Posterior spinal fusion is performed with unit rod
instrumentation6,7.
Special considerations are required when a posterior spinal fusion with a unit
rod is performed in patients with a hyperkyphotic and/or hyperlordotic spinal
deformity.
Patients who are found to have a rigid kyphotic curve on physical
examination or who have a hyperkyphosis that cannot be corrected to within
40° to 60° on stress radiographs are treated with anterior release
(disc resection) prior to the posterior spinal fusion. All patients with
hyperlordosis in this series had a rigid deformity and underwent anterior
closing-wedge osteotomies (disc removal with removal of some of the vertebrae
on either side of the disc). This step is critical in the treatment of rigid
sagittal deformities to restore flexibility, and therefore correctability, to
the spine.
The anesthesiologist should prepare for the possibility of substantial
bleeding (one to two blood volumes is not uncommon). Preoperative preparation
should include placement of an arterial line, two large (18 gauge or larger)
peripheral intravenous lines, and a central venous line and ensuring the
availability of adequate blood, fresh-frozen plasma, and platelets for
replacement. A core body temperature of 35°C should be maintained with a
heating device. In addition, placement of a Foley catheter to monitor urine
output and placement of a nasogastric tube to decompress the stomach are
recommended.
The patient is placed in the prone position on a Relton-Hall (or
equivalent) spinal frame. More recently, a radiolucent frame (Jackson table)
has been helpful to assess placement of the pelvic limbs of the rod. For
patients with hyperlordosis, it is important that their legs hang from the
frame as low as possible to allow for maximum passive correction of anterior
pelvic tilt and the hyperlordosis (Fig.
1).
A routine posterior approach through a midline incision, exposing the spine
from the first thoracic vertebra to the sacrum, is employed. Each vertebra is
exposed subperiosteally out to the lateral tips of the transverse processes.
The posterior outer wing of the ilium is next exposed subperiosteally to the
sciatic notch including the posterior superior iliac spine. The left and right
drill-guides are inserted subperiosteally into the left and right sciatic
notches, respectively. The drill-hole should be started as far inferior on the
posterior superior iliac crest as possible. The lateral handle of the
drill-guide is kept parallel with the pelvis, and the axial handle is kept
parallel with the body axis. Next, a drill-hole is made into the pelvis,
aiming it just anterior to the sciatic notch
(Fig. 2). Drill-hole placement
in this portion of the pelvis is important because it provides the strongest
fixation in the bone. The remainder of the pelvis may be very osteoporotic in
many of these children who are frequently not able to walk. The drill-holes
are temporarily plugged with gelfoam to assist with hemostasis.
Next, the spinous processes are cut off to expose the ligamentum flavum in
a manner to maximally preserve the laminae, especially the superior aspect of
each vertebra in the thoracic and lumbar spine
(Fig. 3). This is extremely
important to minimize wire pullout. The ligamentum flavum is removed to expose
the sublaminar space, and a single 16-gauge double Luque wire is passed at
each level from T2 to L4 (Fig.
4). Two double wires are passed at T1 and L5. The rod is available
in lengths ranging from 270 to 450 mm in 2-cm increments. The distance from T1
to the drill-hole entrance site is measured, and the proper rod size is
selected (see the special considerations for rod length selection described
later) (Fig. 5). Next, the
pelvic limbs of the unit rod are crossed and inserted into the drill-holes
(Fig. 6). The rod limbs are
alternately gradually impacted into the pelvis (see the special considerations
for hyperlordosis described later). A rod holder should be used to guide the
pelvic limb of the rod in the correct direction of the previously drilled
holes so as to avoid penetration of the pelvic cortex.
With a hyperkyphotic deformity, the space between the posterior elements of
the vertebrae increases as the facet joints slide apart from one another. As
the hyperkyphosis is corrected, the facet joints overlap again and the spine
shortens, returning to its normal length
(Fig. 7). Accordingly, the size
of the unit rod required for implantation should be approximately 1 to 2 cm
shorter than the measured length of the spine. In addition, patients with a
hyperkyphotic deformity may have an impeded venous return to the heart when
the surgeon applies pressure to the apex of the curve during correction of the
deformity. This causes a drop in the mean arterial pressure. In these
patients, decreasing the pressure on the curve apex while increasing the
administration of intravenous fluids allows the mean arterial pressure to
return to baseline. After the blood pressure has remained normalized for five
to ten minutes, corrective pressure at the apex of the curve is gradually
increased again to allow for stretching of the soft tissues and equilibration
of the hemodynamic status. If the blood pressure drops again, the steps are
repeated. None of our patients have required abortion of the surgery because
of hemodynamic instability.
INDICATIONS:
This procedure is indicated for symptomatic children with cerebral palsy
who have severe sagittal plane deformities (=70°), i.e., hyperkyphosis
and hyperlordosis. The two most common symptoms include severe seating
problems that cannot be rectified with wheelchair modifications and severe
back pain that cannot be relieved with conservative measures. Other less
common indications are superior mesenteric artery syndrome that is refractory
to conservative treatment and loss of bowel and bladder control, when either
has been shown with diagnostic workup to be caused by severe hyperlordosis.
More severe rigid sagittal plane deformities as previously described must
undergo anterior release prior to the posterior procedure.
CONTRAINDICATIONS:
Relative contraindications include severe medical and nutritional
compromise. Nutritionally compromised children may benefit from tube feeding
and a prolonged period of improving the nutritional status before any surgical
procedure is considered. In patients with severe hyperlordosis and a superior
mesenteric artery syndrome, however, surgery may be a part of the solution to
obtain proper nutrition. Some quadriplegic children may have severe
osteopenia, making it very difficult to obtain wire fixation to the vertebrae.
In these children, pretreatment for several months with intravenous
bisphosphonates may improve bone quality.
PITFALLS:
The most important pitfalls are perioperative in nature. Severe
intraoperative hypotension means that the anesthesiologist has gotten behind
on fluid, blood, and blood product replacement. Communication between the
anesthesiologist and the surgeon is critical. A type and cross match for one
and one-half to two times the blood volume of the child is recommended as is
the availability of fresh-frozen plasma and platelets. Hypothermia is another
potential pitfall. The room temperature should be kept high, and the patient
should be well covered. A heated ventilator, warming of intravenous fluids and
blood, and a blanket-heating device are all helpful in keeping the patient
warm.
Technical pitfalls are possible, but they can be minimized with meticulous
attention to the details of the procedure. In particular, sagittal plane
deformities that are extremely stiff must have adequate anterior release. If
this is not performed, the surgeon will not be able to correct the deformity
adequately with posterior instrumentation alone and the stiffness will
predispose to wire breakage or cutout.
Severe lordosis is the most difficult sagittal plane deformity to address.
The difficulty develops when the surgeon tries to insert the rod and cannot
get the rod far enough anteriorly to align the pelvic legs of the rod with the
holes made in the pelvis. This creates a risk that the pelvic legs of the rod
will anteromedially perforate through the pelvis. Rod penetration into the
sciatic notch is also possible. Methods for dealing with excessive lordosis
are described above. Proper alignment of the pelvic limbs can be checked
intraoperatively with fluoroscopy. If pelvic penetration is recognized, the
penetrated rod limb should be cut, reinserted into the proper position, and
reconnected with an end-to-end or a side-to-side connector.
Another technical pitfall is misjudgment of the length of the rod. If the
rod is too long and prominent in the cervical region, it can be cut at the
level of T1 after first cross-linking the two rods together. It is unusual for
a rod to be too short by more than two levels, but, if it occurs, it is due to
improper measurement. If the rod is short by more than two levels, the top of
another unit rod can be cut and connected to it with use of end-to-end or
side-to-side connectors. This is more important in hyperkyphosis to prevent
drop-off of the spine over the top of the rod.
When wires cut through the lamina or break, it is usually because the
surgeon pulled too hard on the wires when twisting them or tried to use the
wire to pull the spine to the rod or it may be because of inadequate anterior
release of a very rigid deformity. Removing too much bone from the lamina,
especially removing bone from the superior aspect of the lamina when opening
the ligamentum flavum or when removing the spinous process, also weakens the
lamina. Avoiding these problems almost always prevents wire pullout, even if
the bone is small and osteoporotic. In cases of severe osteoporosis, medical
treatment of the osteoporosis as mentioned above may be helpful. In case of
wire pullout over a long segment, especially the apex in hyperlordosis and the
top of the rod in the case of hyperkyphosis, pedicle screws can be used to
salvage fixation.
AUTHOR UPDATE:
Recently, we found that children with severe lordosis may have better
fixation and correction with pedicle screws in the lordotic segment
(Figs. 12-A through 12-D).
Long-posted pedicle screw heads allow the lordotic segment to be pulled up to
the rod as the nut is tightened down onto the posted screw. Great care should
be taken in soft osteoporotic bone to prevent screw pullout from the
vertebrae. Side connectors to connect the pedicle screws to the unit rod are
usually necessary, as the screws do not always line up perfectly with the rod.
No other modifications have been made since publication of the original
article.
In a hyperlordotic deformity, the space between the posterior elements of
the vertebrae decreases (the vertebrae become "crowded").
Correction of the deformity restores the space between the posterior elements
and effectively lengthens the spine (the opposite of the effect with kyphosis)
(Fig. 7). Therefore, the size
of the unit rod required for implantation is 1 to 2 cm longer than the
measured length of the spine. A second consideration in the treatment of a
patient with hyperlordosis is that the increased anterior pelvic tilt
increases the likelihood of the rod penetrating through the inner iliac table
during its insertion into the pelvis. Therefore, special attention must be
paid when the distal ends of the unit rod are directed into the pelvis in
these patients. The distal pelvic limbs must be directed very posteriorly, and
it is often very difficult to aim the rod in the direction required
(Fig. 8). When the lordosis is
severe, prepadded blocks can be placed at the apex of the curve by an
unscrubbed assistant to facilitate correction, and, in some cases, the
assistant further aides in the correction by pushing on the apex of the curve
from under the sterile drapes (see Fig.
1). If the iliac ends of the rod still cannot be properly
inserted, one or both ends of the rod are cut, inserted separately, and
reconnected with end-to-end or side-to-side connectors (Figs.
9-A, 9-B, and
9-C). An alternative is to
prebend excessive lordosis into the rod, insert the pelvic limbs of the rod,
and then gradually reestablish normal lordosis into the rod with use of rod
benders in situ. One must be extremely careful when using this technique in
patients with osteoporotic bone as the rod limbs may penetrate the inner table
of the pelvis during restoration (bending back the rod) of normal lordosis to
the rod. Postoperatively, it is helpful to make oblique and inlet radiographs
of the pelvis to ascertain proper placement of the rod limbs in the
pelvis.
The surgeon corrects both hyperlordotic and hyperkyphotic deformities by
applying gradual manual pressure to slowly bring the rod to the spine with a
rod pusher (Fig. 10). The
surgeon should not try to force the correction in one move. The unit rod is
used for rigid fixation of the curve after the spine has been corrected
manually. Wire tightening should not be used to pull the rod to the spine.
This may cause the wires to break or pull through the often osteoporotic soft
bone of the spine. The wires are tightened only in a stepwise manner, one
level at a time, as the spine is manually corrected to the rod. After all of
the wires are tightened, they are cut approximately 1 cm in length and are
bent over to the midline. A large amount of crushed corticocancellous
allograft bone (at least 240 cc) is placed after facetectomies and
decortication of the spine out to the transverse processes have been performed
(Fig. 11). The fascia is
closed meticulously as is the remainder of the incision. No drain is used.
Postoperatively, the patient is kept intubated and ventilated for
twenty-four to forty-eight hours in the intensive care unit. Fluid management
to avoid hypotension is important, and urine output should be maintained at a
minimum of 0.5 mL/kg/hr. Children who are nutritionally compromised are
started on central hyperalimentation at twenty-four to forty-eight hours
postoperatively or on tube feedings through a feeding jejunostomy. The child
is gotten up in a chair after extubation; however, the child should not be
placed back into his or her own wheelchair until a physical therapist has
evaluated and made adjustments to the chair. Placing the child back into a
wheelchair that has not been adjusted may lead to decubitus ulcers.