## 1 Introduction  
Hip osteoarthritis (OA) is a frequent musculoskeletal degenerative disease [1], initially causing pain during movement, which then progresses on to also include pain at rest. While the mechanism of the development of hip OA is not fully understood, some studies claim that anatomical deviations, especially of the acetabulum and the geometric relation between the head and the shaft, may play a role in the initiation and the course of hip OA [2, 3]. In hip OA patients, [PERSON] et al. [4] reported changes in leg alignment with a significantly higher sacral slope and a higher femoral mechanical angle. More studies have already reported on the causal relationship between leg alignment and knee OA. A varus alignment of the knee seems to worsen OA in the medial compartment, whereas a valgus alignment contributes to the development of lateral knee OA [5; 6]. Furthermore, a smaller femoral offset (FO) and a larger neck-shaft angle (NSA) have been correlated to the incidence of OA in the lateral knee compartment [7]. Measurements of clinical leg parameters from radiographic images can be affected by how the leg is positioned, i.e., horizontal dimensional parameters, such as the femoral offset, are highly influenced by the rotation of the femur [8]. These measurement errors can be minimized by using 3D EOS(r) technology [9]. An important benefit of EOS is that images are captured with patients in an upright standing position, unlike in conventional hip X-rays, where patients are supine. This upright standing position allows for a more accurate recording of the patient's functional weight-bearing leg alignment [10]. Furthermore, EOS images provide information on the whole leg, not just a pelvic overview, and the patient is exposed to a lower radiation dose than with a conventional X-ray [11; 12]. Hence, it is possible to extract clinical leg parameters that have not been previously studied in relation to gait. Hip OA patients usually adjust their gait pattern in order to reduce pain, which leads to altered joint kinematics and kinetics [13; 14; 15]. It has been shown that hip OA patients show a significantly lower knee adduction moment (KAM) on the involved side compared to healthy subjects [14] and compared to their non-involved side [14; 15]. It is suggested that a reduced KAM shifts the knee joint load from the medial to the lateral compartment [14; 16], which supports the finding that lateral knee OA is often associated with hip OA [17]. Beyond the lower frontal knee moments, lower hip adduction moments (HAMs) on the involved side were also shown in unilateral hip OA patients [14; 18]. Joint kinematics have a direct influence on the joint load. This has not only been shown in various simulation studies [19; 20] but also in multiple studies in hip OA patients [14; 21]. Patients walking with less hip adduction showed a reduced HAM [21]. Regarding the KAM, it was found that patients walking with a reduced knee flexion-extension and a greater foot progression angle (FPA) had a smaller KAM [14]. [PERSON] et al. [14] found that the knee range of motion (RoM) in the sagittal plane and the FPA explained 39% of the KAM alterations of the involved limb during the second half of the stance phase. Another aspect that can directly influence the external joint moments during gait is leg morphology. Leg alignment has previously been presented as a key factor in the load distribution of the knee. [PERSON] et al. [22] pointed out that the mechanical axis (varus/valgus configuration, calculated via the hip, knee, and ankle joints) is the best single predictor for alterations in the KAM. The abovementioned results point toward an influence of leg alignment on the joint load distribution of the knee and likely also of the hip. The question of whether leg alignment and femoral parameters explain the pathological joint adduction moments in unilateral hip OA patients has not been answered yet.