The Patellofemoral Entry Point–Transition Point Angle: Digitalization and Reliability Analysis

Abstract

Background: Trochlear dysplasia is important in recurrent patellofemoral instability (PFI); reliable metrics are essential to understand this condition and help define optimal treatment. One such measurement is the entry point to transition point (EP-TP) angle, which describes 2 components of patellar tracking from full knee extension to flexion: (1) mediodistal movement and (2) pure distal movement. Currently, measurement of this metric relies on 3-dimensional (3D) models. Purpose: To further develop the EP-TP angle metric to allow its digital measurement, to establish cohort and interrater reliability values, and to evaluate its appropriateness for clinical use. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: A data set composed of 30 recurrent PFI patient knees and 30 control knees was acquired. Each knee came with anterior views of the distal femur and curvature maps, which highlighted relevant structures such as the trochlear groove and the medial and proximal ridge. Three raters measured EP-TP angles utilizing a digital tool with and without the curvature maps. Significance of differences in means, presence of a TP, and interrater reliability with intraclass correlation coefficients were tested and evaluated according to an established guideline (<0.5, poor reliability; >0.5-0.75, moderate; >0.75-0.9, good; >0.9 excellent reliability). A significance level of .05 corrected with the Bonferroni method was assumed. Results: PFI knees had a significantly higher EP-TP angle than the control cohort (48.9° ± 10.8° vs 11.8° ± 14.6°; P < .001). Patients had a greater TP prevalence than controls (96.7% vs 10.0%). Interrater reliability for the EP-TP angle with curvature maps was significantly better than without (0.84 [95% CI, 0.76-0.89] vs 0.67 [95% CI, 0.55-0.78]; P < .001). Conclusion: The EP-TP angle is significantly different between PFI patients and controls. This study presents a digital approach for measuring the EP-TP angle, in contrast with the initially described method, which required 3D prints. This digital approach, when aided with curvature mapping, yields better reliability.