## 4 Discussion
Other studies have reported MRI-detected muscle atrophy around MoM hip articulations [44; 45; 46; 47; 48]. [PERSON] et al. [45] investigated 74 MoM hip arthrolasties, 51 implanted by a posterior approach, 21 by a lateral approach, and 2 unknown. [PERSON] et al. found that 75% had muscle atrophy of gluteus minimus and 50% had muscle atrophy of gluteus medius, with a median MRI-scan interval of 11 months. Likewise, [PERSON] et al. [48] investigated 20 patients with MoM hip arthrolasties, mainly implanted by the posterior approach and a few by the antero-lateral approach, and reported that 45% had muscle atrophy of the gluteus minimus, and 40% had muscle atrophy of the gluteus medius. Besides the theory of muscle atrophy in MoM hip arthrolasties being surgery-induced, some studies suggest that it may be related to an exaggerated inflammatory response to the metal wear debris created by MoM hip articulations, which is thought to reduce function [44; 45]. Unfortunately, our data failed to point out which theory is most likely. [PERSON] et al. [49] also found persistent muscle atrophy around the hip joint two years after MoP THA with a posterior surgical access compared to the contralateral hip. We chose a step test, which mimics the physical performance of getting into a bus or climbing stairs. Stepping up and down requires balance and a greater range of motion, and is able to identify inter-limb compensations in comparison to level walking [50]. In a step test it is not possible to compensate by shifting weight toward the unaffected leg. Instead, patients compensate with higher trunk flexion and vertical and antero-posterior acceleration to decrease the single leg loading time while stepping up. We found that patients undergoing MoM THA/HRA had significantly less time asymmetry compared to patients undergoing MoP THA when ascending the step bench (11.6% vs. 20.6%). [PERSON] et al. [31] reported step time asymmetry in 20% of patients with knee osteoarthritis compared to 11% in healthy subjects, and these results from the healthy subjects are similar to our results from MoM THA/HRA. This supports our hypothesis that MoM THA/HRA contributes to superior outcomes in performance-based tests compared to MoP THA. However, it could also reflect that patients undergoing MoM THA/HRA had more muscle power in both legs compared with MoP THA, inherently allowing movements that are more symmetrical and less compensation during stepping. However, our data failed to demonstrate differences in step test rotation and acceleration asymmetry between MoM THA/HRA and MoP THA. This was surprising because the patients undergoing MoMTHA/HRA in our study had more muscle mass and muscle power compared to MoP THA. Furthermore, our MoM THA/HRA group was younger and included more male patients than the MoP THA group. A possible explanation for this could be that the inter-limb difference in muscle mass of the hip and thigh ROI and muscle power were similar between groups, and that the inter-limb difference is of greater importance related to asymmetry than the absolute muscle mass and power found in each leg in performance-based tests. Five patients did not perform the step test. One patient from the MOM HRA group had recently had a knee operation. Four patients experienced balance problems, where one was from the MoM HRA group and three from the MoP THA group. Data from another four patients in the MoM THA/HRA group, and one in the MoP THA group, were missing due to technical problems with the IMU recorder. Thus, the results of the step test remained from 64 patients, 45 in the MoM THA/HRA group and 19 in the MoP THA group. Ten patients did not use the 40 cm step bench, but they accomplished the 30 cm step bench, four in the MoM THA/HRA group and six in the MoP THA group. The scores for PROMs, HHS, and HAGOS, were similar between the two groups, MoM THA/HRA and MoP THA, which is in line with findings by [PERSON] et al. [51]. In a nationwide, population-based cross-sectional study, [PERSON] et al.