Evaluation of ground reaction forces and centers of pressure predicted by AnyBody Modeling System during load reaching/handling activities and effects of the prediction errors on model-estimated spinal loads.

Full-body and lower-extremity human musculoskeletal models require feet ground reaction forces (GRFs) and centers of pressure (CoPs) as inputs to predict muscle forces and joint loads. GRFs/CoPs are traditionally measured via floor-mounted forceplates that are usually restricted to research laboratories thus limiting their applicability in real occupational and clinical setups. Alternatively, GRFs/CoPs can be estimated via inverse dynamic approaches as also implemented in the Anybody Modeling System (AnyBody Technology, Aalborg, Denmark). The accuracy of Anybody in estimating GRFs/CoPs during load-handling/reaching activities and the effect of its prediction errors on model-estimated spinal loads remain to be investigated. Twelve normal- and over-weight individuals performed total of 480 static load-handling/reaching activities while measuring (by forceplates) and predicting (by AnyBody) their GRFs/CoPs. Moreover, the effects of GRF/CoP prediction errors on the estimated spinal loads were evaluated by inputting measured or predicted GRFs/CoPs into subject-specific musculoskeletal models. Regardless of the subject groups (normal-weight or overweight) and tasks (load-reaching or load-handling), results indicated great agreements between the measured and predicted GRFs (normalized root-mean-squared error, nRMSEs < 14% and R2 > 0.90) and between their model-estimated spinal loads (nRMSEs < 14% and R2 > 0.83). These agreements were good but relatively less satisfactory for CoPs (nRMSEs < 17% and 0.57 < R2 < 0.68). The only exception, requiring a more throughout investigation, was the situation when the ground-foot contact was significantly reduced during the activity. It appears that occupational/clinical investigations performed in real workstation/clinical setups with no access to forceplates may benefit from the AnyBody GRF/CoP prediction tools for a wide range of load-reaching/handling activities.

Effect of obesity on spinal loads during load-reaching activities: A subject- and kinematics-specific musculoskeletal modeling approach.

Obesity has been associated to increase the risk of low back disorders. Previous musculoskeletal models simulating the effect of body weight on intervertebral joint loads have assumed identical body postures for obese and normal-weight individuals during a given physical activity. Our recent kinematic-measurement studies, however, indicate that obese individuals adapt different body postures (segmental orientations) than normal-weight ones when performing load-reaching activities. The present study, therefore, used a subject- and kinematics-specific musculoskeletal modeling approach to compare spinal loads of nine normal-weight and nine obese individuals each performing twelve static two-handed load-reaching activities at different hand heights, anterior distances, and asymmetry angles (total of 12 tasks × 18 subjects = 216 model simulations). Each model incorporated personalized muscle architectures, body mass distributions, and full-body kinematics for each subject and task. Results indicated that even when accounting for subject-specific body kinematics obese individuals experienced significantly larger (by âˆ¼38% in average) L5-S1 compression (2305 ± 468 N versus 1674 ± 337 N) and shear (508 ± 111 N versus 705 ± 150 N) loads during all reaching activities (p < 0.05 for all hand positions). This average difference of âˆ¼38% was similar to the results obtained from previous modeling investigations that neglected kinematics differences between the two weight groups. Moreover, there was no significant interaction effect between body weight and hand position on the spinal loads; indicating that the effect of body weight on L5-S1 loads was not dependent on the position of hands. Postural differences alone appear, hence, ineffective in compensating the greater spinal loads that obese people experience during reaching activities.

Effects of eight different ligament property datasets on biomechanics of a lumbar L4-L5 finite element model.

Ligaments assist trunk muscles in balancing external moments and providing spinal stability. In absence of the personalized material properties for ligaments, finite element (FE) models use dispersed data from the literature. This study aims to investigate the relative effects of eight different ligament property datasets on FE model responses. Eight L4-L5 models distinct only in ligament properties were constructed and loaded under moment (15Nm) alone or combined with a compressive follower load (FL). Range of motions (RoM) of the disc-alone model matched well in vitro data. Ligament properties significantly affected only sagittal RoMs (∼3.0-7.1° in flexion and ∼3.8-5.8° in extension at 10Nm). Sequential removal of ligaments shifted sagittal RoMs in and out of the corresponding in vitro ranges. When moment was combined with FL, center of rotation matched in vivo data for all models (3.8±0.9mm and 4.3±1.8mm posterior to the disc center in flexion and extension, respectively). Under 15Nm sagittal moments, ligament strains were often smaller or within the in vitro range in flexion whereas some posterior ligament forces approached their failure forces in some models. Ligament forces varied substantially within the models and affected the moment-sharing and internal forces on the disc and facet joints. Intradiscal pressure (IDP) had the greatest variation between models in extension. None of the datasets yielded results in agreement with all reported measurements. Results emphasized the important role of ligaments especially under larger moments and the need for their accurate representation in search for valid spinal models.

Finite element analysis of mechanical behavior of human dysplastic hip joints: a systematic review.

Developmental dysplasia of the hip (DDH) is a common condition predisposing to osteoarthritis (OA). Especially since DDH is best identified and treated in infancy before bones ossify, there is surprisingly a near-complete absence of literature examining mechanical behavior of infant dysplastic hips. We sought to identify current practice in finite element modeling (FEM) of DDH, to inform future modeling of infant dysplastic hips. We performed multi-database systematic review using PRISMA criteria. Abstracts (n = 126) fulfilling inclusion criteria were screened for methodological quality, and results were analyzed and summarized for eligible articles (n = 12). The majority of the studies modeled human adult dysplastic hips. Two studies focused on etiology of DDH through simulating mechanobiological growth of prenatal hips; we found no FEM-based studies in infants or children. Finite element models used either patient-specific geometry or idealized average geometry. Diversities in choice of material properties, boundary conditions, and loading scenarios were found in the finite-element models. FEM of adult dysplastic hips demonstrated generally smaller cartilage contact area in dysplastic hips than in normal joints. Contact pressure (CP) may be higher or lower in dysplastic hips depending on joint geometry and mechanical contribution of labrum (Lb). FEM of mechanobiological growth of prenatal hip joints revealed evidence for effects of the joint mechanical environment on formation of coxa valga, asymmetrically shallow acetabulum and malformed femoral head associated with DDH. Future modeling informed by the results of this review may yield valuable insights into optimal treatment of DDH, and into how and why OA develops early in DDH.

Micro-scale finite element modeling of ultrasound propagation in aluminum trabecular bone-mimicking phantoms: A comparison between numerical simulation and experimental results.

The present study investigated the accuracy of micro-scale finite element modeling for simulating broadband ultrasound propagation in water-saturated trabecular bone-mimicking phantoms. To this end, five commercially manufactured aluminum foam samples as trabecular bone-mimicking phantoms were utilized for ultrasonic immersion through-transmission experiments. Based on micro-computed tomography images of the same physical samples, three-dimensional high-resolution computational samples were generated to be implemented in the micro-scale finite element models. The finite element models employed the standard Galerkin finite element method (FEM) in time domain to simulate the ultrasonic experiments. The numerical simulations did not include energy dissipative mechanisms of ultrasonic attenuation; however, they expectedly simulated reflection, refraction, scattering, and wave mode conversion. The accuracy of the finite element simulations were evaluated by comparing the simulated ultrasonic attenuation and velocity with the experimental data. The maximum and the average relative errors between the experimental and simulated attenuation coefficients in the frequency range of 0.6-1.4 MHz were 17% and 6% respectively. Moreover, the simulations closely predicted the time-of-flight based velocities and the phase velocities of ultrasound with maximum relative errors of 20 m/s and 11 m/s respectively. The results of this study strongly suggest that micro-scale finite element modeling can effectively simulate broadband ultrasound propagation in water-saturated trabecular bone-mimicking structures.

Symmetry analysis of talus bone: A Geometric morphometric approach.

OBJECTIVE: The main object of this study was to use a geometric morphometric approach to quantify the left-right symmetry of talus bones. METHODS: Analysis was carried out using CT scan images of 11 pairs of intact tali. Two important geometric parameters, volume and surface area, were quantified for left and right talus bones. The geometric shape variations between the right and left talus bones were also measured using deviation analysis. Furthermore, location of asymmetry in the geometric shapes were identified. RESULTS: Numerical results showed that talus bones are bilaterally symmetrical in nature, and the difference between the surface area of the left and right talus bones was less than 7.5%. Similarly, the difference in the volume of both bones was less than 7.5%. Results of the three-dimensional (3D) deviation analyses demonstrated the mean deviation between left and right talus bones were in the range of -0.74 mm to 0.62 mm. It was observed that in eight of 11 subjects, the deviation in symmetry occurred in regions that are clinically less important during talus surgery. CONCLUSIONS: We conclude that left and right talus bones of intact human ankle joints show a strong degree of symmetry. The results of this study may have significance with respect to talus surgery, and in investigating traumatic talus injury where the geometric shape of the contralateral talus can be used as control. Cite this article: Bone Joint Res 2014;3:139-45.

The finite element method for micro-scale modeling of ultrasound propagation in cancellous bone.

Quantitative ultrasound for bone assessment is based on the correlations between ultrasonic parameters and the properties (mechanical and physical) of cancellous bone. To elucidate the correlations, understanding the physics of ultrasound in cancellous bone is demanded. Micro-scale modeling of ultrasound propagation in cancellous bone using the finite-difference time-domain (FDTD) method has been so far utilized as one of the approaches in this regard. However, the FDTD method accompanies two disadvantages: staircase sampling of cancellous bone by finite difference grids leads to generation of wave artifacts at the solid-fluid interface inside the bone; additionally, this method cannot explicitly satisfy the needed perfect-slip conditions at the interface. To overcome these disadvantages, the finite element method (FEM) is proposed in this study. Three-dimensional finite element models of six water-saturated cancellous bone samples with different bone volume were created. The values of speed of sound (SOS) and broadband ultrasound attenuation (BUA) were calculated through the finite element simulations of ultrasound propagation in each sample. Comparing the results with other experimental and simulation studies demonstrated the capabilities of the FEM for micro-scale modeling of ultrasound in water-saturated cancellous bone.

Mechanical loading effects on isthmic spondylolytic lumbar segment: finite element modelling using a personalised geometry.

Biomechanics of the isthmic spondylolysis was investigated by using a nonlinear 3D-finite element model (FEM). A personalised in vivo pediatric geometry of L5-S1 low-grade spondylolisthesis patient was used to develop a L5-pelvis motion segment model that took into consideration vertebrae, disc and ligaments. The stress distribution in the affected motion segment under axial force only, and for a combination of flexion and extension was evaluated. Predicted results showed that, under all loading conditions, stresses were much higher on the pedicle and in the dorsal wall of the pars interarticularis due to the abnormal geometry which is consistent with clinical observations.

Effect of load position on muscle forces, internal loads and stability of the human spine in upright postures.

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.

Spinal muscle forces, internal loads and stability in standing under various postures and loads--application of kinematics-based algorithm.

This work aimed to evaluate trunk muscle forces, internal loads and stability margin under some simulated standing postures, with and without external loads, using a nonlinear finite element model of the T1-S1 spine with realistic nonlinear load-displacement properties. A novel kinematics-based algorithm was applied that exploited a set of spinal sagittal rotations, initially calculated to minimize balancing moments, to solve the redundant active-passive system. The loads consisted of upper body gravity distributed along the spine with or without 200 N held in the hands, either in the front of the body or on the sides. Nonlinear and linear stability/perturbation analyses at deformed, stressed configurations with a linear stiffness-force relationship for muscles identified the system stability and critical muscle stiffness coefficient. Predictions were in good agreement with reported measurements of posture, muscle EMG and intradiscal pressure. Minimal changes in posture (posterior pelvic tilt and lumbar flattening) substantially influenced muscle forces, internal loads and stability margin. Addition of 200 N load in front of the body markedly increased the system stability, global muscle forces, and internal loads, which reached anterior shear and compression forces of approximately 500 N and approximately 1,200 N, respectively, at lower lumbar levels. Co-activation in abdominal muscles (up to 3% maximum force) substantially increased extensor muscle forces, internal loads and stability margin, allowing a smaller critical muscle coefficient. A tradeoff existed between lower internal loads in passive tissues and higher stability margins, as both increased with greater muscle activation. The strength of the proposed model is in accounting for the synergy by simultaneous consideration of passive structure and muscle forces under applied postures and loads.

Muscle force evaluation and the role of posture in human lumbar spine under compression.

Using two nonlinear finite element models of the lumbar spine, the concept of optimal posture is explored by minimizing the segmental sagittal moments required for the equilibrium of the passive lumbar spine under a total of 2800 N axial compression while varying the pelvic tilt and lumbar lordosis. The redundant active-passive system is subsequently solved for this posture using a novel kinematics-based muscle calculation algorithm along with minimization approach. Some flattening in the lumbar spine substantially reduces the required moments and internal passive shear forces under 2800 N axial compression force. Small muscle forces are calculated for this optimal posture. The role of flattening in the lumbar lordosis and posterior pelvic tilt in diminishing the lumbar muscle activities in neutral postures is demonstrated. Without such changes in posture, the required moments probably exceed the moment-generating capability of local lumbar muscles. Consideration of such active-passive synergy and lack of its restriction may prove crucial in many activities. Moreover, a kinematics-based algorithm is proposed for the solution of spinal redundancy that fully accounts for the existing passive-active synergy while simultaneously satisfying all kinematics and equilibrium conditions along the length of the spine.