High-Grade Trochlear Dysplasia Confounds Patellar Height Assessment Using the Magnetic Resonance Imaging-Derived Patellotrochlear and Radiograph-Derived Caton-Deschamps Indices.

PURPOSE: To compare patellotrochlear index (PTI) and Caton-Deschamps index (CDI) measurements between normal and dysplastic trochleae, to evaluate the ability of PTI to discriminate patella alta, and to examine the relation of both indices with functional engagement as represented by the instability resolution angle (IRA). METHODS: Surgical patients diagnosed with patellofemoral instability between 2016 and 2024 who underwent preoperative magnetic resonance imaging and true lateral radiographs were retrospectively reviewed. Patellar height was measured using PTI on magnetic resonance imaging and CDI on radiographs, and IRA was recorded intraoperatively. Trochlear dysplasia was graded by Dejour classification and stratified into low-grade (normal/type A) and high-grade (types B-D). Differences in PTI and CDI across dysplasia grades and associations among CDI, PTI, and IRA were evaluated using mixed-effects modeling; IRA analyses were limited to knees with intraoperative measurements. RESULTS: A total of 152 knees (96 women, mean age 23.3 ± 9.8 years) were included. PTI differed across Dejour grades (P = .001) and was higher in type A and D trochleae relative to normal trochleae. CDI also differed across grades (P = .004) and was lower in type A and C trochleae relative to normal trochleae. Using thresholds of CDI ≥ 1.2 and PTI ≤ 0.28, 38.2% of knees showed discordant patella alta classification. In nondysplastic knees, all PTI-defined patella alta cases were also CDI-defined patella alta, but concordance declines in high-grade dysplasia (61.1%). The CDI-PTI association differed by dysplasia stratum (P = .30), with a significant inverse association in normal/type A but not high-grade dysplasia. Neither index was associated with IRA (P > .05). CONCLUSIONS: PTI and CDI show strong agreement in nondysplastic knees but diverge significantly in high-grade trochlear dysplasia, where PTI fails to correspond with CDI in identifying patella alta. LEVEL OF EVIDENCE: Level III, retrospective comparative cohort study.