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.

Maltracking or subluxation is one of the complication of patellofemoral arthroplasty (PFA). The purpose of this investigation was to measure femoral component rotational alignment in PFA using a standard computed tomography (CT) scanner. Second, apply this technique to two groups; a control group of patients with well functioning PFA and a study of a group of patients with patellofemoral problems as maltracking or subluxation.

Data was analyzed from our center that has continuously performed PFA for isolated patellofemoral degenerative disease since 1978. Patients were included if they had a minimum four year follow up. A total of 124 patients (149 knees) were treated with PFA. There were 39 men and 85 women who had a mean age of 64 years (range, 46 to 78 years). A pre-operative and post-operative CT scan is performed in our center for all the patients since this period to assess femoro-patellar malalignment. The trochlear twist angle was determined using the single axial CT image through the femoral epi-condyles. To determine whether the femoral component was in excessive internal or external rotation, measurements were done on the post-operative CT scan and the trochlear twist angle of the femoral component was compared to the pre-operative trochlear twist angle.

At a mean follow up of 13 years (range, 4 to 30 years), overall prosthetic survival and preservation was 91 per cent. There were 112 knees (75 per cent) with good or excellent clinical results (Knee Society score of 80 points or more). Revision to total knee replacement for femoro-tibial disease progression was necessary in 9 knees (6 per cent). Complications related to the patellofemoral arthroplasty (28 knees) included : residual pain or mechanical symptom 10 (7 per cent) requiring other ancillary procedures ; maltracking or subluxation 18 (12%) with component revision in 10 knees; Radiographic findings show 2 component loosenings and 1 patella fracture. There was no incidence of infection or component wear.

The group with patellofemoral complications had excessive (p less than 0.01) femoral internal component rotation. This excessive combined internal rotation was directly proportional to the severity of the patellofemoral complication. Small amounts of internal rotation (1–4 degrees) correlated with pain. Moderate combined internal rotation (5–10 degrees) with lateral tracking and patellar tilting. Large amounts of combined internal rotational (10–17 degrees) correlated with patellar sub-luxation, early patellar dislocation or late patellar prosthesis failure (fracture of the patella or loosening of the patella button). The control group (112 knees without complications) was in external rotation (10-0 degrees).

This study showed that increasing amounts of excessive internal rotational malalignment resulted in more severe patellofemoral complications.