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ne between the two implants in some oftheir edentulous patients rehabilitated using two-implant-retained overdentures.31However, due to the scarcity ofliterature concerning the effects of implant number on stressdistribution in the upper structure of the overdenture, furtherexperimental stress analysis and long-term clinical researchneeds to be carried out.It can be postulated that forces, both axial and lateral,generated by an overdenture on a single implant, have thepotential to be greater than those produced by a multipleimplant-retained/supported overdenture, resulting in a risk ofloss of osseointegration. However, Maeda et al. evaluated thebiomechanical rationale for single-implant mandibular over-dentures using magnetic and ball attachments in an in vitromodel and found that single-implant overdentures hadbiomechanical properties similar to two-implant overdenturesin terms of lateral forces to the abutment and denture basemovements under functional molar loads.32Our study showedthat stress in the abutment of model A was lower than in theother three models under three loading conditions. Moreover,when functioning with anterior teeth, the overdenture an-chored by a single implant rotated over the implant from oneside to another and randomly inclined to one side, which in ourcase happened to be the left side. A similar effect happenedunder VM with the same model. Thus, the left side of the wholealveolar ridge took the role of bearing the occlusal load. Thecontact area between the denture and mucosa in model A wastherefore much larger than in the other three models, causingless pressure on the mucosa. In addition, under VI, themaximum equivalent strain in peri-implant cortical bone inmodel A was much lower than in the other three models,indicating that denture loading did not cause any apparentincrease of strain in peri-implant bone and that the implantmainly took the role of retention rather than support. Therefore,our results suggest that use of a single-implant overdenturedoes not lead to strain concentration in the bone around theimplant and could be a feasible choice for edentulous patients.Clinical studies also suggest that mandibular single-implantoverdentures are a successful and beneficial treatment optionfor older edentulous adults with minimal financial outlay.6–11Two-implant overdenture has been considered a firstchoice for the treatment of edentulous patients worldwide.However, we found that under anterior loading, the dentureshowed more obvious rotation than it did in models C or D.This agrees with another study showing that the application ofthree or four implants may create an angular instead of astraight-line relationship between the implants, preventing the intrusion of the anterior portion of the denture tissue-ward.14Clinical studies have also suggested that one of thechief concerns of patients wearing two-implant overdenturesis denture rotation.31Therefore, it has been recommended bysome investigators that clinicians should use three or fourimplants when increased retention is required.15Cliniciansshould also pay attention to the appropriate extension of thedenture base and occlusal harmony to prevent excessiverotational movement.31There has been some concern that with three-implantoverdentures, the strain in the bone around the middle implantmay be high, especially when functioning with the posteriorteeth. Nevertheless, our results showed that during simulationof grinding food with the posterior teeth, the maximumequivalent strain in the cortical bone was located around theleft implant, on the same side as the load. Therefore, it could beconcluded from our results that overdenture anchored by threeimplants did not cause any strain burden in the cortical bonearound the middle implant. Geckili et al. also found that themarginal bone loss around the central implants of three-implant mandibular overdentures, when using ball or barattachments, was lower than around the implants on the leftand right sides.33Furthermore, as mentioned above, three-implant overdenture was more stable than the two-implantmodel in our study. For patients who complain about rotationalmovement around the fulcrum line of their two-implantmandibular overdenture, adding a third implant in the midlineof the jaw could theoretically improve denture stability.5. ConclusionWithin the limitations of this in vitro study, the followingconclusions can be drawn: The maximum strain values in peri-implant bone werewithin physiological limits in all four models. The single-implant model demonstrated the features of lowstrain in peri-implant bone, low stress in the abutments andcompromised denture stability. It provides a cost-effectivetreatment alternative for patients with limited economicresources. When simulating the action of cutting food with the anteriorteeth, the two-implant model demonstrated relatively highstrain in peri-implant bone, high stress in the abutmentsand compromised denture stability. A third implant placedbetween the original two could provide a possible solutionwhen patients rehabilitated using two-implant overden-tures report constant and obvious denture rotation aroundthe fulcrum line. When functioning with the anterior teeth, three- and four-implant models were steadier than the two-implant model.No strain burden was found in the cortical bone around themiddle implant in the three-implant model.AcknowledgementThis work was supported by the National Key Technology R&DProgramme (no. 2007BAI18B05).r e f e r e n c e s1. Engquist B, Bergendal