The Morphological Analysis of the Superior Articular Facet of the adult Human Atlas vertebra
Correspondence Address :
Dr. Monika Lalit, 24, Lane-5, Gopal Nagar, Majitha Road, Amritsar-
143001, Punjab, INDIA
E Mail: monika.lalit@yahoo.com
The ‘Atlas’ is first cervical vertebra, supporting the “globe” of the head. The sound knowledge of of the Atlas vertebra is a well established entity in Anatomy. In a number of 30 atlas vertebrae, 60 superior articular facets were studied for the presence of a constriction or a groove and a tendency to separate i.e complete or incomplete separation. On the basis of this, different shapes of the superior articular facets were also studied This study was performed in the Department of Anatomy, Government MedicalCollege, Amritsar. The results were compared with those of other studies and were statistically analysed. The clinical significance of this study has been thus discussed with a reference to its possible effect on the restriction of the neck at the atlanto-occipital joint.
Superior articular facet, Presence of constriction, Groove, Tendency for separation
Injuries of the upper cervical spine which cause severe disabilities following trauma, have always been an interesting focus for anatomists (1). The Atlas is the first cervical vertebra which supports the “globe” of the head (2). If the atlas was morphologically similar to other vertebrae, death would be a common result of fracture. Anatomically, the atlas is embedded in the neck muscles and is therefore protected from injury.The unique structure and the anatomical location of the atlas forms a safety mechanism (3). Superior articular facets which are present on the atlas vertebrae face superomedially and are admirably accepted for nodding movements and also for the weight bearing of the head (4). These are also accepted for the reception of the condyles of the occipital bone to form an atlantooccipital joint (5). Thus, the assessment of these structures and the morphological relationship of the superior articular facet of the atlas is done before performing any clinical procedure in its proximity.
Study Design
An evaluation of the superior articular facet on the atlas vertebrae, by using dry, bony and non-pathological vertebrae.
Inclusion Criteria
1. The bones were dry, macerated and thoroughly cleaned. 2. The vertebrae were complete in all respects, in order to give the correct observations 3. The bones were non-pathological. The instruments which were used during the present study were: 1. Hand lens 2. Lead pencil 3. Markers
The study was conducted in the Department of Anatomy, Government Medical College, Amritsar. The material for the study comprised of 30 adult human atlas vertebrae of unknown age and sex. These were obtained by a maceration of the cadavers, which were made available for the purpose of dissection. All the atlas vertebrae were thoroughly boiled, cleaned and numbered from 1-30. Each bone was meticulously examined for the following important morphological features:
1. The presence of a constriction: It may be present on one or both the sides of the vertebra and shows important features like the presence of a constriction on the medial or the lateral side, a constriction on both the sides, and the absence of a constriction on both the sides of the facet (6). (Table/Fig 1). (Table/Fig 2), [Table/ Fig 3]
2. The presence of a groove:
A groove of variable depth and breadth was seen to be present in the facets throughout the whole or part of the breadth of the facets i.e. in the centre or on the medial or the lateral side. Sometimes the groove is well defined and deep along the constrictions and may divide the facet into two (6). See(Table/Fig 4)
3. A tendency of separation:
The division of the facet into two or a tendency towards it was indicated by the presence of a constriction or a groove or both. The amount of tendency of separation, was represented as absent, present, marked and well marked or as a separation into two i.e. complete and incomplete categories (6).(Table/Fig 5), (Table/Fig 6)
4. The shape of the Superior Articular Facet (SAF):
The presence of a constriction or a groove and a tendency to separate gives the SAF different shapes which are defined as follows; Oval shape – no constriction, Kidney shape – constriction present on one side of the facet, Dumb-bell shape – constrictions on both the sides of the facet and a groove connecting these constrictions and the Fig. of eight shape – the extension of these constrictions towards the centre of the facet (6), (7). [See Table/Fig 1-3], (Table/Fig 7)
5. The presence of an accessory foramen transversarium was observed[See Table/Fig 4].
6. The formation of a retroarticular canal was also observed
[See Table/Fig 4].
In 30 atlas vertebrae, the 60 superior articular facets were studied for the presence of a constriction or a groove and a tendency to separate i.e. complete or incomplete separation. Based on these observations, the percentage was calculated for each parameter and different shapes of the superior articular facets were also studied. The shape of the SAF was also statistically analysed by applying the Chi- Square test. The readings were noted and the results were compared with the findings of other workers, as shown in [Table/Fig 8-11].
Of the 60 superior articular facets which were examined, 17 facets (56.6%) showed no constriction and these were defined as oval shaped. 12 facets (40%) showed a constriction only on one side and were considered as kidney shaped. 21 facets (70%) were found to have a constriction on both the sides, which were defined as dumb-bell shaped and 10 facets (33.3%) showed a constriction on both the sides along with a groove in the centre of the facet, which were considered as the ‘figure of 8’ shape. The Chi-Square test was applied to evaluate the percentages of the types of the shapes on the right and left sides of the superior articular facet and the difference between these was found to be statistically insignificant (p=0.536) Incomplete separation was seen on 2 facets (6.6%) on the right and left sides individually, whereas a complete separation was seen only on one facet (3.3%) on the left
sideFor comparing the results of the present study with those which were reviewed, only the findings of the authors who studied skeletal material were taken into account and these are shown in the (Table/Fig 8), [Table/Fig 9], [Table/Fig 10] and [Table/Fig 11] as follows:
1. The presence of a constriction:
It is clear from Table/Fig 8, that the presence of a constriction on the individual sides of the facets had a higher value in the present study, as compared to the results recorded by Singh, whereas a constriction which was observed on both sides of the facets, in the present study had a lower value as compared to that which was observed by Singh (8), (9).
The Superior articular facets face superomedially (4) and for the reception of the condyles of the occipital bone to form an atlantooccipital joint (5). An atlanto occipital transarticular approach is useful for the anterior extradural lesions of the cranio vertebral junction at this point (9). An abnormal hypertrophy of the articular facets is again a recognized cause for the narrowing of the vertebral canal and consequent neurological feature (10). In severe cases of ossification, sometimes a complete bridge is formed across the posterior arch of the atlas surrounding the vertebral artery, which is called as the retroarticular canal or the vertebral artery ring of the atlas vertebra (11) This arterial ring may sometimes be bridged by fibrous tissue i.e. the oblique ligament of the atlas (11) can ossify completely or incompletely, thus forming a complete or incomplete bony ring (12) as seen in Table/Fig 4
2. The presence of the groove:
As can be evaluated from the comparative analysis of Table/Fig 9, in the present study, the grooves on the right and left facets were found to be in 3 cases each (10%), which was comparable with the work of Singh, but the groove on both the facets was seen in 11 cases (36.6%), thus carrying a lesser value in the north Indian population. No groove was found in 13 cases (43.3%), thus showing a higher value than that which was reported by Singh. These grooves may give rise to pressure facets which are smooth circular impressions which are present on the medial sides of the articular surfaces. These pressure facets indicate a greater pressure at these sites during movement at the atlanto-occipital joints (6). At the junction of the posterior arch, with the lateral masses holding the superior articular facets on each side, the posterior arch is hollowed out above, to form a smooth groove for the lodgment of the first part of the vertebral artery with the venous plexus around it and the suboccipital nerve (dorsal ramus of the first cervical nerve). The groove is occasionally converted into a foramen or a short canal by a bony arch (13). (as seen in Table/Fig 4) The knowledge of the quantitative anatomyof the vertebral artery groove on the posterior arch of the atlas will prove useful to the surgeons who perform operative procedures in this region and will thus help in avoiding vascular complications14 like vertebrobasilar insufficiency. There is a reduction in the blood flow in the vertebrobasilar circulation (15) and this may mainfest as a variety of signs and symptoms, from dizziness to unconsciousness and death (15). Such a compromised blood flow may be due to the serpentine course and the multiple loops of the vertebral artery with respect to the atlas (16). The vertebral artery, the first cervical nerve and the vertebral plexus which passes through the vertebral ring can be compressed, thus causing basilar insufficiency and symptoms like: pain in the temporal region and in the back of the eye, occipital headache, periodic photophobia, paraesthaesia of the hands and a sensation of pressure on the hands, thus leading to a diagnosis of: psychogenic tension , functional headache, depression, cervical pain and stress (17).
3. Tendency for Separation:The findings of the present study were compared with the previous observations, as shown in the Table/Fig 10. The tendency to separate in the present study, showed no significant difference between the right and left facets (33.3%-Lt.facet; 30%-Rt.facet and13.3%-both facets). Incomplete separation in the present study, was found to be the same in the right and left facets (6.6%), but complete separation into two was more frequent on the left side as compared to the right facet. The results of the study by Singh account for a higher value in the amount of separation in the Varanasi population as compared to those of the present study. The tendency of the SAF of the atlas to split into two (as seen in Table/Fig 5 and 6) is an indication of the further restriction of the movement at the atlanto-occipital joint (6). The incidence of the division of the SAF of the atlas is of anthropological interest (18).
4. The Shape of the Superior Articular Facet (SAF):
Table 11 depicts the different shapes of the SAF as was observed by different authors. The present study thus reported the shape of the SAF in a majority of the cases as a dumb-bell shape followed by an oval shape, a reniform and a figure of eight (F8). (See Figs.1, 2, 3 and 4) Francis (1955) gave no comparative data but defined only the shapes, whereas Singh (1965) reported a combined dumbbell and F8 as the commonest shape and Gupta (2000) defined only two shapes and found the oval as the commonest. The difference between the percentages of the types of shapes on the right and left sides of the superior articular facet was found to be statistically insignificant (p=0.536). None of the earlier authors have compared the parameters of both the sides in the available accessible literature. The occurrence of osteophytes in the vertebral column is well known to anatomists, clinicians and anthropologists. Osteophytes may appear on the atlas characteristically in the region of the superior articular facets (as seen in the Table/Fig 4). Vertebrobasilar ischaemia due to the compression of the vertebral arteries, which is caused by the osteophytes, may also occur (19).
SUMMARY AND CONCLUSIONS
The difference between the values of the present study and those observed by other authors may be due to racial factors, different living conditions or the environment. The difference between the percentages of the types of shapes on the right and left side of the superior articular facet was found to be statistically insignificant (p=0.536). None of the earlier authors have compared the parameters which are related to the shapes on both the sides in the available accessible literature. To conclude, it can be stated that: 1. Division of one facet into two or a tendency towards it is indicated by the presence of a constriction or a groove or both 2. These grooves may give rise to pressure facets which are smooth circular impressions which are present on the medial sides of the articular surfaces. In some cases, these pressure facets are so marked that the rest of the articular area looks non articular. These pressure facets indicate a greater pressure at these sites during movement at the atlanto-occipital joints. 3. The tendency of the superior articular facet of the atlas to split into two, is an indication of restricted movements at the atlanto-occipital joint 4. The superior articular facet of the atlas with a different shape i.e an oval kidney dumb-bell shape, is also an indication of the furthur restriction of the movements at the atlanto-occipital joint. 5. Congential osseous craniovertebral junction malformations are the most common compressive lesions. The presence of myriad bony configurations like the angultion or the rotation of the atlas and the asymmetry of the facet joint may lead to incomplete decompression (20). 6. The occurrence of osteophytes in the vertebral column is well known to anatomists, clinicians and anthropologists. Osteophytes may appear on the atlas characteristically in the middle of the anterior arch, either on the superior or the inferior border. They may also occur on the posterior arch. Vertebrobasilar ischaemia due to the compression of the vertebral arteries, which is caused by the osteophytes, may also occur (16). 7. Superior and inferior articular processes form a solid articular pillar that transmits some weight from one vertebra to the next lower vertebra (21). Abnormal hypertrophy of the articular facets is again a recognized cause for the narrowing of the vertebral canal and consequent neurological features (22). 8. The vertebral and the basilar arteries contribute blood not only to the brain, but also to the inner ear and their spasm may be manifested not only by neurological symptoms, but also by labyrinthine or hearing disturbances (17). 9. It is possible that extreme rotation movements at the cervical spine in an individual with an associated bony canal may result in the compression of the vertebral artery and compromised blood flow. Thus, these anatomical features should be taken into account in the therapeutic manipulation of the cervical spine (15). 10. The knowledge of the quantitative anatomy of the vertebral artery groove on the posterior arch of the atlas will prove useful to the surgeons who perform operative procedures in this region and will thus help in avoiding vascular complications (14). 11. In recent years, considerable innovations in the internal fixation techniques have created a need for a more detailed quantitative description of the anatomy of this bone (23). 12. Thus, the anatomy of the cervical vertebrae or the spine is of high clinical importance to surgeons, as a surgical procedure may be done through the anterior or posterior cervical spine with gratifying results (24).
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