Anterolateral C1-C2 transarticular fixation for atlantoaxial arthrodesis: Landmarks, working area, and angles of approach

BACKGROUND: An alternative route must be used for atlantoaxial arthrodesis to avoid the risks of transoral route or when posterior approaches are contraindicated. OBJECTIVE: To assess relevant quantitative anatomic parameters for C1-C2 anterolateral transarticular fixation and to demonstrate the nuances of an anterolateral approach to the upper cervical spine. METHODS: Five cadaveric necks were dissected bilaterally to demonstrate anatomic landmarks and surgical technique. The C2 pars interarticularis was used as the entry for inserting screws toward the C1 lateral mass. Ten computed tomography scans were analyzed to quantify working area and optimal angles of approach. RESULTS: The medial surface of sternocleidomastoid muscle was dissected extensively but not divided. The C2 transverse process was a landmark for guiding dissection posterior to the carotid sheath. In all specimens, the gray ramus communicans from the superior cervical ganglion to the C2 nerve was a landmark for locating the C2 pars. Slightly below that branch, the longus capitis muscle could be displaced medially to reach the C2 pars. The ideal angles for screw placement were 22.9 ± 5.7° medial to the sagittal plane and 25.3 ± 7.4° posterior to the coronal plane. The mean working area was 71.2 mm (range, 49-103 mm). CONCLUSION: We propose a new anterolateral stabilization technique for atlantoaxial instability based on less traumatic dissection of the upper cervical region, different instrumentation, and guidance by reliable landmarks. For anterolateral transarticular C1-C2 screw fixation, the gray ramus communicans to the C2 nerve is a reliable landmark for locating the entry for a screw on the C2 pars.