Stress and strain distributions of tibia due to Total Ankle Replacement: The effects of implant orientation and implant-bone inter-facial condition

Introduction One of the causes of aseptic loosening of Total Ankle Replacement (TAR) is due toimplant induced adaptive bone remodelling [1]. Implant orientations and implant-boneinterfacial conditions of the TAR components might alter stress-strain distribution, whichlead to increase loosening rate of TAR components due to bone remodelling. The aimof this study to investigate the effects of implant orientationand implant-bone interfacialconditions on stress and strain distribution of tibia bone due to TAR.Materials & MethodsFinite Element (FE) models of intact and implanted right ankle were developed usingcomputed tomography (CT) data set. The cortical bone was assigned with Young’smodulus of 19GPa and Poisson’s ratio of 0.3, respectively. The cancellous bonematerial property assigned as heterogeneous and material property was distributedusing CT-scan data sets. Scandinavian Total Ankle Replacement (S.T.A.R@TM)prosthesis was chosen for TAR and positioned according to surgical guidelines. Inorder to determine the effect of implant orientations on bone stress-strain distibution,four other FE models of implanted ankle were developed separately, which consists ofvariation of varus and valgus angle of 5ᵒ and 10ᵒ respectively (Fig.1). Young’s modulusand Poisson’s ratio of tibial and talar components were taken as 210GPa and 0.3, whilefor meniscal bearing it was 1174MPa and 0.4, respectively. Three positions of ankleduring gait, dorsiflexion, neutral and plantar flexion were considered as applied loadingcondition. Both debonded and bonded conditions were studied.Results and DiscussionImplantation (optimally positioned) headed to increase in stress and strain in the tibiabone near the vicinity of the tibial component and decrease in stress and strain awayfrom the implant vicinity, when both debonded and bonded condition was assumed. Incomparison with debonded condition, stress and strain were generally increased in thetibia bone when bonded condition was assumed. As compared to optimally positionedimplant, around 5%-25% and 5%-32% reduction in stresses were observed away fromthe implant vicinity in the tibia due to orientation of 5ᵒ and 10ᵒ varus angle and 5%-35% and 5%-45% for 5ᵒ and 10ᵒ valgus angle. Decrease in strain was observed around 10%to 30 % and 10% to 40% away from the implant vicinity, cooresponding to varus andvalgus orientations. Implant orientation causes stress/strain shielding away from theimplant vicinity due to less load transfer through these regions.ConclusionIt can be concluded from the present study that proper orientation and implant-boneinterfacial condition of the implant is suggested for long term performance of theimplant. The present study is beneficial for pre-clinical evaluation of TAR.