Dysplastic Hips in Young Adults Demonstrate Different Relationships Between Acetabular Coverage, Joint Congruity, and Contact Mechanics Than Asymptomatic Hips.

This study investigated the relationship between three-dimensional (3D) acetabular coverage and contact mechanics in dysplastic and ostensibly normal hips. Fifty asymptomatic hips previously imaged with CT scans/angiograms were matched on a 2:1 basis to 25 dysplastic hips with previous CT imaging, based on age, gender, weight, and BMI. CT imaging was used to create 3D patient-specific hip models from which the 3D coverage metrics of subchondral arc angle (i.e., acetabular weight-bearing morphology) and hip joint coverage angle (i.e., femoral head coverage), and the congruity metrics of acetabular sphericity index $\mathrm{SI}$ (i.e., sphericity of the acetabulum) and joint congruity index $\mathrm{CI}$ were assessed globally and in five octants spanning the weight-bearing acetabulum. Discrete element analysis was used to calculate hip contact mechanics, with results assessed globally and subdivided into the same five octants. Increasing superior-anterior subchondral arc angle was associated with increasing superior-anterior mean chronic contact stress-time exposure in dysplastic hips, which was significantly (p < 0.001) different from asymptomatic hips where increasing superior-anterior subchondral arc angle was associated with decreasing superior-anterior mean chronic contact stress-time exposure. Similarly, increasing joint congruity $\mathrm{CI}$ anteriorly was associated with increasing anterior mean chronic contact stress-time exposure in dysplastic hips, which was significantly (p = 0.003) different from the trend of decreasing anterior mean exposure with increasing anterior $\mathrm{CI}$ in asymptomatic hips. These results indicate fundamental differences in how contact mechanics in asymptomatic and dysplastic hips respond to differences in acetabular coverage and joint congruity, suggesting that asymptomatic hips follow the expected geometry-based trend, while dysplastic hips do not.