RESEARCH ARTICLE |
https://doi.org/10.5005/jp-journals-11003-0128 |
Arcuate Foramen: An Anatomic Variant of Atlas Vertebra and Its Clinical Considerations
1-4Department of Anatomy, Army College of Medical Sciences, Delhi, India
Corresponding Author: Ruchi Dhuria, Department of Anatomy, Army College of Medical Sciences, Delhi, India, e-mail: ruchidhuria22@yahoo.com
Received on: 03 May 2023; Accepted on: 29 May 2023; Published on: 28 June 2023
ABSTRACT
The ponticulus posticus (PP) or arcuate foramen (AF) is a bony bridge arising from lateral mass of atlas, which transforms the groove of vertebral artery into a canal on posterior arch of atlas. The presence of an AF and its relevance was seen in vertebral artery stretching, dissection, canine impactions, in some unexplained headaches and neck pains.
Objective: The aim is to study the presence of AF in atlas vertebrae, their dimensions, and clinical implications.
Materials and methods: A total of 80 dry atlas vertebra specimens were obtained from the Anatomy Department of Army College of Medical Sciences and North Delhi Municipal Corporation Medical College, Delhi, India, and were studied for the presence and dimensions of the AF and the incidences were compared.
Result and conclusion: In our observational study, the AF was found in 13.75% of specimens. The AF was classified as complete, partial, and bony spicules. The observed dimensions of the complete AF in the present study were comparatively less. Interestingly, in some specimen’s minimum thickness of dorsal arch of atlas vertebra was found to be less than the minimum thickness required for the surgical screw placement into the lateral masses. Knowledge about these anatomical variations will be of help to neurosurgeons, radiologists, and general medical practitioners.
How to cite this article: Dubey A, Dhuria R, Rustagi SM, et al. Arcuate Foramen: An Anatomic Variant of Atlas Vertebra and Its Clinical Considerations. J Med Acad 2023;6(1):3-7.
Source of support: Nil
Conflict of interest: Dr Shaifaly M Rustagi is associated as the Executive Editor of this journal and this manuscript was subjected to this journal’s standard review procedures, with this peer review handled independently of this editorial board member and her research group.
Keywords: Arcuate foramen, Atlas screw placement, Atlas vertebra, Ponticulus posticus
INTRODUCTION
Atlas, cervical vertebra 1 (C1) is the superior most cervical vertebra, which gives support to the skull and spinal cord. It also gives attachment to the muscles of neck. It is named atlas due to its unique position, where it holds the skull at the top of the vertebral column.1
Unlike most cervical vertebrae, the ring-shaped atlas has no vertebral body and spinous process. It consists of two articulating lateral masses, which are connected by short anterior and posterior arches and two transverse processes. The centrum of atlas is fused with the axis to form the odontoid process or dens of the axis. The anterior arch articulates with the dens of axis vertebra (C2) and form a pivot type of synovial median atlantoaxial joint. Each lateral mass consists upper kidney-shaped and lower circular articular facets. Upper facet articulates with the occipital condyles and forms atlantooccipital joint. Lower facet articulates with the corresponding facet of axis to form lateral atlantoaxial joint. Transverse processes surround the transversarium foramen through which vertebral artery and vein pass. Just behind the lateral mass, the upper surface of posterior arch of atlas represents a wide groove on both sides for the lodgement of first cervical spinal nerve and third part of vertebral artery.2
The arcuate foramen (AF), an anatomical variation of atlas, is formed by the presence of a bony bridge due to ossification of posterior atlantooccipital membrane over the passage of the vertebral artery.3 It is known by many names such as foramen arcuate atlantis, ponticulus posticus (PP) or posterior ponticle, or Kimmerle’s anomaly.4 This anomaly is reported to be present in approximately 3–15% of the population.5,6 AF is found more common in females than in males,7 while some authors reported no difference in prevalence on the basis of sex and age.8
Types of Arcuate Foramina
Different types of classification of formation of AF were given by the authors over time. Cederberg et al., designated four grades of ossification to classify the AF—(1) No calcification, (2) less than half calcified, (3) At least half calcified, and (4) Complete calcification.9 Santhi et al., described the AF formation as posterior, lateral, and posterolateral. This was given on the basis of the direction of bony bridge formed on the vertebral groove.10 Hasan et al., described the arcuate foramina into six classes—(1) class 1—if an impression for the vertebral artery was noticeable on the posterior arch, (2) class 2—the impression for the artery was deeper, (3) class 3—if a partial posterior ponticulus or a bony spicule was present, (4) class 4—if a complete posterior ponticulus was found, (5) class 5—when a bridge extends from the lateral mass to the transverse process of atlas, and (6) class 6—where a relatively more extensive posterolateral tunnel was found.11
The existence of an AF and its relevance in stretching of vertebral artery against lateral mass of atlas has been described in literature. In such conditions, the opposite side vertebral artery becomes narrow and elongated, which would lead to impaired blood flow.2 Presence of osseous bridges and AF triggered higher incidence of vertebral artery dissection because of tethering.12 The existence of AF was also found to be associated with unexplained headaches (cervicogenic headache) and neck pain.13 Occurrence of PP was also seen to be associated with maxillary canine impaction and palatal impaction of canines.14
Surgically, occurrence of an AF can pose a great threat while placing screws in the lateral masses of atlas by producing an incorrect impression of a wider posterior arch.13 In such cases, the screws can lead to the impairment of vertebral artery and/or the suboccipital nerve. The number of patients treated with screws through the posterior arch of atlas has dramatically increased in couple of years; therefore, it is substantial to understand the morphology of AF and recognize its presence prior to placement of the screws and its clinical relevance.
MATERIALS AND METHODS
Arcuate foramen (AF) has clinical, radiological, as well as surgical implications. Therefore, it is important to know AF, prevalence, types, measurements, and their variations.
In the present study, 80 dry human atlases were obtained from the Department of Anatomy, Army College of Medical Sciences, Delhi Cantt and North Delhi Municipal Corporation Medical College, Delhi, India for the incidences and the presence of AF. All these atlas vertebrae were of unknown age and sex. The specimens exhibiting the AF were classified as complete foramen (bilateral and unilateral), partial ponticulus (bilateral and unilateral), and bony spicules. Dimensions of the foramen, height (rostrocaudal plane), and length (ventrodorsal plane) were recorded with the help of manual vernier calipers. Maximum dimensions of the AF were compared with the dimensions of the groove for the vertebral artery. Data were recorded for both sides separately, and the incidence of AF on the basis of the side was calculated.
RESULTS
Dimensions
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Complete bilateral AF type I—in this type, a bony canal was found running from the transverse foramen to the AF, which is open dorsally (n = 4) (Fig. 1).
-
Complete bilateral AF type II—in this type, a complete bridge is formed between the lateral mass of atlas and posterior bar of transverse process immediately next to the transverse foramen (n = 1) (Fig. 2).
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Partial ponticulus bilateral—in this type, a partial bony bridge is present between the posterior margin of superior articular facet and posterior arch of atlas (n = 3) (Fig. 3).
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Bony spicules type I—in this type, there is a presence of small bony spicules from the lateral masses of atlas towards its posterior arch (n = 2) (Fig. 4).
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Bony spicules type II—in this type, there is a presence of small bony spicules originating from the posterior arch of atlas at the level of vertebral foramen (n = 1) (Fig. 5).
Figs 1A and B: (A) Complete AF with bony canal; (B) Bony canal open dorsally extending from foramen transversarium to AF
Fig. 2: Complete AF formed between the lateral mass of atlas and posterior bar of transverse process
Fig. 3: The presence of osseous bridge over the vertebral groove in atlas
Figs 4A and B: Shows bony spicule originating from the lateral masses of atlas (A) lateral view and (B) superior view
Fig. 5: Bony spicule originating from the posterior arch of atlas
DISCUSSION
The presence of AF, either complete or partial, is of significant clinical relevance to neurosurgeons. In the absence of AF, vertebral artery runs upwards through the foramen transversarium of C6–C2 vertebrae. It almost runs a straight course to the axis vertebra, and after that artery enters through the foramen transversarium of atlas. Later, it turns medially and posteriorly behind the lateral masses to reach the groove on the superior surface of the posterior arch of atlas. Then it enters the vertebral canal by crossing under the posterior atlantooccipital membrane. In case of complete AF, vertebral artery passes through a bony bridge present over the vertebral groove.8
Elliot and Tanweer, in their study, conveyed that the overall prevalence of AF in 16.7% of cases. Out of which, 18.8% were cadaver studies, 17.2% were computed tomography studies, and 16.6% were X-ray studies. They also observed complete AF in 9.3% of cases and incomplete or partial foramen in 8.7% of cases.7
In our study, it is not only a foramen but an actual tunnel running from the transverse foramen to the vertebral groove (average length—1.5 mm on left and 1.7 mm on right side, average height—5 mm on left and 4 mm on right side) and then a complete bony bridge is present over the vertebral groove (height—3.5–5.7 mm right side and 4.0–6.0 mm left side, length—5.5–7.0 mm right side, and 7.0–7.5 mm left side). The canal is open posterolaterally. Presence of a bony canal was also previously explained by Afsharpour et al.8 (Tables 1 to 3).
| S. No. | Type | Bilateral | Unilateral | |
|---|---|---|---|---|
| Right | Left | |||
| 1 | Complete | 5 (6.25%) | 0 | 0 |
| 2 | Partial ponticulus | 3 (3.75%) | 0 | 0 |
| 3 | Bony spicules | 0 | 1 (1.25%) | 2 (2.5%) |
| Right side (in mm) | Left side (in mm) | |
|---|---|---|
| Height of foramen | 3.5–5.7 | 4.0–6.0 |
| Length of foramen | 5.5–7.0 | 7.0 - 7.5 |
| Right side | Left side | |
|---|---|---|
| Average height of bony canal | 4.0 mm | 5.0 mm |
| Average length of bony canal | 1.7 mm | 1.5 mm |
In the previous literature, the average length of AF was explained as 5.28–9.56 mm on left and 9.35–10.4 mm on right. The range of height was 5.24–7.36 mm on left and 6–6.90 mm on right side.15 In present study, height of the AF was less in comparison to the earlier study.
These dimensions are a matter of importance for surgeons performing screw placement in the lateral masses using the dorsal arch of atlas, especially when the occurrence of AF gives a false appearance of wide dorsal arch.8 To pass a 3.5 mm screw safely through the dorsolateral arch, a minimum thickness of 5 mm should be present. In the present study, the minimum thickness of the dorsal arch in complete AF was found to be 3.0 mm, and in case of partial AF was 4.0 mm (Tables 4 to 9).
| Right side | Left side | |
|---|---|---|
| Height of foramen | 4.5 mm | 4.0 mm |
| Length of foramen | 4.0 mm | 5.0 mm |
| Right side (in mm) | Left side (in mm) | |
|---|---|---|
| Height | 4.5–6.0 | 5.0–7.0 |
| Average distance from transverse foramen | 1.2 | 1.0 |
| Left side (in mm) | |
|---|---|
| Length of spicule | 4.0–4.5 |
| Right side | |
|---|---|
| Length of spicule | 3.5 mm |
| Maximum thickness (mm) | Minimum thickness (mm) | |||
|---|---|---|---|---|
| Left | Right | Left | Right | |
| Complete bilateral | 6.0 | – | – | 3.0 |
| Partial bilateral | 5.0 | – | – | 4.0 |
| Bony spicules | – | 4.5 | – | 3.0 |
| S. No. | Authors | Incidences (%) |
|---|---|---|
| 1 | Krishnamurthy et al., 2007 | 8.33 |
| 2 | Tubbs et al., 2007 | 5 |
| 3 | Baeesa et al., 2012 | 16.10 |
| 4 | Cakmak et al., 2005 | 11.2 |
| 5 | Santhi et al., 2017 | 8.60 |
| 6 | Present study | 6.25 |
Apart from the surgical point of view, presence of AF poses many other clinical risks, such as compression of vertebral artery, which could lead to certain neurological conditions like vertebrobasilar arterial insufficiency.16,17Mitchell j also stated that compressed vertebral arteries could result in stenosis from hyperextension of the head or manual pressure on this region.18 However, some other authors believe that there was no risk of stenosis. Cushing et al., in 2001, stated with clinical verdicts that presence of AF enhances the risk and frequency of vertebral artery dissection within the AF due to tethering of the artery during trauma, especially with neck rotation.12 All these data suggest that the risk of vertebral artery damage in presence of AF mainly depends on the dimensions and the structure of the foramen.
Arcuate foramen (AF) is progressively being observed as a leading variation of the atlas recently. In six specimens of our study, a small bony bridge/bony spicule was found between the lateral masses and the posterior arch of atlas (Tables 5 to 7). In previous literature, this is called PP. The presence of ponticulus was found to be associated with migraine without aura in chiropractic patients. Although the mechanism for this is still ambiguous, but it may probably be associated with ischemic contraction of vertebral artery or by dural tension at the craniocervical junction.19
Presence of PP also gives the impression of extensive dorsal arch and may mislead the clinician. In such cases, surgeon may insert the screw into the AF and injure the vertebral artery. It can also lead to stroke or even death by embolism, thrombosis, and/or arterial dissection.7
Hence, it will not be wrong to say that such anatomical variations should be investigated and considered as differential diagnosis of chronic orofacial pain. Correlation of AF with Barré–Liéou syndrome has also been reported. Its characteristic symptoms are headache, retro-orbital pain, vasomotor disturbance of the face, swallowing and phonation caused by alteration of blood flow within the vertebra and recurrent disturbances of vision.20 Therefore, AF is a potential anatomical variant of the atlas.
CONCLUSION
Surgeons, neurologists, and the entire medical community in general should have acquaintance with the presence and variations of AF. Clinicians should consider the presence of the complete AF or the bony spicules with symptoms suggesting the major cause of compression of vertebral artery. So, the awareness of such anomalies before surgical procedures on the atlas is very crucial. We strongly support preoperative screening for detecting the presence of AF.
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