Understanding lumbar facet joint involvement and biomechanical changes post spinal fusion is limited. This study aimed to establish an in vitro model assessing mechanical effects of fusion on human lumbar facet joints, employing synchronized motion, pressure, and stiffness analysis. Seven human lumbar spinal units (age 54 to 92, ethics 15/YH/0096) underwent fusion via a partial nucleotomy model mimicking a lateral cage approach with PMMA cement injection. Mechanical testing pre and post-fusion included measuring compressive displacement and load, local motion capture, and pressure mapping at the facet joints. pQCT imaging (82 microns isotropic) was carried out at each stage to assess the integrity of the vertebral endplates and quantify the amount of cement injected. Before fusion, relative facet joint displacement (6.5 ± 4.1 mm) at maximum load (1.1 kN) exceeded crosshead displacement (3.9 ± 1.5 mm), with loads transferred across both facet joints. After fusion, facet displacement (2.0 ± 1.2 mm) reduced compared to pre-fusion, as was the crosshead displacement (2.2 ± 0.6 mm). Post-fusion loads (71.4 ± 73.2 N) transferred were reduced compared to pre-fusion levels (194.5 ± 125.4 N). Analysis of CT images showed no endplate damage post-fusion, whilst the IVD tissue: cement volume ratio did not correlate with the post-fusion behaviour of the specimens.Objectives
Methods and Results
The purpose of this study was to develop a novel, minimally invasive therapy for nucleus pulposus augmentation without the need for major surgical incision. Two optimum patented self-assembling peptides based on natural amino acids were mixed with glycosaminoglycans (GAGs) to form reversible, tunable hydrogels that mimic the vital biological osmotic pumping action and aid in swelling pressure of the intervertebral disc (IVD). Separate peptide and GAG solutions can be switched from fluid to gel upon mixing inside the body. The gels were analysed using a series of complementary techniques (FTIR, TEM & rheometry) to determine their cross-length scale structure and properties. Approaches to developing a clinical product were then developed including the incorporation of a fluorescent probe and a CT contrast agents to aid visualization of the gels, and a semi-automatic syringe driver rig, incorporating a pressure sensor, for the delivery of the solutions into the intervertebral discs. The efficacy of the procedure in restoring disc height and biomechanics was examined using chemically degenerated bovine caudal samples. It was found the presence of the GAGs stabilized the peptides forming stiffer gels, even upon injection through a long (∼10cm) small gauge needle. The injected gels were easily visualized post injection by microCT and by eye during dissection under visible and UV light. It was also noted that following injection, the disc height of the degenerated samples was restored to a similar level of that observed for native discs. A hydrogel has been developed that is injected through a narrow bore needle using a semi-automatic delivery rig and forms a self-assembled gel in situ which has shown to restore the disc height. Further tests are now underway to examine their biomechanical performance across more physiological time periods.
