Kyphectomy in Spina Bifida

 

 

 

 

DEFINITION

Kyphosis in the patient with myelomeningocele can occur at the thoracolumbar junction, the midlumbar spine, or the lumbosacral junction.

The different types of kyphosis have some bearing on the treatment needed for the repair, but whether the origin is congenital, developmental, or paralytic, the consequences can be devastating for the child with this condition.

Skin breakdown over the apex of the kyphosis can cause deep wound infections and lead to central nervous system infections.

Secondary changes in other organ systems can create compromise in the gastrointestinal or genitourinary systems or even potentially disastrous kinking of the great vessels due to compromise in the abdominal height. Diminished absorption frequently occurs in the gastrointestinal tract, and renal calculi may develop from poor urinary drainage.

Secondary effects on the pulmonary capacity produce thoracic insufficiency syndrome because the abdominal contents are pushed into the thoracic cage. Further worsening the respiratory compromise is a thoracic lordosis cephalad to the kyphosis.

Bracing generally leads to problems from skin pressure and ultimately does not solve the problem.

 

ANATOMY

 

The kyphotic angle can be a gradual slope or an acute gibbus.

 

The paraspinal musculature is segmentally innervated, thus partially active in a flexion position lateral to the bony ridges owing to lack of posterior migration from an embryologic origin. In this position, they contribute to forward flexion of the spinal column.

 

 

The bony ridges laterally in the area of the diastasis leave meager bone for fusion mass on the posterior side of the vertebral column.

 

The midline defect from the original myelomeningocele characteristically is covered by a fragile dura separated from the overlying skin by a thin layer of subcutaneous tissue.

 

 

The soft tissue coverage is made worse by poorly vascularized scarred skin.

 

One of the most reliably formed vertebral structures is the sacral ala.

 

The great vessels generally do not follow the kyphotic contours into the kyphotic apex.

 

PATHOGENESIS

 

Embryologically, the notochord is covered dorsally by closure of the ectoderm, progressing in a cephalad to caudal direction. In myelomeningocele, the closure is incomplete, usually at the caudal end.

 

Less common types of myelomeningocele occur in the thoracic and cervical area. Thoracolumbar, lumbar, and lumbosacral kyphosis is the most commonly seen type and occurs because of lack of posterior migration of the

ectoderm surrounding the notochord, leaving the neural placode in a vulnerable position resulting in an exposed myelomeningocele sac at birth.

 

 

Congenital bony defects occurring in this area lead to an early-onset kyphosis that can pose significant problems for the neurosurgeon's closure at birth. This has led some experts to encourage neonatal correction of the kyphosis.

 

With further growth and an upright sitting posture, the paraspinal musculature, which has formed in a lateral and anterior position, pulls the upper torso into a more kyphotic position, both actively through muscle contracture and secondarily from gravity.

 

 

 

This can lead to further skin compromise and pressure in the soft tissues overlying the kyphosis. Skin breakdown can be a serious problem over the apex of the kyphosis.

 

The C7 lateral plumb line shows the upper torso to be far out of balance in a forward-flexed posture, leading to thoracic insufficiency from the abdominal contents pressing under the diaphragm.

 

 

This can render the child a “functional quadriplegic” because he or she uses the upper extremities for balance and to unweight the diaphragm by pivoting on the extended arms for breathing purposes (marionette maneuver; FIG 1).

 

From a developmental standpoint, this can further limit the young child's upper extremity interaction with his or her environment, which is essential for the development of normal intelligence.

 

 

 

FIG 1 • Functional quadriplegia supporting position with arms in myelokyphosis; lumbar kyphosis with thoracic lordosis.

 

 

 

NATURAL HISTORY

P.748

 

The natural history of unpublished cases of severe kyphosis is one of respiratory compromise from thoracic insufficiency syndrome, progressive decline in pulmonary capacity, and death.

PATIENT HISTORY AND PHYSICAL FINDINGS

 

A careful history and physical examination should elicit possible signs and symptoms of associated anomalies, including the following:

 

 

 

 

Chiari malformation Tethered cord Respiratory compromise

 

 

Gastrointestinal malabsorption Urinary hydrostasis and lithiasis

 

Physical examination of the child should include flexibility tests of the curve by physically supporting the child under the armpits to suspend him or her against gravity. Bending back supine on the examining table can also indicate the extent of lumbar flexibility.

 

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

Standard radiographs, anteroposterior (AP) and lateral views of the full spine, in the upright sitting posture assess the effects of gravity on the curve (FIG 2A,B).

 

 

Supine radiographs are helpful for visualization of bony definition.

 

Flexibility films with traction, manual push, or back bending over a bolster with a shoot-through lateral film are helpful adjuncts.

 

Computed tomography (CT) scans, especially three-dimensional CT scans, offer the best delineation of the anatomy.

 

 

 

FIG 2 • A,B. A 13-year-old child with myelokyphosis with diastasis beginning at T6 with 127 degrees of kyphosis. C. Preoperative MRI in a 9-year-old child with myelomeningocele before undergoing kyphectomy with growing construct.

 

 

Magnetic resonance imaging (MRI) is critical for assessment of the intrathecal structures and assessing for Chiari malformation, syringomyelias, and tethering (FIG 2C).

 

DIFFERENTIAL DIAGNOSIS

Congenital versus developmental kyphosis Sacral agenesis

Charcot joints secondary to vertebral column breakdown across the apex of the kyphosis

 

 

NONOPERATIVE MANAGEMENT

 

Bracing has no place in the treatment of this disorder.

 

Occasionally, traction is helpful to stretch the kyphosis, especially in a developmental type, to aid in correction at the time of surgery.

 

This can be done with cervical traction or halo traction, and some authors have promoted the use of this traction during surgery to aid in the correction.

 

SURGICAL MANAGEMENT

Preoperative Planning

 

Vascular monitoring devices are an important adjunct during surgery, and either arterial lines or pulse oximeters on both feet are important to monitor blood supply to the lower extremities at the time of correction.

 

A great deal of tension can come to bear on the aorta at the time of kyphosis realignment. Thus, arterial and central venous lines are necessary to monitor central pressure and allow for rapid administration of medication and fluids.

 

Areas of skin breakdown should be addressed prior to kyphectomy.

 

 

Preoperative planning may include consultation with a plastic surgeon and possible placement of tissue expanders in the posterolateral axillary margins to aid in skin closure at surgery (FIG 3).

 

As a part of the preoperative planning, all imaging studies are carefully reviewed to assess flexibility, the adequacy of the vertebral bodies to tolerate pedicle screws, and planning for which levels will need to be decancellized or removed.

 

 

It is recommended that these plans be recorded on a “blueprint” that can be placed on the operating room wall outlining the location of implants, osteotomies, and order of progression for the surgical plan.

 

Assessment by neurosurgery is necessary preoperatively regarding shunt functioning and review of the MRIs.

 

 

 

FIG 3 • Myelomeningocele in an 11-month-old child with tissue expanders placed bilaterally before delayed closure and kyphectomy.

 

 

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Preoperative antibiotics are essential, including gram-negative coverage for urinary pathogens. These are continued postoperatively for 6 to 12 weeks.

 

Nutritional status is maximized and may require hyperalimentation via a gastrostomy tube button months ahead of surgery to maximize postoperative healing.

 

Positioning

 

During positioning, careful padding with extra foam is essential to protect delicate skin during a prolonged operation.

 

Eye protection to guard against intraoperative ocular compromise and a spinal frame that allows for suspension of the abdominal structures will diminish the epidural vascular pressure.

 

Preoperative assessment of the hips is important to anticipate the intraoperative positioning, and if the flexion contractures about the hips are too severe, a preliminary release of contractures done a few weeks ahead of time may be necessary to allow for proper positioning of the legs at the time of the kyphectomy.

 

Approach

 

The surgical incision may involve excision of compromised skin lesions or scars, although this is best addressed before surgery.

 

The previous incisions on a myelomeningocele back may not be midline or ideally placed.

 

 

The best skin incision for kyphectomy should follow the previous skin incisions to maximize blood supply to the skin edges at closure. Maximum skin and subcutaneous tissue coverage is important for good closure.

 

If soft tissue quality is poor, then previously placed tissue expanders may be removed from the midline at closure and the expanded tissue brought to the midline.

 

There may be times when the poorly healed, convoluted scars from previous neurosurgical interventions may be source of bacteria that can impair healing or may contribute toward postoperative infection. Preliminary excision of the scars by plastic surgery may afford the best defense against outside-in infections.

 

 

TECHNIQUES

• Incision and Lumbar Dissection

  The incision—either straight or curvilinear—follows the previous scars. It is extended deep to the spinous processes in those sections where they exist.

  The caudal portion of the incision is made to the level of the dura, with care taken to avoid laceration of the fragile dura.

  The surgical plane is then deviated to the right and left side superficial to the dura while palpating for the lateral bony elements. The deep portion of the incision is directed toward the bone.

  It is desirable to maintain as much subcutaneous thickness as possible.

  If there are lacerations of the dura, it is best to stop and sew them as one proceeds because the thinned dura may require a flap of adjacent tissue for a watertight closure. Sometimes, it is necessary to sew in a piece of Duragen sealant to ensure a watertight closure.

  A 4-0 Nurolon on a small tapered needle works quite well for an incidental durotomy repair.

  As one proceeds from distal to proximal in the lumbar spine, the lateral elements are palpated, and with the use of electrocautery, the soft tissues are incised to bone.

  The muscle and soft tissue attachments to the laterally positioned lumbar bones are released to reveal the underlying bony elements that embryologically should have progressed posteriorly to become the lamina

  and facets (TECH FIG 1A-C).

   

  The rudiments of these bones can be visualized along with a transverse process at each level in the bony ridge of the diastasis.

   

  The medial neural placode is left intact because it acts as third-space filler and padding for the implants.

   

  There may be instances in which the neuroplacode has to be mobilized, and this is done by releasing nonfunctioning roots on one side and reflecting the dura laterally to gain access to the disc space and underlying vertebrae (TECH FIG 1D).

   

   

   

  TECH FIG 1 • A. Intraoperative dissection with neuroplacode left in place and forceps placed on bilateral bony ridges. B. Paraspinal muscles are dissected away from the kyphosis, with frequent irrigation of neural elements. C. In a different patient, the kyphosis has been dissected out. (continued)

   

   

  P.750

   

   

  TECH FIG 1 • (continued) D. The neuroplacode can be left in place, mobilized to one side by releasing nonfunctioning nerve roots over four levels, or resecting to the level of the diastasis and oversewing.

• Thoracic Dissection

   

  Once the lumbar spine is dissected, the thoracic area is approached.

   

  If one is contemplating a fusion of the thoracic spine, such as in a child older than 8 years of age, full dissection out to the tips of the transverse processes should be accomplished.

   

  If a growing rod construct is being used, such as in a child younger than 8 years, this is done with minimal dissection in order to preserve growth.

   

  In these cases, the muscle and soft tissue attachments are cleaned from the sides of the spinous processes as far as the facet joints.

   

  One needs to be able to visualize the ligamentum flavum sufficiently to pass sublaminar wires for the Luque trolley portion of the “growing” construct.

   

  Generally, four thoracic levels for wires are all that is necessary.

   

  In the lumbar spine, soft tissues should be cleaned from bone sufficiently to allow for fusion between the lateral elements and to the sacrum.

• Pedicle Screw Placement

   

  At this point in the operation, radiographic C-arm guidance is helpful for placement of pedicle screws.

   

  The entrance point for the lumbar dysplastic pedicles is in a lateral position, with the pedicles directed obliquely toward the vertebral body (TECH FIG 2).

   

  Bilateral screws can be obliquely placed for fixation.

   

  Fixation to the pelvis can be done with multiple types of fixation devices, including S rods, S hooks, and iliac threaded bolts.

   

  Fusion to the sacrum is essential to firmly plant the rod on the pelvis and allow for growth off the top of the rods in the thoracic spine.

   

  The C-arm is used in both AP and lateral positions to confirm satisfactory placement of the screws in bone. Bicortical fixation is generally not necessary because of the strong fixation supplied by the triangulation of the screws.

   

  Polyaxial screws are desirable through the lumbar segments.

   

   

   

  TECH FIG 2 • Screws in place and bilateral curettes in pedicles to decancellize at L3 before doing the same at T12.

   

   

   

• Decancellization

  P.751

   

  Decancellization can be accomplished at multiple levels, leaving adequate vertebral levels for fixation and correction. Ideally, it is accomplished at one or two levels in a location that will leave sufficient midlumbar fixation points to push the vertebrae forward to create lordosis.

   

  The levels chosen for decancellization are approached after screw placement, based on the preoperative planning.

   

  The decancellization begins with a burr at the entrance to the pedicle. It continues with enlarging sizes of curettes, saving the bone for the fusion.

   

  The inside of the vertebral body is completely cored out, and when bleeding points are encountered, the pedicle can be filled with FloSeal and, if necessary, further packed with some rolled Gelfoam to stop the

  bleeding.

   

   

   

  TECH FIG 3 • Decancellization. A. Lateral view. B. Cross-sectional view.

   

   

  Care is taken to avoid violating the posterior cortex of the vertebral body until the very end because this is where the epidural vessels are most prolific.

   

  The lateral margins of these vertebral bodies are removed, including the transverse process and posterolateral bone.

   

  The decancellization should be thorough, leaving only the cortex. This is carried out bilaterally, followed by implosion of the posterior cortex with an Epstein curette, pushing the bone fragments into the cavity of the vertebral body (TECH FIG 3).

   

  Bleeding points are stabilized.

   

  In most instances, decancellization alone at select levels is all that is necessary to gain the mobility for correction.

• Horizontal Resection

   

  Occasionally, removal of a vertebral segment may be indicated. If so, it can be accomplished while maintaining the neuroplacode.

   

  The vertebral section for removal in that instance would be taken from that section of the curve that is horizontal and cephalad to the apex of the kyphosis (TECH FIG 4).

   

   

   

  TECH FIG 4 • A. If bone is to be resected (due to extreme stiffness), this should be done in the horizontal section at the top of the kyphosis, not at the apex. B. After horizontal resection, the bone is pushed forward to realign. C. After resection, realignment and fixation are accomplished.

   

   

   

• Rod Placement

  P.752

   

  Once these corrective maneuvers have been completed, rods linked to the sacrum can then be placed bilaterally to push the vertebral bodies anteriorly into a straight or preferably a lordotic position (TECH FIG 5A).

   

  Through gradual approximation of the rod forward toward the thoracic fixation points, the lumbar segments are brought into alignment and the rods gradually tightened to the wires of the thoracic spine (TECH FIG 5B,C).

   

  Physiologic kyphosis can be contoured into the thoracic component of the rods to correct the thoracic lordosis.

   

   

   

  TECH FIG 5 • A. Bilateral rods are anchored to L4 and S hooks in preparation for reduction. B. Decancellized levels are crushed by compressing adjacent screw heads. C. In a different patient, gradual reduction with wires and provisional tightening are accomplished using a growing construct. D. Completed reduction in patient in A and B. Allograft and autograft have been applied to decorticated bone for fusion. E. Completed reduction in patient with growing construct in C.

   

   

  Generally, the rods are left one level long at the top to allow for growth in the thoracic spine.

   

  The final tightening should produce some distraction between the lowest lumbar segment fixation point and the S hooks pushed against the sacral ala (TECH FIG 5D,E). This will set the S hooks in place securely.

   

  Final contouring with the in situ benders can allow for further lordosis of the lumbar spine if desired.

• Assessing and Managing Lower Extremity Hypoperfusion

   

  Frequently, the initial maneuvers for correction across the kyphosis are accompanied by a decrease in blood flow to the lower extremities. Therefore, it is important to do this corrective maneuver gradually in small increments.

   

  The baroreceptors in the aorta can accommodate to the change in alignment and stretch. If the blood flow to the feet is unable to accommodate to the new position of the spine, further decancellization or vertebral body removal will be necessary.

   

  This decision is based on the flow to the lower extremities reflected in the pulse oximeter or arterial catheters in the feet.

• Closure

   

  As part of the closure, it is important to grasp the paraspinal musculature with clamps to pull the muscles toward the midline by elevating and mobilizing the muscle layer with a Cobb elevator.

   

  Sometimes, release of the fascia on the posterior side of the musculature is necessary, and this is best done in the posterior axillary line with a vertical cut in the fascia.

   

  The paraspinal musculature should be brought as close as possible toward the midline on both sides and sewn down (TECH FIG 6).

   

  At least one and more likely two Hemovac drains should be left, one in the deep and one in the superficial layers, for drainage over 1 week to 10 days postoperatively.

  Subcuticular closures can be used, but they should be reinforced with external suture of some kind, either clips or interrupted nylon sutures on a temporary basis.

  P.753

   

  TECH FIG 6 • The paraspinal muscle flaps are brought to midline for final closure.

   

   

   

  PEARLS AND PITFALLS
  	Evaluation ▪ The surgeon can assess flexibility of the curve by physically supporting the child under the armpits to suspend him or her against gravity. Bending back supine on the examining table can also indicate the extent of lumbar flexibility.     Preoperative ▪ Areas of skin breakdown should be addressed and healed before kyphectomy. preparation     Intraoperative ▪ Vascular monitoring of the lower extremities is a critical part of the intraoperative monitoring monitoring.     Preoperative ▪ Preoperative antibiotics are essential, including gram-negative coverage for preparation urinary pathogens. These are continued postoperatively for 6-12 weeks.     Managing ▪ A 4-0 Nurolon on a small taper needle in a running fashion works quite well for an incidental incidental durotomy repair. Duragen can be sewn over the repair, and occasionally, dural tears the use of a sealant (Tisseel) is necessary.     Preventing ▪ Care is taken to avoid violating the posterior cortex of the vertebral body until the epidural very end because this is where the epidural vessels are most prolific. bleeding

   

   

  • The final tightening should produce some distraction between the lowest lumbar segment fixation point and the S hooks pushed against the sacral ala. This will set the S hooks in place securely.

  Setting the S hook

   

  Postoperative care

• All reasonable measures must be taken to avoid any pressure on the wound or extremities in the postoperative period. All areas of insensate skin must be protected from excessive pressure with frequent change in position on a soft surface. The dressings should be covered with a waterproof covering to protect against secondary contamination from stool.

 

POSTOPERATIVE CARE

 

For recovery, patients are placed on their back with an extra-thick foam on top of the mattress to avoid excessive skin pressure.

 

 

Logrolling is instituted 6 hours postoperatively and repeated every 2 hours. Recovery occurs in the intensive care unit until the patient is sufficiently stable.

 

Although postoperative immobilization is not necessary, if desired, it can be accomplished with careful molding of a bivalved jacket with a Plastazote soft lining.

 

OUTCOMES

Improved sitting

Improved respiratory function Better blood supply to skin

 

 

COMPLICATIONS

Skin breakdown

Infection, superficial or deep

Vascular compromise to feet with stretch on aorta Loosening of spinal implants

Pseudarthrosis

 

 

SUGGESTED READING

1. McCarthy RE. Myelokyphosis. Shriners Hospitals for Crippled Children, Symposium on Caring for the Child with Myelomeningocele. Rosemont, IL: American Academy of Orthopaedic Surgeons, 2002.