ePoster #727 - ISHA Annual Scientific Meeting 2016

Extent Of Head Coverage Is The Principle Determinant Of Hip Motion To Impingement

Joshua Choi, BS, Houston, TX UNITED STATES
Ryan Blackwell, BS, Houston, TX UNITED STATES
Sabir K. Ismaily, PhD, Houston, TX UNITED STATES
Rathika Mallepalley, BS, Houston, TX UNITED STATES
Philip C. Noble, PhD, Houston, TX UNITED STATES

The Institute of Orthopedic Research and Education, Houston, TX, UNITED STATES

FDA Status Not Applicable

Summary: Our study has identified extent of femoral head coverage as key morphologic parameter that can act as a strong predictor of FAI in preoperative assessments, the results of which indicate that acetabular and not femoral parameters are significant predictors of hip motion in cases of mixed FAI - this suggests that new morphologic parameters derived from MR imaging should be explored to identify FAI.

Introduction: Previous studies have shown that osseous morphology is a principal determinant of hip range of motion (ROM), though the relative importance of femoral vs acetabular morphology is presently undetermined. In the case of femoro-acetabular impingement (FAI), reduced hip motion has been attributed to two distinct entities, cam-type FAI and pincer-type FAI, characterized by specific femoral and acetabular deformities. However, as most patients present with a combination of the cam and pincer-morphology, it is our hypothesis that a compound indicator derived from both femoral and acetabular parameters will provide more accurate predictions of the functional impairment of joint motion. In this study, we examine the accuracy of alternative parameters derived from single reformatted MR slices as predictors of joint ROM in cases of mixed cam and pincer FAI.

Method: Fifteen 3D computer models of the hip joint were prepared by reconstructing CT scans of cases with mixed cam- and pincer type FAI (Average alpha angle:75.0±12.6°, average lateral center edge angle (LCE): 43.9±4.7°). The models of the femur and pelvis were loaded into custom software that allowed relative motion simulating an anterior impingement test in which the femur was initially placed in 90° of flexion and 15° adduction, and internally rotated to impingement. The point of impingement was marked on the femur and the acetabulum. As potential predictors of joint motion, we measured 8 morphologic parameters - 3 femoral (Alpha angle, the anterior head-neck offset (AO), the anterior offset ratio (AOR)), and 5 acetabular - using CAD and customized image analysis software. A single slice was taken through each computer model perpendicular to the acetabular rim passing through the point of impingement. Stepwise linear regression was performed on the morphologic parameters against ROM data to find the determinants that contributed significantly to overall variance.

Results: None of the parameters defining femoral morphology was significantly correlated with anterior ROM, including the alpha angle (r= 0.201, p=0.236), the head-neck offset (AO; r= -0.087, p=0.379), and the anterior offset ratio (r= 0.009, p=0.487). The Anterior ROM (average value: 14.0±10.0°) was significantly correlated (p=0.006) with two variables, the angle subtended by the acetabulum (A; average: 193.6±5.6°) and the distance between the center of the acetabulum and the point of impingement (B; average value: 44.6±3.2 mm). The contribution of each to anterior ROM for each significant parameter was A: 34% and B: 50%.

Discussion: Previous studies have suggested various morphologic parameters as primary determinants of hip ROM, such as the femoral alpha angle. However, the results of this study identifies two acetabular parameters, A and B, as the principle determinants of anterior hip ROM. Both parameters relate directly to femoral head coverage, which could have clinical significance in the diagnosis of FAI. The fact that no femoral parameters significantly influenced ROM in this study suggests more attention should be directed to acetabular morphologies in future correlative studies.