To evaluate the long-term changes in patellar cartilage thickness in TPX-100 versus placebo-treated knees in subjects who participated in Study TPX-100-1 or TPX-100-2.

No drug intervention. Study to collect 1 additional MRI of each knee treated in either TPX-100-1 or TPX-100-2.

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.

Subjects with bi-lateral knee OA that have previous been treated in TPX-100-1 or TPX-100-2