosthetic complications.1 Even when tightened with a mechanical torqueing device, screws still become loose, with single-tooth implant restorations being most affected.2 Loose screws may fracture under occlusal loads if they are not retightened, and managing fractured screws is a difficult task for clinicians.3 Screw loosening occurs if the joint separating forces are greater in magnitude than the clamping forces.4 Several factors, however, have been shown to contribute to screw loosening, one factor is misfit of the implant/ abutment hexagon that results in the loss of the friction fit between the implant and
ABSTRACT Statement of problem. Many aftermarket abutments for cement-retained crowns are available for the tapered screw-vent implant. Aftermarket abutments vary widely, from stock to custom abutments and in materials such as zirconia, titanium, or a combination of the two. How these aftermarket abutments perform under occlusal loads with regard to strain distribution is not clear. Purpose. The purpose of this in vitro study was to measure and compare the different strains placed upon the bone around implants by 9 different abutments for cement-retained crowns on an implant with an internal hexagonal platform. Material and methods. Nine 4.1×11.5-mm tapered screw-vent implants were placed into a 305×51×8-mm resin block for strain measurements. Five abutment specimens of each of the 9 different abutments (N=45) were evaluated with 1 of the 9 implants. Monolithic zirconia crowns were then fabricated for each of the 9 different abutments, the crowns were cyclically loaded (maximum force 225 N) at 30 degrees, twice at a frequency of 2 Hz, and the strain was measured and recorded. The strain to the resin block was determined using a 3-dimensional digital image correlation (3D DIC) technique. Commercial image correlation software was used to analyze the strain around the implants. Data for maximal and minimal principal strains were compared using analysis of variance with a Tukey-Kramer post hoc test (a=.05). Results. Strain measurements showed no significant differences among any of the abutments for minimal (compression) principal strains (P>.05). For maximal (tensile) principal strains, the zirconia abutment showed the highest, and the patient-specific abutment showed the second-highest strain around the implant, with the zirconia being significantly greater than all abutments, with the exception of the patient-specific abutment, and the patient-specific abutment being significantly greater than the straight contoured abutment in titanium and also zirconia (P<.05). Conclusions. The patient-specific and zirconia abutments conferred the most tensile strain to the implants. When selecting an abutment for a cement-retained crown on a tapered screw-vent implant, practitioners should consider zirconia or titanium as well as aftermarket brands versus the implant name brand being used. (J Prosthet Dent 2016;-:---)
Supported by the Ohio State University Implant Research Fund. a Former Chief Resident, Advanced Prosthodontics Graduate Program, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. b Associate Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. c Research Scientist, Department of Mechanical and Aerospace Engineering, The Ohio State University College of Engineering, Columbus, Ohio. d Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. e Professor, Division of Periodontics, The Ohio State University College of Dentistry, Columbus, Ohio. f Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio.
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Clinical Implications The results of this study suggest that the selection of abutments for internal hexagonal implants may play a role in the transmission and location of forces upon loading of the prosthesis; this may increase prosthetic complications such as screw loosening or screw fracture.
abutment surfaces.4 Other joint-separating forces are also working on the screw-joint interface to cause screw loosening, such as excursive contacts and off-axis central contacts. In most situations, these forces on implant restorations cannot be avoided, meaning only the fit of the abutment and implant can be controlled by the operator, making abutment selection an important factor in the prevention of screw loosening. In addition to screw loosening, bone loss around the implant is a major complication that could lead to loss of the implant. The forces that act on a prosthesis are referred to as stresses, and these stresses are then transferred to the bone in the form of strain.5-7 These stresses and strains, in addition to causing prosthetic problems such as screw loosening or breakage, can also cause biological problems such as bone loss. Although, physiologically, bone needs a certain amount of strain to stay viable and avoid atrophy, too much strain can result in microfractures and bone loss.8 Excessive strain has been shown to result in bone loss around dental implants and even result in loss of osseointegration around the implant beyond the physiologic threshold.9-11 The internal hexagon, including the tapered screwvent (TSV) implant (Zimmer Dental) design, has been marketed by many of the leading implant manufacturers. This implant features a 1.5-mm-deep internal hexagon below a 0.5-mm-wide 45-degree lead-in bevel.12,13 Clinicians using the standardized design of the TSV implant have an abundant choice of abutments for a cement-retained restoration. Stock and custom abutments are available from a number of different manufacturers and in different materials. Manufacturers of abutments for cement-retained crowns include Atlantis (Dentsply Implants), Glidewell Laboratories, Implant Direct, and Zimmer Dental. In addition to different manufacturers, different materials also can be used for the abutment, including titanium (commercially pure grades 1-4 and alloy grade 5) and zirconia, with zirconia being chosen for improved esthetics. Although cost and marketing play an important part in what clinicians select, some abutment types may provide advantages, such as the patient-specific abutment (PSA; Zimmer Dental) “friction fit” between the implant and the abutment interface. Such a friction fit
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may result in a more stable screw joint and may confer less harmful strains to the bone surrounding the implant. Some nonoriginal manufacturer components for implants, abutments included, have lower cost, making them an attractive treatment option. However, nonoriginal manufacturer components do not require independent fatigue testing if the design is based on substantial equivalence based on the U.S. Food and Drug administration 510(k) guidelines,14,15 despite studies showing decreased fatigue resistance of nonoriginal manufacturer components and variations in machining tolerances.16 The purpose of this in vitro study was to test the strain on resultant bone after cyclically loading crowns fabricated to fit on 9 different abutments for cement-retained restorations on the TSV implant after a 30-NCm torque was applied. The null hypothesis was that no differences in strain would be found among the 9 different abutments. MATERIAL AND METHODS A 305×51×8-mm resin block (ABS transparent resin; DSM Somos) with a similar elastic modulus (2000 MPa) to cancellous bone (1507 MPa)16 was fabricated by a (3D Systems Healthcare). Nine pilot holes were drilled 5 mm in depth on the top surface 25 mm apart from each other and a drill press was used for parallelism. The implant sites were then prepared by a periodontist with a 3.8-mm tap drill and a surgical implant handpiece (Surgical Motor System; Zimmer Dental) to a 12-mm depth. The threads of 4.1×11.5-mm Tapered Screw-Vent implants (n=9) (Zimmer Dental) were coated with a metal bonding cyanoacrylate adhesive (Permabond 910; Permabond LLC) and the implants were placed using the same handpiece set to 35 Ncm. The implants were driven into the prepared implant sites manually with a wrench which was attached to a digital torque indicator (Model M5i; Mark-10 Corp) and the implant platforms were leveled with the resin surface. Nine different abutments for cement-retained crowns were tested for strain based on the available options for the Tapered Screw-Vent implants from a dental laboratory (Slagle and Kiser Dental Ceramics) (Table 1). After abutment fabrication, monolithic zirconia crowns were milled for each group of abutments, based on an initial waxing and their digital scan (Fig. 1). The abutments were grouped into 4 categories: Stock Titanium (Astra, Implant Direct), Stock Zirconia (Astra, Implant Direct), Custom Titanium (Atlantis Dentsplyimplants, Zimmer Dental, Glidewell Laboratories), and Custom Zirconia (Atlantis Dentsplyimplants, Glidewell Laboratories). The abutments were different based on material, manufacturer, stock or custom, type of material at the connection to the implant, and cost.
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Table 1. Specifications of 9 abutments for cement-retained prostheses Stock or Custom
Connection to Implant
Abutment Name
Manufacturer Material
Atlantis titanium (AtlTi)
Dentsply Implants
Titanium Custom
Titanium
Atlantis zirconia (AtlZr)
Dentsply Implants
Zirconia
Zirconia
Inclusive custom implant abutment titanium (GlTi)
Glidewell Laboratories
Titanium Custom
Titanium
Inclusive custom implant abutment zirconia (GlZr)
Glidewell Laboratories
Zirconia
Zirconia
Legacy straight contoured abutment (IDTi)
Implant Direct Titanium Stock
Titanium
Legacy zirconia straight contoured abutment (IDZr)
Implant Direct Zirconia
Stock
Titanium insert
AstraTech ZirDesign abutment (AstZr)
AstraTech
Zirconia
Stock
Zirconia
AstraTech TiDesign abutment (AstTi)
AstraTech
Titanium Stock
Titanium
Zimmer patient-specific abutment (PSA)
Zimmer Dental
Titanium Custom
Friction fit titanium
Custom
Custom
Figure 1. Crown fabrication.
An abutment type from the test groups was assigned to only 1 of the 9 implants. A 3-dimensional digital image correlation technique was used to measure strain to the transparent resin when loading forces were applied to the crown/abutment interface. Commercial image correlation software (VIC3D Version 7.2.4, build 194, Correlated Solutions, Inc.) was used to analyze data from images collected by 2 digital cameras (GRAS-20S4M-C; Point Grey Research) (Fig. 2). The cameras were in 2048×2048 pixel resolution and equipped with 35-mm lenses (Schneider-Kreuznach; Jos. Schneider Optische Werke GmbH). A custom fixture was used to mount the cameras on a tripod. The fixture was directed at the specimens on the resin block that was fixed to an x-y table at 30 degrees to the load cell. Each camera was independently calibrated. For each calibration image captured, a system of equations, which related the sensor position of the grid to the camera parameters, was formed. The parameters were provided to the cameras through the solutions of these equations to transform camera sensor coordinates to a common world coordinate system, which provided the basis for relating image positions in both cameras to a common 3dimensional location.17-19 A high-contrast, non-repetitive dot pattern was implemented on the external surface of the resin block model. A white spray paint (Rustoleum Painter’s Touch) base coat was applied as the first layer and allowed to dry. A second, contrasting black spray paint was spattered lightly on the white base layer and this second layer was also allowed to dry. Five abutments per group were first randomly (www.random.org) hand-tightened and then tightened to the manufacturer recommended torque Salaita et al
Figure 2. Abutments tightened into implants placed in resin block.
value using a torque wrench (Zimmer Dental). The abutments were also spray painted using the same protocol. The 2 digital cameras recorded images of the resin as the crowns were cyclically loaded with a torque at a 30-degree angle to the abutment. Two force cycles at a maximum of 225 N at 2 Hz were used to load the fabricated zirconia crowns with a steel hemispherical loading apparatus attached to a hydraulic load frame (Instron 1321; Instron Corp) (Fig. 3). This was done 5 times for each abutment for a total of 45 trials. Force measurements were recorded from the load cell equipped on the load frame. After the images were captured, the data were processed with a software (VIC-3D Version 7.2.4, build 194, Correlated Solutions, Inc.). First, the displacement field was calculated from the images, and then the resultant strain field was calculated from the resultant displacements. A rectangular area directly apical to the implant platform was selected, and strain measurements during the 2 cycles of loading were extracted from the data. Specifically, minimal principal strains and maximal principal strains at 100 N were examined for each of the 9 abutments. THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 3. Strain measurement arrangement.
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Denominator df
F-Ratio
P
Abutment
8
44
27.84
<.001
Direction
1
44
2050.8
<.001
Abutment×direction
8
44
3.61
.003
The principal strains were determined at rise and fall, loading and unloading, respectively, at 100 N, and the strain measurements were repeated once for each specimen. The means of these 4 determinations for every specimen was used for the interpretive analysis, which was accomplished with a repeated measures analysis of variance. The maximum likelihood estimation method and the Satterthwaite degrees of freedom method were used over all abutments for the 2 principal strain directions. Pairwise comparisons among the abutments within each principal strain direction were accomplished by Bonferroni corrections of t test probabilities. Statistical software (SAS Proprietary Software v9.3, PROC MIXED and PROC MULTTEST; SAS Institute Inc) was used for the statistical analyses (a=.05). RESULTS Maximal principal strains were generally positive (tensile), whereas minimal principal strains were generally negative (compressive) (Fig. 4). For maximal (tensile) principal strains, significant differences were found (Table 2, Fig. 5). No significant differences were found among any of the abutments minimal (compressive) principal strains (P>.001) (Fig. 6). The Atlantis zirconia abutment conferred the highest strains, which were significantly higher than all other abutments except for the Zimmer PSA (P<.001) (Fig. 5). The Zimmer PSA abutment had the second highest strain values, which were significantly greater than 3 other abutments: the
0.0008 0.0007
Principal Strain e1
Numerator df
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Figure 4. Digital image correlation information of strain data. Red indicates highest amount of strain and purple indicates lowest.
Table 2. Type 3 tests of fixed effects for repeated measures ANOVA of principal strains Effect
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0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000
I I H H G G F F E E D D C C B B A A AstTi AstZir AtlTi AtlZir GlTi GlZir lDTi lDZir PSA
Abutment Figure 5. Mean maximal (tensile) principal strains and 95% confidence limits around implants. Letters in bars indicate pairs found to be statistically significantly different (P<.05). AtlTi, Atlantis titanium; AstTi, AstraTech TiDesign abutment; AstZr, AstraTech ZirDesign abutment; ATlZr, Atlantis zirconia; G1Ti, Inclusive custom implant abutment titanium; G1Zr, Inclusive custom implant abutment zirconia; IDTi, Legacy straight contoured abutment; IDZr, Legacy zirconia straight contoured abutment; PSA, Zimmer patient-specific abutment.
AstraTech ZirDesign zirconia abutment, the Legacy straight contoured abutment in titanium, and the Legacy straight contoured abutment in zirconia (P<.001). DISCUSSION The null hypothesis, that all abutments would impart equal amounts of strain around the implant, was rejected because the Atlantis zirconia abutment exerted significantly greater principal tensile strains around the implant than all other abutments, except for the Zimmer PSA abutment, which had the second highest tensile strains around the implant (P<.001). The Zimmer PSA abutment had significantly higher principal tensile strains than the AstraTech ZirDesign abutment and Legacy straight contoured abutment in titanium and zirconia (P<.001). Salaita et al
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0.0000
Principal Strain e2
–0.0001 –0.0002 –0.0003 –0.0004 –0.0005 –0.0006 –0.0007 –0.0008 –0.0009
AstTi AstZir AtlTi AtlZir GlTi GlZir lDTi lDZir PSA
Abutment Figure 6. Mean minimal (compressive) principal strains and 95% confidence limits around implants. AtlTi, Atlantis titanium; AstTi, AstraTech TiDesign abutment; AstZr, AstraTech ZirDesign abutment; ATlZr, Atlantis zirconia; G1Ti, Inclusive custom implant abutment titanium; G1Zr, Inclusive custom implant abutment zirconia; IDTi, Legacy straight contoured abutment; IDZr, Legacy zirconia straight contoured abutment; PSA, Zimmer patient-specific abutment.
Strain distribution around implants has previously been studied, primarily with strain gauges or photoelastic measurements.6,7 However, these techniques have their limitations, with strain gauges giving only quantitative data and photoelastic studies providing only qualitative data. Recently, digital image correlation has been used for implant biomechanics, providing both visual and quantitative data for the entire surface of a model.20,21 The magnitude of strains around the implants was examined for 9 different abutments attached to TSV implants in this study. Both the compressive and the tensile principal strains were measured. The minimal principal (compressive) strains measured around the implants were found not to be statistically significant when the 9 different abutments were compared. However, 2 abutments showed significantly higher maximal tensile strains than the other abutments: the Atlantis zirconia abutment showed the highest magnitude of principal tensile strains, whereas the Zimmer PSA showed the second highest magnitude of strains. Although excessive strain to the bone induces microfractures and bone loss, insufficient strain results in loss of bone homeostasis and bone atrophy.8 It is beyond the scope of this paper to determine whether any of the abutments conferred a strain that was beyond the homeostatic point, which has been estimated to be around 50 MPa. However, the force of 100 N that was analyzed relates to the average occlusal force on a single anterior tooth in a woman and is unlikely to induce strains that would result in microfractures to the bone around the implant.22 Also, bone has less tolerance for tensile forces than compressive forces, with bones normally tolerating 4000 me (electron magnetic dipole moment) in Salaita et al
compression and 2500 me in tension.23,24 More strain to the area around the implant may indicate a tighter connection or friction-fit of the abutment to the implant. By having a tight connection of the abutment to the implant during loading, the strain may be transferred directly to the bone around the implant, as if the implant and abutment were all one unit. Lower strains may indicate more force transmission to the abutment/ connection complex versus the resultant bone around the implant, which would support the manufacturer’s claims of a friction fit for the Zimmer PSA, as well as indicate a precise fit of the Atlantis Zirconia abutment. For the strain tests, all abutments had crowns fabricated for them, which standardized the overall contour. However, this was an in vitro model in which a bonesimulating resin was used instead of human bone and in which the research was limited to a single implant system. The strains to the bone were measured only on the external surface of the model, as opposed to a 3D approach. In addition, 2 cycles were carried out for the strain measurements, whereas thermocycling with accelerated aging could have more closely replicated a clinical situation and helped analyze the implant abutment complex over time. Clinical studies of different implant systems and abutments would help to analyze strains in a 3D manner. CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. No significant differences were found among compressive strains in the implant abutments studied. 2. However, for tensile strains, the Atlantis Zirconia abutment and Zimmer PSA abutment showed the most strains around the implant. 3. The Atlantis Zirconia abutment had tensile strains that were significantly higher than all other abutments, except for the Zimmer PSA abutment; and the Zimmer PSA abutment had significantly higher strains than the AstraTech ZirDesign and the Legacy Straight Contour abutments in titanium and zirconia. REFERENCES 1. Dailey B, Jordan L, Blind O, Tavernier B. Axial displacement of abutments into implants and implant replicas, with the tapered cone-screw internal connection, as a function of tightening torque. Int J Oral Maxillofac Implants 2009;24:251-6. 2. Jemt T, Lacey WR, Harris D, Henry PJ, Krogh PH Jr, Polizzi G, et al. Osseointegrated implants for single tooth replacement: a 1-year report from a multicenter prospective study. Int J Oral Maxillofac 1991;6:29-36. 3. Yilmaz B, McGlumphy E. A technique to retrieve fractured implant screws. J Prosthet Dent 2011;105:137-8. 4. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9:149-60.
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5. Hollinger JO. Bone dynamics: morphogenesis, growth modeling, and remodeling. In: Lieberman JR, Friedlaender GE, editors. Bone regeneration and repair: biology and clinical applications. Totowa, NJ: Humana Press; 2005:15. 6. Akça K, Fanuscu MI, Caputo AA. Effect of compromised cortical bone on implant load distribution. J Prosthodont 2008;17:616-20. 7. Cehreli M, Duyck J, De Cooman M, Puers R, Naert I. Implant design and interface force transfer. A photoelastic and strain-gauge analysis. Clin Oral Implants Res 2004;15:249-57. 8. Frost HM. A 2003 update of bone physiology and Wolff’s Law for clinicians. Angle Orthod 2004;74:3-15. 9. Isidor F. Influence of forces on peri-implant bone. Clin Oral Implants Res 2006;(suppl 2):8-18. 10. Chambrone L, Chambrone LA, Lima LA. Effects of occlusal overload on periimplant tissue health: a systematic review of animal-model studies. J Periodontol 2010;81:1367-78. 11. Vidyasagar L, Apse P. Biological response to dental implant loading/overloading. Implant overloading: empiricism or science? Stomatologica 2003;5: 83-9. 12. Niznick GA. The Core-Vent implant system. The evolution of the osseointegration implant. Oral Health 1983;73:13-7. 13. Niznick GA. The implant abutment connection: the key to prosthetic success. Compendium 1991;12:932, 934-38. 14. Cashman PM, Schneider RL, Schneider GB, Stanford CM, Clancy JM, Qian F. In vitro analysis of post-fatigue reverse-torque values at the dental abutment/ implant interface for a unitarian abutment design. J Prosthodont 2011;20: 503-9. 15. Binon PP, Weir DJ, Marshall SJ. Surface analysis of an original Brånemark implant and three related clones. Int J Oral Maxillofac Implants 1992;7: 168-75. 16. Seidt JD. Plastic deformation and ductile fracture of 2024-T351 aluminum under various loading conditions [dissertation]. Columbus: the Ohio State University; 2010. 17. Takahashi N, Kitagami T, Komori T. Analysis of stress on a fixed partial with a blade-vent implant. J Prosthet Dent 1978;40:186-91.
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18. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8:162-78. 19. Schmidt C, Tyson A, Galanulis K. Full field dynamic displacement and strain measurement using advanced 3D image correlation photogrammetry. Exp Tech 2003;27:47-50. 20. Gilat A, Schmidt TE. Full field strain measurements in compression and tensile split Hopkinson bar experiment. Exp Mech 2009;49:291-302. 21. Morita Y, Quian L, Todo M. Strain distribution around dental implants in cortical/cancellous bone models using DIC method. Paper presented at: Society for Experimental Mechanics Annual SEM conference; Albuquerque, NM; June 2009. 22. Sugiura T, Horiuchi K, Sugimura M, Tsutsumi S. Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate. J Musculoskelet Neuronal Interact 2000;1:165-70. 23. Pattin CA, Caler WE, Carter DR. Cyclic mechanical property degradation during fatigue loading of cortical bone. J Biomech 1996;29:69-79. 24. Burr DB, Milgrom C, Fyhrie D, Forwood M, Nyska M, Finestone A, et al. In vivo measurement of human tibial strains during vigorous activity. Bone 1996;18:405-10. Corresponding author: Dr Burak Yilmaz The Ohio State University College of Dentistry 305 W 12th Ave Columbus, OH 43210 Email: [email protected] Acknowledgments The authors thank Dr. William Johnston for help with statistical analysis of this study. Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Strain analysis of 9 different abutments for cement-retained crowns on an internal hexagonal implant Louai G. Salaita, DDS, MS,a Burak Yilmaz, DDS, PhD,b Jeremy D. Seidt, PhD,c Nancy L. Clelland, DMD, MS,d Hua-Hong Chien, DDS, PhD,e and Edwin A. McGlumphy, DDS, MSf Although the predictability of implant treatment has increased significantly, loosening of the abutment screw still occurs more frequently than other prosthetic complications.1 Even when tightened with a mechanical torqueing device, screws still become loose, with single-tooth implant restorations being most affected.2 Loose screws may fracture under occlusal loads if they are not retightened, and managing fractured screws is a difficult task for clinicians.3 Screw loosening occurs if the joint separating forces are greater in magnitude than the clamping forces.4 Several factors, however, have been shown to contribute to screw loosening, one factor is misfit of the implant/ abutment hexagon that results in the loss of the friction fit between the implant and
ABSTRACT Statement of problem. Many aftermarket abutments for cement-retained crowns are available for the tapered screw-vent implant. Aftermarket abutments vary widely, from stock to custom abutments and in materials such as zirconia, titanium, or a combination of the two. How these aftermarket abutments perform under occlusal loads with regard to strain distribution is not clear. Purpose. The purpose of this in vitro study was to measure and compare the different strains placed upon the bone around implants by 9 different abutments for cement-retained crowns on an implant with an internal hexagonal platform. Material and methods. Nine 4.1×11.5-mm tapered screw-vent implants were placed into a 305×51×8-mm resin block for strain measurements. Five abutment specimens of each of the 9 different abutments (N=45) were evaluated with 1 of the 9 implants. Monolithic zirconia crowns were then fabricated for each of the 9 different abutments, the crowns were cyclically loaded (maximum force 225 N) at 30 degrees, twice at a frequency of 2 Hz, and the strain was measured and recorded. The strain to the resin block was determined using a 3-dimensional digital image correlation (3D DIC) technique. Commercial image correlation software was used to analyze the strain around the implants. Data for maximal and minimal principal strains were compared using analysis of variance with a Tukey-Kramer post hoc test (a=.05). Results. Strain measurements showed no significant differences among any of the abutments for minimal (compression) principal strains (P>.05). For maximal (tensile) principal strains, the zirconia abutment showed the highest, and the patient-specific abutment showed the second-highest strain around the implant, with the zirconia being significantly greater than all abutments, with the exception of the patient-specific abutment, and the patient-specific abutment being significantly greater than the straight contoured abutment in titanium and also zirconia (P<.05). Conclusions. The patient-specific and zirconia abutments conferred the most tensile strain to the implants. When selecting an abutment for a cement-retained crown on a tapered screw-vent implant, practitioners should consider zirconia or titanium as well as aftermarket brands versus the implant name brand being used. (J Prosthet Dent 2016;-:---)
Supported by the Ohio State University Implant Research Fund. a Former Chief Resident, Advanced Prosthodontics Graduate Program, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. b Associate Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. c Research Scientist, Department of Mechanical and Aerospace Engineering, The Ohio State University College of Engineering, Columbus, Ohio. d Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio. e Professor, Division of Periodontics, The Ohio State University College of Dentistry, Columbus, Ohio. f Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, Ohio.
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Clinical Implications The results of this study suggest that the selection of abutments for internal hexagonal implants may play a role in the transmission and location of forces upon loading of the prosthesis; this may increase prosthetic complications such as screw loosening or screw fracture.
abutment surfaces.4 Other joint-separating forces are also working on the screw-joint interface to cause screw loosening, such as excursive contacts and off-axis central contacts. In most situations, these forces on implant restorations cannot be avoided, meaning only the fit of the abutment and implant can be controlled by the operator, making abutment selection an important factor in the prevention of screw loosening. In addition to screw loosening, bone loss around the implant is a major complication that could lead to loss of the implant. The forces that act on a prosthesis are referred to as stresses, and these stresses are then transferred to the bone in the form of strain.5-7 These stresses and strains, in addition to causing prosthetic problems such as screw loosening or breakage, can also cause biological problems such as bone loss. Although, physiologically, bone needs a certain amount of strain to stay viable and avoid atrophy, too much strain can result in microfractures and bone loss.8 Excessive strain has been shown to result in bone loss around dental implants and even result in loss of osseointegration around the implant beyond the physiologic threshold.9-11 The internal hexagon, including the tapered screwvent (TSV) implant (Zimmer Dental) design, has been marketed by many of the leading implant manufacturers. This implant features a 1.5-mm-deep internal hexagon below a 0.5-mm-wide 45-degree lead-in bevel.12,13 Clinicians using the standardized design of the TSV implant have an abundant choice of abutments for a cement-retained restoration. Stock and custom abutments are available from a number of different manufacturers and in different materials. Manufacturers of abutments for cement-retained crowns include Atlantis (Dentsply Implants), Glidewell Laboratories, Implant Direct, and Zimmer Dental. In addition to different manufacturers, different materials also can be used for the abutment, including titanium (commercially pure grades 1-4 and alloy grade 5) and zirconia, with zirconia being chosen for improved esthetics. Although cost and marketing play an important part in what clinicians select, some abutment types may provide advantages, such as the patient-specific abutment (PSA; Zimmer Dental) “friction fit” between the implant and the abutment interface. Such a friction fit
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may result in a more stable screw joint and may confer less harmful strains to the bone surrounding the implant. Some nonoriginal manufacturer components for implants, abutments included, have lower cost, making them an attractive treatment option. However, nonoriginal manufacturer components do not require independent fatigue testing if the design is based on substantial equivalence based on the U.S. Food and Drug administration 510(k) guidelines,14,15 despite studies showing decreased fatigue resistance of nonoriginal manufacturer components and variations in machining tolerances.16 The purpose of this in vitro study was to test the strain on resultant bone after cyclically loading crowns fabricated to fit on 9 different abutments for cement-retained restorations on the TSV implant after a 30-NCm torque was applied. The null hypothesis was that no differences in strain would be found among the 9 different abutments. MATERIAL AND METHODS A 305×51×8-mm resin block (ABS transparent resin; DSM Somos) with a similar elastic modulus (2000 MPa) to cancellous bone (1507 MPa)16 was fabricated by a (3D Systems Healthcare). Nine pilot holes were drilled 5 mm in depth on the top surface 25 mm apart from each other and a drill press was used for parallelism. The implant sites were then prepared by a periodontist with a 3.8-mm tap drill and a surgical implant handpiece (Surgical Motor System; Zimmer Dental) to a 12-mm depth. The threads of 4.1×11.5-mm Tapered Screw-Vent implants (n=9) (Zimmer Dental) were coated with a metal bonding cyanoacrylate adhesive (Permabond 910; Permabond LLC) and the implants were placed using the same handpiece set to 35 Ncm. The implants were driven into the prepared implant sites manually with a wrench which was attached to a digital torque indicator (Model M5i; Mark-10 Corp) and the implant platforms were leveled with the resin surface. Nine different abutments for cement-retained crowns were tested for strain based on the available options for the Tapered Screw-Vent implants from a dental laboratory (Slagle and Kiser Dental Ceramics) (Table 1). After abutment fabrication, monolithic zirconia crowns were milled for each group of abutments, based on an initial waxing and their digital scan (Fig. 1). The abutments were grouped into 4 categories: Stock Titanium (Astra, Implant Direct), Stock Zirconia (Astra, Implant Direct), Custom Titanium (Atlantis Dentsplyimplants, Zimmer Dental, Glidewell Laboratories), and Custom Zirconia (Atlantis Dentsplyimplants, Glidewell Laboratories). The abutments were different based on material, manufacturer, stock or custom, type of material at the connection to the implant, and cost.
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Table 1. Specifications of 9 abutments for cement-retained prostheses Stock or Custom
Connection to Implant
Abutment Name
Manufacturer Material
Atlantis titanium (AtlTi)
Dentsply Implants
Titanium Custom
Titanium
Atlantis zirconia (AtlZr)
Dentsply Implants
Zirconia
Zirconia
Inclusive custom implant abutment titanium (GlTi)
Glidewell Laboratories
Titanium Custom
Titanium
Inclusive custom implant abutment zirconia (GlZr)
Glidewell Laboratories
Zirconia
Zirconia
Legacy straight contoured abutment (IDTi)
Implant Direct Titanium Stock
Titanium
Legacy zirconia straight contoured abutment (IDZr)
Implant Direct Zirconia
Stock
Titanium insert
AstraTech ZirDesign abutment (AstZr)
AstraTech
Zirconia
Stock
Zirconia
AstraTech TiDesign abutment (AstTi)
AstraTech
Titanium Stock
Titanium
Zimmer patient-specific abutment (PSA)
Zimmer Dental
Titanium Custom
Friction fit titanium
Custom
Custom
Figure 1. Crown fabrication.
An abutment type from the test groups was assigned to only 1 of the 9 implants. A 3-dimensional digital image correlation technique was used to measure strain to the transparent resin when loading forces were applied to the crown/abutment interface. Commercial image correlation software (VIC3D Version 7.2.4, build 194, Correlated Solutions, Inc.) was used to analyze data from images collected by 2 digital cameras (GRAS-20S4M-C; Point Grey Research) (Fig. 2). The cameras were in 2048×2048 pixel resolution and equipped with 35-mm lenses (Schneider-Kreuznach; Jos. Schneider Optische Werke GmbH). A custom fixture was used to mount the cameras on a tripod. The fixture was directed at the specimens on the resin block that was fixed to an x-y table at 30 degrees to the load cell. Each camera was independently calibrated. For each calibration image captured, a system of equations, which related the sensor position of the grid to the camera parameters, was formed. The parameters were provided to the cameras through the solutions of these equations to transform camera sensor coordinates to a common world coordinate system, which provided the basis for relating image positions in both cameras to a common 3dimensional location.17-19 A high-contrast, non-repetitive dot pattern was implemented on the external surface of the resin block model. A white spray paint (Rustoleum Painter’s Touch) base coat was applied as the first layer and allowed to dry. A second, contrasting black spray paint was spattered lightly on the white base layer and this second layer was also allowed to dry. Five abutments per group were first randomly (www.random.org) hand-tightened and then tightened to the manufacturer recommended torque Salaita et al
Figure 2. Abutments tightened into implants placed in resin block.
value using a torque wrench (Zimmer Dental). The abutments were also spray painted using the same protocol. The 2 digital cameras recorded images of the resin as the crowns were cyclically loaded with a torque at a 30-degree angle to the abutment. Two force cycles at a maximum of 225 N at 2 Hz were used to load the fabricated zirconia crowns with a steel hemispherical loading apparatus attached to a hydraulic load frame (Instron 1321; Instron Corp) (Fig. 3). This was done 5 times for each abutment for a total of 45 trials. Force measurements were recorded from the load cell equipped on the load frame. After the images were captured, the data were processed with a software (VIC-3D Version 7.2.4, build 194, Correlated Solutions, Inc.). First, the displacement field was calculated from the images, and then the resultant strain field was calculated from the resultant displacements. A rectangular area directly apical to the implant platform was selected, and strain measurements during the 2 cycles of loading were extracted from the data. Specifically, minimal principal strains and maximal principal strains at 100 N were examined for each of the 9 abutments. THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 3. Strain measurement arrangement.
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Denominator df
F-Ratio
P
Abutment
8
44
27.84
<.001
Direction
1
44
2050.8
<.001
Abutment×direction
8
44
3.61
.003
The principal strains were determined at rise and fall, loading and unloading, respectively, at 100 N, and the strain measurements were repeated once for each specimen. The means of these 4 determinations for every specimen was used for the interpretive analysis, which was accomplished with a repeated measures analysis of variance. The maximum likelihood estimation method and the Satterthwaite degrees of freedom method were used over all abutments for the 2 principal strain directions. Pairwise comparisons among the abutments within each principal strain direction were accomplished by Bonferroni corrections of t test probabilities. Statistical software (SAS Proprietary Software v9.3, PROC MIXED and PROC MULTTEST; SAS Institute Inc) was used for the statistical analyses (a=.05). RESULTS Maximal principal strains were generally positive (tensile), whereas minimal principal strains were generally negative (compressive) (Fig. 4). For maximal (tensile) principal strains, significant differences were found (Table 2, Fig. 5). No significant differences were found among any of the abutments minimal (compressive) principal strains (P>.001) (Fig. 6). The Atlantis zirconia abutment conferred the highest strains, which were significantly higher than all other abutments except for the Zimmer PSA (P<.001) (Fig. 5). The Zimmer PSA abutment had the second highest strain values, which were significantly greater than 3 other abutments: the
0.0008 0.0007
Principal Strain e1
Numerator df
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Figure 4. Digital image correlation information of strain data. Red indicates highest amount of strain and purple indicates lowest.
Table 2. Type 3 tests of fixed effects for repeated measures ANOVA of principal strains Effect
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0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000
I I H H G G F F E E D D C C B B A A AstTi AstZir AtlTi AtlZir GlTi GlZir lDTi lDZir PSA
Abutment Figure 5. Mean maximal (tensile) principal strains and 95% confidence limits around implants. Letters in bars indicate pairs found to be statistically significantly different (P<.05). AtlTi, Atlantis titanium; AstTi, AstraTech TiDesign abutment; AstZr, AstraTech ZirDesign abutment; ATlZr, Atlantis zirconia; G1Ti, Inclusive custom implant abutment titanium; G1Zr, Inclusive custom implant abutment zirconia; IDTi, Legacy straight contoured abutment; IDZr, Legacy zirconia straight contoured abutment; PSA, Zimmer patient-specific abutment.
AstraTech ZirDesign zirconia abutment, the Legacy straight contoured abutment in titanium, and the Legacy straight contoured abutment in zirconia (P<.001). DISCUSSION The null hypothesis, that all abutments would impart equal amounts of strain around the implant, was rejected because the Atlantis zirconia abutment exerted significantly greater principal tensile strains around the implant than all other abutments, except for the Zimmer PSA abutment, which had the second highest tensile strains around the implant (P<.001). The Zimmer PSA abutment had significantly higher principal tensile strains than the AstraTech ZirDesign abutment and Legacy straight contoured abutment in titanium and zirconia (P<.001). Salaita et al
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0.0000
Principal Strain e2
–0.0001 –0.0002 –0.0003 –0.0004 –0.0005 –0.0006 –0.0007 –0.0008 –0.0009
AstTi AstZir AtlTi AtlZir GlTi GlZir lDTi lDZir PSA
Abutment Figure 6. Mean minimal (compressive) principal strains and 95% confidence limits around implants. AtlTi, Atlantis titanium; AstTi, AstraTech TiDesign abutment; AstZr, AstraTech ZirDesign abutment; ATlZr, Atlantis zirconia; G1Ti, Inclusive custom implant abutment titanium; G1Zr, Inclusive custom implant abutment zirconia; IDTi, Legacy straight contoured abutment; IDZr, Legacy zirconia straight contoured abutment; PSA, Zimmer patient-specific abutment.
Strain distribution around implants has previously been studied, primarily with strain gauges or photoelastic measurements.6,7 However, these techniques have their limitations, with strain gauges giving only quantitative data and photoelastic studies providing only qualitative data. Recently, digital image correlation has been used for implant biomechanics, providing both visual and quantitative data for the entire surface of a model.20,21 The magnitude of strains around the implants was examined for 9 different abutments attached to TSV implants in this study. Both the compressive and the tensile principal strains were measured. The minimal principal (compressive) strains measured around the implants were found not to be statistically significant when the 9 different abutments were compared. However, 2 abutments showed significantly higher maximal tensile strains than the other abutments: the Atlantis zirconia abutment showed the highest magnitude of principal tensile strains, whereas the Zimmer PSA showed the second highest magnitude of strains. Although excessive strain to the bone induces microfractures and bone loss, insufficient strain results in loss of bone homeostasis and bone atrophy.8 It is beyond the scope of this paper to determine whether any of the abutments conferred a strain that was beyond the homeostatic point, which has been estimated to be around 50 MPa. However, the force of 100 N that was analyzed relates to the average occlusal force on a single anterior tooth in a woman and is unlikely to induce strains that would result in microfractures to the bone around the implant.22 Also, bone has less tolerance for tensile forces than compressive forces, with bones normally tolerating 4000 me (electron magnetic dipole moment) in Salaita et al
compression and 2500 me in tension.23,24 More strain to the area around the implant may indicate a tighter connection or friction-fit of the abutment to the implant. By having a tight connection of the abutment to the implant during loading, the strain may be transferred directly to the bone around the implant, as if the implant and abutment were all one unit. Lower strains may indicate more force transmission to the abutment/ connection complex versus the resultant bone around the implant, which would support the manufacturer’s claims of a friction fit for the Zimmer PSA, as well as indicate a precise fit of the Atlantis Zirconia abutment. For the strain tests, all abutments had crowns fabricated for them, which standardized the overall contour. However, this was an in vitro model in which a bonesimulating resin was used instead of human bone and in which the research was limited to a single implant system. The strains to the bone were measured only on the external surface of the model, as opposed to a 3D approach. In addition, 2 cycles were carried out for the strain measurements, whereas thermocycling with accelerated aging could have more closely replicated a clinical situation and helped analyze the implant abutment complex over time. Clinical studies of different implant systems and abutments would help to analyze strains in a 3D manner. CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. No significant differences were found among compressive strains in the implant abutments studied. 2. However, for tensile strains, the Atlantis Zirconia abutment and Zimmer PSA abutment showed the most strains around the implant. 3. The Atlantis Zirconia abutment had tensile strains that were significantly higher than all other abutments, except for the Zimmer PSA abutment; and the Zimmer PSA abutment had significantly higher strains than the AstraTech ZirDesign and the Legacy Straight Contour abutments in titanium and zirconia. REFERENCES 1. Dailey B, Jordan L, Blind O, Tavernier B. Axial displacement of abutments into implants and implant replicas, with the tapered cone-screw internal connection, as a function of tightening torque. Int J Oral Maxillofac Implants 2009;24:251-6. 2. Jemt T, Lacey WR, Harris D, Henry PJ, Krogh PH Jr, Polizzi G, et al. Osseointegrated implants for single tooth replacement: a 1-year report from a multicenter prospective study. Int J Oral Maxillofac 1991;6:29-36. 3. Yilmaz B, McGlumphy E. A technique to retrieve fractured implant screws. J Prosthet Dent 2011;105:137-8. 4. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9:149-60.
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5. Hollinger JO. Bone dynamics: morphogenesis, growth modeling, and remodeling. In: Lieberman JR, Friedlaender GE, editors. Bone regeneration and repair: biology and clinical applications. Totowa, NJ: Humana Press; 2005:15. 6. Akça K, Fanuscu MI, Caputo AA. Effect of compromised cortical bone on implant load distribution. J Prosthodont 2008;17:616-20. 7. Cehreli M, Duyck J, De Cooman M, Puers R, Naert I. Implant design and interface force transfer. A photoelastic and strain-gauge analysis. Clin Oral Implants Res 2004;15:249-57. 8. Frost HM. A 2003 update of bone physiology and Wolff’s Law for clinicians. Angle Orthod 2004;74:3-15. 9. Isidor F. Influence of forces on peri-implant bone. Clin Oral Implants Res 2006;(suppl 2):8-18. 10. Chambrone L, Chambrone LA, Lima LA. Effects of occlusal overload on periimplant tissue health: a systematic review of animal-model studies. J Periodontol 2010;81:1367-78. 11. Vidyasagar L, Apse P. Biological response to dental implant loading/overloading. Implant overloading: empiricism or science? Stomatologica 2003;5: 83-9. 12. Niznick GA. The Core-Vent implant system. The evolution of the osseointegration implant. Oral Health 1983;73:13-7. 13. Niznick GA. The implant abutment connection: the key to prosthetic success. Compendium 1991;12:932, 934-38. 14. Cashman PM, Schneider RL, Schneider GB, Stanford CM, Clancy JM, Qian F. In vitro analysis of post-fatigue reverse-torque values at the dental abutment/ implant interface for a unitarian abutment design. J Prosthodont 2011;20: 503-9. 15. Binon PP, Weir DJ, Marshall SJ. Surface analysis of an original Brånemark implant and three related clones. Int J Oral Maxillofac Implants 1992;7: 168-75. 16. Seidt JD. Plastic deformation and ductile fracture of 2024-T351 aluminum under various loading conditions [dissertation]. Columbus: the Ohio State University; 2010. 17. Takahashi N, Kitagami T, Komori T. Analysis of stress on a fixed partial with a blade-vent implant. J Prosthet Dent 1978;40:186-91.
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18. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8:162-78. 19. Schmidt C, Tyson A, Galanulis K. Full field dynamic displacement and strain measurement using advanced 3D image correlation photogrammetry. Exp Tech 2003;27:47-50. 20. Gilat A, Schmidt TE. Full field strain measurements in compression and tensile split Hopkinson bar experiment. Exp Mech 2009;49:291-302. 21. Morita Y, Quian L, Todo M. Strain distribution around dental implants in cortical/cancellous bone models using DIC method. Paper presented at: Society for Experimental Mechanics Annual SEM conference; Albuquerque, NM; June 2009. 22. Sugiura T, Horiuchi K, Sugimura M, Tsutsumi S. Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate. J Musculoskelet Neuronal Interact 2000;1:165-70. 23. Pattin CA, Caler WE, Carter DR. Cyclic mechanical property degradation during fatigue loading of cortical bone. J Biomech 1996;29:69-79. 24. Burr DB, Milgrom C, Fyhrie D, Forwood M, Nyska M, Finestone A, et al. In vivo measurement of human tibial strains during vigorous activity. Bone 1996;18:405-10. Corresponding author: Dr Burak Yilmaz The Ohio State University College of Dentistry 305 W 12th Ave Columbus, OH 43210 Email: [email protected] Acknowledgments The authors thank Dr. William Johnston for help with statistical analysis of this study. Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.
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