A proposed technique for intraoperative measurement of cervical spine stiffness

OBJECTIVE: Classically cervical biomechanical data have been obtained using cadaver models. Few methods exist for in vivo measurement of cervical biomechanics. A technique for intraoperative measurement of cervical motion segment stiffness (load-axial displacement) has been developed, enabling load/displacement data to be obtained during surgery at the spinal levels operated on and at adjacent cervical levels. METHODS: The instrument used for measuring motion segment stiffness was a vertebral retractor equipped with displacement and strain transducers and designed to be placed over the shafts of Caspar distraction screws. Stiffness was measured under simulated anterior cervical surgical conditions. RESULTS: A preliminary study deciphering axial cervical spine load/displacement data on 12 cadavers was performed with this instrument. Using a distraction load of 88.8640 N, axial displacements of 2.110 +/- 0.258 mm (mean +/- standard error of the mean) at C3-C4, 2.367 +/- 0.304 mm at C4-C5, and 2.207 +/- 0.238 mm at C5-C6 were found. When data were evaluated for age, there was a significant decrease in stiffness with advancing age in two of the three spinal segments tested (P < 0.05 for C3-C4, P < 0.04 for C4-C5, P < 0.12 for C5-C6), which correlated with previous studies. CONCLUSION: These results suggest that this technique may be useful in assessing intraoperative motion-segment stability.