Practical aspects for optimal registration (matching) on the lateral skull base 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 with a pointer system or with the microscope. An optical pointer system that uses anatomic landmarks and surface points has been developed in Bern, Switzerland.

Methods: Axial computed tomography of a cadaver skull was performed. The images were processed on a computer workstation. An infrared camera was used. A reference base mounted on the head and a needle pointer, both equipped with light-emitting diodes, were used. Different anatomic landmarks were determined on the computer image of the skull and were compared with the actual anatomic markers taken on the bone or on the skin simulation material, and 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 but an improvement in accuracy with surface matching.

Conclusion: For this navigation system, it is recommended that the following 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 situations, surface matching is recommended.