The anatomy of the first vertebra, namely atlas, has significant clinical implications. Anatomically it is not a typical vertebra due to its shape and absence of body and spinous process. Nevertheless, its function and variations of development are important in clinical settings . Arcuate foramen of atlas is an ossification of the oblique atlanto-occipital ligament superior to the vertebral artery groove of the atlas. The list of names for this anomaly is impressing as the anomaly itself and includes Kimmerle anomaly (variant or deformity), ponticulus posterior (ponticulus posticus) of the atlas, pons posticus, foramen atlantoideum posterius (or vertebrale), canalis arteriae vertebralis, foramen sagitale, retroarticular vertebral artery ring, foramen retroarticular superior, retrocondylar bony foramen, posterior atlantoid foramen, atlas bridging, posterior glenoid process and speculum . The vertebral artery, which passes under these bony projections, can be compressed along with the suboccipital nerve causing a variety of symptoms.
This anomaly is an underestimated structure, which is important to take into account in case of patients with a headache, vertigo, shoulder and arm pain. Therefore, healthcare providers, including neurologists, neurosurgeons and vertebrologists should be aware of this clinically significant variation.
During the examination of 50 atlases, four cases of arcuate foramen were encountered: three partial arcuate foramen (6%; fig. 1) and a case of complete arcuate foramen (2%; fig. 2). The rest of the vertebra had a normal anatomy (92%; fig. 3). The vertebra with partial arcuate foramen had ossified projections of 5-9 mm (medium 7.1 mm). In the rare case of the complete arcuate foramen, there was a narrow foramen for the vertebral artery. The mean anteroposterior length of the atlas was 42.1±4.6 mm and the anteroposterior length of the vertebral canal was 38.2±3.7 mm. The mean transverse dimension of the vertebra was 53.2±3.8 mm and of the vertebral canal 42.2±4.9 mm. There was no statistical difference between the dimensions of the atlases with arcuate foramen compared to those without assessed by Student’s t-test (p>0,05).
Fig 1: Partial arcuate foramen
Fig 2: Complete arcuate foramen
||Fig 3: Normal anatomy of the atlas
The ossification of ligamentous structures in various parts of the body is not uncommon and can affect a variety of structures in the body with most common symptoms as pain, numbness and weakness. Surgery is often the only treatment option for such cases [3, 4]. The arcuate foramen can be complete (fig. 2), when the entire ligament is ossified, or partial (fig. 1), when a bone exostosis is present. The incomplete arcuate foramen is seen in 5.5% of cases, and complete in 7.18% (2.58% on the right and 4.59% on the left). Bilateral complete arcuate foramen as presented in the current case is a rare finding and is seen only in 1.14% of cases . It seems that its incidence increases with age and over a period of time partial bridging can eventually become total . Other risk factors include male sex, ethnicity and being a laborer [7, 8].
The main problem is that the vertebral artery can be entrapped in the bony bridges causing vertebro-basilar arterial insufficiency. This is supported by the fact that in case of partial ponticulus posticus up to 14% of cases can be symptomatic. In complete form, the number of symptomatic patients can be up to 78%, mainly, in the form of migraine or chronic type of headache . The list of associated complains may include also cervical migraine, neurosensory-type hearing loss, neck pain, vertigo, shoulder/arm pain and even loss of postural muscle tone or consciousness . It may even be that repetitive traumas can lead to even more severe complications like tethering of the vertebral artery with its dissection by repetitive trauma and posterior circulation stroke in children .
Patients with arcuate foramen of atlas can be particularly at risk during cervical manual manipulations (compressive pressure, hyperextension) and during surgical procedures in this region (insertion of screws or exposing the horizontal portion of the V3 segment of the vertebral artery) [8, 12].
Anatomy is a field that has an extensive list of developmental anomalies and variations, which often have implications for clinical practice. We presented two anatomical cases of anatomical bringing along with a brief review of its clinical significance for different specialties. It is important to keep in mind these variations since they can cause symptoms in 14-78% of cases and can be often overlooked by healthcare providers.
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