BACKGROUND: Radiological risk factors for an osteochondral fracture (OCF) associated with patellar instability are rarely studied, particularly in patients with recurrent instability. PURPOSE: To identify specific radiological characteristics that relate to the increased prevalence of OCFs associated with patellar instability. STUDY DESIGN: Case-control study
Level of evidence, 3. METHODS: Patient data (n = 730) from the Justifying Patellar Instability Treatment by Early Results (JUPITER) multicenter study were reviewed for radiological findings. Trochlear crossing sign, Caton-Deschamps index (CDI), tibial tubercle-trochlear groove (TT-TG) distance, patellar tilt, trochlear depth, trochlear bump, sulcus angle, and patellar subluxation were measured on radiography or magnetic resonance imaging and classified into 2 categories based on pathological thresholds for general patellar instability. Patients were grouped according to the presence or absence of an OCF and analyzed with the Mann-Whitney test, chi-square test, and multivariate regression. RESULTS: A high CDI was associated with a 0.43 decreased odds of an OCF, while a high TT-TG distance was a risk factor for an OCF, with a 2.17 times increased odds. Although a first-time dislocation increased the odds of an OCF by 4.72 times, recurrent instability was found to have the same predictive relationship of CDI and TT-TG distance with fracture incidence. A lower CDI, a shallower trochlear depth, a smaller trochlear bump, and the presence of a patellar subluxation were more common in patients with OCFs. A lower CDI, positive trochlear crossing sign, and shallower trochlear depth were more common in the subset of patients with recurrent instability and associated OCFs. CONCLUSION: Patella alta was protective of OCFs in patients with first-time and recurrent instability, while a lateralized tibial tubercle was a risk factor. These radiological characteristics should guide health care professionals on the risk of future OCFs during treatment planning after a patellar instability event.