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Various localizing mechanisms for intraoperative navigation with frameless stereotaxy have been developed, including proprioceptive arms, freehand probes with infrared optical tracking systems, and integrated microscope/stereotactic systems that map the microscope's focal point. During a 5-year period, 1,212 procedures were performed on 1,155 patients using five navigational devices, and the accuracy and clinical performance of the systems were compared. Operative pathology included 193 gliomas, 128 convexity meningiomas, 117 skull-base tumors, 113 epilepsy operations, 106 cavernous malformations, 65 arteriovenous malformations, 6 aneurysms, 25 acoustic neuromas, 25 ventriculoperitoneal shunt procedures, 45 metastases, 338 other or unspecified primary brain tumors, and 51 miscellaneous lesions. The proprioceptive arm was used in 1,112 procedures (mean error, 1.86 mm); the freehand probe (1-dimensional cameras) was used in 29 procedures (mean error, 1.72 mm); the freehand probe (2-dimensional cameras) was used in 40 procedures (mean error, 2.64 mm); the MKM microscope system (Carl Zeiss, Thornwood, NY) was used in 16 procedures (mean error, 3.08 mm); and the SMN microscope system (Carl Zeiss) was used in 15 procedures (mean error, 2.30 mm). According to a survey of performance characteristics and surgeons' preferences, the proprioceptive arm system had the best apparatus, computer software, tracking mechanism, and display. It was accurate, reliable, and the easiest of the five systems to use. Freehand systems are hampered by tracking interference, and integrated microscope systems are still in the early stages of development.

Frameless stereotactic image-guided techniques may be used in spinal surgery to better define anatomical landmarks, to precisely localize pathological lesions and their borders, and to assist in the precise placement of instrumentation. This technology may be applied to surgery involving the entire spinal axis. Currently, it is best suited for dorsal approaches. However, modifications will soon be available that will vastly expand its application into the ventral region as well.