Poster 291: Entry Point-Transition Point Angle as a Novel 3-Dimensional Metric of Patellar Instability

Abstract

Objectives: Three-dimensional (3D) modeling has increasing utility in identifying differences in trochlear morphology in patients with patellar instability when compared to historical 2-dimensional (2D) attempts. Distortions in the proximal medial ridges of trochleas are a distinguishing feature of trochlear dysplasia. Recently, the entry-point to transition-point (EP-TP) angle was established as a novel 3D metric to quantify degree of dysplasia, as it captures the tracking of the patella within the trochlea. However, previous attempts relied on 3D prints, which are not widely accessible. The purpose of this study is to validate this new measurement and compare differences between patients with patellar instability and the general population. Methods: Forty-three patients with recurrent patellar dislocations who were treated by a single surgeon between January 2020 and July 2023 were included. The study received an institutional review board exemption. Individuals from the New Mexico Decedent Image Database with no history of patellar instability or patellofemoral pain were used to obtain computed tomography (CT) images from the general population. Stereolithography (STL) models of distal femurs in 3D were generated from CT images using ScanIP. Minimum and maximum principal curvature maps were taken as screenshots from maps generated in MeshLab. Two people independently conducted measurements using a custom measuring tool displaying anteroposterior views and curvature maps of the distal femur. The measurers were asked to locate the medial and lateral ridges, transition point, and apex of the trochlea groove for each femur. A Mann–Whitney U test was performed to evaluate whether the recurrent dislocators had higher EP-TP values. P values below 0.01 were regarded as significant. Interclass correlation for both raters (ICC (1, k), reliability of values within this study) and single rater (ICC (1,1), reliability of measurement within clinical practice) and the 95% CI thereof, was computed for the 2 independent measurements. Results: A total of 100 femurs were measured, including 43 recurrent dislocators and 57 population femurs. The recurrent dislocator cohort had a mean EP-TP of 40.9° with a standard deviation of 16.4°. The general population cohort had a mean EP-TP of 11.3° with a standard deviation of 12.3°. EP-TP for dislocators and general population was significantly different. (p < 0.001). ICC (1, k) was 0.81 with a 95%-confidence interval of 0.72 to 0.87. ICC (1,1) was 0.69 with a 95% CI of 0.57 to 0.78. Conclusions: EP-TP angle is a novel metric that helps quantify trochlear dysplasia, which can help clinicians better understand patellar instability. In this study, we validate EP-TP angle by showing that EP-TP angle was significantly different between femurs of recurrent dislocators and the general population.