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What’s New in Pediatric Spine Growth Modulation and Implant Technology for Early-Onset Scoliosis?

Wessell, Nolan M. MD*; Martus, Jeffrey E. MD; Halanski, Matthew A. MD; Snyder, Brian MD, PhD§; Truong, Walter MD∥,¶

Journal of Pediatric Orthopaedics: January 2018 - Volume 38 - Issue 1 - p e3–e13
doi: 10.1097/BPO.0000000000000830
POSNA Review Articles

Background: Early-onset scoliosis (EOS) affects roughly 1 to 2 out of 10,000 live births per year. Because this subset of patients has a yet to achieve a majority of their skeletal growth, a number of treatment challenges need to be addressed before surgical intervention. If left untreated, EOS can cause a number of problems throughout the patient’s lifespan, particularly in regards to the growth of the thorax and pulmonary development. A wide variety of surgical systems and techniques are available to the treating surgeon.

Methods: A review of the orthopaedic literature from 2010 to 2015 relating to pediatric spine growth modulation was performed. Ninety-eight papers were identified and, following exclusion criteria, a total of 31 papers were selected for further review.

Results: This paper summarizes the recently published literature regarding growth-friendly spinal implants, the status of their Food and Drug Administration approval labeling as well as the indications, applications, and complications associated with their implementation.

Conclusions: There are a growing number of options at the surgeon’s disposal when treating patients with EOS. As surgeons, we must continue to be vigilant in our demand for sound clinical evidence as we strive to provide optimal care for our patients. The rapidly advancing field of spinal growth modulation is exciting. More work must be done to further enhance our ability to predictably modulate growth in the pediatric spine.

*Department of Orthopaedic Surgery, Henry Ford Health System, Detroit, MI

Vanderbilt University School of Medicine, Nashville, TN

University of Wisconsin School of Medicine & Public Health, Madison, WI

§Boston Children’s Hospital, Harvard Medical School, Boston, MA

Gillette Children’s Specialty Healthcare, St Paul

Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, MN

None of the authors received financial support for the production of this manuscript.

M.A.H. has received implant donations from Medtronic Inc. for previous studies unrelated to the work contained herein. W.T. has previously received a 1-day travel stipend from DePuy/Synthes to observe live operative demonstrations. The other authors declare no conflicts of interest.

Reprints: Nolan M. Wessell, MD, Department of Orthopaedic Surgery, Henry Ford Health System, 2799 W. Grand Blvd, CFP-6, Detroit, MI 48202. E-mail: nwessel1@hfhs.org.

According to the National Scoliosis Foundation, the incidence of scoliosis ranges between 2% and 3% among the general population.1 A subset of these patients (1 to 2 per 10,000 births) present at an age at which the majority of musculoskeletal growth has yet to occur.1 Early-onset scoliosis (EOS) represents a group of congenital and acquired conditions that affect the growth and development of the spine and thorax in children. Besides affecting the height, posture, and functional mobility of affected children, the resultant spinal deformity has a significant impact on their overall health by limiting the volume of the thorax and the subsequent ability for lung growth.2–4 The treatment of scoliosis in children is evolving. The previous paradigm of making a “crooked” spine “straight” by instrumenting multiple levels and fusing the spine early has been shown to inhibit the growth and development of the lungs and decrease pulmonary function.5,6 Scoliosis is not just a spine deformity; it represents a 3-dimensional skeletal malformation that directly affects the volume, symmetry, and function of the thorax and indirectly affects lung growth and function.7–9 Recently, devices for preserving and potentially modulating spine and thoracic growth have become available and are now approved for use in growing children. The purpose of this paper is to summarize the recently published literature (within the last 5 y) regarding growth-friendly spinal implants, the status of their Food and Drug Administration (FDA) approval labeling, as well as to discuss the indications, applications, and complications associated with their implementation.

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NORMAL SPINE DEVELOPMENT

The vertebral column morphology begins to take shape early in embryologic development. During weeks 3 to 5, somites begin to form and segmentation takes place shortly thereafter during weeks 5 to 6.10,11 Growth of the spinal column continues at a rapid pace throughout the early childhood years as nearly two thirds of the adult sitting height is achieved by age 5.11 This rate of growth decelerates between ages 5 and 10.12 Concomitant with the expansion of the vertebral column is the growth of the thoracic cavity and development of the lungs. By the age of 5, nearly 30% of the adult thoracic volume is attained and this increases to 50% by age 10.12 Abnormal growth of the spinal column is often concomitant with restricted growth of the thoracic cavity thereby resulting in severe compromise of respiratory function by limiting excursion of the diaphragm and restricting inspiratory expansion of the rib cage.5,13 In an effort to develop a unifying principle that emphasizes how structural deformities of the spine and rib cage can degrade respiratory function and lung development, Campbell2,3 introduced the concept of thoracic insufficiency syndrome (TIS), which is defined as the inability of the thorax to support normal respiration or lung growth. TIS represents a novel form of postnatal pulmonary hypoplasia and restrictive respiratory disease that occurs in patients with congenital, infantile, or neuromuscular scoliosis and congenital or acquired anomalies of the ribs and chest wall that induce prolonged mechanical inhibition of respiration and/or pulmonary growth. Several studies have demonstrated poor outcomes in patients who underwent early fusion of the spine due to the development of TIS.6,9,14

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HISTORY OF SPINAL IMPLANTS IN EOS

Beginning with Harrington’s use of growth rods in the 1960s, the treatment of spinal deformity in the young patient has changed drastically. Expanded knowledge of the ill effects of early spinal fusion has led to the development of growth-friendly instrumentation systems. Recently, a variety of technologies that attempt to modulate spinal growth while allowing continued physiological growth of the pulmonary and cardiovascular systems have become available for use in clinical practice. These systems are largely based on the Hueter-Volkmann principle that suggests mechanical factors influence longitudinal bone growth and remodeling: compressive forces inhibit growth, whereas tensile forces stimulate growth. This principle of mechanotransduction has been implicated in the progression of scoliosis, and may be harnessed for its treatment.15 We organized the various options for growth-friendly implants to include the mechanism of growth modulation based on: (1) loading mode—distraction (tension) versus compression versus guided growth; (2) anatomic location—anterior versus posterior; (3) anchoring mode—spine versus rib; and (4) means of lengthening—nonoperative versus operative versus innate growth. This functional classification scheme is currently being utilized by the American Society for Testing and Materials in the development of test protocols for nonfusion growth-modulating spinal implants as part of the FDA approval process.

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METHODS

The PubMed and Medline databases were searched using the following terms: scoliosis, growing rods, VEPTR, MAGEC, magnetic expansion control, guided growth, SHILLA, vertebral tethers, vertebral stapling, nitinol staples, Hemibridge Clip, and approved FDA labeling. Ninety-eight peer-reviewed papers published in English from January 1, 2010 through June 30, 2015 were assessed; the authors identified 31 papers as contributing important new information (Fig. 1). A number of papers discussed outcomes related to multiple devices. Articles were excluded if they were determined by the authors to not offer new, clinically relevant information. The work contained herein was approved by the Pediatric Orthopaedic Society of North America (POSNA) publications committee and the POSNA presidential line.

FIGURE 1

FIGURE 1

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GROWTH-FRIENDLY SPINE INSTRUMENTATION SYSTEMS

Distraction-based, Posteriorly Placed, Spine-based, Operatively Lengthened Implants

Traditional Growing Rods

Growing rod constructs use 1 or 2 rods that are fixed to the spine above and below the scoliosis using hooks, wires, and/or pedicle screw anchors.16 Dual-rod constructs have been shown to offer better curve correction as compared with single-rod constructs.17 Sequential lengthening of the rods is required every 6 to 9 months to progressively correct the scoliosis and maintain/stimulate spine growth (Figs. 2A, B). This technique is indicated and approved by the FDA for the treatment of EOS in patients younger than 10 years of age with moderate to severe scoliosis (≥60 degrees).18

FIGURE 2

FIGURE 2

Retrospective analyses of clinical series have established the efficacy of posteriorly placed growing rod systems for correcting the magnitude of EOS (∼40% improvement in the major curve), while preserving thoracic (T1-12) and overall spine (T1-S1) height over the entire course of sequential lengthening of the rods (∼Δ4 cm thoracic height, ∼Δ8 cm spine height).19,20 However, a number of complications have been associated with this procedure including failure of the device itself (rod breakage or disassembly), failure at the bone-implant interface (pullout or fracture of vertebral anchors), neurological injury, and surgical site infection (SSI). Review of a multicenter registry by Kabirian et al21 revealed that 11.1% of 379 growing rod patients developed at least 1 SSI, of which 52.4% of these patients eventually required implant removal. Risk factors for SSI included the use of stainless steel implants, nonambulatory status, and the number of surgical procedures. Growing rod patients undergoing at least 8 surgical procedures had a 50% risk of deep infection. Another multicenter retrospective study observed complications to be associated with increased magnitude of upper thoracic scoliosis and kyphosis as well as the number of rod-lengthening procedures.22

Another important issue affecting the overall performance of growing rods is the “Law of diminishing growth.”23 This refers to the observation that with each sequential rod lengthening, there is a significant increase in the distractive force required and a corresponding decrease in the spine height gained. This is due to the decrease in moment arm available as the spine straightens and the progressive stiffness of the spine. This manifestation is explained by basic spine biomechanics. The moment required to correct a scoliotic curve is the product of the distractive force applied to the ends of the rod spanning the concavity of the curve and the distance from the apex of the curve to the line of action of the distraction force (ie, the moment arm). With each sequential distraction, as the magnitude of the scoliosis decreases, so does the lateral offset to the apex of the curve, thereby decreasing the moment arm over which the corrective moment is acting. To maintain a constant corrective moment, the distractive force must increase in proportion to the decrease in the length of the moment arm. Once the scoliosis is fully corrected, tension is being applied to a “straight” spine, and the distractive force reflects the intrinsic stiffness of the paravertebral muscles, interspinous ligaments, intervertebral disk, and facet joint capsule. The upper limit of the distractive force that can be tolerated is determined by the material properties of the bone-anchor interface. In vivo measurement of the peak force applied during growing rod distraction approach 500 N,24 which may exceed the strength of attachment of sublaminar hooks and pedicle screws, depending on the bone mineral density and structural geometry of the vertebra.25

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Distraction-based, Posteriorly Placed, Spine-based, Nonoperatively Lengthened Implants

MAGnetic Expansion Control (MAGEC)

The MAGEC system (Ellipse Technologies Inc., Irvine, CA) is posteriorly based, adjustable growing rod system that can be dynamically lengthened (or shortened) by virtue of a magnetic, screw drive actuator implanted within either a 4.5- or 5.5-mm-diameter spinal rod (Fig. 3). A hand-held, electronically powered, external remote controller uses cylindrical magnets rotated in opposition to the implanted magnetic actuator to noninvasively distract (or retract) the implanted rod. The major advantage of externally adjusted rods is the elimination of the open procedures required to manually lengthen the rod. Expansion of the rod occurs in the outpatient setting, without anesthesia, and there are no published reports of pain and intolerance by the child. A cost analysis by Rolton et al26 concluded that avoiding repeated lengthening procedures saves an estimated $12.5k per patient. At present, the FDA has approved (510K designation) the use of the MAGEC system for skeletally immature patients below 10 years of age with progressive spinal deformities (ie, major curve ≥30 degrees and/or thoracic height <22 cm) associated with or at risk of TIS. The MAGEC device is contraindicated for use in patients with an implanted electronic device, those who might require an MRI during the duration of rod implantation, or a patient weighing <11.4 kg.

FIGURE 3

FIGURE 3

There are relatively few long-term outcome studies of the MAGEC device. Hickey et al27 monitored 8 patients treated for EOS with MAGEC rods for a minimum of 23 months. Six patients had dual-rod constructs; 2 patients had single-rod constructs. The MAGEC rod was implanted as a primary procedure in 4 patients and as a revision procedure in 4 patients. Among EOS patients treated initially with MAGEC rods, the mean preoperative major curve of 74 degrees improved to 42 degrees postoperatively. However, for patients revised with MAGEC rods, there was little change in the extent of spinal deformity (major curve angles of 45 vs. 42 degrees preoperatively vs. postoperatively, respectively). The device was not without complications: 2 MAGEC rods failed to maintain distraction, there was 1 instance of rod fracture, and 1 case of proximal pedicle screw pullout.

A prospective study of 34 patients, mean age 8 years, treated for progressive EOS with the MAGEC system over 15 months demonstrated only modest improvement in the mean major curve from 47 to 41 degrees.28 Complications included loss of distraction, superficial wound infections, prominent hardware, rod fracture, and pullout of a laminar hook. Concomitant with the decrease in spinal deformity was an enhancement in pulmonary function. In a small case series of 6 EOS patients treated with MAGEC, Yoon et al29 observed a statistically significant increase in the mean predicted forced vital capacity (FVC) and forced expiratory volume (FEV1) of 14.1% and 17.2%, respectively. Additional prospective studies are required to fully establish the efficacy of the device in addressing the all manifestations of EOS.

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Distraction-based, Posteriorly Placed, Rib-based, Operatively Lengthened Implants

Vertical Expandable Prosthetic Titanium Rib (VEPTR)

The VEPTR (DePuy Synthes, West Chester, PA) is an implant specifically designed to address rib and spine deformities associated with TIS. Now in its second design iteration, the VEPTR system can be attached to the thorax, spine, or pelvis (rib-to-rib, rib-to-spine, or rib-to-pelvis) at multiple fixation points, using a variety of anchor types (rib cradles, sublaminar hooks, pedicle screws, iliac crest “S” hooks). Lengthening of the spine and thorax is performed surgically every 6 to 9 months through a limited exposure to progressively correct the thoracic deformity associated with TIS and improve thoracic volume. Sequential distraction of a curvilinear rib sleeve attached to 1 or more superior rib cradles above is achieved by incrementally sliding the construct along a curved rail that fastens either to another inferior rib cradle or transitions distally into a rod for attachment to the spine or the ilium. Initially approved by the FDA in 2004 for the treatment of TIS under a human device exemption, it is now also approved for the treatment of EOS in patients between the age of 6 months and skeletal maturity with moderate to severe scoliosis (≥60 degrees).30

Numerous retrospective studies have shown that VEPTR treatment improves scoliosis by 6 to 15 degrees.31–34 However, although implantation of a VEPTR device decreased the vertebral major curve, Gadepalli et al35 revealed that there was no increase in lung volumes as measured on postoperative CT scans of the thorax. Another retrospective case series demonstrated that the VEPTR device was successful at improving FVC in patients with TIS, but that the increase was not maintained during continued growth.36 The predicted FVC decreased from 77% to 58% over the course of 6-year follow-up and may be associated with an increase in chest wall stiffness.34 Although the VEPTR did improve the volume of the thorax in an experimental animal model of TIS, the rib cage remained stiff, decreasing thoracic compliance.37

Owing to the medically complex patient population treated with VEPTR, it has a high complication rate including the occurrence of wound dehiscence, skin flap compromise, SSI, implant loosening, device migration, and device failure.31–34 Sankar et al38 reported a total of 45 complications in 19 patients treated with VEPTR (2.37 complications per patient). The rate of infection in patients treated with VEPTR was greater than that observed in similar patients treated with spine-based growing rods or hybrid constructs. Another study followed 54 patients for nearly 2 years, and exposed a complication rate of 137% per patient.39 The increased occurrence of complications in these EOS/TIS patients may be related to the severity of the thoracic deformity, poor nutritional status, compromised pulmonary function, nonambulatory status, and extended duration of hospitalization after the procedure.40

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Guided-Growth, Posteriorly Placed, Spine-based Implant, Lengthened by Innate Growth Implants

SHILLA

The SHILLA growth guidance system (Medtronic Inc., Minneapolis, MN) is predicated on the so-called “Luque trolley” concept of correcting a scoliosis using multiple sublaminar wires as segmental anchors capable of sliding along posteriorly placed spine rods spanning the curve, which align the vertebrae and act as a rail to guide longitudinal spine growth. The current iteration of the device uses pedicle screws with standard locking set-screws to rigidly fix the vertebrae forming the apex of the curve to dual rods spanning the scoliosis to achieve immediate deformity correction and local arthrodesis; the remaining vertebrae, proximal and distal to the apex of the deformity, are captured using multiaxial pedicle screws with nonlocking, “flanged” set-screws that can slide along the rods, allowing guided longitudinal growth while maintaining alignment of these nonfused vertebrae.41 This approach avoids the “Law of diminishing growth” associated with distraction-based systems, as it does not require sequential rod lengthening to correct the scoliosis and maintain spine growth. This technique is indicated and approved by the FDA for the treatment of EOS in patients younger than 10 years of age with moderate to severe scoliosis (≥60 degrees).42

In a retrospective review of 10 patients followed for at least 2 years after implantation of the SHILLA device, McCarthy et al43 showed an average major curve angle correction from 70.5 degrees pretreatment to 27 degrees at follow-up. Coincident with a 12% increase in spine height, the space available for the lungs increased by 13%. Overall, there was a 50% complication rate that included SSI and device-related issues requiring revision or rod exchange.43 A multicenter, case-matched, retrospective review of 72 patients comparing treatment of EOS using the SHILLA growth guidance system to standard, distraction-based, dual growing rod constructs demonstrated that patients treated with dual growing rods experienced better correction of their spine deformity (Δ Cobb angle 36 vs. 23 degrees), and increased spine height (Δ T1-S1 length, 8.8 vs. 6.4 cm).44 There were no reported differences in the rate of complications between the 2 groups; however, the children treated with SHILLA required fewer surgeries (2.8/patient) compared with children treated with growing rods (7.4/patient).44 Further longer-term retrospective comparative studies and prospective studies are required to better understand the effectiveness and reliability of this system and to identify the patient population that will benefit most from this technique.

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Compression-based, Anteriorly Placed, Spine-based, Lengthened by Innate Growth Implants

Predicated on the Heuter-Volkmann principle, staple hemiepiphyseodesis has been used for decades to modulate growth of the appendicular skeleton to correct angular malalignment. Similar compression-based systems are now available to correct spinal deformity. Vertebral body tethers spanning multiple functional spine units (FSU) or vertebral body staples spanning single FSUs are implanted along the convexity of a scoliotic curve to inhibit growth of the relatively longer portion of the spine, while permitting continued growth of the shorter, concave portion of the spine.45 Successful treatment necessitates an understanding of the growth potential of the vertebrae comprising the spine deformity and the ability to predictably modulate the vertebral growth remaining to equilibrate the relative length discrepancy between the convex and concave segments of the contorted spine. The perceived advantages of these anteriorly placed devices are: that they can be inserted using minimally invasive, thoracoscopically assisted, surgical techniques; that they preserve motion of the instrumented segments; and that they allow continued spine growth without the need for sequential lengthening procedures.46

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Staples

Predicated on the Heuter-Volkmann principle, compression devices modulate growth by placing a compressive force on the convex side of curves over single or multiple growth areas known as FSUs.45 Vertebral body stapling is a fusionless method of spine growth modulation first described in 1951 using a canine scoliosis model.47 One current version uses a staple composed of a nickel and titanium metal alloy that allows the staple to alter its configuration depending on temperature (Fig. 4A).45 At room temperature, to facilitate insertion, the ends of the staple are straight, but at body temperature, after vertebral body penetration, the ends curl in to compress the vertebral endplates and interposed intervertebral disk (Fig. 4B). These staples have been shown to provide sufficient compressive strength to induce hemiepiphyseodesis in a goat model of scoliosis.48,49 However, some biomechanical studies suggest that growth inhibition may be due to tissue damage within the vertebral endplate apophysis as opposed to sustained compression from the staple itself.50 Regardless of the mechanobiology, recent retrospective clinical studies have demonstrated that vertebral body stapling can effectively control progression of moderate thoracic and/or lumbar scoliosis (major curve angles 25 to 45 degrees).51 Defining success as maintenance or improvement of scoliosis to within 10 degrees of preoperative measurements, Lavelle and colleagues showed vertebral body stapling to be successful in 77% of patients with preoperative major curve angles <35 degrees and 85% of patients with preoperative major curves <20 degrees. Theologis and colleagues followed 12 patients under 10 years of age with moderate idiopathic thoracic or lumbar scoliosis (major curve angles 30 to 39 degrees) treated with vertebral body stapling for an average of 3.4 years. The staples controlled curve progression an average of 23.0 degrees, thereby delaying the need for definitive spine fusion.52 Considering the positive results of the prospective, case-controlled, BrAIST study,53 which proved that bracing successfully mitigated progression of moderate thoracic and lumbar scoliosis, Kaymaz et al54 and others have argued that the moderate nature of the scoliosis effectively treated with vertebral body staples could just as easily have been treated with a thoracolumbar orthosis without exposure to the risks associated with the surgical insertion of vertebral staples. Currently, vertebral body staples are recommended for the treatment of moderately severe, flexible curves (major curve <40 degrees); these devices have not been approved by the FDA for this indication and are used at the surgeon’s discretion.45

FIGURE 4

FIGURE 4

There are additional devices that function similarly to a vertebral staple, with the addition of screws above and below the tines for additional fixation in the vertebrae (Fig. 5).55 In the United States, this device has FDA approval for the treatment of moderately severe scoliosis (major curve <40 degrees) under the Humanitarian Use Device clause, but it is approved for use in Europe.56 Preclinical animal models have demonstrated encouraging results57–59; however, the clinical trials are ongoing.

FIGURE 5

FIGURE 5

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Tethers

Vertebral body tethers represent an adaptation of Dwyer cables. In its application as a fusionless spine implant, a tensioned polyester cable threaded through vertebral body screw anchors inserted into each of the contiguous FSUs along the convexity of the scoliosis functions to obtain immediate partial correction of the deformity and to modulate spine growth (Figs. 6A, B). Supported by histologic studies conducted by Chay et al60 in a porcine model, preservation of growth potential 20 weeks after placement of the vertebral body tethering device, tethers have the theoretical advantage of growth and mobility preservation while modulating spine growth. Samdani and colleagues published a retrospective case review of 32 patients with a minimum 1-year follow-up using an anteriorly applied, vertebral body tether for the treatment of adolescent idiopathic scoliosis. On average, thoracic curve magnitudes improved from 42.8±8.0 degrees to 21.0±8.5 degrees after instrumentation; the average curve magnitude was 17.9±11.4 degrees at 1-year follow-up. Lumbar curves achieved similar improvement, decreasing from 25.2±7.3 degrees preoperatively to 18.0±7.1 degrees postoperatively and 12.6±9.4 degrees at 1-year follow-up.61 After 2 years of follow-up in 11 children, these promising results were sustained; patients exhibited 70% correction of their thoracic or lumbar scoliosis.62 Several complications were encountered including persistent atelectasis requiring bronchoscopy (1 patient), overcorrection of the scoliosis by 5 to 13 degrees (3 patients), and return to the operating room for loosing of the tether to correct 10 degrees of overcorrection (2 patients). Currently vertebral body tethers are recommended for the treatment of moderately severe, flexible curves (major <60 degrees); these devices have not been approved by the FDA for this indication and are used at the surgeon’s discretion.

FIGURE 6

FIGURE 6

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PREVENTION OF COMPLICATIONS

There is a high rate of complications associated with these procedures. A multifaceted approach may help reduce the incidence of SSI including preoperative assessment of patient nutrition and appropriate intervention, diligent use of preoperative skin cleansing, prophylactic use of perioperative antibiotics to cover MRSA and gram-negative bacteria, instillation of vancomycin powder into the wound at closure, and meticulous postoperative wound care.39,40 Device fatigue failure can be prevented by using the largest rod diameter possible (flexural and torsional rigidity of the rod vary as the radius raised to the 4th power; eg, a 5-mm-diameter rod is 1.5× stiffer than 4.5-mm-diameter rod). Minimizing the number of connections between members helps prevent stress risers and crevice corrosion. A growing body of evidence supports the notion that patients with low bone mineral density and failure at the bone-implant interface may be reduced with the preoperative administration of calcium and vitamin D supplements and the judicious use of bisphosphonates to inhibit bone resorption.63–67 The stress applied to each bone-anchor interface can be reduced by distributing the applied load between dual posterior rods fixated with 6 bone anchors (ie, 3 pedicle screws, sublaminar hooks, and/or rib cradles per rod) at both the upper and lower end of the construct.68,69 Using finite element analysis to determine the optimal interval between sequential distractions to minimize device failure, Agarwal et al70 calculated that shorter time intervals between subsequent lengthening decreased the applied stress for the same height gain. Lengthening a posteriorly placed, distraction-based system every 2 months over a 2-year interval decreased the stress generated by 50% to 75% compared with lengthening every 6 months over the same 2-year interval to obtain the same gain in spine height.

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SUMMARY

Over the past 2 decades, the treatment of scoliosis in young children has evolved. The previous paradigm of making a “crooked” spine “straight” by instrumenting and fusing the spine early has been replaced by implanting a fusionless device that improves the 3-dimensional thoracic deformity while preserving pulmonary function and increasing trunk height. Several of these devices are now approved for use in growing children, with a few more in the midst of clinical trials, subject to critical evaluation. These devices must maintain correction of the thoracic deformity, modulate growth of the spine and rib cage, and preserve pulmonary function without failing mechanically for an indeterminate number of years. With the myriad of options available, there are no established performance criteria for nonfusion spinal instrumentation systems nor are there protocols for optimizing the growth of the spine and thorax while simultaneously ameliorating the associated deformity. There are endless, unique considerations in children that complicate successful application of these devices. While it is possible to safely engineer devices that meet the progressive mechanical demands of growing children, the pathophysiological processes that contribute to spine and thoracic deformity and the mechanobiological principles that govern growth of the spine and thorax in health and disease have yet to be elucidated.

Beyond developing implant systems that incrementally straighten the spine and/or rib cage, basic and applied research is required to better understand how to predictably modulate growth of the spine and thorax in children using these devices. Several retrospective clinical studies revealed apparent overcorrection or undercorrection of scoliosis, thereby highlighting our imprecision in optimizing patient outcomes with minimal morbidity. Before these devices can be implemented reliably for the treatment of EOS, it will be necessary to: (1) characterize normal versus abnormal spine and thoracic growth; (2) define what metrics we should be measuring to predict the remaining growth of the spine and thorax; (3) develop analytic models of spine growth that accurately predict progression of the deformity and specific interventions (eg, inhbition of growth by applying compression along the convexity of a scoliosis vs.stimulation of growth by applying distraction across the concavity of a scoliosis) over how many vertebral segments and for what time duration to achieve the desired clinical outcome. This will require collaboration among clinicians caring for these patients, scientists investigating the biological processes that contribute to spinal deformity, and the mechanobiologist researching the mechanisms of mechanotransduction to optimize instrumentation systems and treatment protocols to predictably regulate growth of the spine and rib cage to correct thoracic deformity and preserve pulmonary function.

International research collaboration under the guidance of the Growing Spine Study Group, the Children’s Spine Study Group, and the International Congress on Early Onset Scoliosis is needed. Emphasis should be placed on the evaluation of prospective studies and the use of multicenter patient registries to better understand and improve patient outcomes following implantation of spinal growth-modulating devices. The future of pediatric spine surgery is rapidly advancing and continued surgeon involvement in implant design and research will offer new solutions to complex problems and most importantly provide better patient care.

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Keywords:

early-onset scoliosis; spine growth; growth modulation; implant technology

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