Method and device used in arthroscopy for the diagnosis or treatment of a joint (2) surrounded at least partially by a capsule (3). The device (1) includes a plate (6) and a traction element (10). The method comprises the steps of inserting the device (1) through the capsule (3) and into the joint (2), allowing the traction element (10) to protrude, and pulling the traction element (10) so that the plate (6) exerts an intra-articular traction of the capsule (3) tissues, creating a space (13) under the plate (6). The formation of this space (13) helps to improve visibility inside the joint (2) without the need for extensive incisions in the capsule (3). Therefore, the capsule (3) is less damaged after the arthroscopy and post-operative complications are minimized for the patient.