Lower Trochlear Facet Asymmetry Ratio Is Associated with Objective Patellar Instability in a Retrospective MRI Case-Control Study with Patella Alta-Positive Controls

Background/Objectives: Because patella alta frequently accompanies objective patellar instability, separating trochlear morphology from patellar height is difficult. We evaluated whether the trochlear facet asymmetry ratio was associated with instability among knees with patella alta. Methods: This retrospective case-control MRI study included 123 knees: 43 with objective patellar instability, 43 normal-height controls, and 37 patella alta-positive controls without instability identified by re-screening the eligible control pool. Nine morphologic parameters were measured. The primary analysis compared the instability group with patella alta-positive controls; a supportive analysis used the full cohort. Results: The facet asymmetry ratio showed a graded pattern across groups (instability, 42.7 ± 12.4%; patella alta-positive controls, 59.2 ± 10.7%; normal-height controls, 70.3 ± 11.4%; p < 0.001). In the primary comparison, it had the highest observed apparent area under the curve (0.855), although its confidence interval overlapped with those of quantitative trochlear depth and tibial tubercle-trochlear groove (TT-TG) distance; the Insall–Salvati ratio showed weaker discrimination (0.679). After adjustment for the Insall–Salvati ratio and age, a lower facet asymmetry ratio remained associated with instability (odds ratio per 1-percentage-point increase, 0.903; 95% confidence interval, 0.854–0.956; p < 0.001). This association was attenuated after additional adjustment for quantitative trochlear depth or for tibial tubercle-trochlear groove distance together with Wiberg type. Interobserver reliability was excellent (intraclass correlation coefficient, 0.994). Conclusions: In this selected cohort, a lower trochlear facet asymmetry ratio was associated with objective patellar instability after adjustment for patellar height and age. The internally derived 49% threshold is exploratory and requires external validation before clinical use. © 2026 by the authors.