Association between sagittal alignment and loads at the adjacent segment in the fused spine: a combined clinical and musculoskeletal modeling study of 205 patients with adult spinal deformity

Fusion surgery; Sagittal alignment; Spine surgeryCirugía de fusión; Alineación sagital; Cirugía de columnaCirurgia de fusió; Alineació sagital; Cirurgia de columnaPurpose Sagittal malalignment is a risk factor for mechanical complications after surgery for adult spinal deformity (ASD). Spinal loads, modulated by sagittal alignment, may explain this relationship. The aims of this study were to investigate the relationships between: (1) postoperative changes in loads at the proximal segment and realignment, and (2) absolute postoperative loads and postoperative alignment measures. Methods A previously validated musculoskeletal model of the whole spine was applied to study a clinical sample of 205 patients with ASD. Based on clinical and radiographic data, pre-and postoperative patient-specific alignments were simulated to predict loads at the proximal segment adjacent to the spinal fusion. Results Weak-to-moderate associations were found between pre-to-postop changes in lumbar lordosis, LL (r =  − 0.23, r =  − 0.43; p < 0.001), global tilt, GT (r = 0.26, r = 0.38; p < 0.001) and the Global Alignment and Proportion score, GAP (r = 0.26, r = 0.37; p < 0.001), and changes in compressive and shear forces at the proximal segment. GAP score parameters, thoracic kyphosis measurements and the slope of upper instrumented vertebra were associated with changes in shear. In patients with T10-pelvis fusion, moderate-to-strong associations were found between postoperative sagittal alignment measures and compressive and shear loads, with GT showing the strongest correlations (r = 0.75, r = 0.73, p < 0.001). Conclusions Spinal loads were estimated for patient-specific full spinal alignment profiles in a large cohort of patients with ASD pre-and postoperatively. Loads on the proximal segments were greater in association with sagittal malalignment and malorientation of proximal vertebra. Future work should explore whether they provide a causative mechanism explaining the associated risk of proximal junction complications.Study funding was provided by Maxi Foundation. Open access funding was provided by Swiss Federal Institute of Technology Zurich

The effect of muscle ageing and sarcopenia on spinal segmental loads

Purpose The interrelations between age-related muscle deterioration (sarcopenia) and vertebral fractures have been suggested based on clinical observations, but the biomechanical relationships have not been explored. The study aim was to investigate the effects of muscle ageing and sarcopenia on muscle recruitment patterns and spinal loads, using musculoskeletal multi-body modelling. Methods A generic AnyBody model of the thoracolumbar spine, including > 600 fascicles representing trunk musculature, was used. Several stages of normal ageing and sarcopenia were modelled by reduced strength of erector spinae and multifidus muscles (ageing from 3rd to 6th life decade: ≥ 60% of normal strength; sarcopenia: mild 60%, moderate 48%, severe 36%, very severe 24%), reflecting the reported decrease in cross-sectional area and increased fat infiltration. All other model parameters were kept unchanged. Full-range flexion was simulated using inverse dynamics with muscle optimization to predict spinal loads and muscle recruitment patterns. Results The muscle changes due to normal ageing (≥ 60% strength) had a minor effect on predicted loads and provoked only slightly elevated muscle activities. Severe (36%) and very severe (24%) stages of sarcopenia, however, were associated with substantial increases in compression (by up to 36% or 318N) at the levels of the upper thoracic spine (T1T2–T5T6) and shear loading (by up to 75% or 176N) along the whole spine (T1T2–L4L5). The muscle activities increased for almost all muscles, up to 100% of their available strength. Conclusions The study highlights the distinct and detrimental consequences of sarcopenia, in contrast to normal ageing, on spinal loading and required muscular effort.ISSN:0940-6719ISSN:1432-093

Ceramics in total disc replacements: A scoping review

Background: Ceramics are used in Total Disc Replacements (1) in articulating surfaces for their wear resistance and biocompatibility and (2) on endplates to promote osseointegration. They furthermore exhibit MRI and CT compatibility. These properties address main challenges associated with non-ceramic Total Disc Replacements i.e. wear, migration and postoperative imaging. While brittleness of ceramics caused fear of fracture in the past, improvements of ceramic materials were made and considerable clinical experience with ceramic Total Disc Replacements was gained. This review aims to assess the evidence on the use of ceramics in Total Disc Replacements and compare safety and effectiveness of ceramic Total Disc Replacements to spinal fusion and Total Disc Replacements in general. Methods: We conducted a scoping review on the use of ceramics in Total Disc Replacements using Scopus, Web of Science and PubMed. The review includes 36 clinical, ex vivo and nonhuman in vivo, tribological and mechanical studies and case reports. Findings: Ceramics are used in cervical Total Disc Replacements, with safety and efficacy confirmed in clinical studies, with up to 10 and 3.3 years follow-up, for articulation and osseointegration applications, respectively. Clinical evidence shows that ceramic Total Disc Replacements (alike non-ceramic ones) restore segmental motion and result in non-inferior and possibly superior outcomes to spinal fusion. In vivo studies show osseointegration comparable to non-ceramic devices. Tribological studies suggest appropriate wear properties. Interpretation: We found no indications of systematic problems with the use of ceramics in Total Disc Replacements. Ceramics are suitable materials for Total Disc Replacements.ISSN:0268-0033ISSN:1879-127

Accounting for Biomechanical Measures from Musculoskeletal Simulation of Upright Posture Does Not Enhance the Prediction of Curve Progression in Adolescent Idiopathic Scoliosis

A major clinical challenge in adolescent idiopathic scoliosis (AIS) is the difficulty of predicting curve progression at initial presentation. The early detection of progressive curves can offer the opportunity to better target effective non-operative treatments, reducing the need for surgery and the risks of related complications. Predictive models for the detection of scoliosis progression in subjects before growth spurt have been developed. These models accounted for geometrical parameters of the global spine and local descriptors of the scoliotic curve, but neglected contributions from biomechanical measurements such as trunk muscle activation and intervertebral loading, which could provide advantageous information. The present study exploits a musculoskeletal model of the thoracolumbar spine, developed in AnyBody software and adapted and validated for the subject-specific characterization of mild scoliosis. A dataset of 100 AIS subjects with mild scoliosis and in pre-pubertal age at first examination, and recognized as stable (60) or progressive (40) after at least 6-months follow-up period was exploited. Anthropometrical data and geometrical parameters of the spine at first examination, as well as biomechanical parameters from musculoskeletal simulation replicating relaxed upright posture were accounted for as predictors of the scoliosis progression. Predicted height and weight were used for model scaling because not available in the original dataset. Robust procedure for obtaining such parameters from radiographic images was developed by exploiting a comparable dataset with real values. Six predictive modelling approaches based on different algorithms for the binary classification of stable and progressive cases were compared. The best fitting approaches were exploited to evaluate the effect of accounting for the biomechanical parameters on the prediction of scoliosis progression. The performance of two sets of predictors was compared: accounting for anthropometrical and geometrical parameters only; considering in addition the biomechanical ones. Median accuracy of the best fitting algorithms ranged from 0.76 to 0.78. No differences were found in the classification performance by including or neglecting the biomechanical parameters. Median sensitivity was 0.75, and that of specificity ranged from 0.75 to 0.83. In conclusion, accounting for biomechanical measures did not enhance the prediction of curve progression, thus not supporting a potential clinical application at this stage.ISSN:2296-418

The influence of spinal fusion length on proximal junction biomechanics: a parametric computational study

Purpose Proximal junctional kyphosis and failure are frequent complications in adult spinal deformity surgery with long fusion constructs. The aim of this study was to assess the biomechanics of the proximal segment for fusions of various lengths. Methods A previously established musculoskeletal model of thoracolumbar spine was used to simulate full-range flexion task for fusions (modeled by introduction of rigid constraints) with lower instrumented vertebrae at L5 or S1 and upper instrumented vertebrae (UIV) at any level above, up to T2. Inverse dynamics simulations with force-dependent kinematics were performed for gradually increasing spinal flexion in order to predict global and segmental range of flexion, maximum passive moment, segmental compression and shear forces, which were compared to the uninstrumented case. Results For long fusions, with the UIV at T11 or higher, the model predicted an increase in segmental flexion (by 33–860%, or 1.6°–4.7°) and passive moment (by 39–1370%, or 13–31 Nm) at the proximal junction—generally increasing with fusion length. While the maximum shear force was 57–239% (135–283 N) higher for the proximal junction at the upper thorax (UIV at T6 or above), the compression forces were reduced by up to 44% (375 N). Conclusions The length of the instrumentation has an important effect on the proximal segment biomechanics. Despite the limitations of the current model, musculoskeletal modeling appears to be a promising and versatile method to support planning of spinal instrumentation surgeries in the future.ISSN:0940-6719ISSN:1432-093

Assessment of trunk muscle activation and intervertebral load in adolescent idiopathic scoliosis by musculoskeletal modelling approach

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity of the spine, the aetiology and pathogenesis of which are poorly understood. Unfortunately, biomechanical data describing trunk muscle activation and intervertebral load, which can contribute to understanding the pathomechanics of the AIS spine, cannot be measured in vivo due to the invasiveness of the procedures. The present study provides the biomechanical characterization of the spinal loads in scoliotic subjects by exploiting musculoskeletal modelling approach, allowing for calculating biomechanical measures in an assigned posture. A spine model with articulated ribcage previously developed in AnyBody software was applied. The predicted outcomes were evaluated in the upright posture, depending on scoliosis severity and curve type, in a population of 132 scoliotic subjects with mild, moderate, and severe scoliosis. Radiographic-based three dimensional reconstruction of vertebral orientations and scaling of body segments and trunk muscle cross-section area guaranteed geometrical subject-specificity. Validation analysis supporting the application of the model was performed. Trunk muscles were found more activated in the convex side of the scoliotic curve, in agreement with reference in vivo measurements, with progressive increase with scoliosis severity. The intervertebral lateral shear was found positively correlated with the severity of the scoliosis, demonstrating that the transferred load is not a priori orthogonal to vertebral endplate in the frontal plane, and thus questioning the assumption of the 'follower load' approach in case of experimental or computational study on the scoliotic spine. The study opens the way for the subject-specific characterization of scoliosis in assigned loading and motion conditions