The Importance of Platybasia and the Palatine Line in Patient Selection for Endonasal Surgery of the Craniocervical Junction: A Radiographic Study of 12 Patients

Objective

Ventral decompressive surgery of the craniocervical junction is performed to manage a variety of conditions, including basilar invagination, which can be associated with platybasia. We have noted that the anatomic changes of platybasia could affect the height of the odontoid over a line drawn along the nasal cavity floor, the palatine line (PL). This anatomic change may influence the use of nasal endoscopic surgery for patients with platybasia who also have basilar invagination. We investigated whether the height of the craniocervical junction is elevated over the PL in patients with and without platybasia.

Methods

We conducted a retrospective review of consecutive craniovertebral junction surgical cases during a 14-month period. During that time we treated 12 patients, including 4 with platybasia and 8 without. The average age was 50 years (range, 18-64 years). Preoperative and postoperative radiographic images were evaluated and charts reviewed.

Results

The mean height of the odontoid over the PL without platybasia was 3.5 mm (range, 0-19.0 mm). In those with platybasia, it was 15.5 mm (range, 7- 26.0 mm; P = .021). There was a statistically significant increase in the height of the clival tip and C1 ring in patient with platybasia as well.

Conclusions

Platybasia is associated with an increase in the odontoid and craniocervical junction over the PL. This increase in height has implications for endoscopic approach selection in patients with platybasia. Platybasia patients with basilar invagination may be better suited to a transnasal approach.

Introduction

Platybasia, i.e., congenital flattening of the skull base, is occasionally seen in association with basilar invagination (3, 16, 23). Compromise of the foramen magnum, reduction of the posterior fossa volume, and protrusion of the odontoid process into the ventral brain stem may cause neurological symptoms in some of these patients. Anterior surgical decompression of the craniocervical junction may be thus considered to treat this problem (9, 13, 22).

Surgical approaches to the craniocervical junction have traditionally been achieved transorally (with or without splitting of the soft palate) (15, 19, 20, 22). More recently, several endoscopic approaches have been introduced, including a transnasal approach (1, 5, 14, 17, 21, 25, 26), transcervical endoscopic approach (18, 24), and a combined endonasal endo-oral approach (8). Few data exist in the literature that describe the anatomic relationships of the craniocervical junction from an endoscopic perspective. In particular, there is currently no staging system or identification of factors to aid in the endoscopic approach selection to the craniocervical junction. Previously accepted anatomic references for the odontoid are McGregor's line and Chamberlain's line (6, 23). These lines are described as being drawn from the posterior edge of the hard palate to the occiput. However, these two lines do not adequately describe the view of the odontoid from an endoscopic approach because the occiput, like the posterior edge of the palate, can vary in its relationship to the craniocervical junction. Thus, the line drawn from the posterior palate will change its relationship to the odontoid as the occiput is moved up or down and the height of the craniocervical junction over these lines does not directly relate to the view achieved from an endonasal approach.

We sought another marker to help establish the minimal plane of approach needed to access the tip of the odontoid with the use of an endoscopic approach. We have found that a line drawn along the floor of the nasal cavity extending from the premaxilla parallel to the hard palate at the craniocervical junction, the palatine line (PL), provides a better planning tool for an endoscopic approach to the craniocervical junction (Figure 1).

In this study, we aimed to determine whether the condition of platybasia is associated with an increased height of the craniocervical junction and odontoid over the plane of the palate. Hypothetically, preoperative assessment of the height of the surgical target over the palate can identify cases appropriate for an endonasal versus an endo-oral approach.