- Email: [email protected]
Photoelastic stress analysis of external versus internal implant-abutment connections
Photoelastic stress analysis of external versus internal implant-abutment connections Pattapon Asvanund, DDS MSD,a and Steven M. Morgano, DMDb Mahidol University, Bangkok, Thailand; Goldman School of Dental Medicine, Boston University, Boston, Mass Statement of problem. Common complications of implant restorations are abutment screw loosening and screw fracture. The geometry of the implant-abutment connection may affect stresses generated from loading, and these stresses may have a role in screw loosening or fracture. Purpose. This study compared the load transfer characteristics of a complete-arch restoration supported by 4 implants with external and internal implant-abutment connections. Loads were applied to the prostheses in 3 positions. Material and methods. Two-dimensional photoelastic models were used to simulate bone. Two types of implants (ReplaceSelect Internal-Interface Tapered Implants and ReplaceSelect External-Interface Tapered Implants) were placed in the photoelastic models. Complete-arch metal frameworks were fabricated on the abutments. Artificial teeth were arranged on the framework, and the prosthesis was screwed onto the abutments. The specimens were analyzed at 2 levels (implant-abutment level and apical to the implant level) with 3 loading conditions (4-point load; 2-point anterior load; and 2-point lateral load). The numbers of fringe orders were recorded and compared. Results. With the 4-point load, no stress differences occurred between the external-implant abutment connection and internal-implant abutment connection at the connection level and at the apical level. With the 2-point anterior load, the internal-implant abutment connection resulted in lower stresses at the connection level both in the loaded and non-loaded areas. With the 2-point lateral load, the internal-implant abutment connection resulted in lower stresses at the connection level at the non-loaded area. Conclusions. When loaded off-center, the internal-implant abutment connection produced less stress when compared with the external-implant abutment connection. (J Prosthet Dent 2011;106:266-271)
Clinical Implications
The internal-implant abutment connection could potentially reduce stresses within the connection when off-center loads are applied. The success of implant treatment depends partly on the biomechanical load on the dental implants.1 Functional loads applied on the prosthesis are transferred through the implants to the surrounding bone. Factors such as location and angulation of the implant, as well as geometry of the implant-abutment connection have been reported to influence stress and strain around implants.2-5 The increase in the load on an implant is directly pro-
portional to stress and strain developed at the bone-implant interface,6 and some studies have reported a relationship between occlusal overload and loss of osseointegration.7,8 The microgap between the abutment and the implant has been studied.9 Steinebrunner et al10 evaluated microbial leakage of different designs of the implant-abutment connections in an in vitro model. The authors reported that bacterial penetration occurred at
the implant-abutment interface during mastication. This microgap may allow micromovement between the implantabutment connection during clinical function, and this micromovement could result in localized inflammation with subsequent crestal bone loss.11 The stability of an implant-retained prosthesis depends on the integrity of the screw connection.12 Yousef et al13 reported that off-axis loading may result in screw and abut-
Instructor of Prosthodontics, Director Prosthodontic Residency Training Program, Faculty of Dentistry, Mahidol University. Professor and Director, Division of Postdoctoral Prosthodontics, Department of Restorative Sciences and Biomaterials, Goldman School of Dental Medicine.
a
b
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October 2011 ment deformation with subsequent screw loosening. Implants featuring a short external hexagon at the prospective connection with the abutment seemed to be especially prone to screw loosening. Jemt et al14 and Becker and Becker15 reported that a high incidence of screw loosening (up to 40%) was found for the external abutment connection. Screw loosening seemed to occur most often with single-tooth implant-supported restorations16 and Levine et al17 reported a far lower rate of abutment screw loosening (3.6 to 5.3%) with internal abutment connections. The external implant-abutment connection and the internal implant-abutment connection use different mechanical principles of function. For the external connection, the axial preload of the abutment screw is the determining factor for stability of the connection.18 This screw secures the abutment under horizontal loading. The optimal preload corresponds theoretically to the yield point of the screw. For the internal connection, internal form fit and friction are the basic mechanical principles. Lateral loading is resisted by the frictional interface.19 In a finite element analysis (FEA) model, Wang et al20 reported elongation of the abutment screw during the application of a torque moment, which increased preload in the implant abutment complex. The author’s of a systematic review
of clinical studies on abutment screwloosening in single-implant restorations reported that screw loosening was rare so long as proper procedures were followed when making the implant-abutment connections.21 Other clinical studies22-25 reported complications regarding implant-abutment connections. Balshi26 and Wei27 reported loosening or fracture of prosthetic screws related to parafunctional habits or an improperly designed or adjusted occlusion. Although research has concentrated on the influence of stresses developed around implants,28-32 the authors identified no research that evaluates the stresses developed by complete-arch restorations comparing external and internal implant-abutment connections. The photoelastic model has previously been used to evaluated stress about implants with various types of prostheses and variables.33-37 This technique is useful for evaluating the stresses responsible for failure of a structure, especially a structure with irregular form. The model is examined in a field of polarized light with loads applied. The photoelastic model, under carefully controlled conditions, can indicate internal stresses.38 Nevertheless, the photoelastic model has limitations when predicting the response of biologic systems to applied loads, similar to the limitations of other modeling systems, such as FEA, and strain gauge studies.20
1 Artificial teeth arranged on duplicate model of photoelastic model.
Asvanund and Morgano
The purpose of this study was to compare, through photoelastic analysis, the load transfer characteristics of complete-arch restorations supported by 4 implants, with external and internal implant-abutment connections.
MATERIAL AND METHODS Two photoelastic models (PL-4M; Vishay Micro-Measurements, Raleigh, NC) with a modulus of elasticity of 3.1 GPa were fabricated to replicate an edentulous human mandible. The photoelastic model was then duplicated with a silicone mold (Speedex; Coltène/ Whaledent AG, Alstätten, Switzerland) and poured with ADA Type IV dental stone (Vel-Mix; Kerr, Romulus, Mich). Artificial teeth (CosmoHXL; Dentsply Intl York, Pa) were arranged on the duplicate model to the mandibular first molar (Fig. 1). A 3-mm thick polyethylene sheet (Copyplast; Scheu-Dental GmBH, Iserlohn, Germany) was vacuum adapted (Biostar; Scheu-Dental GmBH) over the tooth arrangement. Four holes were drilled with a 2.0-mm twist drill (NobelBiocare USA, Yorba Linda, Calif ) through the polyethylene sheet at the canine and second premolar regions, bilaterally (Fig. 2). This sheet served as a positioning template for drilling the simulated osteotomies in the photoelastic models. The photoelastic models were positioned within the polyethylene sheet, and twist drills with 2.0 mm,
2 Vacuum-formed polyethylene sheet with holes at canines and second premolars bilaterally to be used as drill guides on photoelastic model.
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3 Four implants (4.3 x 13 mm) placed in photoelastic model.
3.5 mm, and then 4.3 mm diameters were used sequentially to drill at 800 rpm to a depth of 13 mm. A screw tap (NobelBiocare USA) was used at 50 rpm under water coolant. Four implants (4.3 × 13 mm) (Replace Select Internal-Interface Tapered Implant; NobelBiocare) were placed in each prepared hole in 1 photoelastic model, representing the internal connection model. Another 4 implants (4.3 × 13 mm) (Replace Select External-Interface Tapered Implant; NobelBiocare) were placed in each prepared hole in the other photoelastic model, representing the external connection model (Fig. 3). Patterns for the representative mandibular prosthetic frameworks were prepared with pink baseplate wax (Cavex Set Up Wax; Cavex Holland BV, Haarlem, The Netherlands), shaped to conform to the photoelastic block. The gold cylinders (PME Coping, Gold; NobelBiocare USA) were connected to the wax patterns. Retentive beads with a diameter of 0.6 mm. (Renfert GmbH; Hilzingen, Germany) were placed on each wax pattern, and the wax pattern was sprued (Sprues;Yeti Dentalproduct; Engen, Germany). Then the pattern was invested with phosphate bonded investment (Deguvest-Impact; Degussa-Hule, Hanau, Germany) at a ratio of powder to distilled water of 100 g:40 ml. The casting ring was placed in an electric burn-out
4 The processed prosthesis connected to implant replicas prior to seating on photoelastic model.
furnace (Midtherm TH; Bego, Germany). The temperature was raised from room temperature to 300°C within 1 hour, increased to 700°C in another hour, and held for 45 minutes. The framework was cast with silver-palladium casting alloy (Palliag M; Degussa-Hule) with a vacuum pressurecasting machine (NautilusT; Bego). The castings were then airborne-particle abraded with 50-µm aluminum oxide (Topstar 2; Bego) at 0.2 MPa pressure. Each cast framework was connected to the implants by guide pins (NobelBiocare) and the casting was evaluated for complete, passive seating. The artificial teeth (Cosmo HXL; Dentsply Intl) were arranged on the framework guided by the vacuum-formed polyethylene sheet. The canines and second premolars were drilled to produce channels that provided screw access. The prosthesis was processed with heat polymerized acrylic resin (Rodex; Rodont, Srl Milan, Italy) (Fig. 4). For the internal connection model, the processed prosthesis was connected to the abutments (PME Transmucosal Abutment 3 mmH; NobelBiocare) with 35 N-cm of torque. For the external connection model, the second processed prosthesis was connected to the abutments (PME Abutment 3.25 mmH; NobelBiocare) also with 35 N-cm of torque. Each screw-accesses channel for both prostheses was filled with a cotton
The Journal of Prosthetic Dentistry
pellet (2 mm), and composite resin (Clearfil; Kuraray America, New York, NY). The composite resin was light polymerized at 600 mW/cm2 (Demetron LC; Kerr Corporation, Orange, Calif ) for 40 seconds. A universal testing machine (Instron Model 5580; Norwood, Mass) with a custom-made puncture probe was used to apply a vertical load of 250 N. The crosshead speed was 0.125 mm/min on the prostheses. This relatively slow speed was used for the purpose of making photographs of the stresses generated in the photoelastic block. There were 3 loading conditions: (1) 4-point load with force applied to the canines and the premolars (4-P); (2) 2-point anterior load with force applied to the left and right canines (2-P-ANT), and (3) 2-point lateral load with force applied to the right canine and premolar (2-P-LAT). The observation of stress distribution was demonstrated by the formation of fringe orders38 at the implant-abutment connection and apical to the implant level. Color photographs were made upon reaching the desired loads. The distribution and stress magnitudes were shown as isochromatic fringe orders. The interpretation of fringe order values are: yellow, order 0.25; yellow-red, order 0.5; yellow-redblue, 1.0; yellow-red-blue-yellow-redgreen, 2.0; yellow-red-blue-yellow-redgreen-yellow-red-green, 3.0 respectively.
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October 2011 RESULTS In condition 4-P, the values of fringe order on each implant at the implant-abutment connection (I/A) and at the apical area of implant (Ap) are presented in Table I. Figure 5 dem-
onstrates the values of fringe order, under 4-point load, from left to right, right second premolar, right canine, left canine, left second premolar. In condition 2-P-ANT, the values of fringe order on each implant at the implant-abutment connection and
at the apical area of the implant are presented in Table I. Figure 6 demonstrates the values of fringe order, under 2-point anterior load, from left to right, right second premolar, right canine, left canine, left second premolar. For the internal connection (Fig.
Table I. Fringe order of loading condition 4-P, 2-P-ANT and 2-P-LAT at I/A level and Ap level of internal connection and external connection.
Condition
Internal Connection 4-P 2-P-ANT 2-P-LAT I/A Ap I/A Ap I/A Ap
External Connection 4-P 2-P-ANT 2-P-LAT I/A Ap I/A Ap I/A Ap
Lcanine
0.75
1.00 0.50 1.00 0.25
1.00
1.00
1.00 1.00 1.00 1.00
1.00
Lpremolar
1.00
1.00 0.25 0.25 0.25
0.25
1.00
1.00 1.00 1.00 1.00
1.00
Rcanine
1.00
1.00 0.50 1.00 0.50
1.00
1.00
1.00 1.00 1.00 1.00
1.00
Rpremolar
1.00
1.00 0.25 1.00 1.00
1.00
1.00
1.00 1.00 1.00 1.00
1.00
A
A
B
B
5 Four-point load: from left to right, right second premolar, right canine, left canine, left second premolar. A. Fringe orders at I/A and Ap of internal connection model. B. Fringe orders at the I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
6 Two-point anterior load: from left to right, right second premolar, right canine, left canine, left second premolar. A, Fringe orders at I/A and Ap of internal connection model. B, Fringe orders at the I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
A
B 7 Two-point right lateral load: from left to right, right second premolar, right canine, left canine, left second premolar. A, Fringe orders at I/A and Ap of internal connection model. B, Fringe orders at I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
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Volume 106 Issue 4 6A), the value of fringe order at the I/A level was 0.5 on the canines when loaded, and 0.25 on the non-loaded points (premolars). The fringe order at the Ap level was 1.00 for both the loaded and non-loaded points. For the external connection (Fig. 6B), the fringe order values were 1.00 at the I/A level and Ap level for both the loaded and non-loaded points. In condition 2-P-LAT, the values of fringe order on each implant at the implant-abutment connection and at the implant apex are presented in Table I. Figure 7 demonstrates the value of fringe order, under 2-point right side load, from left to right, right second premolar, right canine, left canine, left second premolar. For the internal connection (Fig. 7A), the value of fringe order at the I/A level was 1.00 (right premolar), 0.5 (right canine) when loaded, and 0.25 on non-loaded points (left canine and left premolar). The fringe order at the Ap level was 1.00 for both the loaded and non-loaded points. For the external connection (Fig. 7B), the fringe order values were 1.00 at the I/A and Ap levels of both the loaded and nonloaded points.
DISCUSSION In the present study, the photoelastic model consisted of a complete-arch prosthesis supported by implants with either external connections or internal connections. The rationale for evaluating stress at the implant-abutment connection was to determine whether different connection designs had any effect on stresses at the marginal bone level. Loading condition 4-P represented full-arch force acting on the prosthesis. When loads were applied uniformly on the cross-arch prosthesis, the stresses generated at the implant-abutment connection and at the apical area of the implant were not different for either connection. This finding was similar to an in vitro study that compared force transmissions of several conical implant-abutment connec-
tion designs at the implant interface in a photoelastic block.29 The authors concluded that under the same loading conditions, all stresses had similar stress characteristics.29 Thus, in the present study under a uniformly distributed load on the completearch prosthesis, the stresses generated around the implant-abutment connection of both the internal and external connection were similar. In loading condition 2-P-ANT, there was an anterior force acting on the complete-arch prosthesis. This condition aimed to simulate the effect of masticating or incising in the anterior region of the prosthesis, or the effect of hyperocclusion on the anterior teeth as a result of wear of the posterior teeth. The stress generated at the implant connection at the loading point of the internal connection design was less when compared with the external connection design. These differences appear to be caused by the change in the load transfer mechanism related to the different abutment connection types, based on the fringe order values observed in condition 2. In load condition 2-P-LAT, representing unilateral force acting on the complete-arch prosthesis, this condition aimed to simulate the effect of unilateral mastication. Parafunctional habits were found in patients with fractured implants.26 In this study, 6 of 8 failed implants were supporting posterior prostheses. 26 A clinical study by Wei27 concluded that the majority of failures occurred in implantsupported restorations occluding with complete dentures and having poor occlusion. The stresses generated at the implant connection at the non-loading point of the internal connection design were less than those of the external connection design. Kraisat et al32 compared the effect of joint design on the fatigue strength and failure mode of 2 implant systems, the Brånemarkstyle implant, which has an external hexagonal connection, and the ITI implant, which has an internal con-
The Journal of Prosthetic Dentistry
nection. The authors reported that the internal abutment connection showed fatigue resistance that was superior to the external connection. Another in vitro study by Mollersten et al30 also favored the internal abutment connection. The authors reported that the external abutment connection was less resistant to bending moments. When compared with previous in vitro studies, the results of the present study showed less stress generated by the internal-implant abutment connection. However, more prospective clinical studies are suggested to evaluate the stress distribution of these 2 types of implant-abutment connections. This investigation evaluated the effect of axial loading only, and only 2 models were used, 1 for the external connection and 1 for the internal connection. Fringe orders were used to quantitatively evaluate the stresses generated for each of the models for the 2 connections, and statistical analysis of the results was not conducted. This type of study design is common among photoelastic analyses, whereby there is 1 model per condition and no statistical analysis of the fringe order values.33-37 Lack of oblique loading is a limitation of this study. The direction of loading may result in different stresses generated. Thus, further investigations could be beneficial in determining the stress transfer under angled loading at the implant-abutment connection.
CONCLUSION This study was conducted to investigate the stresses generated by an external-implant abutment connection and an internal-implant abutment connection. The fringe orders in the photoelastic model at the implant-abutment connection and at the apical area of the implants were recorded. Within the limitations of this study, the following conclusions were drawn: 1. The loading point caused direct relative stress distribution at the implant-abutment connection level.
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October 2011 2. There were more stresses generated at the implant-abutment connection level by the external-implant abutment connection than by the internal-implant abutment connection, when the prosthesis was loaded anteriorly and unilaterally. 3. There were no differences in stresses generated at the apical level of the implants at both the loading and non-loading points.
REFERENCES 1. Esposito M, Hirsch JM, Lekholm U, Thomsen P. Biological factors contributing to failures of osseointegrated oral implants. (II). Etiopathogenesis. Eur J Oral Sci 1998;106:721-64. 2. Hobkirk JA, Havthoulas TK. The influence of mandibular deformation, implant numbers, and loading position on detected forces in abutments supporting implant superstructures. J Prosthet Dent 1998;80:169-74. 3. Clelland NL, Lee JK, Bimbenet OC, Brantley WA. A three-dimensional finite element stress analysis of angled abutments for an implant placed in the anterior maxilla. J Prosthodont 1995;4:95-100. 4. Holmgren EP, Seckinger RJ, Kilgren LM, Mante F. Evaluating parameters of osseointegrated dental implants using finite element analysis--a two dimensional comparative study examining the effects of implant diameter, implant shape, and load direction. J Oral Implantol 1998;24:80-8. 5. Assif D, Marshak B, Horowitz A. Analysis of load transfer and stress distribution by an implant-supported fixed partial denture. J Prosthet Dent 1996;75:285-91. 6. Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants 1993;8:19-31. 7. Lindhe J, Berglundh T, Ericsson I, Liljenberg B, Marinello CP. Experimental breakdown of peri-implant and periodontal tissues. A study in the beagle dog. Clin Oral Implants Res 1992;3:9-16. 8. Miyata T, Kobayashi Y, Araki H, Ohto T, Shin K. The influence of controlled occlusal overload on peri-implant tissue. Part3: A histologic study in monkeys. Int J Oral Maxillofac Implants 2000;15:425-31. 9. Jansen VK, Conrads G, Richter EJ. Microbial leakage and marginal fit of the implantabutment interface. Int J Oral Maxillofac Implants 1997;12:527-40. 10.Steinebrunner L, Wolfart S, Bossmann K, Kern M. In vitro evaluation of bacterial leakage along the implant-abutment interface of different implant systems. Int J Oral Maxillofac Implants 2005;20:875-81.
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11.Geramy A, Morgano SM. Finite element analysis of three designs of an implantsupported molar crown. J Prosthet Dent 2004;92:434-40. 12.Piermatti J, Yousef H, Luke A, Mahevich R, Weiner S. An in vitro analysis of implant screw torque loss with external hex and internal connection implant systems. Implant Dent 2006;15:427-35. 13.Yousef H, Luke A, Ricci J, Weiner S. Analysis of changes in implant screws subject to occlusal loading: a preliminary analysis. Implant Dent 2005;14:378-82. 14.Jemt T, Laney WR, Harris D, Henry PJ, Krogh PH, Polizzi G, Zarb GA, Herrmann I. Osseointegrated implants for single tooth replacement: a 1-year report from a multicenter prospective study. Int J Oral Maxillofac Implants 1991;6:29-36. 15.Becker W, Becker BE. Replacement of maxillary and mandibular molars with single endosseous implant restorations: a retrospective study. J Prosthet Dent 1995;74:51-5. 16.McGlumphy EA, Mendel DA, Holloway JA. Implant screw mechanics. Dent Clin North Am 1998;42:71-89. 17.Levine RA, Clem DS 3rd, Wilson TG Jr, Higginbottom F, Solnit G. A multicenter retrospective analysis of the ITI implant system used for single-tooth replacements: results of loading for 2 or more years. Int J Oral Maxillofac Implants 1999;14:516-20. 18.Sakaguchi RL, Borgersen SE. Linear contact analysis of preload in dental implant screws. Int J Oral Maxillofac Implants 1995;10:295-302. 19.MaedaY, Satoh T, Sogo M. In vitro differences of stress concentrations for internal and external hex implant-abutment connections: a short communication. J Oral Rehabil 2006;33:75-8. 20.Wang RF, Kang B, Lang LA, Razzoog ME. The dynamic natures of implant loading. J Prosthet Dent 2009;101:359-71. 21.Theoharidou A, Petridis HP, Tzannas K, Garefis P. Abutment screw loosening in single-implant restorations: a systematic review. Int J Oral Maxillofac Implants 2008;23:681-90. 22.Kreissl ME, Gerds T, Muche R, Heydecke G, Strub JR. Technical complications of implant-supported fixed partial dentures in partially edentulous cases after an average observation period of 5 years. Clin Oral Implants Res 2007;18:720-6. 23.Norton MR. Multiple single-tooth implant restorations in the posterior jaws: maintenance of marginal bone levels with reference to the implant-abutment microgap. Int J Oral Maxillofac Implants 2006;21:777-84. 24.Vogel RE, Davliakos JP. Spline implant prospective multicenter study; interim report on prosthetic screw stability in partially edentulous patients. J Esthet Rest Dent 2002;14:225-37.
25.Mangano C, Bartolucci EG. Single tooth replacement by Morse taper connection implants: a retrospective study of 80 implants. Int J Oral Maxillofac Implants 2001;16:675-80. 26.Balshi TJ. An analysis and management of fractured implants: a clinical report. Int J Oral Maxillofac Implants 1996;11:660-6. 27.Wie H. Registration of localization, occlusion and occluding materials for failing screw joints in the Brånemark implant system. Clin Oral Implants Res 1995;6:47-53. 28.Rangert B, Krogh PH, Langer B, Van Roekel N. Bending overload and implant fracture: a retrospective clinical analysis. Int J Oral Maxillofac Implants 1995;10:326-34. 29.Norton MR. An in vitro evaluation of the strength of an internal conical interface compared to a butt joint interface in implant design. Clin Oral Implants Res 1997;8:290-8. 30.Möllersten L, Lockowandt P, Lindén LA. Comparison of strength and failure mode of seven implant systems: an in vitro test. J Prosthet Dent 1997;78:582-91. 31.Cehreli M, Duyck J, De Cooman M, Puers R, Naert I. Implant design and interface force transfer. A photoelastic and straingauge analysis. Clin Oral Implants Res 2004;15:249-57. 32.Khraisat A, Stegaroiu R, Nomura S, Miyakawa O. Fatigue resistance of two implant/abutment joint systems. J Prosthet Dent 2002;88:604-10. 33.Ochiai KT, Williams BH, Hojo S, Nishimura R, Caputo AA. Photoelastic analysis of the effect of palatal support on various implant-supported overdenture designs. J Prosthet Dent 2004;91:421-7. 34.Celik G, Uludag B. Photoelastic stress analysis of various retention mechanisms on 3-implant-retained mandibular overdentures. J Prosthet Dent 2007;97:229-35. 35.White SN, Caputo AA, Anderkvist T. Effect of cantilever length on stress transfer by implant-supported prostheses. J Prosthet Dent 1994;71:493-9. 36.Sadowsky SJ, Caputo AA. Stress transfer of four mandibular implant overdenture cantilever designs. J Prosthet Dent 2004;92:328-36. 37.Akça K, Fanuscu MI, Caputo AA. Effect of compromised cortical bone on implant load distribution. J Prosthodont 2008;17:616-20. 38.Mahler DB, Peyton FA. Photoelasticity as a research technique for analyzing stresses in dental structures. J Dent Res 1955;34:831-8. Corresponding author: Dr Steven M. Morgano Boston University Goldman School of Dental Medicine 100 East Newton Street, Rm G219 Boston, MA 02118 Fax: 617-638-5429 E-mail: [email protected] Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications
The internal-implant abutment connection could potentially reduce stresses within the connection when off-center loads are applied. The success of implant treatment depends partly on the biomechanical load on the dental implants.1 Functional loads applied on the prosthesis are transferred through the implants to the surrounding bone. Factors such as location and angulation of the implant, as well as geometry of the implant-abutment connection have been reported to influence stress and strain around implants.2-5 The increase in the load on an implant is directly pro-
portional to stress and strain developed at the bone-implant interface,6 and some studies have reported a relationship between occlusal overload and loss of osseointegration.7,8 The microgap between the abutment and the implant has been studied.9 Steinebrunner et al10 evaluated microbial leakage of different designs of the implant-abutment connections in an in vitro model. The authors reported that bacterial penetration occurred at
the implant-abutment interface during mastication. This microgap may allow micromovement between the implantabutment connection during clinical function, and this micromovement could result in localized inflammation with subsequent crestal bone loss.11 The stability of an implant-retained prosthesis depends on the integrity of the screw connection.12 Yousef et al13 reported that off-axis loading may result in screw and abut-
Instructor of Prosthodontics, Director Prosthodontic Residency Training Program, Faculty of Dentistry, Mahidol University. Professor and Director, Division of Postdoctoral Prosthodontics, Department of Restorative Sciences and Biomaterials, Goldman School of Dental Medicine.
a
b
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October 2011 ment deformation with subsequent screw loosening. Implants featuring a short external hexagon at the prospective connection with the abutment seemed to be especially prone to screw loosening. Jemt et al14 and Becker and Becker15 reported that a high incidence of screw loosening (up to 40%) was found for the external abutment connection. Screw loosening seemed to occur most often with single-tooth implant-supported restorations16 and Levine et al17 reported a far lower rate of abutment screw loosening (3.6 to 5.3%) with internal abutment connections. The external implant-abutment connection and the internal implant-abutment connection use different mechanical principles of function. For the external connection, the axial preload of the abutment screw is the determining factor for stability of the connection.18 This screw secures the abutment under horizontal loading. The optimal preload corresponds theoretically to the yield point of the screw. For the internal connection, internal form fit and friction are the basic mechanical principles. Lateral loading is resisted by the frictional interface.19 In a finite element analysis (FEA) model, Wang et al20 reported elongation of the abutment screw during the application of a torque moment, which increased preload in the implant abutment complex. The author’s of a systematic review
of clinical studies on abutment screwloosening in single-implant restorations reported that screw loosening was rare so long as proper procedures were followed when making the implant-abutment connections.21 Other clinical studies22-25 reported complications regarding implant-abutment connections. Balshi26 and Wei27 reported loosening or fracture of prosthetic screws related to parafunctional habits or an improperly designed or adjusted occlusion. Although research has concentrated on the influence of stresses developed around implants,28-32 the authors identified no research that evaluates the stresses developed by complete-arch restorations comparing external and internal implant-abutment connections. The photoelastic model has previously been used to evaluated stress about implants with various types of prostheses and variables.33-37 This technique is useful for evaluating the stresses responsible for failure of a structure, especially a structure with irregular form. The model is examined in a field of polarized light with loads applied. The photoelastic model, under carefully controlled conditions, can indicate internal stresses.38 Nevertheless, the photoelastic model has limitations when predicting the response of biologic systems to applied loads, similar to the limitations of other modeling systems, such as FEA, and strain gauge studies.20
1 Artificial teeth arranged on duplicate model of photoelastic model.
Asvanund and Morgano
The purpose of this study was to compare, through photoelastic analysis, the load transfer characteristics of complete-arch restorations supported by 4 implants, with external and internal implant-abutment connections.
MATERIAL AND METHODS Two photoelastic models (PL-4M; Vishay Micro-Measurements, Raleigh, NC) with a modulus of elasticity of 3.1 GPa were fabricated to replicate an edentulous human mandible. The photoelastic model was then duplicated with a silicone mold (Speedex; Coltène/ Whaledent AG, Alstätten, Switzerland) and poured with ADA Type IV dental stone (Vel-Mix; Kerr, Romulus, Mich). Artificial teeth (CosmoHXL; Dentsply Intl York, Pa) were arranged on the duplicate model to the mandibular first molar (Fig. 1). A 3-mm thick polyethylene sheet (Copyplast; Scheu-Dental GmBH, Iserlohn, Germany) was vacuum adapted (Biostar; Scheu-Dental GmBH) over the tooth arrangement. Four holes were drilled with a 2.0-mm twist drill (NobelBiocare USA, Yorba Linda, Calif ) through the polyethylene sheet at the canine and second premolar regions, bilaterally (Fig. 2). This sheet served as a positioning template for drilling the simulated osteotomies in the photoelastic models. The photoelastic models were positioned within the polyethylene sheet, and twist drills with 2.0 mm,
2 Vacuum-formed polyethylene sheet with holes at canines and second premolars bilaterally to be used as drill guides on photoelastic model.
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Volume 106 Issue 4
3 Four implants (4.3 x 13 mm) placed in photoelastic model.
3.5 mm, and then 4.3 mm diameters were used sequentially to drill at 800 rpm to a depth of 13 mm. A screw tap (NobelBiocare USA) was used at 50 rpm under water coolant. Four implants (4.3 × 13 mm) (Replace Select Internal-Interface Tapered Implant; NobelBiocare) were placed in each prepared hole in 1 photoelastic model, representing the internal connection model. Another 4 implants (4.3 × 13 mm) (Replace Select External-Interface Tapered Implant; NobelBiocare) were placed in each prepared hole in the other photoelastic model, representing the external connection model (Fig. 3). Patterns for the representative mandibular prosthetic frameworks were prepared with pink baseplate wax (Cavex Set Up Wax; Cavex Holland BV, Haarlem, The Netherlands), shaped to conform to the photoelastic block. The gold cylinders (PME Coping, Gold; NobelBiocare USA) were connected to the wax patterns. Retentive beads with a diameter of 0.6 mm. (Renfert GmbH; Hilzingen, Germany) were placed on each wax pattern, and the wax pattern was sprued (Sprues;Yeti Dentalproduct; Engen, Germany). Then the pattern was invested with phosphate bonded investment (Deguvest-Impact; Degussa-Hule, Hanau, Germany) at a ratio of powder to distilled water of 100 g:40 ml. The casting ring was placed in an electric burn-out
4 The processed prosthesis connected to implant replicas prior to seating on photoelastic model.
furnace (Midtherm TH; Bego, Germany). The temperature was raised from room temperature to 300°C within 1 hour, increased to 700°C in another hour, and held for 45 minutes. The framework was cast with silver-palladium casting alloy (Palliag M; Degussa-Hule) with a vacuum pressurecasting machine (NautilusT; Bego). The castings were then airborne-particle abraded with 50-µm aluminum oxide (Topstar 2; Bego) at 0.2 MPa pressure. Each cast framework was connected to the implants by guide pins (NobelBiocare) and the casting was evaluated for complete, passive seating. The artificial teeth (Cosmo HXL; Dentsply Intl) were arranged on the framework guided by the vacuum-formed polyethylene sheet. The canines and second premolars were drilled to produce channels that provided screw access. The prosthesis was processed with heat polymerized acrylic resin (Rodex; Rodont, Srl Milan, Italy) (Fig. 4). For the internal connection model, the processed prosthesis was connected to the abutments (PME Transmucosal Abutment 3 mmH; NobelBiocare) with 35 N-cm of torque. For the external connection model, the second processed prosthesis was connected to the abutments (PME Abutment 3.25 mmH; NobelBiocare) also with 35 N-cm of torque. Each screw-accesses channel for both prostheses was filled with a cotton
The Journal of Prosthetic Dentistry
pellet (2 mm), and composite resin (Clearfil; Kuraray America, New York, NY). The composite resin was light polymerized at 600 mW/cm2 (Demetron LC; Kerr Corporation, Orange, Calif ) for 40 seconds. A universal testing machine (Instron Model 5580; Norwood, Mass) with a custom-made puncture probe was used to apply a vertical load of 250 N. The crosshead speed was 0.125 mm/min on the prostheses. This relatively slow speed was used for the purpose of making photographs of the stresses generated in the photoelastic block. There were 3 loading conditions: (1) 4-point load with force applied to the canines and the premolars (4-P); (2) 2-point anterior load with force applied to the left and right canines (2-P-ANT), and (3) 2-point lateral load with force applied to the right canine and premolar (2-P-LAT). The observation of stress distribution was demonstrated by the formation of fringe orders38 at the implant-abutment connection and apical to the implant level. Color photographs were made upon reaching the desired loads. The distribution and stress magnitudes were shown as isochromatic fringe orders. The interpretation of fringe order values are: yellow, order 0.25; yellow-red, order 0.5; yellow-redblue, 1.0; yellow-red-blue-yellow-redgreen, 2.0; yellow-red-blue-yellow-redgreen-yellow-red-green, 3.0 respectively.
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October 2011 RESULTS In condition 4-P, the values of fringe order on each implant at the implant-abutment connection (I/A) and at the apical area of implant (Ap) are presented in Table I. Figure 5 dem-
onstrates the values of fringe order, under 4-point load, from left to right, right second premolar, right canine, left canine, left second premolar. In condition 2-P-ANT, the values of fringe order on each implant at the implant-abutment connection and
at the apical area of the implant are presented in Table I. Figure 6 demonstrates the values of fringe order, under 2-point anterior load, from left to right, right second premolar, right canine, left canine, left second premolar. For the internal connection (Fig.
Table I. Fringe order of loading condition 4-P, 2-P-ANT and 2-P-LAT at I/A level and Ap level of internal connection and external connection.
Condition
Internal Connection 4-P 2-P-ANT 2-P-LAT I/A Ap I/A Ap I/A Ap
External Connection 4-P 2-P-ANT 2-P-LAT I/A Ap I/A Ap I/A Ap
Lcanine
0.75
1.00 0.50 1.00 0.25
1.00
1.00
1.00 1.00 1.00 1.00
1.00
Lpremolar
1.00
1.00 0.25 0.25 0.25
0.25
1.00
1.00 1.00 1.00 1.00
1.00
Rcanine
1.00
1.00 0.50 1.00 0.50
1.00
1.00
1.00 1.00 1.00 1.00
1.00
Rpremolar
1.00
1.00 0.25 1.00 1.00
1.00
1.00
1.00 1.00 1.00 1.00
1.00
A
A
B
B
5 Four-point load: from left to right, right second premolar, right canine, left canine, left second premolar. A. Fringe orders at I/A and Ap of internal connection model. B. Fringe orders at the I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
6 Two-point anterior load: from left to right, right second premolar, right canine, left canine, left second premolar. A, Fringe orders at I/A and Ap of internal connection model. B, Fringe orders at the I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
A
B 7 Two-point right lateral load: from left to right, right second premolar, right canine, left canine, left second premolar. A, Fringe orders at I/A and Ap of internal connection model. B, Fringe orders at I/A and Ap of external connection model. Yellow=0.25, yellow-red=0.5, yellow-red-blue=1.0
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Volume 106 Issue 4 6A), the value of fringe order at the I/A level was 0.5 on the canines when loaded, and 0.25 on the non-loaded points (premolars). The fringe order at the Ap level was 1.00 for both the loaded and non-loaded points. For the external connection (Fig. 6B), the fringe order values were 1.00 at the I/A level and Ap level for both the loaded and non-loaded points. In condition 2-P-LAT, the values of fringe order on each implant at the implant-abutment connection and at the implant apex are presented in Table I. Figure 7 demonstrates the value of fringe order, under 2-point right side load, from left to right, right second premolar, right canine, left canine, left second premolar. For the internal connection (Fig. 7A), the value of fringe order at the I/A level was 1.00 (right premolar), 0.5 (right canine) when loaded, and 0.25 on non-loaded points (left canine and left premolar). The fringe order at the Ap level was 1.00 for both the loaded and non-loaded points. For the external connection (Fig. 7B), the fringe order values were 1.00 at the I/A and Ap levels of both the loaded and nonloaded points.
DISCUSSION In the present study, the photoelastic model consisted of a complete-arch prosthesis supported by implants with either external connections or internal connections. The rationale for evaluating stress at the implant-abutment connection was to determine whether different connection designs had any effect on stresses at the marginal bone level. Loading condition 4-P represented full-arch force acting on the prosthesis. When loads were applied uniformly on the cross-arch prosthesis, the stresses generated at the implant-abutment connection and at the apical area of the implant were not different for either connection. This finding was similar to an in vitro study that compared force transmissions of several conical implant-abutment connec-
tion designs at the implant interface in a photoelastic block.29 The authors concluded that under the same loading conditions, all stresses had similar stress characteristics.29 Thus, in the present study under a uniformly distributed load on the completearch prosthesis, the stresses generated around the implant-abutment connection of both the internal and external connection were similar. In loading condition 2-P-ANT, there was an anterior force acting on the complete-arch prosthesis. This condition aimed to simulate the effect of masticating or incising in the anterior region of the prosthesis, or the effect of hyperocclusion on the anterior teeth as a result of wear of the posterior teeth. The stress generated at the implant connection at the loading point of the internal connection design was less when compared with the external connection design. These differences appear to be caused by the change in the load transfer mechanism related to the different abutment connection types, based on the fringe order values observed in condition 2. In load condition 2-P-LAT, representing unilateral force acting on the complete-arch prosthesis, this condition aimed to simulate the effect of unilateral mastication. Parafunctional habits were found in patients with fractured implants.26 In this study, 6 of 8 failed implants were supporting posterior prostheses. 26 A clinical study by Wei27 concluded that the majority of failures occurred in implantsupported restorations occluding with complete dentures and having poor occlusion. The stresses generated at the implant connection at the non-loading point of the internal connection design were less than those of the external connection design. Kraisat et al32 compared the effect of joint design on the fatigue strength and failure mode of 2 implant systems, the Brånemarkstyle implant, which has an external hexagonal connection, and the ITI implant, which has an internal con-
The Journal of Prosthetic Dentistry
nection. The authors reported that the internal abutment connection showed fatigue resistance that was superior to the external connection. Another in vitro study by Mollersten et al30 also favored the internal abutment connection. The authors reported that the external abutment connection was less resistant to bending moments. When compared with previous in vitro studies, the results of the present study showed less stress generated by the internal-implant abutment connection. However, more prospective clinical studies are suggested to evaluate the stress distribution of these 2 types of implant-abutment connections. This investigation evaluated the effect of axial loading only, and only 2 models were used, 1 for the external connection and 1 for the internal connection. Fringe orders were used to quantitatively evaluate the stresses generated for each of the models for the 2 connections, and statistical analysis of the results was not conducted. This type of study design is common among photoelastic analyses, whereby there is 1 model per condition and no statistical analysis of the fringe order values.33-37 Lack of oblique loading is a limitation of this study. The direction of loading may result in different stresses generated. Thus, further investigations could be beneficial in determining the stress transfer under angled loading at the implant-abutment connection.
CONCLUSION This study was conducted to investigate the stresses generated by an external-implant abutment connection and an internal-implant abutment connection. The fringe orders in the photoelastic model at the implant-abutment connection and at the apical area of the implants were recorded. Within the limitations of this study, the following conclusions were drawn: 1. The loading point caused direct relative stress distribution at the implant-abutment connection level.
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October 2011 2. There were more stresses generated at the implant-abutment connection level by the external-implant abutment connection than by the internal-implant abutment connection, when the prosthesis was loaded anteriorly and unilaterally. 3. There were no differences in stresses generated at the apical level of the implants at both the loading and non-loading points.
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