Arthroscopic lavage means washing out any blood, fluid or loose debris from inside the joint cavity during a keyhole procedure.
Debridement and lavage
Lavage is generally combined with arthroscopic debridement, where fronds of joint material or degenerative tissue is sucked out using a small 'hoover' - ie. both washing (rinsing) and sucking. 'Arthroscopic' means that this is done using keyhole surgery, where a thin 'scope' is pushed into the joint (arthro=joint) to visualise the structures.
Arthroscopic Lavage and Debridement for Osteoarthritis of the Knee. Medical Advisory Secretariat. Ont Health Technol Assess Ser. 2005; 5(12): 1–37.