Biomechanical analysis of posterior instrumentation systems after decompressive laminectomy. An unstable calf-spine model.

Mechanical non-destructive cyclical testing in rotation, axial compression, and flexion were performed on twelve fresh spinal segments from calves. Each segment contained five motion segments. Each spine was destabilized with bilateral laminectomy and facetectomy of the fourth and fifth lumbar vertebrae, resection of the pars interarticularis of the fourth lumbar vertebra, and resection of the disc between the fourth and fifth lumbar vertebrae. Sequential stabilization of each spine was used to compare the stiffness of: (1) Harrington distraction instrumentation of five levels, (2) Luque rectangular instrumentation of five levels, (3) modified Steffee transpedicular notched-rod instrumentation of three and five levels, and (4) Cotrel-Dubousset transpedicular instrumentation of three and five levels with and without transverse approximating rods. This in vitro study of a calf-spine model led to three reproducible conclusions: (1) after laminectomy and discectomy, the instrumented spine was more unstable in rotation and flexion than when it was subjected to axial compressive loads; (2) the most rigid implant was the Cotrel-Dubousset transpedicular instrumentation of five vertebral levels (p less than 0.05); and (3) with the Steffee or the Cotrel-Dubousset transpedicular instrumentation of three vertebral levels, it was possible to restore torsional, compressive, and flexural rigidity to the destabilized spine of the calf. Furthermore, transpedicular fixation of only three vertebral levels provided more in vitro stability than either traditional Harrington or Luque rectangular instrumentation, which require fixation of five vertebral levels to stabilize a spine after laminectomy.