More Spine Fusion Levels, More Hip Problems?

Is the number of spine fusion levels an underappreciated, possibly ignored risk factor for subsequent hip problems?

Researchers from the Kyoto University Graduate School of Medicine in Japan set out to examine whether the number of lumbar spinal fusion levels has the effect of increasing force in the hip joint leading to accelerated wear of the joint space.

Their work, “Number of Levels of Spinal Fusion Associated with the Rate of Joint-Space Narrowing in the Hip,” appears in the June 2, 2021, edition of The Journal of Bone and Joint Surgery.

Citing the novelty of their work was co-author Toshiyuki Kawai, M.D., Ph.D., who told OTW, “To date, there has been no report that examined the effects of long fusion [constructs] on the degeneration of the hip joint. But there was a related article by Lum et al. that was a type of database analysis in which authors calculated the ratio of patients who underwent total hip arthroplasty after spinal fusion and demonstrated that female patients who underwent spinal fusion of seven levels or more were more likely to require total hip arthroplasty afterward.”

The research team enrolled 205 patients who had been treated with lumbar or lumbo-sacral fusion from July 2011 to September 2018 and collected data for:

• degenerative disc disease (n = 134),
• degenerative scoliosis (n = 39),
• spondylolisthesis (n = 25),
• osteoporotic vertebral collapse (n = 4), and
• spondylolysis (n = 3).

The team then categorized each patient according to the number of levels fused: (single, 1 level; short, 2 to 3 levels; middle, 4 to 6 levels; and long, ‡7 levels).

According to Dr. Kawai, the team found that the rate of joint-space narrowing for all patients was 0.114 ± 0.168 mm/year. More specifically, the team documented clear differences between patients with single level fusion and multiple level fusion.

To quote from the study paper: “The narrowing rate for single- level fusion was 0.062 ± 0.087 mm/year, whereas that for fusion of ‡7 levels was 0.307 ± 0.254 mm/year. In the multivariate regression analysis, only the length of fusion (standardized coefficient [SC] = 0.374, p < 0.0001) was associated with an increased narrowing rate.”

“When the narrowing rate was normalized by height, female sex was another risk factor for increased narrowing (SC = 0.109, p = 0.023). Secondary regression modeling performed with patients who underwent spinal fusion for degenerative disc disease showed that the length of fusion (SC = 0.454, p < 0.0001) and female sex (SC = 0.138, p = 0.033) were associated with increased joint-space narrowing.”

Dr. Kawai told OTW, “The joint narrowing rate in the hip was statistically significantly increased after longer spinal fusion even after adjustments were made for all the confounding variables including sex, age and diagnosis. Patients with hip arthritis, hip dysplasia, rheumatoid arthritis had been excluded from analysis.”

“The hip joint can be regarded as the adjacent joint to the lumbar spine. If patients undergo spinal fusion that may limit the mobility of the sacrum, which can increase the mechanical force on the hip joint. Spinal fusion is an established procedure and widely performed these days.”