Experimental Modelling of Viscoelastic Self-Heating in Healthy and Degenerate Bovine Intervertebral Discs

Objectives Low back pain (LBP) is an increasing drain on developed economies due to direct medical costs and lost working days. The majority of medical costs can be attributed to long-term problems resulting from specific physiological conditions. Acute injury and/or chronic degeneration of the intervertebral disc (IVD) has been linked with long term pain with high levels of nerve in-growth in degenerate IVDs. The fact that disc degeneration is a structural failing and not just a pathogenesis of pain may lead to reduced mobility and quality of life (QOL). Degenerate IVDs have elevated levels of heat shock proteins (HSPs) and HSPs elevated temperatures and/or heat shock within the IVD is a potential mechanism for HSP upregulation. Is it possible that high temperatures are a precursor to degeneration? Could activities of daily living (ADL) result in elevated temperatures in the IVD? This study aims to determine if there is a significant generation of heat within the IVD when subjected to cyclic loading at levels and frequencies relevant to ADL and whether this is effected by degeneration. Materials and Methods Bovine coccygeal discs were removed whole from tail sections and half of the discs were injected with a 2mg/ml collagenase solution and incubated at 37°C for 2 hours to simulate moderate degeneration. Discs were then subjected to sinusoidal loading at 2Hz at force levels equivalent to those in the human spine during locomotion. Mechanical data was analysed with MATLAB software to determine the energy dissipated by the discs for each cycle of loading and an idealised thermal model was generated to predict temperature change within the disc. Results Under axial loading equivalent to that in the lumbar spine during walking degenerate discs showed greater average compression than healthy discs (0.108mm and 0.024mm respectively) and therefore substantially lower average stiffness (714N/mm and 3149N/mm). Average heat generation in degenerate IVDs (2.79mW) was lower than that in healthy discs (4.13mW). An idealised 3D model of heat loss from the disc showed no significant increase in disc temperature in either healthy or degenerate disc condition. Conclusions Heat generated due to dissipated energy from axial loading of intervertebral discs at loading equivalent to that in the lumbar spine when walking is not enough to induce significant temperature increases within the disc. If elevated levels of HSPs within degenerate discs are the result of high temperatures, heat generated within the disc due to activities of daily living such as walking are not the cause