In this study, we found that the patients with Crowe IV DDH showed significantly greater SS, LL, SSA , C7T, and lower SVA(C7) than controls, which was more obvious in the patients with bilateral Crowe IV DDH. Thirty-one (63.2%) Crowe IV DDH patients reported LBP. However, there were no significant differences in prevalence or VAS between unilateral and bilateral Crowe IV DDH group.
Several studies reported the impact of hip disease on spine-pelvic alignments, and their results were inconsistent. In a prospective study, Weng reported a significantly larger SS and smaller PT in patients with hip OA [6]. Piazzolla reported that patients with hip OA and low back pain have significantly increased SS, LL, and forward inclination of the trunk [17]. Yoshimoto chose individuals with low back pain as controls and reported that patients with hip OA showed significantly higher PI, SS and LL [7]. However, Sariali found that patients with hip OA had significantly lower SS than asymptomatic healthy controls [18]. In patients with secondary OA with hip dysplasia, Okuda found that pelvic inclination tended to increase in pre/early-stage OA patients. With aging, patients with OA maintained the lumbar lordotic angle and did not develop a posterior sacral slope angle [19].
Crowe IV DDH may represent one of the most severe conditions among the spectrum of hip pathology. One previous study found that total sagittal alignment of the spine in patients with bilateral hip dislocations was compensated for by anterior tilt of the pelvis and lumbar hyperlordosis [12]. However, the parameter SI, which was used to evaluate sacral rotation, was not frequently reported in the recent literature.
The present study used SS to evaluate pelvic rotation. Our results revealed that the patients with Crowe IV DDH had significantly larger SS than that of controls. As a parameter closely correlated with SS, the LL angle showed the similar results. Our results also indicated that even with unilateral dislocated hip, the spinal-pelvic alignment can be altered. However, with one non-dislocated hip sustaining the trunk, the pelvic orientation in unilateral cases may not change as much as bilateral cases.
SVA(C7) was measured in the present study to assess the global spinal balance. The spine is considered slightly unbalanced if C7PL located between the femoral head and the posterior edge of the sacral plate, and severely unbalanced if it is located anterior to the femoral heads. Our results revealed the patients with Crowe IV DDH had an average lower SVA than control group. This means the patients with Crowe IV DDH present with a tendency toward a backward-leaning trunk, which was in accordance with the description of the relationship between femoral head, central gravity line, and C7PL [20].
Other parameters related to C7 were C7T and SSA. C7T is a functional parameter that reflects the global orientation of spine. SSA is a parameter that quantifies the global kyphosis of the entire spine and pelvis. C7T, SSA and SS are linked by the following equation: SSA = C7T+SS [16]. In the present study, the average SSA and C7T in the patients with Crowe IV DDH were significantly larger than those in the control group. This result also indicated that patients with Crowe IV DDH present with a more backward global orientation of the spine, and besides, a decreased global kyphosis of the entire spine-pelvis.
Hip diseases would lead to the change of sagittal spinal-pelvic alignment, while different diseases may have different mechanisms for this change. Hip flexion contracture has been reported as one cause of pelvic anteversion and lumbar hyperlordosis in patients with severe hip OA [21]. However, severe dysplasia hips often reveal a greater range of motion (ROM) than healthy hips. Therefore, spinal alignment changes in Crowe IV DDH cannot be attributed to hip flexion contracture. In the Crowe IV DDH patients, the femoral head is dislocated upward and backward, and the trunk supporting function of hip joint loses. Meanwhile, anatomical course of soft tissue also changed. It is the two factors that make the pelvic rotation anteriorly and thereby increase SS. Lumbar lordosis will then be increased to compensate for the anterior pelvic tilt and to meet the criterion of central gravity line passing approximately through the femoral head.
LBP caused by hip disease was termed ‘‘secondary’’ Hip-Spine Syndrome (HSS) by Offierski and MacNab [21]. They suggested that hyperlordosis of the lumbar spine may result in abnormal force on the posterior facets and LBP. Parvizi reported that 49% of patients with end-stage hip arthritis had LBP [22]. In the patients with Crowe IV DDH, the current authors considered that several factors may contribute to LBP, the first factor was the overloaded shear and compressing stresses on the posterior facet joints; the second factor was the asymmetric load distribution on posterior facet joints; the third factor was the fatigue of back muscles. Therefore, although the average age of the Crowe IV DDH patients in our study was only 40 years old, the observed LBP prevalence was 63.2%.
There are several limitations in the present study. Firstly, several parameters related to hip centers were not measured, including PI and PT. Measuring PT and PI in the Crowe ⅣDDH patients were inaccurate due to the high dislocated hip joints. Secondly, though ages in our groups varied quite a bit, and the status of the contralateral hip differed in patients with unilateral hip dislocation, we did not set subgroups because of the small sample size. Thirdly, we did not attempt to explore the correlation between lumbago and the spinal-pelvic alignment. LBP can be caused by multiple factors, which would be taken into account in future research.