Practical aspects for optimal registration (matching) on the lateral skullbase with an optical frameless computer-aided pointer system

Hypothesis: Paired-point matching and surface matching are highly accurate when used on the lateral skull base with an optical computer-aided surgery system. Background: Computer-aided surgery on the lateral skull base can be done wi th a pointer system or with the microscope. An optical pointer system that uses anatomic landmarks and surface points has been developed in Fern, Swit zerland. Methods: Axial computed tomography of a cadaver skull was performed. The im ages were processed on a computer workstation. An infrared camera was used. A reference base mounted on the head and a needle pointer, both equipped w ith light-emitting diodes, were used. Different anatomic landmarks were det ermined on the computer image of the skull and were compared with the actua l anatomic markers taken on the bone or on the skin simulation material, an d the target error was defined. In a second step, additional surface points on different regions of the skull were taken, and the inaccuracy from the target was redetermined. Results: The authors found a mean average error in accuracy from the target with paired-point matching alone in the best series of 0.79 mm. Under skin simulation, they found a deterioration with paired-point matching alone bu t an improvement in accuracy with surface matching. Conclusion: For this navigation system, it is recommended that the followin g five anatomic points be selected for matching of the lateral skull base: the tip of the mastoid, the mastoid foramen, the umbo, the frontozygomatic suture, and the anterior nasal spine. For additional accuracy in clinical s ituations, surface matching is recommended.