The ponticulus posticus of the first cervical vertebra.

Abstract

The evaluation of anatomic landmarks is an important task of the radiologist because significant changes in these structures may be an index to underlying disease processes. The variations and asymmetrical deviations of such landmarks in size, shape, and contour are often many and confusing. For this reason they must be studied with care to insure adequate differentiation from disease processes. The extensive variations which may occur in the region of the craniocervical junction have been reviewed by McRae (6). Hoare (3) has also pointed out the importance of evaluating the grooves and foramina in the region of the posterior and lateral margins of the atlas and axis in vascular lesions of the posterior fossa and in lesions involving the cerebellum. It appeared desirable to us to evaluate the sulcus formed by the vertebral artery as it crosses the posterolateral laminar margin of the first cervical vertebra. After its exit from the transverse foramen, the vertebral artery passes over the posterior arch of the atlas, forming a groove called the vertebral artery sulcus (Fig. 1). This sulcus, situated on the posterolateral margin of the posterior arch of the atlas varies in size and in some instances is very deep. To the posterior arch of the atlas is attached, inferiorly and posteriorly, the posterior occipital ligament, which is connected above with the posterior margin of the foramen magnum. This ligament is a broad but thin membranous sheet intimately blending with the dura. Its lateral divisions are known as the oblique atlanto-occipital ligaments. These ligaments are incomplete at their inferior margins, affording with the vertebral artery sulcus an opening for the passage of the vertebral artery and suboccipital nerve (Fig. 2). At times an anomalous ossification center occurs in the ligament, and bridges the sulcus. This bony arch—the ponticulus posticus—encloses the foramen arcuale or posterior atlantoid foramen, and through this the suboccipital nerve and vertebral artery pass as they course over the upper surface of the first vertebra. The ponticulus has been described as Kimmerle's anomaly (4, 5). As late as 1955 one American text on the spine erroneously designated the structure as the foramen transversarium (1). Writers in general do not evaluate its variations in size, its occurrence in the general population, or its sex incidence. Our interest in this anatomic structure was stimulated by its incidental demonstration in various skull examinations. We therefore undertook a review of the roentgenograms of 300 normal skulls, taken for the most part from routine studies or examinations for possible metastases, which had proved negative. Cases with definite evidence of intracranial or calvarial lesions were excluded. Of the 300 patients represented, 170 (56.6 per cent) were females and 130 males. The females ranged in age from newborn to eighty-seven years, the average being fifty-two and the median forty-nine years.