Assured Safety Drill with Bi-stable Bit Retraction Mechanism

A handheld, portable cranial drilling tool for safely creating holes in the skull without damaging brain tissue is presented. Such a device is essential for neurosurgeons and mid-level practitioners treating patients with traumatic brain injury. A typical procedure creates a small hole for inserting sensors to monitor intra-cranial pressure measurements and/or removing excess fluid. Drilling holes in emergency settings with existing tools is difficult and dangerous due to the risk of a drill bit unintentionally plunging into brain tissue. Cranial perforators, which counter-bore holes and automatically stop upon skull penetration, do exist but are limited to large diameter hole size and an operating room environment. The tool presented here is compatible with a large range of bit diameters and provides safe, reliable access. This is accomplished through a dynamic bi-stable linkage that supports drilling when force is applied against the skull but retracts upon penetration when the reaction force is diminished. Retraction is achieved when centrifugal forces from rotating masses overpower the axial forces, thus changing the state of the bi-stable mechanism. Initial testing on ex-vivo animal structures has demonstrated that the device can withdraw the drill bit in sufficient time to eliminate the risk of soft tissue damage. Ease of use and portability of the device will enable its use in unregulated environments such as hospital emergency rooms and emergency disaster relief areas. INTRODUCTION Gaining access to the inside of the skull is an important step for diagnosing and treating many traumatic head injuries. Head trauma can cause traumatic brain injury (TBI) and a number of negative side effects such as swelling of brain tissue, blood hemorrhaging, or cerebrospinal fluid buildup. All of these effects can result in a large increase in pressure inside the cranial cavity. This pressure increase can even cause herniation, which is a potentially fatal condition where pressure build-up forces brain tissue into different sections of the skull, such as through the foramen magnum [1]. Physicians can monitor pressure buildup by drilling a hole through the skull and draining excess fluid or placing sensors (such as the Camino monitor) inside the skull for pressure monitoring [2]. Typically a skilled neurosurgeon or a trained mid-level practitioner is required to make the hole in the skull due to the risk in damaging delicate brain tissue under the dura. Without proper training, a clinician may inadvertently advance the spinning drill bit beyond the underside of the skull by several millimeters and into brain tissue [3]. This is called “plunging” and can lead to permanent brain damage [4]. Clinicians treating TBI must develop the skills to detect drill bit penetration without plunging despite highly variable skull thickness because different areas of the skull range in thickness from 3mm to over 10mm with mean thickness 5-7mm [5]. As shown in Fig. 1, even for one patient the skull thickness could vary by several millimeters over the span of a small space.