Biomechanical rigidity of cadaveric cervical spine with posterior versus combined posterior and anterior instrumentation: Laboratory investigation

Object. In patients with cervical stenosis with myelopathy, posterior instrumentation following cervical laminectomy has been shown to reduce the incidence of postoperative instability and kyphosis. However, the indications for posterior plus anterior instrumentation are not always obvious, and using both posterior and anterior instrumentation routinely is unnecessary and excessive. This study examines the rigidity of the intact isolated cadaveric cervical spine, after C4-6 laminectomy, with posterior lateral mass instrumentation, and following posterior plus anterior instrumentation. Methods. Ten fresh-frozen human cadaveric cervical spines from C1 to T2 were potted in the neutral position, and retroreflective markers were placed on C-3 and C-7. Specimens were mounted on a biomechanical testing frame, and angular rotations of C3 relative to C7 were measured. Pure moments of 0, 0.3, 0.6, 0.9, and 1.2 Nm were applied at C2 in all 3 planes. Each specimen was load tested as follows: 1) in the intact state; 2) after C4-6 laminectomy; 3) with C3-7 lateral mass instrumentation; and 4) with C3-7 posterior plus anterior instrumentation. Results. Laminectomy was not associated with a significant increase in motion compared with the intact state with any load or in any direction. Instrumentation was associated with reduction in motion in all directions, and there was no significant difference in posterior versus combined posterior and anterior instrumentation. Conclusions. Rigidity imparted to the cervical spine by a 5-level posterior lateral mass fixation is not augmented by anterior instrumentation.