In this study, we evaluated how lumbosacral fusion surgery affects joint space narrowing of the hip and found that 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.
The relationship between spinal alignment and hip OA progression has been receiving increasing attention recently, with various articles reporting a correlation between spinal parameters and hip OA. As LL decreases in the degenerating lumbar spine, the pelvis tilts more posteriorly and the acetabular roof coverage of the hip decreases. Decreased acetabular roof coverage correlates with an increase in the hip joint load and leads to the progression of hip OA. Tateuchi et al. reported that larger anterior inclination and decreased mobility of the spine were predictors of radiographic progression of hip OA, because the internal hip extension caused by the forward tilting of the trunk consequently increases the mechanical load on the hip[21]. Damm et al. also reported that tilting the trunk doubles the load on the hip joins[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.
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 lumbar-pelvic 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 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 men compared to 2.19 among women. Kawai et al. also reported that longer spinal fusion was associated with the progression of hip joint narrowing[32]. Their findings are similar to ours. However, our study assessed not only joint space narrowing but also pre- and postoperative spinal alignment simultaneously, revealing that a long fusion had a greater influence on joint space narrowing of the hip compared to the influence of spinal parameters. 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. On the other hand, no statistically significant difference was observed in postoperative sagittal alignment between the OA-progression and non-progression groups (Table 4). It is reported that 16.5% and 36.1% of patients with posterior lumbar arthrodesis had adjacent segmental degeneration at five and ten years, respectively[33]. 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 significantly 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 women, 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, the number of patients who underwent THA after spine fusion was too small, and we could not evaluate the clinical assessment of the hip. Thus, we cannot show whether spine fusion affects conversion to THA or not. We plan to conduct a clinical assessment of the hip before and after spine fusion to elucidate how much spine fusion affects the conversion to THA. Second, we set minimum follow-up duration at 12 months from baseline X-ray measurements, similar 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 that in other reports[21, 34], 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 definition of hip OA progression is not unified across studies. Some authors defined the progression of hip OA as an increase in the Kelgren-Laurence grade of more than one[35–37], 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, 38]. 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.