Subluxation of the patella. Investigation by computerized tomography.

Subluxation of the patella may produce pain and instability in young people. However its classification, diagnosis and treatment are controversial. We have used computerized tomography (CT) to measure the amount of lateral shift of the patella and the angle of tilt to the femur in patients with subluxation. The mean lateral shift was 31.4% and the angle of tilt 31.8 degrees. These values are significantly greater than those obtained from conventional axial radiographs or in arthrographic axial views. We have obtained interesting evidence that the rate of lateral shift apparently increased on voluntary contraction of the quadriceps muscle (Q-contracted). With the quadriceps contracted, the mean rate of the lateral shift in the affected knees showed a significant increase (+27.7%) compared to that in normal knees (+14.0%). We have reviewed 40 knees treated by operative realignment with a mean follow-up of 27 months. The mean rates of lateral shift measured from CT-image in the knees before operation improved significantly to the normal range at follow-up, both with the quadriceps relaxed and contracted. The results evaluated by Insall's criteria were "excellent" in 13 knees; "good" in 22; "fair" in 3 and "poor" in 2. The satisfactory group (those with excellent and good results) revealed an improvement in lateral shift when the quadriceps was contracted, while the unsatisfactory group (fair and poor results) did not.