360 Degree And Posterior Spinal Fusion Segmental Biomechanics: A Cadaveric Model

Significance: Selective segmental arthrodesis of the spine is occasionally associated with premature degeneration of adjacent motion segments known as transition zone syndrome (TZS). The presumed etiology is the transference of additional stresses and motion from the immobilized segment to the adjacent levels.

Purpose: To determine, within the instrumented and the adjacent uninstrumented levels, the amount of cantilever bending induced by a posterior spinal fusion (PSF) and a 360° fusion using a human cadaveric spine model.

Methods: Four 4–segment (L2 to the sacrum) fresh human lumbar cadaveric spines were tested intact, with posterior pedicle screw instrumentation (PSF) at L5–S1 and with pedicle screw instrumentation augmented by a simulated anterior interbody fusion (360° fusion) at L5–S1 as well. Testing was conducted on intact spines and then repeated using both PSF and 360° fusion techniques. Testing was performed using an MTS 809 biaxial servohydraulic biomechanical testing system. Axial loads were delivered to the rostral end of the spine via specially designed biomechanical grips. The spines were constrained at both the caudal and rostral ends. Spines were preloaded to 20 kgf (196 N) and cycled sinusoidally at an amplitude of 3 mm (6 mm total excursion). Segmental axial motion was measured anteriorly and posteriorly at each level using an extensometer.

Results: Motion within instrumented segment: Anterior motion decreased by 44% using a PSF technique (p = 0.02) and 66% using a 360° technique (p = 0.004). Posterior motion decreased 47% (p = 0.015) using a PSF technique and 62% using a 360° technique (p=0.004). Adjacent segment motion: Anterior motion increased by 33% using a PSF technique (p=NS) and 51% using the 360° technique (p=NS). Posterior motion decreased by 50% in the PSF model (p=0.087) and decreased by 57% (p=0.027) in the 360° model.

Conclusions: Compared to the intact spines, both the PSF and the 360° fusion techniques appear to have a significant stiffening effect on both the anterior and posterior columns of the instrumented segment. At the adjacent segment, however, there was a statistically significant decrease in posterior column motion in both the PSF and 360° fusion models. Anteriorly, there was a trend of increasing adjacent segment motion with increasing construct stiffness. The advantage of interbody fusion is the ability to lordose the spine, but there has been concern that anterior spinal fusions in the lumbar spine may create such a rigid fusion that increased stresses and motion will be transferred to the adjacent level(s), creating accelerated degeneration of the discs (TZS). This data suggests that an anterior interbody fusion with a posterior pedicle screw construct does increase the adjacent segment motion in this model.