Estimation of shear load sharing in moderately degenerated human lumbar spine

• Published in: Journal of biomechanics 2013-02, Vol.46 (4), p.651-657
• Main Authors: Skrzypiec, Daniel M, Bishop, Nicholas E, Klein, Anke, Püschel, Klaus, Morlock, Michael M, Huber, Gerd
• Format: Article
• Language: English
• Subjects: Adult; Biomechanical Phenomena; Bone density; Design engineering; Humans; In-vitro; Intervertebral disc; Intervertebral Disc Degeneration - pathology; Intervertebral Disc Degeneration - physiopathology; Legal medicine; Load; Load sharing; Lumbar spine; Lumbar Vertebrae - pathology; Lumbar Vertebrae - physiopathology; Male; Physical Medicine and Rehabilitation; Shear; Shear Strength - physiology; Spondylolysis - pathology; Spondylolysis - physiopathology; Stress, Mechanical; Vertebrae; Weight-Bearing - physiology; Young Adult
• ISSN: 0021-9290; 1873-2380
• DOI: 10.1016/j.jbiomech.2012.11.050

Abstract Shear load sharing between intervertebral discs and apophyseal joints was investigated experimentally in human lumbar motion segments with moderately degenerated intervertebral discs. ‘Motion-Segments’ (21–42 years, n =6) and ‘Disc-Segments’ (22–42 years, n =6) were subjected to shear in 0° flexion, using a modified materials testing machine, while immersed in a Ringer bath at 37 °C. Initially, two cycles of anterior and posterior shear loading up to 200 N (50 N/s) were applied, to evaluate stiffnesses in both directions. Specimens were then exposed to 15 mm of anterior displacement at a rate of 0.5 mm/s. A physiological compressive load of 500 N was applied throughout. The initial 5 mm of the load–displacement curves were approximated with 6th order polynomials for evaluation of the mean behaviour in each group. ‘Disc-Segments’ were 66% ( p =0.002) and 43% ( p =0.026) less stiff than ‘Motion-Segments’ for anterior and posterior shear directions, respectively. ‘Disc-Segments’ exhibited 44% lower peak shear load ( p =0.015) than ‘Motion-Segments’. All specimens in the ‘Disc-Segments’ group showed damage either at the interface between the endplates and the disc. The intervertebral disc contributes 38% to initial anterior shear load-bearing, increasing to 66% at 5 mm displacement. Some over-estimation of disc load-bearing might have been caused by the comparison of segments from different levels. The apophyseal joints make a substantial contribution (65–55%) to anterior shear load-bearing over the initial 2 mm of shear displacement but this decreases with increasing shear displacement.