WEAR OF POLYETHYLENE IS LESS IN LATERAL THAN IN MEDIAL UNICOMPARTMENTAL TIBIAL COMPONENTS

Abstract

Introduction: This study evaluated the creep and true wear in 55 medial and in 35 lateral fixed bearing uni-compartmental implants that had a flat articular surface at the time of implantation.

Materials and Methods: All the polyethylene components had the same design, the same sterilization, and were retrieved from 11 to 244 months after their implantation. The postoperative deformity had been measured on weight-bearing radiographs of the whole limb (hip-knee-ankle angle). The retrieved implants were placed in a coordinate measuring machine.Using this system, a three dimensional scaled image was used to calculate the total penetration of the femoral implant in relation with true wear and creep. To separate plastic deformation from true wear, the volume of true wear was calculated by wheighing the tibial components and comparing the results with non implanted components. Difference between the penetration determined by the coordinate machine and penetration determined by wheighing was considered to be in relation with creep.

Results: Total linear penetration rates ranged from 0.2 to 2.6 mm/year (mean 0.19 mm/year) and was significantly less in lateral (mean 0.14 mm/year) than in medial implants (mean 0.25 mm/year). Linear penetration rates in relation with wear ranged from 0.1 to 1.4 mm/year (mean 0.13 mm/year), and penetration in relation with creep ranged from 0.1 to 1.9 mm/year (mean 0.12 mm/year). The linear penetration of the femoral condyle in relation with true wear was negatively correlated with length implantation in both medial and lateral implants. The linear penetration in relation with creep was higher in the first two years after the implantation versus the subsequent years in both medial and lateral implants. Using multiple linear regression analyses to remove the confounding effects of age, weight, gender and thickness of the implant, we found that an increase of the postoperative deformity was in relation (p = 0.03) with an increase of creep and an increase of true wear for medial implants. But an increase of the postoperative deformity was not in relation (p = 0.34) with an increase of creep or an increase of true wear for lateral implants.

Discussion: the postoperative deformity has a high influence on the penetration rate of the femoral condyle in the polyethylene of medial unicompartmental fixed bearing tibial implants. This phenomenon was not observed for the lateral fixed bearing implants and wear was significantly (p= 0.01) less in lateral than in medial implants. This phenomenon (important for the surgical technique and the choice of implant designs) may be in relation with different kinematics in the two compartments.