tailieunhanh - Spinal Disorders: Fundamentals of Diagnosis and Treatment Part 8

Spinal Disorders: Fundamentals of Diagnosis and Treatment Part 8. Spinal disorders are among the most common medical conditions with significant impact on health related quality of life, use of health care resources and socio-economic costs. Spinal surgery is still one of the fastest growing areas in clinical medicine. | BiomechanicsoftheSpine Chapter 2 45 Figure 2. Load transfer in normal and degenerated discs a The intervertebral disc consists of a gel-like nucleus surrounded by a fibrous anulus consisting of multiple concentric lamellae. b In the healthy disc left compressive loads create a hydrostatic pressure within the fluid nucleus which is resisted by tensile stresses in the outer anulus. c Loads are transferred through the central portion of the vertebral endplate causing substantial deflection of the endplate up to mm . d e In the degenerated disc the nucleus is dehydrated and compressive loads are transferred by compressive stresses in the anulus. This may lead to an inward bulge of the inner anulus buckling of the lamellae and cleft formation. Endplate loading is reduced as stresses are transferred through the stronger and stiffer outer endplate region. flexibility at low loads and increasing stiffness at high loads 98 . Likewise a highly non-linear response of disc to torsion has been demonstrated 28 . Very little torque is required for the first 0-3 of rotation between 3 and 12 rotation there is a linear relationship between torque and rotation and failure of the anulus fibers occurs at a rotation of more than 20 rotation. Measurements of internal disc displacements during loading 80 90 have shown a characteristic motion of the nucleus away from the direction of applied bending load . a posterior shift of the anulus during flexion . Nucleus pressurization and displacement results in heterogenous disc bulging. Posterior disc bulging is greatest during extension and least during flexion which has implications for the most common disc injury disc protrusion and prolapse. Extrusion of nuclear material through the anulus usually occurs in the posterolateral direction and can cause compression of the dura and or nerve The nucleus shifts depending on the loading direction Nucleus extrusion usually occurs posterolaterally 46 Section Basic Science Combined axial .