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I research _ restoration 10 I implants4_2010 isexpected.Contrarilymorestresseswerefoundonthe goldcrownplacedontwoimplantsduetoitsvolumere- duction (less material under the same load). This is clearly seen in increasing stresses on the two implants, thatmoreloadeffectwastransferredthroughtheweak crown to the two implants. That showed maximum stressesintheareaunderthecrown,whilethewideim- plant showed maximum stresses at its tip. Looking to energy**absorptionandstressconcentrationonwhole system starting from coating to cortical and spongy bone,althoughthestresslevelsfoundwastoolowand farfromcrackingdanger,thefollowingconclusionscan be pointed out; the total results favourise the two im- plantsinspongyboneandthewideimplantinthecor- ticallayer,butthealveolarboneconsistsofspongybone surrounded by a layer of cortical bone. It’s also well knownthataccordingtothedegreeofbonedensitythe alveolarboneisclassifiedtoD1,2,3,4 23 inadescendingor- der. So, provided that the edentulous space after the molar extraction permits, it’s recommended in the harderbonequality(D1,2)touseonewidediameterim- plant and in the softer bone (D3,4) quality two average sized implants. Therefore more detailed study to com- promise between the two implants size/design and in- termediate space can put this stress values in safe, ac- ceptable,andcontrollableregionunderhigherlevelsof loading. **Theareaunderthe - curveuptoagivenvalueofstrainistheto- tal mechanical energy per unit volume consumed by the material in strainingittothatvalue(Fig.9).Thisiseasilyshownasfollowsinequa- tion2: _Summary Restoration of single molar using implants en- countersmanyproblems;mesio-distalcantileverdue to very wide occlusal table is the most prominent. An increased occlusal force posteriorly worsens the problemandincreasesfailures.Toovercometheover- load, the use of wide diameter implants or two regu- lar sized implants were suggested. The aim of this studywastoverifythebestsolutionthathasthebest effect on alveolar bone under distributed vertical loading. Therefore, a virtual experiment using Finite Element Analysis was done using ANSYS version 9. A simplifiedsimulationofspongyandcorticalbonesof thejawastwoco-axialcylinderswasutilized.Fullde- tailed with high accuracy simulation for implant, crown, and coating was implemented. The compari- sonincludeddifferenttypesofstressesanddeforma- tions of both wide implant and two regular implants under the same boundary conditions and load appli- cation. The three main stresses compressive, tensile, shear and the equivalent stresses in addition to the verticaldeformityandthetotaldeformitieswerecon- sidered in the comparison between the two models. The results were obtained as percentages using the wideimplantasareference.Thespongyboneshowed about 5% less stresses in the two implants model than the one wide diameter implant. The exceptions are the relatively increase in maximum compressive stressesanddeformationsoforder12%and0.3%re- spectively.Thestressesanddisplacementsonthecor- tical bone are higher in the two implant model due to havingtwocloseholes,whichresultsinweakareain- between. The spongy bone response to the two im- plants was found to be better considering the stress distribution (energy absorbed by spongy bone**). Therefore, it was concluded that, using the wide di- ameterimplantortwoaverageonesasasolutionde- pends on the case primarily. Provided that the avail- able bone width is sufficient mesio-distally and bucco-lingualy, the choice will depend on the type of bone. The harder D1,2 types having harder bone qual- ity and thicker cortical plates are more convenient to thewideimplantchoice.TheD3,4 typesconsistofmore spongyandlesscorticalbone,aremoresuitabletothe two implant solution. Editorial note: The literature list can be requested fromtheauthor. Fig. 9_Strain energy = area under stress strain curve. Fig. 10_Equation 2 (stress energy). Fig. 8b Fig. 9 Amr A.Azim Professor,Faculty of Dentistry,Cairo University E-mail:drazim@link.net Amani M.Zaki GBOI.2009,Egypt E-mail:amani.m.zaki@gmail.com Mohamed I.El-Anwar Researcher,Mechanical Engineering Department, National Research Center,Egypt E-mail:anwar_eg@yahoo.com _contact implants Fig. 10