Radiographic research has uncovered many factors attributed to the progression of a bilateral spondylolytic defect to spondylolisthesis, with specific attention given to spino-pelvic alignment and balance. Sacro-pelvic parameters have been studied extensively and are believed to be implicated in the progression of vertebral slippage. However, the role of L5-S1 disc morphology in slippage progression remains poorly understood. Furthermore, while lytic spondylolisthesis is often associated with disc degeneration at the index level, it is not clear whether disc degeneration is the cause or the consequence of the slippage. Using computational techniques, the main objectives of my research were to examine:
1) The role of sacral slope and secondary changes to the L5-S1 disc morphology in the progression of bilateral L5 spondylolysis to spondylolisthesis
2) The role of different grades of disc degeneration in the progression of bilateral L5 spondylolysis to spondylolisthesis
Finite element (FE) models were generated from high-resolution computed tomography (CT) scans and simulated in flexion and extension bending. The FE analysis demonstrated that both high and low sacral slope produce abnormal disc shapes that are responsible for separate pathomechanisms of progression involving localised posterior and anterior disc stresses respectively. Congenital abnormalities in spino-pelvic alignment are likely to cause stress concentrations that place an individual at higher risk of progression to spondylolisthesis. FE models were also built to examine the role of different grades of disc degeneration in the progression of L5 spondylolysis to spondylolisthesis. The models represented mild and severe material degeneration with and without a 50% disc height collapse. A mildly degenerate disc experienced greater motion instability compared with severe degeneration. Disc height collapse, with or without degenerative changes, is one of the plausible re-stabilisation mechanisms available to the L5-S1 motion segment to mitigate increased intervertebral motions and shear stresses due to a spondylolytic defect.