Persistent post-surgical pain is a significant adverse effect after total knee arthroplasty, present in around 20% of the patients. Central sensitization may contribute to developing and maintaining pain. Therefore studies should investigate if pain processing altered mechanisms are present in this population.

Numerous studies suggest that peripheral and central sensitization in patients with symptomatic knee osteoarthritis could be an important factor capable of explaining part of these poor outcomes after knee arthroplasty. Quantitative sensory testing has been frequently used to investigate this sensitization of the central nervous system and assess pain hypersensitivity locally and away from the affected region in patients with chronic knee osteoarthritis pain. Pressure algometry is a method described to determine pain thresholds to pressure and thus evaluate the sensitivity to pain due to tissue pressure. However, another aspect associated with sensitization is the downward modulation of peripheral inputs. This occurs in neurons in the posterior horn of the medulla and can be either inhibitory or facilitatory. The conditioned pain modulation test is a manifestation of this phenomenon, which can be easily assessed in patients, and is characterized by the modification of the response to the application of a painful stimulus when a second conditioning painful stimulus is applied. Conditioned pain modulation has been shown to be altered in patients with symptomatic knee osteoarthritis.

The literature suggests that although one of the main components of pain perpetuation with people with knee osteoarthritis is sensitization of the nervous system, successful knee arthroplasty is able to normalize its functioning. This implies that the maintenance of sensitization can be triggered by peripheral nociceptive stimuli. When performing a knee arthroplasty, the structures capable of generating these peripheral nociceptive stimuli are eliminated, so that the pain-generating mechanisms cannot be the same as before in those who manifest persistent post-surgical pain. For this reason, one of the hypotheses is that the mechanism responsible for causing this pain is a central mechanism and not a peripheral one. Although this hypothesis has not been fully tested, there are indications that people with chronic post-surgical pain may suffer peripheral and central sensitization processes, very similar to those that can be observed in people with symptomatic osteoarthritis of the knee that is not operated on, even though the structures involved are no longer present. This fact reinforces the theory of the involvement of a central mechanism in the perpetuation of pain after surgery.

Therefore, the present study aims to investigate the involvement of the central sensitization process in the perpetuation of postoperative pain in individuals undergoing total knee arthroplasty for primary symptomatic knee osteoarthritis. From a clinical point of view, current postoperative treatments focus mainly on improving the patient's functional outcomes (e.g., physical function and joint range of motion), and the status of the descending pain modulator system is rarely taken into account. The present study seeks to place importance on the central mechanisms in the perpetuation of pain after knee arthroplasty, to inspire new treatment protocols that take these processes into account.

Inclusion criteria:

- Patients who underwent total knee arthroplasty due to painful primary osteoarthritis.

Exclusion criteria:

• Patients operated due to a revision surgery.
• Patients with a unicompartmental knee arthroplasty
• Pateints operated due to a fracture.
• Patients planning to undergo another lower extremity operation during the following 12 months.
• Patients with inflammatory or rheumatic diseases such as: rheumatoid arthritis, psoriatic arthritis, systemic lupus erythematosus or ankylosing spondylitis.
• Patients with mental health conditions and/or major depression.

Subjects who underwent total knee arthroplasty due to symptomatic, end-stage knee ostheoarthritis.