Multilevel Lumbar Fusion And Sacral Fusion Affect Joint Space Narrowing of The Hip


 Background: This study aimed to elucidate the effect of lumbosacral fusion on joint space narrowing of the hip. Methods: We retrospectively studied 511 hips of 261 patients who had undergone lumbar fusion. Whole-spine X-ray was performed for all the patients before surgery and at final follow-up. Center edge angle, joint space of the hip, sagittal vertical axis, thoracic kyphosis, lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt, and sacral slope were measured. The number of lumbar fusion levels, inclusion of sacral fusion, follow-up duration, and wear of the hip joint (mm/year) were also recorded. Multi regression analysis was performed to identify the risk factors for joint space narrowing.Results: The female sex (P=0.04), number of fixed lumbar levels (P=0.002), sacral fusions (P=0.039), and follow-up period (P<0.001) were independent risk factors for joint space narrowing of the hip. The patients who underwent four or more levels of lumbar fusion experienced more rapid wearing of the hip joint space than that experienced by patients with less than three levels of lumbar fusion (P=0.044). Conclusion: Surgeons should pay attention to joint space narrowing of the hip after performing multiple lumbar fusions or sacral fusion in females.


Background
Osteoarthritis (OA) of the hip is not a life-threatening disease, but it greatly affects quality of life (QOL) and mental health. The number of total hip arthroplasty (THA) surgeries performed worldwide is estimated to increase 174% from 2005 to 2030 (1), and the prevention of hip OA is increasing in importance. Hip OA is caused by multiple factors such as age, sex, and body weight (2,3), but the correlation between spinal sagittal alignment and the hip joint gained attention after O erski and MacNab (4) reported on hip-spine syndrome. Spinal sagittal alignment affects not only the spinal and hip muscles (5, 6) but also physical ability (7), QOL (8- 13), and activities of daily living that can lead to locomotive syndrome (14). Thus, correction of sagittal alignment may improve the function of the hip.
Patients with hip OA frequently present with spinal sagittal malalignment (15). Compared to the incidence of lumbar spondylolisthesis of 5% in a cadaver study (16) and 8.9% in the elderly population (17), the incidence of lumbar spondylolisthesis among patients with hip OA was 22% (18). This suggests a strong association between hip OA and spinal deformity, with some patients undergoing both hip and spinal surgeries. Esposito et al. postulated that decreased spinal motion causes excessive mechanical loading of the hip (19), and inappropriate mechanical loading of the joint has been shown to be a risk factor for the progression of hip OA (20). Tateuchi et al. also reported that sagittal alignment and mobility of the lumbar spine affect the progression of secondary hip OA (21). Overloading of the hip may be improved by correcting spinal deformity, but spinal fusion decreases spinal mobility and may increase loading of the hip joint. Spinal fusion that includes the pelvis restricts spinopelvic mobility and can induce dislocation after THA by causing impingement of the femoral head on the acetabular rim (22,23).
These past reports suggest the in uence of spinal fusion on the progression of hip OA.
Although spinal fusion is widely performed, the impact of spinal fusion on joint space narrowing of the hip is still unclear. The aim of this study was to elucidate the in uence of lumbar fusion levels and sacral fusion on joint space narrowing of the hip. We hypothesized that the number of lumbar fusion levels and sacral fusion accelerates joint space narrowing of the hip.

Patient Enrollment
This study retrospectively examined 511 hip joints of 265 patients (190 hips of 95 males and 321 hips of 170 females) that had undergone primary lumbar fusion between May 2010 and May 2020. All procedures were approved by the Institutional Review Board of Tokai University Hospital (21R-043). Inclusion criteria were as follows: (1) patients aged over 50 years who had undergone a lumbar fusion surgery; (2) patients who had their whole-spine standing X-ray evaluated before and after surgeries; and (3) patients with a follow-up period of over 1 year. Exclusion criteria were as follows: (1) patients who had undergone a previous hip surgery; (2) patients with connective tissue disease; (3) patients who had no hip joint space during patient recruitment; (4) patients without whole-spine standing X-ray; and (5) patients with no follow-up data. Baseline parameters of the patients included in this study are shown in Table 1. Surgical indication for spine fusion Spine fusion was performed for patients with foraminal stenosis, segmental instability, or global sagittal malalignment. Short fusion (one to two xations) was performed to decompress foraminal stenosis or correct segmental instability. Long fusion was performed to correct global sagittal malalignment.

Radiographic Evaluation
Radiographic evaluations were performed both preoperatively and more than 1 year after surgery. Sagittal alignment was assessed using whole-spine X-rays in which patients stood relaxed at a shoulder-width stance looking straight ahead, with elbows bent and knuckles placed in the bilateral supraclavicular fossae (24). The following parameters were evaluated as spinal parameters: sagittal vertical axis (SVA), thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS)

Results
Multiple regression analysis revealed that the female sex (P=0.04), the number of fusion levels (P=0.002), sacral fusion (P=0.039), and the follow-up period (P<0.0001) were independent risk factors for joint space narrowing (  Table 3). The number of lumber fusion levels in the OA-progression group was signi cantly higher than that in the non-progression group (P=0.009; Table 4) In contrast, no statistically signi cant difference was observed in the CE angle and sagittal alignment between the two groups ( Table 4). The hip joint space of patients who underwent four or more levels of lumbar fusion wore more rapidly (0.15 ± 0.26 mm/year) than that of those who underwent less than three levels of lumbar fusion (0.1 ± 0.17 mm/year; P=0.044).

Discussion
In this study, we evaluated how lumbosacral fusion surgery affects joint space narrowing of the hip and found that the female sex, the number of lumbar fusion levels, and sacral fusion were independent risk factors for joint space narrowing of the hip. In particular, the hip joint space of patients who underwent four or more levels of lumbar fusion wore more rapidly than that of those who underwent up to three levels of lumbar fusion surgery.  (29), but this study included only patients in the pre-, early, and advanced stages of OA and excluded people with normal hip joints. Although we agree that sagittal alignment could affect the progression of hip OA, our study indicated that sagittal alignment was not an independent risk factor for the progression of hip OA. This discrepancy may arise because our study included participants with normal hip joints along with those in the pre-and early hip OA stages; the difference in the hip OA progression likely affected the results. Our results showed that most spinal parameters (SVA, TK, LL, and PT) improved through surgery (Table 3), which would theoretically decrease the load on the hips. No statistically signi cant difference was observed in the postoperative sagittal alignments between the OAprogression and non-progression groups ( Table 4), suggesting that postoperative spinal parameters did not affect the joint space narrowing of the hip.
Compared to healthy individuals, the mobility of the thoracolumbar spine is decreased in patients with hip OA (30). Lumbar fusion surgery directly affects the mobility of the thoracolumbar spine and lumbarpelvic structure. Spinal mobility worsens as the number of spinal fusion levels increases, which may lead to progression of hip OA. Lum et al. reported that the female sex and a longer fusion increase the risk of hip OA progression requiring THA (31). They reported that the relative risk of undergoing THA after fusion of >7 spinal levels was 1.03 among males compared to 2.19 among females, which was similar to our result. It is reported that 16.5% and 36.1% of patients with posterior lumbar arthrodesis had adjacent segmental degeneration at ve and ten years, respectively (32). The lumbar spine and hip joints are adjacent segments especially when the fusion construct extends down to the sacrum and ilium, so the number of lumbar fusion levels and inclusion of sacral fusion signi cantly affect joint space narrowing of the hip. Our study revealed that although spinal alignment is improved by spinal fusion, it greatly affects joint space narrowing of the hip. Furthermore, the female sex is more susceptible to joint space narrowing of the hip after long spinal fusion surgery. Therefore, spine surgeons should pay attention to joint space narrowing of the hip after performing long fusion surgery in females, especially when performing sacral fusions.
This study has several limitations. First, this study was retrospective, and four hips of four patients underwent THA after spine fusion. However, patients who underwent THA after spine fusion were too small, and we could not assess clinical assessment of the hip. Thus, we can not mention whether spine fusion affect conversion to THA or not. We are going to assess clinical assessment of the hip before and after spine fusion to elucidate how much spine fusion affect conversion to THA. Second, we set minimal follow-up duration at 12 months from baseline X-ray measurements similarly to a previous report (21), and we observed a gradual narrowing of the hip joint space as time progressed. Our mean follow-up duration of 2.9 ± 1.7 years is relatively shorter than other reports (21,33), so a higher rate of hip OA may be revealed with longer observation periods. However, only a few reports have investigated the effect of lumbar and sacral fusions on joint space narrowing of the hip, and we believe that this study adds important information to the literature. Third, this study did not include a control group. However, multiple regression analysis is more appropriate than comparative analysis to elucidate multiple unknown factors.
Fourth, the de nition of hip OA progression is not uni ed across studies. Some authors de ned the progression of hip OA as an increase in the Kelgren-Laurence grade of more than one (34-36), but we found Kelgren-Laurence grading to be highly variable. Yearly joint space narrowing has also been used to assess progression of hip OA (28, 37). Although the measurement of joint space may also be variable, we found the intraclass reliability from three separate examiners measuring joint space to be highly reliable.

Conclusion
The female sex, the number of lumbar fusion levels, and sacral fusion were risk factors for joint space narrowing of the hip. Careful follow-up is needed for female patients with joint space narrowing of the hip after surgeons perform lumbosacral fusion. In particular, four or more lumbar fusion levels affect joint space narrowing of the hip to a higher degree compared to three lumbar fusion levels. Availability of data and materials All data generated or analysed during this study are included in this published article and its supplementary information les.

Competing interests
The authors declare that they have no competing interests.

Funding
This research received no speci c grant from any funding agency in the public, commercial, or not-forpro t sectors.
Authors' contributions TU conceptualized and designed this study; TU, KY, and HO acquired and analyzed the data; TU drafted the article; HK and MW critically revised the important intellectual content of the manuscript; All authors read and approved the nal manuscript. Figure 1 Measurements of spinal parameters.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. appendix.xlsx