The impact of bone mineral density and disc degeneration on shear strength and stiffness of the lumbar spine following laminectomy

Abstract

PurposeLaminectomy is a standard surgical procedure for elderly patients with symptomatic degenerative lumbar stenosis. The procedure aims at decompression of the affected nerves, but it also causes a reduction of spinal shear strength and shear stiffness. The magnitude of this reduction and the influence of bone mineral density (BMD) and disc degeneration are unknown. We studied the influence of laminectomy, BMD, and disc degeneration on shear force to failure (SFF) and shear stiffness (SS).MethodsTen human cadaveric lumbar spines were obtained (mean age 72.1 years, range 53–89 years). Laminectomy was performed either on L2 or L4, equally divided within the group of ten spines. BMD was assessed by dual X-ray absorptiometry (DXA). Low BMD was defined as a BMD value below the median. Intervertebral discs were assessed for degeneration by MRI (Pfirrmann) and scaled in mild and severe degeneration groups. Motion segments L2–L3 and L4–L5 were isolated from each spine. SFF and SS were measured, while loading simultaneously with 1,600 N axial compression.ResultsLow BMD had a significant negative effect on SFF. In addition, a significant interaction between low BMD and laminectomy was found. In the high BMD group, SFF was 2,482 N (range 1,678–3,284) and decreased to 1,371 N (range 940–1,886) after laminectomy. In the low BMD group, SFF was 1,339 N (range 909–1,628) and decreased to 761 N (range 561–1,221). Disc degeneration did not affect SFF, nor did it interact with laminectomy. Neither low BMD nor the interaction of low BMD and laminectomy did affect SS. Degeneration and its interaction with laminectomy did not significantly affect SS.ConclusionsIn conclusion, low BMD significantly decreased SFF before and after lumbar laminectomy. Therefore, DXA assessment may be an important asset to preoperative screening. Lumbar disc degeneration did not affect shear properties of lumbar segments before or after laminectomy.

Torsion biomechanics of the spine following lumbar laminectomy: a human cadaver study

“…Farfan et al [9,11] measured TMF under compression (maximum of 573 N) in degenerated segments; their results were comparable to the present results. We used a constant compressive load level of 1,600 N to allow for comparison with the load levels found in daily physiological loading [16] and to compare with previous work [5,6]. While this force may seem high, it is not very high compared to that estimated in vivo compression.…”

“…Consequently, segments were able to find their physiological motion patterns irrespective of possible differences in embedding. During application of torsion moments, segments were loaded with a continuous purely axial compressive force of 1,600 N applied using a pneumatic cylinder [5,6]. Calibration of axial compression was performed using a load cell (Hottinger Baldwin Messtechnik Ó , Force Transducer Type C2, Darmstadt, Germany).…”

“…Calibration of axial compression was performed using a load cell (Hottinger Baldwin Messtechnik Ó , Force Transducer Type C2, Darmstadt, Germany). The 1,600 N preload was selected to allow for comparison with load levels found in daily physiological loading [16] and to compare with previous work [5,6], without causing compressive failure [7]. Subsequently, torsion load was applied with a constant rate of 3.0°per min by pulling on a metal wire, which was securely fixed to the part of the casting mold that contained the caudal vertebral body (Fig.…”

“…The effect of lumbar laminectomy on intervertebral shear stiffness and shear force to failure is well-known [5,6]. However, during daily activities such as asymmetric lifting [17], the lumbar spine is not only subjected to shear forces but also to torsion moments and the resulting axial rotation.…”

The Michel Benoist and Robert Mulholland yearly European Spine Journal Review

“…The paper by Bisshop et al [3] published in March calls attention to the impact of bone mineral density (BMD) on shear strength and stiffness of the lumbar spine following laminectomy. It is admitted that laminectomy without fusion can be a source of instability, postoperative spondylolisthesis, and possibly fracture of the pars interarticularis.…”