Stéphane Lavallée

Computer-assisted spine surgery

When inserting screws into a vertebral pedicle, the surgeon usually exposes the back part of the vertebra and uses his or her anatomic knowledge to align the drill in the proper direction. A slight error in direction may result in an important error in the position of the tip of the screw. This is done with no direct visibility of crucial structures (spinal cord, pleura, vessels). Statistical analysis of a series of surgical procedures has shown that 10% to 40% of the screws are not installed correctly. To reduce the risk of complication, a computer assisted method is proposed that enables the surgeon to place a screw at a position preoperatively defined in 3 dimensions using computed tomography images. This allows the surgeon to align a standard surgical drill with the optimal position and direction. The depth of the pilot hole during drilling also is monitored by the system to prevent penetration of the anterior cortex of the vertebral body. Using this procedure, in vitro tests were performed and showed that an accuracy of less than 1 mm can be obtained. Clinical trials were done in 10 patients who suffered severe scoliosis or spondylolisthesis. The trajectory of the holes drilled in L2, L3, L4, and L5 vertebrae were checked for all clinical tests. Postoperative radiographs and computed tomography scans showed that the screws were well inserted in each plane for each pedicle. This technique also can be used to perform osteosynthesis at the thoracic and cervical levels.

Building a Complete Surface Model from Sparse Data Using Statistical Shape Models: Application to Computer Assisted Knee Surgery System

This paper addresses the problem of extrapolating very few range data to obtain a complete surface representation of an antomical structure. A new method that uses statistical shape models is proposed and its application to modeling a few points manually digitized on the femoral surface is detailed, in order to improve visualization of a system developped by TIMC laboratory for computer assisted anterior cruciate ligament (ACL) reconstruction. The model is built from a population of 11 femur specimen digitized manually. Data sets are registered together using an elastic registration method of Szeliski and Lavallee based on octree-splines. Principal Components Analysis (PCA) is performed on a field of surface deformation vectors. Fitting this statistical model to a few points is performed by non-linear optimisation. Results are presented for both simulated and real data. The method is very flexible and can be applied to any structures for which the shape is stable.

An Overview of Computer-Integrated Surgery and Therapy

Computer-integrated surgery and therapy (CIST): Methods and systems to help the surgeon or the physician use multimodality data (mainly medical images) in a rational and quantitative way, in order to plan but also to perform medical interventions through the use of passive, semi-active, or active guiding systems.

Treatment of Pelvic Ring Fractures: Percutaneous Computer Assisted Iliosacral Screwing

This paper describes the development and preliminary testing of an image-guided system for the placement of iliosacral screws to stabilize pelvic ring fractures percutaneously, with the aim of decreasing the incidence of surgical complications and increasing the accuracy of screw placement. Pre-operative planning of screw trajectories is performed on a 3D model of the pelvis constructed from CT scans. During surgery, a 6D optical localizer is used to track the positions and orientations of an ultrasound probe, a surgical drill and a reference frame fixed to the posterior iliac crest. Registration of the pre-operative model with curves segmented from the ultrasound scans is performed using a surface-based algorithm. The drill tip and axis are displayed relative to the desired screw trajectories in real time. The accuracy of the system has been verified in four cadaver specimens by comparing actual screw trajectories to the desired trajectories and by measuring the distance from each screw to important anatomical landmarks on post-operative CT scans. All screws were considered to be in correct position.

Computer Assisted Orthognathic Surgery

This paper presents a surgical simulator for orthognathic surgery based on the integration of dental models and 3D cephalometry. The objective of dental models integration is to make coherent informations gathered from different sources (occlusal analysis and CT scan), and for that purpose, a system using a 3D optical localizer is used. The 3D Cephalometry analysis is used for the detection of dysmorphosis and surgical planning. This cephalometry integrates the Inferrence process for improving the surgical system. Both elements of our simulator have been implemented and technically validated with success.

Image-guided spinal surgery: Technology, operative technique, and clinical practice

The complications associated with misplaced pedicle screws are mostly neurological or vascular. Previous studies of surgical procedures have shown a significant rate of incorrect placement of pedicle screws, ranging from 15% to 40%. To increase the safety of screw placement, a technique that combines preoperative computed tomography (CT) imaging with intraoperative passive navigation is proposed. A combination of registration algorithms is used to match the preoperative model of the vertebra with intraoperative points that are obtained by using a pointer and a 3-dimensional optical localizer. Images and optimal trajectories are then reported in the intraoperative space. The tip and axis of any surgical tool or guide is visualized in real-time in the volume of preoperative CT images to perform the computer-assisted drilling procedure. Ninety-six pedicle screws have been inserted in lumbar and thoracic vertebrae (from T10 to L5) for various spine disorders with the computer-assisted system. Surgery was followed by postoperative radiographs and CT scans, on which measurements of screw position relative to pedicle could be performed. Nine percent to 12% penetration is obtained with the computer-assisted technique; 44% is obtained with the manual insertion. Preoperative errors from the CT scan data (image gap) and intraoperative errors that occur during the collecting of point coordinates may explain the computer-assisted technique failures. The results clearly show that the computer-assisted surgery technique provides much better safety and accuracy than manual insertion. This technology is evolving rapidly and many new extensions will occur in the years to come.