Finite Element Analysis Of Implant-assisted Removable .

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RESEARCH AND EDUCATIONFinite element analysis of implant-assisted removable partialdentures: Framework design considerationsReza Shahmiri, MengStMedTecha and Raj Das, PhDbThe use of an implant with aABSTRACTtooth- and tissue-supportedStatement of problem. Connecting an acrylic resin base to both a metal framework and a rigidlyKennedy class I removablefixed implant may affect the rotational displacement of the prosthesis during loading.partial denture (RPD) hasPurpose. The purpose of this finite element analysis study was to analyze the effect of connecting abeen suggested as a way todenture base metal framework to an implant with the aim of decreasing the rotational movementreduce loading on the edenof an implant-assisted removable partial denture.tulous ridge.1 Kennedy class IMaterial and methods. A mesial occlusal rest direct retainer and a distal occlusal rest directRPDs often fail to achieveretainer were modeled and adapted to incorporate a modified denture base metal framework in thepatient satisfaction.2 Althoughconnection area for each model. The stress and deformation patterns of the prosthesis structuremandibular RPDs opposing awere determined using finite element analysis and compared for both situations.maxillary complete dentureResults. A maximum von Mises stress of 923 MPa was observed on the metal framework of thereduced the potential load onprosthesis with a mesial occlusal rest, and the maximum value was 1478 MPa for the distal occlusalthe extension base RPD andrest. A maximum von Mises stress of 17 MPa occurred on the acrylic resin denture base for theled to fewer patient commesial occlusal rest, and a maximum von Mises stress of 29 MPa occurred for the distal occlusal rest.1plaints, the downward growthConclusions. The distal occlusal rest direct retainer is stiffer than the mesial design and undergoesof maxillary tuberosities, papilapproximately 66% less deformation. The modified denture base framework with an I-bar and distallary hyperplasia, resorption ofocclusal rest design provides more effective support to the acrylic resin structure. (J Prosthet Dentthe premaxilla, overeruption of2017;118:177-186)the mandibular anterior teeth,failure have been reported when attachments wereand resorption of the posterior mandibular ridge haveincorporated into an IARPD.7,8 The clinician needs tobeen reported to be complications and are known as3decide whether to design the prosthesis with contactcombination or Kelly syndrome. In addition, the lack ofbetween the implant and the IARPD on the acrylic resinany mandibular posterior tooth support compromisesor on a customized metal casting of a metal frameworkarch integrity, which also leads to combinationcovering the attachments.1 The influence of metalsyndrome.4framework design modification on the prosthesis strucImplant rehabilitation has been shown to be anture and its performance needs to be investigated. Theeffective treatment for these conditions.5 An implantpurpose of this study was to evaluate the structuralassisted removable partial denture (IARPD) could proresponse and integrity as characterized by the inducedvide additional posterior support to prevent the resorpstress and deformation of a modified metal frameworktion of the anterior maxilla and reduce the risk ofdesign in the denture base area using finite elementcombination syndrome.6 However, fractured metalanalysis (FEA).frameworks or acrylic resin denture base and late implantSupported by EngDental MasterTech.aGeneral Manager, EngDental MasterTech, Sydney, New South Wales, Australia.bAssociate Professor, School of Engineering, Royal Melbourne Institute of Technology, Melbourne, Victoria, Australia.THE JOURNAL OF PROSTHETIC DENTISTRY177

178Volume 118 Issue 2Clinical ImplicationsRedesigning the denture base framework of animplant-assisted partial removable dental prosthesiswith an I-bar and distal positioning of the occlusalrest improve support during the loading of theacrylic resin denture base.MATERIAL AND METHODSAll components used in this study were created bycomputer-aided design (CAD) modeling that includedindividual part geometry creation, followed by an assembly process, as described in previous publications9-11(Fig. 1). Both the distal occlusal rest direct retainer andthe mesial occlusal rest direct retainer metal frameworkmodels were modified to allow them to have directcontact with a titanium ball attachment.The new denture base framework over the implantwas modeled in CAD software (Solidworks 2008), using acombination of input data from the scanned geometryand CAD (geometric) models of the metal framework.This was subsequently exported as a Parasolid XT file.The matching missing section of the denture base metalframework was generated by creating a copy of the metalframework model. Likewise, the matching matrix sectionof the denture base metal framework was generated bycreating a copy of the matrix and implant componentmodel. The matrix feature was added to the metalframework and merged with the denture base metalframework. The parts were saved as multiple parts inSolidworks software to position different solid models asone entity in relation to the mandibular model. Thematrices were accommodated in the prosthesis andaligned with the axis of the implant to avoid misalignment of the matrix position (Fig. 2).Additional identical CAD (geometric) models of theprosthesis denture bases were required in order toanalyze 2 types of metal framework designs. Thus, 2copies of the denture bases were generated (1 for eachmetal framework design). A copy of the metal frameworkwas made and used as a cutting tool to remove thecommon metal framework volumes from the prosthesisdenture base. This procedure was repeated for each sideof the denture base (Fig. 3). All final components weresaved as separate solid part files and then exported asindividual Parasolid XT files (Fig. 4).The interaction between the individual componentswas specified through contact behavior. Contact surfacesbetween the teeth and periodontal ligament (PDL), PDLand mandible bone, mandible bone and soft tissue,implant body and mandible bone, resin denture base andmetal framework were set to be perfectly bonded. Frictional contacts were modeled using the correspondingTHE JOURNAL OF PROSTHETIC DENTISTRYFigure 1. Assembled model of individual parts. A, Mandible, implant,teeth, periodontal ligament. B, Soft tissue, framework. C, Resin denturebase and highlighted loading area.friction coefficients (m) for all other component interfacesbetween the matrix and the implant (m 0.36), theocclusal rest direct retainer and the teeth (m 0.1), and thecontact surface between the resin denture base and thesoft tissue (m 0.01).12,13 The PDL and dentin wereconsidered elastic solids to incorporate realistic movements and deformations of the teeth. The interfacebetween the root and PDL was considered bonded. Thecementum layer was not considered as cementum andShahmiri and Das

August 2017179Table 1. Mechanical properties of materialsTitanium(Implant odontalLigamentdYoungmodulus(GPa)1102.202114168.90 10 5Poisson Ratio0.330.310.300.300.45CharacteristicaITI product catalog. bVertex product catalog. cWironit product catalog. dData from ref. 22.Figure 2. Titanium matrix inserted into modified denture base metalframework.dentin have similar mechanical properties.14 A tetrahedral mesh was generated with each node having 3 degrees of freedoms, the translations in 3 directions. Theresulting finite element mesh had 216 780 elements and376 975 nodes. The stress analysis was conducted withbilateral pressure applied on the surface of the teeth.A static linear elastic FEA was used in this study. Thebone was assumed to be transversely isotropic,15,16 andall materials followed linearly elastic behavior, complyingwith the Hook law, using 2 physical properties, theYoung modulus (E) and the Poisson ratio (n) (Table 1).Although anatomic locations are fixed in oral structures,experimental data have shown that the mechanicalproperties of bone are nearly constant in the transverseplane.15,16 Therefore, elastic deformation analysis underphysiological loading can be conducted by assumingtransversely isotropic linear elastic equations.15,16 Theoverall stiffness of the bone was approximated by takingthe average value based on the relative volume fraction ofcortical and trabecular bone. The Young moduli valueswere 14.7 GPa for cortical bone and 0.49 GPa fortrabecular bone, and the Poisson ratio for both thecortical and trabecular bone was taken to be 0.3(Table 1).17The forces were applied on the model as equivalentsurface pressures by evenly distributing them over theocclusal surface. The loaded areas were 198.92 mm2 onthe left and 183.01 mm2 on the right side of the prosthesis. For the applied load of 120 N on each side, thepressures applied were 0.60 MPa on the left and 0.66MPa on the right.RESULTSStresses and deformations were determined in thedifferent structures of the metal framework, the acrylicresin denture base, and the regions of the underlyingtissue. The corresponding maximum values of thestresses were subsequently determined to assess thecriticality and proneness to failure of the structures.A maximum von Mises stress of 923 MPa wasobserved on the metal framework of the prosthesis with amesial occlusal rest, and the maximum value of the stresswas 1478 MPa for the model with a distal occlusal rest.Hence the maximum stress in the distal occlusal restmetal framework was approximately 60% higher thanthat in the mesial occlusal rest. In the IARPD with amesial occlusal rest, the stress was highest where thelingual bar was connected to the denture base area(Fig. 5).The stress pattern was distinct for the acrylic resindenture base structure on the framework for the distalocclusal rest direct retainer (Fig. 6). In the mesialFigure 3. Implant-assisted removable partial denture with acrylic resin denture base inserted. A, Mesial occlusal rest. B, Distal occlusal rest.Shahmiri and DasTHE JOURNAL OF PROSTHETIC DENTISTRY

180Figure 4. Multiple solid parts (implants, IARPD metal framework parts,IARPD acrylic resin structures, and attachments components) positionedas one entity in relation to mandibular model (exported as Parasolid XTfile). IARPD, implant-assisted removable partial denture.occlusal rest direct retainer situation, the metal framework had a stress distribution where the stressed regionextended up to the end of the finishing line, the junctionbetween the metal framework and the acrylic resindenture base.The principal stress distribution in the distal occlusalrest direct retainer situation showed that the maximumprincipal stress occurred on the fitting surface of theproximal plates and indirect retainers and that the minimum principal stress occurred on the upper side of theocclusal rest direct retainer and indirect retainers(Fig. 7B). A similar principal stress distribution wasobserved in the mesial occlusal rest direct retainer. Suchstress variation and resultant gradient could cause highdeformation and potential failure of the retainer. However, the high-stress region extended in the upper surfaceof the denture base metal framework up to the junctionbetween the metal framework and the acrylic resindenture base (Fig. 7A).Volume 118 Issue 2The maximum total displacements were 507 mm forthe mesial occlusal rest direct retainer and 306 mm for thedistal occlusal rest direct retainer. This demonstrates thatthe distal occlusal rest direct retainer was much stifferand underwent approximately 66% less deformationthan the mesial design. The deformation pattern in themetal framework structure highlights the fact that themaximum deformation occurred on the infrabulge retainers (I-bars) in both the mesial and the distal occlusalrest direct retainer designs (Fig. 8).The maximum von Mises stress on the acrylic resinsurface was 17 MPa for the mesial occlusal rest directretainer and occurred on the lingual side of the prosthesis near the finishing line, and from this region, thestress pattern extended to the lingual surface up to themesiobuccal surface of the second molar (Fig. 9A). Amaximum von Mises stress of 29 MPa for the distalocclusal rest direct retainer was observed in themesiolingual surface of the second premolar. This stresspattern further extended up to the distal occlusal surfaceof the second premolar (Fig. 9B). Hence the distalocclusal rest direct retainer design had a higher stress(approximately 72% greater) than the mesial occlusalrest.The total displacement was found to be 130 mm forthe mesial occlusal rest direct retainer design and wasmuch larger (276 mm) for the distal occlusal rest directretainer design (Fig. 10). The total deformation pattern inthe IARPD structure highlights the fact that themaximum deformation occurred on the mesial area of thedenture base and infrabulge retainers (I-bars) in bothmatrix designs. Also, the rotational effect improved onthe distobuccal surface of the distal occlusal rest directretainer design, and buccal flange rotational effects werenot noticed (Fig. 10B).The deformation pattern of the abutment teeth, bone,and soft tissue (Fig. 11) was subsequently analyzed.Maximum deformation was found in the abutment teethFigure 5. von Mises stress distribution showing maximum stress on metal framework of prosthesis (MPa). A, Mesial occlusal rest. B, Distalocclusal rest.THE JOURNAL OF PROSTHETIC DENTISTRYShahmiri and Das

August 2017181Figure 6. High-stress regions at junction between metal framework and acrylic resin structure of prosthesis (MPa). A, Mesial occlusal rest. B, Distalocclusal rest.Figure 7. Tensile stresses (red) and compressive stresses (blue) on upper side of direct retainers. A, Mesial occlusal rest. B, Distal occlusal rest.Figure 8. Deformation (total displacement) of metal framework surface of prosthesis showing maximum displacement locations (mm). A, Modifieddenture base metal framework of mesial occlusal rest. B, Modified denture base metal framework of distal occlusal rest.on the mesial occlusal rest direct retainer (Fig. 11A). Themaximum deformation occurred in all anterior teeth andthe symphysis region of the mandible for the distalocclusal rest direct retainer (Fig. 11B). The values of theShahmiri and Dasmaximum total displacements were 220 mm for themesial occlusal rest direct retainer and 182 mm forthe distal occlusal rest direct retainer designs. A summaryof the results with the maximum stress and displacementTHE JOURNAL OF PROSTHETIC DENTISTRY

182Volume 118 Issue 2Figure 9. Stress distributions showing maximum von Mises stress on acrylic resin surface of prosthesis (MPa). A, IARPD with mesial occlusal rest.B, IARPD with distal occlusal rest. IARPD, implant-assisted removable partial denture.Figure 10. Total deformation showing maximum displacement locations (mm). A, Modified denture base metal framework of mesial occlusal rest directretainer model. B, Modified denture base metal framework of distal occlusal rest direct retainer model.values is presented in Table 2, which also includes themaximum values of the displacement of all components.DISCUSSIONMandibular Kennedy class I RPDs exhibit complexbiomechanical behavior because of the supportingstructures of the teeth and mucosa, each with differentproperties.18An osseointegrated implant has been suggested as a means to provide additional support andretention, which would reduce biomechanical complexities.12 It is expected that an implant incorporated into amandibular Kennedy class I RPD would reduce therotational movement of the prosthesis.19 However, nosignificant change of movement between the RPD andIARPD was identified in this study.20 Changing theposition of the occlusal rest direct retainer from themesial side of the abutment teeth to the distal side didreduce stress in the IARPD structure,10 although therotational movement still existed. A combination of aTHE JOURNAL OF PROSTHETIC DENTISTRYmismatch of deformation between the mesial and distalareas of the denture base and the movement of theabutment teeth leads to the rotational behavior of thedenture base of IARPDs.Connecting an acrylic resin base to both the metalframework and the rigidly fixed implant often contributesto the rotational displacement of the prosthesis duringloading. This study was conducted to analyze the effectof connecting the denture base metal framework to theimplant with the aim of decreasing the rotationalmovement of the IARPD. The analysis revealed a significant increase in stress on the distal occlusal restdirect retainer metal framework structure, where thedenture base metal framework is connected to theimplant (Fig. 5B). In addition, the high-stress regionremained primarily around the distal occlusal rest; withthe mesial occlusal rest, the stressed regions extendedup to the lingual bar and the denture base metalframework, exceeding the finishing line. This made theacrylic resin structure prone to failure by cracking. ThisShahmiri and Das

August 2017183Figure 11. Deformation (displacement) pattern of prosthesis underlying tissue (mm). A, Modified denture base metal framework of mesial occlusal rest.B, Modified denture base metal framework of distal occlusal rest.Table 2. Stress and displacement values for metal and acrylic resin structuresMaximum von Mises Stress(MPa)Maximum FrameworkDisplacement (mm)MetalAcrylic ResinMetalMaximum TotalDisplacement (mm)Maximum Abutment ToothDisplacement nTable data summarize stress and displacement values for metal and acrylic resin structures of implant-assisted removable partial denture with modified meshwork.was more noticeable when tensile stresses weregenerated on the upper surface of the denture basemetal framework of the IARPD (Fig. 7A). The maximumdeformation of the metal framework occurred in theI-bar. The deflection of the I-bar tip was found to exertload and hence generate stresses in the surroundingmaterial. For this reason, specific attention is requiredduring the fabrication of a metal framework in terms ofthe taper and the radius of curvature ratio between thevertical and horizontal sections in order to reduce fatigue in the clasp arm.21,22The acrylic resin structure showed an increase inmaximum stress in the distal occlusal rest direct retainerdesign. The maximum stress was well beyond the flexuralstrength of 75 MPa of the acrylic resin material (VertexDental. r). Although the maximum stress was higher forthe distal occlusal rest direct retainer design, this stressremained on the surface of the premolar teeth and didnot extend to the buccal flange of the acrylic resinstructure. In contrast, the mesial occlusal rest directretainer design had the maximum stress on the lingualside of the prosthesis near the finishing line, and, fromthis region, the stressed areas extended to the lingualsurface up to the mesiobuccal surface of the secondmolar. The distal occlusal rest direct retainer with themodified denture base metal framework appeared toprovide better support to the acrylic resin structure byreducing the stress on the surface of the acrylic resinbase.Shahmiri and DasA reduced rotation effect in the distal position of theocclusal rest direct retainer was observed (Fig. 10B). Therotation effect in the distobuccal area was not perceivedon the IARPD with the distal occlusal rest and was onlyfound in the distolingual region. An increase in themaximum total deformation in the distal occlusal restdirect retainer along with a reduced rotational movementcan explain why there was more vertical rather thanlateral displacement. However, the mesial occlusal restdirect retainer underwent less deformation. The alteration of the denture base metal framework did notreduce rotational movements in the mesial occlusal restdirect retainer design.The maximum displacement was observed on theunderlying structure of the distal occlusal rest directretainer and was greater than that on the mesial occlusalrest direct retainer. Large deformations occurred in all ofthe mandibular anterior teeth and extended to thesymphysis region of the mandible in the modified denture base metal framework of the IARPD with a distalocclusal rest. This may indicate resistance to the lateraltransverse bending (“wishboning”).23 Wishboning of themandible occurs in the thinnest area, providing sites forstress concentration. This was manifested in the mesial ofthe canine, symmetrically through the mental fossae(anterior buccal depression) in the buccal aspect of themandible, which is highlighted in Figure 12A (in creamcolor). On the lingual side, the uniform surface transitionprevented deformity (Fig. 12B). If wishboning happens,deposition on the lingual side of the anterior mandibleTHE JOURNAL OF PROSTHETIC DENTISTRY

184Volume 118 Issue 2Figure 12. Wishboning highlighted with soft tissue removed. A, Buccal aspect. B, Lingual aspect.Figure 13. Implant-assisted removable partial denture contact surface between attachment and framework. A, Center contact. B Wide 2ABCD10.63Res3001000796.4Framin denturebaseewo2001000A0TotaBl deCDrkADeformation5001500(μm)1477.5Abutment deformationformFramdef eworormkationationBFigure 14. Comparison of maximum values of stress and deformation in implant-assisted removable partial dentures with different metal frameworkdesigns. A, Von Mises stresses values of metal framework and resin denture base. B, Displacement values of total, metal framework and abutmenttooth. *All resin denture base stress values were multiplied by 10 for clear visualization.and resorption on the buccal side would be expected. Abone microstrain exceeding 1500 m causes bone deposition, and a microstrain below 100 m results in boneresorption.23 However, this study identified a microstrainTHE JOURNAL OF PROSTHETIC DENTISTRYrange between 140 and 180 m in the symphysis region.Further study is required to investigate the response ofmandibular bone during the loading of an IARPD with adistal occlusal rest.Shahmiri and Das

August 2017Lower deformation was found to occur on abutmentteeth with the distal occlusal rest direct retainer designthan that with the mesial occlusal rest direct retainerdesign. The abutment teeth displacement occurs throughthe PDL. The PDL is assumed to be made of a homogenous material in most FEA studies24,25 The homogenousmaterial assumption of the PDL in an FEA might makethe structural analysis less accurate.26 However, theresults of the present study are in accord with the findings of the nonhomogeneous PDL of FE studies, such asthose by Rocha et al27 and Archangleo et al,18 showingthat stress on the abutment teeth remains high with anIARPD, regardless of its homogenous or nonhomogeneous nature.Brudvik1 stated that where clinicians choose to makea custom metal casting framework over the ball attachment, the center of the ball surface only needs to becontacted (Fig. 13A). The reasoning behind this designwas not explained. One possible explanation could bethat a larger portion of the hemisphere of the ballcomponent interacts with the matrix for the wide contact,thus leading to relatively higher wear (Fig. 13B). Inaddition, a close contact with little or no clearance between the ball and the matrix can lead to edge loading(Fig. 13B). Close contact of the ball and the matrix resultsin a greater surface matrix articular arc angle. However,the force is not exerted from the ball component; rather itis exerted onto the ball attachment. Therefore, smallcenter contact can transfer nonaxial forces to the implant.Further study is needed to investigate the effect of contact surfaces on force transfer to an implant and underlying tissues.The use of a resilient attachment in conjunction withan implant limits the movement of an IARPD in thesagittal plane, regardless of the matrix design.10 Therotational movement of the prosthesis toward tissueneeds to be limited in order to protect the abutment teethfrom undesirable torques and forces.2 Occlusal rest seatsthat direct occlusal forces along the long axis of thesupporting teeth can provide resistance to movementtoward tissue.2 Distal positioning of an occlusal rest of anIARPD significantly decreased the displacement of theabutment teeth and the metal framework.9 This is animportant improvement, because it helps distribute theforces exerted on the components of the prosthesis(metal framework and acrylic resin) and distribution offorces more uniformly than with a mesial occlusal restdesign (Fig. 14A). Although the total displacement ofthe denture base of the modified framework withdistal occlusal rest remained relatively high (Fig. 14B),this could be attributed to a higher degree of resiliencythat allows the prosthesis to distribute forces morefavorably.Shahmiri and Das185CONCLUSIONSWithin the limitation of the current study, the followingconclusions were drawn:1. Modifying the denture base metal framework withinfrabulge retainers (I-bars) and the distal position of the occlusal rest of abutment teeth makethe structure much stiffer; it undergoes approximately 66% less deformation than with the mesialdesign.2. The metal framework provides support more effectively to the acrylic resin structure in the modifieddenture base metal framework with a distal occlusalrest direct retainer design than that provided by themesial occlusal rest direct retainer design.3. Moving the position of the occlusal rest from themesial to distal side of the abutment tooth andmodifying the denture base metal frameworkreduced the displacement of the metal frameworkand abutment teeth.REFERENCES1. Brudvik JS. Advanced removable partial dentures. 15th ed. Chicago: Quintessence; 1999. p. 164.2. Carr A, Brown D. McGivney G. McCracken’s removable partial denture. 13thed. Chicago: Quintessence; 2015. p. 37.3. Kelly E. Changes caused by a mandibular removable partial denture opposinga maxillary complete denture. J Prosthet Dent 2003;90:213-39.4. Stern N, Brayer L. Collapse of the occlusion-aetiology, symptomatology andtreatment. J Oral Rehabil 1975;2:1-19.5. Tolstunov L. Combination syndrome: classification and case report. J OralImplantol 2007;33:139-51.6. Keltjens HM, Kayser AF, Hertel R, Battistuzzi PG. Distal extension removablepartial dentures supported by implants and residual teeth: considerations andcase reports. Inter J Oral Maxillofac Implants 1993;8:208-13.7. Payne AGT, Tawse-Smith A, De Silva RK, Ma S. Multicentre prospectiveevaluation of implant-assisted mandibular removable partial dentures:surgical and prosthodontic outcomes. Clin Oral Implant Res 2017;28:116-25.8. De Freitas RFCP, De Carvalho Dias K, Da Fonte Porto Carreiro A,Barbosa GAS, Ferreira MAF. Mandibular implant-supported removablepartial denture with distal extension: a systematic review. J Oral Rehabil2012;39:791-8.9. Shahmiri R, Das RN, Aarts JM, Bennani V. Finite element analysis of animplant-assisted removable partial denture during bilateral loading: occlusalrests position. J Prosthet Dent 2014;112:1126-33.10. Shahmiri R, Aarts JM, Bennani V, Atieh MA, Swain MV. Finite elementanalysis of an implant-assisted removable partial denture. J Prosthodont2013;22:550-5.11. Shahmiri R, Das R. Finite element analysis of an attachment of implantassisted removable partial denture with different matrix design duringbilateral loading. Int J Oral Maxillofac Implants 2016;31:e116-27.12. Natali NA. Dental biomechanics. Boca Raton: CRC Press; 2003. p. 304.13. Curtis RV, Watson F. Dental biomaterials. London: Woodhead PublishingSeries; 2008. p. 528.14. Geramy A, Sharafoddin F. Abfraction: 3D analysis by means of the finiteelement method. Quintessence Int 2003;34:526-33.15. Ichim I, Swain M, Kieser JA. Mandibular biomechanics and development ofthe human chin. J Dent Res 2006;85:638-42.16. Ichim I, Kieser JA, Swain MV. Functional significance of strain distribution inthe human mandible under masticatory load: numerical predictions. ArchOral Biol 2007;52:465-73.17. Yettram AL, Wright KW, Houston WJ. Centre of rotation of a maxillarycentral incisor under orthodontic loading. Br J Orthod 1977;4:23-7.18. Archangelo CM, Rocha EP, Pereira JA, Martin Junior M, Anchieta RB, FreitasJúnior AC. Periodontal ligament influence on the stress distribution in aTHE JOURNAL OF PROSTHETIC DENTISTRY

18619.20.21.22.23.24.Volume 118 Issue 2removable partial denture supported by implant: a finite element analysis.J Appl Oral Sci 2012;20:362-8.Todorovi A, Radovi K, Grbovi A, Rudolf R, Maksimovi I, Stamenkovi D.Stress analysis of a unilateral complex partial denture using the finiteelement method. Mater Tech 2010;44:41-7.Ohkubo C, Kobayashi M, Suzuki Y, Hosoi T. Effect of implant support ondistal-extension removable partial dentures: in vivo assessment. Int J OralMaxillofac Implants 2008;23:1095-101.Sato Y, Yuasa Y, Akagawa Y, Ohkawa S. An investigation of preferable taperand thickness ratios for cast circumferential clasp arms using finite elementanalysis. Int J Prosthodont 1995;8:392-7.Sato Y, Tsuga K, Abe Y, Asahara S, Akagawa Y. Finite element analysis onpreferable I-bar clasp shape. J Oral Rehabil 2001;28:413-7.Dobson SD, Trinkaus E. Cross-sectional geometry and morphology of themandibular symphysis in Middle and Late Pleistocene homo. J Hum Evol2002;43:67-87.Groning F, Fagan MJ, O’Higgins P. The effects of the periodontal ligament onmandibular stiffness: a study combining finite element analysis and geometric morphometrics. J Biomech 2011;29(44):1304-12.25. Meijer HJ, Starmans FJ, Steen WH, Bosman F. A three-dimensional, finiteelement analysis of bone around dental implants in an edentulous humanmandible. Arch Oral Biol 1993;38:491-6.26. Frost HM. Bone’s mechanostat: a 2003 update. Anat Rec A Discov Mol CellEvol Biol 2003;275:1081-101.27. Rocha EP, Luersen MA, Pellizzer EP. Distal extension removable partialdenture associated with an osseointegrated implant. Study

occlusal rest direct retainer and indirect retainers (Fig. 7B). A similar principal stress distribution was observed in the mesial occlusal rest direct retainer. Such stress variation and resultant gradient could cause high deformation and potential failure of the retainer. How-e

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