Ligamentous tethering and intradiscal pressure affecting the mechanical environment of scoliotic spines.

Despite several theories have been proposed to explain the progression of Adolescent Idiopathic Scoliosis (AIS), there is no consensus on the mechanical factors that control the spinal deformities. Prominent biomechanical notions focus on the geometrical asymmetry and differential growth, however, the correlation between these phenomena remains unclear. We postulate that intradiscal pressure and its connection with the supporting ligamentous structures are the reasons behind the asymmetric growth in AIS. To investigate this hypothesis, a numerical 3D patient-specific model of a scoliotic spine is constructed to carry upper body weight. Four analyses are performed: control simulation with no ligaments followed by 3 simulations, in each, a different and stiffer set of ligaments is employed. The analyses showed that intradiscal pressure is relatively high in the spine's higher-deformity region. Moreover, the stiffness effect of the ligamentous tethering correlated directly to intradiscal pressure; the stiffer the ligaments, the higher the intradiscal pressure. Due to geometrical asymmetry, the pressure is eccentric toward the concave region of deformed vertebral units. As a result, the deformed annulus fibrosus generated uplifts in the convex side of deformed vertebral units. The eccentric pressure and the uplift are opposite in location and direction creating an imbalanced mechanical environment for the spine during growth.

Inter-screw index as a novel diagnostic indicator of tether breakage.

PURPOSE: Tether breakage is the most common complication of Vertebral Body Tethering (VBT) occurring in up to 52% of Adolescent Idiopathic Scoliosis (AIS) patients and risks continued progression and revision. Radiographical diagnosis of tether breakage is commonly defined by a 5° increase in inter-screw angle and associates breakage with loss of correction. However, the sensitivity of this method was 56% only, suggesting that tethers can break without an increase in angulation, which was supported by other studies. To our knowledge, current literature lacks a method merely focusing on the diagnosis of tether breakage radiographically that does not associate the breakages with loss of correction. METHODS: This was a retrospective review of prospectively collected data of AIS patients who underwent VBT. The "inter-screw index" is defined as the percentage increase in inter-screw distance since post-op, with ≥ 13% increase defined as tether breakage as suggested by our mechanical tests. CTs were reviewed to identify the breakages and compared with inter-screw angle and inter-screw index. RESULTS: 94 segments from 13 CTs were reviewed, and 15 tether breakages were identified. Use of inter-screw index correctly identified 14 breakages (93%), whereas ≥ 5° increase in inter-screw angle only identified 12 breakages (80%). CONCLUSION: Use of inter-screw index is proven to be more sensitive than inter-screw angle in identifying tether breakages. Therefore, we propose the use of inter-screw index to diagnose tether breakages radiographically. Tether breakages were not necessarily accompanied by a loss of segmental correction leading to an increase in inter-screw angle, especially after skeletal maturity. LEVEL OF EVIDENCE: Level 3.

Biomechanics of the tether breakage: tensile behaviour of a single-unit vertebral body tethering construct.

PURPOSE: Tether breakage was reported as the most common complication of vertebral body tethering. However, as the literature suggests the physiological loads do not have the potential to cause the failure of the tether. Currently, the biomechanical reason behind the tether breakage is unknown. The current study aims to elucidate the effects of the tension forces on the failure mechanisms of the VBT and provide mechanical justification for how it can be identified radiographically. METHODS: Tensile tests (20%/min strain rate) were performed on single-unit VBT samples. Failure modes and mechanical characteristics were reported. RESULTS: The failure took place prematurely due to the slippage of the tether at the screw-tether junction where the tether is damaged significantly by the locking cap. Slippage was initiated at 10-13% tensile strain level where the tensile stress and tension force were 50.4 ± 1.5 MPa and 582.2 ± 30.8 N, respectively. CONCLUSION: The failure occurs because of high-stress concentrations generated within the locking region which damages the tether surface and leads to the slippage of the tether. We observed that the loads leading to failure are within the physiological limits and may indicate the high likelihood of the tether breakage. The failure mode observed in our study is shown to be the dominant failure mode, and a design improvement on the gripping mechanism is suggested to avoid failure at the screw-tether junction. We observed that the tether elongates 10-13% prior to the breakage, which can be employed as a diagnostic criterion to screen for tether breakages radiographically.

Yeditepe spine mesh: Finite element modeling and validation of a parametric CAD model of lumbar spine.

Finite element analysis is a powerful tool that is often used to study the biomechanical response of the spine. The primary objective of this study was to illustrate the mechanical behavior of a previously proposed parametric CAD spine model in comparison with a segmented FSU model and the literature. In this study, two finite element models of the L4-L5 spinal level were developed from the same patient's CT scan data. The first was developed using well-known segmentation methods, whereas the second was developed from the new by using a novel parametric CAD model. Both models were subjected to the same loading and boundary conditions to perform flexion, extension, lateral bending and axial rotation motions. The segmented finite element model was observed to be in good agreement with the literature. The parametric finite element model results were also observed to be in good agreement with the segmented finite element model and with the literature except under extension.

Sensitivity of muscle and intervertebral disc force computations to variations in muscle attachment sites.

The current paper aims at assessing the sensitivity of muscle and intervertebral disc force computations against potential errors in modeling muscle attachment sites. We perturbed each attachment location in a complete and coherent musculoskeletal model of the human spine and quantified the changes in muscle and disc forces during standing upright, flexion, lateral bending, and axial rotation of the trunk. Although the majority of the muscles caused minor changes (less than 5%) in the disc forces, certain muscle groups, for example, quadratus lumborum, altered the shear and compressive forces as high as 353% and 17%, respectively. Furthermore, percent changes were higher in the shear forces than in the compressive forces. Our analyses identified certain muscles in the rib cage (intercostales interni and intercostales externi) and lumbar spine (quadratus lumborum and longissimus thoracis) as being more influential for computing muscle and disc forces. Furthermore, the disc forces at the L4/L5 joint were the most sensitive against muscle attachment sites, followed by T6/T7 and T12/L1 joints. Presented findings suggest that modeling muscle attachment sites based on solely anatomical illustrations might lead to erroneous evaluation of internal forces and promote using anatomical datasets where these locations were accurately measured. When developing a personalized model of the spine, certain care should also be paid especially for the muscles indicated in this work.