Middle patellar tendon to posterior cruciate ligament (PT-PCL) and normalized PT-PCL : new magnetic resonance indices for tibial tubercle position in patients with patellar instability

BACKGROUND: To demonstrate whether the distance between the middle point of the patellar tendon and posterior cruciate ligament (PT-PCL) calculated on a single axial MR image could be an alternative measure to tibial tubercle-PCL (TT-PCL) distance for TT lateralization without the need of imaging processing. To show that normalization of PT-PCL (nPT-PCL) against the maximum diameter of the tibial plateau may help to identify patients with patellar instability (PI). METHODS: MR scans of 30 patients (13 females, age 32 ± 13 years) with known PI and 60 patients (31 females, age 39 ± 19 years) with no history of PI were reviewed. Two operators calculated TT-PCL, and PT-PCL nPT-PCL. Intraclass correlation coefficient, Student's t-test, Receiver Operator Characteristic curves, Spearman's Rho and McNemar's test were used. RESULTS: Interobserver reproducibility was 0.894 for PT-PCL for TT-PCL (95% CI = 0.839-0.930) and 0.866 for TT-PCL (95% CI = 0.796-0.912). The PT-PCL was 23.5 ± 3.8 mm in patients and 20.0 ± 2.7 mm in controls (P < 0.001). The TT-PCL was 22.9 ± 3.9 mm in patients and 20.5 ± 2.7 mm in controls (P = 0.002). Correlation between the PT-PCL and TT-PCL was R = 0.838, P < 0.001. The PT-PCL had 66.6% (95% CI = 0.542-0.790) diagnostic yield. The nPT-PCL was significantly higher in patients (0.302 ± 0.03) than controls (0.271 ± 0.03; P < 0.001) with 73.9% (95% CI = 0.628-0.851) diagnostic yield. CONCLUSION: The PT-PCL correlated with TT-PCL, with 66.6% diagnostic yield. The nPT-PCL may represent an additional index, with 73.9% diagnostic yield.