Keywords
atlas - vertebral artery groove - posterior screw placement - lateral mass - superior
articular facet
Introduction
Atlas and axis are having unique anatomical features as compared with the rest of
the cervical vertebrae. A large number of different surgical techniques such as interlaminar
clamp, interspinous wiring, plates, and screw fixation have been currently employed
to correct the instability of the atlantoaxial complex or occipitocervical junction
caused by numerous traumatic and nontraumatic conditions. Recently, transarticular
and transpedicular screws fixation have been widely used in stabilizing the cervical
column.[1]
[2]
[3]
[4]
[5]
[6] In spite of the benefits conferred by transpedicular screw fixation in the cervical
column, controversy exists regarding its potential risks. Incorrect insertion of pedicle
screws can cause damage to adjacent vital structures such as the spinal cord, nerve
roots, cranial nerves, and vertebral arteries. Fusion of occipitocervical or atlantoaxial
spine is an accepted treatment option in upper cervical instability caused by trauma
or various disorders.[7]
[8]
[9]
[10]
[11]
[12] Clinically, iatrogenic injury to the vertebral artery during an approach to the
atlantoaxial region is rare, but it has a potential hazard.[13] Posterior screw placement techniques to the atlas lateral mass have been recently
introduced to avoid the inevitable loss of occipitocervical motion in occipitocervical
fusion and to enable posterior C1-C2 fusion in patients who are not suitable for transarticular
screw fixation due to anatomic variations, as for instance, of the vertebral artery.[8]
[9]
[12]
[14] To further improve posterior C1-C2 fusion techniques, some recent publications evaluated
the application of screws to C1 via the posterior arch.[15]
[16]
[17]
[18] Two of these studies focused on a morphometric characterization of the atlas to
minimize intraoperative malposition of the so-called C1 pedicle screws.[16]
[18] However, these studies explicitly differ in the results concerning some key anatomic
measurements, necessitating an additional evaluation of anatomic landmarks and safe
zone for the screw placement through the posterior arch of C1. Therefore, this study
was aimed to evaluate the various dimensions of the atlas quantitatively and analyze
their relationship with the vertebral artery foramen, in addition to determining the
safe sites for different surgical approaches.
Materials and Methods
This study was performed on 30 adult atlas vertebrae of Indian origin, from the department
of Anatomy. All samples were examined to ensure that the vertebrae were intact and
free from any other bony abnormalities before measurements were made. All distances
were measured by digital vernier caliper, accurate to 0.1 mm for linear measurements.
In this study, we have taken the following measurements. All the atlases studied belong
to adult cases which were measured with digital vernier caliper, accurate to 0.1 mm
for linear measurements. The different anatomical parameters measured have been shown
in [Figs. 1] and [2]. ([Table 1])
Table 1
Showing different anatomical parameters measured
|
Sr. No
|
Description of parameter
|
|
A
|
Distance between both tips of transverse process
|
|
B
|
Distance between both lateral most edge of transverse foramen, Tf–Tf
|
|
C
|
Distance between medial edge of transverse foramen, Tf–Tf
|
|
D
|
Distance from midline to medial most edge of vertebral artery groove outer cortex
(right)
|
|
E
|
Distance from midline to medial most edge of vertebral artery groove outer cortex
(Left)
|
|
F
|
Distance from midline to medial most edge of vertebral artery groove on inner cortex
(right)
|
|
G
|
Distance from midline to medial most edge of vertebral artery groove on inner cortex
(left)
|
|
H
|
Max transverse diameter of vertebral canal
|
|
I
|
AP dimension of vertebral canal, maximum
|
|
J
|
AP dimension of vertebral canal, minimum
|
|
K
|
superior articular facet length, left
|
|
L
|
superior articular facet breadth, left
|
|
M
|
superior articular facet length, right
|
|
N
|
superior articular facet width, right
|
|
O
|
Inferior articular facet length, left
|
|
P
|
Inferior articular facet breadth, left
|
|
Q
|
Inferior articular facet length, right
|
|
R
|
Inferior articular facet breadth, right
|
|
S
|
Thickness of vertebral artery groove, right
|
|
T
|
Thickness of vertebral artery groove, left
|
|
U
|
Foramen transversarium anterior posterior diameter, right
|
|
V
|
Foramen transversarium transverse diameter, right
|
|
W
|
Foramen transversarium anterior posterior diameter, left
|
|
X
|
Foramen transversarium transverse diameter, left
|
Fig. 1 (Showing the measured distances A–N) (A) Distance between both tips of transverse
process; (B) distance between both lateral most edge of transverse foramen, Tf–Tf;
(C) distance between medial edge of transverse foramen, Tf–Tf; (D) distance from midline
to medial most edge of vertebral artery groove outer cortex (right); (E) distance
from midline to medial most edge of vertebral artery groove outer cortex (left); (F)
distance from midline to medial most edge of vertebral artery groove on inner cortex
(right); (G) distance from midline to medial most edge of vertebral artery groove
on inner cortex (left); (H) Max transverse diameter of vertebral canal; (I) AP dimension
of vertebral canal, maximum; (K) superior articular facet length (left); (L) superior
articular facet breadth (left); (M) superior articular facet length (right); and (N)
superior articular facet width (right).
Fig. 2 (Showing the measured distances W, X, U, and V). (U) Foramen transversarium anterior
posterior diameter (right); (V) foramen transversarium transverse diameter (right);
(W) foramen transversarium anterior posterior diameter (left); and (X) foramen transversarium
transverse diameter (left).
Observations and Results
The observations and results have been recorded in [Table 2].
Table 2
Showing the result of measured parameters
|
Sr. No
|
Description of parameter
|
Mean (mm) ± SD
|
Range (mm)
|
|
A
|
Distance between both tips of transverse process
|
71.98 ± 4.6
|
64.28–81.1
|
|
B
|
Distance between both lateral most edge of transverse foramen, Tf–Tf
|
58.18 ± 4.26
|
51.80–65.87
|
|
C
|
Distance between medial edge of transverse foramen, Tf–Tf
|
45.38 ± 3.25
|
39.78–50.95
|
|
D
|
Distance from midline to medial most edge of vertebral artery groove outer cortex
(right)
|
24.85 ± 2.78
|
20.20–32.8
|
|
E
|
Distance from midline to medial most edge of vertebral artery groove outer cortex
(left)
|
24.39 ± 2.06
|
20.9–29.00
|
|
F
|
Distance from midline to medial most edge of vertebral artery groove on inner cortex
(right)
|
10.73 ± 2.92
|
07.8–20.00
|
|
G
|
Distance from midline to medial most edge of vertebral artery groove on inner cortex
(left)
|
09.72 ± 2.56
|
06.00–17.80
|
|
H
|
Max transverse diameter of vertebral canal
|
27.31 ± 2.74
|
22.70–34.46
|
|
I
|
AP dimension of vertebral canal, maximum
|
29.44 ± 2.54
|
23.11–35.32
|
|
J
|
AP dimension of vertebral canal, Minimum
|
28.58 ± 2.27
|
24.60–33.73
|
|
K
|
superior articular facet length (left)
|
21.84 ± 2.11
|
16.68–25.49
|
|
L
|
superior articular facet breadth (left)
|
12.19 ± 1.58
|
09.55–15.51
|
|
M
|
superior articular facet length (right)
|
22.13 ± 2.26
|
16.99–25.80
|
|
N
|
superior articular facet width (right)
|
11.82 ± 1.79
|
09.52–18.11
|
|
O
|
Inferior articular facet length (left)
|
16.67 ± 1.84
|
12.62–20.36
|
|
P
|
Inferior articular facet breadth (left)
|
16.39 ± 1.93
|
13.14–20.06
|
|
Q
|
Inferior articular facet length (right)
|
16.24 ± 1.44
|
13.67–21.16
|
|
R
|
Inferior articular facet breadth (right)
|
15.84 ± 1.83
|
12.69–19.95
|
|
S
|
Thickness of vertebral artery groove (right)
|
03.79 ± 1.08
|
01.70–07.58
|
|
T
|
Thickness of vertebral artery groove (left)
|
04.05 ± 0.86
|
02.70–06.92
|
|
U
|
Foramen transversarium anterior posterior diameter (right)
|
07.40 ± 1.13
|
05.49–09.45
|
|
V
|
Foramen transversarium transverse diameter (right)
|
05.91 ± 1.03
|
04.29–08.60
|
|
W
|
Foramen transversarium anterior posterior diameter (left)
|
06.97 ± 0.98
|
05.00–08.99
|
|
X
|
Foramen transversarium transverse diameter (left)
|
05.53 ± 0.72
|
04.13–07.02
|
Discussion
As surgical techniques and instrumentation for the treatment of unstable cervical
spine as a result of traumatic, congenital, or neoplastic disorders continue to evolve,
more knowledge about bones and surrounding anatomy is required.[19] The relationship between the vertebral arteries, atlas, and axis vertebrae have
a determining role in planning an operative approach. Various techniques such as interlaminar
clamp and hook plating, lateral screw and plate fixation, and interspinous wiring
have been described for treating cervical instability.[19] Transpedicular screw fixation is one of the most sophisticated procedures currently
in use to treat atlas and axis instabilities. Current posterior fixation techniques
at the upper cervical spine might include C1 lateral mass screws as well as stabilization
techniques through the posterior arch of C1. Regarding the biomechanical characteristics
of C1-C2 instrumentation techniques, recent investigation proved that C1 lateral mass
screws in conjunction with C2 pedicle screws achieved a similar stability compared
with Magrel C1-C2 transarticular screw fixation technique.[12]
[20] Use of transpedicular screws has been reported for treating spinal trauma, extensive
laminectomies, and destruction of bony elements by neoplasm. Although pedicle screw
has been found to provide superior fixation with the least likelihood of hardware
loosening in comparison with other surgical techniques, controversy exists regarding
its potential risks.[21] The rate of recognized vertebral artery injury was identified as 2% in the report
by Gupta and Goel,[3] 4.1% in the study by Wright and Lauryssen,[22] and 8% in the study by Madawi et al.[5] However, the actual incidence of vertebral artery injury may be higher than those
reported because of the low survey response and the possibility of unrecognized vertebral
artery injury. The actual risk of neurological deficit was only 0.2% per patient because
the contralateral uninjured vertebral artery circulation was adequate and no ischemia
was observed.[6] Gupta and Goel[3] reported that they encountered bleeding probably through a vertebral artery laceration
in 2 of 106 cases in which plate and screw technique was applied, and bleeding was
stopped after the tightening of screw in these cases.
Unicortical and bicortical lateral mass screws are inserted into the atlas directly
underneath the base of the superior arch. Even though the bicortical C1 lateral mass
screws have a higher pull out strength than unicortical lateral mass screws, one has
to consider the potential risk of an injury of the hypoglossal nerve or the internal
carotid artery from bicortical screws.[23]
[24] Screws inserted through the posterior arch of C1 into the lateral mass have a longer
trajectory compared with lateral mass screws. Because of this, the so-called pedicle
screws inserted through the posterior arch of C1 has a superior biomechanical stability
than lateral mass screws.[17] In addition to the larger pull out strength screws placed through the posterior
arch into C1, a main argument for preferring this instrumentation technique is to
avoid an excessive venous bleeding from the venous plexus around the C2 root during
the classical subarcuate procedure in the placement of lateral mass screws.
According to our study, the mean distance between both transverse processes of atlas
was 71.98 ± 4.6 mm with a range from 64.28 to 81.10 mm. The mean distance between
the outermost edges of the transverse foramens was 58.18 ± 4.26 mm with a range from
51.80 to 65.87 mm; the mean distance between the innermost edges of the transverse
foramens was 45.38 mm with a range from 39.78 to 50.95 mm. Lang[25] reported that the mean distance between the transverse processes was 78.2 mm; the
mean distance between the outermost edges of the transverse foramens was 64 mm; and
the mean distance between the innermost edges of the transverse foramens was 52.3
mm. The transverse foramen through which the vertebral artery passes lies lateral
to the transverse process of C1. Immediately behind the superior articular facet is
a transverse groove for the vertebral artery. The articular process usually overhangs
this groove anteriorly. There is often a bony bridge over the course of vertebral
artery.[25] Ebraheim et al[13] suggested that dissection of soft tissue attachments on the posterior arch of C1
was limited to 8 to 12 mm. Anatomically, the bony groove on the superior surface of
the posterior arch of C1 represents the exact location of the vertebral artery. Damage
to the vertebral artery can be avoided, if exposure of the posterior arch of C1 remains
medial to the groove.[13] In our study, we found the thickness of the vertebral artery groove on C1 to be
03.79 ± 1.08 mm with a range of 01.70 to 07.58 on right side and 04.05 ± 00.86 mm
with a range of 02.70 to 06.92 mm on left side. This thickness is satisfactory for
applying some fixation techniques such as clamp and hook plating, and anatomical wiring.
It implies the role of adequate study of microanatomy by fine computed tomographic
cuts to choose the size of the screw or plan the type of fixation which needs to be
chosen from case to case. Thickness of the vertebral artery groove on the atlas was
found by Ebraheim et al to be 3 to 5 mm.[13] Sengul and Kadioglu[19] revealed that the range from the sagittal midline to the inner most edge of the
vertebral artery was 11 mm for left side with a minimum of 9 mm for both the sides
and suggested that the dissection on the posterior arch of the C1 should be limited
to 10 mm to prevent injury to the vertebral artery during dissection through the posterior
approach. According to our study, the range from the sagittal midline to the innermost
edge of the vertebral artery groove is found to be from 07.8 to 20.00 mm with a mean
of 10.73 ± 2.92 mm on the right side and from 06.00 to 17.8 mm with a mean of 09.72 ± 2.56 mm
on the left side. From our study, we can say that dissection on the posterior arch
of C1 can be extended to 12 mm from the midline through the posterior approach.
The shape of the superior facet of the atlas was generally ovoid. Miller and Ramage
et al[4] stated that a kidney-shaped facet of C1 was not frequent and they were not symmetric
as mirror images on both sides. Gupta and Goel[3] found kidney-shaped superior facets in 24% of facets and they were not mirror symmetric.
Sengul and Kadioglu[19] found 72% of superior facets to be of oval-shaped; only 28% were kidney-shaped and
none of the facets were exactly similar to each other, in their study. In our study,
74% are oval in shape and 26% are kidney-shaped. These also are not mirror symmetric.
This is in accordance with the previous study. Doherty and Heggeness[26] studied the vertebral canal and the arches of 88 dried human C1 vertebrae. They
found that the canal diameter ranged from 32 mm in transverse trajectory, and 29 mm
in AP trajectory. In the study by Lang,[25] these dimensions were 30.2 and 34.5 mm, respectively. In the study presented here,
the transverse diameter of vertebral canal of C1 had a mean of 27.31 ± 2.74 mm with
a range of 22.70 to 34.46 mm; and the maximum AP dimension of the vertebral canal
had a mean of 29.44 ± 2.54 mm with a range of 23.11 to 35.32 mm.
Conclusion
We found that the transverse diameter of vertebral canal of C1 had a mean of 27.31 ± 2.74 mm
with a range of 22.70 to 34.46 mm; and the maximum AP dimension of the vertebral canal
had a mean of 29.44 ± 2.54 mm with a range of 23.11 to 35.32 mm. The thickness of
the vertebral artery groove on C1 is 03.79 ± 1.08 mm with a range of 01.70 to 07.58 mm
on the right side and 04.05 ± 00.86 mm with a range of 02.70 to 06.92 mm on the left
side. The range from sagittal midline to the innermost edge of the vertebral artery
groove is found to be 07.8 to 20.00 mm with a mean of 10.73 ± 2.92 mm on the right
side and 06.00 to 17.8 mm with a mean of 09.72 ± 2.56 mm on the left side. Overall,
74% of superior articular facets are oval in shape and 26% in kidney shape. None of
them are exactly similar to each other. On the basis of these findings, we concluded
that this thickness is satisfactory for applying some fixation techniques such as
clamp and hook plating, and anatomical wiring and the dissection on the posterior
arch of C1 can be extended to 12 mm from the midline through the posterior approach.
