PART XII THE SPINE
1644
TABLE 37.1
Ascending and Descending (Motor) Tracts
NUMBER ( FIG. 37.3 ) PATH FUNCTION SIDE OF BODY
1 Anterior corticospinal tract Skilled movement Opposite
2 Vestibulospinal tract Facilitates extensor muscle tone Same
3 Lateral corticospinal (pyramidal tract) Skilled movement Same
4 Dorsolateral fasciculus Pain and temperature Bidirectional
5 Fasciculus proprius Short spinal connections Bidirectional
6 Fasciculus gracilis Position/fine touch Same
7 Fasciculus cuneatus Position/fine touch Same
8 Lateral spinothalamic tract Pain and temperature Opposite
9 Anterior spinothalamic tract Light touch Opposite
Modified from Patton HD, Sundsten JW, Crill WE, Swanson PD, editors: Introduction to basic neurology , Philadelphia, 1976, WB Saunders.
every level. Screw placement in the pedicles at C3, C4, and C5
requires smaller screws (<4.5 mm) and more care in placement
than those of the other cervical vertebrae. CT measurements of
cervical pedicle morphology found that C2 and C7 pedicles had
larger mean interdiameters than all other cervical vertebrae, and
that C3 had the smallest mean interdiameter. ¢e outer pedi-
cle width-to-height ratio increased from C2 to C7, indicating
that pedicles in the upper cervical spine (C2-4) are elongated,
whereas pedicles in the lower cervical spine (C6-7) are rounded.
It also is crucial to know that cervical pedicles angle medially at
all levels, with the most medial angulation at C5 and the least at
C2 and C7. ¢e pedicles slope upward at C2 and C3, are parallel
at C4 and C5, and are angled downward at C6 and C7.
¢e vertebral artery from C3 to C6 is at signi£cant risk for
iatrogenic injury during pedicle screw placement. ¢e pedicle
cortex is not uniformly thick. ¢e thinnest portion of the cor-
tex (the lateral cortex) protects the vertebral artery, and the
medial cortex toward the spinal cord is almost twice as thick
as the lateral cortex. Variations in the course of the vertebral
artery also place it at risk during placement of pedicle screws.
At the C2 and C7-T1 levels, the vertebral artery is less at risk
during pedicle screw £xation. ¢e vertebral artery follows a
more posterior and lateral course at C2, whereas at C7-T1 it
is outside the transverse foramen.
Pedicle dimensions and angles change progressively
from the upper thoracic spine distally. A thorough knowl-
edge of these relationships is important when considering
the use of the pedicle as a screw purchase site. A study of
2905 pedicle measurements made from T1 to L5 found that
pedicles were widest at L5 and narrowest at T5 in the hori-
zontal plane ( Fig. 37.4 ). ¢e widest pedicles in the sagittal
plane were at T11, and the narrowest were at T1. Because
of the oval shape of the pedicle, the sagittal plane width was
generally larger than the horizontal plane width. ¢e larg-
est pedicle angle in the horizontal plane was at L5. In the
sagittal plane, the pedicles angle caudal at L5 and cephalad
at L3-T1. ¢e depth to the anterior cortex was signi£cantly
longer along the pedicle axis than along a line parallel to the
midline of the vertebral body at all levels except T12 and L1.
¢e thoracic pedicle is a convoluted, three-dimensional
structure that is £lled mostly with cancellous bone (62% to
79%). Panjabi et§al. showed that the cortical shell is of variable
density throughout its perimeter and that the lateral wall is
signi£cantly thinner than the medial wall. ¢is seemed to be
true for all levels of thoracic vertebrae.
¢e locations for screw insertion have been identi£ed
and described in several studies. ¢e respective facet joint
space and the middle of the transverse process are the most
important reference points. An opening is made in the ped-
icle with a drill or handheld curet, a¥er which a self-tapping
screw is passed through the pedicle into the vertebral body.
¢e pedicles of the thoracic and lumbar vertebrae are tube-
like bony structures that connect the anterior and posterior
columns of the spine. Medial to the medial wall of the pedicle
lies the dural sac. Inferior to the medial wall of the pedicle is
the nerve root in the neural foramen. ¢e lumbar roots usu-
ally are situated in the upper third of the foramen; it is more
dangerous to penetrate the pedicle medially or inferiorly as
opposed to laterally or superiorly.
We use three techniques for open localization of the pedi-
cle: (1) the intersection technique, (2) the pars interarticularis
technique, and (3) the mammillary process technique. It is
important in preoperative planning to assess individual spinal
anatomy with the use of high-quality anteroposterior and lat-
eral radiographs of the lumbar and thoracic spine and axial CT
or MRI at the level of the pedicle. In the lumbar spine, coaxial
¡uoroscopy images are a reliable guide to the true bony cortex
of the pedicle. ¢e intersection technique is perhaps the most
commonly used method of localizing the pedicle. It involves
dropping a line from the lateral aspect of the facet joint, which
intersects a line that bisects the transverse process at a spot
overlying the pedicle ( Fig. 37.5 ). ¢e pars interarticularis is the
area of bone where the pedicle connects to the lamina. Because
the laminae and the pars interarticularis can be identi£ed easily
at surgery, they provide landmarks by which a pedicular drill
starting point can be made. ¢e mammillary process technique
is based on a small prominence of bone at the base of the trans-
verse process. ¢is mammillary process can be used as a start-
ing point for transpedicular drilling. Usually, the mammillary
process is more lateral than the intersection technique starting
point, which also is more lateral than the pars interarticularis
starting point. ¢us, di¦erent angles must be used when drill-
ing from these sites. With the help of preoperative CT scanning
or MRI at the level of the pedicle and intraoperative ¡uoros-
copy, the angle of the pedicle to the sagittal and horizontal
planes can be determined.