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| ORIGINAL ARTICLE |
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| Year : 2021 | Volume
: 70
| Issue : 3 | Page : 151-155 |
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Variation of the superior articular facet of atlas and their significance
Neeru Goyal, Anjali Jain
Department of Anatomy, Christian Medical College, Ludhiana, Punjab, India
| Date of Submission | 06-Sep-2020 |
| Date of Acceptance | 25-Jul-2021 |
| Date of Web Publication | 23-Sep-2021 |
Correspondence Address:
Dr. Neeru Goyal
Department of Anatomy, Christian Medical College, Ludhiana, Punjab
India
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/JASI.JASI_125_19
Introduction: Atlas helps in complex biomechanical movements of the skull along with weight transmission of skull to spine. Recent developments in fixation technologies and minimally invasive surgical approaches have encouraged further studies of the region. Objectives of this study are to explore the shape, size, and symmetry of the superior articular facet of atlas. Good knowledge of the variations of the facet is important for orthopedicians, physiotherapists, and neurosurgeons. Material and Methods: The study was conducted on 110 dried adult atlas vertebrae. Shape, size, and number of the superior articular facets on each atlas were recorded. Results: Different shapes observed were kidney-shaped, oval-shaped, irregular, rectangular, comma-shaped, sinuous, sole/8-shaped, and two/three separate facets. Length and width of the facet were similar on two sides. In 31.82% of cases, the facets on the two sides were not symmetrical. Discussion and Conclusion: Variations of the superior articular facets have been extensively described. While planning treatment plans in cases of craniovertebral joint dysfunction, morphology and variations of the region should be kept in mind.
Keywords: Atlas, spine, superior articular facet, vertebra
How to cite this article:
Goyal N, Jain A. Variation of the superior articular facet of atlas and their significance. J Anat Soc India 2021;70:151-5 |
| Introduction | |  |
Atlas occupies the most important and vital position interfacing skull cranially and axis caudally constituting atlanto-occipital and atlantoaxial joints. Hence, it helps in complex biomechanical movements of the skull and neck along with weight transmission of skull to spine. Besides this, it paves the way for spinomedullary junction through the vertebral foramen. Vertebral artery and first cervical nerve pass over the superior surface of its posterior arch, making it more vulnerable.[1] The atlas consists of two lateral masses connected by a shorter anterior and a longer posterior arch. The lateral masses are ovoid, their long axes converging anteriorly. Each bears a kidney-shaped superior articular facet for the respective occipital condyle, which is sometimes completely divided into a larger anterior and a smaller posterior part.[2]
The atlanto-occipital joints are responsible for about one-half of all cervical flexion and extension.[3] The stability of the cervical spine is violated by various traumatic and nontraumatic causes. Instability of craniovertebral junction needs surgical correction or long-term immobility to attain a solid fusion. Therefore, reduction and rebuilding of the stability of this complex are important. A short-segment posterior fixation technique is often adopted to preserve the motion of the craniovertebral junction.[4] Morphometric characterization of the atlas has been found to be useful in operative management of atlantoaxial complex and occipitocervical instability.[5],[6]
Most of the surgical theories and techniques consider the atlas to be a source or contributing factor to many pathologies, including cervicogenic headaches, disequilibrium, and otalgia when the biomechanics of the atlanto-occipital and/or atlantoaxial joints are disturbed.[7] Fitz-Ritson[8] reported a high correspondence of upper cervical joint fixations in patients suffering from cervicogenic vertigo.
Variations of the superior articular facet have been extensively described.[4],[9],[10],[11],[12],[13],[14],[15] Still, more is being added to it day by day due to its contribution to the movements of head. Recent developments in fixation technologies and minimally invasive surgical approaches have encouraged further studies of the region. The objectives of this study are to explore the shape, size and symmetry of the superior articular facet of atlas. Good knowledge of the variations of the facet is important for orthopedicians, physiotherapists, and neurosurgeons.
| Material and Methods | | |
The study was conducted on 110 dried adult atlas vertebrae available in the department of anatomy. Only the bones with intact superior articular facets were included, while damaged and pathologically abnormal bones were excluded. Shape, size, and number of the superior articular facets on each atlas were recorded. Length (anteroposterior dimension) and width (transverse dimension) were measured using digital Vernier calipers.
| Results | | |
Shape
Shape of the superior articular facet was found to be quite variable [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]. Different shapes observed were kidney-shaped (25.9%), oval (41.8%), irregular (8.1%), rectangular (5.4%), comma-shaped (5.4%), sinuous (1.8%), and sole/8-shaped (4.5%) facets. Two (3.6%)/three (3.6%) separate facets were also seen. Comma-shaped facets were narrow anteriorly and wide posteriorly. In sinuous superior articular facets, the facet was S shaped. In sole/8-shaped facets, deep indentations were seen on both the sides of the facet, dividing the facet into two parts; if the two parts were equal, the facet was 8 shaped; however, if the anterior part was larger, the facet was called to be sole shaped. In some bones, the superior articular facet was divided into two or three smaller facets by narrow nonarticular areas. | Figure 1: Superior aspect of atlas showing comma-shaped superior articular facets
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 | Figure 2: Superior aspect of atlas showing 8-shaped superior articular facets
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 | Figure 3: Superior aspect of atlas showing rectangular superior articular facet
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 | Figure 4: Superior aspect of atlas showing sinuous superior articular facet
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 | Figure 5: Superior aspect of atlas showing superior articular facets divided into two smaller facets by narrow nonarticular area
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Size
[Table 1] shows the average length (anteroposterior dimension) and width (transverse dimension) of the superior articular facet on the right and left sides.
Asymmetry
In 31.82% of cases, the shape of the superior articular facet was found to be different on the two sides.
| Discussion | | |
The atlas supports the skull and is uniquely positioned in the atlantoaxial complex. As new surgical techniques and instruments for the treatment of unstable cervical spine continue to evolve, detailed knowledge about this bone becomes even more essential.[14] Standard textbooks describe superior articular facet to be kidney shaped or oval in shape. Many authors have commented on the shape of the facet, and there is a lot of discrepancy in the literature about the incidence of different shapes of the facet. Some authors[14],[16] described only oval- and kidney-shaped facets. Senegul and Kodiglu[14] observed 72% of facets to be oval and 28% to be kidney shaped, while Gosavi and Vatsalaswamy[16] found 74% of facets to be oval and 26% to be kidney shaped. Many other authors have described different shapes of the facet.
Lalit et al.[17] observed 28.3% oval-shaped, 20% kidney-shaped, 35% dumbbell-shaped, and 16.6% 8-shaped facets. They even put the completely separated facets into 8-shaped facets category. Kaore et al.[18] and Suman et al.[19] also described similar shapes, but they included completely separated facets in a separate category. Gupta et al.[10] described many different shapes including oval-shaped, 8-shaped, kidney-shaped, bi-lobed, tri-lobed, irregular, triangular, V-shaped, and leaf-shaped facets, but they included facets with osteophytes into irregular-shaped facets. Motagi and Ranganath[20] observed maximum (39%) facets to be irregular, but Lalit et al.[17] and Kaore et al.[18] did not describe any irregular-shaped facets.
Apart from the shape of the facet, there is a lot of difference in the incidence of the various shapes. Most authors[10],[14],[16],[17],[18],[19] observed oval-shaped superior articular facets to be most common. However, Singh reported a combined dumbbell and 8-shaped as the most common shape. Motagi and Ranganath[20] reported maximum facets to be irregular, while Patil and Kumar[21] found maximum facets to be dumbbell shaped. In the present study, oval facet was the most common type. We also observed kidney-shaped, irregular-shaped, sole/8-shaped, rectangular, comma-shaped, and sinuous facets.
The atlas vertebra develops from the caudal half of occipital somite 4 and the cranial half of cervical somite 1. It ossifies from three centers – two appear in the lateral masses at the 7th week and they gradually extend into the posterior arch where they unite between the 3rd and 4th years, usually directly but occasionally through a separate center. A separate centre appears for the anterior arch at about the end of first year. This unites with the lateral masses between sixth and eighth year, the lines of union extending across anterior parts of the superior articular facets.Occasionally, the anterior arch is formed by the extension and ultimate union of centers in the lateral masses and sometimes from two lateral centers in the arch itself.[2] The posterior part of the superior articular facet is developed by the posterior arch. This different embryological development of the two parts of the superior articular facets explains their partial or complete dissociation.[22]
Phylogenetically, the formation of craniocervical joint has been the result of many trials. Every imaginable combination has been present and is still seen in some fish. The joint has developed depending on the mode of life of the creature. There are many different ways in which creatures use their heads and each habitual motion reacts upon the articulating units. The primitive triple condyle of the occipital has all the units of equal size, i.e. the basioccipital with a ventral median condyle nearly circular and the lateral occipitals with elongated lateral condyles situated dorsally on each side. This has been followed either by preponderance of the lateral units as in amphibia or the gradual enlargement of the median unit combined with the recession of the lateral ones until the single condyle of the birds is reached. In mammals, the large paired, lateral condyles are the prominent feature and the basioccipital has withdrawn from the odontoid. The bicondylar joint thus restricts the movements of the head to nodding, the turning to the one side being possible at the other vertebral joints. The tendency of the superior articular facets of the atlas to split into two separate facets seems a step in that direction, and this tendency is probably an indication of further restriction of movements at the atlanto-occipital joint.[15]
In the present study, the average length and width of the superior articular facet were found to be 21.99 mm and 11.64 mm, respectively. Different authors have given different dimensions of the facet [Table 2]. We did not observe any significant difference in the dimensions on right and left sides, while some authors have found significant difference in these measurements. Salahuddin et al.[13] attributed this difference to the handedness. They stated that when right-handed people lift weights using their right hand, head is tilted to left. Left facets are subjected to greater force. Hence, left dimensions become larger. Transarticular screw fixation has become one of the primary treatment options for cervical spine instability. The knowledge of the anteroposterior and transverse dimensions of the superior articular facet can help in the safe planning of these screw placements.[18] | Table 2: Comparison of dimensions of superior articular facet in various populations
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In the present study, although the dimensions were similar on the two sides, in 31.82% of cases, the facets on the two sides were not mirror images of each other and varied in shape on the two sides of the same vertebra. Historically to interpret the craniovertebral joint function, the superior articular facets are considered to be symmetrical. Symmetrical functions are possible only in the presence of symmetrical anatomical structures. Since the facets are not symmetrical in many of the atlas vertebrae, considering them, symmetrical during assessment of joint function may mislead the examiner. This may cause the implementation of incorrect treatment plans.[13] Asymmetry should be kept in mind while implementing the treatment plans for joint dysfunction. As the age advances, the physical anthropometry of the joint may variate that may cause either symptomatic or asymptomatic clinical conditions. It is a debatable task to postulate that variant changes in the articulating surfaces of atlanto-occipital joint are the responsible and reasonable factors for neck strains that involve the joint.[20]
| Conclusion | | |
Hence, we conclude that although the superior articular facet has gained considerable attention, considering the discrepancy in the description by the various authors, comparison of the results with the available literature should be done very carefully. While planning treatment plans in cases of craniovertebral joint dysfunction, morphology and variations of the region should be kept in mind.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
[27]
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]
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