In Vitro Biomechanics of Human Cadaveric Cervical Spines With Mature Fusion.

Background: This study sought to compare index and adjacent-level biomechanics of cadaveric specimens with mature fusion versus normal spines in intact and acutely fused conditions.

Methods: Eight human cadaveric cervical spines with mature fusion across 1 to 3 levels were studied. Intervertebral angular range of motion (ROM) was determined at fused and adjacent levels during pure moments inducing flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Mature fusion data were compared to data from normal spine specimens tested intact and then with a 1-level anterior plate/graft (fresh fixation). Bone qualities were compared using dual-energy x-ray absorptiometry.

Results: Mean bone mineral density was significantly greater in mature fusion spines (0.632 ± 0.239 g/cm) than in normal spines (0.489 ± 0.195 g/cm) ( < .001). Mean ROM for levels with mature fusion was 42% (FE), 42% (LB), and 29% (AR) of the mean same-level ROM in freshly fixated specimens ( .045). The mean adjacent-level ROM in spines with mature fusion was less than in normal spines (matched levels) in all directions, with the greatest difference 1 level below fusion (FE: -38%, < .001; LB: -42%, < .001; AR: -49%, .001), followed by 1 level above fusion (FE: -23%, .04; LB: -22%, .07; AR: -28%, .02) and 2 levels above fusion (FE: -20%, .08; LB: -18%, .11; AR: -31%, .009). Mature fusion reduced the magnitude of coupled LB during AR at C6-7 and C7-T1 ( .03).

Conclusion: Cervical spine segments with mature fusion have higher bone mass, are less flexible than freshly fixed spines, and have reduced mobility at adjacent levels.