A Pivotal Study of a Facet Replacement System to Treat Spinal Stenosis

Spinal Stenosis continues to be a major cause of back and leg pain. The condition is attributed to narrowing of the space around the nerves in the lumbar spine. This is often caused by the degenerative process in the spine and the facet joints. The current treatment calls for removal of bone around the affected nerve including the facet joints and fusing the posterior of the spine to ensure the segments remain stable. The ACADIA® Facet Replacement System (AFRS) allows for an anatomic reconstruction of the facet joint after decompression and removal of the degenerated facet. Like the original facet joint, the replacement implant is designed to reproduce facet motion while restoring normal stability and motion. The ACADIA® Facet Replacement System (AFRS) has been designed on the principals that have allowed other total joint replacement procedures to provide significant patient benefits. These guiding principals include: * Anatomically based implant design * Reproducible surgical technique * Elimination of pain The ACADIA® Facet Replacement System allows the surgeon to remove the offending bone while preserving the motion of the facet joint. This study will evaluate the outcomes of patients using the AFRS™ investigation compared to those receiving instrumented posterior fusion procedure. Patients will be required to complete study visits before the procedure and at 6 weeks, 3, 6, 12, 24 months post procedure and annually thereafter as required by FDA. Follow up visits consist of administration of questionnaires, radiographs and neurological assessment.