Methodology to Calibrate Disc Degeneration in the Cervical Spine During Cyclic Fatigue Loading

Prolonged exposure to vibrational working conditions can cause neck, back, and shoulder pain. Mechanical degradation of soft tissues resulting from this type of fatigue was experimentally shown to contribute to endplate and compression fractures. However, effects of repetitive subfailure loading on intervertebral disc (IVD) behavior have not been well defined. This manuscript describes a methodology to experimentally characterize changes in cervical spine IVD material properties under fatigue. Bone-disc-bone spinal units with intact ligaments obtained from human cervical spines were obtained and a lack of bony or soft tissue degeneration was confirmed using X-ray and MRI scans. Cranial and caudal specimen extents were fixed in PMMA to facilitate attachment to testing devices. Baseline response was quantified using flexion/extension pure moment protocols. Specimens were immersed in a 34-deg-C saline bath and allowed to acclimate for one hour. A stress-relaxation test was then performed and viscoelasticity quantified using a quasi linear viscoelastic (QLV) material model. Fatigue testing was performed for up to 50,000 cycles with intermittent viscoelasticity, pure moment testing, and imaging scans performed to quantify cycle-dependent changes in disc properties. Preliminary results demonstrated progressive changes in viscoelasticity and bending response of cervical spine segments with increasing number of load cycles. This procedure will be used to quantify degradation of the IVD under repetitive compressive loads, focusing on effects of loading magnitude and frequency.