Posterior Spinal Fusion versus 360 Fusion in a Calf Spine Model: A Biomechanical Analysis of Motion at the Instrumented and Adjacent Uninstrumented Segments.

Purpose: To determine, within the instrumented and the adjacent uninstrumented levels, the amount of segmental flexion and extension observed in a posterior spinal fusion (PSF) and a 360° fusion utilizing a calf spine 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.

Methods: Eight 4–segment lumbar calf spines were tested intact, with posterior pedicle screw instrumentation (PSF) and with pedicle screw instrumentation augmented by a simulated anterior interbody fusion (360° fusion). 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 the caudal end and unconstrained at the rostral end. Segmental axial motion was measured anteriorly and posteriorly at each level using an extensometer. The spines were preloaded to 40 kgf (392 N) and cycled sinusoidally at an amplitude of 2 mm (4 mm total displacement from peak to trough).

Results: Motion within the instrumented segment: Anterior motion decreased by 47% using a PSF technique (p = 0.057) and 69% using a 360° technique (p = 0.059) when comparing intact to post–op specimens. Posterior motion decreased 60% (p = 0.04) using a PSF technique and 75% using a 360° technique (p=0.01). Adjacent segment motion: Anterior motion increased by 75% using a PSF technique (p=0.068) and 100% using 360° technique (p = 0.03) when compared to intact specimens. Posterior motion was unchanged with the PSF technique and slightly increased with the 360° technique (p=NS) when compared to intact specimens.

Conclusions: In this model, both the PSF and 360° techniques provide similar segmental stabilization. Posterior motion is reduced most significantly in the 360° technique(p = 0.01). The 360° fusion technique promotes more anterior adjacent segmental motion (p=0.03) than does the PSF. Therefore a 360° fusion technique may be more likely to promote TZS in an otherwise healthy adjacent uninstrumented disc and may not be the optimal selective fusion technique in patients with preexisting adjacent segment degenerative disease.