Hip and lumbar spine disorders often coexist, and in 1983, Offierski and MacNab proposed the close relationship between them as the hip–spine syndrome (HSS) [1]. Since then, related studies have been conducted from various perspectives. The hip joint and lumbar spine are adjacent to each other via the pelvis, and special consideration of their relationship is required. The pelvis, which connects the hip joint and spine, experiences changes in its orientation due to the alignment of both joints, and its orientation might affect each other. Rivière et al. defined the interaction between the lumbopelvic complex and hip joint as the spine–hip relations (SHR) [2]. They stated that excessive posterior tilt of the pelvis might relatively reduce the femoral head coverage and induce osteoarthritis, thus raising the issue of spine–hip syndrome (SHS), in which malalignment of the spine affects the hip joint [[3], [4], [5]].

Hip overload due to decreased head coverage becomes a serious problem in hip dysplasia. If pelvic malalignment such as its posterior tilt relatively reduce the femoral head coverage and causes a similar pathology, the issue should be investigated from a preventive perspective.

To investigate the effect of pelvic orientation on the hip joint, the stress distribution of the hip joint can be simulated using finite-element analysis (FEA). Several studies using FEA have focused on the sagittal plane pelvic tilt and hip joint stress distribution and reported that the posterior pelvic tilt increases the stress in the load-bearing part of the anterosuperior acetabulum [6,7]. Pelvic tilt on the sagittal plane has been widely discussed; however, the pelvis is oriented not only on the sagittal plane but also on coronal and horizontal planes, and these orientations might change the femoral head coverage and hip stress distribution, leading to hip overload. Discussion about the effect of these orientations on these planes is limited [5]. Pelvic obliquity on the coronal plane was studied in relation to leg-length discrepancy and its effect on the hip joint [8]. The hip joint stress distribution on the coronal plane regarding lateral center-edge angles in periacetabular osteotomy has been reported recently [9]. In the horizontal plane, the acetabular anteversion angle is used as an index of the anterior coverage of the acetabulum [10]; however, whether pelvic rotation on the horizontal plane might relatively affect the hip joint coverage and stress distribution is not clear.

Studies on pelvic tilt and hip stress distribution using FEA are often aimed at cases of hip dysplasia, not at cases of intact hip joint with relatively decreased head coverage as per SHS researches. In addition, the orientation of the pelvis was investigated in one axis of motion in previous studies, not in all three axes of motion.

We hypothesized that specific pelvic orientation in the coronal and horizontal planes of the intact hip joint might lead to local stress accumulation in the acetabulum. This study aimed to examine how stress distribution of the intact hip joint changes under the three-dimensional orientation of the pelvis. Changes in the orientation as changes up to a maximum angle of 20° using 10° increments in each plane were analyzed.