Rib to Pelvis Vertical Expandable Prosthetic Titanium Rib Insertion to Manage Neuromuscular Scoliosis

DEFINITION

Vertical expandable prosthetic titanium rib (VEPTR) has gained enormous worldwide popularity in the last decade as an effective method to manage potentially lethal chest and spine deformity in children.

Initially, VEPTR was used primarily to expand the congenitally abnormal chest, particularly in circumstances where there were fused, absent, or hypoplastic ribs.

As surgeons gained familiarity with the technique, the VEPTR device was adopted as a very effective nonfusion method to manage severe early-onset neuromuscular scoliosis in conditions such as spinal muscular atrophy (SMA), spina bifida, and cerebral palsy.

With refinement, the rib to pelvis bilateral VEPTR technique has become one of the most commonly used initial implant strategies for these conditions because the technique can be done through a few small incisions, the spine is left untouched (and thus hopefully, unfused), and the expansions for growth are

simple and of low morbidity.7

ANATOMY

The thoracic cage is composed of 12 pairs of ribs, the sternum, and 12 thoracic vertebrae. The ribs are interspaced with intercostal muscles. The external intercostals arise from the inferior border of one rib and attach to the superior border of the caudal rib with the external intercostal membrane lying posterior to it. Below this is the internal intercostal which has its fibers running at right angles to the external intercostal and the internal intercostal membrane deeper to the internal intercostal.

The neurovascular structures which are composed of the intercostal vein, artery, and nerve live beneath the internal intercostal muscle. It is important to understand that the vein lies superior to the subcostal groove, whereas the artery and the nerve reside inferior to the subcostal groove. This nerve innervates the adjacent intercostal muscles. Further deeper to the internal intercostal but above the parietal pleura is the innermost intercostal muscle, which becomes the transversus thoracic muscle anteriorly.

A normal thoracic development during childhood is essential for lung growth. Hence, respiratory function will depend on the lung growth and ability of the thorax to act as a dynamic pump to facilitate inhalation and exhalation.

The complexity of thoracic growth is not well understood but it is believed that the thoracic spine plays a critical role and contributes to vertical growth of the rib cage.

The thoracic spine grows 1.4 cm per year from birth to 5 years of age, 0.6 cm per year from 6 to 10 years of age, and 1.2 cm per year from 11 to 15 years of age.1, 3

The expected shortening of the thoracic spine can be calculated in congenital scoliosis or early spine fusion but the complex relationship between loss of thoracic volume and thoracic spine shortening and its indirect

adverse effect on lung volume and expansion has yet to be quantified and remains unclear.

However, studies done on a natural history model of spondylothoracic dysplasia (Jarcho-Levin syndrome),4 in which the thoracic spine is only one-fourth of normal height in adults, have shown most surviving adults to have restrictive lung disease with an average vital capacity of only 27%.

Symmetric growth and the correct orientation of the ribs for the age of the child contributes to the width and depth of the rib cage and helps to maximize volume expansion and make respiration more effective. The thoracic cross-sectional volume is directly proportional to the length of the ribs and the degree of rib obliquity. At birth, infants have a square-shaped thoracic cross-section, which becomes a more rectangular-shaped thoracic cross-section in adults.

The ribs are oriented horizontally and the growth of the ribs occurs primarily at the anterior physis. At birth, the thoracic volume is only 6.7% of the adult volume. By 2 years of age, the ribs grows downward more obliquely and the thoracic cross-section changes to an oval shape. By 5 years of age, the thoracic volume increases to 30% of the adult size and to 50% of the adult size by 10 years of age. From age 10 years to skeletal maturity, the thorax grows rapidly and doubles itself to the adult size.

Almost 85% of lung alveolar cells are formed immediately after birth, with only slight increase in the first 2 years of life. The end age of alveolar cell formation is controversial and so is the concept of alveolar cell hypertrophy. The lungs still continue to increase in size even if the alveolar cell formation ceases due to compensatory lung growth triggered by a lung “stretch reflex.” Experimental pneumonectomy in young animals and partial pneumonectomy in children aged 30 months to 5 years showed compensatory lung growth by alveolar cell multiplication.

PATHOGENESIS

VEPTR is currently widely accepted treatment for volume depletion deformities of the thorax (VDD). VDD can be classified into three main types:

Type I: absence of ribs and exotic scoliosis Type II: ribs that are fused and exotic scoliosis

Type IIIa: hypoplastic and foreshortened thoracic cavity as in Jarcho-Levin syndrome

Type IIIb: transverse constricted as in Jeune thoracic dystrophy or the windswept thorax of scoliosis

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NATURAL HISTORY

Thoracic insufficiency may initially present with early occult respiratory deficiency when the child begins to show signs of limited pulmonary capacity. They fatigue easily with play activities and compensate with an increased respiratory rate at rest but do not yet require oxygen support. The final sequela of progressive thoracic insufficiency syndrome is respiratory insufficiency.

An increase in the ventilation requirements reflects an exponential clinical deterioration in respiratory function with important consequences for the family and the child.

Early-onset scoliosis in neuromuscular conditions such as SMA begins in the first decade of life due to truncal weakness. It then progress steadily unless managed surgically and can become lethal. It occurs in 100% of the type I SMA and most of the type II SMA when they become nonambulators.

PATIENT HISTORY AND PHYSICAL FINDINGS

Physical examination should include careful evaluation of the spinal deformity, pelvic obliquity, sitting balance, and shoulder balance.

Respiratory effort is assessed by determining the rate of respiration, lung auscultation for abnormal breath sounds, anthropomorphic measurements (height, weight, chest circumference at the nipple line, and limb length) looking for failure to thrive, and the thumb excursion test to evaluate thoracic wall expansion during respiration.

IMAGING AND OTHER DIAGNOSTIC STUDIES

The radiographic assessment includes standard posteroanterior (PA) (FIG 1) and lateral views of the spine with rib cage. The Cobb angle is measured for the scoliosis, and the head and truncal decompensation is assessed. The space available for the lungs to expand is measured by the ratio between the concave heights to the convex height.

FIG 1 • Preoperative radiograph of a 10-year-old boy with SMA and progressive scoliosis. The preoperative Cobb angle was 56 degrees and a 3.5-cm pelvic tilt.

If cervical spine instability is a possibility, a cervical spine series including flexion and extension views is performed.

A formal consultation with a pediatric pulmonologist, including pulmonary function testing, is very valuable to establish baseline function and counsel the family appropriately regarding risk of postoperative pulmonary complications.

Magnetic resonance imaging (MRI) of the entire spine should be ordered to evaluate the spinal cord and those children who have any physical findings concerning for tethered cord.

Computed tomography lung volumes can be compared with normative values to provide more information but

such results may not be available.

Dynamic MRI may be performed to assess the function of the diaphragm, and a screening MRI is routinely performed to look for abnormalities such as cord tethering or syrinx.

Echocardiogram can be done to detect early-onset cor pulmonale.

NONOPERATIVE MANAGEMENT

The primary nonoperative option is observation, with serial radiographs and close monitoring of pulmonary function.

In some cases, braces or serial casting can be offered, although it is often impractical in children with neuromuscular spinal deformity at risk for thoracic insufficiency.

SURGICAL MANAGEMENT

Bilateral rib to pelvis VEPTR technique offers a relatively minimally invasive way to manage early-onset neuromuscular scoliosis associated with thoracic insufficiency. The technique can be performed with relatively low blood loss and morbidity and with much less risk of autofusion as is seen in standard dual growing rod techniques.

The bilateral rib to pelvis VEPTR technique is generally contraindicated in ambulatory children because a recent report shows a significant incidence of crouched gait, likely due to mechanical factors at the

lumbosacral junction.7

Indications

A rapidly progressive curve with pelvic obliquity and nonambulatory patients. Thoracic insufficiency may or may not be present.

A greater than 10% reduction in the height of the hemithorax on the concave side of the curve compared with the height of the contralateral hemithorax (space available for the lung is <90%)

Progressive thoracic insufficiency syndrome

An age of at least 6 months up to skeletal maturity. The younger the patient, the more likely there will be beneficial lung growth with thoracic expansion.

Concurrent approval of the previously mentioned indications by a pediatric orthopaedist, a pediatric general surgeon, and a pediatric pulmonologist

Contraindications

Inadequate soft tissue coverage for the devices. Generally, this correlates clinically with a child with a body weight below the 25th percentile.

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Inadequate rib bone stock for device attachment, such as in a patient with severe osteogenesis imperfecta

Absence of cephalad osseous ribs for attachment of the devices

Inability to undergo repetitive episodes of general anesthesia because of cardiac disease, pulmonary disease, or other medical conditions

Active pulmonary infection

Absent diaphragm function

Preoperative Planning

At our institution, all patients with possible thoracic insufficiency syndrome are evaluated by a multidisciplinary team of clinicians including a pediatric orthopaedic surgeon, a pediatric general surgeon, and a pediatric pulmonologist.

Preoperative full-length spinal radiographs should be reviewed, with careful attention to the anatomy of the ribs and pelvis, which will become anchor fixation sites.

Preoperative nutritional evaluation is valuable for children with very low body mass index (BMI).

Positioning

The patient is placed in the prone position over gel bolsters. The upper extremities are draped out of the field with the shoulders flexed no more than 90 degrees.

Upper and lower extremity intraoperative multimodal spinal cord monitoring is performed.

A pulse oximeter is placed on the hand on the side of the operation. Both an arterial line and a central venous line are established. Prophylactic antibiotics are given intravenously.

TECHNIQUES

• Exposure

The C-arm image intensifier is used to mark out four longitudinal incisions.

As in all cases of VEPTR surgery, it is critically important to avoid placing the actual incision directly over the anchor site. Incision should always be either medial or lateral to the anchor site by a centimeter or two.

This fundamental principle of VEPTR surgery will markedly decrease the risk of postoperative wound breakdown for your patients, especially those with poor nutrition and limited subcutaneous tissue.

The two proximal longitudinal paraspinous incisions are for the proximal cradles to the right and left upper thoracic rib segments.

Two longitudinal distal incisions are placed for the pelvic hooks on the right and left ilium (TECH FIG 1).

TECH FIG 1 • Ten-year-old boy with a history of SMA and progressive scoliosis. He was placed in prone position over gel bolster and incision sites marked under C-arm. Two proximal incision on either side of T4-T5 rib segments at the top of the picture and two distal incisions over the right and left ilium for the pelvic hooks is seen at the bottom of the picture.

• Rib Cradle Insertion

   

  The two upper rib cradles are placed first. Optimal positioning is about 2 cm lateral to the costotransverse junction. A narrow window in the trapezius and rhomboid muscles is made and the rib is exposed.

   

  Care should be taken not to divide the blood supply of the rib by stripping it of surrounding soft tissue.

   

  In most cases, we use two adjacent ribs with a double rib cradle. Depending on the sagittal contour, this can be T4-T5, T3-T4, or even in some cases T2-T3.

   

  Because implant migration is so common after prolonged distraction in medically fragile children, the surgeon should plan for the next step if there is migration. Therefore, using more caudal ribs (T4-T5) allows the surgeon to simply move the cradle to the next most cephalad pair of ribs (T2-T3) if there is bone failure at the initial anchor site.

   

  It is always easier to reattach more cephalad than more caudad because the latter requires collapsing the device rather than expanding the device.

   

  The first rib should never be used, as cephalad migration of this device places the brachial plexus at risk.

   

  Once the rib is exposed, a towel clip is placed on some adjacent soft tissue (do not place the towel clip on the rib, it may damage smaller, more fragile ribs) and image intensifier is used to confirm the site where the rib cradle will be placed.

   

  The narrow entry point for the curved Freer elevator is over the intercostal space proximal and distal to the rib (TECH FIG 2A).

   

  The Freer elevator is used to carefully create an entry point for the rib cradle anteriorly and guide it along the curve of the rib, carefully protecting the pleura beneath it (TECH FIG 2B).

   

  The rib trial is inserted, and the superior rib cradle is then encircled around the rib by sweeping it from medial to lateral. The cradle is finally locked into place by the cap (TECH FIG 2C).

   

   

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  TECH FIG 2 • A. A curved narrow Freer is passed around the rib surface before a rib cradle trial to free the periosteum around the rib and to prevent puncturing the pleura. B. Cross-sectional anatomy of the rib with the intercostal muscles and the neurovascular bundle. Placement of the rib cradle is demonstrated. C. A cephalad rib cradle was placed over the right T4 rib segment. A similar construct was placed over the left side corresponding rib segment. The placement was finally checked with C-arm.

• Insertion of Pelvic Hooks

   

  A longitudinal incision is made and carried down to the paraspinous muscles to the iliac crest. A small window is made in the paraspinous muscle and fascia at its insertion on the iliac crest and gentle digital dissection is used along the inner table of the crest at this location, which will later be the site of the internal portion of the pelvic hook.

   

  Every effort should be made to keep the soft tissue window as small as possible, helping to minimize the risk of immediate migration of the hook to an unsatisfactory location.

   

  The optimal location on the pelvis is just a few millimeters medial to the most cephalad crest of the pelvis (TECH FIG 3A).

   

  Placing the hook lateral to the crest will cause it to ride off the lateral edge into the soft tissues and placing it medially risks the sacroiliac joint and perhaps the lumbar nerve root.

   

  The hook is then inserted through small window in the fascia, coming to rest on the iliac crest, with cartilage left intact on the ilium. The portion of the iliac apophysis caudad to the hook prevents acute inferior drift into the pelvis.

   

  After all four anchors are in place, image intensifier should be used to confirm optimal position of both rib cradles and both pelvic hooks, before proceeding to implant placement (TECH FIG 3B,C).

   

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  TECH FIG 3 • A. Hemipelvis with the placement of S hook about 2 to 3 cm posterior to the highest crest prominence and lateral to the sacroiliac joint (SI). This prevents the migration of the hook into the SI joint. B. Intraoperative fluoroscopy to assess the placement of the rib cradle. C. Intraoperative fluoroscopy to assess the placement of the pelvic hooks.

• Concave Device Creation and Insertion

   

  The concave VEPTR is placed first.

   

  Gentle digital dissection and uterine forceps are used to develop a subcutaneous tunnel between the proximal rib cradle site and the distal pelvic hooks site.

   

  The direction of this dissection, and the subsequent placement of devices, should always be proximal to distal to avoid inadvertent abdominal or chest intrusion, which can be lethal (TECH FIG 4).

  Sizing

   

  Perhaps the most challenging aspect of building the construct is getting the length correct.

   

  Only through a good understanding of the deformities flexibility and a significant amount of experience can the surgeon create a properly sized device consistently.

   

  It is generally wise to plan a device several centimeters longer than measured with the rod template because it is always easier to remove a centimeter of the rod than to realize that the construct is much too small and needs to be discarded for a larger device.

   

   

   

  TECH FIG 4 • A. Submuscular tunneling of the uterine artery forceps. Note the pointed end of the forceps always faces the skin and should be palpated throughout the tunneling process. B. A size 20F chest tube is clamped to the uterine artery forceps caudally and pulled out cephalad. The rod is then placed submuscular through the tunnel by gently pulling and rotating the chest tube from the caudal end.

   

   

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  Begin estimating the concave construct length by placing a rod template into the proximal rib cradle, through the subcutaneous tunnel and down to the pelvic hook.

   

  Be certain that the pelvic hook is fully seated on the iliac crest.

   

  Once the template is in place, manually maximally correct the deformity to get a better estimation of the length. Then add a couple centimeters to your measurement to account for the additional length that invariably is needed.

   

  Select a VEPTR size that allows the lengthening portion of the device to sit appropriately based on the sagittal contour of the deformity (ie, avoid having the lengthening portion of the VEPTR device over sharp kyphosis).

   

  Allow 2 to 3 cm of rod proximal to the lengthening portion of the device.

   

  It is wise to contour this proximal portion into kyphosis, as it will diminish the forces that will, over time, cause the rib cradle to erode through the rib.

  Contouring

   

  Distally, a large portion of the rod can be contoured into satisfactory lumbar lordosis.

   

  Creating proper lordosis is essential, and can diminish postoperative hip flexor spasms and other problems, especially in children with spasticity associated with their neuromuscular scoliosis.

  Insertion

   

  After the construct is created, attached by an expansion clip, and properly contoured, it is threaded through the subcutaneous tunnel with the aid of a chest tube as a shuttle. After attaching the device to the proximal rib cradle, a side-to-side connector is attached to the end of the device and to the pelvic hook. This may be somewhat challenging unless the deformity is corrected as the concave device is inserted; until the deformity is corrected, the concave device will be much too long and it will be difficult to

  fit it in the distal portion of the incision.

   

  Ideally, after the concave device is inserted with maximal manual deformity correction, there will still be 2 to 3 cm of rod distal to the rod-to-rod connector. This is the site where further correction can be obtained during this initial insertion.

   

  With distraction, and manual correction of the deformity, this last bit of rod is brought up through the rod-to-rod connector, ideally leaving it flush.

   

  It is imperative that the communication be maintained with their monitoring team during this aggressive distraction and correction.

   

  After the first device is inserted and fully lengthened, the surgeon should visually inspect both anchor sites to assure that the rib is intact proximally and the pelvic hook is not subsiding into the pelvis (rare).

• Convex Device Creation and Insertion

   

   

  Once the concave device is in place, a similar technique is used to build the convex device. There are two important considerations for the convex device: sagittal contour and length.

   

  The sagittal contour of the convex device may need to be a bit more kyphotic, and this should be built into the sagittal contour. Sometimes, the lengthening segment needs to be a bit shorter on this side to accommodate convex kyphosis.

   

  The convex device should be built to be only slightly longer than original templates because at this point, most of the deformity is already corrected.

   

  Inevitably, the pelvic hook is not completely seated and there is flexibility in the ribs proximally, so the surgeon should still make the convex device a little bit longer than indicated by the template measurement.

   

  Once both devices are inserted and fully seated and out to length, and monitoring is normal, the anchor sites are again visually inspected to ensure their integrity.

• Completion and Wound Closure

After final tightening, all incisions are aggressively irrigated with both Betadine solution and normal saline solution.

We prefer the use of vancomycin powder in all patients who weigh more than 30 kg. We do this as an adjunct to our intravenous antibiotics that are designed to cover both gram-positive and gram-negative in these neuromuscular cases.

Meticulous soft tissue handling and wound closure is essential. We do not use drains for these incisions, which generally are very dry. We use Dermabond and a silver-impregnated dressing to give us maximal protection against surgical site infection, which is known to be a high risk in these cases.

Full-length, high-quality intraoperative radiographs (both PA and lateral) should be taken in the operating room (OR) and reviewed carefully prior leaving the OR (TECH FIG 5).

TECH FIG 5 • Postoperative radiograph of the patient in FIG 1 shows excellent correction of the scoliosis. Cobb angle is 16 degrees and a fully corrected pelvic tilt.

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POSTOPERATIVE CARE

Most patients are extubated, and ventilator support is not needed unless they have significant comorbidities.

In cases with a pleural tear, a chest tube is used and it is removed when drainage is less than 20 to 25 mL/day.

Weight-bearing anteroposterior and lateral radiographs are made postoperatively. No external bracing is necessary after surgery. Patients are allowed to return to full activities 6 weeks after hospital discharge.

OUTCOMES

The outcomes for rib to pelvis VEPTR as reported by Smith7 provides good insight that VEPTR can give excellent correction of scoliosis as the mean Cobb values of the primary and secondary curves including the kyphosis were shown to be significantly corrected.

In a series of 37 patients, 18 ambulatory patients underwent 139 procedures and the 19 nonambulatory patients underwent 100 procedures with an average follow-ups of 84 and 64 months, respectively. The nonambulators did significantly better than their ambulatory counterparts who developed crouch gait (7/18) and often needed conversion to rib-spine constructs (39%).

The overall rate of adverse events per procedure was reported to be 13%. The rate of adverse events in the nonambulatory group was 15%.

COMPLICATIONS

Complications for VEPTR include infection, skin sloughing, displacement and migration of devices through the rib, fatigue fracture, and neurologic issues such as brachial plexus neurapraxia or spinal cord

injury. Sankar et al,5 in a retrospective series, compared the complication rates of different systems. Dual growing rods had an average of 0.52 per year complications, “hybrid growing rods” had 0.36 per year complications, and VEPTR patients had 0.52 per year complications.

Campbell and Smith2 in 201 VEPTR patients (1412 surgeries) reported 3.3% infection rate per surgery. Skin slough rate was 8.5% and 27% rib cradle migration rate with complete cutout in 3 years.

Upper extremity neurologic injury is more common during VEPTR surgery than the lower extremity. The rates of potential neurologic injuries during primary implantation of the VEPTR are 2.8% and during

exchange of the VEPTR is 1.3%.6

REFERENCES

1. Butler JP, Loring SH, Patz S, et al. Evidence for adult lung growth in humans. N Engl J Med 2012;367:244-247.

    

    

2. Campbell RM Jr, Smith MD. Thoracic insufficiency syndrome and exotic scoliosis. J Bone Joint Surg 2007;89(suppl 1):108-122.

    

    

3. Davies G, Reid L. Effect of scoliosis on growth of alveoli and pulmonary arteries and on the right ventricle. Arch Dis Child 1971;46:623-632.

    

    

4. Ramírez N, Cornier AS, Campbell RM Jr, et al. Natural history of thoracic insufficiency syndrome: a spondylothoracic dysplasia perspective. J Bone Joint Surg Am 2007;89(12):2663-2675.

    

    

5. Sankar WN, Acevedo DC, Skaggs DL. Comparison of complications among growing spinal implants. Spine 2010;35(23):2091-2096.

    

    

6. Skaggs DL, Choi PD, Rice C, et al. Efficacy of intraoperative neurologic monitoring in surgery involving a vertical expandable prosthetic titanium rib for early-onset spinal deformity. J Bone Joint Surg Am 2009;91(7):1657-1663.

    

    

7. Smith JT. Bilateral rib-to-pelvis technique for managing early-onset scoliosis. Clin Orthop Relat Res 2011;469(5):1349-1355.