1. Patients must be at least six months post-operative.

2. Potential subjects will have a body weight of less than 250 lbs.

3. Candidates must have an AKS score \>70 post-operatively.

4. Patients must have passive flexion of at least 100.

5. Implanted group must have either an LCS PS RP TKA, having an anatomical patella, or Sigma PS RP TKA; or normal knee group must have normal knee.

6. Must be willing to sign both Informed Consent and HIPAA forms.

In Vivo Determination of 3D Patellofemoral Mechanics