SELF CENTERING MECHANISM IN BIPOLAR ENDOPROSTHESIS – IS IT EFFECTIVE?

Abstract

Bipolar hip arthroplasty was introduced to alleviate the problems of hip pain, acetabular protrusion and femoral stem loosening associated with unipolar prosthesis. Earlier generation bipolar endoprosthesis used to cause varus fixation of outer head which led to the unacceptably high incidence of dislocation, component disassembly and fractures of the polyethylene bearing insert. Second generation endoprosthesis with a self centering mechanism were introduced to overcome these problems. This new design incorporates a polar offset by setting the center of rotation of the inner head proximal to the center of rotation of the outer head, which generates a valgus producing moment at the outer cup. There is a controversy whether this mechanism works in vivo, more so in indigenous prosthesis.

A retrospective observational study was done on 37 subjects, which included 21 males and 16 females. The first radiograph was taken with the patient standing and bearing full weight over the endoprosthetic leg and abducting the contra lateral limb as much as possible. The second radiograph was taken with the patient standing neutral and bearing weight on both the legs. Abduction and adduction views were then taken in supine position. The radiographs were analyzed using the method similar to that of Drinker and Murray. The adductive motion from abduction to neutral position is within the range of inner bearing oscillation. Modified Harris Hip Score was used to evaluate the patients clinically. Results were analyzed using Wilcoxon Matched-Pairs Signed-Ranks Test, Students t-test and Karl Pearson correlation statistics.

The mean outer head alignment changed from 42.46 degrees ±13.62 (range 10 to 72 degrees) to 31.93 degrees ± 10.59 (range 8 to 50 degrees) in moving from abduction to neutral position in weight bearing position. The analysis showed that 68.66% of the total motion occurred at the outer bearing in weight bearing position whereas 73.86% of the total motion occurred at the outer bearing in supine position. The difference between distribution of motion between supine and weight bearing position was not found to be statistically different using Wilcoxon Matched-Pairs Signed-Ranks Test (p = 0.3164) and unpaired students t test (p = 0.35). No correlation was found between weight of the patient and time of follow up with outer head alignment and differential distribution of motion.

Self centering mechanism of bipolar endoprosthesis works in vivo under physiological loads and aligns the cup in neutral or valgus position till an average follow up of 10 months. Though the motion occurs at both the bearing surfaces outer bearing motion clearly predominated in both weight bearing as well as supine position.