EXAMINING ADJACENT DISC DISEASE: A NOVEL IN-VITRO TISSUE APPROACH

When a patient experiences extreme degeneration of an intervertebral disc, a spinal fusion may be considered as a viable treatment plan. However, any implementation of hardware will change the biomechanics of the spine. This premise has led to the debate around adjacent disc disease (ADD) which is defined as the disc adjacent to a fused functional spine unit experiencing an accelerated rate of degeneration. There is a large controversy regarding ADD as a relevant pathology; however, it is largely unknown how spinal fusions effect the mechanical properties of the adjacent annulus fibrosus. Therefore, this thesis aims to examine the effect of spinal fusions on the mechanical properties of the annulus in the adjacent cranial disc of a fused motion segment using a novel in-vitro tissue model. Fifty-two porcine cervical spine C3-C5 segments were used for the current study. For each sample, the C4/5 disc was either injured or left intact and either fused or not fused resulting in the following four conditions: 1) control (not injured or fused); 2) injured only; 3) fused only; and 4) injured+fused. Injuries were created using an 18.5-gauge needle and piercing through the posterolateral aspect of the spine to the nucleus. Spinal fusions were accomplished by wrapping 18-gauge steel wire around the transverse and spinous processes laterally on both sides. After injury/fusion, each specimen was subjected to a 15-minute preconditioning period under 300 N axial compression followed by a cyclic compression protocol of a 0.5 hertz sinusoidal waveform ranging from 300-1200 N for 2 hours (3600 cycles) (MTS, Eden Prairie, MN). Post-compression, two annular samples were dissected from the C3/4 disc (adjacent to the injured/fused level). From these samples, a single annular layer tensile test was conducted to measure the strength of the intralamellar matrix (adhesion matrix within a single annular layer) in both the anterior and posterior. In addition, a peel test was conducted to measure the adhesion strength of the interlamellar matrix (adhesion matrix between each layer) of the posterolateral aspect of the annulus. A significant interaction between the injured and fused was observed in all of the stress values for the intralamellar matrix strength (toe region stress (p\u3c0.001), initial failure stress (p=0.03), and ultimate stress (p=0.004)). In contrast, there were no significant main effects or interactions (p\u3e0.05) found for any interlamellar matrix adhesion property. The control and injured+fused conditions were subsequently replicated in eight soft fixed (ethanol) human lumbar spines and similar findings were observed. It appears that both fusion and disc injury negatively impact the material properties of the annulus in the adjacent segment confirming the notion that disrupting a disc does directly impact the biomechanics of the adjacent level. Reduced intralamellar matrix strength is associated with many disc pathologies, thereby providing a potential mechanism for the occurrence of ADD. These findings may prompt spine surgeons to be more conservative when deciding if a spinal fusion is right for a patient as there seems to be a negative holistic effect on the spine