Growing Rods
Single and Dual Growing Rods
As discussed throughout this symposium, posterior instrumentation without
fusion, with the use of single or dual growing rods, has been used to treat
progressive early onset scoliosis in young children. The goals of treatment
have been to achieve correction of the spinal deformity, maintain correction
during the subsequent growth period, allow spinal growth and lung development,
and avoid or eliminate the need for definitive fusion of the spine at an early
age.
A recent study has demonstrated improved surgical results with use of the
dual growing rod
technique1. These
improvements are primarily due to the ability to achieve a more stable
construct and to the performance of more frequent lengthening procedures. The
results of a recent long-term study that examined the outcomes of the dual
growing rod technique have confirmed that there is significantly better
correction of scoliosis (p = 0.0256) and better achievement of
growth1.
Despite these technical improvements, the surgical treatment of progressive
early onset scoliosis remains complex and is associated with a high rate of
complications, including anchor dislodgment or rod breakage, wound problems,
and alignment issues. Furthermore, these children are generally at a higher
risk because of comorbidities (especially pulmonary difficulties) and the long
period of treatment from initial surgery to final fusion. Even though
lengthening may be done as an outpatient procedure, multiple procedures and
anesthesias are required.
Ideally, the growing rod technique should provide a stable construct
spanned over the shortest possible number of segments necessary to stop the
progression of the deformity and maintain correction in all three dimensions.
The implants should have a low profile and be able to be used in small
children. The procedure should require less invasive methods and less frequent
lengthenings, preferably without the need for anesthesia (perhaps with use of
remote lengthening methods). Ideally, if the deformities are controlled until
skeletal maturity, it may be possible to remove the implants and allow maximal
mobility of the spine without the need for spinal fusion.
In order to make an accurate evaluation of the results of different types
of treatment, it will be necessary to develop methods of outcome evaluation
that are specific to patients with early onset scoliosis. Long-term,
society-wide research and multicenter studies should be developed to document
the effectiveness of the treatment methods in comparison with the natural
history of these diseases.
VEPTR, Thoracic Insufficiency Syndrome, and Pulmonary Growth
VEPTR: Thoracic reconstruction procedures made possible by the
vertical, expandable prosthetic titanium rib (VEPTR) represent a shift in
emphasis from focusing exclusively on spinal deformity alone to addressing all
related components of thoracic deformity of the growing child, including the
spine, the rib cage, and the diaphragm, without compromising thoracic growth.
The classic orthopaedic approach has been correction of the angular deformity
of the spine through implanted instrumentation. In the VEPTR procedure,
however, the surgical strategy is radically different. The thoracic deformity
is first corrected as much as possible through rib osteotomies or intercostal
muscle lysis, thus enlarging the constricted thorax, and then the expandable
ribs are added, not to correct deformity but rather to stabilize the
correction obtained by the reconstruction. The devices are then lengthened
every six months. As with spinal instrumentation, VEPTR does maintain
correction of the Cobb angle, but the similarity stops there as the ambitious
goal of VEPTR surgery is to restore volume and symmetry to the growing
malformed thorax, in hopes of stabilizing or improving the biomechanics of the
thoracic "engine of respiration," so that there is maximum room
for growth of the underlying lungs.
Like most instrumentation, VEPTR will undergo incremental technical
modifications to improve ease of use and effectiveness. The greatest challenge
will be to provide a self-expansion capability to VEPTR constructs so that the
cost and morbidity associated with repeat operative procedures for device
expansion can be minimized. VEPTR strategies, however, regardless of technical
improvements, address only the volume-depletion deformity of the malformed
thorax, not thoracic function deficits. The real task ahead is to better
understand the prime indication for VEPTR with regard to thoracic
insufficiency syndrome, and to understand how VEPTR will impact the health and
longevity of the child.
Thoracic insufficiency syndrome: Thoracic insufficiency syndrome
is the inability of the thorax (the combined biomechanical unit of the
thoracic spine, the rib cage, the sternum, and the diaphragm) to support
normal respiration or lung growth in the
child2.
Three-dimensional volume-depletion distortion of the thorax, as a result of
combined spine and chest-wall malformation, is the anatomic cause of this
syndrome. Outcome measures to assess this deformity and its linkage to lung
growth and pulmonary function are almost completely lacking. Even though the
Cobb angle is the orthopaedic gold standard for assessment of spinal
deformity, it has limited relevance as a static two-dimensional measurement of
this dynamic, three-dimensional problem. Not only is there limited knowledge
about the natural history of the thoracic insufficiency syndrome component of
diseases of the growing spine, but treatment outcomes have been assessed only
by the efficacy of correction of the angular deformity of the spine without
assessment of the effect on growth, volume, and biomechanical effectiveness of
the thorax. It is difficult to understand a disease without first
understanding what is normal, but, other than the growth in length of the
thoracic spine as measured radiographically and caliper estimates of thoracic
volume, knowledge about the growing thorax is limited. How the thorax grows,
how it gains the critical volume to accommodate lung growth through either
spinal growth or a change in cross section, how rib orientation affects the
biomechanics of the costovertebral joints, and how expansion of the chest wall
contributes to vital capacity are all poorly understood areas. It is crucial
to understand how VEPTR or any surgical procedure on the spine will influence
all of these variables in the growing child, but the tools of assessment
currently do not exist.
Future research will need to address many important questions. Long-term
longitudinal studies of the growth of the thoracic spine in children, from the
time of birth until the time of skeletal maturity, are needed to obtain
comprehensive normative data in the tradition of the Green-Anderson
limb-growth studies3
so that treatment decisions can be more evidence-based. Advanced technology
assessment can complement radiographic and physical measurements of these
children, with lung volumes, diaphragmatic area and function, and intercostal
muscle chest-wall expansion assessed with techniques such as dynamic magnetic
resonance imaging and then correlated with infant pulmonary function testing
or standard spirometry.
Pulmonary growth: A better understanding of normal lung growth and
its relationship to thoracic growth will require an animal model. The use of
nonhuman primates, such as baboons, may provide us with a better understanding
of the timing and importance of two mechanisms of lung growth: the addition of
new alveolar cells, and hypertrophy of existing alveoli, thus providing
insight into how normal lungs grow with age. Once normal development is
understood, then abnormal processes can be better studied, and the effect of
thoracic volume-depletion deformity on the lungs can be addressed. Analogous
to the growing femoral head and acetabulum, the lungs and the growing chest
probably have a yet-to-be defined symbiotic relationship, but the only safe
assumption at this point is that lungs will only grow to the size permitted by
the thorax. A small thorax constricted by malformation and deformity will
contain small lungs, and pulmonary morbidity and mortality may follow. A
non-human primate study of lung histology and size as well as growth of the
thorax following surgical simulation of rib absence, rib fusion, and early
spinal fusion would help clarify these issues. Advanced imaging techniques
could provide important information about the biomechanical dynamics of
thoracic performance in these nonhuman primate models.
Future studies: Once the tools of assessment for thoracic growth
and function are developed, the deformities that cause thoracic insufficiency
syndrome can be better characterized. Recently, untreated spondylothoracic
dysplasia, a cause of severe thoracic insufficiency syndrome, was studied with
use of computed tomographic scanning analysis and pulmonary function testing
in addition to traditional
radiography4. The
mean forced vital capacity was only 29.5% of predicted normal, which was a
reflection of thoracic volume losses from a reduction in height (mean thoracic
spinal length, 24.2% of predicted normal) and was also the result of the loss
of contribution of chest-wall expansion to vital capacity as a result of
bilateral rib fusions. Lung volumes, compared with the recently published
normative values of Gollogly et
al.5, were below the
third percentile. The three-dimensional thoracic deformity caused by
spondylothoracic dysplasia, and the effect of that deformity on pulmonary
function, was characterized for the first time by the authors of this landmark
study, and thus this condition may be relevant as a natural history model for
patients who are undergoing early spinal fusion for the treatment of
deformity. The natural history of other thoracic malformations also needs to
be studied with use of the three-dimensional approach, with the addition of
dynamic magnetic resonance imaging or other techniques to assess thoracic
biomechanics.
Idiopathic scoliosis, without rib abnormalities, is classically considered
a pure angular deformity of the spine. The goal of surgery has remained
unchanged since 1962, when Paul Harrington wrote that spinal surgery should
"correct the curve and stabilize the treated segments in the corrected
position by skeletal
fixation."6 He
added that instrumentation alone is preferred for those nine years of age and
younger, while fusion should be added to the instrumentation for older
patients. That surgical strategy remains essentially unaltered forty-four
years later. To better accomplish Harrington's goal, technological advances in
spinal surgery instrumentation have been impressive, with many primary
improvements made with regard to the strength of implants through advances in
metallurgy and in design innovation through the introduction of dual posterior
rods, segmental vertebral fixation (now available with thoracic pedicle
screws), and rigid anterior instrumentation implantable through either open or
thoracoscopic techniques. These improvements have made it possible to achieve
better correction of the deformity of scoliosis (as seen with measurement of
the Cobb angle), have helped decrease the risk of pseudarthrosis, and have
created less need for postoperative immobilization. Incremental improvements
in instrumentation will continue. The primary outcome measure for all of these
advances remains correction of the Cobb angle. But is scoliosis just an
angular deformity of the spine, or is it a three-dimensional deformity of the
thorax that is only poorly demonstrated by plain radiographs? What does the
Cobb angle tell us about the effect of scoliosis on thoracic biomechanics,
lung growth, health, and longevity?
The effect of idiopathic scoliosis on thoracic volume, function, and growth
and the resultant effect of these variables on pulmonary function and
development is poorly understood, both in terms of natural history as well as
with regard to the treatment effect of the classic tried-and-true spinal
fusion or growth-sparing surgery (e.g., VEPTR and growing rods) and other
methods currently under development. Numerous questions remain unanswered or
even unconsidered. What is the effect of spinal fusion on the normal loss of
vital capacity with aging at the time of skeletal maturity? What is the
minimum length of thoracic spine needed at skeletal maturity for adequate
thoracic volume and vital capacity, and how should this be factored into the
timing of an early spinal fusion for a young child? Is pulmonary function
adversely affected by early spinal
fusion7, or is it
only affected by severe spinal deformity and thoracic distortion? Does the
limited thoracic spinal fusion that is necessary in the use of
"growth-sparing" techniques, such as dual or single growing rods,
significantly compromise thoracic volume? Can a child with scoliosis and a
stiff rib hump with a windswept deformity of the chest tolerate as much
thoracic spinal shortening from fusion as can a child with a similar curve
with a spacious thorax and a mobile chest wall? What are the limits of normal
thoracic spinal rotation and kyphosis in providing sufficient thoracic volume
for diaphragmatic breathing and costovertebral joint alignment and/or mobility
for secondary breathing by chest wall expansion? Can a lung that is thin from
compression in a convex hemithorax, and surrounded by a stiff rib hump, be
adequately expanded with only a distorted hemidiaphragm at its base? What is
the "point of no return," when spinal rotation permanently
disables rib motion of the convex rib cage and the constricted lung will no
longer compensate with new growth even if thoracic volume and symmetry are
restored? Growth-sparing scoliosis treatments would best be performed before
reaching this point of no return. Currently being investigated are genetic
screening tests that may identify patients whose small curves will almost
certainly progress and will be brace resistant, so that growth-sparing
surgical intervention can be considered earlier, when deformity is minimal,
with probable greater effectiveness. All of these issues are very important,
but much research and advancement in assessment techniques are needed before
the answers will be available.
The basic proof of the efficacy of a treatment method is that the result is
better than the natural history of the disease process, as measured by both
primary and secondary outcome measures. Other than Cobb angle correction,
treatment of the child with spinal deformity is assessed subjectively.
Akbarnia has coined the term "opinion-based-medicine," and
although this term is currently clinically relevant for the care of children
with spinal deformity by the experienced orthopaedist, the goal of the future
is to replace the current anecdotal approach with an evidence-based medicine
model of disease study and treatment, thus developing outcome measures that
not only define angular deformity of the spine but also acknowledge the
complex variables of dynamic thoracic dysfunction in thoracic insufficiency
syndrome.
Spinal Growth Modulation
Potential Role of Staples, Tethers, and Other Fusionless Devices
Spine staples: Intervertebral stapling as an alternative to
bracing for the treatment of progressive scoliosis in juvenile and adolescent
idiopathic scoliosis is a logical extension of growth-sparing techniques in
pediatric spinal deformity. Although there is a fairly extensive clinical
experience with a Nitinol shape memory alloy staple (Medtronic Sofamor Danek,
Memphis, Tennessee) that inhibits convex apophyseal growth, the absolute
indications for this operative procedure are unclear. One major issue is the
difficulty in determining when an operative intervention should be applied to
patients who currently are being offered nonoperative treatment with either
observation or a brace. Two major advances that should occur in the next five
to ten years may help to resolve this dilemma. First, better data will be
forth-coming from the Bracing in Adolescent Idiopathic Scoliosis (BrAIST)
study, which is due to start in 2007 and is being funded by the National
Institutes of Health. This study will explore the efficacy of bracing the
scoliotic spine in immature North American patients. Second, it is hoped that
advances will occur in the realm of genetic understanding of the risk of
progression in idiopathic scoliosis. It is entirely probable that, at a not
too distant time in the future, skeletally immature patients with idiopathic
curves in the 20° to 40° range will have a "progression
risk" genetic screening test performed that will help determine what the
best treatment (if any) will be. In this way, it will be possible to harness
future scoliotic growth by stabilizing the curve, and it may even be possible
to reverse some of the deformity. Those at highest risk of progression may be
offered more aggressive treatments to prevent progression of the curve, while
those at lowest risk may just be observed. Either way, there may be an upper
limit of Cobb measurement or clinical deformity in which some form of
intervertebral stapling is rendered ineffective. Many other issues remain,
including determining the optimal levels to staple, the type of staple to
utilize, the number and position of the staples on the vertebrae, and the most
appropriate curve pattern (thoracic, thoracolumbar and/or lumbar) to treat
with this method. Lastly, postoperative protocols and activities for these
patients will need to be defined and evaluated.
Spinal tethers: There is a group of devices called spinal tethers
that can limit growth on the convex side of progressive curves, both those in
the frontal plane, involving scoliosis, and those in the sagittal plane,
involving kyphosis. Although the theory is somewhat similar to the technique
of intervertebral stapling, tethering devices have the theoretical advantage
of applying corrective forces onto the deformed spinal column during
placement. Thus, they may be indicated for the treatment of deformities of
larger magnitudes in immature patients. It may be possible to place vertebral
body screws on both sides of a convexity and then attach a tethering type of
"rope" material, which is then shortened and tightened. In a
similar manner, a kyphosis may be corrected by placing several sets of
posterior pedicle screws cephalad and caudad to a progressive kyphosis in an
immature patient, along with the subcutaneous placement of a similar tether
material. With continued growth, the convex scoliotic and kyphotic tether
would encourage further "growth" on the concave (scoliotic) and
anterior (kyphotic) regions, thereby lessening or reversing the deformity.
Many factors have to be determined, including the optimal levels of fixation,
the optimal tethering material that will provide long-term stability without
total immobilization of the spinal column, and the best timing for the
application of these techniques.
Other fusionless procedures: Other growth modulation techniques
that may be pursued in the future include methods to directly control the
growth plates of the deforming vertebrae. If the concave portion of scoliotic
vertebral growth plates could be isolated and some form of inhibitor (such as
a laser) could be applied to limit future growth, future curve progression
could be curtailed. This would require enhanced intraoperative imaging for the
purpose of specifically isolating the optimal region of the growth plate to be
ablated among many or all levels of the scoliotic curve. It also may require
mechanical means of inhibition, such as stapling or tethering, to produce a
maximum benefit. Once again, knowledge regarding which immature curves have a
high risk of progression, thus warranting these types of prophylactic
interventions, would be quite beneficial. It is possible to envision a menu of
nonfusion treatments that would be applied over time, depending on the
efficacy of each treatment and on the extent of modulation that occurs.
Lastly, even if fusion of certain progressive deformities is required,
limiting the fusion area by applying tethers or other devices to regions
cephalad and caudad to the fused levels may lessen the long-term morbidity
that can result from a spinal fusion. Such an approach could also lessen the
transition stresses from the unfused spine to the unoperated, mobile spine.
The future of these fusionless devices appears promising, and it will be very
important to perform clinical studies to help sort out the best use of these
newer technologies.
In conclusion, early onset scoliosis should be separated from other forms
of scoliosis. Questions regarding etiology and treatment are different and
need to be addressed specifically. The Scoliosis Research Society, the
Pediatric Orthopaedic Society of North America, Shriners Hospitals for
Children, and the American Academy of Orthopaedic Surgeons need to encourage
multicenter studies, particularly in the areas of natural history, spinal
growth and lung development, treatment, and outcomes measurement. This type of
research will require appreciable funding from the National Institutes of
Health as well as from industry, foundations, and societies to further explore
the genetic aspects of the cause and possible prevention of this disease.
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