Relative and absolute patella height measurements are required in moderate and severe trochlea dysplasia for surgical planning.

Abstract

This study assessed the accuracy of the patellotrochlea index (PTI) in identifying relative patella alta in trochlea dysplasia. We hypothesise that PTI fails to appreciate relative patella alta in trochlea dysplasia, the role of both absolute and relative patella height measurements was assessed. This is a retrospective comparative radiographic study. Pre-operative knee radiographs and Magnetic Resonance Imaging (MRI) were reviewed. Cases were grouped using the Oswestry Bristol Classification (OBC). Patella instability cases were reviewed, alongside MPFL, TTO and trochleoplasty; ACL reconstruction patients were reviewed for normal trochlea morphology. 120 included. Patella height measurements were PTI and the Caton-Deschamps ratio (CD). Statistical analysis included McNemar's tests, linear regression, one-way ANOVA, and Pearson's correlation, with significance set at p < 0.05. In normal and mild dysplastic trochleas there was no significant difference in patella alta identified by CD and PTI (p > 0.05). In moderate (OBC 3) and severe (OBC 4) trochlea dysplasia there was only 30% and 40% agreement, respectively (p < 0.05) for CD and PTI. Sensitivity and specificity analysis showed 30% had a diagnostic CD with a negative PTI. Linear regression analysis indicated that a PTI of approximately 0.42 is a potential novel diagnostic point for relative patella alta in moderate and severe dysplasia. PTI fails to diagnose relative patella alta in moderate and severe trochlea dysplasia. We would advocate the use of both CD and PTI in preoperative assessment of patella instability. The potential novel diagnostic threshold of 0.42 to diagnose relative patella alta needs further research with outcome correlation.