The potential of proximal junctional kyphosis prevention using a novel tether pedicle screw construct: an in silico study comparing the influence of standard and dynamic techniques on adjacent-level range of motion and load pattern.

OBJECTIVE: A tether pedicle screw (TPS) enables individual stepless pretensioning and is placed at one or two levels above the upper instrumented vertebra (UIV+1 and UIV+2, respectively). This study aimed to evaluate a novel customized TPS for the prevention of proximal junctional kyphosis (PJK) and to investigate the potential to generate a smoother force transition from cranial to long fusion during trunk flexion, instead of an abrupt change at the UIV, following adult spinal deformity surgery. METHODS: A finite element model was designed based on an adult patient with spinal deformity instrumented from T10 to S1. Five different sagittal balance types and implant configurations were tested. The proximal range of motion (ROM) and intervertebral stress were examined, with a special focus on their respective discontinuities. RESULTS: Tension shielding at UIV/UIV+1 by the TPS was consistent irrespective of sagittal profiles. The use of TPSs at UIV+1 and UIV+2 increased the efficacy in reducing spinal ROM discontinuity at UIV/UIV+1, as compared with the use of TPSs at UIV+1 only. Through the use of two pairs of TPSs cranial to the UIV, the optimal tension configuration could be defined to avoid a reduction effect at UIV+1. Neither the addition of transition rods to the TPSs nor the use of transition rods in combination with standard pedicle screws improved the junctional mechanics when compared with TPSs at UIV+1/UIV+2. CONCLUSIONS: A smoother motion discontinuity at the UIV can be achieved via implementation of a TPS strategy. This new technology shows favorable in silico mechanics for reducing the risk of PJK.

A Critical Comparison of Comparators Used to Demonstrate Credibility of Physics-Based Numerical Spine Models.

The ability of new medical devices and technology to demonstrate safety and effectiveness, and consequently acquire regulatory approval, has been dependent on benchtop, in vitro, and in vivo evidence and experimentation. Regulatory agencies have recently begun accepting computational models and simulations as credible evidence for virtual clinical trials and medical device development. However, it is crucial that any computational model undergo rigorous verification and validation activities to attain credibility for its context of use before it can be accepted for regulatory submission. Several recently published numerical models of the human spine were considered for their implementation of various comparators as a means of model validation. The comparators used in each published model were examined and classified as either an engineering or natural comparator. Further, a method of scoring the comparators was developed based on guidelines from ASME V&V40 and the draft guidance from the US FDA, and used to evaluate the pertinence of each comparator in model validation. Thus, this review article aimed to score the various comparators used to validate numerical models of the spine in order to examine the comparator's ability to lend credibility towards computational models of the spine for specific contexts of use.

Finite element analysis comparing a PEEK posterior fixation device versus pedicle screws for lumbar fusion.

BACKGROUND: Pedicle screw loosening and breakage are common causes of revision surgery after lumbar fusion. Thus, there remains a continued need for supplemental fixation options that offer immediate stability without the associated failure modes. This finite element analysis compared the biomechanical properties of a novel cortico-pedicular posterior fixation (CPPF) device with those of a conventional pedicle screw system (PSS). METHODS: The CPPF device is a polyetheretherketone strap providing circumferential cortical fixation for lumbar fusion procedures via an arcuate tunnel. Using a validated finite element model, we compared the stability and load transfer characteristics of CPPF to intact conditions under a 415 N follower load and PSS conditions under a 222 N preload. Depending on the instrumented levels, two different interbody devices were used: a lateral lumbar interbody device at L4-5 or an anterior lumbar interbody device at L5-S1. Primary outcomes included range of motion of the functional spinal units and anterior load transfer, defined as the total load through the disk and interbody device after functional motion and follower load application. RESULTS: Across all combinations of interbody devices and lumbar levels evaluated, CPPF consistently demonstrated significant reductions in flexion (ranging from 90 to 98%), extension (ranging from 88 to 94%), lateral bending (ranging from 75 to 80%), and torsion (ranging from 77 to 86%) compared to the intact spine. Stability provided by the CPPF device was comparable to PSS in all simulations (range of motion within 0.5 degrees for flexion-extension, 0.6 degrees for lateral bending, and 0.5 degrees for torsion). The total anterior load transfer was higher with CPPF versus PSS, with differences across all tested conditions ranging from 128 to 258 N during flexion, 89-323 N during extension, 135-377 N during lateral bending, 95-258 N during torsion, and 82-250 N during standing. CONCLUSION: Under the modeled conditions, cortico-pedicular fixation for supplementing anterior or lateral interbody devices between L4 and S1 resulted in comparable stability based on range of motion measures and less anterior column stress shielding based on total anterior load transfer measures compared to PSS. Clinical studies are needed to confirm these finite element analysis findings.

Real-time biomechanics using the finite element method and machine learning: Review and perspective.

PURPOSE: The finite element method (FEM) is the preferred method to simulate phenomena in anatomical structures. However, purely FEM-based mechanical simulations require considerable time, limiting their use in clinical applications that require real-time responses, such as haptics simulators. Machine learning (ML) approaches have been proposed to help with the reduction of the required time. The present paper reviews cases where ML could help to generate faster simulations, without considerably affecting the performance results. METHODS: This review details the ML approaches used, considering the anatomical structures involved, the data collection strategies, the selected ML algorithms, with corresponding features, the metrics used for validation, and the resulting time gains. RESULTS: A total of 41 references were found. ML algorithms are mainly trained with FEM-based simulations in 32 publications. The preferred ML approach is neural networks, including deep learning in 35 publications. Tissue deformation is simulated in 18 applications, but other features are also considered. The average distance error and mean squared error are the most frequently used performance metrics, in 14 and 17 publications, respectively. The time gains were considerable, going from hours or minutes for purely FEM-based simulations to milliseconds, when using ML. CONCLUSIONS: ML algorithms can be used to accelerate FEM-based biomechanical simulations of anatomical structures, possibly reaching real-time responses. Fast and real-time simulations of anatomical structures, generated with ML algorithms, can help to reduce the time required by FEM-based simulations and accelerate their adoption in the clinical practice.

Comparison of the biomechanical 3D efficiency of different brace designs for the treatment of scoliosis using a finite element model.

The biomechanical influence of thoraco-lumbo-sacral bracing, a commonly employed treatment in scoliosis, is still not fully understood. The aim of this study was to compare the immediate corrections generated by different virtual braces using a patient-specific finite element model (FEM) and to analyze the most influential design factors. The 3D geometry of three patients presenting different types of curves was acquired with a multi-view X-ray technique and surface topography. A personalized FEM of the patients' trunk and a parametric model of a virtual custom-fit brace were then created. The installation of the braces on the patients was simulated. The influence of 15 design factors on the 3D correction generated by the brace was evaluated following a design of experiments simulation protocol allowing computing the main and two-way interaction effects of the design factors. A total of 12,288 different braces were tested. Results showed a great variability of the braces effectiveness. Of the 15 design factors investigated, according to the 2 modalities chosen for each one, the 5 most influential design factors were the position of the brace opening (posterior vs. anterior), the strap tension, the trochanter extension side, the lordosis design and the rigid shell shape. The position of the brace opening modified the correction mechanism. The trochanter extension position influenced the efficiency of the thoracic and lumbar pads by modifying their lever arm. Increasing the strap tension improved corrections of coronal curves. The lordosis design had an influence in the sagittal plane but not in the coronal plane. This study could help to better understand the brace biomechanics and to rationalize and optimize their design.

Biomechanical Comparison of the Load-Sharing Capacity of High and Low Implant Density Constructs With Three Types of Pedicle Screws for the Instrumentation of Adolescent Idiopathic Scoliosis.

STUDY DESIGN: Biomechanical numerical simulation analysis of implant design and density in adolescent idiopathic scoliosis posterior instrumentation. OBJECTIVES: To evaluate the combined effect of pedicle screw design and density on deformity correction and construct load-sharing capacity. SUMMARY OF BACKGROUND DATA: Screw density is an area of popular study because of the impact of cost and potential patient morbidity of higher-density constructs. Using fewer screws raises concern about reduced correction and greater forces on each screw. METHODS: Personalized spinal numerical models were created for five patients. The correction techniques from five spine surgeons using both a high- and a low-density implant pattern (2 vs. 1.4 ± 0.22 screws/level) with uniaxial, multiaxial, and favored angle screws were simulated. The predicted correction and forces sustained by the implants were compared. The postoperative load-sharing capacity of a high- and a low-density construct, with or without crosslinks, was compared by simulating daily activities motions. RESULTS: The major coronal curve correction was similar with high- and low-density constructs (73% ± 10% vs. 72% ± 10%; p > .05) but was higher when using uniaxial (77% ± 8%) compared to multiaxial (69% ± 11%) and favored angle screws (71% ± 10%; p = .009). High- and low-density constructs sustained similar intraoperative peak forces (305 ± 61 N vs. 301 ± 73 N; p = .23) regardless of screw design (all p > .05). Multiaxial and favored angle screws reduced the peak axial force by 23% and 38% compared to uniaxial screws (p = .007). The high-density construct reduced the postoperative loads sustained by each implant by 31% (p = .006). Crosslinks had no effect on load sharing (p = .23). CONCLUSION: High- and low-density implant patterns achieved similar coronal correction with equivalent capacity to share corrective forces regardless of the screw design. Increased degrees of freedom of the screw head reduces the capacity to correct coronal deformity but generates lower bone-screw forces. The reduced number of screws increased the postoperative forces sustained by each screw, but its effect on potential complications requires further investigations. LEVEL OF EVIDENCE: Level 4.

Biomechanical simulations of costo-vertebral and anterior vertebral body tethers for the fusionless treatment of pediatric scoliosis.

Compression-based fusionless tethers are an alternative to conventional surgical treatments of pediatric scoliosis. Anterior approaches place an anterior (ANT) tether on the anterolateral convexity of the deformed spine to modify growth. Posterior, or costo-vertebral (CV), approaches have not been assessed for biomechanical and corrective effectiveness. The objective was to biomechanically assess CV and ANT tethers using six patient-specific, finite element models of adolescent scoliotic patients (11.9 ± 0.7 years, Cobb 34° ± 10°). A validated algorithm simulated the growth and Hueter-Volkmann growth modulation over a period of 2 years with the CV and ANT tethers at two initial tensions (100, 200 N). The models without tethering also simulated deformity progression with Cobb angle increasing from 34° to 56°, axial rotation 11° to 13°, and kyphosis 28° to 32° (mean values). With the CV tether, the Cobb angle was reduced to 27° and 20° for tensions of 100 and 200 N, respectively, kyphosis to 21° and 19°, and no change in axial rotation. With the ANT tether, Cobb was reduced to 32° and 9° for 100 and 200 N, respectively, kyphosis unchanged, and axial rotation to 3° and 0°. While the CV tether mildly corrected the coronal curve over a 2-year growth period, it had sagittal lordosing effect, particularly with increasing initial axial rotation (>15°). The ANT tether achieved coronal correction, maintained kyphosis, and reduced the axial rotation, but over-correction was simulated at higher initial tensions. This biomechanical study captured the differences between a CV and ANT tether and indicated the variability arising from the patient-specific characteristics. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:254-264, 2018.

Virtual prototyping of a brace design for the correction of scoliotic deformities.

Based on a three-dimensional patient-specific finite element model of the spine, rib cage, pelvis and abdomen, a parametric model of a thoraco-lumbo-sacral orthosis (TLSO) was built. Its geometry is custom-fit to the patient. The rigid shell, pads and openings are all represented. The interaction between the trunk and the brace is modeled by a point-to-surface contact interface. During the nonlinear simulation process, the brace is opened, positioned on the patient and strap tension is applied. A TLSO similar to Boston brace system was built for a right-thoracic scoliotic patient. The influences of the trochanter pad and strap tension on the 3-D geometrical corrections and on the forces generated by the brace were evaluated. The role of the trochanter pad as a lever arm is confirmed by the model. The brace induces a reduction of the lordosis and pelvic tilt. The reduction of the frontal curvature is about 20% for a strap tension of 60 N. Axial rotation does not significantly change and rib hump is worsened. By using an explicit brace model and a contact interface, a more realistic simulation of orthotic treatment of scoliosis can be achieved. The stabilization of the brace on the patient can be represented and less restrictive boundary conditions can be applied. This model could be used to study the effect of design parameters on the brace efficiency.

Biomechanical modeling of brace design.

OBJECTIVE: To study the biomechanical effectiveness of brace design parameters in right thoracic idiopathic scoliosis. METHODS: A finite element model (FEM) of the spine, rib cage, pelvis and abdomen was adapted to the geometry of 8 patients with right-thoracic idiopathic scoliosis using a multi-view radiographic reconstruction technique. A detailed parametric FEM of a thoraco-lumbo-sacral orthosis and a Box, Hunter & Hunter experimental design method were used to analyze the contribution of brace design parameters (brace size, number of straps, strap tension, position of the thoracic pad, lordosis reduction design) and of patient's spine stiffness. RESULTS: The mean Cobb angle correction of the thoracic curve was 5.1 degrees (0 degrees to 16 degrees). The most influential parameters were, in descending order, the strap tension, lordosis reduction design and spine stiffness. Their effects are independent and remain weak (-3 degrees when strap tension increases from 20 N to 60 N). Changing the position of the thoracic pad (slightly above or below the apex) doesn't have a significant effect. No significant correction of the axial rotation and rib hump was obtained. DISCUSSION & CONCLUSION: Frontal curve correction varied significantly, which justifies the need for an adequate adjustment of the brace. A more efficient design for the correction of transverse deformities remains to be found. The "active" correction component by the muscles was not included, but one can anticipate that its action would be concurrent to the passive brace mechanisms, enabling supplementary correction. A new tool simulating brace treatment has been developed, which allows rational design of braces.

Biomechanical Assessment of Providence Nighttime Brace for the Treatment of Adolescent Idiopathic Scoliosis.

STUDY DESIGN: Biomechanical study of the Providence brace for the treatment of adolescent idiopathic scoliosis (AIS). OBJECTIVES: To model and assess the effectiveness of Providence nighttime brace. SUMMARY OF BACKGROUND DATA: Providence nighttime brace is an alternative to traditional daytime thoracolumbosacral orthosis for the treatment of moderate scoliotic deformities. It applies three-point pressure to reduce scoliotic curves. The biomechanics of the supine position and Providence brace is still poorly understood. METHODS: Eighteen patients with AIS were recruited at our institution. For each patient, a personalized finite element model (FEM) of the trunk was created. The spine, rib cage, and pelvis geometry was acquired using simultaneous biplanar low-dose radiographs (EOS). The trunk surface was acquired using a three-dimensional surface topography scanner. The interior surface of each patient's Providence brace was digitized and used to generate an FEM of the brace. Pressures at the brace/skin interface were measured using pressure sensors, and the average pressure distribution was computed. The standing to supine transition and brace installation were computationally simulated. RESULTS: Simulated standing to supine position induced an average curve correction of 45% and 48% for thoracic and lumbar curves, while adding the brace resulted in an average correction of 62% and 64% (vs. real in-brace correction of 65% and 70%). Simulated pressures had the same distribution as measured ones. Bending moments on apical vertebrae were mostly annulled by the positioning in the supine position, and further overcorrected on average by 10% to 13%, but in the opposite direction. CONCLUSIONS: The supine position is responsible for the major part of coronal curve correction, while the brace itself plays a complementary role. Bending moments induced by the brace generated a rebalancing of pressure on the growth plates, which could help reduce the asymmetric growth of the vertebrae. LEVEL OF EVIDENCE: Level II.

Biomechanical simulation and analysis of scoliosis correction using a fusionless intravertebral epiphyseal device.

STUDY DESIGN: Computer simulations to analyze the biomechanics of a novel compression-based fusionless device (hemistaple) that does not cross the disc for the treatment of adolescent idiopathic scoliosis. OBJECTIVE: To biomechanically model, simulate, and analyze the hemistaple action using a human finite element model (FEM). SUMMARY OF BACKGROUND DATA: A new fusionless growth sparing instrumentation device (hemistaple), which locally compresses the growth plate without spanning the disc, was previously developed and successively tested on different animal models. METHODS: Patient-specific FEMs of the spine, rib cage, and pelvis were built using radiographs of 10 scoliotic adolescents (11.7 ± 0.9 yr; Cobb thoracic: 35° ± 7°, lumbar: 24° ± 6°). A validated algorithm allowed simulating the growth (0.8-1.1 mm/yr/vertebra) and growth modulation process (Hueter-Volkmann principle) during a period of 2 years. Four instrumentation configurations on the convex curves were individually simulated (Config 1: 5 thoracic vertebrae with hemistaples on superior endplates; Config 2: same as Config 1 with hemistaples on both endplates; Config 3: same as Config 1 + 4 lumbar vertebrae; Config 4: same as Config 2 + 4 lumbar vertebrae). RESULTS: Without hemistaples, on average the thoracic and lumbar Cobb angles, respectively, progressed from 35° to 56° and 24° to 30°, whereas the vertebral wedging at curve apices progressed from 5° to 12°. With the hemistaple Config 1, the Cobb angles progressed but were limited to 42° and 26°, whereas the wedging ended at 8°. With Config 3, Cobb and wedging were kept nearly constant (38°, 21°, 7°). With hemistaples on both endplates (Config 2, Config 4), the Cobb and wedging were all reduced (thoracic Cobb for Config 2 and 4: 24° and 15°; lumbar Cobb: 21° and 11°; wedging: 2° and 1°). CONCLUSION: This study suggests that the hemistaple has the biomechanical potential to control the scoliosis progression and highlights the importance of the instrumentation configuration to correct the spinal deformities. It biomechanically supports the new fusionless device concept as an alternative for the early treatment of idiopathic scoliosis. LEVEL OF EVIDENCE: 5.

Braces Optimized With Computer-Assisted Design and Simulations Are Lighter, More Comfortable, and More Efficient Than Plaster-Cast Braces for the Treatment of Adolescent Idiopathic Scoliosis.

STUDY DESIGN: Feasibility study to compare the effectiveness of 2 brace design and fabrication methods for treatment of adolescent idiopathic scoliosis: a standard plaster-cast method and a computational method combining computer-aided design and fabrication and finite element simulation. OBJECTIVES: To improve brace design using a new brace design method. SUMMARY OF BACKGROUND DATA: Initial in-brace correction and patient's compliance to treatment are important factors for brace efficiency. Negative cosmetic appearance and functional discomfort resulting from pressure points, humidity, and restriction of movement can cause poor compliance with the prescribed wearing schedule. METHODS: A total of 15 consecutive patients with brace prescription were recruited. Two braces were designed and fabricated for each patient: a standard thoracolumbo-sacral orthosis brace fabricated using plaster-cast method and an improved brace for comfort (NewBrace) fabricated using a computational method combining computer-aided design and fabrication software (Rodin4D) and a simulation platform. Three-dimensional reconstructions of the torso and the trunk skeleton were used to create a personalized finite element model, which was used for brace design and predict correction. Simulated pressures on the torso and distance between the brace and patient's skin were used to remove ineffective brace material situated at more than 6 mm from the patient's skin. Biplanar radiographs of the patient wearing each brace were taken to compare their effectiveness. Patients filled out a questionnaire to compare their comfort. RESULTS: NewBraces were 61% thinner and had 32% less material than standard braces with equivalent correction. NewBraces were more comfortable (11 of 15 patients) or equivalent to (4 of 15 cases) standard braces. Simulated correction was simulated within 5° compared with in-brace results. CONCLUSIONS: This study demonstrates the feasibility of designing lighter and more comfortable braces with correction equivalent to standard braces. This design platform has the potential to further improve brace correction efficiency and its compliance.

New brace design combining CAD/CAM and biomechanical simulation for the treatment of adolescent idiopathic scoliosis.

BACKGROUND: A numerical based brace design platform, including biomechanical simulation, Computer Aided Design and Computer Aided Manufacturing (CAD/CAM) was developed to rationalize braces for the treatment of adolescent idiopathic scoliosis. The objective of this study was to test the feasibility of the approach and assess the effectiveness of braces issued from this platform as compared to standard brace design. METHODS: The biomechanical finite element model was built using the 3D reconstruction of the trunk skeleton from bi-planar radiographs and of the torso surface from surface topography. The finite element model is linked to a CAD/CAM software (Rodin4D), allowing the iterative design and simulation of the correction provided by the brace, as well as predicting pressures exerted on the torso. The resulting brace design was then fabricated using a numerical controlled carver. A brace designed using this platform (New Brace) as well as a standard thoraco-lumbo-sacral orthosis (Standard Brace) were built for six scoliotic patients. Both brace effectiveness was assessed using radiographs and compared to the simulations. FINDINGS: The New Brace corrected on average the spine deformities within 5° of Cobb angle of the simulated correction and with a similar correction as compared to the Standard Brace (average correction of 16° vs. 11° (MT); P=0.1 and 13° vs. 16° (TL/L); P=0.5 for the Standard Brace and the New Brace respectively). The two braces had a similar 10° lordosing effect of the thoracic curve. The coronal balance was quite similar (7.3 vs. 6.8mm balance improvement respectively for New Brace vs. Standard Brace). INTERPRETATION: These first clinical results showed the feasibility of building computer-assisted braces, equivalent to standard orthosis. An extended study on more cases is under way to fully assess this new design paradigm, which in the long term would allow improving brace design and rationalize the conservative treatments of scoliosis.

Biomechanical modeling of brace treatment of scoliosis: effects of gravitational loads.

The biomechanics of bracing in adolescent idiopathic scoliosis is still not fully understood. Finite element models (FEM) have been used but the gravity forces were not included and the production of spinal stresses not evaluated. An improved FEM to simulate brace treatment was thus developed. The 3D geometry of the spine, rib cage, pelvis, and of the trunk external surface of five scoliotic patients was acquired using a multi-view X-ray technique and surface topography. A FEM of the patient's trunk including gravity forces was created. Custom-fit braces were modeled and their installation simulated. Immediate geometrical corrections and pressures were computed and validated. The resulting compressive loads on the vertebral endplates were quantified. The influence of the strap tension, spine stiffness, and of the gravity forces was evaluated. Results showed that the brace biomechanical action was importantly to prevent the scoliotic spine from bending under the gravity forces. The immediate correction depended on the strap tension and spine stiffness. The distribution and amplitude of computed pressures were similar to those measured with the real braces. After the brace installation, the coronal asymmetrical compressive loading on the vertebral endplates was significantly reduced. In conclusion, the model developed presents improvements over previous models and could be used to better understand and optimize brace treatment.

A new method to include the gravitational forces in a finite element model of the scoliotic spine.

The distribution of stresses in the scoliotic spine is still not well known despite its biomechanical importance in the pathomechanisms and treatment of scoliosis. Gravitational forces are one of the sources of these stresses. Existing finite element models (FEMs), when considering gravity, applied these forces on a geometry acquired from radiographs while the patient was already subjected to gravity, which resulted in a deformed spine different from the actual one. A new method to include gravitational forces on a scoliotic trunk FEM and compute the stresses in the spine was consequently developed. The 3D geometry of three scoliotic patients was acquired using a multi-view X-ray 3D reconstruction technique and surface topography. The FEM of the patients' trunk was created using this geometry. A simulation process was developed to apply the gravitational forces at the centers of gravity of each vertebra level. First the "zero-gravity" geometry was determined by applying adequate upwards forces on the initial geometry. The stresses were reset to zero and then the gravity forces were applied to compute the geometry of the spine subjected to gravity. An optimization process was necessary to find the appropriate zero-gravity and gravity geometries. The design variables were the forces applied on the model to find the zero-gravity geometry. After optimization the difference between the vertebral positions acquired from radiographs and the vertebral positions simulated with the model was inferior to 3 mm. The forces and compressive stresses in the scoliotic spine were then computed. There was an asymmetrical load in the coronal plane, particularly, at the apices of the scoliotic curves. Difference of mean compressive stresses between concavity and convexity of the scoliotic curves ranged between 0.1 and 0.2 MPa. In conclusion, a realistic way of integrating gravity in a scoliotic trunk FEM was developed and stresses due to gravity were explicitly computed. This is a valuable improvement for further biomechanical modeling studies of scoliosis.

A biomechanical study of the Charleston brace for the treatment of scoliosis.

STUDY DESIGN: A biomechanical study of the Charleston brace. OBJECTIVE: To model the nighttime Charleston brace treatment and study its biomechanical action. SUMMARY OF BACKGROUND DATA: The Charleston brace has been proposed as an alternative to the traditional daytime thoracolumbosacral orthosis for the treatment of moderate scoliotic deformities. It is worn at night and imposes a supine side-bending to reduce the major scoliotic curve. The biomechanics of the Charleston brace is still poorly understood. METHODS: The geometry of the spine, pelvis, rib cage, and of the external trunk surface of 2 scoliotic patients were acquired using a 3-dimensional multiview radiograph reconstruction technique and surface topography. A finite element model of each patient's trunk was created. Two sets of mechanical properties (stiff and normal) of the spine were tested. For each case, the transition from standing to supine position was first simulated by modifying the direction of the gravity forces acting on the patients' spine. Supine bending was simulated by applying a lateral displacement on the first thoracic vertebra. A custom-fit Charleston brace was modeled and positioned on the patient model. Tension was applied in the straps. Efficiency of the simulated Charleston braces was studied by computing geometrical corrections and effects on the internal stresses of the spine. RESULTS: The reduction of the major scoliotic curve varied between 58% and 97% and was in the range of published clinical data. Internal compressive stresses up to 1 MPa were generated on the convex side of the major scoliotic curve and tensile stresses up to 1 MPa on its concavity. In contrast, increased compressive stresses were exerted on the concavity of the secondary curves and added tensile stresses in their convexity. CONCLUSION: This study quantified the Charleston brace's biomechanical effect, which consists in inverting the asymmetrical compressive loading in the major scoliotic curve. It also highlighted that the Charleston brace worsens the asymmetrical compressive loading in the compensatory curves. The finite element model developed could help studying different brace designs and optimizing brace efficiency.

Correlation between immediate in-brace correction and biomechanical effectiveness of brace treatment in adolescent idiopathic scoliosis.

STUDY DESIGN: Multiple brace designs were simulated using a finite element model and their biomechanical effect was evaluated. OBJECTIVE: To study correlations between immediate in-brace correction of coronal curves and bending moments acting on the apical vertebrae. SUMMARY OF BACKGROUND DATA: Immediate in-brace correction has often been deemed as fundamental to long-term brace effect but the biomechanical explanation is unclear. METHODS: Three-dimensional geometry of 3 patients was acquired using multiview radiographs and surface topography techniques. A finite element model of the patients' trunk including gravitational forces and a parametric brace model were created. Two sets of mechanical properties of the spine (stiff and flexible) were tested. Installation of the brace on the patients was simulated. Using an experimental design framework including fourteen design factors, 1024 different virtual braces were tested for each patient. For each brace, immediate in-brace correction of the coronal Cobb angles and the bending moment acting on the apical vertebrae were computed and their correlation was studied. RESULTS: Immediate correction of coronal curves and corresponding impact on the apical vertebrae bending moments were linearly correlated (mean R = 0.88). The amount of immediate correction necessary to nullify the bending moment ranged between 19% and 61% with average 48% (flexible spine model) and 27% (stiff spine model). The braces corrected the apical vertebrae bending moment more in the flexible spine model. In the framework of the Hueter-Volkmann principle, the correlation between coronal immediate in-brace correction and corresponding apical bending moment can be interpreted as a correlation between immediate in-brace correction and long-term treatment outcome. The amount of immediate correction necessary to invert the bending moments, and in theory counteract the progression of the scoliotic deformity, depends on spine stiffness and spine segment. CONCLUSION: This study confirms the importance of immediate in-brace correction to predict long-term outcome of the treatment and provides insights in the understanding of brace biomechanics.