Joint distraction for osteoarthritis: clinical evidence and molecular mechanisms

Joint distraction, the prolonged mechanical separation of the bones at a joint, has emerged as a joint-preserving treatment for end-stage osteoarthritis, with the gradually growing promise of implementation in regular clinical practice. Joint distraction of the knee has been most extensively studied, with these studies showing prolonged symptomatic improvement in combination with repair of cartilage tissue in degenerated knee joints, supporting the concept that cartilage repair can translate into real clinical benefit. The reversal of tissue degeneration observed with joint distraction could be the result of one or a combination of various proposed mechanisms, including partial unloading, synovial fluid pressure oscillation, mechanical and biochemical changes in subchondral bone, adhesion and chondrogenic commitment of joint-derived mesenchymal stem cells or a change in the molecular milieu of the joint. The overall picture that emerges from the combined evidence is relevant for future research and treatment-related improvements of joint distraction and for translation of the insights gained about tissue repair to other joint-preserving techniques. It remains to be elucidated whether optimizing the biomechanical conditions during joint distraction can actually cure osteoarthritis rather than only providing temporary symptomatic relief, but even temporary relief might be relevant for society and patients, as it will delay joint replacement with a prosthesis at an early age and thereby avert revision surgery later in life. Most importantly, improved insights into the underlying mechanisms of joint repair might provide new leads for more targeted treatment options. Knee joint distraction (KJD) is used to stimulate joint repair in advanced osteoarthritis. In the Review, Jansen and Mastbergen provide an overview of the clinical trial results of KJD treatment, current knowledge of the molecular mechanisms underlying joint repair in KJD, and future directions and unanswered questions in KJD research and treatment. Key points Joint distraction can induce tissue structure modifications in degenerated knee joints, accompanied by prolonged symptomatic improvement that supports the concept that cartilage repair can translate into real clinical benefit. It remains to be elucidated whether achieving optimal biomechanical and molecular conditions during distraction can lead to cure in osteoarthritis rather than providing only temporary symptomatic relief. Partial unloadi