## 4 Discussion
found similar mean scores when comparing MoM THA/HRA and MoP THA in the PROMs: the five HOOS subscales, the EQ-5D index, EQ VAS score, and the UCLA activity score. Overall, this suggests that MoM THA/HRA is not superior to MoP THA in outcomes that are important to the patients. The clinical application of the results in this study are that muscle mass and power were reduced in the affected lower limb compared to the non-affected lower limb for bothothesis types, indicating that both groups may benefit from a strength training intervention in their rehabilitation. The inter-limb differences in muscle mass were not statistically significant in thigh-, hip and thigh-, and calf area, but only in the hip area, indicating that patients with MoM/HRA THA may need more specific hip strength training than patients with MoP THA. Furthermore, the study highlights that absolute muscle strength and power may not determine the functional outcome in patients with MoM/HRA THA or MoP THA. The MoM/HRA THA group had more muscle mass in each area compared to the MoP THA group and could produce significantly more muscle power. However, their inter-limb differences or relative muscle strength and power were not significantly different. Therefore, physiotherapists should in the rehabilitation target the resistance training on inter-limb differences in muscle mass or power rather than focusing on absolute muscle mass and power  
Our study has several limitations. First, the MoM THA/HRA group was generally younger and included more males than the MoP THA group, and the two groups also differed in years since operation. Age- and gender-matched groups with comparable follow-up time might have changed the results. Even if we could not verify our hypothesis of superior functional outcomes in MoM THA/HRA compared to MoP THA, this does not seem to have affected the conclusion of our results. However, a comparative strength of the groups presented in this study is that patients were of similar nationality, they were included in the same institution with the same indication for surgery, operated by the same surgeons, investigated using the same equipment, and investigated by the same observers. Secondly, the lack of a healthy subject control group prevented comparisons to normative data. Thirdly, our study lacked comparisons with preoperative data, and the fact that any osteoarthritis of the non-prosthetic hip also affects the function. However, we expect that this would bias our results equally in both groups and thus not affect the between-group comparisons. Fourthly, physical and functional capabilities embodying concepts such as muscle strength and power can be confounded by covariates such as sex and age, which are the two covariates we have used to explain differences between the prostheses (MoM/HRA THA vs. MoP THA) [52]. However, inclusion of other covariates such as patient preoperative functional status should have been included to provide a more accurate explanation of the differences [53, 54]. Fifthly, DXA-scan is not a gold standard but a reference standard for lean mass quantification as it cannot quantify fatty infiltration of muscle [55]. Fatty infiltration of muscles after THA is well established in the literature [56; 57; 58]. Thus, potential differences or similarities in muscle mass or power between the two prostheses might be due to imprecise quantification of lean mass. However, a strength of the DXA-scan is its high correlation with its more expensive alternatives, MRI-scan and CT-scan, for measures of muscle mass. Sixthly, the effect of the surgical approach (i.e., posterior or antero-lateral) with detachment of different muscles has not been accounted for in this study. Most patients underwent a posterior surgical approach (MoM THA, \(n=18\), MoM HRA THA, \(n=22\) and MoP THA, \(n=23\)) and a few underwent an antero-lateral surgical approach (MoM HRA THA, \(n=11\)). The evidence of long term implications comparing surgical approaches on clinical, functional, and patient-reported outcomes are sparse [59].