 Participants in this study will be randomly assigned to one of two treatment groups. Group A will be treated with mechanical distraction with motion; Group B will be treated with mechanical distraction without motion. Participants will be followed for 28 months and will have 11 study visits. Most of the study visits will occur during the first half of the study. State-of-the-art techniques for clinical assessment, articular surface imaging, biomechanical modeling, and biochemical testing will be used in this study. 