Low back pain and neck pain adversely affect the quality of life of millions of people.  Changes in spinal motion can be a useful indicator for the severity of disease and also serve as an important clinical outcome measurement for assessing treatments.  Accurately measuring the motion of the individual vertebrae in the spine is a challenge due to the complex geometry of the bones and the 3-dimensional motions that are required of the spine.  This project has developed and validated a novel methodology for measuring spine motion and applied the technique to measure the motion in a cervical spine with a motion preserving implant.  This technique will allow clinicians and researchers to accurately quantify the performance of the normal spine as well as new implants designed to restore motion to the spine.  This methodology also has applications for other joints, such as the shoulder, ankle, knee and hip.