In order for humans to maintain an upright standing posture, it is essential for the center of gravity(CG) line to project through the feet in the sagittal plane. “Conus of Economy”, first described by Dubousset [16], means that the standing person maintains a center of mass within a narrow range of sway in relation to the feet with minimal energy expenditure. To achieve this, the spine, pelvis, legs and their compensatory mechanisms must be recruited in the event that any pathological changes occur [17]. For example, the trunk of patients with a degenerative spinal deformity has a tendency to lean forward due to the loss of lumbar lordosis; however, standing balance can still be maintained via posterior tilt of the pelvis, hyperextension of the hip joint, and even knee flexion and ankle hyperdorsiflexion.
The impact of hip disease on the alignments of the spine and pelvis has been documented in a few studies, and the results were inconsistent. In a prospective study in which the comparison group consisted of normal controls, Weng et al. reported a significantly larger SS, smaller PT, and comparable LL and PI in patients with hip OA [9]. Piazzolla et al.[18]reported that patients with unilateral hip OA and low back pain have significantly increased SS, LL, and forward inclination of the trunk. In addition, the authors also found increased femoral neck anteversion in the arthritic hip when compared to the contralateral healthy hip. Yoshimoto et al[10] chose individuals with low back pain as controls and reported that patients with hip OA showed significantly higher PI, SS and LL. Conversely, Sariali et al.[19] found that patients with hip OA had significantly lower SS than asymptomatic healthy controls. In patients with secondary OA with hip dysplasia, Okuda et al.[20]found that patients with early-stage OA had comparable lumbar lordosis and sagittal pelvic alignment, with a trend toward greater anterior pelvic inclination. With aging, patients with hip OA did not develop decreased LL and SS. However, in these reported literature, hip lesions in the enrolled patients exhibited primary OA or secondary OA with mild hip dysplasia.
Crowe type IV developmental dysplasia of the hip may represent one of the most severe conditions among the spectrum of hip pathology. High dislocated hips have an impaired ability to stabilize and control body posture in the sagittal plane. Patients with uni- or bilateral dislocated dysplastic hip often present with severe functional disorders, such as a waddling gait, limp, and duck-like posture in standing. To date, the literature has focused on spinal-pelvic alignment of patients with Crowe type IV developmental dysplasia of the hip, which was reported by Matsuyama in 2004[12]. Matsuyama 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. However, the parameter used to evaluate sacral rotation is SI, which is measured as the angle between the line along the posterior border of the S1 body and the reference vertical line. This parameter is not frequently reported in the literature, while sacral slope is more commonly accepted.
The present study utilizing SS revealed that patients with bilateral dislocated dysplastic hips had an average SS of 54.3°±2.8°, which was significantly larger than that of patients with a unilateral dislocated dysplastic hip and asymptomatic volunteers. As a parameter closely correlated with SS, the LL angle showed the same results, Patients with bilaterally dislocated dysplastic hips had higher LL angle than patients with unilateral hip dysplasia or asymptomatic controls. These result indicate that even with a unilateral hip dislocation, the spinal-pelvic alignment can be altered. However, with one hip sustaining the trunk at a location that is as close as possible to the native limb, pelvic orientation may not change as much as in cases where there is bilateral dislocation.
For the assessment of the global spinal balance, SVA(C7) (horizontal distance between the C7 plumb line[C7PL] and posterior point of the sacral endplate) was measured. The C7PL is generally located over or behind the sacral end plate[21]. If the C7PL is located between the femoral head and the posterior edge of the sacral plate, the spine is considered slightly unbalanced. When it is located anterior to the femoral heads, it suggests that the spine is severely unbalanced. This study was unable to detect any cases of C7PL in Crowe type IV DDH patients being located anterior to the sacral endplate; all were balanced based on the criteria in the literature. However, these criteria are sorted to examine the patients with spinal deformity with the trunk primarily leaning forward. In Matsuyama’s report[12], the author indicated that the femoral head shifts posteriorly, which plays an important role in the spinal alignment of patients with bilateral Crowe type IV DDH. He observed that the C7PL is consistently 2 cm behind the femoral head in order to maintain balance when standing. This is in accordance with the description of the relationship between femoral head, gravity line, and C7PL(with a balanced upright posture, the CG line approximately passes through the femoral head and 1.8 cm anterior to the C7PL[22]).We also observed a tendency toward a backward-leaning trunk in patients with Crowe type IV DDH, with an average SVA significantly lower than control group. Other parameters related to C7 were C7T and SSA. C7T is a functional parameter that reflects the global orientation of the spine, and 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[15]. In the present study, the average SSA and C7T in the patients with Crowe type IV DDH were significantly larger than those in the control group. This result also indicated that patients with Crowe type IV DDH present with a more backward-leaning global orientation of the spine and decreased global kyphosis of the entire spine- pelvis.
Hip flexion contracture has been reported as the cause of pelvic anteversion and lumbar hyperlordosis in patients with severe HOA. Offierski and MacNab[23] proposed that, in patients with hip flexion contracture, if the legs are kept in a relatively vertical position, the pelvis must be anteverted due to the loss of hip extension. Consequently, hyperlordosis of the lumbar spine may occur as a means of compensating for the forward orientation of the trunk to maintain an upright standing posture. However, hips with high dislocated dysplasia often reveal a greater range of motion(ROM) than a healthy hip given the lack of normal joint restriction. Therefore, spinal alignment changes in Crowe type IV DDH cannot be attributed to hip flexion contracture. Given that the shape of the pelvis expanded superiorly and anteriorly, combined with a lack of restriction in the posterior direction in hip flexion to fulfill daily activity, it was expected that the femoral head would shift posteriorly while the hip dislocated superiorly. The soft tissues attached to the proximal femur will pull the inferior portion of the pelvis superiorly and posteriorly, facilitating pelvic rotation anteriorly and thereby causing the SS to increase. Lumbar lordosis will then be increased to compensate for the anterior pelvic tilt and to meet the criterion of the CG line passing approximately through the femoral head.
Low back pain caused by hip disease was termed ‘‘secondary’’ Hip-Spine Syndrome (HSS) by Offierski and MacNab[23]. They suggested that hyperlordosis of the lumbar spine may result in abnormal force on the posterior facets and LBP. Parvizi et al.[24] reported that 49% of patients with end-stage hip arthritis had LBP. In a study of 91 primary hip OA patients who underwent THA, Piazzolla[18]reported that 47 patients had LBP. However, both the mechanisms of LBP in patients with hip OA and whether LBP is correlated with spinal-pelvic alignment remain controversial. In patients with Crowe type IV DDH, the current authors considered several factors that may contribute to LBP: the overloaded shear and compressive stresses on the posterior facet joints caused by increased SS and LL; the asymmetric load distribution on posterior facet joints caused by compensatory lumbar scoliosis due to unilateral hip dislocation; and the fatigue of back muscles resulting from maintenance of the trunk in an upright position. Although the average age of the patients in the current study with Crowe type IV DDH was only 40 years, the observed LBP prevalence was 63.2%.
Pelvic tilt has been and remains a concern for hip surgeons because anterior or posterior pelvic tilt has a substantial impact on the orientation of the acetabular cup. For every 1°of posterior pelvic tilt, there is a corresponding increase of 0.7°-0.8° of acetabular anteversion[6, 25, 26]. Several authors also reported that the anterior pelvic plane (APP) described by Lewinnek[27] was not always neutral[6, 25, 26] and varied widely when the patient was in supine, standing, lateral decubitus, and sitting positions[6, 25, 26, 28, 29]. THA patients with a spinal deformity[30], as well as those who have had previous spinal deformity correction[7] or lumbar fusion surgeries [31]were reported to have significantly higher rates of dislocation. Spinal-pelvic mobility when moving between standing and sitting positions[32, 33] and the presence of an unbalanced deformity[33]were reported as factors considered during the adjustment of acetabular cup anteversion during THA surgery. We did not try to adjust acetabular cup anteversion during surgery using these data of standing spinal-pelvic alignment because Crowe type IV dysplastic hip often present with a hypoplastic, triangular acetabulum. In such cases, the basic rule of acetabular reconstruction is to place the cup in the location with the best bone, in addition to protecting the anterior wall. The shell may not be placed in a “safe zone” or in an adjusted orientation based on spinal pathology. A modular stem, such as the S-ROM (DePuy Synthes, Warsaw, Indiana, USA), is the optimal choice in this situation to achieve ideal, combined anteversion. Second, The sagittal spinal-pelvic alignment we described is in standing position, with hips sustaining body weight. In lateral decubitus position when performing surgery, the pelvic tilt may be changed. Third, the standing sagittal spinal-pelvic alignment may also be changed after THA due to the restoration of normal hip biomechanical functions. Although the results of this study may not be relevant for guiding acetabular reconstruction in patients with Crowe type IV hip dysplasia, an understanding of the unique spinal-pelvic alignment caused by a high dislocated hip can lead to a more comprehensive understanding of the interrelationship between the spine, pelvis and hip joint.
There are limitations in the present study. First, several parameters related to hip centers were not measured, including pelvic incidence (PI) and pelvic tilt (PT). PI, the most important sagittal morphological parameter associated with the pelvis, was reported to be the key factor related to PT, SS, and LL. The formulas PI = PT + SS, LL = PI ± 9°[34],PT= − 7° + 0.37PI[35], and SS = 7.3° + 0.63PI[35], which were generated from data obtained from healthy volunteers, have been reported in literature [34, 35]. Specifically, given a PI value, the ideal value of PT, SS, and LL can be estimated. Although a larger SS and LL were observed than in the control group, it would be more convincing if the PI was obtained to detect mismatches between the actual values of the SS and LL and the ideal values that are derived from the PI. Second, the APP angle, an angle formed by the APP and coronal planes, was not measured in this study. Knowing whether this angle had increased may assist in the understanding of pelvic orientation. However, the APP angle was not measured because the image of the anterior superior iliac spine and the symphysis pubis could not be accurately determined on full spine radiographs. Third, we did not attempt to explore the relationship between lumbago and the spinal-pelvic alignment. Low back pain can be caused by multiple factors; as such, confounders that may have an effect on LBP must be taken into account in future research to assist with a more accurate analysis. Forth, neither radiographic anterversion and inclination of the acetabular shell, nor clinical results of these patients after THA were evaluated in the present study. Longer followup are needed to assess the clinical and radiographic results, and whether spinal-pelvic alignment can be adjusted after THA in patients with Crowe type IV DDH .