## 4 Discussion
It might be that hip OA patients stiffen the knee on the involved side and flex the knee more on the non-involved side to compensate for leg length differences [14; 43]; however, an accurate determination of leg length difference was not part of this study and thus this hypothesis cannot be confirmed. The gait kinematics showed only a few correlations to the joint adduction moments: the peak hip adduction corresponded strongly to the HAM in the corresponding part of the stance phase and explained 58% and 54% of HAM1 and HAM2, respectively. These correlations were shown previously by [PERSON] et al. [20], who showed in a simulation study that increased hip adduction increases the hip adduction moments, as well as the hip contact forces. An increased contralateral pelvic drop had a negative impact (it leads to increased adduction moments) on both KAM1 [44] and HAM1 [45]. The contralateral pelvic drop increased the model for KAM1, and in combination with the FO and the HKA, it explained 78% of the KAM1 alteration. Although a significant correlation was found between the contralateral drop and the external knee joint moments, no differences were found between the patients and healthy controls. Our patients showed a gait pattern with an increased ipsilateral trunk lean that was associated with a reduced pelvic drop. A gait compensation pattern of a stable or elevated pelvis, in combination with an increased trunk lean toward the involved side, was shown by [PERSON] [46] in individuals with hip OA and by [PERSON] et al. [47] in children with Legg Calve Perthes disease as a compensating strategy to reduce the hip load. However, an increased contralateral trunk lean associated with an increased pelvic drop was also shown in individuals with hip OA [46; 48]; hence, the presence of these different combined movements may have confounded the correlation between the single movements and the external hip joint moments. Furthermore, [PERSON] et al. [49] found that only a thorough biomechanical analysis using a principal component analysis could reveal differences in the trunk and pelvic leans. This study represents the first step in a detailed analysis of the influence of leg alignment on joint adduction moments in unilateral hip OA patients. Nevertheless, our results should be read in light of some limitations. The small sample size used might have restricted the ability to detect significant differences between our groups. Due to the low number of cases, we were not able to take gender into account in our study, as was done by [PERSON] et al. [34], who found differences between the sexes in terms of leg alignment. Furthermore, [PERSON] et al. [50] and [PERSON] et al. [48] were able to report higher hip adduction moments in women compared to men with hip OA. The hip and knee adduction moments are still the gold standards for assessing the load in the frontal plane; however, the joint contact force, calculated using musculoskeletal modeling, is likely a better indicator of joint load compared to joint moments. In the present study, only the effects morphology and kinematics have on joint adduction moments in hip OA patients are discussed. A joint load expressed by joint moments can also be influenced by the abductor muscle force [50] and leg length differences [51] (due to the destructive effect of OA on the hip joint); however, including leg length and abductor function were beyond the scope of the present study. The descriptive models are only valid for the included cohort of patients. In recognizing our limitations to conclusively discuss causality, we recommend that future longitudinal studies be designed to test the development of the leg alignment in patients with hip OA and its effects on hip and knee adduction moments.