Do the spherical femoral head and the hemispherical acetabulum origin from the same center of rotation?
Nadine Hendriks, Fleur Boel, Michiel M. van Buuren, Noortje S. Riedstra, Harbeer Ahedi, Vahid Arbabi, Nigel Arden, Cindy Boer, Flavia Cicuttini, Timothy F. Cootes, Kay M. Crossley, David Felson, Willem Paul Gielis, Josh Heerey, Graeme Jones, Stefan Kluzek, Nancy E. Lane, Claudia Lindner, John A. Lynch, Joyce v. Meurs, Andrea Mosler, Amanda E. Nelson, Michael Nevitt, Edwin Oei, Jinchi Tang, Harrie Weinans, Carl Johan Tiderius, Sita Bierma-Zeinstra, Jos Runhaar, Rintje Agricola
DOI: https://doi.org/10.1016/j.joca.2024.02.535
Purpose (the aim of the study):
The Center of Rotation (CoR) of a hip joint is a critical parameter in understanding the joint’s biomechanics and function. In research, the CoR of the hip is important for the calculation of different angles, for example the alpha angle (cam morphology) and the center edge angle of Wiberg (hip dysplasia). Both these morphologies are important risk factors for developing hip osteoarthritis (OA). There are two methods often applied in research to determine the CoR of the hip joint on two-dimensional (2D) radiographs. One is based on the center of the circle made by the sphericity of the femoral head and the other is based on the center of the circle made by the concave of the acetabulum. Surprisingly, these methods have never been compared head-to-head. Therefore, the aim of this study was to determine and compare the CoR obtained from the convexity of the femoral head to the CoR obtained from the concave of the acetabulum in hips free of OA.
Methods:
For this study, we used data from the Worldwide Collaboration on Osteoarthritis prediCtion for the Hip (World COACH). World COACH includes 9 prospective cohort studies that have longitudinal hip imaging data available with at least 4 years apart. To rule out potential influence of OA on the CoR, participants with no signs of OA in the standardized anteroposterior radiographs of the hip joints were selected (KL/croft grade = 0). Radiographic images without millimeter indications were excluded. The contour of the femoral head and acetabulum were outlined with points automatically placed using Bonefinder. From this point set, the CoR based on the convexity of the femoral head and the acetabulum were calculated for each hip, based on the best fitting circle method. The circle fit was optimized by performing calculations for the best fitting circle using different combinations of points on the convexity of the femoral head and acetabulum to obtain the circle fit with the smallest radius and with the smallest root mean square error of the distances between the points. With the x-, and y-coordinates of the CoRs of both methods, the directional distance between the points was calculated for each participant.
Results:
Radiographs of 13,635 hips have been included in this study. The mean age of the included participants was 59.0 ± 8.2 years, mean BMI 28.2 ± 4.8 kg/m2, and 61% was female. The mean radius of the circle fit of the femur was 27.25 mm [95% CI: 27.20 – 27.29]. The mean radius of the circle fit based on the acetabulum was 34.17 mm [95% CI: 34.10 – 34.24]. An example of the circle fit and CoR for both methods is visualized in Fig. 1. The mean difference between the x-coordinates of both methods was 1.76 mm [95% CI: 1.74 – 1.77], the mean difference between the y-coordinates of both methods was -2.66 mm [95% CI: -2.71 – -2.61], with a mean directional distance of 3.89 mm [95% CI: 3.85 – 3.92].
Conclusions:
Based on the mean difference in x-, and y-coordinates per hip, the CoR based on the femoral head method is placed 1.76 mm more medially and 2.66 mm towards cranial direction compared to the CoR using the acetabulum method, with a mean directional distance of 3.89 mm. Due to the fact that the CoR of the hip is important for the calculation of different hip joint angles, these differences in CoRs might influence the estimated prevalence of hip pathologies associated with OA development.
