Inclusion Criteria:

• Female patients evaluated to be relatively in good health
• 40 years old and above
• Negative pregnancy test

Additional inclusion criteria for OA patients:

• Frequent knee symptoms in the past year, defined as:
• pain, aching, or stiffness on most days of a month during the past year
• use od medication (all types) for treatment of knee pain on most days of a month during the past year
• Kellgren & Lawrence Grades 2 or 3 of the study knee (with either the same or less severe OA , or no OA in the contralateral knee)
• Body mass index (BMI) of 30 and above
• Medial joint space width of 2 mm and above

Additional inclusion criteria for age-matched controls:

• BMI of 28 and above
• No evidence of knee OA in either knee (i.e., Kellgren & Lawrence Grade 0 diagnosed by x-ray on AP view; infrequent knee pain, aching, stiffness during the past year; infrequent use of medication (all types) for treatment of knee pain during the past year.

Exclusion Criteria:

• Anticipated need for knee surgery in the next 2 years
• OA predominantly in the lateral tibiofemoral compartment measured by PA x-ray view of the study knee
• Intra-articular steroids to study joint (3 month washout), intra-articular hyaluronic acid to the study joint (3 month-washout), doxycycline (3 month washout), or arthroscopy within the last 6 months
• Medial joint space width less than 2 mm measured by PA view of knee x-ray
• History of joint replacement, intra-articular fracture, osteotomy, arthroplasty, or meniscectomy of the study knee
• History of other diseases that may involve the study joint including inflammatory joint diseases, crystalline disease, endocrinopathies, metabolic, diseases, knee infection of the study knee, neuropathic disorders, avascular necrosis, Paget's disease, or tumors (chondrocalcinosis is acceptable; treated thyroid disease is acceptable)
• Systemic inflammatory diseases (e.g., Rheumatoid arthritis)
• Other pain that the principal investigator feels can not be distinguished from knee pain (e.g., lumbar pain, hip pain, trochanteric bursitis)
• Musculoskeletal pain that may preclude the subject from remaining motionless for the MRI acquisitions
• Unable to undergo an MRI exam of the knee because of contraindications
• Unable to receive gadopentate contrast agent injection because of contraindications. A study subject who displays any sign of an allergic reaction to the contrast agent will be withdrawn from the dGEMRIC part of the protocol and will not receive any subsequent contrast agent injection. The subject may continue to complete the remainder fo the study, which includes MR morphometry scan, collection of blood and urine sample for biomarker assays, and completion of study questionnaires.
• Clinical chemistry laboratory values more than 2 times the upper limit of normal
• Participation in clinical trial with a study during in the past 14 days
• Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with trial participation or may interfere with the interpretation of trial results and, in the judgment of the investigator, would make the subject inappropriate for entry in the this trial