BACKGROUND: The tibial tuberosity-trochlear groove (TT-TG) distance is an important reference for the evaluation of patellar instability. However, measurement of the TT-TG distance has disadvantages with relatively low reproducibility. This study aimed to investigate the reliability of patellar tendon lateral deviation angle (PTLD-A) measured on a single computed tomography (CT) slice and the clinical significance for predicting patellar instability. METHODS: Seventy-eight knees with recurrent patellar dislocation were included as the study group, and 76 normal knees in the control group. The PTLD-A and the TT-TG distance were measured on CT images, inter- and intra-observer reproducibility were assessed, and correlation was analyzed, and compared between the groups. The predictive value of both measurements for patellar instability was examined using the receiver operating characteristic curve, and the cut-off value was predicted using the Youden index. RESULTS: Inter- and intra-observer reproducibility of PTLD-A was better than TT-TG distance in both groups as well as across all extents of trochlear types. The correlation between the two measurements was strong (r = 0.756, P <
0.001). Notably, both measurements were significantly higher in the study group than in the control group (P <
0.05). PTLD-A showed high predictive value for patellar instability, whereas TT-TG distance showed medium predictive value. A PTLD-A value of ≥ 13.7° was the threshold for diagnosis of patellar instability. CONCLUSION: PTLD-A measured on a single computed tomography slice of the distal femoral trochlear groove is more reliable than TT-TG distance for prediction of patellar instability. A PTLD-A ≥ 13.7° predicts patellar instability.