Exploring interactions between force, repetition and posture on intervertebral disc height loss and bulging in isolated porcine cervical functional spinal units from sub-acute-failure magnitudes of cyclic compressive loading

Most in vitro studies are limited in the ability to partition intervertebral disc (IVD) height loss from total specimen height loss since the net changes in the actuator position of the materials testing system simply reflect net changes to functional spinal units (FSUs) used for testing. Three levels of peak compressive force, three cycle rates and two dynamic postural conditions were examined using a full-factorial design. Cyclic compressive force was applied using a time-varying waveform with synchronous flexion/extension for 5000 cycles. Surface scans from the anterior aspect of the IVD were recorded in a neutral and flexed posture before and after the cyclic loading protocol using a 3D laser scanner to characterise changes in IVD height loss and bulging. A significant three-way interaction (p=0.0092) between the magnitude of peak compressive force, cycle rate and degree of postural deviation was observed in cycle-varying specimen height loss data. A significant main effect of peak compressive force (p=0.0003) was also observed in IVD height loss calculated from the surface profiles of the IVD. The relative contribution of IVD height loss (measured on the anterior surface) to total specimen height loss across experimental conditions varied considerably, ranging from 19% to 58%. Postural deviation was the only factor that significantly affected the magnitude of peak AF bulge (p=0.0016). This investigation provides evidence that total specimen height loss is not an accurate depiction of cycle-varying changes in the IVD across a range of in vivo scenarios that were replicated with in vitro testing.