Extracranial application of the frameless stereotactic operating microscope: experience with lumbar spine

The frameless stereotactic operating microscope has expanded the potential application of modern stereotaxis to procedures outside of the intracranial compartment by removing the constraint of a rigid frame. We studied seven patients all of whom had a history, examination, and imaging studies consistent with lumbosacral spinal pathology for which they subsequently underwent surgery with the operating microscope. The ability of the frameless stereotactic system with preoperative computed tomography data to locate the level of the lesion as well as define the boundary of the spinal pathology intraoperatively was assessed. In parallel with this application of the frameless system, we analyzed the relationship between the lumbar intervertebral disc spaces (L3-L4, L4-L5, L5-S1) and skin surface fiducials using lateral radiographs. In seven patients with extracranial cases (six herniated lumbar discs and one lumbar spondylolysis with Grade I spondylolisthesis) who underwent operations by this system, the accuracy of the digitization component of the system with respect to localization of an independent test fiducial was 3.28 mm (SD, 0.61). The accuracy of the entire system in locating the independent fiducial within the viewing plane was 6.05 mm (SD, 4.04). Disc space localization had a far greater error of 28.81 mm (SD, 7.49). There was no consistent pattern to the magnitude or direction of the displacement of the lumbar intervertebral discs with respect to the fiducial markers in the sagittal plane. Although accuracy at the level of the fiducial plane was similar to that of intracranial applications, paraspinal tissue and vertebral column deformations rendered poorer accuracy with deeper structures.