Anatomical parameters of subaxial percutaneous transfacet screw fixation based on the analysis of 50 computed tomography scans

Clinical article

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Object

Cervical transfacet screw placement has been described in the literature. Although the technique shows promise for percutaneous application, parameters for screw placement have not been well delineated. This study used reconstructed CT scans with imaging software to assess the feasibility of percutaneous transfacet screw placement, analyzing potential entry angles, transfacet lengths, and sex differences at each subaxial level.

Methods

Fifty consecutive cervical CT scans (obtained in 26 males and 24 females [mean age 41.5 years]) were reformatted using OsiriX software, and transfacet lengths, entry angles, and potential occipital clearance were analyzed at all subaxial levels. Statistical analyses were used to determine the differences, if any, between transfacet lengths, entry angle, and occipital clearance across individual cervical levels. Repeatability was quantified by calculating the intraclass correlation coefficient and Cohen kappa value.

Results

A total of 200 transfacet lengths and 200 entry angles in 50 patients were analyzed. The mean transfacet lengths were 17.9 ± 2.6, 17.6 ± 3.2, 16.3 ± 3.6, and 13.1 ± 2.2 mm at C3–4, C4–5, C5–6, and C6–7, respectively, with mean entry angles at 52.7° ± 7.8°, 56.5° ± 8.0°, 55.0° ± 8.8°, and 53.0° ± 8.7°, respectively. Analysis of variance revealed a significant difference between the mean transfacet lengths, while post hoc analysis revealed significantly larger transfacet lengths in the upper 2 cervical levels (C3–4 and C4–5) than in the lower 2 cervical levels (C5–6 and C6–7). Analysis of variance demonstrated no significant difference between the entry angles. Males had significantly larger transfacet lengths at C5–6 (17.4 vs 15.1 mm) and C6–7 (13.7 vs 12.4 mm) than females. The occiput would have blocked percutaneous screw placement in 86%, 78%, 54%, and 20% of the cases at C3–4, C4–5, C5–6, and C6–7, respectively. Transfacet lengths may accommodate longer screws in the upper cervical spine, but potential screw sizes decrease in the lower subaxial levels. A transfacet entry angle of approximately 50° or greater was associated with a higher incidence of occipital clearance. Additionally, the occiput may pose a significant obstruction to percutaneous transfacet fixation in upper subaxial levels. Interrater reliability was poor for screw angle and length measurements, but was satisfactory in intrarater analysis in 6 of 8 measurements. There was moderate to good agreement of occipital clearance in all but one measurement.

Conclusions

Cervical transfacet screw placement is possible from C-3 to C-7. Because occipital clearance can be difficult at C3–4 and C6–7, the use of curved or flexible instruments may be necessary to obtain the appropriate screw trajectory. Screw lengths varied with spinal level and the sex of the patient.

Abbreviation used in this paper:ICC = intraclass correlation coefficient.

Article Information

Address correspondence to: Joseph O'Brien, M.D., M.P.H., Department of Orthopaedic Surgery, George Washington University, Medical Faculty Associates, 2150 Pennsylvania Avenue NW, Washington, DC 20037. email: jobrien@mfa.gwu.edu.

Please include this information when citing this paper: published online April 20, 2012; DOI: 10.3171/2012.3.SPINE11449.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Sagittal reconstruction used to measure screw entry angle (between arrow and Line z). The x and y axes were rotated so that x is perpendicular to the facet joint. Note in this example the occiput would not block the projected screw trajectory. Filled circles indicate the approximate location of exiting nerve roots.

  • View in gallery

    Reformatted plane through y axis. This image was used to measure screw length. Note that length is limited ventrally by the vertebral artery.

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