Operative Techniques in Orthopaedic Surgery (4 Volume Set) 1st Edition

174. S-Rod Fixation to the Pelvis

Richard E. McCarthy

DEFINITION

images 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.

images One of the most reliable structures in the formation of the spine, even in the dysplastic setting of myelomeningocele, is the sacral ala.

images The S-rods are contoured to press-fit over the sacral ala.1,3,4

images Historically the elongated Harrington hooks were used in a similar manner.

ANATOMY

images The sacral ala in children is a structure 1.5 to 2 cm in depth (front to back) and 2 to 3 cm wide.

images 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.

images Immediately inferior to the pedicle of L5 the nerve transgresses anterior to the sacral ala, separated by a distance of 1.5 cm.

images Besides the L5 root, the tissue anterior to the sacral ala is retroperitoneal fat.

images

FIG 1  Anterior view showing position of S-rod or S-hook with reference to L5 nerve root.

images Of key importance is identification and release of the ileotransverse ligament traversing between the iliac wing and the L5 transverse process.

images To ensure clear access to the sacral ala, this ligament must be released from its attachment to the transverse process (FIG 1).

images 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

images The multiple types of neuromuscular scoliosis requiring fixation to the pelvis include cerebral palsy, dystrophic muscle conditions, spina bifida, and many others.

images The types of pelvic abnormalities associated with spinal deformities include pelvic obliquity, pelvic rotation, and flexion and extension of the sacrum.

NATURAL HISTORY

images The natural history is one of progression and worsening of these deformities.

IMAGING AND OTHER DIAGNOSTIC STUDIES

images Special imaging and other diagnostic studies are only occasionally necessary.

images The sacral ala can usually be clearly visualized as a horseshoe-shaped outline on upright or supine lateral radiographic films.

images The Ferguson view (45-degree angle) in the frontal plane provides the clearest view of the width.

images If there is doubt, a CT scan can better elucidate the exact configuration.

SURGICAL MANAGEMENT

images The surgical management, in terms of preoperative planning, positioning, and approach, is the same as for neuromuscular scoliosis.

images The techniques include cleaning of the soft tissues from the sacral ala with release of the ileotransverse ligament.

images The sizing of the hook to the size of the sacral ala in its front-to-back diameter can be done at surgery.

images 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).

images

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

images  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).

images 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.

images  The rod is removed from the wound and the three-point bender applied to produce the proper sagittal contours.

images  If the S-hook is used instead, the sagittal contours can be made in the rod independent of the hook position.

images  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).

images  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).

images  The correction of the spinal deformity can then occur above this area.

images

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

images  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).

images Both Allen set screws are firmly tightened.

images  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.

images A transverse rod connector above the S-hooks will supply further stability.

images  The pelvis can then be pivoted by grasping the rods above and correcting the pelvic deformity in one maneuver.

images  The final fixation of the S-hook is completed with both set screws firmly tightened.2

images

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.

images

POSTOPERATIVE CARE

images  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.

images No physical therapy is done about the hips for the first 6 months.

OUTCOMES

images 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).

images

FIG 4  Neuromuscular spinal deformity: spastic lordoscoliosis with S-hooks.

COMPLICATIONS

images Rod migration into myelo pelvis with a growth rod

images 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.



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