Abstract

Migration of the acetabular and femoral implants after THR is a better index of the stability of the bone-implant interfaces than are clinical or radiological results. Roentgenstereophotogrammetry (RSA) studies 3-D migration of the implants with high accuracy (0.15 to 0.28 mm for linear migrations). RSA presents several drawbacks which restrict its use to prospective studies on small numbers of patients. Simpler methods have therefore been developed to assess 2-D migration on standard films in retrospective studies. The precision of these "simple" methods is limited, due to several factors: the difficulty to define reliable landmarks on femur or pelvis, sometimes even on implants, measurement errors, related to variations in radiographic technique (focal distance, beam centering, patient positioning). Sutherland, Wetherell and Nunn have proposed methods with an accuracy around 2-3 mm. It appears impossible to correct migration measurements for distorsions due to patient positioning; the EBRA method was therefore developed to reject non-comparable films using a comparability algorithm. A precision of 0.20 to 0.32 mm can thus be reached for the study of cup migration. The same pitfalls are encountered in assessment of migration of the femoral implant; a preliminary theoretical study is mandatory for every implant studied. The data presently available show that migration at 2 years is predictive of the long-term evolution of an implant; for the cup, migration of 1 mm or more at 2 years is predictive of late failure, and similar conclusions can be drawn regarding the femoral implant. The 2-D assessment of implant migration using a correct "simple" method provides a mean to evaluate a new implant or an innovative technical modification in a reasonable amount of time, on a limited number of patients.