This study is being conducted to evaluate the TOPS™ SP System from ("TOPS™ System" or "TOPS"). TOPS is an alternative to spinal fusion and is designed to stabilize but not fuse the affected vertebral level to alleviate pain stemming from degenerative spondylolisthesis (abnormal/excessive movement of the vertebrae causing pain in the lower back and legs) and spinal stenosis (narrowing of the spinal canal resulting in compression of nerves producing symptoms of pain, numbness and tingling in the legs).