StabilimaxNZ) versus simulated fusion: evaluation of adjacent-level effects.

Rationale behind motion preservation devices is to eliminate the accelerated adjacent-level effects (ALE) associated with spinal fusion. We evaluated multidirectional flexibilities and ALEs of StabilimaxNZ and simulated fusion applied to a decompressed spine. StabilimaxNZ was applied at L4-L5 after creating a decompression (laminectomy of L4 plus bilateral medial facetectomy at L4-L5). Multidirectional Flexibility and Hybrid tests were performed on six fresh cadaveric human specimens (T12-S1). Decompression increased average flexion-extension rotation to 124.0% of the intact. StabilimaxNZ and simulated fusion decreased the motion to 62.4 and 23.8% of intact, respectively. In lateral bending, corresponding increase was 121.6% and decreases were 57.5 and 11.9%. In torsion, corresponding increase was 132.7%, and decreases were 36.3% for fusion, and none for StabilimaxNZ ALE was defined as percentage increase over the intact. The ALE at L3-4 was 15.3% for StabilimaxNZ versus 33.4% for fusion, while at L5-S1 the ALE were 5.0% vs. 11.3%, respectively. In lateral bending, the corresponding ALE values were 3.0% vs. 19.1%, and 11.3% vs. 35.8%, respectively. In torsion, the corresponding values were 3.7% vs. 20.6%, and 4.0% vs. 33.5%, respectively. In conclusion, this in vitro study using Flexibility and Hybrid test methods showed that StabilimaxNZ stabilized the decompressed spinal level effectively in sagittal and frontal planes, while allowing a good portion of the normal rotation, and concurrently it did not produce significant ALEs as compared to the fusion. However, it did not stabilize the decompressed specimen in torsion.