We sought to assess the changes in and associations of spinal alignment with LBP after THA. In general, coronal alignment significantly improved, whereas sagittal parameters slightly changed. About half of the patients with preoperative LBP showed improvement postoperatively; however, we did not find any relationship between improvement in LBP and either sagittal or coronal alignment changes.
Hip-spine syndrome was originally described by Offierski and MacNab more than 3 decades ago. 1 The original concept of this syndrome was based on the fact that patients with hip OA experienced pain relief in the back after being treated for hip OA. The researchers showed that flexion contracture of the hip joint led to increased pelvic forward tilt, lumbar lordosis, and LBP as a result. Because contracture and range of motion of the hip joint improve after THA, it is reasonable to speculate that spinal alignment will change after THA. In this study, we showed that spino-pelvic sagittal parameters slightly changed after THA. PI slightly increased, although it is thought to be an individually constant value.14 15 This finding was reasonable considering that the center of the hip joint would shift caudally after THA in patients who experienced central migration of the femoral head preoperatively. Nevertheless, PT slightly increased, whereas SS slightly decreased, which reflected reduced anteversion of the pelvis caused by decreased contracture of the hip joint. These changes were consistent with previous findings, although the difference in the angles was small and might not be clinically significant.
The presence of LBP has been reported in patients with hip OA. According to previous reports, 21.2–56.5% of patients treated with THA had LBP before surgery,12 16 17 18 19 which was almost the same as reported in our study (36.6%). We speculate that the relatively wide range of incidence rates reported in the literature was because of differences in the definition of LBP used among the studies Nevertheless, the incidence of LBP in patients with hip OA is considered relatively high. Moreover, many patients have shown pain relief in the lower back after THA, which accounts for 54–100% in the literature.7 12 16 17 18
It is reasonable to speculate that changes in the spino-pelvic alignment might reduce the tension in the back muscles and relieve LBP. 20 21 22 However, the precise mechanism remains elusive. To explain why LBP is relieved after THA, 2 reports focused on the spino-pelvic alignment changes before and after surgery. Weng et al. investigated the effect of THA on sagittal spinal alignment in 69 patients treated with THA. 12 In their study, 39 (56.5%) patients complained of LBP before surgery, 17 of whom reported complete resolution and 22 of whom reported significant relief. Although the researchers concluded that the improvement in abnormal sagittal spinal-pelvic-leg alignment helped improve preoperative LBP, they did not show any difference in the radiographic parameters between patients with and without preoperative LBP. Eyvazov et al. investigated the effects of THA on spinal sagittal alignment and static balance in 28 patients. 11 They showed that LBP and the Oswestry Disability Index (ODI) significantly improved after surgery, but they did not find any significant correlations between postoperative changes in spinal sagittal alignment or postural balance and improvement in LBP and ODI scores. Considering the results from these 2 reports, preoperative LBP improved to some extent after THA; however, the involvement of spinal sagittal malalignment with improvement in LBP remained uncertain.
It is well known that sagittal imbalance can cause LBP. 23 24 25 In our study, patients with preoperative LBP tended to show decreased LL and consequently PI minus LL mismatch as compared to those without LBP. However, although 54% of the patients with preoperative LBP showed improvement after THA, none of the spinal sagittal parameters were significantly correlated. Therefore, we assume that, although preoperative spinal sagittal malalignment might in part have affected the presence of preoperative LBP, factors other than changes in the spinal sagittal alignment must have influenced the improvement in such LBP. Our results do not necessarily eliminate the possibility of an effect caused by slight changes in the sagittal alignment because the number of patients with preoperative LBP was relatively small. Tiny changes in the pelvic anteversion could have influenced the muscle tonus around the lumbar spine and pelvis.
Compared with the changes in the spinal sagittal alignment, coronal balance improved noticeably after THA. This was expected because pelvic obliquity can be mostly corrected after THA as a result of improvement in the leg length discrepancy. It is well known that coronal imbalance can also cause LBP. Eguchi et al. reported that reduction in scoliosis was correlated with an improvement in the Roland-Morris Disability Questionnaire (RDQ) scores in 30 patients undergoing THA. 9 Although we anticipated that the degree of improvement in the coronal balance would affect the LBP relief after THA, this effect was not observed in our study. We speculate that this finding might be because RDQ can be affected by disorders in the hip joint and in the lumber spine.
Another possibility that could explain the LBP relief after THA is the change in the susceptibility to pain. 26 Patients with hip OA are always bothered by coxalgia, which could lead to hypersensitivity to pain. In this study, the patients whose preoperative LBP did not improve after THA showed worse quality-of-life outcomes in general, specifically on the EQ-5d and PCS. Although the postoperative mental component summary (MCS) of the SF-12 was not significantly different (p = 0.10), it was possible that physical and mental disorders related to the hip joint disorders might have affected the degree of LBP after THA in such patients.
This study has several limitations. First, only one investigator measured the radiographic parameters in this study. Because measurement errors can occur in such cases whose hip OA is severe, examinations by 2 or 3 investigators would have increased the accuracy of the results. Second, although we used a body figure to demonstrate to the patients specifically where the low back was located, the patients might have had difficulty in completely distinguishing LBP from coxalgia. Third, we did not consider pain medications, which would have affected the pain status. Further investigation will be necessary to elucidate these problems.