Painful Knee Prosthesis. Relationship Between Endogenous Analgesia and Persistent Post Surgical Pain.

This is a prospective, observational study, aimed to establish the relationship between an inefficient endogenous pain modulation before surgery (total knee arthroplasty; TKA) and the probability to develop chronic pain after surgery (persistent post surgical pain). Endogenous analgesia efficiency will be measured during the month previous to surgery using quantitative sensory testing (QST). Persistent post surgical pain will be defined as presence of pain in movement greater than 3 points in a 0-10 numerical scale in the operated knee, 6 months after surgery.