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Effects of Screw- and Cement-Retained Implant-Supported Prostheses on Bone: A Nonlinear 3-D Finite Element Analysis Guilherme Carvalho Silva, DDS, MS, PhD,* Guilherme Martins de Andrade, MechEng,† Rodrigo Carvalho Pinto Coelho, DDS, MS,‡ Tulimar Machado Cornacchia, DDS, MS, PhD(Eng),§ Cláudia Silami de Magalhães, DDS, MS, PhD,§ and Allyson Nogueira Moreira, DDS, PhD§
ral rehabilitation with implantsupported prostheses is an accepted treatment modality.1 The original protocol of the pioneering studies of Brånemark et al1 was based exclusively on screw-retained prostheses, and numerous studies have confirmed the success of the application of this concept, particularly in fully edentulous patients.2 However, with the development of new implant systems and new rehabilitation techniques, cement-retained prostheses have become a treatment option, particularly in cases of single and fixed partial prostheses. Currently, cement-retained prostheses are frequently used, with a high level of success.3–6 The advantages and disadvantages of screw- and cement-retained prostheses have particularly received attention from researchers in aspects such as passive fit, quality of marginal adaptation, retention, occlusal factors,
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*Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. †Graduate, Department of Mechanical Engineering, School of Engineering, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. ‡PhD Student, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. §Associate Professor, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.
Reprint requests and correspondence to: Guilherme Carvalho Silva, DDS, MS, PhD, Avenida Afonso Pena 4334, Belo Horizonte, MG 30130-009, Brazil, Phone: +55 31 32259335, E-mail: [email protected] ISSN 1056-6163/15/00000-001 Implant Dentistry Volume 0 Number 0 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/ID.0000000000000278
Purpose: To compare the stresses and displacements on perimplant bone generated by screw- and cement-retained prostheses using the finite element method. Materials and Methods: Two models were constructed: partial fixed implant-supported prostheses with three elements retained by screws (SFP) or cement (CFP). Vertical and oblique loads of 100 N were applied on the models. Bone was analyzed by the principal stresses s1 and s3. The displacement between the implant and the bone was identified by the penetration and gap. Results: Results showed a similar pattern in the distribution of the principal stresses between both prostheses. Under the s1 stresses, the SFP showed similar values in the
bone compared with the CFP. The analysis of the s3 showed stress peaks 28% higher in the SFP, considering vertical and oblique loads. Displacement analysis showed a similar pattern and similar values between the prostheses for penetration and gap under both loads. Conclusions: There were no important differences in the s1 analysis and the displacement between the SFP and CFP. The differences in marginal bone level reported between SFP and CFP in some clinical studies may not be related to a mechanical factor. (Implant Dent 2015;0:1–8) Key Words: biomechanics, finite element method, implant-supported dental prosthesis, marginal bone loss
esthetics, reversibility, ease of manufacture, costs, maintenance of bone level, and gingival health and survival.4,6–8 The biomechanical behavior of screw-retained and cement-retained prostheses regarding the generation of stress on tissues and prosthetic abutments has been compared superficially using distinct methods.9–17 One of the most important aspects analyzed in these studies is the effect of the retaining method (screw or cement) on the periimplant bone, as bone preservation is essential to implant longevity.
Dynamic maintenance and remodeling of the bone structure is dependent on mechanical stimuli that cause stress and strain on living tissue.18–20 Peak stress on supporting bone may cause marginal bone loss and even complete implant failure.21–24 For that reason, an important mechanical component seems to be related to the preservation of periimplant bone. Several aspects may influence the intensity and patterns of stresses and displacements in the bone-implant interface, such as
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Table 2. Friction Coefficient of the Interfaces Between Different Materials Friction Coefficient
Interface
Fig. 1. Three-dimensional models of the prostheses. A, Screw-retained. B, Cement-retained.
Titanium abutment and titanium screw Gold abutment and titanium screw Titanium abutment and titanium implant Gold abutment and titanium implant Titanium screw and titanium implant Titanium implant and bone Zinc phosphate cement and titanium abutment Zinc phosphate cement and gold
0.16 0.20 0.16 0.20 0.16 0.30 0.20 0.20
Zinc phosphate cement interfaces30; other interfaces.31
Fig. 2. Coronal section of the meshes. A, Screw-retained: 1,097,527 elements. B, Cementretained: 1,146,675 elements.
the retention method of the implantsupported prosthesis.11 Consequently, a biomechanical analysis of the mechanism of the stresses and displacements generation is essential to predict the success of implant restorations by identifying possible risks of marginal bone loss. Therefore, the aim of this study was to compare the stresses and displacements in the bone-implant interfaces of implantsupported screw- or cement-retained prostheses using the finite element method in a three-dimensional (3D) nonlinear analysis. Table 1. Mechanical Properties of the Materials
Material Titanium Gold Porcelain Zinc phosphate cement Composite resin 28
Young’s Modulus (MPa)
Poisson Ratio
117,000 100,000 68,900 17,000
0.30 0.30 0.28 0.35
7000
0.20
29
Zinc phosphate cement ; other materials.
MATERIALS
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Two 3D models were developed: screw- or cement-retained porcelain fused to metal fixed partial prostheses with three elements supported by implants in the area of the lower 2nd premolar and 2nd molar. Initially, a section of a human partially edentulous mandible was modeled. Cone-beam computerized tomography (CT) (i-CAT, Imaging Sciences International, Hatfield, PA) of the mandible of a partially edentulous patient was obtained from the files of patients treated by the researcher. The images of 3D CT reconstructions in STL format (3D Systems, Rock Hill, SC) were transferred to 3D computer-aided design software (SolidWorks, Dassault Systèmes SolidWorks Corporation, Santa Monica, CA) for editing and refinement. The edentulous region between the 2nd premolar and 2nd molar was sectioned, and the mandibular canal was eliminated to reduce the volume and the complexity of the computational model. The cortical portion of the bone was lineated in the form of a 1.50-mm thick homogeneous
layer simulating a type II bone,25 and all the contours of the mandible segment were rounded and smoothed. The following components, which were obtained directly from the manufacturer, were modeled, as previously described26: the dental implant (NobelReplace 4.3 3 13 mm; Nobel Biocare, Gothenburg, Sweden), UCLA abutment for screw-retained multiple-unit prostheses (GoldAdapt Non-Engaging, Nobel Biocare, Gothenburg, Sweden), customized engaging titanium abutment for cement-retained prostheses (Esthetic Abutment, Nobel Biocare, Gothenburg, Sweden), and titanium abutment screw (29,475 TorqTite, Nobel Biocare, Gothenburg, Sweden). For modeling of the outer contour of the prostheses, the crown of the 2nd Table 3. Bone Properties Properties
Cortical Bone
Cancellous Bone
Ey Ex Ez Gyx Gyz Gxz nyx nyz nxz
12,500 26,600 17,900 4500 7100 5300 0.18 0.28 0.31
210 1148 1148 68 68 434 0.055 0.055 0.322
Ei, Young’s Modulus (MPa); Gi, Shear modulus (MPa); n, Poisson ratio. x axis is anteroposterior; y axis is apicocoronal; z axis is lateromedial. Cortical bone32; cancellous bone.33
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Fig. 3. Occlusal view of s1 stresses on bone under vertical load. A, Screw-retained. B, Cement-retained.
Fig. 4. Occlusal view of s3 stresses on bone under vertical load. A, Screw-retained. B, Cement-retained.
Fig. 5. Occlusal view of s1 stresses on bone under oblique load. A, Screw-retained. B, Cement-retained.
Fig. 6. Occlusal view of s3 stresses on bone under oblique load. A, Screw-retained. B, Cement-retained.
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molar was modeled and posteriorly replicated three times. The crowns were joined in the proximal region by cylindrical connectors. After completion of the external design of the prostheses, 1.50 mm was determined as the outer layer of the entire volume that corresponded to the ceramic layer. For the cement-retained prosthesis, the entire area just below the ceramic layer before contact with the abutment represented the gold infrastructure. For the screwretained prosthesis, gold was used for the entire inner area under the ceramic layer because it would fuse to the abutments. The screw access was obliterated with composite resin. For the cement-retained prosthesis, a homogeneous layer of 25 mm thickness representing zinc phosphate cement was modeled5,27 between the gold infrastructure and the abutments (Fig. 1, A and B). The mesh was generated with computer-aided engineering software (Ansys 14, ANSYS Inc., Canonsburg, PA) (Fig. 2, A and B). A material with mechanical properties was assigned for each volume28,29 (Table 1) and a coefficient of friction for each material30,31 (Table 2). The bone was considered anisotropic32,33 (Table 3), linear elastic, and homogeneous, and the other materials were defined as isotropic, linear elastic, and homogeneous. The contact areas among the different sections of the models were defined using nonlinear contact elements. The behavior of the contact surface was different according to the interface of the materials. Between the implants and bone, a “rough” contact was selected, which allowed the formation of microspaces without sliding between the elements. An identical contact was selected in the area between the abutments and cement and between the cement and metallic infrastructure in the cement-retained prosthesis because there is no chemical bonding and only mechanical imbrication between the cement and the metal. Between the screws and implants, implants and abutments, and abutments and screws, a “standard” contact was selected, which allowed the formation of microspaces and a small degree of sliding between the surfaces, a common
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Table 4. Maximum Principal s1 Stresses Under Vertical and Oblique Loadings Type of Loading
Screw-Retained: s1 Stresses (MPa)
Cement-Retained: s1 Stresses (MPa)
Difference (%)
Vertical Oblique Mean
33 122 77
28 130 79
17 6 2
gap were identified. A smaller displacement at the interface corresponds with a lower risk of failure. The qualitative analysis was made by comparing the pattern of the distribution of the stresses and displacements. The quantitative analysis was given for the difference in the percentage of the maximum values found for each criterion used.
Table 5. Maximum Principal s3 Stresses Under Vertical and Oblique Loadings Type of Loading
Screw-Retained: s3 Stresses (MPa)
Cement-Retained: s3 Stresses (MPa)
Difference (%)
Vertical Oblique Mean
68 165 116
64 116 90
6 42 28
Table 6. Maximum Penetration Between Implant and Bone Type of Loading
Screw-Retained: Penetration (mm)
Cement-Retained: Penetration (mm)
Difference (%)
Vertical Oblique
0.004 0.009
0.005 0.010
25 11
Table 7. Maximum Gap Between Implant and Bone Type of Loading
Screw-Retained: Gap (mm)
Cement-Retained: Gap (mm)
Difference (%)
Vertical Oblique
0.123 0.116
0.122 0.106
0.8 9
occurrence between metal surfaces. The remaining volumes were considered bonded because they showed characteristics of a cohesive union: cortical and cancellous bone, ceramic and gold infrastructure, and resin and ceramic. The models were constricted in the mesiodistal (anteroposterior) and buccolingual (lateral to medial) directions. The load simulations were identical for the models and were performed in three steps. First, before the imposition loads on the models, the abutment screws received a torque of 35 N$cm, a value recommended by the manufacturer, to represent the settling of the prostheses. For the torque simulation, contact elements were selected on the six inner sides of the hexagonal screw head. Next, a master node was determined in the spatial center of the screw head. A moment of 35 N$cm was applied clockwise on the master node in a rotation of all the nodes of the contact elements on the inner sides of the
screw head, resulting in screw tightening. Finally, the rotation moment was finalized, indicating the end of the tightening. With the screw pretensioned, vertical and oblique (45° in the buccolingual direction), 100 N loads were applied to the occlusal areas of the teeth in each prosthesis at different times. The periimplant bone and the implant were plotted to observe the stresses and displacements externally and internally. The results of the 2nd premolar were presented because the region of the 2nd molar showed almost identical results. The principal stresses s1 and s3 were selected because bone is a fragile structure that distinctly responds to the action of different stresses, thereby exhibiting a greater risk of failure under shear and tensile stresses compared with compressive stresses.34 A lower stress corresponds with a lower risk of failure. For the analysis of the displacement between the contact surfaces, the penetration and
RESULTS The tensile stresses, which were prevalent in the analysis of s1, were plotted with a positive value, whereas the compressive stresses presented negative values. In the analysis of s3, the compressive stresses, which were predominant, were plotted with a negative value, whereas the tensile stresses exhibited positive values. Qualitatively, the results showed a similar stress pattern between the screw- and the cement-retained prostheses. In the vertical load in both models, there was a dispersion of stresses around the entire periimplant cervical area, with the highest stresses concentrated on the buccal surface, particularly when analyzing the compressive stresses (Figs. 3 and 4). In the oblique loading, the analysis of the s1 stresses showed peaks in the buccal area and areas of concentrated tensile stresses in the distal region (Fig. 5, A and B). The analysis of the s3 stresses showed the concentration of compressive stress in the lingual region in the direction opposite to the applied load. These stresses were located on the top and inner surface of the lingual cortical bone. This result was because of compression from the vertical component of the load and from bending of the structure (Fig. 6, A and B). Regarding the intensity of the s1 stresses, there was no important difference when comparing the peaks between the models. The average peak of the stresses, considering the vertical and oblique loads, was only 2% lower in the screw-retained prosthesis (Table 4). However, the s3 peak stresses were on average 28% higher in the screw-retained prostheses, when considering both loads. There was a greater difference in the oblique loading; the s3 peak stress was 42% higher
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Fig. 7. Implant penetration on bone under vertical load. A, Screw-retained. B, Cementretained.
Fig. 8. Gap between implant and bone under vertical load. A, Screw-retained. B, Cementretained.
Fig. 9. Lateral view of implant penetration on bone under oblique load. Left side of the implant: buccal; right side: lingual. A, Screw-retained. B, Cement-retained.
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in the screw-retained prosthesis (Table 5). In both models, under the two loads, the stresses were always located on the cervical region of the cortical bone, with lower or absent stresses in almost all of the cancellous bone. The displacements between the contact surfaces of the implant and bone were identified and measured by plotting the penetration and gap of the implant relative to the bone. The gaps were plotted with negative values. In both prostheses, under the vertical load, the highest penetration was observed in the apical region of the implant; this parameter was 25% higher in the cement-retained prosthesis, but all of the values were very low (Fig. 7, A and B) (Table 6). In the gap analysis, the pattern was similar between the prostheses, with the largest displacement in the buccal region (Fig. 8, A and B). The difference in the extent of the gap was less than 1% (Table 7). In the oblique loading, the areas of greatest penetration were in the lingual cervical area in both prostheses. The displacement in this region was 11% higher in the cement-retained prosthesis. A greater displacement in the buccal face of the bone, compared with the lingual face, was noted in both prostheses (Fig. 9, A and B). In the gap analysis, the areas of greatest displacement were found in a range in the cervical buccal area in both prostheses, with the maximum gap being 9% higher in the screw-retained prosthesis. In both prostheses, a higher gap in the lingual face of the bone was observed compared with the vestibular face, which was the opposite of the penetration pattern (Fig. 10, A and B).
DISCUSSION
Fig. 10. Gap between implant and bone under oblique load. Lateral view: Left side of the implant: buccal; right side: lingual. A, Screw-retained. B, Cement-retained.
The relevance of biomechanical evaluation of the periimplant bone relates to the fact that excessive loads can induce bone resorption, which may lead to implant failure and the consequent failure of the entire prosthetic rehabilitation.22,23 Furthermore, the periimplant bone loss is also related to marginal gingival recession,35 which generates an esthetic risk, particularly in anterior prostheses. In this study,
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the risk of cervical bone resorption was analyzed, but not the risk of complete fracture of the bone around an osseointegrated implant, which has not been reported in the literature. It is supposed that constant stress and strain above a certain limit can lead to microcracks or microdeformations in the bone structure; if microfractures occur more rapidly than repair in the continuous damage process, bone loss can occur before complete fracture of the bone.18,19 The finite element method was selected for this comparative analysis because it is not invasive or destructive but a computational numeric technique, which allows the analysis of various types of internal or external stresses, strains, and displacements among other applications and in any area of the studied structure. Using this method, it is possible to identify patterns, stresses, and displacements in areas of the prosthetic implant-bone complex that are inaccessible by other methods of biomechanical studies. The finite element method is frequently used in implant dentistry and applied in a wide variety of simulations.29,31,36 However, this research method, as any other, has limitations, particularly when trying to extrapolate the results of this numerical technique for the clinical field. One frequent simplification is related to bone modeling. Bone is a dynamic and complex living tissue, without a pattern, whose characteristics vary in the same individual and among individuals. Therefore, a bone model that represents a generalized situation is practically impossible.34,36 A perfect fit without gaps between the abutment and the implant and between the prosthesis and the abutments adopted in this study was also a simplification because minimum spaces typically occur in the adaptation of these structures. However, the absence of gaps was important for the analysis because the comparison was independent of the quality of fit, as the mismatch itself generates stresses.12,13 The precision of the adaptation is dependent on numerous variables inherent in the process of laboratory work, such as the type of metal alloy used, the welding, technical experience, and impression technique.12,13 All
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factors dependent on the prosthesis manufacturing process were excluded, allowing this study to search specifically for the effect of the retention method (screw or cement). Although these assumptions were made, they are not expected to compromise the results, as identical criteria were used impartially in both models. The collected absolute values of the stresses should not be compared with the yield of the rupture limits of the materials involved in the study. Such care in interpreting the results should be taken in any finite element analysis. Clinical inferences may be made, including observing that one type of prosthesis shows better biomechanical behavior or identifying the most likely site for mechanical failure. Implants and prosthetic components, such as screws and abutments, present a pattern determined by the manufacturer, well-defined mechanical properties, and reproducible geometry, and consequently, they should be modeled exactly according to the specifications of the manufacturer without simplification.29,31 In this study, the implant and prosthetic components were carefully modeled using reverse engineering techniques. To search for an analysis as close as possible to the clinical reality, some of the interactions between different components of the bone-implant-prosthesis system were simulated by nonlinear contact elements. Many parts that contact in the bone-implant-prosthesis system (eg, the bone and implant, implant and abutment, abutment and screw, screw and implant, abutment and cement, cement and gold infrastructure) do not present a perfectly cohesive union. Therefore, it is important to incorporate the contact elements at the interface between these structures, allowing the occurrence of sliding and microspaces. Regarding the risk of biological complications, this study compared the values of the principal s1 and s3 stresses in the periimplant bone and the displacement-penetration and gap between the implant and bone. The satisfactory long-term functional and esthetic performance of implant-supported rehabilitation is conditional on maintaining the periimplant bone tissue. From a biomechanical viewpoint and excluding the
aspects related to periimplantitis from this discussion, the preservation of the bone structure is dependent on the degree of stresses and strains to which the bone is subjected. According to the mechanostat theory,18 bone is a dynamic tissue in which remodeling is dependent on mechanical stimulation. Stimulation below a certain threshold (50–100 microstrains) could lead to bone resorption as a result of disuse; stimulation within the physiological limit (1000–1500 microstrains) could cause remodeling to preserve bone tissue; stimulation near the physiological limit (3000 microstrains) could lead to bone hypertrophy; and stimulation above a pathological threshold (25,000 microstrains) could result in fracture.18 Bone tissue can operate under high loads and be subjected to extensive stresses and strains that may cause harm because it has a complex mechanism of repair that is not fully understood.20 The loads imposed on the prostheses and transmitted to the implant anchored into bone should result in stresses within its physiological limits, allowing for maintenance and remodeling. The deleterious effects of excessive load and stresses in periimplant bone have been demonstrated. Animal studies have shown that occlusal overload is capable of promoting periimplant bone loss, which may lead to the total failure of osseointegration.22,23 Studies have shown a positive relationship between high bite force or occlusal overload and periimplant bone loss.21,24 Therefore, the stresses and displacements must be minimized to decrease the risk of failure. The prosthesis retention method influences the behavior of the stresses and displacements on the periimplant bone.11,13,16,17 The present analysis did not demonstrate the superiority of any prosthesis when the s1 stresses, predominantly tensile stresses, were compared because the difference between the peak values of the stress was only 2% between the models considering both loadings. It is known that tensile stresses are more deleterious to bone.34 In the displacement analysis, which evaluated the risk of micromotion between implant and bone and, subsequently, the risk of bone loss,18–20 the patterns and values were similar
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IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015 between the prostheses. In the analysis of the s3 stresses, which were predominantly compressive, considering the vertical and oblique loads, the screwretained prosthesis exhibited 28% higher mean peaks. Compressive stresses are less deleterious to bone than tensile and shear stresses.34 Higher periimplant stresses in the screw-retained prosthesis were found in other comparative mechanical studies.11,13,16,17 The cement layer, the separation of the abutment from the prosthetic structure, and the abutment with greater mechanical imbrication may have been positive influences in the transfer of compressive stresses to the bone. However, other comparative mechanical studies found no differences between the types of prostheses in relation to the generated stresses.9,10,12,14,15 In this study, the lower risk of bone failure in the cement-retained prostheses subjected to compressive stresses that was observed did not align with the clinical findings reported in the literature. The results of clinical studies consider the microbiological role in marginal periimplant bone resorption and the possible biomechanical effect; nevertheless, it is important to draw an exploratory parallel between the numerical results and the clinical findings. Although one clinical study reported a higher marginal bone preservation in cementretained compared with screw-retained prostheses,6 systematic reviews and long-term clinical studies found no differences in the periimplant marginal bone level between single or partial screw- and cement-retained prostheses.3,4,8,37 Moreover, a systematic review reported the marginal bone level to be better in screw-retained prostheses.7 In addition, some studies have reported more favorable gingival results in screw-retained compared with cementretained prostheses.4,38 Most likely, the superior gingival and periimplant bone response observed in these studies refers to the absence of subgingival cement in the screw-retained prosthesis and not to a biomechanical factor. An excess of cement may lead to periimplantitis, resulting in marginal bone loss.39 Therefore, the displacement results and the analysis of the tensile stresses in this study which showed no advantage with
either type of prosthesis appeared to corroborate with most of the mechanical and clinical studies that did not define one type of prosthesis as being superior to another, regarding periimplant bone maintenance. The mechanical effect alone does not seem to affect bone structure in the comparison of screw- and cementretained prostheses. The marginal bone loss seems to be related to a combined effect,40 with several variables influencing the dynamics of periimplant bone. The causes that led to reports of bone level differences between the screwand cement-retained prostheses6,7 are probably biological or associated with an occlusal overload specific to each patient and not related to the retention method.
CONCLUSIONS There were no important differences in the s1 analysis and the displacement between the SFP and CFP. The differences in marginal bone level reported between SFP and CFP in some clinical studies may not be related to a mechanical factor.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
REFERENCES 1. Brånemark PI, Hansson BO, Adell R, et al. Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period. Scand J Plast Reconstr Surg Suppl. 1977;16:1–132. 2. Astrand P, Ahlqvist J, Gunne J, et al. Implant treatment of patients with edentulous jaws: A 20-year follow-up. Clin Implant Dent Relat Res. 2008;10:207–217. 3. Jemt T. Cemented CeraOne and porcelain fused to TiAdapt abutment single-implant crown restorations: A 10-year comparative follow-up study. Clin Implant Dent Relat Res. 2009;11:303–310. 4. Sherif S, Susarla SM, Hwang JW, et al. Clinician- and patient-reported long-term evaluation of screw- and cement-retained implant restorations: A 5-year prospective study. Clin Oral Investig. 2011;15:993–999. 5. Chaar MS, Att W, Strub JR. Prosthetic outcome of cement-retained
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implant-supported fixed dental restorations: A systematic review. J Oral Rehabil. 2011;38:697–711. 6. Nissan J, Narobai D, Gross O, et al. Long-term outcome of cemented versus screw-retained implant-supported partial restorations. Int J Oral Maxillofac Implants. 2011;26:1102–1107. 7. Sailer I, Mühlemann S, Zwahlen M, et al. Cemented and screw-retained implant reconstructions: A systematic review of the survival and complication rates. Clin Oral Implants Res. 2012;23:163–201. 8. de Brandão ML, Vettore MV, Vidigal Júnior GM. Peri-implant bone loss in cement- and screw-retained prostheses: Systematic review and meta-analysis. J Clin Periodontol. 2013;40:287–295. 9. Kim WD, Jacobson Z, Nathanson D. In vitro stress analyses of dental implants supporting screw-retained and cement-retained prostheses. Implant Dent. 1999;8:141–151. 10. Pietrabissa R, Gionso L, Quaglini V, et al. An in vitro study on compensation of mismatch of screw versus cement-retained implant supported fixed prostheses. Clin Oral Implants Res. 2000;11:448–457. 11. Guichet DL, Caputo AA, Choi H, et al. Passivity of fit and marginal opening in screw- or cement-retained implant fixed partial denture designs. Int J Oral Maxillofac Implants. 2000;15:239–246. 12. Heckmann SM, Karl M, Wichmann MG, et al. Cement fixation and screw retention: Parameters of passive fit. An in vitro study of three-unit implant-supported fixed partial dentures. Clin Oral Implants Res. 2004;15:466–473. 13. Karl M, Taylor TD, Wichmann MG, et al. In vivo stress behavior in cemented and screw-retained five-unit implant FPDs. J Prosthodont. 2006;15:20–24. 14. Karl M, Wichmann MG, Winter W, et al. Influence of fixation mode and superstructure span upon strain development of implant fixed partial dentures. J Prosthodont. 2008;17:3–8. 15. Akça K, Kokat AM, Sahin S, et al. Effects of prosthesis design and impression techniques on human cortical bone strain around oral implants under load. Med Eng Phys. 2009;31:758–763. 16. Pellizzer EP, Tonella BP, Ferraço R, et al. Photoelastic stress analysis in screwed and cemented implant-supported dentures with external hexagon implants. J Craniofac Surg. 2010;21:1110–1113. 17. Aguiar FA Jr, Tiossi R, Macedo AP, et al. Photoelastic analysis of stresses transmitted by universal cast to long abutment on implant-supported single restorations under static occlusal loads. J Craniofac Surg. 2012;23:2019–2023. 18. Frost HM. A 2003 update of bone physiology and Wolff’s Law for clinicians. Angle Orthod. 2004;74:3–15.
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19. Isidor F. Influence of forces on periimplant bone. Clin Oral Implants Res. 2006;17:8–18. 20. Taylor D, Hazenberg JG, Lee TC. Living with cracks: Damage and repair in human bone. Nat Mater. 2007;6:263–268. 21. Quirynen M, Naert I, van Steenberghe D. Fixture design and overload influence marginal bone loss and fixture success in the Brånemark system. Clin Oral Implants Res. 1992;3:104–111. 22. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res. 1996;7:143–152. 23. Miyata T, Kobayashi Y, Araki H, et al. The influence of controlled occlusal overload on peri-implant tissue. Part 3: A histologic study in monkeys. Int J Oral Maxillofac Implants. 2000;15:425–431. 24. Geckili O, Mumcu E, Bilhan H. The effect of maximum bite force, implant number, and attachment type on marginal bone loss around implants supporting mandibular overdentures: A retrospective study. Clin Implant Dent Relat Res. 2012; 14:e91–e97. 25. Lekholm U, Zarb G. Patient selection and preparation. In: Brånemark PI, Zarb G, Albrektsson T, eds. Tissue-Integrated Prostheses. Osseointegration in Clinical Dentistry. Chicago, IL: Quintessence; 1985:199–210. 26. Carvalho Silva G, Cornacchia TPM, Las Casas EB, et al. A method for obtain-
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ing a three-dimensional geometric model of dental implants for analysis via the finite element method. Implant Dent. 2013;22: 309–314. 27. Rosenstiel SF, Land MF, Crispin BJ. Dental luting agents: A review of the current literature. J Prosthet Dent. 1998; 80:280–301. 28. Li ZC, White SN. Mechanical properties of dental luting cements. J Prosthet Dent. 1999;81:597–609. 29. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant dentistry: A review of the literature. J Prosthet Dent. 2001;85:585–598. 30. Tillitson EW, Craig RG, Peyton FA. Friction and wear of restorative dental materials. J Dent Res. 1971;50:149–154. 31. Wang RF, Kang B, Lang LA, et al. The dynamic natures of implant loading. J Prosthet Dent. 2009;101:359–371. 32. Schwartz-Dabney CL, Dechow PC. Accuracy of elastic property measurement in mandibular cortical bone is improved by using cylindrical specimens. J Biomech Eng. 2002;124:714–723. 33. O’Mahony AM, Williams JL, Spencer P. Anisotropic elasticity of cortical and cancellous bone in the posterior mandible increases peri-implant stress and strain under oblique loading. Clin Oral Implants Res. 2001;12:648–657. 34. Van Eijden TM. Biomechanics of the mandible. Crit Rev Oral Biol Med. 2000;11:123–136.
35. Miyamoto Y, Obama T. Dental cone beam computed tomography analyses of postoperative labial bone thickness in maxillary anterior implants: Comparing immediate and delayed implant placement. Int J Periodontics Restorative Dent. 2011;31:215–225. 36. Natali AN, Carniel EL, Pavan PG. Modelling of mandible bone properties in the numerical analysis of oral implant biomechanics. Comput Methods Programs Biomed. 2010;100:158–165. 37. Jung RE, Zembic A, Pjetursson BE, et al. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean followup of 5 years. Clin Oral Implants Res. 2012;23:2–21. 38. Weber HP, Kim DM, Ng MW, et al. Peri-implant soft-tissue health surrounding cement- and screw-retained implant restorations: A multi-center, 3-year prospective study. Clin Oral Implants Res. 2006; 17:375–379. 39. Thomas GW. The positive relationship between excess cement and periimplant disease: A prospective clinical endoscopic study. J Periodontol. 2009; 80:1388–1392. 40. Qian J, Wennerberg A, Albrektsson T. Reasons for marginal bone loss around oral implants. Clin Implant Dent Relat Res. 2012;14:792–807.
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Effects of Screw- and Cement-Retained Implant-Supported Prostheses on Bone: A Nonlinear 3-D Finite Element Analysis Guilherme Carvalho Silva, DDS, MS, PhD,* Guilherme Martins de Andrade, MechEng,† Rodrigo Carvalho Pinto Coelho, DDS, MS,‡ Tulimar Machado Cornacchia, DDS, MS, PhD(Eng),§ Cláudia Silami de Magalhães, DDS, MS, PhD,§ and Allyson Nogueira Moreira, DDS, PhD§
ral rehabilitation with implantsupported prostheses is an accepted treatment modality.1 The original protocol of the pioneering studies of Brånemark et al1 was based exclusively on screw-retained prostheses, and numerous studies have confirmed the success of the application of this concept, particularly in fully edentulous patients.2 However, with the development of new implant systems and new rehabilitation techniques, cement-retained prostheses have become a treatment option, particularly in cases of single and fixed partial prostheses. Currently, cement-retained prostheses are frequently used, with a high level of success.3–6 The advantages and disadvantages of screw- and cement-retained prostheses have particularly received attention from researchers in aspects such as passive fit, quality of marginal adaptation, retention, occlusal factors,
O
*Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. †Graduate, Department of Mechanical Engineering, School of Engineering, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. ‡PhD Student, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. §Associate Professor, Department of Restorative Dentistry, School of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.
Reprint requests and correspondence to: Guilherme Carvalho Silva, DDS, MS, PhD, Avenida Afonso Pena 4334, Belo Horizonte, MG 30130-009, Brazil, Phone: +55 31 32259335, E-mail: [email protected] ISSN 1056-6163/15/00000-001 Implant Dentistry Volume 0 Number 0 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/ID.0000000000000278
Purpose: To compare the stresses and displacements on perimplant bone generated by screw- and cement-retained prostheses using the finite element method. Materials and Methods: Two models were constructed: partial fixed implant-supported prostheses with three elements retained by screws (SFP) or cement (CFP). Vertical and oblique loads of 100 N were applied on the models. Bone was analyzed by the principal stresses s1 and s3. The displacement between the implant and the bone was identified by the penetration and gap. Results: Results showed a similar pattern in the distribution of the principal stresses between both prostheses. Under the s1 stresses, the SFP showed similar values in the
bone compared with the CFP. The analysis of the s3 showed stress peaks 28% higher in the SFP, considering vertical and oblique loads. Displacement analysis showed a similar pattern and similar values between the prostheses for penetration and gap under both loads. Conclusions: There were no important differences in the s1 analysis and the displacement between the SFP and CFP. The differences in marginal bone level reported between SFP and CFP in some clinical studies may not be related to a mechanical factor. (Implant Dent 2015;0:1–8) Key Words: biomechanics, finite element method, implant-supported dental prosthesis, marginal bone loss
esthetics, reversibility, ease of manufacture, costs, maintenance of bone level, and gingival health and survival.4,6–8 The biomechanical behavior of screw-retained and cement-retained prostheses regarding the generation of stress on tissues and prosthetic abutments has been compared superficially using distinct methods.9–17 One of the most important aspects analyzed in these studies is the effect of the retaining method (screw or cement) on the periimplant bone, as bone preservation is essential to implant longevity.
Dynamic maintenance and remodeling of the bone structure is dependent on mechanical stimuli that cause stress and strain on living tissue.18–20 Peak stress on supporting bone may cause marginal bone loss and even complete implant failure.21–24 For that reason, an important mechanical component seems to be related to the preservation of periimplant bone. Several aspects may influence the intensity and patterns of stresses and displacements in the bone-implant interface, such as
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Table 2. Friction Coefficient of the Interfaces Between Different Materials Friction Coefficient
Interface
Fig. 1. Three-dimensional models of the prostheses. A, Screw-retained. B, Cement-retained.
Titanium abutment and titanium screw Gold abutment and titanium screw Titanium abutment and titanium implant Gold abutment and titanium implant Titanium screw and titanium implant Titanium implant and bone Zinc phosphate cement and titanium abutment Zinc phosphate cement and gold
0.16 0.20 0.16 0.20 0.16 0.30 0.20 0.20
Zinc phosphate cement interfaces30; other interfaces.31
Fig. 2. Coronal section of the meshes. A, Screw-retained: 1,097,527 elements. B, Cementretained: 1,146,675 elements.
the retention method of the implantsupported prosthesis.11 Consequently, a biomechanical analysis of the mechanism of the stresses and displacements generation is essential to predict the success of implant restorations by identifying possible risks of marginal bone loss. Therefore, the aim of this study was to compare the stresses and displacements in the bone-implant interfaces of implantsupported screw- or cement-retained prostheses using the finite element method in a three-dimensional (3D) nonlinear analysis. Table 1. Mechanical Properties of the Materials
Material Titanium Gold Porcelain Zinc phosphate cement Composite resin 28
Young’s Modulus (MPa)
Poisson Ratio
117,000 100,000 68,900 17,000
0.30 0.30 0.28 0.35
7000
0.20
29
Zinc phosphate cement ; other materials.
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Two 3D models were developed: screw- or cement-retained porcelain fused to metal fixed partial prostheses with three elements supported by implants in the area of the lower 2nd premolar and 2nd molar. Initially, a section of a human partially edentulous mandible was modeled. Cone-beam computerized tomography (CT) (i-CAT, Imaging Sciences International, Hatfield, PA) of the mandible of a partially edentulous patient was obtained from the files of patients treated by the researcher. The images of 3D CT reconstructions in STL format (3D Systems, Rock Hill, SC) were transferred to 3D computer-aided design software (SolidWorks, Dassault Systèmes SolidWorks Corporation, Santa Monica, CA) for editing and refinement. The edentulous region between the 2nd premolar and 2nd molar was sectioned, and the mandibular canal was eliminated to reduce the volume and the complexity of the computational model. The cortical portion of the bone was lineated in the form of a 1.50-mm thick homogeneous
layer simulating a type II bone,25 and all the contours of the mandible segment were rounded and smoothed. The following components, which were obtained directly from the manufacturer, were modeled, as previously described26: the dental implant (NobelReplace 4.3 3 13 mm; Nobel Biocare, Gothenburg, Sweden), UCLA abutment for screw-retained multiple-unit prostheses (GoldAdapt Non-Engaging, Nobel Biocare, Gothenburg, Sweden), customized engaging titanium abutment for cement-retained prostheses (Esthetic Abutment, Nobel Biocare, Gothenburg, Sweden), and titanium abutment screw (29,475 TorqTite, Nobel Biocare, Gothenburg, Sweden). For modeling of the outer contour of the prostheses, the crown of the 2nd Table 3. Bone Properties Properties
Cortical Bone
Cancellous Bone
Ey Ex Ez Gyx Gyz Gxz nyx nyz nxz
12,500 26,600 17,900 4500 7100 5300 0.18 0.28 0.31
210 1148 1148 68 68 434 0.055 0.055 0.322
Ei, Young’s Modulus (MPa); Gi, Shear modulus (MPa); n, Poisson ratio. x axis is anteroposterior; y axis is apicocoronal; z axis is lateromedial. Cortical bone32; cancellous bone.33
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Fig. 3. Occlusal view of s1 stresses on bone under vertical load. A, Screw-retained. B, Cement-retained.
Fig. 4. Occlusal view of s3 stresses on bone under vertical load. A, Screw-retained. B, Cement-retained.
Fig. 5. Occlusal view of s1 stresses on bone under oblique load. A, Screw-retained. B, Cement-retained.
Fig. 6. Occlusal view of s3 stresses on bone under oblique load. A, Screw-retained. B, Cement-retained.
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molar was modeled and posteriorly replicated three times. The crowns were joined in the proximal region by cylindrical connectors. After completion of the external design of the prostheses, 1.50 mm was determined as the outer layer of the entire volume that corresponded to the ceramic layer. For the cement-retained prosthesis, the entire area just below the ceramic layer before contact with the abutment represented the gold infrastructure. For the screwretained prosthesis, gold was used for the entire inner area under the ceramic layer because it would fuse to the abutments. The screw access was obliterated with composite resin. For the cement-retained prosthesis, a homogeneous layer of 25 mm thickness representing zinc phosphate cement was modeled5,27 between the gold infrastructure and the abutments (Fig. 1, A and B). The mesh was generated with computer-aided engineering software (Ansys 14, ANSYS Inc., Canonsburg, PA) (Fig. 2, A and B). A material with mechanical properties was assigned for each volume28,29 (Table 1) and a coefficient of friction for each material30,31 (Table 2). The bone was considered anisotropic32,33 (Table 3), linear elastic, and homogeneous, and the other materials were defined as isotropic, linear elastic, and homogeneous. The contact areas among the different sections of the models were defined using nonlinear contact elements. The behavior of the contact surface was different according to the interface of the materials. Between the implants and bone, a “rough” contact was selected, which allowed the formation of microspaces without sliding between the elements. An identical contact was selected in the area between the abutments and cement and between the cement and metallic infrastructure in the cement-retained prosthesis because there is no chemical bonding and only mechanical imbrication between the cement and the metal. Between the screws and implants, implants and abutments, and abutments and screws, a “standard” contact was selected, which allowed the formation of microspaces and a small degree of sliding between the surfaces, a common
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Table 4. Maximum Principal s1 Stresses Under Vertical and Oblique Loadings Type of Loading
Screw-Retained: s1 Stresses (MPa)
Cement-Retained: s1 Stresses (MPa)
Difference (%)
Vertical Oblique Mean
33 122 77
28 130 79
17 6 2
gap were identified. A smaller displacement at the interface corresponds with a lower risk of failure. The qualitative analysis was made by comparing the pattern of the distribution of the stresses and displacements. The quantitative analysis was given for the difference in the percentage of the maximum values found for each criterion used.
Table 5. Maximum Principal s3 Stresses Under Vertical and Oblique Loadings Type of Loading
Screw-Retained: s3 Stresses (MPa)
Cement-Retained: s3 Stresses (MPa)
Difference (%)
Vertical Oblique Mean
68 165 116
64 116 90
6 42 28
Table 6. Maximum Penetration Between Implant and Bone Type of Loading
Screw-Retained: Penetration (mm)
Cement-Retained: Penetration (mm)
Difference (%)
Vertical Oblique
0.004 0.009
0.005 0.010
25 11
Table 7. Maximum Gap Between Implant and Bone Type of Loading
Screw-Retained: Gap (mm)
Cement-Retained: Gap (mm)
Difference (%)
Vertical Oblique
0.123 0.116
0.122 0.106
0.8 9
occurrence between metal surfaces. The remaining volumes were considered bonded because they showed characteristics of a cohesive union: cortical and cancellous bone, ceramic and gold infrastructure, and resin and ceramic. The models were constricted in the mesiodistal (anteroposterior) and buccolingual (lateral to medial) directions. The load simulations were identical for the models and were performed in three steps. First, before the imposition loads on the models, the abutment screws received a torque of 35 N$cm, a value recommended by the manufacturer, to represent the settling of the prostheses. For the torque simulation, contact elements were selected on the six inner sides of the hexagonal screw head. Next, a master node was determined in the spatial center of the screw head. A moment of 35 N$cm was applied clockwise on the master node in a rotation of all the nodes of the contact elements on the inner sides of the
screw head, resulting in screw tightening. Finally, the rotation moment was finalized, indicating the end of the tightening. With the screw pretensioned, vertical and oblique (45° in the buccolingual direction), 100 N loads were applied to the occlusal areas of the teeth in each prosthesis at different times. The periimplant bone and the implant were plotted to observe the stresses and displacements externally and internally. The results of the 2nd premolar were presented because the region of the 2nd molar showed almost identical results. The principal stresses s1 and s3 were selected because bone is a fragile structure that distinctly responds to the action of different stresses, thereby exhibiting a greater risk of failure under shear and tensile stresses compared with compressive stresses.34 A lower stress corresponds with a lower risk of failure. For the analysis of the displacement between the contact surfaces, the penetration and
RESULTS The tensile stresses, which were prevalent in the analysis of s1, were plotted with a positive value, whereas the compressive stresses presented negative values. In the analysis of s3, the compressive stresses, which were predominant, were plotted with a negative value, whereas the tensile stresses exhibited positive values. Qualitatively, the results showed a similar stress pattern between the screw- and the cement-retained prostheses. In the vertical load in both models, there was a dispersion of stresses around the entire periimplant cervical area, with the highest stresses concentrated on the buccal surface, particularly when analyzing the compressive stresses (Figs. 3 and 4). In the oblique loading, the analysis of the s1 stresses showed peaks in the buccal area and areas of concentrated tensile stresses in the distal region (Fig. 5, A and B). The analysis of the s3 stresses showed the concentration of compressive stress in the lingual region in the direction opposite to the applied load. These stresses were located on the top and inner surface of the lingual cortical bone. This result was because of compression from the vertical component of the load and from bending of the structure (Fig. 6, A and B). Regarding the intensity of the s1 stresses, there was no important difference when comparing the peaks between the models. The average peak of the stresses, considering the vertical and oblique loads, was only 2% lower in the screw-retained prosthesis (Table 4). However, the s3 peak stresses were on average 28% higher in the screw-retained prostheses, when considering both loads. There was a greater difference in the oblique loading; the s3 peak stress was 42% higher
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Fig. 7. Implant penetration on bone under vertical load. A, Screw-retained. B, Cementretained.
Fig. 8. Gap between implant and bone under vertical load. A, Screw-retained. B, Cementretained.
Fig. 9. Lateral view of implant penetration on bone under oblique load. Left side of the implant: buccal; right side: lingual. A, Screw-retained. B, Cement-retained.
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in the screw-retained prosthesis (Table 5). In both models, under the two loads, the stresses were always located on the cervical region of the cortical bone, with lower or absent stresses in almost all of the cancellous bone. The displacements between the contact surfaces of the implant and bone were identified and measured by plotting the penetration and gap of the implant relative to the bone. The gaps were plotted with negative values. In both prostheses, under the vertical load, the highest penetration was observed in the apical region of the implant; this parameter was 25% higher in the cement-retained prosthesis, but all of the values were very low (Fig. 7, A and B) (Table 6). In the gap analysis, the pattern was similar between the prostheses, with the largest displacement in the buccal region (Fig. 8, A and B). The difference in the extent of the gap was less than 1% (Table 7). In the oblique loading, the areas of greatest penetration were in the lingual cervical area in both prostheses. The displacement in this region was 11% higher in the cement-retained prosthesis. A greater displacement in the buccal face of the bone, compared with the lingual face, was noted in both prostheses (Fig. 9, A and B). In the gap analysis, the areas of greatest displacement were found in a range in the cervical buccal area in both prostheses, with the maximum gap being 9% higher in the screw-retained prosthesis. In both prostheses, a higher gap in the lingual face of the bone was observed compared with the vestibular face, which was the opposite of the penetration pattern (Fig. 10, A and B).
DISCUSSION
Fig. 10. Gap between implant and bone under oblique load. Lateral view: Left side of the implant: buccal; right side: lingual. A, Screw-retained. B, Cement-retained.
The relevance of biomechanical evaluation of the periimplant bone relates to the fact that excessive loads can induce bone resorption, which may lead to implant failure and the consequent failure of the entire prosthetic rehabilitation.22,23 Furthermore, the periimplant bone loss is also related to marginal gingival recession,35 which generates an esthetic risk, particularly in anterior prostheses. In this study,
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the risk of cervical bone resorption was analyzed, but not the risk of complete fracture of the bone around an osseointegrated implant, which has not been reported in the literature. It is supposed that constant stress and strain above a certain limit can lead to microcracks or microdeformations in the bone structure; if microfractures occur more rapidly than repair in the continuous damage process, bone loss can occur before complete fracture of the bone.18,19 The finite element method was selected for this comparative analysis because it is not invasive or destructive but a computational numeric technique, which allows the analysis of various types of internal or external stresses, strains, and displacements among other applications and in any area of the studied structure. Using this method, it is possible to identify patterns, stresses, and displacements in areas of the prosthetic implant-bone complex that are inaccessible by other methods of biomechanical studies. The finite element method is frequently used in implant dentistry and applied in a wide variety of simulations.29,31,36 However, this research method, as any other, has limitations, particularly when trying to extrapolate the results of this numerical technique for the clinical field. One frequent simplification is related to bone modeling. Bone is a dynamic and complex living tissue, without a pattern, whose characteristics vary in the same individual and among individuals. Therefore, a bone model that represents a generalized situation is practically impossible.34,36 A perfect fit without gaps between the abutment and the implant and between the prosthesis and the abutments adopted in this study was also a simplification because minimum spaces typically occur in the adaptation of these structures. However, the absence of gaps was important for the analysis because the comparison was independent of the quality of fit, as the mismatch itself generates stresses.12,13 The precision of the adaptation is dependent on numerous variables inherent in the process of laboratory work, such as the type of metal alloy used, the welding, technical experience, and impression technique.12,13 All
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factors dependent on the prosthesis manufacturing process were excluded, allowing this study to search specifically for the effect of the retention method (screw or cement). Although these assumptions were made, they are not expected to compromise the results, as identical criteria were used impartially in both models. The collected absolute values of the stresses should not be compared with the yield of the rupture limits of the materials involved in the study. Such care in interpreting the results should be taken in any finite element analysis. Clinical inferences may be made, including observing that one type of prosthesis shows better biomechanical behavior or identifying the most likely site for mechanical failure. Implants and prosthetic components, such as screws and abutments, present a pattern determined by the manufacturer, well-defined mechanical properties, and reproducible geometry, and consequently, they should be modeled exactly according to the specifications of the manufacturer without simplification.29,31 In this study, the implant and prosthetic components were carefully modeled using reverse engineering techniques. To search for an analysis as close as possible to the clinical reality, some of the interactions between different components of the bone-implant-prosthesis system were simulated by nonlinear contact elements. Many parts that contact in the bone-implant-prosthesis system (eg, the bone and implant, implant and abutment, abutment and screw, screw and implant, abutment and cement, cement and gold infrastructure) do not present a perfectly cohesive union. Therefore, it is important to incorporate the contact elements at the interface between these structures, allowing the occurrence of sliding and microspaces. Regarding the risk of biological complications, this study compared the values of the principal s1 and s3 stresses in the periimplant bone and the displacement-penetration and gap between the implant and bone. The satisfactory long-term functional and esthetic performance of implant-supported rehabilitation is conditional on maintaining the periimplant bone tissue. From a biomechanical viewpoint and excluding the
aspects related to periimplantitis from this discussion, the preservation of the bone structure is dependent on the degree of stresses and strains to which the bone is subjected. According to the mechanostat theory,18 bone is a dynamic tissue in which remodeling is dependent on mechanical stimulation. Stimulation below a certain threshold (50–100 microstrains) could lead to bone resorption as a result of disuse; stimulation within the physiological limit (1000–1500 microstrains) could cause remodeling to preserve bone tissue; stimulation near the physiological limit (3000 microstrains) could lead to bone hypertrophy; and stimulation above a pathological threshold (25,000 microstrains) could result in fracture.18 Bone tissue can operate under high loads and be subjected to extensive stresses and strains that may cause harm because it has a complex mechanism of repair that is not fully understood.20 The loads imposed on the prostheses and transmitted to the implant anchored into bone should result in stresses within its physiological limits, allowing for maintenance and remodeling. The deleterious effects of excessive load and stresses in periimplant bone have been demonstrated. Animal studies have shown that occlusal overload is capable of promoting periimplant bone loss, which may lead to the total failure of osseointegration.22,23 Studies have shown a positive relationship between high bite force or occlusal overload and periimplant bone loss.21,24 Therefore, the stresses and displacements must be minimized to decrease the risk of failure. The prosthesis retention method influences the behavior of the stresses and displacements on the periimplant bone.11,13,16,17 The present analysis did not demonstrate the superiority of any prosthesis when the s1 stresses, predominantly tensile stresses, were compared because the difference between the peak values of the stress was only 2% between the models considering both loadings. It is known that tensile stresses are more deleterious to bone.34 In the displacement analysis, which evaluated the risk of micromotion between implant and bone and, subsequently, the risk of bone loss,18–20 the patterns and values were similar
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IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015 between the prostheses. In the analysis of the s3 stresses, which were predominantly compressive, considering the vertical and oblique loads, the screwretained prosthesis exhibited 28% higher mean peaks. Compressive stresses are less deleterious to bone than tensile and shear stresses.34 Higher periimplant stresses in the screw-retained prosthesis were found in other comparative mechanical studies.11,13,16,17 The cement layer, the separation of the abutment from the prosthetic structure, and the abutment with greater mechanical imbrication may have been positive influences in the transfer of compressive stresses to the bone. However, other comparative mechanical studies found no differences between the types of prostheses in relation to the generated stresses.9,10,12,14,15 In this study, the lower risk of bone failure in the cement-retained prostheses subjected to compressive stresses that was observed did not align with the clinical findings reported in the literature. The results of clinical studies consider the microbiological role in marginal periimplant bone resorption and the possible biomechanical effect; nevertheless, it is important to draw an exploratory parallel between the numerical results and the clinical findings. Although one clinical study reported a higher marginal bone preservation in cementretained compared with screw-retained prostheses,6 systematic reviews and long-term clinical studies found no differences in the periimplant marginal bone level between single or partial screw- and cement-retained prostheses.3,4,8,37 Moreover, a systematic review reported the marginal bone level to be better in screw-retained prostheses.7 In addition, some studies have reported more favorable gingival results in screw-retained compared with cementretained prostheses.4,38 Most likely, the superior gingival and periimplant bone response observed in these studies refers to the absence of subgingival cement in the screw-retained prosthesis and not to a biomechanical factor. An excess of cement may lead to periimplantitis, resulting in marginal bone loss.39 Therefore, the displacement results and the analysis of the tensile stresses in this study which showed no advantage with
either type of prosthesis appeared to corroborate with most of the mechanical and clinical studies that did not define one type of prosthesis as being superior to another, regarding periimplant bone maintenance. The mechanical effect alone does not seem to affect bone structure in the comparison of screw- and cementretained prostheses. The marginal bone loss seems to be related to a combined effect,40 with several variables influencing the dynamics of periimplant bone. The causes that led to reports of bone level differences between the screwand cement-retained prostheses6,7 are probably biological or associated with an occlusal overload specific to each patient and not related to the retention method.
CONCLUSIONS There were no important differences in the s1 analysis and the displacement between the SFP and CFP. The differences in marginal bone level reported between SFP and CFP in some clinical studies may not be related to a mechanical factor.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
REFERENCES 1. Brånemark PI, Hansson BO, Adell R, et al. Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period. Scand J Plast Reconstr Surg Suppl. 1977;16:1–132. 2. Astrand P, Ahlqvist J, Gunne J, et al. Implant treatment of patients with edentulous jaws: A 20-year follow-up. Clin Implant Dent Relat Res. 2008;10:207–217. 3. Jemt T. Cemented CeraOne and porcelain fused to TiAdapt abutment single-implant crown restorations: A 10-year comparative follow-up study. Clin Implant Dent Relat Res. 2009;11:303–310. 4. Sherif S, Susarla SM, Hwang JW, et al. Clinician- and patient-reported long-term evaluation of screw- and cement-retained implant restorations: A 5-year prospective study. Clin Oral Investig. 2011;15:993–999. 5. Chaar MS, Att W, Strub JR. Prosthetic outcome of cement-retained
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implant-supported fixed dental restorations: A systematic review. J Oral Rehabil. 2011;38:697–711. 6. Nissan J, Narobai D, Gross O, et al. Long-term outcome of cemented versus screw-retained implant-supported partial restorations. Int J Oral Maxillofac Implants. 2011;26:1102–1107. 7. Sailer I, Mühlemann S, Zwahlen M, et al. Cemented and screw-retained implant reconstructions: A systematic review of the survival and complication rates. Clin Oral Implants Res. 2012;23:163–201. 8. de Brandão ML, Vettore MV, Vidigal Júnior GM. Peri-implant bone loss in cement- and screw-retained prostheses: Systematic review and meta-analysis. J Clin Periodontol. 2013;40:287–295. 9. Kim WD, Jacobson Z, Nathanson D. In vitro stress analyses of dental implants supporting screw-retained and cement-retained prostheses. Implant Dent. 1999;8:141–151. 10. Pietrabissa R, Gionso L, Quaglini V, et al. An in vitro study on compensation of mismatch of screw versus cement-retained implant supported fixed prostheses. Clin Oral Implants Res. 2000;11:448–457. 11. Guichet DL, Caputo AA, Choi H, et al. Passivity of fit and marginal opening in screw- or cement-retained implant fixed partial denture designs. Int J Oral Maxillofac Implants. 2000;15:239–246. 12. Heckmann SM, Karl M, Wichmann MG, et al. Cement fixation and screw retention: Parameters of passive fit. An in vitro study of three-unit implant-supported fixed partial dentures. Clin Oral Implants Res. 2004;15:466–473. 13. Karl M, Taylor TD, Wichmann MG, et al. In vivo stress behavior in cemented and screw-retained five-unit implant FPDs. J Prosthodont. 2006;15:20–24. 14. Karl M, Wichmann MG, Winter W, et al. Influence of fixation mode and superstructure span upon strain development of implant fixed partial dentures. J Prosthodont. 2008;17:3–8. 15. Akça K, Kokat AM, Sahin S, et al. Effects of prosthesis design and impression techniques on human cortical bone strain around oral implants under load. Med Eng Phys. 2009;31:758–763. 16. Pellizzer EP, Tonella BP, Ferraço R, et al. Photoelastic stress analysis in screwed and cemented implant-supported dentures with external hexagon implants. J Craniofac Surg. 2010;21:1110–1113. 17. Aguiar FA Jr, Tiossi R, Macedo AP, et al. Photoelastic analysis of stresses transmitted by universal cast to long abutment on implant-supported single restorations under static occlusal loads. J Craniofac Surg. 2012;23:2019–2023. 18. Frost HM. A 2003 update of bone physiology and Wolff’s Law for clinicians. Angle Orthod. 2004;74:3–15.
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19. Isidor F. Influence of forces on periimplant bone. Clin Oral Implants Res. 2006;17:8–18. 20. Taylor D, Hazenberg JG, Lee TC. Living with cracks: Damage and repair in human bone. Nat Mater. 2007;6:263–268. 21. Quirynen M, Naert I, van Steenberghe D. Fixture design and overload influence marginal bone loss and fixture success in the Brånemark system. Clin Oral Implants Res. 1992;3:104–111. 22. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res. 1996;7:143–152. 23. Miyata T, Kobayashi Y, Araki H, et al. The influence of controlled occlusal overload on peri-implant tissue. Part 3: A histologic study in monkeys. Int J Oral Maxillofac Implants. 2000;15:425–431. 24. Geckili O, Mumcu E, Bilhan H. The effect of maximum bite force, implant number, and attachment type on marginal bone loss around implants supporting mandibular overdentures: A retrospective study. Clin Implant Dent Relat Res. 2012; 14:e91–e97. 25. Lekholm U, Zarb G. Patient selection and preparation. In: Brånemark PI, Zarb G, Albrektsson T, eds. Tissue-Integrated Prostheses. Osseointegration in Clinical Dentistry. Chicago, IL: Quintessence; 1985:199–210. 26. Carvalho Silva G, Cornacchia TPM, Las Casas EB, et al. A method for obtain-
SILVA
ET AL
ing a three-dimensional geometric model of dental implants for analysis via the finite element method. Implant Dent. 2013;22: 309–314. 27. Rosenstiel SF, Land MF, Crispin BJ. Dental luting agents: A review of the current literature. J Prosthet Dent. 1998; 80:280–301. 28. Li ZC, White SN. Mechanical properties of dental luting cements. J Prosthet Dent. 1999;81:597–609. 29. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant dentistry: A review of the literature. J Prosthet Dent. 2001;85:585–598. 30. Tillitson EW, Craig RG, Peyton FA. Friction and wear of restorative dental materials. J Dent Res. 1971;50:149–154. 31. Wang RF, Kang B, Lang LA, et al. The dynamic natures of implant loading. J Prosthet Dent. 2009;101:359–371. 32. Schwartz-Dabney CL, Dechow PC. Accuracy of elastic property measurement in mandibular cortical bone is improved by using cylindrical specimens. J Biomech Eng. 2002;124:714–723. 33. O’Mahony AM, Williams JL, Spencer P. Anisotropic elasticity of cortical and cancellous bone in the posterior mandible increases peri-implant stress and strain under oblique loading. Clin Oral Implants Res. 2001;12:648–657. 34. Van Eijden TM. Biomechanics of the mandible. Crit Rev Oral Biol Med. 2000;11:123–136.
35. Miyamoto Y, Obama T. Dental cone beam computed tomography analyses of postoperative labial bone thickness in maxillary anterior implants: Comparing immediate and delayed implant placement. Int J Periodontics Restorative Dent. 2011;31:215–225. 36. Natali AN, Carniel EL, Pavan PG. Modelling of mandible bone properties in the numerical analysis of oral implant biomechanics. Comput Methods Programs Biomed. 2010;100:158–165. 37. Jung RE, Zembic A, Pjetursson BE, et al. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean followup of 5 years. Clin Oral Implants Res. 2012;23:2–21. 38. Weber HP, Kim DM, Ng MW, et al. Peri-implant soft-tissue health surrounding cement- and screw-retained implant restorations: A multi-center, 3-year prospective study. Clin Oral Implants Res. 2006; 17:375–379. 39. Thomas GW. The positive relationship between excess cement and periimplant disease: A prospective clinical endoscopic study. J Periodontol. 2009; 80:1388–1392. 40. Qian J, Wennerberg A, Albrektsson T. Reasons for marginal bone loss around oral implants. Clin Implant Dent Relat Res. 2012;14:792–807.
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