INFLUENCE OF POSTOPERATIVE ALIGNMENT ON POLYETHYLENE WEAR AND RECURRENCE OF LONG-TERM DEFORMATION IN UNICOMPARTMENTAL PROSTHESES

Abstract

Purpose: Postoperative undercorrection is recommended for unicompartmental prostheses. The long-term effects of this undercorrection on polyethylene wear and recurrent deformation have not been evaluated to date. We studied the influence of undercorrection on polyethyl-ene wear and the risk of recurrent deformation in uni-compartmental prostheses reviewed at more than 14 years (14–22 years).

Material and methods: Forty unicompartmental prostheses with a polyethylene plateau without a metal back were evaluated at last follow-up. We assessed radiographs performed under fluoroscopic control to obtain a ray tangential to the polyethylene plateau. This film was used to assess penetration of the femoral component into the polyethylene. Goniometry, performed at last follow-up was compared with the postoperative goniometry to measure recurrent deformation. We retained only unicompartmental prostheses with preservation of the anterior cruciate at implantation in order to rule out possible influence of the absence of this ligament.

Results: There was a significant relationship (p< 0.05) between residual postoperative varus and rate of femoral component penetration into the polyethylene. Mean polyethylene wear was 0.15 mm per year for unicompartmental prosthesis with postoperative varus greater than 10°. There was also a correlation (p< 0.01) between recurrent deformation (difference between the last follow-up and postoperative goniometry) and postoperative varus. Schematically, deformation was correlated with penetration of the femoral component into the polyethylene. Finally, recurrent deformation and rate of penetration of the femoral component into the polyethylene was greater with thinner polyethylene inserts (p< 0.05).

Discussion: While undercorrection appears to be desirable for unicompartmental prostheses, it should be moderate. Excessive postoperative varus raises the risk of more rapid polyethylene wear and recurrent deformation. Furthermore, even for minimal undercorrection, the correction achieved postoperatively does not remain constant and varus defomation tends to recur. This phenomenon probably has a protective effect on the contra-lateral femorotibial compartment but in the long-term exposes to the risk of wear and recurrent deformation.