The risk of osteochondral fracture after patellar dislocation is related to patellofemoral anatomy

Purpose: Despite the comprehensive literature on the anatomical risk factors for patellar dislocation, knowledge on the risk factors for subsequent osteochondral fracture (OCF) remains limited. Methods: Magnetic resonance imaging was used to compare measures of patellofemoral anatomy in patients with OCF after patellar dislocation and propensity score matched patients without OCF. For differing measures, limit values showing a 50% probability for the occurrence of OCF were calculated using predictive logistic regression modelling. Proportions of abnormal measures in the groups were compared using Chi-square test. The association of anatomical measures with OCF location was examined by comparing subgroup mean values in the different OCF locations. Results: Propensity score matching provided a total of 111 matched pairs of patients with OCF and patients without OCF. The patients with and without OCF differed in patellotrochlear index (PTI; 0.54 [95% CI 0.52–0.57] vs. 0.47 [95% CI 0.45–0.49]; p < 0.001), tibial tubercle-posterior cruciate ligament distance (TT-PCL; 21.6 mm [95% CI 21.0–22.3 mm] vs. 20.5 mm [95% CI 20.0–21.1 mm]; p = 0.013), trochlear depth (2.5 mm [95% CI 2.3–2.7 mm] vs. 3.0 mm [95% CI 2.8–3.2 mm]; p < 0.001) trochlear facet asymmetry ratio (0.54 [95% CI 0.51–0.57] vs. 0.43 [95% CI 0.42–0.45]; p < 0.001) and trochlear condyle asymmetry ratio (1.04 [95% CI 1.03–1.04] vs. 1.05 [95% CI 1.04–1.05]; 0.013. Thresholds for increased OCF risk were > 0.51 for PTI > 21.1 mm for TT-PCL < 2.8 mm for trochlear depth > 0.48 for trochlear facet asymmetry ratio and < 1.04 for trochlear condyle asymmetry ratio. Conclusion: In patients with OCF after patellar dislocation, trochlear configuration and patella vertical location were closer to normal anatomy, whereas patella lateralization was more severe when compared to patients without OCF. These anatomical factors contribute to the risk of OCF during patellar dislocation. Level of evidence: III.