Lumbosacral Spinal Fusion A Biomechanical Study

Abstract

The effects of spinal fusion on fused segment and the adjacent, unfused segments play a significant role in the clinical effectiveness of spinal fusion for low-back pain with or without sciatica. Much of the information on this important subject is derived from clinical impressions. The purpose of this biomechanical study is to investigate the altered kinematics and biomechanics of the three different types of spinal fusion (posterior, bilateral-lateral, and anterior) on the adjacent, unfused segment as well as within the fused segment and to investigate their clinical implications. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The test specimens included three motion segments, L3–4, L4–5, and L5-S1. To study the mechanics of the lumbar spine under combined compression and bending loads, a special apparatus was designed. These loads were applied by an MTS machine through two sets of pulley systems. The loads, as well as dispiacement data from both actuators, were recorded. A video camera system was utilized to record the kinematics of the spinal motion segment. The unfused specimen was tested first, and the fused specimen then was retested under the identical loading conditions. A total of 16 spine specimens were tested and evaluated—five posterior, four bilateral-lateral, and seven anterior fusions. All types of fusion resulted in increased bending and axial stiffnesses. Overall, anterior fusion provided the largest increase in stiffness, followed by bilateral-lateral fusion and posterior fusion. The kinematic data obtained from the video system shows cephalad shift of the center of rotation by fusion of L5-S1. This shift was largest in the bilateral-lateral fusion and in the posterior fusion. The posterior fusion produced posterior shift in addition to the cephalad shift. In anterior fusion and bilateral-lateral fusion, there was no detectable motion within the fused segment. But in the case of posterior fusion, a small amount was detected in the anterior markers (or anterior portion of the disc). All types of fusion demonstrated stabilizing effects on the fusion and produced increased stress on the adjacent, unfused segments, especially the facet joints. The bilateral-lateral fusion is the best method providing good stabilization to the fused segment and having the least effect on the adjacent unfused segments. Posterior fusion is the worst type of fusion producing the highest amount of stress in the adjacent segments and allowing a small but significant amount of motion across the retained disc.