Effect of Trochlear Dysplasia on Commonly Used Radiographic Parameters to Assess Patellar Instability

BACKGROUND: Trochlear dysplasia (TD) is an important anatomic risk factor for patellar instability (PI). Several imaging-based risk factors for PI have been established, but it is unclear what effect TD has on these measurements, and the Dejour method of categorizing TD has been shown to be unreliable. The lateral trochlear inclination (LTI) is a quantifiable measurement of proximal trochlear morphology. Recently, a modified technique for measuring LTI referencing the posterior condylar angle has demonstrated near-perfect reliability and may serve as a new standard for quantifying TD. PURPOSE/HYPOTHESIS: The purpose of this study was to evaluate how TD, expressed in terms of LTI, affects historically used measures of PI, including the sulcus angle (SA), tibial tuberosity to trochlear groove (TT-TG) distance, lateral patellar inclination (LPI), and Caton-Deschamps (CD) ratio. We hypothesized that lower LTI (ie, more dysplastic trochlea) will correlate with higher SA and TT-TG distances, increased patellar tilt, and more patella alta. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Patients aged 9 to 18 years treated for PI between January 2014 and August 2017 were queried. On magnetic resonance imaging (MRI), LTI, SA, LPI, and the TT-TG measured at the most proximal (pTT-TG) and distal (dTT-TG) aspects of the fully formed trochlear groove were measured on axial images. CD ratio was measured using sagittal MRI as well as lateral radiographs. All measurements were performed by 2 independent observers. Patients were included if there was a diagnosis of PI that was confirmed with a chart review and if there were appropriate imaging studies to analyze. RESULTS: A total of 65 patients met inclusion criteria for this study. Average pTT-TG distance (15.7 ± 4.5 mm) was not significantly different from dTT-TG distance (15.7 ± 4.2 mm) (P = .94). LTI had a weak but significant inverse correlation with pTT-TG (r = –0.40; β = –0.15 mm; P < .001) and SA (r = –0.43; β = –0.478°; P < .001). There was a moderate and significant inverse correlation of LTI with dTT-TG (r = –0.593; β = –0.21 mm; P < .001) and with LPI (r = –0.69; β = –0.54°; P < .001). There was a weak but significant correlation between LTI and the MRI-based CD ratio (r = 0.279; β = 0.149; P = .005). Radiograph-based CD ratio did not show a significant correlation with LTI (r = –0.189; P = .135). CONCLUSION: Smaller (ie, more dysplastic) LTI values correlated significantly with larger TT-TG, SA, LPI, and MRI-based CD ratio measurements. The relationship between SA and LTI is intuitive, but the relationship between LTI and the other analyzed variables suggests that they are dependent variables to TD and may not be consistent independent risk factors for PI. Together, these suggest that TD alters the radiographic interpretation of TT-TG and patellar tilt. The correlation between TD and patellar height is unclear and warrants further investigation.