Orthopedics addresses musculoskeletal conditions, such as trauma, sports injuries, degenerative diseases, and joint reconstruction. Surgeons routinely install cutting guides, plates, screws, and other devices directly onto bone to aid surgery or restrict movement. Electromechanical drills with various bits are used to create blind pilot holes that end just prior to the back exterior of a bone.
Surgeons rely on experience, anatomy, and tactile and aural feedback when drilling into bone.
A change in applied force, tool vibration, or sound pitch may indicate when the drill-bit tip penetrates certain boundaries, e.g. between cortical (dense) and cancellous (porous) bone or at the cortical-to-exterior (posterior) surface (punch-through, break-through, or plunge). However, such feedback may be minimal, or occur too late or too rapidly to prevent the drill bit from breaking through the posterior surface of a bone and damaging soft tissue. For example, a craniotomy involves drilling a pilot hole into the skull to the dura mater (soft membrane surrounding the brain) with a mechanically clutched bit called a craniotome. Although cutting action disengages when soft tissue is encountered, the dura mater may still be torn or damaged by drill-bit break-through.)
There have been many attempts to develop real-time drilling systems to detect bone boundaries, estimate bone thickness, or measure hole depth, including: manual (probe, depth gauge), semi-manual (fixture, feed rate), mechanical (torque, vernier), electrical (motor current/voltage), impedometric (tissue conductivity), optical (computer vision), ultrasound (A-/B-mode imaging), robotic (reaction force), navigational (spatial tracking), etc. Unfortunately, such methods have proven ineffective, slow, inaccurate, expensive, or cumbersome. This project will investigate ultrasonic vibration as a means to estimate relative bone thickness—or rather thinness—while drilling.
Ultrasonically-Assisted Drilling (UAD) is commonly used to improve machining of hard or brittle materials (reduced surface roughness, faster feed rates, reduced thrust force, reduced torque, improved drilling control, reduced drilling temperature, altered chip or burr formation, etc.), but only recently has its ability to sense material properties been investigated. A rotating drill bit that is axially ultrasonically vibrated against bone will observe varying mechanical impedance—vibratory frequency, amplitude, or phase—depending on the elasticity of the bone. Thick bone is rigid compared to thin bone: elasticity increases as it thins between the drill-bit tip and the posterior surface (beyond which lies soft, elastic tissue). Thus, a feedback loop that monitors and controls the changing vibratory characteristics can detect the proximity of the drill-bit tip to the posterior surface and prevent it from breaking through.
The primary deliverable of this project is a basic proof-of-concept ultrasonically-assisted orthopedic drill that detects when the drill-bit tip approaches the posterior bone surface while drilling and potentially prevents the tip from breaking through. Major subsystems include an ultrasonic transducer, rotational electrical coupling, and a real-time feedback control loop. The proof-of-concept system may assume any form suitable for laboratory experimentation, but the methodology may not rely on external fixtures, guides, jigs, booms, or other manual, obtrusive, or disruptive aides. Students shall research the problem, develop a solution, build a system, and test overall performance.