PATELLOFEMORAL ARTHROPLASTY RESULTS ARE IN RELATION WITH OPTIMIZED ROTATION OF THE FEMORAL COMPONENT

Abstract

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.