Arthroscopy means 'looking into a joint'.
Arthroscopy is a surgical procedure, where the surgeon operates on the knee through several small cuts, and using long, fine instruments.
Light is shone into the knee through an instrument called an arthroscope, which has a small lens at the far end to angle the light beam. The same instrument has attachments to allow an inflow under pressure of sterile water-based fluid to expand and wash the joint, and an outflow under suction to extract any tissue which has been cut away.
Both arthroscope and instruments are introduced into the knee via small cuts - called portals.