Introduction: 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. There is growing evidence that spinopelvic parameters relate to lumbar lordiosis and are pivotal to understanding the occurrence of spondylolisthesis. We posit that high sacral slope is associated with pars fractures.

Methods: To investigate this hypothesis, we retrospectively studied sacral biomechanics in 131 patients who underwent single level fusion in our institution for L5-S1 spondylolisthesis. We measured sacral slope in each patient and compared the sacral inclination in patients with a pars fracture to those of patients without a pars fracture. We also calculated the sacral inclination vector force by multiplying the trigonometric sine of the sacral angle with patients’ weight and compared this vector between the two groups.

Results: We found that patients with pars fractures had steeper sacral slopes (43.2° +/-10.1°) compared to those without pars fractures (36.8° +/- 8.1°) (p = 0.0007). Interestingly, despite having not significant differences in weight (82.7 +/- 17.2 kg vs 87.3 +/- 17.2 kg, p=0.189), patients with pars fractures had 49% higher sacral inclination vector forces compared to those without pars fractures (586 +/- 158 N vs 394 +/- 90 N, p<0.0001).

Conclusions: Taken together, these data suggest a strong association between high sacral slopes and presence of pars fracture, and further shows that sacral biomechanics influences the pathogenesis of spondylolisthesis.

Patient Care: We believe that surgeons should be aware of sacral biomechanics when evaluating patients with spondylolisthesis. High sacral slope is associated with pars fractures, and could predispose patients to progressing from lower grade listhesis to higher-grade listhesis. Patients with high sacral inclination and high sacral inclination vector forces on the pars may also be at increased risk for developing new pars interarticularis fractures, leading to worsening listhesis. Thus, sacral biomechanics should be studied to ascertain risk of instability after decompression and progression of disease in patients with low grade spondylolisthesis as well as considered when selecting patients for fusion.

Learning Objectives: Understand how spinopelvic biomechanics influences pathogenesis of spondylolisthesis

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