Retrieved Unicompartmental Implants with Full PE Tibial Components: The Effects of Knee Alignment and Polyethylene Thickness on Creep and Wear

Creep and true wear of polyethylene are difficult to evaluate on radiographs of knee arthroplasties and for this reason the true rate of polyethylene wear in vivo after unicompartmental arthroplasty is not well known. This study evaluated the creep and true wear in fifty-five medial retrieved unicompartmental implants that had a flat articular surface at the time of implantation. All the full polyethylene tibial components had the same design and were retrieved from eleven to 224 months (mean 152 months) after their implantation. The postoperative varus 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 and the coordinates of a grid of points were obtained. Using this system, a three dimensional scaled image of the implant could be created and was used to calculate the total penetration of the femoral condyle due to true wear and creep. Total linear penetration rates ranged from 0.18 to 2.6 millimeters per year (mean 0.25 millimeters per year). Linear penetration rates due to true wear ranged from 0.08 to 1.4 millimeters per year (mean 0.13 millimeter per year), and penetration due to creep ranged from 0.07 to 1.9 millimeters per year (mean 0.12 millimeters per year). The linear and volumetric penetration rates of the femoral condyle due to true wear were negatively correlated with the duration of implantation. The linear penetration rate due to creep was higher in the first two years after the implantation compared to the subsequent years. 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 varus deformity was due to an increase of creep (p = 0.03) but not with an increase of true wear (p = 0.25). Thinner implants were due to an increase of creep (p = 0.02) but not with an increase of true wear (p = 0.34). Increase in age was in relation with decrease of wear (p = 0.02) and increase of weight with increase in creep (p = 0.03). Plastic deformation had a high influence on the penetration rate of the femoral condyle in full polyethylene implants. There is a risk of an increased penetration and a decrease of the remaining thickness of the tibial plateau when the implant is too thin, the knee mal-aligned and the patient heavy - each of these factors increasing the creep deformation. In conclusion, our study suggests that surgeons using the Lotus Mk I unicompartmental knee replacement for medial tibiofemoral arthritis should beware of the overweight patient (>90kg) with a tibial implant of less than 9 mm. There is a risk of increased penetration and decreased thickness of the tibial implant when it is too thin, the knee malaligned, and the patient heavy. All these factors increase creep deformation.