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/cm²) than in normal spines (0.489 ± 0.195 g/cm²) (P < .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 (P ≤ .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%, P < .001; LB: −42%, P < .001; AR: −49%, P = .001), followed by 1 level above fusion (FE: −23%, P = .04; LB: −22%, P = .07; AR: −28%, P = .02) and 2 levels above fusion (FE: −20%, P = .08; LB: −18%, P = .11; AR: −31%, P = .009). Mature fusion reduced the magnitude of coupled LB during AR at C6-7 and C7-T1 (P ≤ .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.