HEAD-TO-HEAD COMPARISON BETWEEN THE CENTER OF ROTATION BASED ON THE SPHERICAL FEMORAL HEAD AND THE HEMISPHERICAL ACETABULUM

INTRODUCTION

The center of rotation (CoR) on 2D hip radiographs is important for the calculation of different angles, for determining the presence of hip dysplasia. The hip joint CoR can be found as the center of a circle fitted to the femoral head. Since the acetabulum and femoral head are concentric, realistically their CoR projects on the hip joint CoR. The two methods to determine the CoR have surprisingly never been compared head-to-head.

OBJECTIVE

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

Data from the Worldwide Collaboration on Osteoarthritis prediCtion for the Hip (World COACH) was used. Participants with no signs of OA in the standardized anteroposterior radiographs of the hip joints were selected (KL/croft grade = 0). The contour of the femoral head and acetabulum were outlined with points automatically placed using Bonefinder and the CoR based on the convexity of the femoral head and the acetabulum were calculated for each hip. With the x-, and y-coordinates of the CoRs of both methods, the directional distance between the points was calculated. Using the CoRs of both methods the LCEA and WCEA were calculated. In sensitivity analysis, the (mild) dysplasia (WCEA and LCEA<25°) cases, based on the femoral head method, were excluded.

RESULTS

The mean age of the participants (n = 13,683) 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 (SD = 2.79). The mean radius of the circle fit based on the acetabulum was 34.17 mm (SD = 4.15). The mean difference between the x-coordinates of both methods was 1.76 mm (SD = 0.98), the mean difference between the y-coordinates of both methods was -2.66 mm (SD = 3.02), with a mean directional distance of 3.89 mm (SD = 2.27). Based on the CoR using the femoral head method the mean LCEA = 34.80° (SD = 5.89), mean WCEA = 29.67° (SD = 6.29), based on the CoR using the acetabulum method the mean LCEA = 30.41° (SD = 6.65), mean WCEA = 25.11° (SD = 6.80). After exclusion of the (mild) dysplasia cases the mean difference between the x-coordinates of both methods was 1.68 mm (SD = 0.84), the mean difference between the y-coordinates of both methods was -2.62 mm (SD = 2.81), with a mean directional distance of 3.71 mm (SD = 2.08). In Table 1, the number of participants with (mild) dysplastic hips are shown based on the different CoRs.

CONCLUSION

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 concave of the acetabulum, with a mean directional distance of 3.89 mm. The CoR determination is independent of the presence of (mild) dysplasia. However, among (mild) dysplastic, the differences between methods on classification of dysplasia is much larger than in the non-dysplastic hips. In the current form, methods cannot be used interchangeably.