An improvement for pedicle screws would be if the screws can directly transfer their forces to the strong cortical bone of the vertebra, rather than through the softer cancellous bone. This approach requires the creation of a curved trajectory through cancellous bone. The goal of this project is to adapt the flexible oscillating drill design of Müller to a steerable drill that can create a curved hole in artificial cancellous bone.
The oscillating drill tip is a cylinder which oscillates rotationally around its centre axis. The cutting direction is radial. This drill design constantly produces a sideways force since the cutters produce a friction force tangential to the cylindrical wall. The friction force on the forward stroke is currently cancelled out by an equal and opposite force during the reversed stroke. Introducing a difference in phase or amplitude between the two translationally oscillating inputs to the drill head can create a force difference between the forward and reverse stroke, which provides a net sideways steering force over many repetitions.
A prototype drill and accompanying oscillator have been designed and built to further research the effects of the amplitude and phase difference on the drill motion and drilling performance. The oscillator outputs two oscillations that can be individually adjusted in stroke length and their relative phase angle. The output stroke lengths can be set to 0.75, 0.50 and 0.25 mm. The phase difference can be adjusted in 18° increments between 0° and 360°. The drill consists of the cylindrical drill head and two parallel leaf springs that slide longitudinally and thereby transfer the motion of the oscillator to the drill head.
The results showed that the drill got embedded in the drilled hole which is insufficient for measuring a steering effect. Therefore, two alternative drill designs were proposed and evaluated to increase the drilling depth. The design adaptation that provides the most promising result, is to drastically lower the stiffness of the connection between the drill head and the leaf springs. An amplitude or phase difference applied by the oscillator causes the drill head to produce a different motion pattern. The pattern from the amplitude difference shows a decrease in the deviation distance which is expected to have a diminishing effect on the effectiveness of the drill. A phase difference of 144° between the two oscillations, causes the drill to produce an oval motion pattern which is expected to be the most effective in generating a steering effect.
This project provides a great basis for the future development of a drill that can steer without a steering actuator. The next step forward with this design should be to incorporate hinges between the leaf springs and the drill head, to maximise the rotational motion of the drill head.