Position of the prosthesis and the incidence of dislocation following total hip replacement

Background Dislocation is the second most common complication of hip replacement surgery,and impact of theprosthesis is believed to be the fundamental reason.The present study employed Solidworks 2003 and MSC-Nastransoftware to analyze the three dimensional variables in order to investigate how to prevent dislocation following hipreplacement surgery.Methods Computed tomography (CT) imaging was used to collect femoral outline data and Solidworks 2003 softwarewas used to construct the cup model with variabilities.Nastran software was used to evaluate dislocation at differentprosthesis positions and different geometrical shapes.Three dimensional movement and results from finite elementmethod were analyzed and the values of dislocation resistance index (DRI),range of motion to impingement (ROM-I),range of motion to dislocation (ROM-D) and peak resisting moment (PRM) were determined.Computer simulation wasused to evaluate the range of motion of the hip joint at different prosthesis positions.Results Finite element analysis showed:(1) Increasing the ratio of head/neck increased the ROM-I values andmoderately increased ROM-D and PRM values.Increasing the head size significantly increased PRM and to some extentROM-I and ROM-D values,which suggested that there would be a greater likelihood of dislocation.(2) Increasing theanteversion angle increased the ROM-I,ROM-D,PRM,energy required for dislocation (ENERGY-D) and DRI values,which would increase the stability of the joint.(3) As the chamber angle was increased,ROM-I,ROM-D,PRM,Energy-Dand DRI values were increased,resulting in improved joint stability.Chamber angles exceeding 55 resulted in increasesin ROM-I and ROM-D values,but decreases in PRM,Energy-D,and DRI values,which,in turn,increased the likelihoodof dislocation.(4) The cup,which was reduced posteriorly,reduced ROM-I values (2.1—5.3) and increased the DRIvalue (0.073).This suggested that the posterior high side had the effect of 10 anteversion angle.Conclusions Increasing the head/neck ratio increases joint stability.Posterior high side reduced the range of motion ofthe joint but increased joint stability;Increasing the anteversion angle increases DRI values and thus improve jointstability;Increasing the chamber angle increases DRI values and improves joint stability.However,at angles exceeding55,further increases in the chamber angle result in decreased DRI values and reduce the stability of the joint.Chin Med J 2007:120(13):1140-1144