We used 10 cadaver knees to estimate the safe pressure during arthroscopy by measuring the volumes and pressures of irrigation fluid at different flexion angles. Maximum volumes could be contained at 35 degrees of flexion. Pressures of 200 to 450 mmHg were measured, and all the knees ruptured by extension or flexion after they were filled to 100 mmHg at 35 degrees of flexion. Fifty milliliters of irrigation fluid had to be removed if the pressure remained constant when extending from 35 degrees and 70 ml when flexing to 90 degrees. Totally, 100 ml irrigation fluid had to be removed when flexing from 35 degrees to 120 degrees. Our investigation indicates that a pressure of 150 mmHg can be tolerated by all knees. Both flexion and extension from the 35 degrees position must be done gently and slowly using a large bore, wide-open inflow and outflow tubes allowing egress of irrigation fluid to prevent capsular rupture, extravasation of irrigation fluid, vascular compromise, or compartment syndrome.