This study is being conducted to determine the percentage of patients with solid spinal fusions after being treated with Osteocel Plus in a PLIF procedure. Due to the unique processing conditions, Osteocel Plus retains a high concentration of stem cells attached to the allograft bone matrix, so it is hypothesized that the fusion rate with Osteocel Plus will be comparable to published data for autograft. The Osteocel product family has already been used in approximately 20,000 cases worldwide.