[Navigation for placement of scaphoid screws : a new indication for intraoperative 3D navigation-a cadaver study]

Up to now, the use of navigation systems for the placement of scaphoid screws has been impossible, mainly because there have been no ways of fixing the reference markers. Faulty placement rates in internal fixation of the scaphoid show there is a current need for a 3D image-based navigation system and intraoperative monitoring of how successful the procedure will be. For this reason, we have developed a new radiotransparent hand fixation device (Scaph-Splint), which allows reliable and accurate drilling of the scaphoid using 3D navigation. Tests of this device and the simultaneous precise placement of screws securing an internal fixation device are described in this paper.

Relative movements between the wrist and fixation device were measured with a 3-D ultrasound motion analyser system. Five cadaveric upper extremity specimens were then used for further navigated test applications. Each specimen was placed in the fixation device, and both the forearm and hand were secured to the two surfaces, with the wrist in approximately 80 degrees of extension. A reference marker was then securely fixed to the fixation device. A commercial navigation system and 3-D fluoroscopic imaging were used for each trial. Under navigation, the scaphoid was drilled in retrograde fashion, and a screw was placed into the drilled hole. Following screw placement, a 3D scan was performed to evaluate its position. The screw placement was analysed blindly to optimal placement and drill or screw perforation, and the image quality was rated on a visual analog scale (VAS).

There were few artefacts, and the image quality of the 3-D scan was judged as as good (VAS 79). Deviations of >or=0.2 mm between planned trajectory and drilling tunnal were not found in any of the specimens; there were deviations of >or=1 mm in one case, and all other cases showed deviations of

We found that the Scaph-Split allowed complete immobilisation of the hand and carpus. This allowed for adequate reference tracker stability and subsequent successful 3D navigated fluoroscopic drilling and screw placement in the scaphoid. While further tests on fresh-frozen cadavers is warranted, this technique may prove clinically to be very useful for surgeons treating scaphoid fractures.