High Sacral Endplate Inclination Vector Forces Are Associated with Pars Fracture–Mediated Lumbosacral Spondylolisthesis

Abstract

Lumbar spondylolisthesis can be related to facet arthropathy and disc degeneration or to a fracture of the pars interarticularis, but the mechanistic underpinnings of spondylolisthesis remain unclear. We posit that high sacral slope and body weight increase sacral inclination vector forces, which leads to pars fractures and exacerbates risk for spondylolisthesis. To investigate this hypothesis, we measured the sacral slope, body weight, and S1 endplate vector forces for patients who underwent L5-S1 fusion for grade I spondylolisthesis. Patients were stratified based on presence of pars fractures versus facet arthropathy, and statistical analyses were performed to determine whether high sacral endplate inclination vector force is associated with pars fracture-mediated spondylolisthesis. We identified 131 patients who had L5-S1 fusion for spondylolisthesis. The mean age was 56 years, and 57% were female. The body weight of patients ranged from 45.4 to 141.9 kg with an average of 83.8 kg; 32 patients had single-level L5-S1 spondylolisthesis secondary to bilateral L5 pars interarticularis fractures, whereas 99 patients had L5-S1 spondylolisthesis due to facet arthropathy. Patients with pars fractures had steeper sacral slopes (43.2 ± 10.1°) compared with those without pars fractures (36.8 ± 8.1°) (P= 0.0007, odds ratio 2.71). Despite having no significant differences in weight (82.7 ± 17.2 vs. 87.3 ± 17.2 kg, P=0.189), patients with pars fractures had 49% greater sacral inclination vector forces compared with those without pars fractures (586 ± 158 N vs. 394 ± 90 N, P < 0.0001). These data suggest that high sacral endplate inclination vector force is a risk factor for developing pars fracture-mediated spondylolisthesis.