Biomechanical loading of the sacrum in adolescent idiopathic scoliosis

A Novel Dynamic Growth Rod Inducing Spinal Growth Modulation for the Correction of Spinal Deformities

Background

Growth rods are the gold standard for treating early-onset scoliosis (EOS). However, current treatments with growth rods do not optimize spinal growth in EOS patients, and frequent distraction surgeries significantly increase complications, imposing considerable economic and psychological burdens on patients. An improved growth rod is urgently required to address the need for dynamic growth and external regulation.

Methods

This study designed a novel growth rod (NGR) with unidirectional sliding and external regulation capabilities. By establishing a three-dimensional model of the EOS spine, we simulated the implantation of traditional growth rods (TGR) and NGR. We applied a compressive load of 400 N to test axial stiffness and a moment of 1 NM to assess bending stiffness under six different conditions. Additionally, we evaluated the range of motion (ROM) of the spinal joints, and the distribution of Von Mises stress in vertebrae, intervertebral discs, and the growth rods, and calculated the axial force, moment, fatigue life, and strain energy of the device.

Results

NGR exhibits higher axial compression and torsional stiffness than TGR and the Intact group. Additionally, Von Mises stress values for NGR are higher than those for TGR across all operating conditions, albeit with slightly lower total strain energy than TGR. Although Von Mises stress in NGR concentrates near the screw fixation, the fatigue life remains adequate for basic living requirements.

Conclusion

Overall, NGR demonstrates superior stiffness and stress distribution. NGR's distraction-based implant features a unidirectional sliding component with a spring-driven mechanism for dynamic correction and a novel non-invasive extension mechanism to reduce infections. Compared to leading EOS implants, NGR offers improved stability, showing promise for enhancing EOS surgical interventions.

Current models to understand the onset and progression of scoliotic deformities in adolescent idiopathic scoliosis: a systematic review

PURPOSE

To create an updated and comprehensive overview of the modeling studies that have been done to understand the mechanics underlying deformities of adolescent idiopathic scoliosis (AIS), to predict the risk of curve progression and thereby substantiate etiopathogenetic theories.

METHODS

In this systematic review, an online search in Scopus and PubMed together with an analysis in secondary references was done, which yielded 86 studies. The modeling types were extracted and the studies were categorized accordingly.

RESULTS

Animal modeling, together with machine learning modeling, forms the category of black box models. This category is perceived as the most clinically relevant. While animal models provide a tangible idea of the biomechanical effects in scoliotic deformities, machine learning modeling was found to be the best curve-progression predictor. The second category, that of artificial models, has, just as animal modeling, a tangible model as a result, but focusses more on the biomechanical process of the scoliotic deformity. The third category is formed by computational models, which are very popular in etiopathogenetic parameter-based studies. They are also the best in calculating stresses and strains on vertebrae, intervertebral discs, and other surrounding tissues.

CONCLUSION

This study presents a comprehensive overview of the current modeling techniques to understand the mechanics of the scoliotic deformities, predict the risk of curve progression in AIS and thereby substantiate etiopathogenetic theories. Although AIS remains to be seen as a complex and multifactorial problem, the progression of its deformity can be predicted with good accuracy. Modeling of AIS develops rapidly and may lead to the identification of risk factors and mitigation strategies in the near future. The overview presented provides a basis to follow this development.

Kinematic and biomechanical responses of the spine to distraction surgery in children with early onset scoliosis: A 3-D finite element analysis

Periodical and consecutive distraction is an effective treatment for severe early onset scoliosis (EOS), which enables the spinal coronal and sagittal plane deformity correction. However, the rate of rod fractures and postoperative complications was still high mainly related to the distraction process. Previous studies have primarily investigated the maximum safe distraction force without a rod broken, neglecting the spinal re-imbalance and distraction energy consumption, which is equally vital to evaluate the operative value. This study aimed to reveal the kinematic and biomechanical responses occurring after spinal distraction surgery, which were affected by traditional bilateral fixation. The spinal models (C6-S1) before four distractions were reconstructed based on CT images and the growing rods were applied with the upward displacement load of 0–25 mm at an interval of 5 mm. Relationships between the distraction distance, the distraction force and the thoracic and lumbar Cobb angle were revealed, and the spinal displacement and rotation in three-dimensional directions were measured. The spinal overall imbalance would also happen during the distraction process even under the safe force, which was characterized by unexpected cervical lordosis and lateral displacement. Additionally, the law of diminishing return has been confirmed by comparing the distraction energy consumption in different distraction distances, which suggests that more attention paid to the spinal kinematic and biomechanical changes is better than to the distraction force. Notably, the selection of fixed segments significantly impacts the distraction force at the same distraction distance. Accordingly, some results could provide a better understanding of spinal distraction surgery.

A systematic review and meta-analysis of fusion rate enhancements and bone graft options for spine surgery

Our study aimed to evaluate differences in outcomes of patients submitted to spinal fusion using different grafts measuring the effectiveness of spinal fusion rates, pseudarthrosis rates, and adverse events. Applying the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, this systematic review and meta-analysis identified 64 eligible articles. The main inclusion criteria were adult patients that were submitted to spinal fusion, autologous iliac crest (AIC), allograft (ALG), alloplastic (ALP; hydroxyapatite, rhBMP-2, rhBMP-7, or the association between them), and local bone (LB), whether in addition to metallic implants or not, was applied. We made a comparison among those groups to evaluate the presence of differences in outcomes, such as fusion rate, hospital stay, follow-up extension (6, 12, 24, and 48 months), pseudarthrosis rate, and adverse events. Sixty-four studies were identified. LB presented significantly higher proportions of fusion rates (95.3% CI 89.7–98.7) compared to the AIC (88.6% CI 84.8–91.9), ALG (87.8% CI 80.8–93.4), and ALP (85.8% CI 75.7–93.5) study groups. Pseudarthrosis presented at a significantly lower pooled proportion of ALG studies (4.8% CI 0.1–15.7) compared to AIC (8.6% CI 4.2–14.2), ALP (7.1% CI 0.9–18.2), and LB (10.3% CI 1.8–24.5). ALP and AIC studies described significantly more cases of adverse events (80 events/404 patients and 860 events/2001 patients, respectively) compared to LB (20 events/311 patients) and ALG (73 events/459 patients). Most studies presented high risk-of-bias scores. Based on fusion rates and adverse events proportions, LB showed a superior trend among the graft cases we analyzed. However, our review revealed highly heterogeneous data and a need for more rigorous studies to better address and assist surgeons’ choices of the best spinal grafts.

Effects of Growing Rod Technique with Different Surgical Modes and Growth Phases on the Treatment Outcome of Early Onset Scoliosis: A 3-D Finite Element Analysis

Early onset scoliosis (EOS) is emerging as a serious threat to children’s health and is the third largest threat to their health after myopia and obesity. At present, the growing rod technique (GRT), which allows patients to regain a well-balanced sagittal profile, is commonly considered as an invasive surgical procedure for the treatment of EOS. However, the risk of postoperative complications and instrumentation breakage remains high, which is mainly related to the choice of fixed mode. Several authors have studied primary stability and instrumentation loads, neglecting the mechanical transmission of the spinal long-segment model in different growth phases, which is fundamental to building a complete biomechanical environment. The present study aimed to investigate the kinematic and biomechanical properties that occur after GRT, across the long spinal structure and the posterior instrumentation, which are affected by unilateral or bilateral fixation. Accordingly, spinal segments (C6-S1) were loaded under flexion (Flex), extension (Ext), left lateral bending (LB), right lateral bending (RB), left torsion (LT), and right torsion (RT) using 11 established spinal models, which were from three growth phases. The stress distribution, spinal and intervertebral range of motion (ROM), counter torque of the vertebra, and bracing force on the rods were measured. The results showed that bilateral posterior fixation (BPF) is more stable than unilateral posterior fixation (UPF), at the expense of more compensations for the superior adjacent segment (SAS), especially when the superior fixed segment is closer to the head. Additionally, the bracing force of the instrumentation on the spine increases as the Cobb angle decreases. Accordingly, this biomechanical analysis provides theoretical suggestions for the selection of BPF or UPF and fixed segments in different growing phases.

Computational modelling of the scoliotic spine: A literature review

Scoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed.

A reduced-order model of the spine to study pediatric scoliosis

The S-shaped curvature of the spine has been hypothesized as the underlying mechanical cause of adolescent idiopathic scoliosis. In earlier work, we proposed a reduced-order model in which the spine was viewed as an S-shaped elastic rod under torsion and bending. Here, we simulate the deformation of S-shaped rods of a wide range of curvatures and inflection points under a fixed mechanical loading. Our analysis determines three distinct axial projection patterns of these S-shaped rods: two loop (in opposite directions) patterns and one Lemniscate pattern. We further identify the curve characteristics associated with each deformation pattern, showing that for rods deforming in a Loop1 shape the position of the inflection point is the highest and the curvature of the rod is smaller compared to the other two types. For rods deforming in the Loop2 shape, the position of the inflection point is the lowest (closer to the fixed base) and the curvatures are higher than the other two types. These patterns matched the common clinically observed scoliotic curves-Lenke 1 and Lenke 5. Our S-shaped elastic rod model generates deformations that are similar to those of a pediatric spine with the same sagittal curvature characteristics and it can differentiate between the clinically observed deformation patterns.

Investigation of multidirectional hip range of motion and hip motion asymmetry in individuals with idiopathic scoliosis with different curve patterns

BACKGROUND

While some studies of the asymmetry of lower limbs in individuals with idiopathic scoliosis exist, there is a need for multidirectional studies conducted on hip joint range of motion and its relationship to curve patterns in idiopathic scoliosis.

OBJECTIVES

This study analyzes the hip joint range of multidirectional motions, hip motion asymmetry and investigates them according to curve patterns in individuals with idiopathic scoliosis.

METHODS

The sample included 108 females with idiopathic scoliosis. Participants were divided into three groups: double curves, single thoracic curve and single lumbar curve. The range of hip flexion and extension, abduction and adduction, and internal and external rotations were assessed actively and passively with a universal goniometer. The range of motion, left-right asymmetry and the mid-points of the ranges of motion were analyzed.

RESULTS

The passive range of the right hip abduction was higher in the thoracic curve group vs. the lumbar curve group. Active and passive ranges of hip extension were higher in the left hip vs. right hip. Active left-right asymmetry was higher than passive left-right asymmetry.

CONCLUSION

Individuals with idiopathic scoliosis had different hip abduction motions according to curve pattern that originated from single curves. Left-right hip asymmetry was seen for the hip extension motion. Higher left-right asymmetry for active motion than passive motion in hip abduction may indicate a problem in motion perception in individuals with idiopathic scoliosis.

A Semi-Analytic Elastic Rod Model of Pediatric Spinal Deformity

The mechanism of the scoliotic curve development in healthy adolescents remains unknown in the field of orthopedic surgery. Variations in the sagittal curvature of the spine are believed to be a leading cause of scoliosis in this patient population. Here, we formulate the mechanics of S-shaped slender elastic rods as a model for pediatric spine under physiological loading. Second, applying inverse mechanics to clinical data of the subtypes of scoliotic spines, with characteristic 3D deformity, we determine the undeformed geometry of the spine before the induction of scoliosis. Our result successfully reproduces the clinical data of the deformed spine under varying loads, confirming that the prescoliotic sagittal curvature of the spine impacts the 3D loading that leads to scoliosis.

3D Deformation Patterns of S Shaped Elastic Rods as a Pathogenesis Model for Spinal Deformity in Adolescent Idiopathic Scoliosis

Adolescent idiopathic scoliosis (AIS) is a three-dimensional (3D) deformity of the spinal column in pediatric population. The primary cause of scoliosis remains unknown. The lack of such understanding has hampered development of effective preventive methods for management of this disease. A long-held assumption in pathogenesis of AIS is that the upright spine in human plays an important role in induction of scoliosis. Here, the variations in the sagittal curve of the scoliotic and non-scoliotic pediatric spines were used to study whether specific sagittal curves, under physiological loadings, are prone to 3D deformation leading to scoliosis. To this end, finite element models of the S shaped elastic rods, which their curves were derived from the radiographs of 129 sagittal spinal curves of adolescents with and without scoliosis, were generated. Using the mechanics of deformation in elastic rods, this study showed that the 3D deformation patterns of the two-dimensional S shaped slender elastic rods mimics the 3D patterns of the spinal deformity in AIS patients with the same S shaped sagittal spinal curve. On the other hand, the rods representing the non-scoliotic sagittal spinal curves, under the same mechanical loading, did not twist thus did not lead to a 3D deformation. This study provided strong evidence that the shape of the sagittal profile in individuals can be a leading cause of the 3D spinal deformity as is observed in the AIS population.

A biomechanical study on proximal junctional kyphosis following long-segment posterior spinal fusion

Posterior long-segment spinal fusion may lead to proximal junctional kyphosis (PJK). The present study sought to identify the appropriate fusion levels required in order to prevent PJK using finite element analysis. A finite element model was constructed based on the whole-spine computed tomography findings of a healthy adult. Nine commonly used posterior spinal fusion methods were selected. Stress on the annulus fibrosis fibers, the posterior ligamentous complex, and the vertebrae after various spinal fusions in the upright position were compared. This study was divided into two groups: non-fusion and fusion. In the former, the stress between the T10 and the upper thoracic vertebrae was higher. Comparing thoracic and lumbar segments in the fusion group, the peak stress values of the upper instrumented vertebrae (UIV) were mainly observed in T2 and L2 whilst those of the UIV+1 were observed in T10 and L2. After normalization, the peak stress values of the UIV and UIV+1 were located in T2 and L2. Similarly, the peak stress values of the annulus fibrosus at the upper adjacent level were on T10 and L2 after normalization. However, the peak stress values of the interspinal/supraspinal complex forces were concentrated on T11, T12, and L1 after normalization whilst the peak stress value of the pedicle screw was on T2. Controversy remains over the fusion of T10, and this study simulated testing conditions with gravitational loading only. However, further assessment is needed prior to reaching definitive conclusions.

Effect of Asymmetric Tension on Biomechanics and Metabolism of Vertebral Epiphyseal Plate in a Rodent Model of Scoliosis

OBJECTIVE

To investigate the effect of asymmetric tension on idiopathic scoliosis (IS) and to understand its pathogenic mechanism.

METHODS

The rodent model of scoliosis was established using Sprague-Dawley rats with left rib-tethering from T₆ to T₁₂ , tail and shoulder amputation, and high-cage feeding. Vertebrae epiphyseal cartilage plates were harvested from the convex and concave sides. To analyze differences on the convex and concave sides, finite element analysis was carried out to determine the mechanical stress. Protein expression on epiphyseal cartilage was evaluated by western blot. Micro-CT was taken to evaluate the bone quality of vertebral on both sides.

RESULTS

Scoliosis curves presented in X-ray radiographs of the rats. Finite element analysis was carried out on the axial and transverse tension of the spine. Stresses of the convex side were -170.14, -373.18, and -3832.32 MPa (X, Y, and Z axis, respectively), while the concave side showed stresses of 361.99, 605.55, and 3661.95 MPa. Collagen type II, collagen type X, Sox 9, RunX2, VEGF, and aggrecan were expressed significantly more on the convex side (P < 0.05). There was asymmetric expression of protein on the epiphyseal cartilage plate at molecular level. Compared with the convex side, the concave side had significantly lower value in the BV/TV and Tb.N, but higher value in the Tb.Sp (P < 0.05). There was asymmetry of bone quality in micro-architecture.

CONCLUSIONS

In this study, asymmetric tension contributed to asymmetry in protein expression and bone quality on vertebral epiphyseal plates, ultimately resulting in asymmetry of anatomy. In addition, asymmetry of anatomy aggravated asymmetric tension. It is the first study to show that there is an asymmetrical vicious circle in IS.

Pseudarthrosis in adult and pediatric spinal deformity surgery: a systematic review of the literature and meta-analysis of incidence, characteristics, and risk factors

We conducted a systematic review with meta-analysis and qualitative synthesis. This study aims to characterize pseudarthrosis after long-segment fusion in spinal deformity by identifying incidence rates by etiology, risk factors for its development, and common features. Pseudarthrosis can be a painful and debilitating complication of spinal fusion that may require reoperation. It is poorly characterized in the setting of spinal deformity. The MEDLINE, EMBASE, and Cochrane databases were searched for clinical research including spinal deformity patients treated with long-segment fusions reporting pseudarthrosis as a complication. Meta-analysis was performed on etiologic subsets of the studies to calculate incidence rates for pseudarthrosis. Qualitative synthesis was performed to identify characteristics of and risk factors for pseudarthrosis. The review found 162 articles reporting outcomes for 16,938 patients which met inclusion criteria. In general, the included studies were of medium to low quality according to recommended reporting standards and study design. Meta-analysis calculated an incidence of 1.4% (95% CI 0.9-1.8%) for pseudarthrosis in adolescent idiopathic scoliosis, 2.2% (95% CI 1.3-3.2%) in neuromuscular scoliosis, and 6.3% (95% CI 4.3-8.2%) in adult spinal deformity. Risk factors for pseudarthrosis include age over 55, construct length greater than 12 segments, smoking, thoracolumbar kyphosis greater than 20°, and fusion to the sacrum. Choice of graft material, pre-operative coronal alignment, post-operative analgesics, and sex have no significant impact on fusion rates. Older patients with greater deformity requiring more extensive instrumentation are at higher risk for pseudarthrosis. Overall incidence of pseudarthrosis requiring reoperation is low in adult populations and very low in adolescent populations.

Are we simplifying balance evaluation in adolescent idiopathic scoliosis?

BACKGROUND

Clinical evaluation of the postural balance in adolescent idiopathic scoliosis has been measured by sagittal vertical axis and frontal balance. The impact of the scoliotic deformity in three planes on balance has not been fully investigated.

METHODS

47 right thoracic and left lumbar curves adolescent idiopathic scoliosis and 10 non-scoliotic controls were registered prospectively. 13 spinopelvic postural parameters were calculated from the 3-dimantional reconstructions of X-rays. 7 balance variables describing the position and sway of the center of pressure were recorded using a pressure mat. A regression analysis was used to predict sagittal vertical axis and frontal balance from the 7 balance variables. A canonical correlation analysis was performed between all the postural parameters and balance variables and the significant associations between the postural and balance variables were determined.

FINDINGS

sagittal vertical axis and frontal balance were not significantly associated with the position or sway of the center of pressure (p>0.05). Canonical correlation analysis showed significant associations between the postural variables in the 3 planes and center of pressure position (R²=0.81) and sway (R²=0.62), p<0.05.

INTERPRETATION

Frontal Cobbs, apical rotations, distal kyphosis, pelvic incidence, sacral slope, sagittal vertical axis, and frontal balance contributed to the postural balance in the cohort. The compensatory role of the pelvis and distal kyphosis in sagittal plane was underlined. Multidimensional analyses between the postural and balance variables showed the alignment of the thoracic, lumbar, and pelvis in the 3 planes, in addition to the global head-pelvic position impact on adolescent idiopathic scoliosis balance.

The biomechanical effects of spinal fusion on the sacral loading in adolescent idiopathic scoliosis

BACKGROUND

Posterior spinal surgical correction is performed to correct spinal deformities in adolescent idiopathic scoliosis. Although the relative spino-pelvic alignment changes after spinal surgery, pelvis remains unfused in idiopathic scoliosis surgery. The impact of the spinal fusion on the transferred load to the pelvis via sacrum is not documented in the scoliotic subgroups.

METHOD

Bi-planar radiographs of 9 scoliotic subjects before and in average 16 months after spinal instrumentation surgery, and 12 controls were selected retrospectively. Patient-specific 3D reconstruction and finite element models of the spine, ribcage, and pelvis were developed. Spinal parameters (Cobb angles, kyphosis, lordosis), sacro-pelvic parameters (pelvic incidence, pelvic tilt, sacral slope), frontal and sagittal balances, the position of the trunk center of mass, and the centroid of the stress distribution on the sacrum superior endplate were measured and computed before operation and in the last follow-up.

FINDINGS

The position of the stress distribution centroid on the sacrum superior endplate with respect to the central hip vertical axis was significantly different between pre-operative and post-operative patients p<0.05. The distance between the anterior-posterior position of the trunk center of mass and the center of pressure on the superior sacral endplate significantly decreased after the spinal surgery p<0.05.

INTERPRETATION

The impact of the scoliosis spinal fusion on the transferred load between the spine and pelvis was evaluated. The biomechanical loading of the sacrum endplate was related to the post-operative postural balance and compensatory changes in the spino-pelvic alignment after scoliosis surgery.