| Background and objectivesIn recent years,the prevalence of edentulism has been increasing with accelerated aging process in China.Having great masticatory efficiency and esthetic properties,implant-fixed prostheses has become an ideal choice for edentulous patients,and reasonable treatment design has become a hotspot in implantology.A framework with rational design is crucial to the success of implant treatment as prostheses transmits the stress generated by occlusal force to implant and peri-implant bone in chewing process.For the patients with insufficient alveolar bone,the problem can be solved by surgery or prosthetic design.But surgery has some disadvantages such as long treatment cycle,high risk and large trauma,two main prosthetic designs are preferred currently:one is axial implantation with cantilever prostheses and another is oblique implantation with non-cantilever prostheses.However,there are still many controversies between two treatment protocols.For one thing,implant-supported fixed non-cantilever prostheses in mandible can facilitate stress transmission from prostheses to implant and peri-implant bone compared the design with cantilever.And it has been verified that the oblique implantation in distal with non-cantilever prostheses can achieve a high success rate in long-term follow up.For another,the direction of force transmission becomes non-axial changed by the oblique implantation in distal,which also increases stress on peri-implant bone,while the prostheses with logical cantilever can also achieve a reliable and stable clinical outcome.In addition,implant number,bone density,and prosthetic material also affect the biomechanical characteristics of implants and peri-implant bone.However,there are few comprehensively comparative studies on cantilever and noncantilever prostheses in implant-supported fixed restoration,and the roles of implant number,bone density,and prosthetic material are also ambiguous.To provide theoretical basis for clinic,this study aims to compare clinical outcomes and biomechanical characteristics of implantsupported fixed cantilever and non-cantilever prostheses in edentulous mandible,which is significant to achieve good treatment outcome and comfortable treatment experience for edentulous patients.Materials and methods1.Inclusion and data collection of clinical samplesAfter obtaining informed consent,a total of 21 edentulous patients with completed implant-supported fixed prostheses in mandible were included,containing 9 patients with cantilever prostheses and 12 patients with non-cantilever prostheses from January 2016 to December 2021.And clinical data of patients were collected,which included age,gender,smoking history and so on.Referring to the Lekholm and Zarb classification,the alveolar bone of implant site was classified as four bone types through cone-beam computerized tomography(CBCT).2.Clinical retrospective cohort analysisThe implant cumulative survival rates and the technical complication rates were recorded from medical history.Within one to three years after loading,the implant marginal bone loss(MBL)and the peri-implant bone density loss(BDL)were respectively measured in each year of all implants,and the peri-implant regions were the implant cervix,median,root and apex.The data were analyzed comprehensively to study the influences of prosthetic designs and bone types for the edentulous mandible with fixed implant-supported prostheses.3.Three-dimensional finite element analysisEdentulous mandible model,simplified axial and oblique implant model,and one-piece framework prosthesis model were constructed and assembled as All-on-4 with cantilever(C4),All-on-4(A4),All-on-6 with cantilever(C6)and All-on-6(A6).And bone and material properties were assigned to all models,including four bone types and two prosthetic materials.Then,masticatory muscle forces and occlusal loading were simulated on models,and a total of 32 models were finally obtained.To explore the best combination in biomechanical benefits,values and distributions of von Mises stress(VMS)on implant,maximum principal strain(MPS)on bone and prosthesis displacement were analyzed respectively,from four dimensions including cantilever,implant number,bone type and prosthetic material.Results1.Inclusion and data collection of clinical samplesAfter implant surgery,completed implant-supported fixed prostheses were delivered on 21 patients during 0 to 14 months.And four patients were excluded due to the deficiency of radiographic data.A total of 17 patients containing 93 implants were finally included and allocated to two groups:The cantilever prostheses group(n=7)contained 34 implants whereas the non-cantilever prostheses group(n=10)contained 59 implants.And there was no significant difference in gender and age between two groups.All implant sites were classified as type I bone containing 38 implants and type Ⅱ/Ⅲ bone containing 55 implants.2.Clinical retrospective cohort analysisThe three-year cumulative survival rates of the cantilever prostheses group and the noncantilever prostheses group were 100.00%and 98.31%,and the technical complication rates were 14.29%and 30%.All the technical complications were related to restoration,containing porcelain chipping(75%)and loss of resin covering the screw(25%).After loading for three years,the MBL of the cantilever prostheses group and the noncantilever prostheses group were 1.60± 0.73 mm and 0.55 ± 1.04 mm with statistical significance(P<0.05);the MBL of type I bone and type Ⅱ/Ⅲ bone were 1.56±0.76 mm and 0.49 ± 1.04 mm(P<0.05).Within one to three years after loading,most of the peri-implant BDL in the cantilever prostheses group were higher than the non-cantilever prostheses group.In the first year,the differences of BDL were mainly concentrated on three peri-implant regions with statistical significance(P<0.05),which were the implant cervix,root and apex,and the implant root has the most significant differences(P<0.01).Most of the peri-implant BDL in type I bone were higher than type Ⅱ/Ⅲ bone,but the differences were no statistical significance.The BDL had no significant differences between peri-implant regions in type I bone,while in type II/III bone,the BDL was mainly concentrated on the implant cervix.3.Three-dimensional finite element analysisIn terms of VMS values,MPS values and prosthesis displacement values,from large to small,all orders of four designs were C4>A4>C6>A6,and A6 was the best design in four designs.Among four bone types,all orders were Ⅳ>Ⅲ>Ⅱ>Ⅰ,and type Ⅰ bone with the highest cancellous bone density is the best bone type.The differences of VMS values were very little between four cancellous bone densities,and MPS values and displacement values decreased with the increase of cancellous bone density.In two prosthetic materials,three values in zirconia were superior compared to polymethyl methacrylate(PMMA),and prosthetic material had little effect on prosthesis displacement.According to the distribution of VMS,MPS and prosthesis displacement,VMS in C4 and C6 models were mainly concentrated on the distal areas of the joint between implant and abutment in posterior region and the mesio area of the joint between implant and abutment in anterior region.In A4 and A6 models,it was mainly concentrated on the buccal areas of the joint between implant and abutment.And compared to A6,A4 had more extensive stress and strain concentration on implant and bone tissue.In all models,the concentration areas of MPS on cortical bone were in line with VMS,but MPS on cancellous bone were mainly concentrated on the implant cervix distal.Four bone densities had no significant effect on the distribution of VMS,MPS and prosthesis displacement,but the higher bone density was,the greater it dispersed strain to surrounding area,which was conducive to relieve stress concentration.The distal cantilever spread the strain from the implant cervix distal to implant body,root,and apex,but it can be alleviated with PMMA to some extent.The distribution rule of prosthesis displacement in all models showed a gradual decrease from anterior to posterior region,except the PMMA prostheses with cantilever,in which the high displacement was concentrated on the cantilever buccal distal.Conclusions1.In implant-supported fixed restoration for edentulous mandible,both cantilever and noncantilever prosthesis can obtain good clinical outcomes in short-term follow up.2.In short-term follow up,the distal cantilever caused stress concentration on distal implant and peri-implant bone,which aggravated the MBL and the BDL.3.In ideal conditions,type I bone had advantageous biomechanical properties,but clinical outcomes indicated that type I bone had a poor effect on maintaining osseointegration stability.4.In ideal condition,A6 was the best design with biomechanical benefits.Compared to A4,C6 is more beneficial to maintain osseointegration stability for the patients with insufficient alveolar bone.5.High bone density and rigid prosthetic materials such as zirconia were more conducive to the transmission and dispersion of occlusal force. |
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