C1 lateral mass working window size and head position: Correlation of flexion-extension CT scans and cadaver specimens

Background Movement between the posterior elements of C1 and C2 has not been quantified. Cervical flexion and extension have been previously shown to affect the craniovertebral junction (C0-C1-C2). Prior studies have not reported the change in the posterior C1-C2 interval with flexion and extension. Head position may increase the exposure of the C1 lateral mass by opening the posterior C1-C2 interval and increasing the C1 working window. Methods The C1 lateral mass was exposed in four male and five female cadaver specimens. The C2 nerve root was left intact. The C1 working window was measured from the superior aspect of the C2 pars to the overhang of the C1 arch. Cervical flexion and extension were then applied and the intervals were remeasured. The maximal C1 working window was measured in flexion, neutral, and extension with calipers and on CT. The distance from the C1 arch to the pars was measured in each position. Overhang of the C1 arch also was measured. The occipital-cervical angle was measured on CT scan in each position using McRae's line to the inferior endplate of C2. Results On average, flexion increased the C1 working window by 4.9mm in cadavers according to measurement with a caliper. In comparison, CT scan demonstrated an average increase of 6.9mm in the C1 working window in flexion compared with extension. Measurements of the occipital-cervical angle across all ranges of motion correlated with prior published data; therefore the measurements at the C1-C2 interval seem reliable. Conclusions This study demonstrated significant movement between the posterior elements of C1 and C2 in flexion and extension. The C1 working window (C1-C2 interval) increased 145% in flexion compared with extension when measured on CT scan. This finding underscores the importance of head position in posterior cervical surgery. If flexion is not possible because of cervical instability or neurologic compromise, the neutral position is a viable alternative. Positioning in neutral compared with extension increased the C1 working window by 80%. Reducing the overhang also can increase exposure. In this study, the average overhang of the C1 arch was 1.2mm. Using a diamond burr to make the arch flush can yield a 10% increase in working space (in flexion).