Download 0383 - morphology of middle and lower cervical pedicles

MORPHOLOGY OF MIDDLE AND LOWER CERVICAL PEDICLES
*Shin, E; +*Panjabi, M (A-NIH); *Chen, N; *Wang, J
+*Yale University School of Medicine, New Haven, CT. Yale University School of Medicine, Dept. of Orthopaedics and Rehabilitation, Biomechanics Research
Laboratory, P.O. Box 208071, New Haven, CT 06520-8071, (203) 785-2812, Fax: (203) 785-7069, [email protected]
Results
Superior and inferior wall cortical thicknesses were found to be similar (mean
value ranges for both: 1.5-2.4 mm). The medial cortical shell (mean value
range: 1.2-2.0 mm) was measured to be 1.4 to 3.6 times as thick as the lateral
cortical shell (mean value range: 0.4-1.1 mm) across all pedicle thin slices and
vertebral levels (Fig. 1B). The external shape of the cervical pedicle was
observed to be more elliptical than that of the cancellous core, which tended to
be more circular (Fig. 2).
Graphical comparisons of thin slices from C3 to C5 demonstrated that the
cervical pedicle was semicircular, rectangular, or even triangular on cross
section (Fig. 3). Large variability was observed not only between individual
spines and vertebral levels, but also along the pedicle axis. The cancellous
core in such specimens ranged from being finely trabeculated to nearly absent.
These variabilities were reflected in the large standard deviations of our
measurements. Thin slices from C6 and C7 exhibited greater uniformity in
size and an ovoid shape, with the slices becoming narrower as the slices
progressed antero-posteriorly along the pedicle long axis.
Discussion
The lateral cortex was significantly thinner than the medial cortex in all thin
slices and at all vertebral levels. The lateral cortex of cervical pedicles is
responsible for protecting the vertebral artery, with the exception of C7
pedicles. Thus, extreme caution must be exercised when inserting pedicle
screws so as not to perforate the thin lateral cortex of the cervical pedicle. The
extensive variability in pedicle morphometry must also be recognized as the
standard deviations for measured pedicle dimensions were found to be large.
Graphical representations of the pedicle demonstrated its complexity as a
three-dimensional structure whose features, with respect to cortical thickness,
orientation, and overall shape, changed as the slices progressed anteroposteriorly. Characteristics of the cervical pedicle at different spinal levels
must be carefully noted prior to transpedicular screw fixation to avoid cortical
wall violations. Our data shows that the effective diameter which would
permit a given screw size is much less than one would expect on the basis of
simple measurements of pedicle height and width.
CCW
CTM
mm
CTS
CCH
PDH
PDW
CCW
CTL
CTM
6.0
4.0
2.0
Cancellous
Core
0.0
C3
Cortical
Shell
A
C4
C5
C6
Vertebral Level
C7
B
Figure 1. Measured dimensions included: pedicle height (PDH) and width
(PDW); cortical shell thicknesses for the superior (CTS), inferior (CTI),
lateral (CTL), and medial walls (CTM); and the cancellous core height (CCH)
and width (CCW).
2.5
1.58
1.59
2.0
Ratio
Methods
Twenty-six human cervical vertebrae (C3-C7) were secured to a thin
sectioning machine to produce three 0.7 mm-thick pedicle slices along its
axis. The first slice was made at the isthmus of the pedicle. To document
variation along the pedicle axis, two additional slices were obtained: one
anteriorly and another posteriorly to the first slice. Plain film radiographs of
the pedicle slices were scanned and digitized to facilitate measurement of the
internal dimensions. Computer software was specifically developed to
determine the external dimensions (i.e. pedicle height and width) and the
internal dimensions (i.e. cortical shell thicknesses of the superior, inferior,
lateral, and medial walls, and the cancellous core height and width) of cervical
pedicles (Fig. 1A).
8.0
PDW
CTL
CTI
Introduction
Transpedicular screw fixation is one of the most sophisticated procedures
currently being used to stabilize the cervical spine. Anatomic studies have
documented the external dimensions and angular parameters of the cervical
pedicle, thereby providing the spinal surgeon with relevant information for
accurate pedicle screw insertion [1,2]. Data is lacking, however, regarding its
internal architecture and cortical shell thickness along the pedicle axis. It has
been shown that screw stability and pull-out strength depend largely on the
internal characteristics of the pedicle, not on its external dimensions [3]. The
purpose of this study is two-fold: 1) To quantify the internal dimensions and
cortical shell thicknesses of middle and lower cervical pedicles, and 2) To
provide comparative graphical data which would document the variability in
pedicle morphology. Our aim is to improve the percentage of successful
surgical outcomes while minimizing risk to the patient.
1.33
1.28
1.23
1.5
1.0
0.5
CCH/
CCW
1.50
1.16
PDH/
PDW
1.03
1.05
C5
C6
1.05
0.0
C3
C4
C7
Vertebral Level
Figure 2. Ratio of pedicle height to pedicle width and cancellous core height
to cancellous core width. Elliptical shapes above and below the graph are
proportional to the mean values shown at each vertebral level.
Superior
Lateral
Medial
Inferior
Figure 3. Sample bitmapped digitized images of C5 left pedicle slices. The
extensive variability in pedicle shape is observed in these slices.
References
1. Karaikovic EE, Daubs MD, Madsen RW, Gaines RW. Morphologic
characteristics of human cervical pedicles. Spine 1997:22:493-500.
2. Panjabi MM, Duranceau J, Goel VK, Oxland T, Takata K. Cervical
human vertebrae: Quantitative three-dimensional anatomy of the middle
and lower regions. Spine 1991;16:861-9.
3. Hirano T, Hasegawa K, Takahashi H, et al. Structural characteristics of
the pedicle and its role in screw stability. Spine 1997;22:2504-10.
Acknowledgement
Funded by NIH Grant AR-42211.
Poster Session - The Spine - VALENCIA FOYER
46th Annual Meeting, Orthopaedic Research Society, March 12-15, 2000, Orlando, Florida
0383