Image-guided Percutaneous Scaphoid Fixation using Intraoperative 3D Fluoroscopy

Introduction and Background The scaphoid is one of the eight small carpal bones comprising the human wrist, situated just at the base of the thumb. It is the most commonly fractured carpal bone, typically resulting from a fall on an outstretched hand. Casting is the standard treatment for uncomplicated fractures, but limits mobility, and therefore percutaneous internal fixation is gaining popularity as a treatment alternative. The procedure involves drilling a guidewire along the length of the scaphoid over which a cannulated screw is placed. Both the wire and screw are inserted through the skin, requiring liberal x-ray imaging and posing an occupational hazard to surgeons who routinely perform these procedures. The mechanical stability and clinical outcome of the procedure have been linked to central screw placement and minimizing cortical breach, both of which can be difficult to achieve with conventional techniques. Computer-assisted surgery has demonstrated improvements in accuracy and reduction in x-ray exposure in other orthopedic pin insertion procedures; however, the typical computer-assisted workflow does not translate easily to this procedure. A standard tracking device cannot be attached directly to the scaphoid for navigation due of the small size of this bone. Because the procedure is performed with minimal invasion (percutaneously), registering the images for navigation by collecting surface points on the scaphoid (i.e., patient-based registration), is counter-productive. Recent developments in flat-panel fluoroscopy have made 3D cone-beam computed-tomography (CT) feasible for intraoperative use, and an ideal instrument to conduct image-guided navigation. This work aimed to use this technology to develop and test an image-guided navigation system for percutaneous scaphoid pinning and compare this to conventional approaches. Methods Two image-guided approaches were developed and tested: the first using volume-rendering of the 3D images similar to digitally-reconstructed radiographs, and the second using volume-slicing comparable to CT slices. Both methods made use of a 3D digital fluoroscopic C-arm (GE Healthcare, France) that could rotate 360 about an isocentre to take a 3D cone-beam CT image. Navigation was performed using a ceiling-mounted optical tracking camera and active infrared markers attached to the wrist and drill guide. A preoperative calibration was performed to establish a spatial relationship between the internal coordinates of the imager and the tracking system; this allowed an intraoperative image to be used for navigation, without the need for patient-based registration or imager tracking. Each navigated procedure began by taking a 3D image of the wrist and rendering it according to one of the two image-guidance methods. A surgeon used these images to create a preoperative plan by positioning a virtual drill path on the scaphoid. To navigate the plan, the real-time position of the drill was displayed relative to the wrist image on a computer monitor, by applying the preoperative calibration relationship to the pose data of the tracked drill guide (Figure 1).