The most satisfactory site for the insertion of an arthroscope is on the lateral side just above the plane of the joint. With the knee joint flexed at a right angle the lateral joint gap is easy to find on the anterior side. With the thumb one can palpate an anterolateral depression which is bordered medially by the patellar tendon, proximally by the lower border of the patella, and distally by the edge of the head of the tibia. The incision is made in the approximate centre of this lateral depression (Figs. 20 and 21).