 Assessment of segmental spine motion has been and continues to be, a difficult clinical problem. Errors of up to 10 degrees for simple measurements of flexion, extension and side bending have been recorded using conventional radiographs xrays. These errors are usually associated with the inability to acquire three-dimensional positions and inaccurate reference points. It has, therefore, been extremely difficult to measure small changes in vertebral alignment, which may prove to be an early, clinically significant, indicator of potential problems. Radiostereometric Analysis, or RSA, is an accurate in vivo measurement technique using two simultaneous radiographs. It provides researchers with three dimensional motion analyses to look not only at routine flexionextension, but also other rotational and translational changes. The measurement accuracy offered by this technique far exceeds the manual techniques currently used. 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. 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 of the implanted construct. 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? 