Abstract
Introduction
Navigated freehand cutting (NFC) technology simplifies bone cutting in laboratory trials by directly navigating implants and power tools [1]. Experiments showed that NFC bone cutting was faster than with conventional jigs. However, most delays occurred at the start of each cut [2]. Therefore, we further reduced starting times and gained more accuracy with a NaviPen and a ‘smart’ NaviPrinter [3]. There were used to physically mark a line on the bone surface indicating where each cut should start. (Fig. 1). Further gains are targeted with our introduction of the On-Tool Marker (OTM); a touch-less laser marking technology as a standalone device or mounted on the cutting instrument (e.g. on the saw). The OTM points the desired cut by projecting a laser image on the bone. That image (usually a line or cross) changes dynamically, so that for any given cut the line projection remains stationary on the bone regardless of the relative location of the device.
Materials & Methods
The OTM is a standalone wireless module composed of three main parts: a small laser projector, electronics for control and communication (WiFi), and a tracking frame. It is navigated in real-time with a Polaris tracker. Software routines on a proprietary NFC system compute its relative position to the target and dynamically re-calculate the image parameters. Such parameters are sent to the OTM for processing, image generation, and projection (Fig. 2). Bandwidth and data integrity were evaluated through bench tests. To assess accuracy of the projection, a target planar cut was defined on a flat surface (a line drawn on grid paper pasted to a navigated board), and the NFC system was fed with this geometrical information. The OTM was moved within a volume of ∼50cm in diameter (distance to the target plane from 5cm to 50cm), and at various angles up to +/− 80° (in roll, pitch and yaw). The projected line should coincide with the target line on paper regardless of the relative positioning of the OTM. Errors (target vs. projected) were measured on the grid paper.
Results
Well-defined lines were projected at a rate of 17fps. Projected lines remained within +/− 2 mm from the target (average ∼0mm). Errors, largely caused by a lag in the images, were unperceivable after a fraction of a second if OTM remained still. Among different colors tested, green was the most suitable, based on brightness and visibility (Fig. 3).
Discussion and Conclusion
A ‘smart’ navigated laser marker was successfully created and tested. The limited refresh speed and lag was not much of a concern, as common use would not require fast motion. However, further work will focus on improving these, and devise solutions for projection on non-planar bone models. OTM would speed the surgery more as it saves the time to use the NaviPen or the NaviPrinter. We estimate this can reach 2–3 minutes based on some preliminary experiments we conducted and not reported here. Finally, OTM can help reduce the number of instruments in surgery even further (less inventory, less sterilization, less cost and less worries).
