Depth Control for Blind Water Jet Drilling in Bone

Abstract

Abstract - When surgically drilling blind holes in bone using a Water Jet (WJ), control over the resulting depth is a challenging issue of paramount concern. This thesis was part of a project aimed at replacing the awl and mallet technique used in traditional microfracture procedures with an arthroscopic high-pressure WJ instrument is capable of accurately drilling 2–4 mm deep holes in subchondral bone. The focus of this paper was to develop, analyze and evaluate concepts for ensuring the depthwise accuracy of a microfracturing WJ. Research was performed on both WJ systems and the microfracture procedure, and a thorough problem analysis detailing all concerning requirements and parameters was set up. It was determined that due to the strong non-uniformity of human bone, both spatially and between subjects, a WJ capable of monitoring the depth and implementing a closed-loop control system was needed to ensure safe and accurate drilling. To measure the depth of the hole and allow for feedback control, a flexible Nickel Titanium probe concept was devised and tested. The concept featured a 3D printed nozzle with built-in WJ orifice and depth probe, which could be extended down the hole made by the WJ by an ex-vivo actuator featurimg load and displacement sensors. When the load sensor detected a sudden rise in extension resistance, bottom contact was assumed and the hole depth was calculated based on the displacement of the probe. A proof-of-concept experiment showed the viability of using a flexible probe to measure the depth. Additionally, the algorithm produced for calculating the depth was shown to be robust against the hysteresis and backlash exhibited by the setup. When probing holes with depths of 0, 1, 2, 3, 4 and 5 mm and bore diameters of 1, 1.5, and 2 mm drilled in solid PMMA, the prototype managed an error mean of 0.00 mm with a SD of 0.19 mm. To test the probe in holes shaped as expected during microfracture surgery, a virtual interference experiment was carried out using mCT scans of WJ-drilled bones and simulated probes of varying diameters. Seven scans were probed from 4 different angles each; the results suggested that a probe with a 0.2–0.3 mm diameter was optimal in terms of traversing the hole without blockages and without risking over-penetration. Moreover, this thesis produced recommendations on carrying the project further, towards a fully integrated system capable of drilling accurate blind holes in human bone, in a closed-loop depth-controlled manner.BMEBioMechanical EngineeringMechanical, Maritime and Materials Engineerin