Inclusion Criteria:

• men and non-pregnant women, age 40-85 years with primary OA of at least one knee, and a flare of pain after withdrawal of prior therapy with either an oral NSAID or acetaminophen (used at least 3 days per week during the previous month).

Exclusion Criteria:

• secondary arthritis related to systemic inflammatory arthritis (including rheumatoid arthritis, psoriatic arthritis, post-infectious arthritis and metabolic arthritis, traumatic arthritis or surgical joint replacement); corticosteroid use:

  • oral corticosteroid within the previous 14 days, or
  • intramuscular corticosteroid within 30 days, or
  • intraarticular corticosteroid into the study knee within 90 days,
  • intra-articular corticosteroid into any other joint within 30 days, or
  • topical corticosteroid at the site of application within 14 days;
• ongoing use of prohibited medication including NSAID, other oral analgesic, muscle relaxant, or low-dose antidepressant for any chronic pain management; - ongoing use of glucosamine or chondroitin (unless used continuously for 90 days prior to study entry);
• sensitivity to diclofenac, acetylsalicylic acid (ASA) or any other NSAID, acetaminophen, dimethyl sulphoxide, propylene glycol, glycerine or ethanol; clinically-active renal, hepatic or peptic ulcer disease;
• history of alcohol or drug abuse;
• lactation;
• concomitant skin disease at the application site;
• current application for disability benefits on the basis of knee osteoarthritis; fibromyalgia; other painful or disabling condition affecting the knee;