Finite Element Stress Analysis Of The Staggered And Straight Placement Of Implants In The Mandibular Posterior Region

If the placement of the implant in bone is unreasonable, thestress distribution will not well-distributed. The occlusal forcestransmission is unparalleled to the axle of the implant,further, it alsocan impact the prosthesis of supporting structure. The stressdistributes differently in straight placement and staggered placement.Bending moments resulting from non-axial overloading of dentalimplants may cause stress concentrations exceeding the physiologicsupporting capacity of cortical bone. Three-dimensional finiteelement stress analysis is an accepted technique used in the solutionof engineering problems. The method has been extensively used tostudy the biomechanics of dental implants and offers manyadvantages over other methods. The purpose of this study is toanalysis which kind of placement is more reasonable under thelimited alveolar ridge conditionead to various kinds of failures.This study based on the biomechanical point of view, amandibular Kennedy Class II partially edentulous 3-D finite elementmodel was constructed. Nine different partial fixed prosthesessupported by 3 implants which were designed according to 2 mainconfigurations: straight –line or staggered implant placement. In 6 ofthe designs, offset placement of the middle implant buccally andlingually was simulated. In the other 3 models, implants with variousdiameters were placed along a straight line. Both anterior andposterior implants of each constructed are constant. We appliedvertical and 30° lateral loads directions under the condition oflocalized load and dispersed load to analysis the stress of the implantand the bone around it.Location and direction of loading had a great influence on thestress-transfer characteristics of both placement configurations. Thehighest values of tensile stress and compressive stress were recordedon cortical bone in the cervical region of the implants. The staggeredplacement configuration changed the stress distribution,but it didnot reduced the overall stress substantially. The stress can beincreased under the different load. The stress distribution wasinclined to the buccolingual direction of the cervical region of theimplants. In the case of the vertical load the compressive stressincreased. In the case of the lateral dispersive load, the stress oflingual offset configuration was reduced and the stress of buccaloffset configuration was increased. The result of the localizedloading on the middle implant is opposite to the lateral dispersiveload. The stress of lingual offset configuration was increased and thestress of buccal offset configuration was reduced. The change of thestress value was not obvious. The stress was transmitted axiallywhen implant was placed in straight-line. The stresses are all reducedunder the different kinds of loads if the diameter of middle implantwith straight placement is 5mm or 6mm. The relationship betweenstress and diameter is inverse ratio. The stress will be reduced by theincrease of the diameter of the middle implant and waswell-distributed.The present study considered the effect of staggered implantplacement on load transfer in a three-unit fixed posteriorimplant-supported prosthesis. The concept of staggered placement ofimplants in posterior edentulous cases originated from completebone-anchored bridge,where implants are placed in a nonlinear archconfiguration. Based on the long-term clinical success suchnonlinear implant placement for completely edentulous patients,buccal and lingual offset was proposed to be an effective way toreduce the potential risk of overload for posterior partial implant-support prothesis. From the results of the present study,somedifferences in stress production of the supporting structure existbetween the two implant placement configurations,but no clearstress reduction effect of staggered placement was observed. Sincethe width of the edentulous ridge is usually limited because of theresorption of the bone,it is very difficult and impossible to placeimplants in large offset permutation aiming at a greatstress-reduction effect in the majority of cases. Thus,the amount ofoffset would be necessary much smaller for posterior partiallyedentulous situation than for completely edentulous cases.Therefore,offsets that can be made in the posterior region that maynot be sufficient to achieve a substantial differences in stressdistribution and intensity between the two placement configurationswhich were not observed in the present study. Extreme offsets resultin longer buccolingual cantilevers,which may increase bendingmoment and load to overloading. Since the buccolingual width of thealveolus ridge is limited,the offset placement of implants will loadto thinner remaining bone on one side. However,the sufficient bonethickness is necessary around implants to withstand occlusal load.The conclusions derived from this 3-D finite element stressstudy are limited to the assumptions made for the composition of thecomputer model and its boundary conditions. Within the limits ofthis study, lower stresses were observed in cortical bone at thecervical region of implants when wider implants were placed along astraight line rather than in offset placement. Therefore, if thebuccolingual width of the residual bony ridge insufficient forstaggered implant placement, we should choose straight-lineplacement with wide implant to reduce the stress and to make thestress transmit axially.