Key-words:
Axis - lamina - morphometry - pedicle
Introduction
Traumatic upper cervical spine leads to instability in weightbearing and movement.
The results of upper cervical spine injury are neck pain, neurological deficit, vertebral
deformity, and movement disorder. Patients with these injuries cannot live normally.
Multiple surgical management of upper cervical spine injury was developed for a higher
chance of successful fusion and higher fixation rate.
C1–C2 wiring with a bone graft entails a wire between the C1 and C2 cervical spine
and the addition of bone at the posterior part of the C1–C2 for fusion. This method
has a failure rate of about 30%.[[1]]
In C1–C2 transarticular[[2]],[[3]] screw fixation, the screws are inserted from the posterior part of the C2 to the
C1 for fixation. A meta-analysis for this technique had a higher rate of fusion in
94.6% but a highly experienced surgeon is needed due to the risk of vertebral artery
injury in 3.1%.[[4]]
C1 lateral mass–C2 pedicle screws,[[5]] is a technique that starts with screws inserted from the posterior C2 to the pedicle
and vertebral body. The next step is to insert the screws into the lateral mass of
C1 and apply rods for fixation between C1 and C2. A meta-analysis revealed a 97.5%
rate of fusion that required less surgical skill than the C1–C2 transarticular screw
technique. Vertebral injury in this technique was <2% which was lower than C1–C2 transarticular
screw fixation.[[6]]
The newer technique is C1 lateral mass–C2 lamina screw fixation. In this method the
screw is applied at the contralateral posterior spinolaminar junction to the ipsilateral
lamina and C1 lateral mass screws with rods. The C2 lamina screws do not present a
risk for vertebral artery injury but they remain a risk for spinal cord injury. This
alternative method can be used when the C1–C2 transarticular screws or pedicle screws
cannot be used due to the inappropriate size or high-riding vertebral artery that
occurs in 16.5%.[[7]] Parker et al.[[8]] found that intraoperative breach occurred more frequently with C2 pedicle screws
than C2 lamina screws. Even though the 1-year durability of C2 laminar screws might
be inferior to C2 pedicle screws for subaxial fusions, they are equally effective
for axial cervical fusions. From the literature review, lack of evidence determine
intraoperative breach and durability in the C1 lateralmass-C2 lamina screw fixation
technique.
Before C2 pedicle or lamina screw fixation, the surgeon needs to evaluate the C2 morphology,
i.e., width, length, and angle, especially the width which is the most important limitation
for screw fixation because around the screws are important structures near the pedicle
(medially the spinal cord and laterally the vertebral artery) and lamina (medially
the spinal cord). In Thailand, the diameter of the smallest screw is 3.5 mm. Therefore,
the screws cannot be inserted if the width of the pedicle or lamina is smaller than
3.5 mm.
In Thai patients, knowledge on the morphology of the C2 pedicle and lamina is limited.
A previous study in 54 Thai patients reported the morphology of the C2 pedicle but
there is no study on the C2 lamina.[[9]] The purpose of this study was to assess the size and angle of the C2 pedicle and
lamina morphology. The data are vital for the planning of screw fixation to decrease
the risk of injury to the vertebral artery and spinal cord. Furthermore, this information
can be used to develop medical instruments in the future.
Materials and Methods
The study was performed in patients over 15 years old who underwent a computed tomography
(CT) scan of the cervical spine. Data were collected at the Faculty of Medicine, Prince
of Songkla University using the Picture Archiving and Communication System (PACS)
by searching the words “CT_C-spine” from January 1, 2016, to December 31, 2017. A
cervical spine CT scan was performed in all patients using a Toshiba Aquilion Prime
Model TSX-303A. The cervical spine CT scan image cuts were at a maximum of 1 mm intervals
in the axial plane and at a maximum of 3 mm intervals in the sagittal plane. The information
collected included demographic data, i.e., gender, age, weight, height, and the parameters
from the cervical spine CT images. Patients with congenital anomalies or injuries
to the C2 spine or artifacts that affected the evaluation of the cortex border of
the pedicle and lamina were excluded.
Measurements
Measurements of the C2 spine parameters, including width, height, length, and angle
of the pedicle and lamina, were performed using the PACS program (Synapse Workstation
FUJIFILM Medical Systems Stamford Connecticut USA, Inc., version 4.3.221). The measurements
were recorded as millimeters and degrees by a single author. The measurements were
repeated two times.
Pedicle parameters
Axial plane
Width – The axial and sagittal planes were opened at the same time to find the mid-portion
of the pedicle. The measurements were performed at the narrowest portion between the
outer cortex of the medial and lateral pedicle at both sides [[Figure 1]]a.
Figure 1: Measurement techniques of the axis. (a) Pedicle width (dashed line). (b) Pedicle
length (dashed line). (c) Pedicle angle
Length of screws – The length was determined from the anterior outer vertebral body
cortex, pedicle to the posterior outer cortex of the facet that was perpendicular
to the mid-portion of the pedicle width. The length must not exceed two hemivertebrae
body line [[Figure 1]]b.
Angle – The angle was measured from the length line and the two hemivertebrae body
line [[Figure 1]]c.
Sagittal plane
Height – The height was measured from the superior to the inferior outer cortex perpendicular
to the pedicle isthmus [[Figure 1]]d.
Lamina parameters
Axial plane
Width – The axial and sagittal planes were opened at the same time to find the mid-portion
of the lamina. The measurement was determined at the narrowest portion between the
outer cortex of the medial and lateral lamina at both sides [[Figure 1]]e.
Length of screws – The length was determined from the posterior outer cortex of the
contralateral spinolaminar junction to the anterior outer cortex of the facet joint
that was perpendicular to the mid-portion of the lamina width [[Figure 1]]f.
Angle – The angle was measured from the length line and the two hemivertebrae body
line [[Figure 1]]g.
Sagittal plane
Height – The height was measured from the superior to the inferior outer cortex at
the midline of the spinous process [[Figure 1]]h.
Results
From the 847 patients who consecutively had a CT C-spine exam at Songklanagarind Hospital
from 2016 to 2017, 349 patients had a 1 mm axial plane CT C-spine. Finally, 270 patients
were enrolled into the study after applying the inclusion and exclusion criteria [[Figure 2]].
Figure 2: Flow chart of inclusion and exclusion data patients
The 270 patients included 188 (69.6%) males and 82 (30.4%) females. The median age
of all patients was 45 years of age. The body mass index data revealed 107 (39.6%)
patients were normal weight (<23 kg/m2), 122 (45.1%) were overweight (≥23 kg/m2),
and 41 patients had missing data [[Table 1]].
Table 1: Demographic data (n=270)
The C2 pedicle width ranged from 1.46 to 8.72 mm with a mean ± standard deviation
(SD) of 5.51 ± 1.42 mm, the C2 pedicle length ranged from 15.87 to 31.93 mm with a
mean ± SD of 23.78 ± 3.10 mm, the C2 pedicle angle from midline ranged from 13.0°
to 57.0° with a mean ± SD of 39.04° ± 7.31°, and the C2 pedicle height ranged from
4.96 to 12.34 mm with a mean ± SD of 8.65 ± 1.04 mm [[Table 2]].
Table 2: C2 pedicle results
The C2 lamina width ranged from 2.24 to 0.13 mm with a mean ± SD of 5.88 ± 1.30 mm,
the C2 lamina length ranged from 22.39 to 43.77 mm with a mean ± SD of 32.17 ± 3.95
mm, the C2 lamina angle from the midline ranged from 33.0° to 63.5° with a mean ±
SD of 49.46° ± 5.36°, and the C2 lamina height ranged from 7.19 to 59.88 mm with a
mean ± SD of 12.27 ± 3.33 mm [[Table 3]].
Table 3: C2 lamina results
A subgroup analysis in gender revealed mean ± SD values of the C2 pedicle width of
5.74 ± 1.19 mm in males and 4.97 ± 1.20 mm in females, C2 pedicle length of 24.33
± 3.21 mm in males and 22.53 ± 2.39 mm in females, C2 pedicle height of 8.92 ± 1.01
mm in males and 8.03 ± 0.82 mm in females, and C2 lamina length of 32.92 ± 3.98 mm
in males and 30.52 ± 3.37 mm in females. The mean pedicle width, length, height, and
lamina length were greater in males than in females. The differences were statistically
significant (t-test, P < 0.05) [[Figure 3]]. A subgroup analysis in age revealed mean differences that were significant (P
< 0.05). The mean ± SD C2 pedicle left length in patients ≥45 years of age was 24.43
± 3.17 mm which was longer than in patients <45 years of age which was 23.47 ± 2.92
mm. The mean ± SD C2 lamina left length in patients ≥45 years of age was 31.60 ± 4.31
mm which was shorter than in patients <45 years of age which was 32.79 ± 4.12 mm.
A subgroup analysis in body mass index indicated no association in the pedicle and
lamina sizes.
Figure 3: Axis morphometry in males and females. (a) Right pedicle width. (b) Left pedicle
width. (c) Right pedicle length. (d) Left pedicle length. (e) Right pedicle height.
(f) Left pedicle height. (g) Right laminar length. (h) Left laminar length
Twenty-four patients (8.8%) had a pedicle width <3.5 mm (bilateral four patients and
unilateral 20 patients) that would not be safe for placement of pedicle screws. However,
they could be replaced with laminar screws since the lamina width was ≥3.5 mm [[Figure 4]].
Figure 4: Flow chart of pedicle and lamina width data (patients)
Discussion
Two methods are used to measure the sizes of the pedicle and lamina. The first uses
measurements from a CT scan and the second uses measurements directly from a cadaver.
From 33 studies, no statistical differences were observed in measuring the cervical
pedicle by either radiography or directly.[[10]]
In Thai patients, the present study found that the mean values for the C2 pedicle
were width 5.51 mm, length screw 23.78 mm, height 8.65 mm, and angulation medially
39.04° which were similar to a previous study by Bunmaprasert et al.[[9]] They measured the C2 pedicle using CT scans in 53 Thai patients. The results revealed
that the mean values were pedicle width 5.47 mm, height 7.54 mm, and angulation medially
38.95°. In Asian people, Yusof et al.[[11]] used CT scans in 80 Malay patients to measure the C2 pedicle. In 40 male patients
and in 40 female patients the overall mean pedicle width was 4.57 mm. Liu J et al.[[10]] reported the C2 pedicle parameters using CT scanners of European and USA manufacturers
from 3 studies and reported the means of width, length, and angle as 7.04 mm, 28.12
mm, and 38.66° [[Table 4]], respectively. Although a systematic review of C1‒7 by Liu J et al.[[10]] reported a comparison of races, statistically significant differences were found
between the Asian and European/USA populations only in the pedicle axis length at
C3 and C4 but the study included only 40 patients in the C2 pedicle group.
Table 4: Comparison of studies on pedicle measurements
Comparisons in gender of the pedicle width, length, and height in males tended to
be larger than females in both the Asian[[9]],[[11]] and European/USA populations.[[10]] The angles for the inserted pedicle screws in all studies were similar (38°‒ 39°
medially).[[9]],[[10]]
To date, morphology of the C2 lamina has not been reported in Thai people. This is
the first study on lamina morphology to reveal the mean values of laminar width 5.88
mm, screw length 32.18 mm, height 12.27 mm, and angulation laterally 49.46°. Kim et
al.[[12]] measured the C2 lamina in 102 Korean patients from CT scans and the mean ± SD results
were width 5.66 ± 0.22 mm, screw length 33.30 ± 2.53 mm, and angulation laterally
43.20° ± 3.00°. Ma et al.[[13]] in China measured the C2 lamina from 120 cadaveric specimens and the mean ± SD
results of the width, length, height, and angulation laterally were 5.87 mm, 26.68
mm, 12.68 mm, and 48.85°, respectively. Cassinelli et al.[[1]] in the USA measured 420 cadaveric specimens and the mean results were width –5.77
mm, length –24.6 mm, and angulation laterally –48.59° [[Table 5]].
Table 5: Comparison of studies on lamina measurements
This is the first study in lamina morphometry in Thai patients to reveal the mean
width of 5.89 mm; however, compared with previous studies in China, Korea, and the
USA, the results were similar –5.87 mm,[[13]] 5.66 mm,[[12]] and 5.77 mm,[[1]] respectively. However, the trend in the size of the lamina in width, height, and
length in males was larger than in females in all studies.[[1]],[[12]],[[13]]
Conclusion
In the Thai samples, 8.8% had a C2 pedicle width <3.5 mm which would not allow insertion
of screws; however, they could be replaced with laminar screws since the width was
≥3.5 mm. In this study, all of the patients who could not be inserted pedicle can
be replaced with lamina screws inserted. The C2 pedicle and lamina of the males tended
to be larger than in the females.
