pB-C2 line (drawn perpendicular to a line drawn between the basion and the posterior aspect of the C2 vertebral body, at the most posterior extent of the odontoid process at the dural interface). The pB-C2 line is a measure of ventral canal encroachment.

The study of Khaleel et al. demonstrates that the odontoid process in adults is anatomically different from that in children: it is longer, more posteriorly inclined, and has a greater pB-C2 line. Therefore, utilization of these parameters with previously published cutoffs in the pediatric population is not appropriate for surgical planning in adults ¹⁾.

Ladner et al. divided the pB-C2 line length into Grade 0 (< 3 mm) and Grade I (≥ 3 mm). Ventral canal encroachment may explain the symptomatology of selected patients with Chiari I malformation. The clinical findings presented suggest that patients with Grade I pB-C2 lines, with increased ventral canal obstruction, may experience a higher likelihood of syrinx reduction and headache resolution from decompressive surgery with duraplasty than those with Grade 0 pB-C2 lines ²⁾.

For Grabb et al. patients with a pB-C2 measurement of less than 9 mm do not require treatment directed at ventral brain stem compression (VBSC). In select patients with pB-C2 measurements of 9 mm or greater, reduction of VBSC may be prudent before posterior fossa decompression ³⁾.