The purpose of this clinical study is to demonstrate the safety and effectiveness of the Dynesys Spinal System for patients requiring one or contiguous two-level posterior spinal stabilization of the lumbar, and/or sacral spine following decompression. The ability for this implant to maintain spinal alignment and non-fusion of spinal segments, while positively affecting clinical outcomes, will be assessed and compared to a posterior lateral spinal fusion (PLF) procedure using autogenous bone with a semi-rigid, polyaxial posterior spinal fixation system.