RESUMO
PURPOSE OF THE STUDY: To examine the exact position of screws in anterior cervical fusion that were intended to be bicortically anchored at the time of surgery. MATERIAL: A randomly selected, representative sample (26) of our patient series undergoing anterior cervical fusion with bicortical screw fixation (250) during period of 1993-1999 is reviewed in this study with respect to an exact position of the tips of the screws to the posterior vertebral cortex. A total of 109 screws were assessed. METHODS: The position of 109 screws was assessed using an axial CT scan. On the basis of the distance of the screw tip from the posterior cortex of the vertebra, the screws were divided into several categories: mono- and bicortically anchored. Bicortically inserted screws were statistically evaluated using the confidence interval. RESULTS: Statistical analysis shows that the confidence interval of the screws being bicortically anchored lies between 66.7 and 87.5% (alpha value = 0.01). No screw was introduced more than 3 mm behind the posterior vertebral cortex. Monocortical introduction more than 1.5 mm in front of the cortex was observed in 7 screws (6.4%) in lower cervical spine and cervico-thoracic junction. DISCUSSION: All the screws that were classified as being too short were introduced in the cervico-thoracical junction or lower cervical spine known for its reduced X-ray transparency. The majority of screws, primarily described as bicortical, indeed penetrated both vertebral cortices. Considering the results of confidence interval for bicortical screw anchoring we conclude that 1 of 4 screws in monosegmental and 2 of 6 screws in bisegmental stabilization could fail to be bicortically inserted. CONCLUSION: [corrected] Intraoperative methods used for an accurate and safe bicortical screw insertion during anterior cervical fusion, i.e. intraoperative fluoroscopy, peak insertion torgue of the screw and the length measurement of the taped screw canal, are reliable enough to fulfill these goals.
