The flat bottom drill for bone drilling without plunging

Abstract

Drilling holes on the skull or spine bones using a twist drill with a pointed tip is a common procedure in neurosurgery to gain access to the brain or spine. The point tip of the twist drill penetrates beyond the bone before neurosurgeons can rely on their haptic senses to detect the drop in thrust force and retract the drill. The tip of an over-penetrated twist drill may damage the brain or spinal nerve underneath the bone. The bone drilling devices with automatic retraction have the same issue of sharp twist drill over-penetration due to the small force drop at the point of penetration. This research designs a flat bottom drill capable of creating a flat bottom hole in bone drilling to overcome the over-penetration issue. This flat bottom drill has a sudden thrust force drop as a signal to stop and retract the drill without penetrating the bone. The flat bottom drill may also use the color change on the bottom surface of the drilled hole for visual feedback for retraction. The design and geometric features of the flat bottom drill are first introduced. A mathematical model is developed to calculate the rake and inclination angles on cutting edges. Four flat bottom drills are designed, manufactured, and evaluated in drilling the ex-vivo bovine bone. Results of the drilling thrust force and torque as well as the chip formation are compared with those of the twist drill. The flat bottom drill has demonstrated to be much more sensitive than the twist drill in penetration detection by sensing the sudden drop in thrust force at the end of bone drilling.