MUA is an abbreviation of 'Manipulation under anaesthesia'.
What happens in an MUA?
An MUA allows the surgeon an opportunity to break adhesions in a joint that has recently been injured or operated on, and where stiffness has become a problem due to sticky adhesions locking up the tissue folds inside the joint. The patient is given a general anaesthetic, possibly with some muscle relaxation, and the surgeon gently rocks the joint through flexion, extension and rotation, trying with each effort to gain a few more degrees of range of motion. The anaesthetic is to ensure complete relaxation without pain interfering with the process.
Manipulation Under Anaesthesia for Stiffness Following Knee Arthroplasty. Mohammed R, Syed S and Ahmed N. Ann R Coll Surg Engl. 2009 Apr; 91(3): 220–223.