Spine, pelvis and lower limbs are important parts of the human body. Spine and lower limbs are connected to the pelvis through sacroiliac joints and hip joints. Therefore, the pelvis is an important transition area. If pathological changes occur in one part of them, it will lead to corresponding compensatory changes in the other two parts. Therefore, for patients with DDH, pathological changes of hip may lead to corresponding changes of spine and pelvis.
In this study, the SVA of patients with is - 4.1 ± 33.9mm; SVA of healthy adults is-11.3 ± 28.9 mm, which means the C7 plumb line of patients with DDH and healthy adults falls behind the posterior upper angle of sacrum. SVA is important to evaluate sagittal balance. General speaking, C7 plumb line should fall above the posterior upper angle of the sacrum. If SVA > 50mm, it is called sagittal imbalance. In this study, patients with DDH can maintain sagittal balance.
In patients with DDH, PI = 42.9 ± 18.9 °, PT = 4.6 ± 15.6 °, SS = 38.2 ± 13.9 °, LL= 53.3 ± 12.6 °, TK = 22.8 ± 9.2 °, TLK = 6.7 ± 5.7 °. In the control group, PI = 45.6 ± 9.4 °, PT = 9.9 ± 6.8 °, SS = 35.5 ± 7.1 °, LL= 48.4 ± 10.8 °, TK = 27.7 ± 10.4 °, TLK = 5.7 ± 10 °. It can be seen that PT decreases, TK decreases and LL increases in patients with DDH. The reason may be that the femoral head in patients with DDH is dislocated backward and upward, resulting in poor hip axis collinearity, resulting in the decrease of PT.
When the femoral head is dislocated backward, the position of the upper body relative to the femoral head is forward. There is a tendency of sagittal imbalance. In order to compensate it, the curvature of the spine changes, LL increases, TK decreases. Spine is in a state of hyperextension to compensate the tendency of sagittal imbalance.
Matsuyama et al. [10] and Hasegawa et al. [11] reported SS increased in patients with high dislocation DDH. In this study, the average SS of patients with was higher than that of healthy adult, but the difference was not statistically significant. Theoretically, PI = PT+ SS. When PI is constant, PT decreases, which will lead to the increase of SS.
In addition, the effect of THA on spine-pelvic sagittal alignment in patients with DDH has also attracted more attention. Murphy et al. [12] showed that there was no significant change in PT 1 year after THA. Ishida et al. [13] reported that the pelvis of patients with hip dysplasia tends to tilt backward after THA. Eyvazov [14] and Weng [15] reported that there was no significant difference in spine-pelvic parameters after THA. The results of this study are consistent with those of Eyvazov and Weng. There is no significant change in sagittal spino-pelvic parameters of patients with DDH after THA (Figure.4).
In addition to sagittal balance, coronal spino-pelvic balance in patients with DDH is also important for patient’s life quality.
After measuring the length of both lower limbs of patients with DDH, it was found that in this study, the average length of lower limbs on the longer side was 782.8 ± 49.4mm, the average length of lower limbs on the shorter side was 766 ± 50.2mm, p < 0.01, and the average HO = 5.3 ± 5.8 °. It shows that there is leg length discrepancy (LLD) in patients with DDH.
For people with LLD, pelvic obliquity may occur to compensate for coronal balance.
Song et al. [16] believe that people with LLD will have continuous pelvic obliquity to compensate, and the average LLD is between 2 cm and 15.8 cm. Aiona et al. [17] believe that pelvic obliquity is one of the main compensatory mechanisms of people with LLD. And for patients with DDH, their hip is in a state of dislocation, which may lead to pelvic obliquity, too. Therefore, LLD and hip dislocation can contribute to pelvic obliquity together.
Besides, LLD may also have an impact on development of DDH. Tallroth et al. [18] pointed out that LLD may be related to the pathological state of hip joint. When LLD occurs, the hip coverage on the longer side becomes worse, and shorter side may deteriorated. And it can result in aggravation of hip dysplasia. The relationship between LLD and DDH is complex. They can interact with each other.
In this study, 86.1% of DDH patients had a higher position of the femoral head (dislocation side), but a lower position of the pelvis. It shows that the dislocation of hip plays a dominant role in coronal pelvic obliquity. However, this does not mean that the contribution of LLD can be ignored.
In the study, we found that the preoperative Cobb = 11.0 ± 10.7 °, IO = 6.7 ± 4.6 °, SO = 6.4 ± 5.4 °, L5O= 6.1 ± 5.5 °. This shows that patients with DDH do have pelvic obliquity and scoliosis. Through the correlation study, we found that the Cobb angle was correlated with SO and IO (r = 0.610, r = 0.570). The result suggests that there is a correlation between scoliosis and pelvic obliquity in patients with DDH. We can hypothesize the scoliosis may be the compensatory method of pelvic obliquity.
In general, CBD more than 30mm is regarded as coronal imbalance. In this study, the preoperative CBD of DDH patients was 14.2 ± 10.6mm, which showed that there was no overall coronal imbalance in patients with DDH. This may be due to the tendency of coronal imbalance was compensated by scoliosis.
When DDH occurs, there will be pelvic obliquity, which may lead to coronal imbalance tendency. And spine will compensate it, manifested as compensatory scoliosis.
Next, we want to explore the effect of THA on spino-pelvic coronal alignment. In patients with DDH, preoperative IO = 6.7 ± 4.6 °, SO = 6.4 ± 5.4 °, HO = 5.3 ± 5.8 °, L5O = 6.1 ± 5.5 °, Cobb angle was 11.0 ± 10.7 °. After THA, IO = 3.5 ± 3 °, SO = 3.7 ± 3.4 °, HO = 3.7 ± 3.6 °, L5O = 4.3 ± 4.7 °, Cobb angle was 6.8 ± 7.7 °. This study showed that THA can significantly reduce Cobb angle, SO and IO in patients with DDH. THA can directly improve the pelvic obliquity of patients with DDH and indirectly reduce the degree of compensatory scoliosis through acetabular reconstruction (Figure.5).