The objectives of this study are four-fold:

1. To determine the three-dimensional, in vivo kinematics for subjects having either a fixed- or mobile-bearing Press Fit Condylar (PFC) Sigma Total Condylar III Prosthesis, comparing the in vivo kinematics to determine if one implant type leads to a benefit for the patient.
2. To determine if bearing mobility occurs in PFC Sigma Rotating Platform Total Condylar III Prosthesis under in vivo, weight-bearing conditions during multiple activities (gait, stair descent, deep knee bend and chair rise).
3. To determine if there is a correlation between in vivo kinematic data obtained using fluoroscopy, electromyography (EMG) and ground reaction force (GRF) data and determine if variability occurs between these two TKA types.
4. To determine if a clinical benefit is either visibly detected (video camera) or quantifiably determined (questionnaire) for either TKA type.

Inclusion Criteria:

1. Patients must be at least three months post-operative.
2. Potential subjects will have a body weight of less than 250 lbs.
3. Candidates must have had a HSS score >90 post-operatively.
4. Patients must have passive flexion of at least 100.
5. All potential subjects will have a Press Fit Condylar (PFC) Rotating Platform Total Condylar III Prosthesis (Depuy Orthopaedics, Warsaw, IN) or fixed-bearing Total Condylar III Prosthesis.
6. Must be willing to sign both Informed Consent and HIPAA forms.

Exclusion Criteria:

1. Pregnant females.
2. Patients that do not meet study requirements.

Subjects requiring total knee arthroplasty