A Prospective Observational Study to Evaluate Long-term Changes in Cartilage Morphology in Subjects Who Previously Received TPX-100 or Placebo in Study TPX-100-1 for Patellar Osteoarthritis Involving Both Knees

Inclusion Criteria: * Subjects who participated in TPX-100-1 and received 4 injections of 200 mg/injection TPX-100 in one knee, or subjects who participated in TPX-100 * Cruciate and collateral ligament stability as assessed by screening history and clinical examination * Stable medial and lateral menisci as assessed by screening history and clinical examination * Able to read, understand, sign and date the subject informed consent Exclusion Criteria: * Contraindication to MRI, including: metallic fragments, clips or devices in the brain, eye, or spinal canal; implanted devices that are magnetically programmed; weight \> 300 lbs.; moderate or severe claustrophobia; previous intolerance of MRI procedure * Prior surgery in the knees, excluding procedures for debridement only (no previous microfracture procedure) * History of rheumatoid arthritis, psoriatic arthritis, or any other autoimmune or infectious cause for arthritis * Last viscosupplementation (e.g. Synvisc® or similar hyaluronic acid product) injected into either knee \< 3 months before enrollment * Last intra-articular knee injection of corticosteroids \< 3 months before enrollment * Use of any steroids (except inhaled corticosteroids for allergy or respiratory problems) during the previous month before enrollment * History of arthroscopy in either knee in the 3 months before enrollment * History of septic arthritis, gout or pseudo-gout in either knee in the 12 months before enrollment * Clinical signs of acute meniscal tear (locking, new acute mechanical symptoms consistent with meniscal tear) * Active systemic infection * Participation in other clinical osteoarthritis drug studies, except for studies TPX-100-1 and TPX-100-2, within two years prior to enrollment in TPX-100-4.