Radiographic and computed tomographic analysis of the position of the tibial tubercle in recurrent dislocation and subluxation of the patella.

Among the factors that influence the patellofemoral alignment and stability, the position of the tibial tubercle was evaluated using plain radiographs and computed tomography (CT). Radiographs of the Q-angle with the knee in full extension and quadriceps muscle relaxed showed that mean Q-angles in the dislocation or subluxation group and control group were 13.8 degrees and 14.3 degrees, respectively. The major reason for this deceptive measurement was a patella positioned laterally in the dislocation or subluxation group. Better estimation was made with the modified Q-angle measurement by using the most concave point of the intercondylar notch as a reference point instead of the center of the patella. By overshadowing CT images of the levels of the patellofemoral joint and the tibial tubercle, it was possible to evaluate the location of the tibial tubercle in relation to the femoral trochlea. Both the angular measurement (lateral deviation angle [LDA]) and the distance measurement (lateral deviation index [LDI]) showed that the tibial tubercle in the dislocation or subluxation group was rotated externally and shifted laterally (LDA, 36.3+/-7.0 degrees and LDI, 30.1+/-5.6) compared with the control group (LDA, 20.2+/-7.1 degrees and LDI, 15.1+/-5.6). Further study is warranted to elucidate the exact mechanism of recurrent patellar dislocation and subluxation.