Summary
The Entry Point-Transtion Point angle (EP-TP) describes the path the patella takes through the trochlea groove during a flexion cycle.
Abstract
Background
Trochlear dysplasia and corresponding patellar maltracking are important factors in recurrent patellofemoral instability (PFI) and therefore there is a need for reliable metrics to better understand trochlear deformity. One such measurement is the entry point to transition point (EP-TP) angle, which describes two components of patellar tracking from full knee extension to flexion: (1) medio-distal movement and (2) pure distal movement. Currently, measurement of this metric relies on printed 3D models.
Purpose
The purpose of this study was to further develop the EP-TP metric to allow its digital measurement, to establish cohort and interrater reliability values, and to evaluate its appropriateness for clinical use.
Study design
Case-Control Study. Level of evidence III.
Methods
An existing dataset comprising 30 recurrent PFI patient knees, with at least two reported dislocation events, and 30 control knees was acquired. Each knee came with both AP-views of the distal femur and curvature maps, which highlight relevant structures such as the groove and the medial and proximal ridge. Three raters measured EP-TP utilizing a digital tool once with and once without the curvature maps. Significance of differences in means, presence of a transition point, and interrater reliability with interclass correlation coefficients were tested and evaluated according to an established guideline (<0.5 poor reliability, 0.5 to 0.75 moderate, 0.75 to 0.9 good, >0.9 excellent reliability). A 'significance level of 0.05 corrected with the Bonferroni method was assumed.
Results
PFI knees had significantly (p<0.001) higher EP-TP than the control cohort (48.9±10.8 vs. 11.8±14.6 degree). The presence of a transition point was higher in patients (96.7% vs. 10.0%) and the interrater reliability for the EP-TP with curvature maps was significantly (p<0.001) better than without (0.84 [CI95%:0.76-0.89] vs. 0.67 [0.55-0.78]).
Conclusion
The entry point- transition point angle (EP-TP) quantifies trochlear dysplasia. In this study we present a new approach for taking this measurement, which can be conducted purely digitally, by contrast with the initially described method, which required 3D prints. We have also identified This digital approach can be aided with additional information such as curvature mapping to yield better reliability.
Key Terms:
Patellofemoral instability, Trochlea Dysplasia, three-dimensional (3D) analysis