Richard E. McCarthy
DEFINITION
The treatment of neuromuscular spinal deformities frequently requires fusion to the pelvis with firm fixation. This can be accomplished with a number of devices that allow for correction of pelvic obliquity and pelvic rotation while allowing for a solid base on which to attach rods for correction of curves above.
One of the most reliable structures in the formation of the spine, even in the dysplastic setting of myelomeningocele, is the sacral ala.
The S-rods are contoured to press-fit over the sacral ala.1,3,4
Historically the elongated Harrington hooks were used in a similar manner.
ANATOMY
The sacral ala in children is a structure 1.5 to 2 cm in depth (front to back) and 2 to 3 cm wide.
The L5 nerve root traverses anterior to the ala in an oblique direction progressing from posterior to anterior and superior to inferior obliquely from the neural foramina.
Immediately inferior to the pedicle of L5 the nerve transgresses anterior to the sacral ala, separated by a distance of 1.5 cm.
Besides the L5 root, the tissue anterior to the sacral ala is retroperitoneal fat.
FIG 1 • Anterior view showing position of S-rod or S-hook with reference to L5 nerve root.
Of key importance is identification and release of the ileotransverse ligament traversing between the iliac wing and the L5 transverse process.
To ensure clear access to the sacral ala, this ligament must be released from its attachment to the transverse process (FIG 1).
The dissection of the soft tissues around the sacral ala is done posteriorly with a curette; the surgeon must use caution against inserting tools anterior to the sacral ala for fear of injuring the L5 nerve root or plunging into the retroperitoneal space.
PATHOGENESIS
The multiple types of neuromuscular scoliosis requiring fixation to the pelvis include cerebral palsy, dystrophic muscle conditions, spina bifida, and many others.
The types of pelvic abnormalities associated with spinal deformities include pelvic obliquity, pelvic rotation, and flexion and extension of the sacrum.
NATURAL HISTORY
The natural history is one of progression and worsening of these deformities.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Special imaging and other diagnostic studies are only occasionally necessary.
The sacral ala can usually be clearly visualized as a horseshoe-shaped outline on upright or supine lateral radiographic films.
The Ferguson view (45-degree angle) in the frontal plane provides the clearest view of the width.
If there is doubt, a CT scan can better elucidate the exact configuration.
SURGICAL MANAGEMENT
The surgical management, in terms of preoperative planning, positioning, and approach, is the same as for neuromuscular scoliosis.
The techniques include cleaning of the soft tissues from the sacral ala with release of the ileotransverse ligament.
The sizing of the hook to the size of the sacral ala in its front-to-back diameter can be done at surgery.
Small and medium sizes are available, and it is important to use the appropriate size rod or hook (right or left) so the rod lies medial to the ala (FIG 2).
FIG 2 • A. Left-sided S-hook, anterior view. B. Left-sided S-hook, posterior view. C. With a rod clamp positioned to demonstrate the posterior plane, a right-sided S-hook is shown in its correct position. D.Right-sided S-rod, side view.
TECHNIQUES
S-ROD PLACEMENT
Proper alignment of the S-rod necessitates placement of the sagittal bends in the appropriate plane with reference to the best fit of the S-portion on the ala (TECH FIG 1A).
This can be aided by placement of a vise grip on the rod in the plane of the lordosis once the S-portion of the rod is positioned over the sacral ala.
The rod is removed from the wound and the three-point bender applied to produce the proper sagittal contours.
If the S-hook is used instead, the sagittal contours can be made in the rod independent of the hook position.
It is best to leave 1 cm protruding from the lower end of the hook when it is first placed on the ala (TECH FIG 1B).
The S-hook is temporarily tightened to the rod and spread between the L4 pedicle screw and the S-hook by tightening one Allen set screw on the hook (TECH FIG 1C,D).
The correction of the spinal deformity can then occur above this area.
TECH FIG 1 • A. Posterior view of a right-sided Srod on the spine. B. Initial position of the S-hook on the rod for placement purposes, with 1 cm of rod protruding. C,D. The S-hook shown positioned over the sacral ala and up the spine.
FINAL FIXATION
Once the final correction has been achieved, the final tightening of the S-hook occurs by distracting once again between the L4 pedicle screw and the concave side Shook, then the convex side rod, distracting the hooks to the end of the rod (TECH FIG 2).
Both Allen set screws are firmly tightened.
A strong cantilever force can be created to correct pelvic obliquity by using two sagittally contoured rods fixed to S-hooks positioned against the sacral ala distracted against the L4 pedicle screws.
A transverse rod connector above the S-hooks will supply further stability.
The pelvis can then be pivoted by grasping the rods above and correcting the pelvic deformity in one maneuver.
The final fixation of the S-hook is completed with both set screws firmly tightened.2
TECH FIG 2 • A. The final seating of the S-hook, distracting it against the L4 pedicle screw. B. Final position of the S-hook on the rod.
POSTOPERATIVE CARE
It is important to maintain hip flexion at 45 degrees or greater for the first 6 months to avoid levering on the pelvis with physical therapy.
No physical therapy is done about the hips for the first 6 months.
OUTCOMES
This technique has been used in more than 200 cases since 1984. Outcomes are generally solid fusions, ease of caregiving, and attainment of level pelvis for sitting (FIG 4).
FIG 4 • Neuromuscular spinal deformity: spastic lordoscoliosis with S-hooks.
COMPLICATIONS
Rod migration into myelo pelvis with a growth rod
I have not experienced any neurologic impairment of the L5 root.
REFERENCES
· McCarthy RE. Sacral pelvic fixation in neuromuscular deformities. Semin Spine Surg 2004;16:126–133.
· McCarthy RE. S-rod technique. In: Spinal Instrumentation Techniques Manual, ed 2. Scoliosis Research Society, 1998.
· McCarthy RE, Bruffett WL, McCullough FL. S-rod fixation to the sacrum in patients with neuromuscular spinal deformities. Clin Orthop Relat Res 1999;364:26–31.
· McCarthy RE, Dunn H, McCullough FL. Luque fixation to the sacral ala with the Dunn modification. Spine 1989;14:281–283.