Ph. MERLOZ M.D.
University Department of Orthopaedic Surgery
CHU A. Michallon - BP 217 - 38043 GRENOBLE CEDEX 9, France
Pilot holes are prepared and screws are inserted into the pedicle without any direct visual control. A slight error in direction may result in a significant error in the position of the tip of the screw. Drilling is performed at least twice but sometimes 3 or 4 times during surgery, with no direct visibility of the crucial structures (spinal cord, lung, vessels and nerves). To check the correct placement of the screw in the pedicle, image intensification is used, but, because of radiation exposure to the patient and the difficulty of use of this system, it cannot be applied during the entire screw insertion procedure. The variability in width, height, and spatial orientation of spinal pedicles, especially in the surgical procedure of scoliosis, consequently leads to a considerable rate of misplaced screws. Biomechanical studies suggest an optimal position of the screw tip to be as a near to the anterior cortex of the vertebral body as possible. A right pedicular screw insertion has to take 2 parameters into account : the spatial orientation of the screw and its length.
To look for the right spatial orientation on the one hand and the right length of a pedicular screw on the other hand and more generally to look for a crucial anatomical structure without any direct visual control, we developed a novel technique that combines preoperative tomographic imaging with principales of stereotaxis because it is important to increase safety by more precise intervention. This technique follows a general tendency of computer assisted medical interventions. A passive system with only a 3-dimensional optical localizer during surgery for both registration and guidance can be used.
The purpose of the method is to reduce those complications, which proves to be feasible according to the first clinical results that were obtained in the lumbar and thoracic regions. There is currently no limitation for using the method in the low back part of the spine, and therefore, osteosynthesis could be performed in higher levels, including cervical vertebrae, with a high level of confidence. Extension of the method to removing inter vertebral disk or to performing biopsy of the cancellous bone of the vertebral body for tumors or more generally to reach a deep target that cannot be visible directly.
In the future, we believe it will be possible to use such a system percutaneously. The acquisition of radiographic or ultra-sound images obtained in calibrated conditions makes it possible to register preoperative computed tomography images with the surgical space percutaneously. Such work may contribute to the reduction of invasiveness of orthopaedic surgery.