Paper 65: Novel CT-Derived 3D Modeling Assessing Dysplastic Hips Versus Controls Demonstrates Decreased 2D Lateral Center Edge Angle, but not Tönnis Angle, Predicts Decreased 3D Anterolateral Coverage

Objectives: Acetabular coverage has significant implications in both acetabular dysplasia and femoroacetabular impingement syndrome (FAIS), with both undercoverage (dysplasia) and overcoverage (pincer-type FAIS) contributing to symptoms, labral tears, and degenerative joint changes. Measurements of acetabular coverage inferred from 2D imaging have been shown to be unreliable, and they provide a limited view of complex 3D morphology. Consequently, there is a need 1) for a consistent technique to allow for accurate, repeatable measurement of acetabular coverage and 2) to determine whether and to what degree 2D radiographic measures of acetabular coverage are related to 3D measures. Thus, the objectives of this study were to utilize novel 3D modeling techniques to 1) compare 3D acetabular coverage between patients undergoing HA followed by periacetabular osteotomy (PAO) for combined dysplasia and FAIS to patients undergoing HA alone for FAIS and 2) evaluate the association between 2D radiographic measures of acetabular coverage—lateral center edge angle (LCEA) and Tönnis angle (TA, acetabular index)—and acetabular coverage as measured on 3D osseous models. Methods: An IRB-approved retrospective review of all patients who underwent HA+PAO from a large orthopedic practice from 2017-2021 was conducted. Included patients had a preoperative anteroposterior (AP) pelvic radiograph and CT scan of the full pelvis and bilateral proximal femurs. HA+PAO patients were matched 1:1 by sex, age, and body mass index (BMI) to patients who underwent HA alone. LCEA and TA were measured on AP pelvic radiographs. Dysplasia subgroups were defined as severe (LCEA < 15°), moderate (LCEA 15°-19.9°), mild/borderline (LCEA 20°-24.9°), and normal/non- dysplastic (LCEA 25°-39.9°). 3D osseous models of the femur and acetabulum were reconstructed from CT scans (Materialise Mimics v24.0) and uploaded into 3D modeling software (Materialise 3-matic v16.0). Acetabular coverage of the femoral head was measured by projecting the acetabular surface onto the femoral head surface as delineated by the acetabular rim profile. A coronal, transverse, and sagittal plane were made. The coronal plane was established by first defining a reference plane using points on both anterior superior iliac spines and the pubic tubercle on the affected side. A parallel plane was then created that passed through the affected femoral head center (FHC). The transverse plane was created by making a true horizontal plane p