Sustained Loading Generates Stress Concentrations in Lumbar Intervertebral Discs

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Abstract
Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading. To show how sustained “creep” loading affects stress distributions inside intervertebral discs. The central region of an intervertebral disc acts like a hydrostatic “cushion” between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep. Twenty-seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2-6 hours. Creep reduced the hydrostatic pressure in the nucleus by 13-36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep. Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption, and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.