Internal disc stresses and kinematic behavior of lumbar spine after laminotomy and laminectomy

Abstract

Raj Rao, MD, Mei Wang, PhD, Peeush Singhal, MD, Linda M. McGrady, BS, Milwaukee, WI, USA; Santi Rao, MD, Fairfield, CA, USAIntroduction: The biomechanical advantage of laminotomy over the more traditional laminectomy is the preservation of the midline posterior ligaments that are important in maintaining the stability of the spine. Decompression can be effectively achieved in most cases of degenerative spinal stenosis by a laminotomy and medial facetectomy, while still preserving the uninvolved spinous process and interspinous ligaments. There have been no published studies that directly compare the amount of destabilization between the laminotomy and laminectomy procedures. More importantly, the changes in internal mechanical stresses within the intervertebral disc resulting from the two procedures have not been previously quantified. The objective of this study is to verify the presumed increase in instability associated with the laminectomy and to compare the central and peripheral intradiscal stresses after the two procedures.Methods: Nine fresh calf spines were harvested for the study. The specimen was dissected into testing segments from L4 to L6 and potted in dental cement. Four miniature pressure transducers (1.5 mm in diameter, 0.3-mm-thick disc) were inserted into nucleus, anterior annulus and posterolateral annulus to measure the internal disc stress at L4–L5 (surgical level). Pure moments in flexion, extension, lateral bending and axial rotation up to 8.5 Nm were applied to the superior vertebrae in six steps. Three-dimensional motion of each segment was measured with motion analysis system (VICON 370). The spines were tested intact first, then after laminotomy at L4–L5 and again after expanding the laminotomy into laminectomy. The range of motion of the surgical level and the inferior adjacent level were evaluated, and analysis of variance and post-hoc were performed to compare the difference between intact spine and after the two surgeries. The peak magnitudes of the intradiscal transducers for the two procedures were normalized with respect to the values of intact spine, and the difference was evaluated with paired t test.Results: Motion at the surgical level increased in all directions from laminotomy to laminectomy, and statistically significant increases were found in flexion (23%) and axial rotation (18%). The largest increase from intact spine to spines after laminotomy and laminectomy was in axial rotation (102% and 120%), followed by extension (33% and 41%) and flexion (11% and 34%). Lateral bending showed the least increase (7% and 11%). Motion at the intact inferior adjacent level remains unchanged. In flexion, the intact spine showed higher stress at the anterior annulus and the nucleus. The stress at the anterior annulus increased 20% after laminotomy, and 130% after laminectomy (p<.02). In extension, the high-stress regions were in the nucleus and posterolateral annulus. The posterolateral stress increased 22% and 27%, respectively, after each procedure, and the nucleus stress increased 24% and 31%, respectively, after laminotomy and laminectomy. The differences between the two procedures were not significant. The posterolateral stress of the loaded side in lateral bending was the highest of all loading modes. The stress remained unchanged after laminotomy but increases 9% after laminectomy (p<.06). In axial rotation, internal disc stresses were evenly distributed and unchanged after each procedure.Discussion: Previous experimental studies have shown that the posterior ligamentous structures are important in maintaining stability of the spine. Our study found that laminectomy resulted in significantly larger motion in flexion and axial rotation than laminotomy, and a significant increase in anterior annular stresses at the operated disc space, indicating anterior shifting of the loading path after laminectomy. It provides a quantitative measure of the biomechanical advantages of laminotomy over laminectomy. The increase in internal disc stresses after a laminectomy could translate clinically into degenerative changes within the disc. A better understanding of the mechanics of the spinal segment, and in particular their response to these interventions, will help diminish the clinical incidence of late development of instability and the increased need for subsequent fusions in the lumbar spine.