Most spinal curves can be described as standard scoliosis and are
due to adolescent idiopathic scoliosis. In patients with standard scoliosis,
the thorax is usually spacious, having achieved most of its adult volume
through growth, and has near normal vital capacity. Standard scoliosis is
characterized on an anteroposterior radiograph by the level and degree of the
curve and is treated by bracing or definitive spinal fusion to effect a
decrease in the Cobb angle. Treatment has a negligible effect on thoracic
growth or long-term pulmonary outcome.
Exotic scoliosis describes an early-onset spinal deformity that is
more complex in nature, often associated with a thorax that has been distorted
by spinal lordosis and curve rotation, thus having a volume-depletion
deformity as well as thoracic growth inhibition with indirect adverse effects
on lung growth (Fig. 1). Exotic
means "foreign," "outlandish," or "alien,"
and the curves of exotic scoliosis are easily recognizable. In the coronal
plane, this scoliosis is not only a "lateral curve" but, from a
three-dimensional thoracic viewpoint, can be considered a lateral flexion
contracture of the thorax with volume depletion on the concave
side and often additional volume depletion on the convex side, in the
transverse plane, from a windswept deformity of the thorax. Primary rib-cage
abnormalities, such as absent or fused ribs, add further thoracic disability.
The typical treatment approaches to spine deformity may be impractical for the
treatment of exotic scoliosis because of potential spine and thoracic growth
inhibition from early fusion or because of additional comorbidities (e.g., the
bone stock may be insufficient or too osteopenic to hold the instrumentation,
the patient may be too small for standard spinal implants, or the lung
function may be so poor that the patient would not survive surgery). In
addition, a spine fusion may be unable to address the three-dimensional
thoracic deformity. While deformity of the spine historically has been the
focus of orthopaedics, the spine serves only as the posterior pillar
of the thorax. It can be likened to the corner of a room, and it is difficult
to correct the full deformity of the "thoracic room" by only
addressing its corner. Even standard bracing or casting may not be possible in
patients with exotic scoliosis because the ribs (e.g., in patients with
osteogenesis imperfecta or other bone dysplasias) are unable to withstand the
corrective pressures. However, the most serious comorbidity in most patients
with exotic scoliosis is thoracic insufficiency syndrome, which is often due
to a volume-depletion thoracic deformity that either may not be addressed by
standard spinal care or that may even develop later, even though the spinal
treatment was otherwise successful (Figs.
2-A through 2-D). To understand this important complicating factor
of exotic scoliosis, the principles of normal thoracic volume, function, and
growth must be understood.
The thorax, a dynamic chamber of respiration, structurally
consists of the spine, the rib cage, and the sternum, with the diaphragm as
its base. The two characteristics of the thorax—normal, stable volume
and the ability to change that volume—are needed for normal
breathing1. The
thorax is the respiratory pump that accomplishes lung expansion
primarily through contraction of the diaphragm, with the rib cage providing
passive support around the lungs. This mechanism is termed primary
breathing1.
Additional positive thoracic change is accomplished through the mechanism of
secondary breathing, with intercostal muscle contraction providing anterior
and lateral expansion of the rib
cage1. Lung
development is dependent on thoracic growth; for pulmonary growth to proceed
in a normal fashion, the thorax must symmetrically enlarge itself through rib
and spine growth during the childhood and adolescent years
(Fig. 3).
Growth of the Thorax and the Lungs
Thoracic growth is complex, depending on the increase in height of the
thoracic spine, symmetrical enlargement of the rib cage through rib growth,
and the correct orientation of the ribs for the age of the child. The thoracic
spine provides the longitudinal component of thoracic volume. It grows in
height according to the age of the
child2: from birth
until a child is five years of age, the thoracic spine grows 1.4 cm/yr; from
six to ten years of age, 0.6 cm/yr; and from eleven to fifteen years of age,
1.2 cm/yr. The degree of height deficiency of the thoracic spine due to
congenital scoliosis or early spine fusion can be quantified by dividing the
actual measured height of the thoracic spine by the expected height of the
thoracic spine to derive a percent normal thoracic spinal height. The
specific relationship between loss of thoracic volume and thoracic spine
shortening as well as the indirect adverse effect on lung volume remains
undefined. However, marked loss of thoracic spinal height results in severe
reduction in thoracic volume for the lungs. In a natural history model, such
as spondylothoracic dysplasia (Jarcho-Levin
syndrome)3,4,
the thoracic spine is virtually a single, short, block vertebra, commonly only
one-fourth of normal height. This condition carries a high rate of mortality
early in life due to respiratory complications. Adult survivors in Puerto Rico
have gone on to have severe, but minimally symptomatic, restrictive lung
disease with an average vital capacity of only 27% of
normal5. Additional
studies are needed to define this crucial relationship.
The contribution of the width and depth of the rib cage to total thoracic
volume is complex. At birth, the ribs are horizontal in orientation, so growth
of the ribs in length, which occurs primarily at the anterior physis, results
in a direct increase in the diameter of the rib cage and the development of a
square-shaped thoracic cross section. At this stage of growth, the thoracic
volume is 6.7% of the adult
volume2. By two
years of age, however, the rib orientation
changes6, causing
the ribs to course downward obliquely and changing the thoracic cross section
to an oval shape. The thoracic cross-sectional volume now depends both on the
growth of the ribs in length and the degree of rib obliquity. Excessive
downward obliquity folds the thorax flat, decreasing sagittal depth, which
reduces volume. Anterior chest-wall deformity, such as pectus excavatum, can
also reduce cross-sectional volume. Thoracic volume increases to 30% of the
adult size by five years of age but only increases to 50% of the adult size by
ten years of age2.
During the last third of skeletal growth, from age ten years to skeletal
maturity, the thorax grows most rapidly in volume, doubling in
size2, and the final
rectangular-shaped thoracic cross section is seen. Recently, normative
computed tomographic scan lung volumes have been
published7, which
means that thoracic volume depletion deformity in young children, who are
unable to perform standard pulmonary function tests, can now be assessed by
computed tomographic scan lung-volume studies in order to derive a percent
normal computed tomographic scan lung volume.
Approximately 85% of lung alveolar cells are formed after
birth8, with most
added in the first two years of
life9. The end point
of alveolar cell multiplication is controversial, with two years of
age10, eight years
of age11, and
twenty years of
age12 all being
described as the age limit. It is difficult to achieve accuracy when counting
alveolar cells in lung specimens, and the end point of alveolar cell
multiplication may never be clearly
defined8. Through
alveolar cell hypertrophy, which is an important but poorly
understood component of lung growth, the lungs continue to increase in size
after alveolar cell multiplication ceases. Compensatory lung growth has been
shown to be triggered by a lung "stretch
reflex"13,
and compensatory lung growth by alveolar cell multiplication was possible
after experimental pneumonectomy in young
animals14 and
following therapeutic partial pneumonectomy in children who ranged in age from
thirty months to five
years15,16.
A detailed discussion of induced lung growth has recently been
published17. In all
of these clinical series, the stretch reflex of the lung was, in theory,
activated by the extra room created in the thorax by removal of lung tissue.
The activation of the stretch reflex of the lung purely by enlargement of the
thorax, without any removal of lung tissue, remains an untested
hypothesis.
Thoracic Insufficiency Syndrome
Thoracic insufficiency syndrome is the inability of the thorax to support
normal respiration or lung
growth1. It defines
the general disease that affects the biomechanical capabilities of the thorax
to serve as the engine of respiration that can enlarge appropriately
with growth. There is currently no simple laboratory test value to identify
thoracic insufficiency syndrome. It is analogous to the term
"osteomyelitis," which is a useful definition of bone infection,
but the clinical diagnosis of osteomyelitis requires further characterization
of the patient through a complete history, physical examination, radiographic
studies, and laboratory tests, with clinical judgment weighing each factor. In
the future, aspects of thoracic insufficiency syndrome will become better
characterized through clinical research and animal studies, but the diagnosis
will likely still remain clinical, with an emphasis on analysis of disorders
of the two characteristics of the thorax that affect thoracic volume and
function.
Types of Volume-Depletion Deformities of the Thorax
In thoracic insufficiency syndrome, three-dimensional deformity of the
thorax in general reduces the volume available for the lungs, but there are
different anatomic patterns of thoracic deformity that require different
specific corrective volume-enhancing surgical strategies, such as the use of
vertical expandable prosthetic titanium ribs (VEPTR), for effective treatment.
We have developed a volume-depletion deformity classification system
(Table I), on the basis of
radiographs and computed tomography studies of the thorax, that enables us to
choose the best surgical approach for either the unilateral or bilateral
volume deficits. Type-I and type-II deformities are asymmetric, with
unilateral volume-depletion deformities that require unilateral surgical
expansion to restore thoracic volume and symmetry in the coronal plane.
Type-III deformities have a global thoracic volume deficit, which may be due
to either a symmetrical longitudinal constriction such as that seen in
Jarcho-Levin syndrome (type-IIIa deformity) or a symmetrical transverse-plane
constriction such as that seen in Jeune asphyxiating thoracic dystrophy
(type-IIIb deformity). These deformities require bilateral staged surgical
correction to either lengthen or laterally expand the symmetrically
constricted thorax.
Disorders of the First Characteristic of the Thorax
The first characteristic of the thorax is that it must have normal, stable
volume. Congenital loss of thoracic volume can cause severe extrinsic,
restrictive lung diseases with high mortality rates in
children1. The
anatomic thoracic abnormality should be classified as a defined
volume-depletion deformity (see Table
I) so that the proper surgical volume-expansion procedure can be
chosen. Mixed volume-depletion deformities or bilateral deformities will
usually require a staged surgical approach, with the former requiring a
combination of various VEPTR expansion thoracoplasty strategies.
Exotic scoliosis is commonly present in types-I and II volume-depletion
deformities because the three-dimensional thoracic deformity cannot be
addressed with use of standard surgical techniques of the spine. In a
type-IIIa volume-depletion deformity due to spondylothoracic dysplasia,
scoliosis is seldom present, but in spondylocostal dyostosis, exotic scoliosis
is common. In type-IIIb volume-depletion deformity, scoliosis is not present
in Jeune syndrome, but, in windswept deformity, exotic scoliosis is always
present. VEPTR expansion thoracoplasty strategies are designed to address
these individual volume-depletion deformities.
Disorders of the Second Characteristic of the Thorax
The second characteristic of the thorax is that it must have the ability to
change volume. This is defined as thoracic
function1. The
diaphragm serves as the "piston" of respiration while the rib cage
is the "cylinder." Therefore, the diaphragm provides the important
primary breathing mechanism for respiration. While the contribution of
secondary breathing through rib-cage expansion is easily disabled by thoracic
deformity, the diaphragmatic piston can usually still function and compensate
for secondary breathing loss. Only in rare instances is there disruption of
the primary breathing mechanism. Bilateral hemidiaphragmatic compromise, such
as that seen in paralysis from spinal cord injury, can be a lethal event. Even
in unilateral hemidiaphragmatic involvement, the contralateral, normal
hemidiaphragm cannot fully compensate, so these patients are at high risk for
the development of frank respiratory insufficiency. In congenital
diaphragmatic hernia, the involved hemidiaphragm is traditionally repaired
with a synthetic membrane and contraction is poor. In congenital scoliosis
with rib anomalies, there can be anomalous insertion of the diaphragm that
disables full diaphragmatic contraction. The diaphragm may be inserted
proximally within the rib cage, where it is unable to contract properly and
its position blocks off hemithoracic volume for the lung. The diaphragm can
also be inserted anomalously into soft tissues in the absence of ribs, so
contraction serves only to pull the soft-tissue attachment medially without
effective expansion of the lung above. The diaphragm is also compromised in
secondary thoracic insufficiency
syndrome18, in
which the torso is collapsed onto the pelvis and there is an increase in
reactive abdominal pressure, thus limiting downward diaphragmatic excursion.
Secondary thoracic insufficiency syndrome is most commonly seen with lumbar
kyphosis in patients with myelomeningocele. A clinical diagnosis of this is
supported by the presence of the so-called marionette
sign18, in which
the head and torso "bob" in synchrony with respiration, with the
diaphragm doing a "push-up" against body weight. Fortunately,
disorders of primary breathing are extremely rare.
A more common early disturbance in thoracic function involves secondary
breathing1. The rib
cage, the "cylinder" of the respiratory engine, provides simple
support of the lungs while the diaphragmatic "piston" increases
thoracic volume by downward excursion, but this biologic cylinder also can
contribute to a volume increase by expanding itself anterolaterally. Rib hump
deformity easily disables this addition to vital capacity, and, from a
biomechanical standpoint, rib-cage motion is probably lost due to
malorientation at the costovertebral junction. The presence of either fused
ribs or absent ribs can directly disable the ability of the hemithorax wall to
assist in expanding the lung. The rib hump in scoliosis also decreases
hemithoracic volume in a windswept deformity of the chest. In anomalous
transverse orientation of the ribs, the bucket-handle biomechanism of rib-cage
expansion during respiration is also disabled when the ribs are locked in the
"end-inspiration" position; this is commonly seen in
myelomeningocele.
If the thorax cannot support either normal respiration or lung growth, then
thoracic insufficiency syndrome is present. The presence of either component
enables a diagnosis of thoracic insufficiency syndrome, but commonly both
components are present. A frequent misperception is that a child must be
oxygen dependent for the diagnosis of thoracic insufficiency to be made, but
respiratory insufficiency is different from thoracic insufficiency syndrome.
Deficiencies in thoracic performance can degrade respiratory efficiency, but
compensation mechanisms of the child can mask the problem clinically. For
example, a child with unilateral fused ribs cannot expand the involved
hemithorax, so normal respiration on that side is not possible, but an
increased respiratory rate, mediated through the diaphragm, may compensate and
prevent clinical hypoxia. This is known as clinically occult respiratory
insufficiency1.
Thoracic insufficiency syndrome is present, since the thorax cannot support
completely normal respiration unilaterally, but unless the stiff, fused
hemithorax is clinically appreciated and the respiratory rate determined, the
child will appear normal. Frank clinical respiratory insufficiency, for which
the patient requires oxygen support or more invasive means such as continuous
positive airway pressure or a ventilator, is seen when thoracic insufficiency
syndrome is severe, often with intrinsic lung disease such as atelectasis,
bronchiectasis, or fibrosis from recurrent pulmonary infection.
Both the thoracic insufficiency syndrome and the intrinsic lung disease
contribute to low vital capacity. The recognition of thoracic insufficiency
syndrome, therefore, needs to be based on comprehensive history, physical
examination, radiographs, computed tomography scans, lung scans, other imaging
studies, arterial or capillary blood gases, pulse oximetry, echocardiograms,
and, when feasible, pulmonary function testing. Thoracic insufficiency
syndrome needs to be further defined as mild, moderate, or severe and as
progressive or not progressive. Each possible treatment option, whether it
includes observation, bracing or casting, growing spinal rods, spine fusion,
or VEPTR expansion thoracoplasty, needs to be evaluated for its effect on the
present level of thoracic insufficiency syndrome and the long-term effect of a
specific treatment on the level of this disorder.
The final sequela of progressive thoracic insufficiency syndrome is
respiratory insufficiency. It may start with early occult respiratory
insufficiency when the child begins to have limited pulmonary reserves,
fatiguing easily with play activities and compensating with an increased
respiratory rate at rest but not yet requiring oxygen support. Once the
child's compensatory respiratory mechanisms are overwhelmed, however, various
degrees of external respiratory support are needed to maintain oxygenation. It
is useful to grade the degree of respiratory insufficiency functionally
because increasing dependence on respiratory support impacts directly on the
quality of life. The clinical respiratory status of patients with frank
respiratory insufficiency is graded by the multispecialty team at our
institution with use of an assisted ventilation
rating19
(Table II). An increase in the
assisted ventilation rating reflects an exponential clinical deterioration in
respiratory function with important consequences for the family and the child.
If treatment results in an improvement in the assisted ventilation rating,
then the gain in quality of life is also clinically exponential.
Multispecialty Evaluation
At our institution, all patients with suspected thoracic insufficiency
syndrome are evaluated separately by a pediatric orthopaedist, a pediatric
general surgeon, and a pediatric pulmonologist. Afterwards, in a face-to-face
conference, all of the specialists discuss the patient from the perspective of
their respective expertise. The following questions need to be answered:
Is the thorax able to support normal respiration?Is the thorax able to support normal lung growth?Is this condition progressive?Is the thorax really the problem?
Is the thorax able to support normal respiration?
Is the thorax able to support normal lung growth?
Is this condition progressive?
Is the thorax really the problem?
A weighted thoracic insufficiency syndrome profile is then constructed.
Each specialist rates the aspects contributing to thoracic insufficiency
syndrome on a numerical scale from 1 (mild) to 10 (severe). The three sets of
scores are then combined to obtain a cumulative score for each of six aspect
categories: history, physical examination, radiographic results, computed
tomographic results, results of lung scans and pulmonary function tests, and
laboratory results. For example, the history aspect of a patient may include
several episodes of pneumonia, requiring hospitalization with temporary
ventilator support, with increasing frequency of hospitalizations over the
previous year. The pulmonologist may rate this aspect a "10"
because thoracic insufficiency syndrome tends to lead to recurrent pneumonia,
and multiple pulmonary infections may cause scarring of the lungs with the
development of pulmonary insufficiency. The episodes also suggest progression.
The other two specialists may also rank this aspect as serious and score it as
a "10," for a total cumulative score of "30." This
score would strongly support a diagnosis of thoracic insufficiency syndrome in
the history category.
On physical examination, the respiratory rate may be mildly elevated, the
chest circumference may be in the 65th percentile clinically, with a moderate
scoliosis, and there may be limited chest-wall motion on the convex side as
measured by the thumb excursion
test1, but there is
no clubbing of the fingers or perioral cyanosis. The chest circumference and
the elevated respiratory rate are somewhat worrisome, and completely normal
respiration certainly is not possible because the chest wall lacks secondary
breathing on one side; however, as long as the chest hypoplasia is not
progressive, it is probably not serious. Thus, the physical examination aspect
category may merit a cumulative score of "20."
The radiographic aspect may show scoliosis with fused ribs. If this
condition is progressive, it suggests an increasing three-dimensional
deformity of the thorax with development of windswept deformity in the
transverse plane. This may result in a cumulative score of "25"
for this category.
The computed tomography scan transverse sections may confirm severe loss of
hemithorax volume on the convex side from the windswept deformity, not obvious
on plain radiographs, so this aspect may merit a cumulative score of
"30."
Ventilation-perfusion lung scans may only show mild asymmetry, and the
patient is too young to undergo a standard pulmonary function test. Computed
tomography lung volumes, however, can be compared with normative
values7 to provide
more information, but such results may not be available. The cumulative score
for this aspect may be "15."
Laboratory results, such as serum electrolytes and arterial or capillary
blood gases, may be normal, meriting a cumulative score of
"0."
The profile for this theoretical patient is then constructed
(Fig. 4). An aspect category
score of "20" or greater is supportive of a diagnosis of thoracic
insufficiency syndrome. The more aspect categories that involve scores that
are equal to or greater than "20," the more severe the thoracic
insufficiency syndrome becomes.
Use of the profile may aid in making the decision as to whether the
thoracic insufficiency syndrome is mild, moderate, or severe. The next step is
to systematically review all treatment options—VEPTR expansion
thoracoplasty and others, including observation—and determine how each
course of action may affect the individual aspect scores of the thoracic
insufficiency syndrome profile with time. Is the thoracic insufficiency
syndrome too mild to treat? In this case, observation is chosen, but
parameters such as radiographs and computed tomography scans, or serial
pulmonary function tests (once the child is old enough), are followed to
detect progression. Moderate thoracic insufficiency syndrome may require
either further observation or treatment. Is the thoracic insufficiency
syndrome too severe to salvage with treatment? This may be the case in older
patients in whom there is little hope of lung growth, although the thorax can
be expanded. Although subjective, the weighted thoracic insufficiency syndrome
profile serves as a useful tool that helps organize complex patient data in a
systematic way with multiple-specialty weighted input. Our institution accepts
patients for VEPTR treatment only if all specialists agree with the VEPTR
surgery recommendation.
The VEPTR device (Synthes Spine, West Chester, Pennsylvania) is available
for use under a United States Food and Drug Administration humanitarian device
exemption for the treatment of thoracic insufficiency syndrome of skeletally
immature patients who fall into certain diagnostic categories
(Table III). Early onset
infantile scoliosis, neither congenital nor neurogenic in origin, can be
treated if a constrictive chestwall syndrome from a windswept deformity in the
transverse plane is present. Specific VEPTR surgical strategies are based on
the respective volume-depletion deformities to be corrected by various types
of VEPTR expansion thoracoplasty.
Type-I Volume-Depletion Deformity: Absent Ribs and Scoliosis
The VEPTR expansion thoracoplasty for absent ribs and scoliosis
volume-depletion deformity is termed a stabilization expansion thoracoplasty.
It is performed through a thoracotomy with the goal to not only stabilize the
flail chest-wall segment but also to laterally expand and lengthen the
collapsed hemithorax
section19. Two to
three VEPTR devices (radius, 220 mm) are usually used. The operative goal is
to equilibrate the thorax in all planes and to increase the space available
for lung1 to 100%,
with symmetrical width on radiographs and with symmetrical hemithorax volumes
on the computed tomography scan in the transverse plane
(Figs. 5-A and 5-B).
Type-II Volume-Depletion Deformity: Fused Ribs and Scoliosis
The expansion thoracoplasty used for this volume-depletion deformity is an
opening wedge
thoracostomy18,20.
The fused hemithorax is osteotomized transversely and lengthened acutely until
the thorax is equilibrated, with the space available for lung approaching 100%
(Figs. 6-A and 6-B). In
children up to age eighteen months, a single rib-to-rib 220-mm-radius VEPTR is
implanted posteriorly to stabilize the acute correction, whereas, in older
patients, a hybrid device is added medially with a second rib-to-rib device
placed more posterolaterally. Unilateral unsegmented bars appear to lengthen
with time after this procedure, resulting in a growth in length of the
thoracic spine21 to
aid in increasing thoracic volume. The most favorable percent normal vital
capacity at the time of follow-up is seen when these patients have surgery
before two years of age and have no history of spine
surgery18.
Type-IIIa Volume-Depletion Deformity: Jarcho-Levin Syndrome
Staged bilateral opening-wedge
thoracostomies22
are performed for this volume-depletion deformity with the placement of
rib-to-rib (radius, 220 mm) VEPTR devices
(Figs. 7-A and 7-B). The goal
is to lengthen sequentially each constricted hemithorax while correcting any
associated scoliosis by performing the first procedure on the concave
hemithorax.
Type-IIIb Volume-Depletion Deformity: Jeune Syndrome or Infantile
Scoliosis with Windswept Deformity
For the volume-depletion deformity of Jeune syndrome, staged bilateral
dynamic segmental expansion thoracoplasties, stabilized with an acutely curved
VEPTR (radius, 70 mm), are performed to address the circumferential thoracic
constriction23
(Figs. 8-A and 8-B). Most of
the lung tissue lies in the two lateral lobes of the trifold chest, so, to
expand these areas posterolaterally, a large segment of chest wall, containing
six to seven ribs, is mobilized by rib osteotomies anteriorly near the
costochondral junction and by osteotomies posteriorly just lateral to the
transverse processes. Stable ribs are left proximally and distally for
prosthesis attachment. The iatrogenic flail chest segment is then acutely
distracted outward, increasing the thoracic volume, and is attached by
titanium slings to the VEPTR, which functions as an arch of support for the
segment. The diastasis between the osteotomized ribs fills in with new bone
within six weeks, effectively providing new length for the ribs. The newly
created thoracic space acutely fills up with a pleural effusion, with the lung
remaining the same size, but, within six months, lung lucency is seen
extending outward on computed tomography scan to the new borders of the
expanded thorax. The second side is treated three to six months later.
Periodic expansion of the curved VEPTR devices continues to drive the chest
wall outward for continued gains in volume.
In early onset scoliosis, the transverse plane constriction causing the
volume-depletion deformity is due to the wind-swept deformity of the thorax
with some type-II constriction of the concave hemithorax in severe curves. The
concave chest is lengthened through an opening-wedge thoracostomy by lysis of
the intercostal muscles at the apex of the hemithorax constriction, with
preservation of the pleura. The thoracic constriction, often proximal to the
apex of the curve, is defined as persistent narrowing of the intercostal
spaces on a supine radiograph with lateral bending away from the
curve24
(Figs. 9-A and 9-B). The
lengthened hemithorax is usually stabilized by a unilateral hybrid
rib-to-spine or rib-to-pelvis VEPTR, but bilateral hybrid VEPTRs, placed from
rib to spine or rib to pelvis, may be preferable if there are double major
curves requiring control, and a dual rib-to-pelvis hybrid VEPTR construct is
best for associated thoracic kyphosis because a long lever arm is needed. If
open bilateral thoracotomies are necessary, the procedures are staged. If the
convex hemithorax rib-to-pelvis hybrid VEPTR is inserted in the manner of
Smith25 and the
concave hemithorax lengthening is performed openly, then dual devices are
implanted during the same procedure.
The repetitive nature of maintenance surgeries required after either VEPTR
expansion thoracoplasty or implantation of growing spinal rods for the purpose
of lengthening devices to accommodate growth of the patient makes both of
these approaches prone to complications. Each new incision is a new
opportunity for the occurrence of a wound infection or skin slough. Scarring
from repeated surgeries may make the soft tissues more susceptible to
infection, but, to our knowledge, no studies have ever addressed this
hypothesis. The comorbidities that tend to occur in these complicated patients
with thoracic insufficiency sy ndrome probably increase the risk of
complication. The most common comorbidity is myelomeningocele, for which the
risk of complications in spine surgery is as high as
40%26.
The cumulative device-related complications of all patients treated by
VEPTR expansion thoracoplasty at our institution over a fifteen-year period
for all forms of volume-depletion deformities, with numerous comorbidities
such as myelomeningocele, were analyzed
(Table IV). They were compared
with the complications seen in two large growing rod
series27,28.
Our patient numbers and length of follow-up were both larger than those in
either series. Our infection rate per procedure was 3.3%, which was less than
in one series but higher than in the other for unclear reasons. Skin slough
was seen in 8.5% of our patients. Migration of fixation is a common problem
for both VEPTR instrumentation and growing rods, but, to our knowledge, the
effect on length of follow-up on the probability of migration of fixation has
not been studied. It is intuitive that the risk of a metal fixation device
migrating into a bone in active young patients, whether into posterior
elements of the spine or a rib, increases with the passage of time. To
evaluate this risk, we developed a migration index, in which the average risk
of migration per patient per year was determined by dividing the total number
of migration events by the total number of patients and then dividing the
quotient by the total number of patient-years of follow-up. The index thus
represents the risk of a migration per year for each patient. We defined a
migration event as complete passage of a device through the bone; partial
excursion into bone was not counted. This was performed for all three series.
Our migration index was 0.09 migrations per year per patient, equivalent to
that in one growing spinal rod
series27 but higher
than that in another
28. Our migrations
were not acute dislodgments of fixation, but rather a slow cephalad migration
of the superior rib cradle into the rib of attachment or a posteroinferior
migration of the spinal hybrid hook into the posterior spinal elements with
complete migration over an average time of 3.2 years. The two growing spinal
rod series did not evaluate this time factor. Invariably, the rib reformed
after complete migration and it was often possible to reseat the rib cradle on
the original site of insertion. In our series, 27% of patients had migration
events, but almost all were asymptomatic, with the complete migration often
recognized fortuitously on preoperative radiographs made at the time of
elective device expansion. Device breakage was lower in our series than in the
two growing spinal rod series.
Complications always occur in complicated patients. The realistic goal is
to minimize, not eliminate, complications and have effective ways to recognize
and treat the complications that do occur. In considering treatment options,
it is important to weigh the risk of inherent complications of VEPTR treatment
against the risk of complications of alternative treatments or the natural
history.