Bio-Inspired Flexible Vertebral Drill

Abstract

Abstract—Spinal fusion surgery is an operation in which two or more adjacent vertebrae are rigidly connected, with the goal to remedy spinal instability, deformation of the vertebrae, or a herniated intervertebral disc. Vertebrae are conventionally fixated by means of pedicle screws, the downsides of which are accidental cortical wall breaches during drilling and poor holding strength of the screw. The holding strength of the bone anchor may be improved by increasing its contact area with the hard cortical wall of the vertebral body. To accomplish this, a curved hole needs to be made along the inside of the cortical wall. This research presents the design, manufacturing, and testing of a bone drilling device, that is flexible in one plane. To this end, the drill was developed on the basis of the tsetse fly’s proboscis, which is a mechanism that uses a cutting surface with its axis of rotation perpendicular to the drilling direction. Implementing this cutting motion has several advantages over conventional drilling: It facilitates using leaf springs as a flexible transmission, and it is not limited to drilling round holes. A prototype was built and tested on Sawbones closed cell foam, which closely mimics the mechanical properties of the cancellous bone found in human vertebrae. The prototype was capable of effectively cutting through foam with densities up to 10 pounds per cubic foot (PCF) with a feed rate of 50 mm/min. The ability to deflect off and follow a simulated cortical wall was also tested, and proved to be effective up to an insertion angle of 15˚. The bio-inspired drilling device presented in this research opens up new possibilities in the development of flexible drilling for a wide variety of orthopaedic applications.