PART XII THE SPINE
1646
For percutaneous pedicle screw placement, we use ¡uoros-
copy that is orthogonal to the target vertebral body in the antero-
posterior and lateral planes and allows clear visualization of the
medial wall of the pedicle and pedicle/vertebral body junction.
A Jamshidi needle typically is docked on the pedicle of inter-
est at the lamina/pedicle junction on the anteroposterior view (9
oclock position for le¥ pedicles and 3 oclock position for right
pedicles). For the most cephalad screw of a construct in the lum-
bar spine, we prefer to place the starting point slightly below
midline on the anteroposterior view (8 oclock for le¥ pedicles
and 4 oclock for right pedicles) to limit encroachment of the next
cephalad facet joint. Under anteroposterior imaging, the needle
is then advanced down the pedicle 20 to 25 mm at a trajectory
that will allow the tip of the needle to be placed at the pedicle/
vertebral body junction without violating the medial wall of the
pedicle. When seated, the needle should pass obliquely across
the pedicle with the tip just lateral to the medial wall on the
anteroposterior view and just deep to the base of the pedicle
on the lateral view. ¢is will allow passage of a guidewire into
the vertebral body and placement of a cannulated percutaneous
pedicle screw using a Seldinger technique. ¢e technique of con-
necting rod passage depends on the implant manufacturer.
Midline cortical screw placement is another technique that
can be used as a slightly less invasive means of £xation in con-
junction with a spinous process splitting or limited midline
approach and with a posterior lumbar interbody fusion when a
posterolateral fusion is not performed. ¢e initial screw start-
ing point is at the medial caudal border of the target pedicle
on true anteroposterior view of the target vertebrae (Fig. 37.6) .
A burr is used to dimple the cortical bone at the entry site. A
pedicle awl is then used to traverse the pedicle in a medial-to-
lateral and caudal-to-cranial trajectory, taking care not to vio-
late the cortical wall of the pedicle as seen on anteroposterior
and lateral ¡uoroscopy. Once the track is formed, it should be
tapped to the size of the screw that will be inserted to prevent
fracture of the thick cortical bone of the pars. A sound is used
to corm bony continuity along the screw path, and markers
can be inserted to corm position with imaging. ¢e desired
decompression and interbody fusion is then performed and
cortical bone screws placed at the end of the case.
CIRCULATION OF SPINAL CORD
¢e arterial supply to the spinal cord has been determined from
gross anatomic dissection, latex arterial injections, and intercos-
tal arteriography. Dommisse contributed signi£cantly to knowl-
edge of the blood supply, stating that the principles that govern
the blood supply of the cord are constant, whereas the patterns
vary with the individual. He emphasized the following factors:
1. Dependence on three vessels. ¢ese are the anterior median
longitudinal arterial trunk and a pair of posterolateral
trunks near the posterior nerve rootlets.
2. Relative demands of gray matter and white matter. ¢e lon-
gitudinal arterial trunks are largest in the cervical and lum-
bar regions near the ganglionic enlargements and are much
smaller in the thoracic region. ¢is is because the metabolic
demands of the gray matter are greater than those of the
white matter, which contains fewer capillary networks.
3. Medullary feeder (radicular) arteries of the cord. ¢ese arter-
ies reinforce the longitudinal arterial channels. ¢ere are 2
to 17 anteriorly and 6 to 25 posteriorly. ¢e vertebral arter-
ies supply 80% of the radicular arteries in the neck; arteries
in the thoracic and lumbar areas arise from the aorta. ¢e
lateral sacral, the £¥h lumbar, the iliolumbar, and the mid-
dle sacral arteries are important in the sacral region.
4. Supplementary source of blood supply to the spinal cord.
¢e vertebral and posterior inferior cerebellar arteries
are important sources of arterial supply. Sacral medullary
feeders arise from the lateral sacral arteries and accom-
pany the distal roots of the cauda equina. ¢e ¡ow in
these vessels seems reversible and the volume adjustable
in response to the metabolic demands.
5. Segmental arteries of the spine. At every vertebral level, a
pair of segmental arteries supplies the extraspinal and
intraspinal structures. ¢e thoracic and lumbar segmental
arteries arise from the aorta; the cervical segmental arter-
ies arise from the vertebral arteries and the costocervical
and thyrocervical trunks. In 60% of individuals, an addi-
tional source arises from the ascending pharyngeal branch
of the external carotid artery. ¢e lateral sacral arteries and,
to a lesser extent, the £¥h lumbar, iliolumbar, and middle
sacral arteries supply segmental vessels in the sacral region.
6. Distribution point” of the segmental arteries. ¢e seg-
mental arteries divide into numerous branches at the
intervertebral foramen, which has been termed the dis-
tribution point ( Fig. 37.7 ). A second anastomotic network
lies within the spinal canal in the loose connective tissue
of the extradural space. ¢is occurs at all levels, with the
greatest concentration in the cervical and lumbar regions.
¢e presence of the rich anastomotic channels o¦ers alter-
native pathways for arterial ¡ow, preserving spinal cord
circulation a¥er the ligation of segmental arteries.
7. Artery of Adamkiewicz. ¢e artery of Adamkiewicz is the
largest of the feeders of the lumbar cord; it is located on
B
C
D
A
FIGURE 37.6 Midline cortical screw placement. A, Incision. B,
Starting point at intersection of medial and caudal aspect of pedicle.
C, Cortical bone trajectory. D, Proper positioning of marker pins. (From
Mizuno M, Kuraishi K, Umeda Y, et al: Midline lumbar fusion with
cortical bone trajectory screw, Neurol Med Chir [Tokyo] 54:716, 2014.)