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Sacral Slope Measurement Helpful for Determining if Fusion Surgery Needed for Spondylolisthesis

Key findings

  • It would be helpful to identify a risk factor adjunctive to pars fracture that could reliably predict whether patients with low-grade spondylolisthesis will benefit from instrumented fusion
  • Among 131 patients with grade I spondylolisthesis who underwent L5–S1 instrumented fusion, higher sacral slope was a risk factor for pars fractures
  • Average sacral inclination vector force was substantially higher in patients with pars fractures than in those without
  • Sacral slope and sacral inclination vector force are easy to calculate and should be taken into account when patients are being considered for decompression without fusion surgery

A controversy in neurosurgery is whether instrumented fusion is needed, versus decompression alone, for low-grade spondylolisthesis in the absence of a pars fracture. Massachusetts General Hospital neurosurgery residents Robert M. Koffie, MD, PhD, and Vijay Yanamadala, MD, and neurosurgeon Jean-Valery Coumans, MD, have published guidance for surgeons who are managing such patients. They explain in World Neurosurgery that patients with higher sacral slopes are at increased risk of developing pars fractures, probably because of the higher vector forces pulling on the lumbar pars interarticularis.

Dr. Coumans' team retrospectively analyzed the records of 131 patients with grade I degenerative spondylolisthesis who underwent L5–S1 posterior lumbar spine instrumented fusion. Spondylolisthesis was secondary to bilateral pars fractures in 32 patients and facet arthropathy in 99 patients. All patients had 25% or less L5 slippage on S1.

The researchers measured sacral slope using standing plain radiographs in neutral position and without pelvic retroversion. They defined the slope as the acute angle of intersection between the line parallel to the superior end plate of S1 and the line parallel to the horizontal plane.

The average slope was significantly higher for patients with pars fractures, 43.2°, than for those without pars fractures, 36.8°. Patients with pars fractures had 17.3% steeper sacral slopes than those without fractures. In line with that finding, patients with higher sacral slopes were 2.7 times more likely to develop pars fractures.

The team also calculated the sacral inclination vector force (Fs) for each patient. They multiplied body weight (in kilograms) by the trigonometric sine of sacral slope angle and the acceleration of gravity constant 9.8 m/s2.

The average Fs was 573.7 N in the group of patients with pars fractures, nearly 50% higher than the average of 390.8 N in the group without fractures.

Sacral slope is easy to measure, the researchers comment, and does not require full radiographs of the entire spine and pelvis. They encourage colleagues to consider sacral slope and sacral inclination vector force during patient selection for decompression without fusion surgery.

steeper sacral slopes in patients with pars fractures than those without fractures

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