Evaluation of initial mechanical stability of noncemented hip implants under physiological loading

Long-term clinical success of total hip replacement surgery is dependent on initial implant stability. For noncemented, porous coated implants in particular, initial stability is essential for fixation by bone ingrowth to occur. A method for evaluating initial stability by quantifying relative implant-bone micromotion has been developed. Based on the assumption that transverse sections of the implant are rigid bodies, the technique measured motion at proximal and distal sites, two clinically relevant regions of an implant. Mathematical transformations derived from measurements obtained by a series of linear displacement transducers were used to determine the three-dimensional displacement vector of any point on the plane. This method was successful in assessing stability of two different implants under physiological loading using either synthetic or cadaveric femora and different surgical protocols for the preparation of the intramedullary canal. The displacements of points at the implant-bone interface for both permanent and recoverable micromotion were obtained. This method may be of use in evaluating new implants and the effect on stability of alternative surgical protocols