SpineOCT2004
9/23/04
2:41 PM
Page 306
J Neurosurg (Spine 1) 3:306–310, 2004
An anatomical study of the C-2 pedicle
SAIT NADERI, M.D., CANDAN ARMAN, PH.D., MUSTAFA GÜVENÇER, M.D., PH.D.,
ESIN KORMAN, PH.D., MEHMET ŞENOĞLU, M.D., SÜLEYMAN TETIK, M.D.,
AND NURI ARDA, M.D.
Departments of Neurosurgery and Anatomy, Dokuz Eylül University School of Medicine, Izmir, Turkey
Object. The C-2 pedicle plays an important role regarding screw purchase for spinal fixation. The aim of this study was
to measure the C-2 pedicle–related linear and angular parameters.
Methods. Seven parameters in 160 C-2 pedicles (80 dry vertebrae) were measured using a Vernier caliper (accurate to
0.1 mm) and goniometer. The Student t-test was used to determine statistical significance.
The authors found that the C-2 isthmus (pars interarticularis) and the C-2 pedicle are distinct structures. The C-2 isthmus covers the pedicle. The isthmus is present between the superior and inferior articular processes, and the pedicle is the
structure beneath the C-2 isthmus. It connects the lateral mass–inferior articular process to the body of the axis. The heights
of the right and the left C-2 pediculoisthmic components (PICs) were 10.3 6 1.6 and 9.9 6 1.5 mm, respectively. The posterior part of the superior aspect of the PIC was wider than the anterior portion. The widths of the posterosuperior aspect
of the PIC were 11.1 6 2 and 11 6 1.7 mm on the right and left sides, whereas the widths of the anterosuperior aspect of
the PIC were 7.9 6 1.7 and 8.5 6 1.6 mm, respectively. The inferior widths of this component were 6.0 6 1.5 and 5.5 6
1.3 mm on the right and left side, respectively. The lengths of the component were 28.8 6 2.9 mm on the right and
28.8 6 3.4 mm on the left side. The PIC exhibits a lateral-to-medial and an inferior-to-superior angle. Its axial angles were
28.4 6 2.5 and 28.6 6 2.2˚ on the right and left sides, respectively; its sagittal angles were 18.8 6 2.1 and 18.8 6 1.7˚,
respectively.
Conclusions. The C-2 pedicle can be seen in the inferior aspect of the vertebra, and it connects posterior vertebral elements (that is, the lateral mass and inferior articular process) to the axial body. The isthmus drapes the pedicle. The authors
suggest that this be termed “the pediculoisthmic component.”
KEY WORDS • cervical spine • axis • isthmus • pedicle • axial
axial vertebra has been the focus of several decompressive and stabilization surgical procedures
for various craniovertebral junction disorders. A
fixation procedure (for example, screw, wire, hook) involving different implant–bone interfaces may be necessary to stabilize the unstable craniovertebral junction. The
C-2 pedicle has been recently used for screw fixation;1,3
however, there is controversy regarding the exact definition of the C-2 pedicle. The aim of this study is to define
the axial pedicle, axial isthmus (pars interarticularis), and
to determine their linear and angular aspects.
T
HE
Materials and Methods
We measured five linear and two angular parameters in 160 C-2
pedicles (80 dry C-2 vertebrae) by using a Vernier caliper, accurate
to 0.1 mm, and a goniometer. The following C-2 pedicle–isthmus
parameters were measured: anterosuperior width (also known as the
width of anterior part of the pars interarticularis or isthmus) (Fig. 1
left); posterosuperior width (also known as the posterior width of
the pars interarticularis or isthmus) (Fig. 1 left); inferior width (also
known as the pedicle width) (Fig. 1 right); height (Fig. 1 right);
axial (mediolateral) angle (that between the long axis of the PIC, as
described below, and sagittal plane) (Fig. 2); sagittal (rostrocaudal)
Abbreviations used in this paper: PIC = pediculoisthmic component; VB = vertebral body.
306
angle (that between the long axis of PIC and horizontal sagittal
plane) (Fig. 2); and the PIC length (Fig. 1 left).
The length, axial and sagittal orientation of the PIC, and the direction required for screw placement were measured from a consistent point, as previously reported15 (a point 5 mm inferior to the
superior border of the C-2 lamina and 7 mm lateral to the lateral
border of the spinal canal). For the purpose of this study, the length
of the PIC was defined as the distance between the screw entry point
and the junction of the PIC and C-2 VB.
Statistical analysis was performed using the Student t-test.
Results
Axial Pedicle and Isthmus
In this study the C-2 pedicle was defined as a portion of
the axial lateral mass–inferior articular process complex
connecting to the VB of the axis, seen in the inferior aspect
of C-2; the isthmus, on the other hand, was described as a
portion between the anterior and posterior articular processes seen in the superior aspect. Although the C-2 isthmus and pedicle are distinct structures, they are so integrated that it is more appropriate to call them PIC.
The heights of the right and the left C-2 PIC (in millimeters) were 10.3 6 1.6 (range 7.3–15.9) and 9.9 6 1.5
(range 6.3–14.6), respectively (p = 0.137 and p . 0.05)
(Fig. 1 right). The posterior portion of the superior aspect
of the PIC was wider than the anterior part (Fig. 1 left).
J. Neurosurg: Spine / Volume 1 / October, 2004
SpineOCT2004
9/23/04
2:41 PM
Page 307
Anatomical study of the C-2 pedicle
FIG. 1. Left: The superior view of the C-2 vertebra and the measured parameter. The anterosuperior
width (a), posterosuperior width (b), and length (c) of
the C-2 PIC. Right: The inferior (upper) and lateral
(lower) view of the C-2 vertebra and the measured
parameter. a = The PIC inferior width (also known as
pedicle width); b = the PIC height.
The posterosuperior widths (in millimeters) of this component were 11.1 6 2 (range 6.8–16.4) and 11 6 1.7
(range 7.5–15.1) on the right and left side, respectively (p =
0.652 and p . 0.05). The anterosuperior widths of this
component (in millimeters) were 7.9 6 1.7 (range
4.2–11.7) and 8.5 6 1.6 (range 4.7–13.6) on the right and
left sides, respectively (p = 0.036 and p . 0.05) (Fig. 1
left). The inferior widths of this component (in millimeters) were 6 6 1.5 (range 2.4–9.6) and 5.5 6 1.3 (range
2.7–9.3) on the right and left sides, respectively (p =
0.038, p . 0.05) (Fig. 1 right). The length of the component (in millimeters) was 28.8 6 2.9 (range 24–35) on the
right and 28.8 6 3.4 (range 24–36) on the left sides,
respectively (p = 0.54 and p . 0.05) (Fig. 1 left). Axial
angles (in degrees) were 28.4 6 2.5 (range 24–33) and
28.6 6 2.2 (range 26–32) on the right and left sides,
respectively (p = 0.632 and p . 0.05) (Fig. 2). Sagittal
angles (in degrees) were 18.8 6 2.1 (range 16–22) and
18.8 6 1.7 (range 16–21) on the right and left sides,
respectively (p = 0.48 and p . 0.05) (Fig. 2 and Table 1).
Discussion
The location of the C-2 pedicle remains a subject of
controversy. Although some authors have reported that the
pedicle connects the VB to the superior articular process,2,3 others have defined the pedicle as the portion beneath and posterior to the superior facet.4,5,7,8,11–13
A pedicle is a portion of the spine connecting the ventral and dorsal elements. Although this is valid for all subaxial vertebrae, the C-2 pedicles are anatomically unique.
Although the superior articular process is a posterior element of the vertebra (that is, posterior to VB in the axial
plane) in all subaxial vertebrae, an inspection of C-2
reveals that its superior articular process is not anatomiJ. Neurosurg: Spine / Volume 1 / October, 2004
cally posterior to the VB. In other words, the axial superior articular process may not be accepted as a posterior element of C-2. Therefore, we disagree with the belief that
the C-2 pedicle is a portion connecting the VB to the superior articular process. The axial pedicle, in this study, was
described as the aspect between the VB and inferior articular process–lateral mass complex (Fig. 3).
We also found that C-2 does not have an isolated pedicle, which can be observed in the subaxial vertebrae.
There exists a complex containing the C-2 pedicle inferiorly and the isthmus superiorly (Fig. 3). Whereas the isthmus connects the superior and inferior articular process,
the true pedicle connects the lateral mass–inferior articular process to the VB–odontoid process junction. The C-2
pedicle is covered by the facet joint and isthmus (Fig. 3).
Therefore, it is more appropriate to term these two components as the PIC. An embryological study may provide
insight into the process of ossification involving the PIC.
The axial PIC is posterolateral to the VB, medial to the
transverse foramen, originates posterolaterally from the
lateral mass–inferior articular process junction, and ends
anteromedially at the VB–odontoid process junction. It is
grooved laterally by the transverse foramen (Fig. 3).
The PIC exhibits a lateral-to-medial inclination and inferior-to-superior angulations (Fig. 2). The lateral-tomedial inclination of the pedicle was reported to be 35.2˚
by Howington, et al.,9 and 33˚ by Xu, et al.14,15 The projection point of the pedicle in both aforementioned studies
was reported to be 5 mm inferior to the superior border of
the lamina and 7 mm lateral to the lateral border of the
spinal canal. Based on the same posterior projection point,
this angle was found to be 28˚ in the present study; however, when penetration of the transverse foramen in some
specimens is considered, a more (2–3˚) medially oriented
screw projection may be more adequate. The inferior-to307
SpineOCT2004
9/23/04
2:41 PM
Page 308
S. Naderi, et al.
length, and anterior isoaxial length. Howington, et al.,9
examined 10 C-2 segments. They described the pedicle
length as “the distance from the posterior surface of the inferior articular process to the junction of the pedicle with
the body of axis,” which they reported to be 16.6 mm.
In an examination of 50 C-2 specimens, Xu, et al.,15
described pedicle length as “the distance between the
anterior most point of the pedicle axis and the posterior
point of pedicle axis projection,” which they recorded as
25.6 mm in males and 25.5 mm in females.
Karaikovic, et al.,10 reported the following axial measurements: pedicle axis length, 25.4 mm; lateral mass
pedicle length, 16.7 mm; and pedicle length, 7.1 mm.
In our PIC series length was 28.8 mm (Fig. 1 and Table 1).
The C-2 PIC Width
FIG. 2. The axial (upper) and sagittal (lower) angle of the C-2
PIC (a = axial [mediolateral] angle; b = sagittal [rostrocaudal]
angle).
superior angulations of the C-2 pedicle have been reported to be 38.8,9 20.2,14,15 and 18˚ in the present series.
It is clear that the main factors affecting the degree of
the angles include the projection point of the pedicle axis
and the position of the vertebra (that is, the intraoperative
position of the axis in prone position).
Foley7 has reported that, whereas the mean pedicle
screw axial and sagittal trajectories (in degrees) were 45
medial (range 24–56) and 41 cephalad (range 23–58), respectively, the mean values for the pars screws (in degrees)
were 7.3 medial (10 lateral–21 medial) and 58.2 cephalad
(range 49–68), respectively. These measurements differ
from ours.
Knowledge of exact location of the C-2 pedicle is
mandatory to understand its pedicle anatomy (that is,
pedicle length, height, width, and angles). The different
perceptions of the C-2 pedicular parameters affect the
results reported in the literature.
The use of different terminology causes confusion
when attempting to determine width of the pedicle and the
isthmus. Howington, et al.,9 reported the width of the C-2
pedicle (that is, inferior aspect of the PIC) to be 8.6 and
7.9 mm in males and females, respectively, whereas Xu
and Ebraheim14 reported the mean width to be 7.9 mm.
Karaikovic, et al.,10 reported the width of the axial isthmus (that is, superior aspect of the PIC) to be 6.9 mm,
whereas we found this measurement to be 11.1 mm.
In an examination of 40 C-2 specimens, Ebraheim, et
al.,6 evaluated the pedicle and transverse foramen. They
measured the superior and inferior portions of the pedicle
separately and reported that the pedicle was grooved
deeply by the transverse foramen anterolaterally, which
caused a dimensional difference between the superior
aspect of the C-2 “pedicle” (that is, isthmus) and inferior
aspect of the C-2 pedicle (true pedicle). They indicated
that the superior dimension of the C-2 pedicle was approximately 3 mm greater than its inferior diameter.
The results obtained in the present study are in line with
those reported by Ebraheim, et al.6 According to our findings, the superior aspect of the C-2 PIC is wider than its
inferior aspect (11 and 6 mm, respectively). On the other
hand, the posterior part of the superior aspect of the PIC is
also wider than its anterior part (11 and 8 mm, respectively). The variable diameters of its different portions should
be taken into consideration during C-2 pedicle screw fixation.
The C-2 PIC Height
We determined the PIC to be 10.3 and 9.9 mm in height
on the right and the left sides, respectively (Table 1 and
Fig. 2). These values are not different from the other
reported heights. The heights of the C-2 pedicle were reported to be 7.7 and 6.9 mm in males and females, respectively, by Xu, et al.,14 7.2 and 6.1 mm, respectively, by Karaikovic, et al.,10 and 10.5 and 9.1 mm, respectively, by
Howington, et al.9
Surgery-Related Considerations
The C-2 PIC Length
The measured length of the C-2 pedicle may differ
depending on technique. Because the measured lengths
vary, including pedicle axis length, lateral mass pedicle
308
In this original study we focused on the C-2 PIC anatomy in a large number of specimens. Our goal was to
address the confusion regarding the terminology of this
structure. Typically, confusion in terminology arises from
J. Neurosurg: Spine / Volume 1 / October, 2004
SpineOCT2004
9/23/04
2:41 PM
Page 309
Anatomical study of the C-2 pedicle
TABLE 1
Summary of C-2 PIC measurements*
PIC Aspect
width (mm)
anterosuperior
rt
lt
posterosuperior
rt
lt
inferior
rt
lt
height
rt
lt
angle (˚)
axial (mediolateral)
rt
lt
sagittal (rostrocaudal)
rt
lt
length (mm)
rt
lt
Mean
SD
Min
Max
p Value
7.9
8.5
1.7
1.6
4.2
4.7
11.7
13.6
0.036
11.1
11.0
2.0
1.7
6.8
7.5
16.4
15.1
0.652
6.0
5.5
1.5
1.3
2.4
2.7
9.6
9.3
0.038
10.3
9.9
1.6
1.5
7.3
6.3
15.9
14.6
0.137
28.4
28.6
2.5
2.2
24.0
26.0
33.0
32.0
0.632
18.7
18.8
2.1
1.7
16.0
16.0
22.0
21.0
0.480
28.8
28.8
2.9
3.4
24.0
24.0
35.0
36.0
0.540
* SD = standard deviation.
the anatomical confusion. Such confusion may lead to
inaccurate surgical technique.
The definition of the C-2 pedicle and isthmus provides
the surgeon with a better understanding of the anatomy for
safer axial screw fixation. Because of the close relationship between the C-2 pedicle and isthmus, the term transpediculoisthmic fixation is more appropriate than transpedicular fixation.
Xu and Ebraheim14 reported that the safest angles for
transpediculoisthmic screw placement were 35˚ medially
and 20˚ rostrally. Using the same posterior screw entry
points as Xu and Ebraheim, we tested 28˚ axial and 18˚
sagittal orientations for the safe placement of transpediculoisthmic screws; we found that a more axially oriented
screw trajectory may be more appropriate for safe screw
placement. After using approximately the same posterior
screw entry points, Abumi1 recommended placement of
screws parallel or slightly cephalad to the C2–3 disc in the
sagittal plane.
The height of the C-2 PIC is great enough to accommodate a 3.5-mm screw safely. The superior width of the PIC
is almost 2 to 5 mm larger than the inferior width of the
PIC. This difference should be kept in mind during C-2
pedicle screw fixation.
Ebraheim, et al.,6 have reported that the lateral wall of
the C-2 pedicle is thinner than the medial wall and may be
vulnerable to injury during the pedicle screw placement.
This point also requires consideration during C-2 pedicle
screw placement.
Conclusions
The axial pedicle and the isthmus are distinct structures.
Unlike the pedicles of the subaxial vertebrae, the C-2
pedicle is visible on the vertebra’s inferior aspect. It connects the posterior vertebral elements (lateral mass and
inferior articular process) to the axial VB. The pedicle is
covered by and integrated with the isthmus. Therefore, we
suggest the term PIC better defines this structure than the
C-2 pedicle (Fig. 3).
References
FIG. 3. The location of the C-2 pedicle and the isthmus. Note
that the red area indicates the C-2 pedicle and the blue area, the
isthmus: A: Superior view; B: inferior view; C: lateral view.
J. Neurosurg: Spine / Volume 1 / October, 2004
1. Abumi K: Point of view: anatomic consideration of C2 pedicle
screw placement. Spine 21:694–695, 1996
2. Benzel EC: Anatomic consideration of C2 pedicle screw placement. Spine 21:2301–2302, 1996 (Letter)
309
SpineOCT2004
9/23/04
2:41 PM
Page 310
S. Naderi, et al.
3. Borne GM, Bedou GL, Pinaudeau M: Treatment of pedicular
fractures of the axis. A clinical study and screw fixation technique. J Neurosurg 60:88–93, 1984
4. de Oliveira E, Rhoton AL Jr, Peace D: Microsurgical anatomy
of the region of the foramen magnum. Surg Neurol 24:
293–352, 1985
5. Ebraheim NA, Fow J, Xu R, et al: The location of the pedicle
and pars interarticularis in the axis. Spine 26:E 34–E37, 2001
6. Ebraheim NA, Xu R, Lin D, et al: Quantitative anatomy of the
transverse foramen and pedicle of the axis. J Spinal Disord
11:521–525, 1998
7. Ebraheim N, Rollins JR, Xu R, et al: Anatomic consideration of
C2 pedicle screw placement. Spine 21:691–694, 1996
8. Foley KT: A morphometric study of the C2 pedicle and pars—
Part 1. (http://www.spineuniverse.com/displayarticle.php/article
423.html) [Accessed 4 August 2004]
9. Howington JU, Kruse JJ, Awasthi D: Surgical anatomy of the
C-2 pedicle. J Neurosurg (Spine 1) 95:88–92, 2001
10. Karaikovic EE, Daubs MD, Madsen RW, et al: Morphologic
characteristics of human cervical pedicles. Spine 22:493–500,
1997
11. Panjabi MM, Duranceau J, Goel V, et al: Cervical human ver-
310
12.
13.
14.
15.
tebrae. Quantitative three-dimensional anatomy of middle and
lower regions. Spine 16:861–869, 1991
Roy-Camille R, Saillant G, Mazel C: Internal fixation of the
unstable cervical spine by a posterior osteosynthesis with plates
and screws, in Cervical Spine Research Society (eds): The Cervical Spine, ed 2. Philadelphia: JB Lippincott, 1989, pp 390–430
Smith MD, Anderson P, Grady MS: Occipitocervical arthrodesis using contoured plate fixation. An early report in a versatile
fixation technique. Spine 18:1984–1990, 1993
Xu R, Ebraheim NA: Essential bony anatomy of the cervical
spine relative to instrumentation. Neurosurg Q 8:172–179, 1998
Xu R, Nadaud MC, Ebraheim NA, et al: Morphology of the second cervical vertebra and the posterior projection of the C2
pedicle axis. Spine 20:259–263, 1995
Manuscript received December 16, 2003.
Accepted in final form May 25, 2004.
Address reprint requests to: Sait Naderi, M.D., Department of
Neurosurgery, Dokuz Eylul University Hospital, Inciraltı, 35340,
Izmir, Turkey. email: [email protected].
J. Neurosurg: Spine / Volume 1 / October, 2004