External rotation of the patellar tendon insertion line: A novel predictor for recurrent patellar dislocation

Recurrent patellar dislocation (RPD) is a multifactorial disorder, with tibial tuberosity lateralization (TTL) as a major contributor. Although the tibial tuberosity-trochlear groove (TT-TG) distance is widely used to assess TTL, it is influenced by knee flexion and trochlear dysplasia. Based on our clinical observation of increased external rotation of the patellar tendon insertion line in RPD, we hypothesized that this abnormality alters patellar vector forces. Currently, no parameter accounts for the rotational orientation of the patellar tendon insertion line. This study evaluated the patellar tendon insertion angle (PTIA) as a novel parameter, compared it with TT-TG distance, and investigated their associations with femorotibial rotation (FTR) and external tibial torsion (ETT).

Forty-eight patients with RPD and 50 control subjects were retrospectively analyzed using low dose computed tomography. TT-TG distance, PTIA, FTR, and ETT were measured. Diagnostic accuracy was evaluated using receiver operating characteristic curve analysis, and correlations between parameters were analyzed using Pearson's coefficient.

The TT-TG distance and PTIA were significantly greater in the RPD group than controls (TT-TG: 19.3 ± 3.7 mm vs. 16.1 ± 4.1 mm; p < 0.001; PTIA: 17.2 ± 4.5° vs. 9.9 ± 3.3°; p < 0.001). The PTIA showed higher diagnostic accuracy than the TT-TG distance. The TT-TG distance correlated positively with FTR, whereas PTIA correlated negatively. Neither parameter was associated with ETT.

The PTIA, a new tibia-based parameter, is significantly increased in RPD and demonstrates superior diagnostic accuracy compared with the TT-TG distance.

Level IV, radiological cross-sectional study.