## 4 Discussion
Previous research focused on dysplastic hips [32; 33] or in patients operated on periacetabular osteotomy [34; 35]. [PERSON] et al [35] reported a normal hip muscle function during walking after 12 month periacetabular osteotomy, and found that joint force magnitude continued to be higher than normal, justifying this finding with the need for joint structures of a longer time to heal than muscles, with a residual pain alleviating strategy. [PERSON] et al. [36] reported a correlation between muscle atrophy, fatty degeneration and higher hip joint forces. Fatty atrophy was observed after THR contributing to functional outcome deterioration [37]. Considering that the patients included in the present study underwent CT-based planning that allowed targeting more accurately the reconstruction of the center of rotation and other features of the implant, we can suppose that the hip contact force in the Study Group could represent an optimal outcome in terms of recovery of correct loading, also symmetric with the contralateral hip. Nevertheless, in the Control Group, the hip force peak seems to be lower, although not significantly, in the operated limb and significantly higher in the contralateral limb, which suggests a nonoptimal recovery of the operated hip function. According to [PERSON] et al. [35], who found contradictory results assessing hip joint kinetics in DDH patients by isokinetic dynamometry and gait analysis, walking can be considered a low-demand task in terms of muscle recruitment, thus more strenuous activities should be explored in the future for a more robust functional assessment of hip muscles. Actually, the physical demand on the gluteal muscles is higher during other common motor tasks (e.g., stairs, squat), and the effect of a more performing muscle might be more marked [14; 36; 38; 39]. The main limitation of this study is the small sample size of the patients, with relevant differences between the Study Group and the Control Group. Unfortunately, we were not able to control the adherence of the patients to the original protocol after 6 months from surgery. In fact, 10 of the patients randomly included in the subgroup for gait analysis came from a distance, and at 6-month follow up they declined to participate. This limited sample size has undoubtedly limited the significance of the observed differences. However, we believe that this limitation does not invalidate the study, given the absence in the literature of analogous studies and the importance of the topic, as it could bring preliminary results to the discussion. Another limitation might regard the lack of gait analysis data before surgery. It is indeed true that a patient control pre- and postoperation would be of additional help in interpreting the results, but the gait pattern before surgery is usually confounding for the presence of leg length discrepancy and pain, hence this choice might be questionable anyway. Finally, it became evident from the analysis of the results that the chosen motor task (i.e., walking at self-selected speed), besides being important as the most common daily task, might not be the best task to highlight the differences induced in kinematics, joint and muscle loads between the operated and contralateral limb. It is possible that a higher demanding task, such as squat or stairs, might better highlight the differences. This should be kept in mind when designing similar studies in the future.