The aim of this project is to investigate fMRI ability to identify pain central sensitization in chronic knee OA. Patients with high and low central sensitization and healthy control subjects will be included. Central sensitization will be clinically defined based on the evidence of regional spread of pain (spreading sensitization) and increased pain response to repeated stimulation (temporal summation). Operatively, a patient will be assigned to high sensitization group when showing (i) clinical evidence of pain or altered sensations spread beyond the knee joint by manual palpation, (ii) a minimum of 3 anatomical sites around the knee showing a pressure pain threshold below 4 kg/cm2 (tender points), (iii) pain score of 4 points or more in a 11-point scale during 4 kg/cm2 pressure stimulation on the anterior/medial surface of the tibial bone (the site selected for the fMRI experiment) and (iv) increase of at least 1 point in a 11-point scale after 10 repeated pressure stimulation on the tenderest point around the knee.