Torsion biomechanics of the spine following lumbar laminectomy: a human cadaver study

Purpose

Lumbar laminectomy affects spinal stability in shear loading. However, the effects of laminectomy on torsion biomechanics are unknown. The purpose of this study was to investigate the effect of laminectomy on torsion stiffness and torsion strength of lumbar spinal segments following laminectomy and whether these biomechanical parameters are affected by disc degeneration and bone mineral density (BMD).

Methods

Ten human cadaveric lumbar spines were obtained (age 75.5, range 59–88). Disc degeneration (MRI) and BMD (DXA) were assessed. Disc degeneration was classified according to Pfirrmann and dichotomized in mild or severe. BMD was defined as high BMD (≥median BMD) or low BMD (<median BMD). Laminectomy was performed either on L2 (5×) or L4 (5×). Twenty motion segments (L2–L3 and L4–L5) were isolated. The effects of laminectomy, disc degeneration and BMD on torsion stiffness (TS) and torsion moments to failure (TMF) were studied.

Results

Load–displacement curves showed a typical bi-phasic pattern with an early torsion stiffness (ETS), late torsion stiffness (LTS) and a TMF. Following laminectomy, ETS decreased 34.1 % (p < 0.001), LTS decreased 30.1 % (p = 0.027) and TMF decreased 17.6 % (p = 0.041). Disc degeneration (p < 0.001) and its interaction with laminectomy (p < 0.031) did significantly affect ETS. In the mildly degenerated group, ETS decreased 19.7 % from 7.6 Nm/degree (6.4–8.4) to 6.1 Nm/degree (1.5–10.3) following laminectomy. In the severely degenerated group, ETS decreased 22.3 % from 12.1 Nm/degree (4.6–21.9) to 9.4 Nm/degree (5.6–14.3) following laminectomy. In segments with low BMD, TMF was 40.7 % (p < 0.001) lower than segments with high BMD 34.9 Nm (range 23.7–51.2) versus 58.9 Nm (range 43.8–79.2)].

Conclusions

Laminectomy affects both torsion stiffness and torsion load to failure. In addition, torsional strength is strongly affected by BMD whereas disc degeneration affects torsional stiffness. Assessment of disc degeneration and BMD pre-operatively improves the understanding of the biomechanical effects of a lumbar laminectomy.