Ratio of Tibial Tubercle-Trochlear Groove Distance to Patellar Width as a Predictor of Patellar Dislocation: Analysis of Individualized Tibial Tubercle Lateralization Parameters.

Background: Available conventional tibial tubercle lateralization (TTL) parameters fail to account for individual patient size or anatomy.

Purposes: To evaluate the predictive ability of individualized TTL parameters and clarify the best predictor of patellar dislocation (PD) and to determine the relationship of the best predictor with other risk factors of PD with quadriceps isotonic contraction.

Study design: Cohort study (diagnosis); Level of evidence, 2.

Methods: A total of 15 patients with PD (28 knees) and 14 controls (28 knees) underwent 4-dimensional computed tomography, and the image with the knee fully extended and quadriceps isotonically contracted was selected for evaluation. The following TTL parameters were measured on the images: tibial tubercle-trochlear groove (TT-TG) distance, tibial tubercle-Roman arch distance, and tibial tubercle-posterior cruciate ligament distance, as well as their ratios to femoral condylar width and patellar width (PW). In addition, the following parameters were measured: Insall-Salvati ratio, Blackburne-Peel ratio, Caton-Deschamps ratio, modified Insall-Salvati ratio, bisect offset index, congruence angle, lateral patellar tilt, lateral trochlear inclination, sulcus depth, sulcus angle, trochlear groove medialization, patella-patellar tendon angle, patellofemoral axis angle (P-FAA), patellar articular facet-patellar tendon angle, patellar articular facet-femoral axis angle (PA-FAA), and patellar shape according to Wiberg type. The area under the receiver operating characteristic curve (AUC) was calculated to access the diagnostic accuracy of the TTL parameters for PD, and the TTL parameters with high diagnostic ability were evaluated for correlation with the remaining parameters.

Results: The TT-TG/PW ratio had the best diagnostic ability for PD (AUC = 0.890). The TT-TG/PW ratio was positively correlated with bisect offset index, congruence angle, lateral patellar tilt, P-FAA and PA-FAA (r = 0.610, 0.465, 0.635, 0.553 and 0.418, respectively; P≤ .027 for all), and TT-TG/PW ratio was greater in knees with type II versus type III patella (P = .017).

Conclusion: With the knee fully extended and quadriceps isotonically contracted, the TT-TG/PW ratio was found to be the best predictor of PD and reflected individualized TTL, which helps with clinical preoperative planning.