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.