Abstract
Introduction
Computer aided surgery aims to improve surgical outcomes with computer guidance. Navigated Freehand bone Cutting (NFC) takes this further by eliminating the need for cumbersome mechanical jigs, while decreasing cutting time and complexity. To reduce the footprint of the NFC tracking system (currently NDI Polaris) we designed and implemented “On-Tool Tracking” (OTT), a novel miniaturized tracking system that mounts onto the cutting instruments (Fig. 1). This study investigates the accuracy of the 3D-measurements of the OTT system.
Materials and Methods
OTT was designed using off-the-shelf components to communicate as a wireless device. OTT consists of the following:
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Stereo camera rig (each camera transmits images to the PC for processing at 30fps);
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pico-projector (presents visual information to the user);
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power-tool motor controller (stops the motor if the user deviates from the desired plan); and
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touch-screen user interface.
OTT communicates with a main PC using four wireless modules, based on three different technologies: Wi-Fi, Xbee, and UWB-USB.
OTT was secured on the upper actuator of a 5-axis Materials Testing Station (MTS-Systems), while the tracked, active wireless reference frame (RF) was locked in the lower actuator(s) (Fig. 2). The origin of OTT's camera system was aligned with the main vertical axis of the MTS and the RF origin set perpendicular to the cameras, with its origin coinciding with the same main vertical axis.
Using the MTS readings as reference (accuracy: 0.01mm/0.01º) for comparison, OTT software acquired multiple static measurements of the camera-rig vs. the RF pose at each location. X-translations and roll-angles were actuated by the MTS hydraulics; pitch and Y-translation were applied manually, while yaw was kept constant (0º).
Results
The results presented in (Table 1) show an RMS error of 1.02mm (sd: 1.06) in X and 0.204mm (sd: 0.387) Y directions, 1.5mm. Mean angular error was 0.039º (sd: 0.137) for roll and 0.908º (sd: 0.738) for pitch.
Discussion and Conclusion
As expected, the error grew larger as the magnitude of displacement (linear or angular) increased because the RF would face the radially distorted periphery of the wide-angle lenses. Nevertheless, all OTT's readings were accurate along the center of the volume-of-interest of the cameras (actual working area during surgery). Improvements in camera calibration and setup alignment would likely decouple and improve measurements.
The results indicate that OTT could potentially be used as a tracking device in surgery for Navigated Freehand Cutting. OTT presents multiple advantages relative to conventional typical tracking systems:
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Reduces footprint in operating room;
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avoids line-of-sight concerns (the tracker ‘travels’ with the tool, close to the bone);
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improves xy/z ratio due to the cameras proximity to the reference frames; and
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minimizes damage to the bone by RFs pins due to smaller size and lighter RF.
NFC combined with OTT can simplify arthroplasty by reducing complexity, implant/procedure specific instrumentation, and surgical time. Further experiments, including dynamic accuracy, coupled effects, and yaw and Z-displacement assessment will be performed to complete the required data.
