The most important thing a cranial perforator does is stop. Opening a skull is a controlled breakthrough problem. Bone is hard. The dura beneath it is roughly a millimetre thick, and the brain behind that has no mechanical defence at all. A drill that cuts through the inner table and keeps turning has already done the damage. So the entire design brief for the perforator is not cutting. It is knowing when to quit. The bit is a two-part assembly. An inner pilot tip leads, and an outer reamer follows behind it, widening the hole into a countersunk cone. Between the drive shaft and the cutting head sits a spring-loaded mechanical clutch, held engaged by the axial load the surgeon pushes into the bone. The skull cooperates with this arrangement: - The outer table is dense cortical bone and resists hard - The diploe below it is spongy and cuts fast - The inner table is thin, dense, and gives way suddenly The instant the pilot tip punches into the epidural space, the load on it collapse...