The purpose of this study is to determine whether Extracorporeal Focused Shock Wave Therapy reduces knee pain and enhance function in patients with primary knee osteoarthritis.

This study is a placebo controlled Randomized Clinical Trial (RCT) in which the primary outcome is to determine whether Extracorporeal Focused Shock Wave Therapy (ESWT) reduces knee pain comparing the VAS at baseline and one month after the end of the treatment. Function assessment and progression of pain along the treatment are the secondary outcomes. Function will be assessed with the Western Ontario & McMaster Universities Osteoarthritis Index (WOMAC), the Lequesne's Algofunctional Questionnaire for Osteoarthritis of Knee (Lequesne) and the Timed up and Go (TUG).

The ESWT applications will be performed once a week for four consecutive weeks, the patients will be assessed for pain and function before the beginning of the treatment and one week after every application of the ESWT.

The treatment groups will be randomized. The allocation of the patients is sealed and will not be disclosed to the patients nor to the evaluators.

Inclusion Criteria:

• Clinical and radiologic diagnosis of primary knee osteoarthritis (Kellgren & Lawrence I, II or III);
• Capability to understand the Informed Consent Form;
• Chronic pain for at least 3 months prior to inclusion, measured by VAS. (VAS 4 or above);
• Absence of skin injures, infections or tumor in the target knee;
• Availability to comply with the visits.

Exclusion Criteria:

• History of spinal cord stenosis or clinical symptoms of lumbar radiculopathy;
• History or onset neurological diseases;
• Generalized pain or fibromyalgia;
• Inability to walk;
• History of knee surgery in the target knee;
• Secondary causes of osteoarthritis;
• Use of statins and quinolones in the previous year;
• Uncontrolled and ongoing psychiatric diseases;
• Invasive knee treatments with hyaluronic acid infusion, corticosteroids and anaesthetics, in the target knee, up to 6 months previous to study inclusion.