The Efficacy and Integrity of Shape Memory Alloy Staples and Bone Anchors with Ligament Tethers in the Fusionless Treatment of Experimental Scoliosis

Braun, John T. MD; Akyuz, Ephraim MS; Ogilvie, James W. MD; Bachus, Kent N. PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.D.02103
Scientific Articles
Abstract

Background: Scoliosis is a complex three-dimensional deformity with limited treatment options. Current treatments present potential problems that may be addressed with use of fusionless techniques for the correction of scoliosis. However, there are few data comparing the efficacy of different fusionless implant strategies in controlling scoliosis or on the integrity of rigid compared with flexible devices in an in vivo setting over time. The objective of this study was to compare the efficacy and integrity of rigid and flexible anterior thoracic tethers used to treat experimental scoliosis.

Methods: Experimental scoliosis was created in twenty-four Spanish Cross-X female goats and was subsequently treated with either anterior shape memory alloy staples or anterior ligament tethers attached to bone anchors. Serial radiographs were analyzed to determine the efficacy of the implants in controlling scoliosis progression as well as the integrity of the implants at study completion. After the goats were killed, the implants were analyzed with use of three quantitative indices of implant integrity and implant pullout testing.

Results: Over the treatment period, scoliosis progressed from 77.3° to 94.3° in the goats treated with staples and was corrected from 73.4° to 69.9° in the goats treated with bone anchors, with loosening of eighteen of forty-two staples (two of the eighteen dislodged) and evidence of drift in two of forty-nine anchors. Histologic sections revealed a consistent halo of fibrous tissue around the staple tines but well-fixed bone anchors at all sites. Pullout testing demonstrated that bone anchors had greater strength than staples initially and at the study completion, with an increase in bone anchor fixation over the course of the study.

Conclusions: In this scoliosis model, the flexible ligament tethers attached to bone anchors demonstrated greater efficacy and integrity than the more rigid shape memory alloy staples.

Clinical Relevance: Fusionless scoliosis surgery offers many theoretical advantages over instrumented arthrodesis. Improvements in fusionless implant design, with a specific focus on optimizing the fixation to bone and maximizing the tethering effect, may lead to better control of idiopathic scoliosis with lower rates of device loosening and failure.

Author Information

1 Department of Orthopaedics, University Orthopaedic Center, University of Utah, School of Medicine, 590 Wakara Way, Salt Lake City, UT 84108. E-mail address for J.T. Braun: john.braun@hsc.utah.edu

2 Orthopaedic Research Laboratory, Departments of Orthopaedics and Bioengineering, University of Utah, Salt Lake City, UT 84112

Article Outline

Scoliosis is a complex three-dimensional spinal deformity that most often requires treatment to address curve progression during growth. Standard treatment options for progressive scoliosis are essentially limited to bracing or surgery1-5. While brace treatment is noninvasive and preserves growth, motion, and function of the spine, it does not correct deformity and is only modestly successful in preventing curve progression. Success of this form of treatment may also be hampered by patient compliance issues and the negative psychological impact of bracing6-14. In contrast, surgical treatment with an instrumented spinal arthrodesis usually results in better deformity correction but is associated with substantially greater risk. The risks of surgery are related to the invasiveness of spinal arthrodesis, the instantaneous correction of spinal deformity, and the profoundly altered biomechanics of the fused spine15-22.

Fusionless scoliosis surgery may provide substantial advantages over both bracing and arthrodesis23,24. The goal of this new technique is to harness the patient's inherent spinal growth and redirect it to achieve correction, rather than progression, of the deformity. Several terms for the treatment of scoliosis without arthrodesis have evolved and imply different methods of fusionless scoliosis surgery. These include endoscopic vertebral stapling, anterior spinal tethering, convex scoliosis tethering, mechanical modulation of spinal growth, and internal bracing of spinal deformity23-25. By applying implants directly to the spine, anterior fusionless techniques are theoretically more advantageous mechanically than external bracing—as bracing does not directly apply corrective forces to the spine but indirectly transmits forces by means of the ribs, pelvis, and torso—and patient compliance issues are eliminated. Furthermore, minimally invasive tethering of the anterior thoracic spine by means of an endoscopic approach is also less extensive than arthrodesis, with no requirement for discectomies, preparation of the fusion bed, or harvest of bone graft.

All spinal implants that span a spinal motion segment are initially subjected to a high stress that is usually intended to be temporary26-30. With the achievement of fusion, the high stresses are reduced or eliminated, thus preventing instrumentation loosening or failure. Fusionless scoliosis implants are not afforded the luxury of eventual fusion, and therefore issues related to implant loosening or failure over time are important. There are few data available on the longevity of fusionless scoliosis implants, and there are no studies, as far as we know, that have compared different implant strategies objectively with use of several indices of integrity in an in vivo setting over time. To be effective over a prolonged period, these dynamically loaded implants must maintain appropriate fixation to the host bone without loosening or failure.

The purpose of this study was to compare the efficacy and integrity of more rigid shape memory alloy staples and more flexible anterior thoracic tethers for the treatment of progressive experimental scoliosis. Efficacy was defined as the ability of the implant to control the progression of scoliosis. Plain radiographs were used for in vivo assessment of efficacy. Integrity was defined as the ability of the implant to maintain fixation in the host bone during the treatment period. It was hypothesized that the use of a bone anchor attached to a more flexible ligament loop tether would provide better control of scoliosis progression than would the more rigid shape memory alloy staple. It was also hypothesized that the use of a bone anchor attached to a flexible ligament tether would demonstrate greater implant integrity.

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Materials and Methods

Creation and Correction of Deformity

Under a study protocol approved by the Institutional Animal Care and Use Committee, scoliosis was created in twenty-four Spanish Cross-X female goats (six to eight weeks old and weighing 8 to 12 kg) with use of a flexible left posterior asymmetric tether from the T5 to L1 laminae (3.5-mm polyethylene core-polyester sleeve; Medtronic Sofamor Danek, Memphis, Tennessee) as previously described31. Convex rib resection and concave rib-tethering from T8 to T13 were also performed to create an experimental scoliosis (Fig. 1) similar to that described in our original model32. After eight weeks of posterior tethering, goats with progressive curves were randomized into one of three treatment groups. Group I received no treatment and served as scoliosis controls; Group II underwent anterior convex stapling across the six levels of maximum curvature with rigid Nitinol shape memory alloy staples (Medtronic Sofamor Danek); and Group III underwent anterior tethering across the six levels of maximum curvature with flexible ligament loops (3.5-mm polyethylene core-polyester sleeve; Medtronic Sofamor Danek) attached to bone anchors (Medtronic Sofamor Danek). The goats were observed for an additional twelve to sixteen weeks after treatment. The four-week range in the observation period—necessitated by scheduling constraints—was shared equally by all groups. An additional five goats served as growth controls with no induced scoliosis. All animals were killed at the end of the twenty to twenty-four-week total study period.

In Group II, shape memory alloy staples were placed anterolaterally along the maximum curvature, with the staple base spanning the disc space vertically and each tine anchored in adjacent vertebral bodies. Prior to implantation, the staples were soaked in an ice bath causing them to become malleable. The staple tines were then pulled to 90° angles for implantation. Once seated, the staples were deployed by contact of the staple shoulder with an electrocautery for approximately three seconds. This brief heating of the staple shoulder initiated a bending of each staple tine to its former “crimped” shape (a decrease in the staple tine-base angle from 90° at implantation to 70° to 80° after deployment).

In Group III, bone anchors were placed laterally along the maximum curvature. Unlike placement of the shape memory alloy staples, anterolateral placement of the bone anchors was not possible because of the length of the anchor and the geometry of the goat vertebral body. Anterolateral placement of the bone anchor would have resulted in violation of the spinal canal by the tip of the implant. Additionally, whereas shape memory alloy staples were placed without any preparation of the vertebral body, bone-anchor placement required use of a trephine to core a path for the anchor. Bone from this core was packed into the hollow chamber of the anchor prior to implantation. Once in place, each adjacent pair of anchors was compressed and then an appropriately sized ligament loop was positioned across the anchors to maintain the tension in a corrected position. Mushroom-shaped caps were then threaded into the anchors to prevent ligament dislodgement.

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Radiographic Analyses

Serial plain radiographs in the posterior-anterior and lateral planes were used to determine the magnitude of the deformity and the gross integrity of the implants throughout the study. Progression of deformity was defined as an increase in curve magnitude of 5° as measured with the Cobb method33. All staples and anchors that demonstrated evidence of loosening, including radiolucency, drift, or back-out, were noted.

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Histologic Analyses

After the goats were killed, the apical spinal segments (T9 to T11) were harvested and placed in 70% ethanol to preserve the specimens before embedment in polymethylmethacrylate with use of standard techniques34,35. Once the samples were completely embedded, a custom, water-cooled, high-speed, cut-off saw36 was used to section the samples with use of a diamond-edged blade (Rockazona, Peoria, Arizona). Each section was then ground to a 2 to 3-mm thickness and polished to an optical finish with a variable speed grinding wheel (Buehler, Lake Bluff, Illinois) with use of standard techniques37,38. Sections were taken in the coronal plane through the vertebral bodies and discs. Contact radiographs were made of each section and used for analysis. Sections that provided the best representation of the center of the implant and the vertebral body in the coronal plane were selected.

The sectioned vertebrae were initially qualitatively analyzed with use of gross histologic techniques. Any observations regarding general implant integrity were noted. Each 2-mm-thick section was then sputter-coated with a conductive layer of gold for approximately two minutes (Hummer VI-A; Anatech, Alexandria, Virginia) and was examined in a scanning electron microscope (JSM-6100; JEOL, Peabody, Massachusetts) with use of the backscattered electron detector (Tetra; Oxford Instruments, Concord, Massachusetts) to provide a contrast between mineralized and nonmineralized tissues. Three quantitative indices were obtained: an osseointegration index, a bone proximity index, and the bone ingrowth.

The osseointegration index and bone proximity index were determined, with use of an Image-Pro Plus software program (Media Cybernetics, Silver Spring, Maryland), from the histological sections that best represented the center of the implant. In Group II, two images allowed evaluation of the entire staple while six images were required in Group III to capture the entire bone anchor. The osseointegration index was calculated by measuring the length of the implant surface osseointegrated with the surrounding bone and dividing this value by the total length of the implant surface. The resultant percentage was used to indicate the amount of bone osseointegrated with the implant. To quantify the bone proximity index, trace lines were created over the implant surface and the surrounding bone. The average distance between the two traces, calculated by the Image-Pro software, represented the average distance between the host bone and the implant surface. The bone ingrowth analysis was accomplished in a manner similar to that used by Bloebaum et al.39 for bone ingrowth analysis within a porous coating. No bone ingrowth analysis was possible for the staples, as these implants possess no internal chamber. For each animal in Group III, three bone anchors (T9, T10, and T11) were analyzed. Three images of the entire inner confines of the hollow chamber, as well as two images of the host bone cephalad and caudad to the implant, were obtained for each of the anchors. The host bone provided a normal percentage of bone expected within a randomly sampled area of the vertebral body. Link ISIS software (Oxford Instruments) was used to calculate the quantity of bone within a specified area of each image. The percentage of bone within the implant chamber was used to represent bone ingrowth and was compared for reference to areas of surrounding bone.

Statistical analyses were performed on all of the histologic data with use of independent t tests, with a level of significance defined as a p value (alpha) of ≤0.05.

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Implant Pullout Testing

After the goats were killed, the cadaveric specimens (less the apical spinal segments T9 to T11, which had been used for histologic and backscattered electron image analysis) were sealed in air-tight plastic bags (S-1987; Uline, Waukegan, Illinois) and frozen to -20°C. Once thawed, segments T7-T8 and T12-T13 were removed from the intact spinal columns and were prepared for pullout testing. The strength of implant fixation was analyzed at two time-points, the first representing the initial fixation strength immediately after implantation (time zero) and the second representing the final fixation strength at the end of the treatment period (twelve to sixteen weeks after implantation). Vertebral bodies from the untreated animals (Group I and growth controls) were used to generate time-zero data, whereas the treated animals (Groups II and III) were used to generate twelve to sixteen-week data. Specimens used for time-zero pullout testing were cleaned of all soft tissue to expose the bone of the vertebral bodies. Specimens used for the twelve to sixteen-week pullout testing underwent a minimal amount of soft-tissue dissection to allow for application of the pullout fixtures to the implants. For the staples, a sewing needle was used to guide a spider wire through the fibrous tissue and under each shoulder of the staple (Fig. 2). For the bone anchors, fibrous tissue surrounding the exposed portion of the anchor was removed (Fig. 3). Following dissection, specimens were coated with a thin layer of a water-based lubricant to minimize dehydration and were kept moist with use of periodic sprays of 0.9% saline solution. Though staples likely resumed a malleable crystal structure with freezing, no manipulation of the staple occurred, and therefore the overall shape of the staple in the bone was unchanged. Heating of all staples with use of an electrocautery prior to pullout testing merely ensured that the shape memory alloy was in its deployed crystal structure state at the time of pullout testing.

Pullout testing to generate time-zero data was performed on the six untreated scoliotic spines as well as the five growth control goats. Staples were implanted across the T7-T8 and T12-T13 segments in these eleven goats post mortem. After staple testing, bone-anchor pullout testing was accomplished in the same four vertebrae. Because of the small diameter of the tines, staple implantation and pullout testing resulted in a 3-mm cylindrical hole in the lateral vertebral body with minimal change in the surrounding cancellous bone. Subsequent preparation for implantation of the 8-mm-diameter bone anchor involved the use of a 6-mm trephine to core a path for the bone anchor around the previous staple path. This method of bone-anchor implantation essentially mimicked that used during the in vivo portion of the study. This procedure was used to conserve the limited supply of specimens and to increase the power of the statistical analyses. The T7-T8 and T12-T13 vertebrae from the fourteen treated goats were used to determine the pullout strengths at twelve to sixteen weeks after implantation.

Implant pullout testing was performed with use of a servohydraulic materials testing machine (model 8500; Instron, Canton, Massachusetts). Testing of the implanted devices was performed by securely bracing the implanted specimen in a specially designed jig such that the implanted device was aligned with the Instron actuator, allowing for a tensile load to extract the implant. A 5-N preload was placed on each specimen followed by a constant displacement rate of 1 mm/sec until failure occurred.

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Implant pullout data were analyzed with use of a one-way analysis of variance followed by a Tukey-Kramer HSD (honestly significant difference) post-hoc test. Statistical significance was defined as a p value (alpha) of ≤0.05.

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Results

Radiographic Analyses

Of the twenty-four goats that underwent posterior asymmetric tethering with convex rib resection and concave rib-tethering, two (8%) died in the postoperative period because of pulmonary complications. Of the twenty-two goats available for analysis at the end of the eight-week tethering period, twenty (91% of the surviving animals; 83% of all study animals) had progressive, structural, idiopathic-type, lordoscoliotic curves develop convex to the right in the thoracic spine. All twenty goats with progressive scoliotic curves demonstrated radiographic and clinical features characteristic of idiopathic scoliosis. Radiographically, these features included substantial displacement of the apical vertebrae from the midline; wedging of the apical vertebral bodies and discs; and rotation of the apical vertebra, with a grade of 2 or 3 according to the criteria of Nash and Moe40. Clinically, these features included decreased flexibility of the spine (determined with use of a push prone maneuver); and, after rib regeneration, a typical, idiopathic-type, posterior thoracic deformity involving a right rib prominence and flattened left thoracic cage.

Over the eight-week tethering period, curves progressed from an average (and standard deviation) of 57.2° ± 8.3° to 76.5° ± 9.3° in the coronal plane (p < 0.001) and from -18.9° ± 3.8° to -40.9° ± 7.6° in the sagittal plane (p < 0.001). The goats were then randomized into the three treatment groups, at which point no significant differences in curvature were demonstrated in the coronal or sagittal planes between any of the three groups. During the treatment period, the scoliosis progressed in the seven goats treated with staples (Group II) from an average of 77.3° ± 11.5° to 94.3° ± 12.2° (p < 0.05) (Figs. 4-A, 4-B, and 4-C), demonstrating no significant difference in progression of scoliosis (p = 0.90) compared with the six untreated goats (Group I), in which the scoliosis progressed from an average of 79.5° ± 7.6° to 96.8° ± 6.7° (p < 0.05) (Table I). In contrast, the scoliosis in the seven goats with ligament tethers attached to bone anchors (Group III) corrected from an average of 73.4° ± 8.4° to 69.9° ± 9.7° (p = 0.34) (Figs. 5-A, 5-B, and 5-C).

Qualitatively, serial radiographs demonstrated progressive loosening of eighteen (43%) of forty-two staples, with two staples becoming completely dislodged. Only two (4%) of forty-nine anchors demonstrated a slight drift without radiolucency.

At the beginning of the treatment period, the sagittal plane deformity in the untreated group measured an average of -40.3° ± 6.3° and progressed to -61.0° ± 18.8° of lordosis (p < 0.05) over twelve to sixteen weeks. The sagittal plane deformity progressed from an average of -37.3° ± 6.6° to -49.0° ± 14.7° of lordosis (p = 0.03) in the group treated with staples and from -44.4° to -58.9° of lordosis (p < 0.003) in the group treated with bone anchors and ligament tethers over the twelve to sixteen-week treatment period.

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Histologic Analyses

Gross inspection of the histologic sections revealed a consistent halo of fibrous tissue around the staple tines but intimate contact of the surrounding host bone with the bone anchors at all sites, including the two anchors that demonstrated slight drift (Fig. 6). This finding was consistent with the measurements collected from the backscattered electron images (Fig. 7). The average osseointegration index value (and standard error) for the seven goats treated with staples (40.1% ± 17.5%) was significantly less than that calculated for the seven goats treated with bone anchors (76.4% ± 6.8%) (p < 0.001). The average bone proximity index was 702.0 ± 138.0 μm for the staples and 215.0 ± 69.0 μm for the bone anchors; the difference was significant (p < 0.001). In addition, there was an average of 11.0% ± 3.8% bone ingrowth within the hollow chamber of the bone anchor compared with 21.7% ± 4.9% bone in the host region (p = 0.004).

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Implant Pullout Testing

The average staple pullout strength was 101.4 ± 23.0 N for twenty-two staples at time zero and 86.0 ± 48.6 N for thirteen staples at twelve to sixteen weeks after implantation (p = 0.3). (One staple was dislodged and therefore in a suboptimal position for implant pullout testing.) The average bone-anchor pullout strength was 495.4 ± 171.3 N for forty-four anchors at time zero and 639.8 ± 213.4 N for twenty-eight anchors at twelve to sixteen weeks after implantation; the increase was significant (p = 0.004). At both time-points, the bone-anchor pullout strength was significantly greater than that of the staples (p < 0.001) (Fig. 8). The mode of failure differed between staples and anchors. At twelve to sixteen weeks after implantation of the devices, pullout testing resulted in failure of the vertebral body bone for twenty-one (75%) of the twenty-eight anchors, whereas all staples pulled out cleanly with no host-bone failure.

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Discussion

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Although the term fusionless scoliosis surgery is currently used to describe definitive anterior spinal procedures that control the progression of scoliosis during growth, other fusionless treatments have been in existence for years. These more established fusionless procedures most often use posterior implants to control the progression of spinal deformity in younger children41,42. However, fusionless scoliosis surgery with use of anterior implants provides theoretical advantages over posterior procedures. Subcutaneous or submuscular rod techniques and the vertical expandable prosthetic titanium rib41 are not only potentially more invasive than fusionless scoliosis surgery but may be associated with an increased rate of complications. Additionally, the procedures are temporizing and require multiple surgeries throughout growth with the ultimate goal of spinal fusion. Fusionless scoliosis surgery avoids multiple procedures, as well as the requirement for an eventual arthrodesis, by offering a single intervention that may provide a more permanent solution to the spinal deformity23,43. Furthermore, substantial correction of a spinal deformity in the absence of a rigid fusion mass spanning several vertebral motion segments may prevent some of the long-term problems related to spinal arthrodesis with instrumentation. These include altered stress on adjacent unfused segments and spinal imbalance issues16-22,44.

The data from this study demonstrate greater efficacy and integrity of a bone anchor attached to a more flexible ligament loop tether compared with a more rigid shape memory alloy staple in the fusionless treatment of a progressive experimental scoliosis. The greater efficacy of the bone anchor attached to a ligament tether in controlling scoliosis progression was demonstrated in vivo over the course of the twelve to sixteen-week treatment period, and implant pullout testing demonstrated superior fixation of the bone anchor both at the time of initial implantation (time zero) and at the end of the treatment period (twelve to sixteen weeks after implantation). In contrast to the more rigid staple base, the ligament loop used with the bone anchor provided a more flexible tether spanning the disc space that was likely associated with decreased forces during spinal motion. This potentially protected the bone anchor from loosening over the course of the study.

The pullout testing was important not only in highlighting differences between the two implants at given time-points but also in demonstrating changes in integrity within an implant over the course of the study. Whereas the staple demonstrated no significant change in pullout strength between the two time-points, the bone anchor showed a significant increase in pullout strength. It is speculated that this difference was related to two factors: (1) the rigidity of the portion of the implant spanning the motion segment, and (2) the quality of the fixation to bone. The staple, though made of shape memory alloy, has a relatively rigid base spanning the disc space compared with the ligament loop-bone anchor construct. For a given displacement across the disc space, created by the motion of the spine, greater forces were likely generated at the junction of the implant and host bone in the more rigid staple. These higher forces perhaps contributed to the increased loosening and the trend toward decreased fixation strength of the staple over the course of the study. Additionally, the smooth tine is suboptimal for fixation to bone and relies primarily on the mechanical “crimping” effect of the deployed shape memory alloy staple.

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While different methods of implant pullout testing are possible, axial pullout was chosen to allow a simple comparison of a limited number of specimens. Although the axial pullout testing performed at time zero in this study did not approximate the mode of failure for these implants in vivo, it did demonstrate the substantial difference in the initial strength of osseous fixation between the implants. The pullout testing at twelve to sixteen weeks may have provided a more appropriate estimation of the in vivo strength of the implant, as these implants were subjected to physiologic spinal loads in live goats over an extended period of time.

It is possible that the greater efficacy of the bone anchor attached to a flexible ligament loop tether compared with a rigid shape memory alloy staple in controlling scoliosis progression was due in part to the superior integrity of this implant over the course of this study. However, the initial scoliosis correction of 11.4° achieved by this device compared with the initial correction of 1.2° achieved with the staple likely altered the biomechanical environment in favor of the bone anchor-ligament loop construct.

Despite the superior performance of the bone anchor attached to a ligament loop tether in this experimental scoliosis animal model, some caution is appropriate in making comparisons with the treatment of human idiopathic scoliosis. Our model, although it approximates idiopathic scoliosis, does not mimic this condition. Indeed, without a clearly defined etiology45, it is impossible to reproduce a true idiopathic scoliosis. However, our previous work32 has suggested similarities to idiopathic scoliosis clinically, radiographically, histologically, and biochemically. The scoliotic deformities created in this study were of an extreme magnitude and demonstrated a malignancy of progression that is not commonly seen in idiopathic scoliosis. Yet, the mechanical factors related to progression of scoliosis with growth, according to the Hueter-Volkmann principle46-50, are well simulated in this model. The experimental scoliosis created in this study represents a challenging scenario for the testing of fusionless implants.

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Other limitations of this experimental scoliosis model are apparent when contrasting the biomechanics and anatomy of animals and humans32. The postural differences between a quadruped goat and a bipedal human likely create forces on the spine that are not directly comparable. The anatomy of the goat thorax is also more pyramidally shaped and stiffer than the cubical human thorax. However, there are many similarities between the goat thorax and the human thoracic spine, making the goat thorax a reasonable enough approximation of a juvenile human spine that it can provide useful information for the study of progressive scoliosis and its treatment.

Previous attempts to correct scoliosis with anterior fusionless techniques have been disappointing51,52. Nachlas and Borden52 were initially optimistic about their ability to create and correct lumbar scoliosis in a canine model using a (rather weak) staple spanning several vertebral segments. The enthusiasm for this new treatment waned after the application of their staple in three children with progressive scoliosis met with poor results. Other investigators in the past51 have also been dissatisfied with convex stapling as a means of controlling progressive scoliosis.

More recent investigations of convex vertebral body stapling, both in animal models and in juvenile and adolescent scoliosis, however, have offered promising early results with use of improved implants and techniques23,25,43. The use of a shape memory alloy staple tailored to the size of the vertebral body, the application of several staples per level, the instrumentation of all levels of curvature, and the employment of minimally invasive endoscopic approaches all offer substantial improvements over previous fusionless techniques. Patient selection may also play a role in the current success of these fusionless treatments, with perhaps the ideal candidates for this intervention possessing smaller and more flexible single thoracic curves. Yet, with the early clinical success of these stapling procedures, no basic-science data are available to assist in the evaluation of these implants and their effect on the surrounding tissues.

The model used in this study provides a unique environment for the evaluation of novel fusionless techniques with use of objective radiographic, histological, and biomechanical analyses to compare various strategies. Improvements in implant design in this experimental model, with a specific focus on optimizing the fixation to bone and maximizing the tethering effect, may lead to greater control of idiopathic scoliosis in children. ▪

NOTE: The authors would like to thank Michelle Swenson for her technical assistance with the collection of the data.

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Medtronic Sofamor Danek (Memphis, Tennessee). In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Medtronic). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Departments of Orthopaedics and Bioengineering, University of Utah, School of Medicine, Salt Lake City, Utah

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