Finite element analysis of stability and functional stress with implant-supported maxillary obturator prostheses Andréa Alves de Sousa, DDSa and Beatriz Silva Câmara Mattos, DDS, PhDb School of Dentistry, University of São Paulo, São Paulo, Brazil Statement of problem. Maxillary resections jeopardize the stability and functional stress generated by implanted-supported prostheses. Purpose. The purpose of this study was to evaluate the stability and functional stress caused by implanted-supported obturator prostheses in simulated maxillary resections of an edentulous maxilla corresponding to Okay Classes Ib, II, and III, with no surgical reconstruction. Material and methods. Implants were positioned in the residual maxilla, and bar-clip retention systems were designed for each experimental model. The 3-dimensional models of the maxillary resection and corresponding implanted-supported obturator prosthesis, constructed from a computed tomography scan, were used to develop a finite element mesh. Loads were simultaneously applied to the occlusal (80 N) and anterior (35 N) platforms corresponding to the prosthetic teeth. Qualitative analysis was based on the scale of maximum principal stress; values obtained by means of quantitative analysis were expressed in MPa. Results. The implant-supported obturator prostheses tended to rotate toward the surgical resection, the region with no osseous support. Tensile and compressive stresses in the gingival mucosa and in the cortical bone increased as the osseous support and the numbers of implants and clips diminished. Conclusions. All evaluated bar-clip retention systems displayed a tendency toward dislodgment of the obturator prosthesis, increasing as the osseous resection area amplified. The osseous tensile and compressive stresses resulting from the bar-clip retention system for Okay Classes Ib, II, and III maxillectomy may not be favorable to the survival rate of implants. (J Prosthet Dent 2014;-:---)
Clinical Implications The professional must be aware of the stability provided and the unfavorable functional stress that may occur when implant-supported maxillary obturator prostheses and bar-clip retention systems are used. Maxillary obturator prostheses should satisfactorily minimize the functional disturbances that may occur during masticatory function and speech to improve the health-related quality of life and social reintegration of patients. Nevertheless, esthetics are frequently compromised, and the patient may experience psychological
issues after a maxillectomy.1-9 Extensive maxillary bone loss negatively affects the biomechanics of a maxillary prosthesis, demanding specific and complex designs to meet all patient problems and needs.3,10-12 A prosthetic design that is appropriate for specific oral conditions is important to the improvement of the
retention, stability, and durability of the obturator prosthesis. Patients with edentulism represent a challenge, and implant-supported obturators provide better results regarding the functional restoration of mastication and speech.2 The retention system must be evaluated for stability and the implants assessed for the osseointegration and
Portions of this research were presented at the annual meeting of the American Academy of Maxillofacial Prosthetics and the International Academy of Maxillofacial Rehabilitation, Santa Ana Pueblo, New Mexico, October 2013. a
Dentist, Department of Maxillofacial Surgery, Prosthesis and Traumatology. Associate Professor, Department of Maxillofacial Surgery, Prosthesis and Traumatology.
b
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Volume preservation of the periimplant marginal bone. Unfavorable stress conditions generated by occlusal overload, considering intensity, direction, duration, and speed, may lead to the loss of retention elements.13 This study evaluated the biomechanics of implanted-supported obturator prostheses in the simulated maxillary resections of edentulous maxillae corresponding to the Okay classification of the defect, with no previous surgical reconstruction with microvascular surgical techniques: Class Ib, defects that involve any portion of the maxillary alveolus, and dentition posterior to the canines; Class II, defects that involve any portion of the tooth-bearing maxillary alveolus but that also include the canine region and the anterior margin of the defect within the premaxilla; and Class III, defects that involve any part of the tooth-bearing maxillary alveolus and that include both canines, such as anterior transverse palatectomy involving more than half of the palatal surface.14 The Aramany classification considers patients with partial edentulism and maxillary resection, to define the principles of framework design for a partial removable dental prosthesis.15 Thus, maxillary resection in Aramany Class I corresponds to Okay Class II, Aramany Class II to Okay Class Ib, and Aramany Class IV to Okay Class III.14,15 The finite element method is an important research tool for biomechanical analyses in dental research. This method has the advantages of being noninvasive, allowing the visualization of superimposed structures, and enabling the stipulation of the material properties of anatomic craniofacial structures.16 Such a method may establish the location, magnitude, and direction of an applied force, as it may also allocate stress points that can be theoretically measured.17 Additionally, it can be repeated as many times as necessary, given that it does not affect the physical properties of the analyzed materials.17-21 The finite element analysis of maxillary bone stress caused by
Aramany Class IV partial removable dental obturator prostheses found that the tensile stress under posterior and anterior loads was greater than the compressive stress, regardless of bone region. Likewise, the amount of tensile and compressive stress did not exceed the physiologic limits of the maxillary bone tissue.22 Nevertheless, implant-supported rehabilitation in edentulous maxillectomy patients faces another challenge, because occlusal force causes a certain amount of stress on the framework, which is, in turn, transferred to implant and periimplant bone. A single 1-piece superstructure creates bending moments and generates a more complex stress distribution that may increase periimplant stress.13 The cantilever length, the implant design and positioning, and the mechanical properties and morphology of the bone can all interfere with the load transmission mechanisms and result in bone overloading.23 The use of remote implant anchorage, such as the zygomatic implant, to avoid adverse load patterns for the rehabilitation of maxillary defects can provide support for cantilevered prosthetic extensions and reduce stress to implants in native sites.24 The finite element analysis of zygomatic implant techniques for severely atrophic edentulous maxillae found, from a biomechanical perspective, that zygomatic implants give rise to quite homogeneous force transference.20,25 This study evaluated the biomechanics of implanted-supported obturator prostheses in 3-dimensional (3D) digital models, simulating the maxillary resections of edentulous maxilla corresponding to Okay Classes Ib, II, and III with no surgical reconstruction or zygomatic implant. The purpose of this study was to verify if there is a relationship between compressive and tension stress with the bone remaining area and the number of the implants.
MATERIAL AND METHODS After approval was obtained from the Ethics Committee of the University
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of São Paulo Dental School, a young adult man with no congenital or acquired defect in the craniofacial region underwent a computed tomography (CT) scan (LightSpeed 16 Pro; GE Medical Systems). The CT data, recorded as Digital Imaging and Communications in Medicine (DICOM) files, were exported to a software package (InVesalius Software, v1.0; Renato Archer Information Technology Center) for image segmentation and conversion to stereolithography (STL) format. The STL files were transferred to another software package (Magics X Service Pack 2, v1.1.17; Materialise), which was used to evaluate the files for possible image distortion.18,19 They were then imported into another software package (Rhinoceros 3D; Robert McNeel & Associates) and computer processed (Workstation Sun Microsystem ultra 20 M2) to generate 3D computer-aided design (CAD) models. The lining mucosa was edited with the same software. The CAD models produced in the previous steps served as the basis for the creation of a finite element mesh. The data from the CAD model were imported to a software package (Femap with NEi Nastran; NEI Software) in 3 different file formats: Step 214 (*.stp), Acis (*.sat), and Parasolid (*.x_t). This software evaluated the model for possible geometric inconsistencies (such as the absence of surfaces or curves and the formation of spaces between surfaces) and for dimensional inconsistencies, which were standardized in millimeters. Dentoflex provided implant 3D models and universal castable long abutments. The 11.50-mm-long implants were positioned in the lateral incisor, first premolar, and molar regions, and 13.50-mm-long implants were positioned in the canine region. The location of the implants was established according to the 3D models: for Okay Class Ib, 6 implants were inserted in the canine and lateral incisors on both sides and in the left first premolar and molar regions, designated as 1a, 1b, 2a, 2b, 3a, and
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3b (Fig. 1); for Okay Class II, 4 implants were positioned in the left lateral incisor, canine, first premolar,
and molar regions, designated as 2a, 2b, 3a, and 3b (Fig. 2); and for Okay Class III, 2 implants were placed in the
1 Okay Class Ib. Bar-clip retention system: 6 implants inserted in canine and lateral incisors on both sides and in left first premolar and molar regions, designated as 1a, 1b, 2a, 2b, 3a, and 3b.
2 Okay Class II. Bar-clip retention system: 4 implants positioned in left lateral incisor, canine, first premolar, and molar regions, designated as 2a, 2b, 3a, and 3b.
left first premolar and molar regions, designated as 3a and 3b (Fig. 3). Each of the 3D models simulating Okay Classes Ib, II, and III received an implant-supported edentulous maxillary obturator prosthesis with the barclip retention system (Fig. 4). The obturator prosthesis design did not take into account the vertical extension of the defect. The polymeric body of the prosthesis, including the platforms representing the anterior and posterior teeth, was edited with software (Rhinoceros, v4.0; Robert McNeel & Associates). The program used quadratic tetrahedral elements to determine volume.17,21 The finite element mesh was created in a controlled manner, and the geometry reached 126 301 elements and 211 255 nodes in the Okay Class Ib model, 76 721 elements and 135 837 nodes in the Okay Class II model, and 56 216 elements and 99 575 nodes in the Okay Class III model. The elastic modulus (Young modulus) and Poisson ratio were determined for each anatomic structure and material: acrylic resin, artificial teeth, mucosa, cancellous bone, cortical bone, implant, bar, and plastic clip (Table I).26-28 All elements were bodies with isotropic physical characteristics. To process the mesh, it was determined that the model would have to demonstrate linear elastic mechanical behavior.17,21 Simultaneously, a force of 80 N was applied to the occlusal platforms, representing the posterior teeth, and a force of 35 N was applied to the incisal platforms, representing the anterior teeth of the obturator prosthesis. These values represent the average maximum posterior and anterior occlusal force observed for men and women wearing obturator prostheses (Fig. 5).29
RESULTS
3 Okay Class III. Bar-clip retention system: 2 implants placed in left first premolar and molar regions, designated as 3a and 3b.
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Results are based on a qualitative analysis corresponding to the scale of maximum principal stress and represented by a color scale and a
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4 Simultaneous anterior (35 N) and posterior (80 N) platform loads.
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displacement of 6.12 mm of the obturator prosthesis on the region with no osseous support. There was an almost uniform maximum tensile stress distribution in the gingival mucosa (3.03 MPa). Maximum tensile stress reached 50.10 MPa in the distal cortical area corresponding to the implant placed in the first molar tooth region. Cancellous bone had 3.27 MPa of maximum tensile stress in the palatal cervical region for the first premolar and the same value for the vestibular cervical area of the implant located in the first molar region (Fig. 8).
DISCUSSION Young modulus and Poisson ratio for anatomic structures and materials
Table I.
Young Modulus Poisson (MPa) Ratio
Material Acrylic resin26
3000
0.35
Artificial teeth27
2800
0.28
Cancellous bone28
1370
0.30
13 700
0.30
110 000
0.33
218 000
0.35
3000
0.28
Cortical bone28 Implant
28
Bar28 Plastic clip
27
quantitative analysis expressed in MPa.
Okay Class Ib The forces applied to the incisal and occlusal platforms resulted in a maximum dislodgment of 0.45 mm of the prostheses in the direction of the region with no osseous support. The gingival mucosa presented a uniform compressive stress, with a maximum value of 2.36 MPa. A high maximum tensile stress (15.00 MPa) was observed in the mesial area of the cortical bone corresponding to the implant on the lateral incisor region. Additionally, the anterior alveolar bone crest underwent a maximum tensile stress of 6.42 MPa. Regarding the
cancellous bone, the highest maximum tensile stress (2.51 MPa) was observed in the vestibular region of the first implant, near the resection line, corresponding to the right canine tooth (Fig. 6).
Okay Class II Stress was observed as a result of the loads applied to the anterior and posterior platforms at the same time, causing a maximum displacement of the prosthesis of 3.15 mm in the region with no osseous support. Stress distribution on the gingival mucosa was almost uniform, giving rise to a maximum compressive stress of 3.17 MPa. Maximum tensile stress in the vestibular area of the cortical bone corresponding to the implant positioned at the canine region reached 28.76 MPa, and a maximum compressive stress of 7.16 MPa was noticed on the palatal bone. All implants displayed some tensile stress in the cancellous bone, ranging from 2.45 MPa to 1.68 MPa for the implants located in the regions of the lateral incisor and canine and from 0.91 MPa to 0.14 MPa for the implants located in the region of the first premolar and first molar (Fig. 7).
Okay Class III In Okay Class III, the same loads applied before caused a maximum
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The results support that exist exist a relationship between compressive and tension stress with the bone remaining area and number of implants. The maximum occlusal force attained by obturator prosthesis wearers during a clinical assessment of the masticatory function has been found to be 50% lower than that attained by wearers of conventional complete dentures.30 This study applied a force of 80 N to the occlusal platform, representative of the posterior teeth of the obturator prosthesis, and a force of 35 N to the incisal platform, representative of the anterior teeth. These values were based on a previous study23 of patients with prosthetic obturation of maxillary defects and correspond to the average maximum occlusal and incisal forces observed in men and women.29 Overdentures have some advantages over a conventional complete denture, as the preservation of the residual alveolar ridge improves stability, retention, comfort, and masticatory efficiency, leading to greater satisfaction and a better quality of life for patients.27 These advantages become even more evident in maxillectomy patients, because the retention system will dissipate the functional forces to the remaining anatomic structures. The oral mucosa deadens and distributes the masticatory forces applied to the prosthesis to the bone surface and is fully able to tolerate short-term
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pressures during mastication. Yet in patients with no surgical reconstruction, the mucosa covering the defect area is not supposed to absorb the masticatory forces, maximizing the expectation for the remaining oral mucosa. The obturator prosthesis 3D
design did not take into account the extension of the defect or the lateral anatomic structures surrounding the defect. The presence of a defect portion indeed defines the obturator prosthesis and may reduce clinical dislodgment by improving the stability of the
5 Bar-clip retention system.
6 Maximum principal stress (MPa) distribution in Okay Class Ib: compressive stress (blue); tensile stress (red).
7 Maximum principal stress (MPa) distribution in Okay Class II compressive stress (blue); tensile stress (red).
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prosthesis. However, the study design was focused not on the stability of the prosthesis but on evidencing the dissipation of tensile and compressive stresses on the remaining maxillary anatomic structures. An analysis of conventional prosthesis dislodgment found displacement of up to 1 mm. The amplitude of movement in the obturator prosthesis is higher than in a conventional denture and varies from individual to individual.31 The present analysis considered maximum principal stress, which is associated with displacement analysis and may be used to illustrate movement tendencies. All of the retention system designs applied in the obturator prosthesis evaluated in this study had a dislodgment of the prosthesis toward the area with no osseous support, with the rotation axis located close to the anterior resection line; the larger the defect, the wider the displacement of the obturator prosthesis. The maximum vertical dislodgment was lower when there were implants on both sides of the maxilla (Class Ib, 0.45 mm). As the defect increased with removal of a hemimaxilla, the displacement of the prosthesis expanded substantially (Class II, 3.15 mm), and extreme dislodgment occurred when only the posterior part of the unilateral maxilla was preserved during surgery (Class III, 6.12 mm). This indicates that the location and the number of implants have a direct influence on the stability provided by an obturator prosthesis. The maximum compressive stress observed at the gingival mucosa was 2.36 MPa for Class Ib and 3.17 MPa for Class II, whereas the maximum tensile stress of 3.03 MPa was observed for Class III. The tensile stress observed in Class III may result from the greater dislodgment of the prosthesis observed in Class III cases, versus Class II and Class I cases, as a result of a more extensive surgical resection and limited bone tissue area. One of the most relevant purposes of the retention system provided by using osseointegrated implants is to preserve the remaining support structures, given that both osseointegration and the maintenance
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8 Maximum principal stress (MPa) distribution in Okay Class III: compressive stress (blue); tensile stress (red). of the level of the marginal bone are critical for the success of the rehabilitation.2 The vestibular cortical bone had a maximum tensile stress of 0.28 MPa for Class Ib and 0.82 MPa for Class II. The maximum tensile stress of 6.02 MPa observed at the distal vestibular cortical bone and the maximum compressive stress of 2.96 MPa observed in other regions of the Class III model may result from the higher dislodgment presented by the corresponding obturator prosthesis. Although how mechanical loading precisely affects osteoblastic and osteoclastic activity is unknown, studies of the quantification of stress, strain, or strain energy density have been proposed to evaluate mechanical stimuli in bone remodeling. The bone has the ability to change its structure in response to mechanical stress induced by loading an implant within defined physiologic limits. Forces of low intensity in cancellous bone may lead to bone resorption.32,33 The underload threshold is 2 MPa, below which bone resorption occurs. Bone stability occurs under stresses between 2 and 4 MPa, osteogenesis occurs under stresses between 4 and 8 MPa, and stresses greater than 8 MPa can lead once again to resorption.32 Both underload and overload imply biomechanical responses of bone and may determine osseointegration and remodeling after implant placement.34
In this experiment, the Okay Class Ib cortical bone model presented a tensile stress of 0.28 MPa in the vestibular region, 0.71 MPa in the palate, 6.24 MPa in the bone crest, and 18.78 MPa in the cervical region of implant 1b. According to the aforementioned limits,32 only the region of the anterior alveolar bone crest would be at the bone formation stage. The vestibular and palatal regions and the cervical region of implant 1b were susceptible to bone resorption. Only the cancellous bone in the cervical region of implant 1a displayed a tensile stress of 2.51 MPa, corresponding to bone stability. The other regions of implants 1a and 1b were suggestive of bone resorption, because stresses were below the physiologic stimulus for maintaining osseous stability (Fig. 6). The Okay Class II cortical bone had a maximum compressive stress of 7.16 MPa in the palate, suggesting bone formation. The vestibular cervical area of the implant 2b displayed a maximum tensile stress of 28.76 MPa, indicating bone resorption by being far beyond physiologic limits. The maximum tensile stress of 0.82 MPa in the vestibular alveolar bone crest was also suggestive of bone resorption in the present model, as a consequence of being under the load needed to maintain bone physiologic stability.32 Cancellous bone presented a maximum tensile stress of 2.45 MPa in the vestibular cervical region around implant 2a (Fig. 7). Stress
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values compatible with bone stability were observed only in the region of implant 2a. The other values of maximum tensile stress are suggestive of bone resorption, because loads are below those required for physiologic bone stability. The cortical bone of Okay Class III model had a maximum compressive stress of 2.96 MPa in the palate and in the vestibular bone crest regions, suggesting bone stability. In the distal cervical region of implant 3b, the maximum tensile stress was 50.10 MPa, and this value was suggestive of bone resorption as it greatly exceeded the physiologic load needed for bone remodeling (Fig. 8). A maximum tensile stress of 3.27 MPa was observed in the palatal cervical region of implant 3a and in the vestibular cervical region of implant 3b, and a maximum compressive stress of 2.03 MPa was displayed in the apical region of implant 3b. Given that these tensile and compressive stresses were within the physiologic limits of bone tissue, the results are suggestive of osseous stability in such regions.32 The biomechanics of an obturator prosthesis changed according to the number and position of the implants. The highest dissipation of forces occurred in the cervical bone tissue surrounding the second implants, that is, 1b (15.00 MPa), 2b (28.76 MPa), and 3b (50.10 MPa), which were close to the resection line. These high stress values may induce vertical bone loss and must be taken into consideration when designing an implant-supported obturator prosthesis for edentulous patients with maxillary resections but without previous surgical reconstruction with a bar-clip retention system.
CONCLUSION The maximum dislodgment of the obturator prosthesis increased with a decrease in the area of bone support, the number of implants, and the number of clips. The gingival mucosa, the cortical bone, and the cancellous bone were subjected to compressive and
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tensile stress that increased along with the decrease in the area of bone support, the number of implants, and the number of clips in the bar-clip retention system. The high tensile stresses observed in the cervical cortical bone around implants were above the physiologic threshold of bone tissue and suggestive of bone resorption. This fact jeopardizes the preservation of the remaining anatomic structures after maxillectomy and the success of the retention system. The finite element analysis undertaken here may be an important tool in predicting the success of implantsupported prosthetic rehabilitation of patients after maxillectomy.
REFERENCES 1. Rogers SN, Lowe D, McNally D, Brown JS, Vaughan ED. Health-related quality of life after maxillectomy: a comparison between prosthetic obturation and free flap. J Oral Maxillofac Surg 2003;61:174-81. 2. Fukuda M, Takahashi T, Nagai H, Iino M. Implant-supported edentulous maxillary obturators with bar attachments after maxillectomy. J Oral Maxillofac Surg 2004;62: 799-805. 3. Cune M, van Kampen F, van der Bilt A, Bosman F. Patient satisfaction and preference with magnet, bar-clip, and ball-socket retained mandibular implant overdentures: a cross-over clinical trial. Int J Prosthodont 2005;18:99-105. 4. Arigbede AO, Dosumu OO, Shaba P, Esan TA. Evaluation of speech in patients with partial surgically acquired defects: pre and post prosthetic obturation. J Contemp Dent Pract 2006;7:1-9. 5. Oh WS, Roumanas E. Dental implantassisted prosthetic rehabilitation of a patient with a bilateral maxillectomy defect secondary to mucormycosis. J Prosthet Dent 2006;96:88-95. 6. Aydin C, Delilbasi E, Yilmaz H, Karakoca S, Bal BT. Reconstruction of total maxillectomy defect with implant-retained obturator prosthesis. NY State Dent J 2007;73:38-41. 7. Eckardt A, Teltzrow T, Schulze A, Hoppe M, Kuettner C. Nasalance in patients with maxillary defectseReconstruction versus obturation. J Cranimaxillofac Surg 2007;35: 241-5. 8. Rismanchian M, Bajoghli F, Mostajeran Z, Fazel A, Eshkevari P. Effect of implants on maximum bite force in edentulous patients. J Oral Implantol 2009;35:196-200. 9. Leles CR, Leles JL, de Paula Souza C, Martins RR, Mendonça EF. Implant-supported obturator overdenture for extensive maxillary resection patient: a clinical report. J Prosthodont 2010;19:240-4.
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7 10. Myers RE, Mitchell DL. A photoelastic study of stress induced by framework design in a maxillary resection. J Prosthet Dent 1989;61: 590-4. 11. Mericske-Stern R, Venetz E, Fahrländer F, Bürgin W. In vivo force measurements on maxillary implants supporting a fixed prosthesis or an overdenture: a pilot study. J Prosthet Dent 2000;84:535-47. 12. Semper W, Heberer S, Nelson K. Retrospective analysis of bar-retained dentures with cantilever extension: marginal bone level changes around dental implants over time. Int J Oral Maxillofac Implants 2010;25: 385-93. 13. Alvarez-Arenal A, Brizuela-Velasco A, Dellanos-Lanchares H, Gonzalez-Gonzalez I. Should oral implants be splinted in a mandibular implant-supported fixed complete denture? A 3-dimensional-model finite element analysis. J Prosthet Dent 2014;112:508-14. 14. Okay DJ, Genden E, Buchbinder D, Urken M. Prosthodontic guidelines for surgical reconstruction of the maxilla: a classification system of defects. J Prosthet Dent 2001;86: 352-63. 15. Aramany MA. Basic principles of obturator design for partially edentulous patients, part II: design principles. J Prosthet Dent 1978;40: 656-62. 16. Sun J, Jiao T, Tie Y, Wang DM. Threedimensional finite element analysis of the application of attachment for obturator framework in unilateral maxillary defect. J Oral Rehabil 2008;35:695-9. 17. Gao J, Xu W, Ding Z. 3D finite element mesh generation of complicated tooth model based on CT slices. Comput Meth Programs Biomed 2006;28:916-24. 18. Viceconti M, Zannoni C, Testi D, Petrone M, Perticoni S, Quadrani P, et al. The multimod application framework: a rapid application development tool for computer aided medicine. Comput Methods Programs Biomed 2007;85:138-51. 19. Taddei F, Schileo E, Helgason B, Cristofolini L, Viceconti M. The material mapping strategy influences the accuracy of CT-based finite element models of bones: an evaluation against experimental measurements. Med Eng Phys 2007;29:973-9. 20. Ujigawa K, Kato Y, Kizu Y, Tonogi M, Yamane GY. Three-dimensional finite elemental analysis of zygomatic implants in craniofacial structures. Int J Oral Maxillofac Surg 2007;36:620-5. 21. Zhao L, Herman E, Patel PK. The structural implications of a unilateral facial skeletal cleft: a three-dimensional finite element model approach. Cleft Palate Craniofac J 2008;45:121-30. 22. Miyashita ER, Mattos BS, Noritomi PY, Navarro H. Finite element analysis of maxillary bone stress caused by Aramany Class IV obturator prostheses. J Prosthet Dent 2012;107:336-42. 23. Baggi L, Pastore S, Di Girolamo M, Vairo G. Implant-bone load transfer mechanisms in complete-arch prostheses supported by four implants: a three-dimensional finite element approach. J Prosthet Dent 2013;109:9-21.
24. Parel SM, Brånemark PI, Ohrnell LO, Svensson B. Remote implant anchorage for the rehabilitation of maxillary defects. J Prosthet Dent 2001;86:377-81. 25. Wen H, Guo W, Liang R, Xiang L, Long G, Wang T, et al. Finite element analysis of three zygomatic implant techniques for the severely atrophic edentulous maxilla. J Prosthet Dent 2014;111:203-15. 26. Ono T, Kohda H, Hori K, Nokubi T. Masticatory performance in postmaxillectomy patients with edentulous maxillae fitted with obturator prostheses. Int J Prosthodont 2007;20:145-50. 27. Assunção WG, Tabata LF, Barão VAR, Rocha EP. Comparison of stress distribution between complete denture and implantretained overdenture-2D FEA. J Oral Rehabil 2008;35:766-74. 28. Prakash V, DiSouza M, Adhikari R. A comparison of stress distribution and flexion among various designs of bar attachments for implant overdentures: a three dimensional finite element analysis. Indian J Dent Res 2009;20:31-6. 29. Matsuyama M, Tsukiyama Y, Tomioka M. Clinical assessment of chewing function of obturator prosthesis wearers by objective measure of masticatory performance and maximum occlusal force. Int J Prosthodont 2006;19:253-7. 30. Wedel A, Yontchev E, Carlsson GE, Ow R. Masticatory function in patients with congenital and acquired maxillofacial defects. J Prosthet Dent 1994;72:303-8. 31. Tsuchiya AM, Ueno T, Taniguchi H, Ohyama T. Mobility of the obturator prosthesis in hemimaxillectomy edentulous patients. J Med Dent Sci 1998;45:19-27. 32. Li J, Li H, Shi L, Fok AS, Ucer C, Devlin H, et al. A mathematical model for simulating the bone remodeling process under mechanical stimulus. Dent Mater 2007;23: 1073-8. 33. Rahimi A, Bourauel C, Jager A, Gedrange T, Heinemann F. Load transfer by fine threading the implant neckea FEM study. J Physiol Pharmacol 2009;60(suppl 8):107-12. 34. Lin CL, Lin YH, Chang SH. Multi-factorial analysis of variables influencing the bone loss of an implant placed in the maxilla: prediction using FEA and SED bone remodeling algorithm. J Biomech 2010;43:644-51. Corresponding author: Dr Beatriz Silva Câmara Mattos Avenida Lineu Prestes, 2227 Cidade Universitária São Paulo, SP 05508-000 BRAZIL E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications The professional must be aware of the stability provided and the unfavorable functional stress that may occur when implant-supported maxillary obturator prostheses and bar-clip retention systems are used. Maxillary obturator prostheses should satisfactorily minimize the functional disturbances that may occur during masticatory function and speech to improve the health-related quality of life and social reintegration of patients. Nevertheless, esthetics are frequently compromised, and the patient may experience psychological
issues after a maxillectomy.1-9 Extensive maxillary bone loss negatively affects the biomechanics of a maxillary prosthesis, demanding specific and complex designs to meet all patient problems and needs.3,10-12 A prosthetic design that is appropriate for specific oral conditions is important to the improvement of the
retention, stability, and durability of the obturator prosthesis. Patients with edentulism represent a challenge, and implant-supported obturators provide better results regarding the functional restoration of mastication and speech.2 The retention system must be evaluated for stability and the implants assessed for the osseointegration and
Portions of this research were presented at the annual meeting of the American Academy of Maxillofacial Prosthetics and the International Academy of Maxillofacial Rehabilitation, Santa Ana Pueblo, New Mexico, October 2013. a
Dentist, Department of Maxillofacial Surgery, Prosthesis and Traumatology. Associate Professor, Department of Maxillofacial Surgery, Prosthesis and Traumatology.
b
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Volume preservation of the periimplant marginal bone. Unfavorable stress conditions generated by occlusal overload, considering intensity, direction, duration, and speed, may lead to the loss of retention elements.13 This study evaluated the biomechanics of implanted-supported obturator prostheses in the simulated maxillary resections of edentulous maxillae corresponding to the Okay classification of the defect, with no previous surgical reconstruction with microvascular surgical techniques: Class Ib, defects that involve any portion of the maxillary alveolus, and dentition posterior to the canines; Class II, defects that involve any portion of the tooth-bearing maxillary alveolus but that also include the canine region and the anterior margin of the defect within the premaxilla; and Class III, defects that involve any part of the tooth-bearing maxillary alveolus and that include both canines, such as anterior transverse palatectomy involving more than half of the palatal surface.14 The Aramany classification considers patients with partial edentulism and maxillary resection, to define the principles of framework design for a partial removable dental prosthesis.15 Thus, maxillary resection in Aramany Class I corresponds to Okay Class II, Aramany Class II to Okay Class Ib, and Aramany Class IV to Okay Class III.14,15 The finite element method is an important research tool for biomechanical analyses in dental research. This method has the advantages of being noninvasive, allowing the visualization of superimposed structures, and enabling the stipulation of the material properties of anatomic craniofacial structures.16 Such a method may establish the location, magnitude, and direction of an applied force, as it may also allocate stress points that can be theoretically measured.17 Additionally, it can be repeated as many times as necessary, given that it does not affect the physical properties of the analyzed materials.17-21 The finite element analysis of maxillary bone stress caused by
Aramany Class IV partial removable dental obturator prostheses found that the tensile stress under posterior and anterior loads was greater than the compressive stress, regardless of bone region. Likewise, the amount of tensile and compressive stress did not exceed the physiologic limits of the maxillary bone tissue.22 Nevertheless, implant-supported rehabilitation in edentulous maxillectomy patients faces another challenge, because occlusal force causes a certain amount of stress on the framework, which is, in turn, transferred to implant and periimplant bone. A single 1-piece superstructure creates bending moments and generates a more complex stress distribution that may increase periimplant stress.13 The cantilever length, the implant design and positioning, and the mechanical properties and morphology of the bone can all interfere with the load transmission mechanisms and result in bone overloading.23 The use of remote implant anchorage, such as the zygomatic implant, to avoid adverse load patterns for the rehabilitation of maxillary defects can provide support for cantilevered prosthetic extensions and reduce stress to implants in native sites.24 The finite element analysis of zygomatic implant techniques for severely atrophic edentulous maxillae found, from a biomechanical perspective, that zygomatic implants give rise to quite homogeneous force transference.20,25 This study evaluated the biomechanics of implanted-supported obturator prostheses in 3-dimensional (3D) digital models, simulating the maxillary resections of edentulous maxilla corresponding to Okay Classes Ib, II, and III with no surgical reconstruction or zygomatic implant. The purpose of this study was to verify if there is a relationship between compressive and tension stress with the bone remaining area and the number of the implants.
MATERIAL AND METHODS After approval was obtained from the Ethics Committee of the University
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of São Paulo Dental School, a young adult man with no congenital or acquired defect in the craniofacial region underwent a computed tomography (CT) scan (LightSpeed 16 Pro; GE Medical Systems). The CT data, recorded as Digital Imaging and Communications in Medicine (DICOM) files, were exported to a software package (InVesalius Software, v1.0; Renato Archer Information Technology Center) for image segmentation and conversion to stereolithography (STL) format. The STL files were transferred to another software package (Magics X Service Pack 2, v1.1.17; Materialise), which was used to evaluate the files for possible image distortion.18,19 They were then imported into another software package (Rhinoceros 3D; Robert McNeel & Associates) and computer processed (Workstation Sun Microsystem ultra 20 M2) to generate 3D computer-aided design (CAD) models. The lining mucosa was edited with the same software. The CAD models produced in the previous steps served as the basis for the creation of a finite element mesh. The data from the CAD model were imported to a software package (Femap with NEi Nastran; NEI Software) in 3 different file formats: Step 214 (*.stp), Acis (*.sat), and Parasolid (*.x_t). This software evaluated the model for possible geometric inconsistencies (such as the absence of surfaces or curves and the formation of spaces between surfaces) and for dimensional inconsistencies, which were standardized in millimeters. Dentoflex provided implant 3D models and universal castable long abutments. The 11.50-mm-long implants were positioned in the lateral incisor, first premolar, and molar regions, and 13.50-mm-long implants were positioned in the canine region. The location of the implants was established according to the 3D models: for Okay Class Ib, 6 implants were inserted in the canine and lateral incisors on both sides and in the left first premolar and molar regions, designated as 1a, 1b, 2a, 2b, 3a, and
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3b (Fig. 1); for Okay Class II, 4 implants were positioned in the left lateral incisor, canine, first premolar,
and molar regions, designated as 2a, 2b, 3a, and 3b (Fig. 2); and for Okay Class III, 2 implants were placed in the
1 Okay Class Ib. Bar-clip retention system: 6 implants inserted in canine and lateral incisors on both sides and in left first premolar and molar regions, designated as 1a, 1b, 2a, 2b, 3a, and 3b.
2 Okay Class II. Bar-clip retention system: 4 implants positioned in left lateral incisor, canine, first premolar, and molar regions, designated as 2a, 2b, 3a, and 3b.
left first premolar and molar regions, designated as 3a and 3b (Fig. 3). Each of the 3D models simulating Okay Classes Ib, II, and III received an implant-supported edentulous maxillary obturator prosthesis with the barclip retention system (Fig. 4). The obturator prosthesis design did not take into account the vertical extension of the defect. The polymeric body of the prosthesis, including the platforms representing the anterior and posterior teeth, was edited with software (Rhinoceros, v4.0; Robert McNeel & Associates). The program used quadratic tetrahedral elements to determine volume.17,21 The finite element mesh was created in a controlled manner, and the geometry reached 126 301 elements and 211 255 nodes in the Okay Class Ib model, 76 721 elements and 135 837 nodes in the Okay Class II model, and 56 216 elements and 99 575 nodes in the Okay Class III model. The elastic modulus (Young modulus) and Poisson ratio were determined for each anatomic structure and material: acrylic resin, artificial teeth, mucosa, cancellous bone, cortical bone, implant, bar, and plastic clip (Table I).26-28 All elements were bodies with isotropic physical characteristics. To process the mesh, it was determined that the model would have to demonstrate linear elastic mechanical behavior.17,21 Simultaneously, a force of 80 N was applied to the occlusal platforms, representing the posterior teeth, and a force of 35 N was applied to the incisal platforms, representing the anterior teeth of the obturator prosthesis. These values represent the average maximum posterior and anterior occlusal force observed for men and women wearing obturator prostheses (Fig. 5).29
RESULTS
3 Okay Class III. Bar-clip retention system: 2 implants placed in left first premolar and molar regions, designated as 3a and 3b.
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Results are based on a qualitative analysis corresponding to the scale of maximum principal stress and represented by a color scale and a
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4 Simultaneous anterior (35 N) and posterior (80 N) platform loads.
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displacement of 6.12 mm of the obturator prosthesis on the region with no osseous support. There was an almost uniform maximum tensile stress distribution in the gingival mucosa (3.03 MPa). Maximum tensile stress reached 50.10 MPa in the distal cortical area corresponding to the implant placed in the first molar tooth region. Cancellous bone had 3.27 MPa of maximum tensile stress in the palatal cervical region for the first premolar and the same value for the vestibular cervical area of the implant located in the first molar region (Fig. 8).
DISCUSSION Young modulus and Poisson ratio for anatomic structures and materials
Table I.
Young Modulus Poisson (MPa) Ratio
Material Acrylic resin26
3000
0.35
Artificial teeth27
2800
0.28
Cancellous bone28
1370
0.30
13 700
0.30
110 000
0.33
218 000
0.35
3000
0.28
Cortical bone28 Implant
28
Bar28 Plastic clip
27
quantitative analysis expressed in MPa.
Okay Class Ib The forces applied to the incisal and occlusal platforms resulted in a maximum dislodgment of 0.45 mm of the prostheses in the direction of the region with no osseous support. The gingival mucosa presented a uniform compressive stress, with a maximum value of 2.36 MPa. A high maximum tensile stress (15.00 MPa) was observed in the mesial area of the cortical bone corresponding to the implant on the lateral incisor region. Additionally, the anterior alveolar bone crest underwent a maximum tensile stress of 6.42 MPa. Regarding the
cancellous bone, the highest maximum tensile stress (2.51 MPa) was observed in the vestibular region of the first implant, near the resection line, corresponding to the right canine tooth (Fig. 6).
Okay Class II Stress was observed as a result of the loads applied to the anterior and posterior platforms at the same time, causing a maximum displacement of the prosthesis of 3.15 mm in the region with no osseous support. Stress distribution on the gingival mucosa was almost uniform, giving rise to a maximum compressive stress of 3.17 MPa. Maximum tensile stress in the vestibular area of the cortical bone corresponding to the implant positioned at the canine region reached 28.76 MPa, and a maximum compressive stress of 7.16 MPa was noticed on the palatal bone. All implants displayed some tensile stress in the cancellous bone, ranging from 2.45 MPa to 1.68 MPa for the implants located in the regions of the lateral incisor and canine and from 0.91 MPa to 0.14 MPa for the implants located in the region of the first premolar and first molar (Fig. 7).
Okay Class III In Okay Class III, the same loads applied before caused a maximum
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The results support that exist exist a relationship between compressive and tension stress with the bone remaining area and number of implants. The maximum occlusal force attained by obturator prosthesis wearers during a clinical assessment of the masticatory function has been found to be 50% lower than that attained by wearers of conventional complete dentures.30 This study applied a force of 80 N to the occlusal platform, representative of the posterior teeth of the obturator prosthesis, and a force of 35 N to the incisal platform, representative of the anterior teeth. These values were based on a previous study23 of patients with prosthetic obturation of maxillary defects and correspond to the average maximum occlusal and incisal forces observed in men and women.29 Overdentures have some advantages over a conventional complete denture, as the preservation of the residual alveolar ridge improves stability, retention, comfort, and masticatory efficiency, leading to greater satisfaction and a better quality of life for patients.27 These advantages become even more evident in maxillectomy patients, because the retention system will dissipate the functional forces to the remaining anatomic structures. The oral mucosa deadens and distributes the masticatory forces applied to the prosthesis to the bone surface and is fully able to tolerate short-term
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pressures during mastication. Yet in patients with no surgical reconstruction, the mucosa covering the defect area is not supposed to absorb the masticatory forces, maximizing the expectation for the remaining oral mucosa. The obturator prosthesis 3D
design did not take into account the extension of the defect or the lateral anatomic structures surrounding the defect. The presence of a defect portion indeed defines the obturator prosthesis and may reduce clinical dislodgment by improving the stability of the
5 Bar-clip retention system.
6 Maximum principal stress (MPa) distribution in Okay Class Ib: compressive stress (blue); tensile stress (red).
7 Maximum principal stress (MPa) distribution in Okay Class II compressive stress (blue); tensile stress (red).
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prosthesis. However, the study design was focused not on the stability of the prosthesis but on evidencing the dissipation of tensile and compressive stresses on the remaining maxillary anatomic structures. An analysis of conventional prosthesis dislodgment found displacement of up to 1 mm. The amplitude of movement in the obturator prosthesis is higher than in a conventional denture and varies from individual to individual.31 The present analysis considered maximum principal stress, which is associated with displacement analysis and may be used to illustrate movement tendencies. All of the retention system designs applied in the obturator prosthesis evaluated in this study had a dislodgment of the prosthesis toward the area with no osseous support, with the rotation axis located close to the anterior resection line; the larger the defect, the wider the displacement of the obturator prosthesis. The maximum vertical dislodgment was lower when there were implants on both sides of the maxilla (Class Ib, 0.45 mm). As the defect increased with removal of a hemimaxilla, the displacement of the prosthesis expanded substantially (Class II, 3.15 mm), and extreme dislodgment occurred when only the posterior part of the unilateral maxilla was preserved during surgery (Class III, 6.12 mm). This indicates that the location and the number of implants have a direct influence on the stability provided by an obturator prosthesis. The maximum compressive stress observed at the gingival mucosa was 2.36 MPa for Class Ib and 3.17 MPa for Class II, whereas the maximum tensile stress of 3.03 MPa was observed for Class III. The tensile stress observed in Class III may result from the greater dislodgment of the prosthesis observed in Class III cases, versus Class II and Class I cases, as a result of a more extensive surgical resection and limited bone tissue area. One of the most relevant purposes of the retention system provided by using osseointegrated implants is to preserve the remaining support structures, given that both osseointegration and the maintenance
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8 Maximum principal stress (MPa) distribution in Okay Class III: compressive stress (blue); tensile stress (red). of the level of the marginal bone are critical for the success of the rehabilitation.2 The vestibular cortical bone had a maximum tensile stress of 0.28 MPa for Class Ib and 0.82 MPa for Class II. The maximum tensile stress of 6.02 MPa observed at the distal vestibular cortical bone and the maximum compressive stress of 2.96 MPa observed in other regions of the Class III model may result from the higher dislodgment presented by the corresponding obturator prosthesis. Although how mechanical loading precisely affects osteoblastic and osteoclastic activity is unknown, studies of the quantification of stress, strain, or strain energy density have been proposed to evaluate mechanical stimuli in bone remodeling. The bone has the ability to change its structure in response to mechanical stress induced by loading an implant within defined physiologic limits. Forces of low intensity in cancellous bone may lead to bone resorption.32,33 The underload threshold is 2 MPa, below which bone resorption occurs. Bone stability occurs under stresses between 2 and 4 MPa, osteogenesis occurs under stresses between 4 and 8 MPa, and stresses greater than 8 MPa can lead once again to resorption.32 Both underload and overload imply biomechanical responses of bone and may determine osseointegration and remodeling after implant placement.34
In this experiment, the Okay Class Ib cortical bone model presented a tensile stress of 0.28 MPa in the vestibular region, 0.71 MPa in the palate, 6.24 MPa in the bone crest, and 18.78 MPa in the cervical region of implant 1b. According to the aforementioned limits,32 only the region of the anterior alveolar bone crest would be at the bone formation stage. The vestibular and palatal regions and the cervical region of implant 1b were susceptible to bone resorption. Only the cancellous bone in the cervical region of implant 1a displayed a tensile stress of 2.51 MPa, corresponding to bone stability. The other regions of implants 1a and 1b were suggestive of bone resorption, because stresses were below the physiologic stimulus for maintaining osseous stability (Fig. 6). The Okay Class II cortical bone had a maximum compressive stress of 7.16 MPa in the palate, suggesting bone formation. The vestibular cervical area of the implant 2b displayed a maximum tensile stress of 28.76 MPa, indicating bone resorption by being far beyond physiologic limits. The maximum tensile stress of 0.82 MPa in the vestibular alveolar bone crest was also suggestive of bone resorption in the present model, as a consequence of being under the load needed to maintain bone physiologic stability.32 Cancellous bone presented a maximum tensile stress of 2.45 MPa in the vestibular cervical region around implant 2a (Fig. 7). Stress
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values compatible with bone stability were observed only in the region of implant 2a. The other values of maximum tensile stress are suggestive of bone resorption, because loads are below those required for physiologic bone stability. The cortical bone of Okay Class III model had a maximum compressive stress of 2.96 MPa in the palate and in the vestibular bone crest regions, suggesting bone stability. In the distal cervical region of implant 3b, the maximum tensile stress was 50.10 MPa, and this value was suggestive of bone resorption as it greatly exceeded the physiologic load needed for bone remodeling (Fig. 8). A maximum tensile stress of 3.27 MPa was observed in the palatal cervical region of implant 3a and in the vestibular cervical region of implant 3b, and a maximum compressive stress of 2.03 MPa was displayed in the apical region of implant 3b. Given that these tensile and compressive stresses were within the physiologic limits of bone tissue, the results are suggestive of osseous stability in such regions.32 The biomechanics of an obturator prosthesis changed according to the number and position of the implants. The highest dissipation of forces occurred in the cervical bone tissue surrounding the second implants, that is, 1b (15.00 MPa), 2b (28.76 MPa), and 3b (50.10 MPa), which were close to the resection line. These high stress values may induce vertical bone loss and must be taken into consideration when designing an implant-supported obturator prosthesis for edentulous patients with maxillary resections but without previous surgical reconstruction with a bar-clip retention system.
CONCLUSION The maximum dislodgment of the obturator prosthesis increased with a decrease in the area of bone support, the number of implants, and the number of clips. The gingival mucosa, the cortical bone, and the cancellous bone were subjected to compressive and
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tensile stress that increased along with the decrease in the area of bone support, the number of implants, and the number of clips in the bar-clip retention system. The high tensile stresses observed in the cervical cortical bone around implants were above the physiologic threshold of bone tissue and suggestive of bone resorption. This fact jeopardizes the preservation of the remaining anatomic structures after maxillectomy and the success of the retention system. The finite element analysis undertaken here may be an important tool in predicting the success of implantsupported prosthetic rehabilitation of patients after maxillectomy.
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24. Parel SM, Brånemark PI, Ohrnell LO, Svensson B. Remote implant anchorage for the rehabilitation of maxillary defects. J Prosthet Dent 2001;86:377-81. 25. Wen H, Guo W, Liang R, Xiang L, Long G, Wang T, et al. Finite element analysis of three zygomatic implant techniques for the severely atrophic edentulous maxilla. J Prosthet Dent 2014;111:203-15. 26. Ono T, Kohda H, Hori K, Nokubi T. Masticatory performance in postmaxillectomy patients with edentulous maxillae fitted with obturator prostheses. Int J Prosthodont 2007;20:145-50. 27. Assunção WG, Tabata LF, Barão VAR, Rocha EP. Comparison of stress distribution between complete denture and implantretained overdenture-2D FEA. J Oral Rehabil 2008;35:766-74. 28. Prakash V, DiSouza M, Adhikari R. A comparison of stress distribution and flexion among various designs of bar attachments for implant overdentures: a three dimensional finite element analysis. Indian J Dent Res 2009;20:31-6. 29. Matsuyama M, Tsukiyama Y, Tomioka M. Clinical assessment of chewing function of obturator prosthesis wearers by objective measure of masticatory performance and maximum occlusal force. Int J Prosthodont 2006;19:253-7. 30. Wedel A, Yontchev E, Carlsson GE, Ow R. Masticatory function in patients with congenital and acquired maxillofacial defects. J Prosthet Dent 1994;72:303-8. 31. Tsuchiya AM, Ueno T, Taniguchi H, Ohyama T. Mobility of the obturator prosthesis in hemimaxillectomy edentulous patients. J Med Dent Sci 1998;45:19-27. 32. Li J, Li H, Shi L, Fok AS, Ucer C, Devlin H, et al. A mathematical model for simulating the bone remodeling process under mechanical stimulus. Dent Mater 2007;23: 1073-8. 33. Rahimi A, Bourauel C, Jager A, Gedrange T, Heinemann F. Load transfer by fine threading the implant neckea FEM study. J Physiol Pharmacol 2009;60(suppl 8):107-12. 34. Lin CL, Lin YH, Chang SH. Multi-factorial analysis of variables influencing the bone loss of an implant placed in the maxilla: prediction using FEA and SED bone remodeling algorithm. J Biomech 2010;43:644-51. Corresponding author: Dr Beatriz Silva Câmara Mattos Avenida Lineu Prestes, 2227 Cidade Universitária São Paulo, SP 05508-000 BRAZIL E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
