 The purpose of this study is to assess the amount of motion in the lumbar spine in patients with the AST Stabilimax BAR. The AST Stabilimax BAR is designed to allow for motion in the spine while providing the necessary stability to the spinal segment. The amount and degree of this motion is currently unknown clinically, but has been biomechanically tested in vitro. By using tantalum bead markers placed in the spine at the time of the surgery, RSA analysis can determine the amount and degree of motion the implant provides. Two questions will be examined with this data: 1 what is the amount of sagittal and coronal plane motion that occurs in vivo with the AST Stabilimax BAR system? and 2 will this change over the 2 year time frame for this study? 