The Journal of Bone and Joint Surgery, Vol 70, Issue 5 680-691, Copyright © 1988 by Journal of Bone and Joint Surgery, Inc
Biomechanical analysis of posterior instrumentation systems after decompressive laminectomy. An unstable calf-spine model
KR Gurr, PC McAfee and CM Shih
Department of Orthopedic Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
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.