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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.

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.

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.

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. © 2019, © 2019 University of Twente. Published by Informa UK Limited, trading as Taylor & Francis Group.

Biomechanical studies based on anatomical meshes are often generated with various segmentation methods; hence they are fixed, preventing tuning of morphometrical parameters. The primary objective of this study was to develop a methodology that would allow construction of a robust and high-fidelity CAD model from the CT scans of an individual. In this framework, a holistic approach was adopted for the design of the lumbar spine. This involved the breaking-up of the design intent into three levels. From top-down, a master layout sketch of the vertebral curve was used to drive the entire model at the highest level. Secondly, each vertebra was divided into two parts in order to simplify the parameterization procedure. Thirdly, a spiral curve was introduced in order to facilitate the placement of the facet joints. This way, a statically-determinate vertebral stacking sequence during assembly was realized. Hausdorff distance between the CAD and CT meshes was used to evaluate the accuracy of the model. In addition, mean and standard deviation of the errors were also calculated. A new generic lumbar vertebral CAD model was successfully built and tested. This model could lead to dramatic reductions in both time and effort required to prepare patient-specific biomechanical models. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. This study was supported as a research project by the Yeditepe University. We thank Aikaterini Panteli, MD, Neurosurgeon, and Kaan Yaltirik, MD, Neurosurgeon from the Yeditepe University Hospital for providing us with the CT scans.

For those studying the biomechanical response of the lumbar spine, anatomical meshes obtained from medical imaging data is quite important. However, such models are generally fixed and can only represent a single subject’s geometry. The objective of this study was to improve our previous lumbar vertebral CAD model such that the parameters are now extracted from the CT scan using a semi-automatic procedure. To illustrate the procedure, first, the transverse cross-sections of vertebral bodies were obtained from an individual at three levels, superior, middle and inferior. Parametric contour curves were fitted onto the vertebral body boundaries using an optimization procedure and the fitting errors are reported here. Five lumbar vertebral bodies were then formed using the lofting operation. Hausdorff distance between the CAD and segmentation models was used to assess the accuracy of the resulting models. The means and standard deviations of Hausdorff distances are also reported here. The adopted optimization process was observed to be resulting coefficient of determination values as high as 0.978. Our new model is expected to lead to dramatic reductions in both time and effort required to build a patient-specific biomechanical model. © Springer Nature Switzerland AG 2019.