Purpose of the study The purpose of this study was to evaluate the practical application of computed tomography (CT) measurements of the TT TG (tibial tuberosity - trochlear groove) distance in patients undergoing surgery for patellar instability. Material and methods We reviewed retrospectively 42 patients (30 women, 12 men) who underwent surgery for patellar instability between 1989 and 2002. Objective evidence of unilateral instability was present in 36 patients and of bilateral instability in 6. Pre- and postoperative CT scans of both knees were examined for each patient to measure the TT TG distance for both knees (n = 48 knees). We also studied the difference in two consecutive TT TG measurements made on 36 non operated knees. For the 48 operated knees, we compared the CT measurement of medialization (difference between the pre- and postoperative TT TG) and the measurement made intraoperatively. Results For the 36 non-operated knees, the mean difference between two consecutive TT TG measurements was 3.2 mm (range 0-13 mm). This difference was significant. For the 48 operated knees, medialization measured on the CT scan was 8.6 mm on average. Medialization effectively measured intraoperatively was 6.9 mm on average for the same series of knees. Comparing these two types of measurement, the difference expressed in absolute value was 4.6 mm (range 0-20 mm). This corresponded to a 67% difference (4.6/6.9) for the TT-TG measurement compared with surgically performed medialization. Discussion For some authors, preoperative measurement of the TT TG distance remains a useful tool for establishing therapeutic choices for patellar instability. It must be recalled however that the measurement error is to the order of 3.25 mm. Use of the TT-TG for postoperative assessment is however a much more difficult technique since the anatomic landmarks have been modified by surgery. A rigorously applied standard radiographic protocol is required. The surgeon can then conduct a critical analysis of the landmarks retained, the slices used and the values obtained.
