Developing CT based computational models of pediatric femurs

Effect of different constraining boundary conditions on simulated femoral stresses and strains during gait

Finite element analysis (FEA) is commonly used in orthopaedic research to estimate localised tissue stresses and strains. A variety of boundary conditions have been proposed for isolated femur analysis, but it remains unclear how these assumed constraints influence FEA predictions of bone biomechanics. This study compared the femoral head deflection (FHD), stresses, and strains elicited under four commonly used boundary conditions (fixed knee, mid-shaft constraint, springs, and isostatic methods) and benchmarked these mechanics against the gold standard inertia relief method for normal and pathological femurs (extreme anteversion and retroversion, coxa vara, and coxa valga). Simulations were performed for the stance phase of walking with the applied femoral loading determined from patient-specific neuromusculoskeletal models. Due to unrealistic biomechanics observed for the commonly used boundary conditions, we propose a novel biomechanical constraint method to generate physiological femur biomechanics. The biomechanical method yielded FHD (< 1 mm), strains (approaching 1000 µε), and stresses (< 60 MPa), which were consistent with physiological observations and similar to predictions from the inertia relief method (average coefficient of determination = 0.97, average normalized root mean square error = 0.17). Our results highlight the superior performance of the biomechanical method compared to current methods of constraint for  both healthy and pathological femurs.

Immature porcine cortical bone mechanical properties and composition change with maturation and displacement rate

Computational models of mature bone have been used to predict fracture; however, analogous study of immature diaphyseal fracture has not been conducted due to sparse experimental mechanical data. A model of immature bone fracture may be used to aid in the differentiation of accidental and non-accidental trauma fractures in young, newly ambulatory children (0-3 years). The objective of this study was to characterize the evolution of tissue-level mechanical behavior, composition, and microstructure of maturing cortical porcine bone with uniaxial tension, Raman spectroscopy, and light microscopy as a function of maturation. We asked: 1) How do the monotonic uniaxial tensile properties change with maturation and displacement rate; 2) How does the composition and microstructure change with maturation; and 3) Is there a correlation between composition and tensile properties with maturation? Elastic modulus (p < 0.001), fracture stress (p < 0.001), and energy absorption (p < 0.014) increased as a function of maturation at the quasistatic rate by 110%, 86%, and 96%, respectively. Fracture stress also increased by 90% with maturation at the faster rate (p = 0.001). Fracture stress increased as a function of increasing displacement rate by 28% (newborn p = 0.048; 1-month p = 0.004; 3-month p= < 0.001), and fracture strain decreased by 68% with increasing displacement rate (newborn p = 0.002; 1-month p = 0.036; 3-month p < 0.001). Carbonate-to-phosphate ratio was positively linearly related to elastic modulus, and fracture stress was positively related to carbonate-to-phosphate ratio and matrix maturation ratio. The results of this study support that immature bone is strain-rate dependent and becomes more brittle at faster rates, contributing to the foundation upon which a computational model can be built to evaluate immature bone fracture.

Bone Fractures Numerical Analysis in a Femur Affected by Osteogenesis Imperfecta

This work presents a non-invasive methodology to obtain a three-dimensional femur model of three-year-old infants affected with Osteogenesis Imperfecta (OI) type III. DICOM^® Files of a femur were processed to obtain a finite element model to assess the transverse, the oblique, and the comminuted fractures. The model is evaluated under a normal walking cycle. The loads applied were considered the most critical force generated on the normal walking cycle, and the analyses considered anisotropic bone conditions. The outcome shows stress concentration areas in the central zone of the diaphysis of the femur, and the highest levels of stress occur in the case of the comminuted fracture, while the transverse fracture presents the lowest values. Thus, the method can be helpful for determining the bone fracture behavior of certain pathologies, such as osteogenesis imperfecta, osteopenia, and osteoporosis.

Femur morphology in healthy infants and young children

The objective of this study was to characterize femur morphology in healthy infants and young children. Anterior-posterior (AP) radiographs of the femur from children age 0-3 years with no history of bone disease were obtained from two children's hospitals and one medical examiner's office. Femur morphological measures (bone length, minimum diaphysis diameter, growth plate width, and femur radius of curvature) and sectional structural measures were determined. Measures were described and compared based on subject age and mass. Relationships between measures and age and mass were evaluated. The 169 AP femur radiographs were obtained from 99 children (59.6% males, median age = 12.0 months, IQR = 0-27.5 months, median body weight = 10.0 kg, IQR = 4.4-15.6 kg). Femur length (r_s  = 0.97, p < 0.001; r_s  = 0.89, p < 0.001), trochanter width (r_s  = 0.86, p < 0.001; r_s  = 0.85, p < 0.001), minimum diaphysis diameter (r_s  = 0.91, p < 0.001; r_s  = 0.87, p < 0.001), and growth plate width (r_s  = 0.91, p < 0.001; r_s  = 0.84, p < 0.001) increased with age and weight, respectively. Cross-sectional area (r_s  = 0.87; r_s  = 0.86; p < 0.01), polar moment of inertia (r_s  = 0.91; r_s  = 0.87; p < 0.001), moment of inertia (r_s  = 0.91; r_s  = 0.87; p < 0.001), polar modulus (r_s  = 0.91; r_s  = 0.87; p < 0.001) and medullary canal diameter (r_s  = 0.83, p < 0.001; r_s  = 0.73, p < 0.001) at the minimum diaphysis also increased with age and weight, respectively. Changes during rapid bone growth are important to understanding fracture risk in infants and young children as they transition to independent walking. Femur length, trochanter width, minimum diaphysis diameter and growth plate width increased with age and weight. Structural properties associated with fracture resistance also increased with age and weight.

Biomechanics of immature human cortical bone: A systematic review

The whole bone geometry, microstructure, and mechanical properties of mature human bone are widely reported; however, immature bone (0-18 years) has not been similarly robustly characterized. There is an interest in analyzing and predicting the mechanical loading conditions associated with long bone diaphyseal fractures attributed to trauma in children. Thus, understanding the mechanical properties of immature bone in a temporal reference frame is an essential first step to understand diaphyseal fractures of pediatric long bones. The purpose of this systematic review was to ask, what is the state of knowledge regarding the 1) evolution of whole bone geometry and microstructure of immature pediatric bone as a function of maturation and 2) cortical bone density and experimental quasi-static mechanical properties at the tissue level in the diaphyseal region of immature pediatric long bones? The systematic search yielded 36 studies of the whole bone geometry, microstructure, and mechanical properties of immature pediatric long bones. The elastic modulus, yield stress, and ultimate stress were shown to generally increase with maturation, whereas the yield strain was approximately invariant; however, the specific year-to-year progression of these properties could not be characterized from the limited studies available. The results of this systematic search indicate there is a dearth of knowledge associated with the biomechanics of cortical bone from immature pediatric long bones; it also provides a basis for computational studies of immature human long bones. Additional biomechanical studies of immature human bone are necessary to develop a robust catalogue, which can be used in broad applications to understand fracture mechanics, bone pathologies, and athletic injury in the pediatric setting.

Investigation of femur fracture potential in common pediatric falls using finite element analysis

A finite element (FE) model of an 11-month-old child's femur was developed to evaluate fracture risk in short-distance feet-first falls and bed falls. Pediatric material properties were applied to the FE model. Femur loading was derived from previously conducted fall experiments using a child surrogate where fall conditions (e.g., fall height, impact surface) were varied. Fracture thresholds based on principal stress and strain were used to examine potential for fracture. Peak stress/strain were significantly greater for feet-first falls from greater heights and onto harder impact surfaces. Feet-first falls exceeded some, but not all fracture thresholds. Bed falls did not exceed any fracture thresholds.

Predicting bone strength from CT data: Clinical applications

In this review we summarise over 15 years of research and development around the prediction of whole bones strength from Computed Tomography data, with particular reference to the prediction of the risk of hip fracture in osteoporotic patients. We briefly discuss the theoretical background, and then provide a summary of the laboratory and clinical validation of these modelling technologies. We then discuss the three current clinical applications: in clinical research, in clinical trials, and in clinical practice. On average the strength predicted with finite element models (QCT-FE) based on computed tomography is 7% more accurate that that predicted with areal bone mineral density from Dual X-ray Absorptiometry (DXA-aBMD), the current standard of care, both in term of laboratory validation on cadaver bones and in terms of stratification accuracy on clinical cohorts of fractured and non-fractured women. This improved accuracy makes QCT-FE superior to DXA-aBMD in clinical research and in clinical trials, where the its use can cut in half the number of patients to be enrolled to get the same statistical power. For routine clinical use to decide who to treat with antiresorptive drugs, QCT-FE is more accurate but less cost-effective than DXA-aBMD, at least when the decision is on first line treatment like bisphosphonates. But the ability to predict skeletal strength from medical imaging is now opening a number of other applications, for example in paediatrics and oncology.

Investigating rolling as mechanism for humeral fractures in non-ambulant infants: a preliminary finite element study

AIM

To use personalised computed tomography (CT)-based finite element models to quantitatively investigate the likelihood of self-inflicted humeral fracture in non-ambulant infants secondary to rolling.

MATERIALS AND METHODS

Three whole-body post-mortem CT examinations of children at the age of rolling (two 4-month-old and one 6-month-old) were used. The mechanical moment needed by each infant to perform a rolling manoeuvre was calculated and applied to the finite element model in order to simulate spontaneous rolling from the prone to the supine position.

RESULTS

The maximum predicted strains were found to be substantially lower (with a difference of >80%) than the elastic limit of the bone.

CONCLUSION

Results of this study challenge the plausibility of self-inflicted humeral fracture caused by rolling in non-ambulant infants and indicate that it is unlikely for a humeral fracture to result from this mechanism without the assistance of an external force.

Finite element modelling of the developing infant femur using paired CT and MRI scans

Bone finite element (FE) studies based on infant post-mortem computed tomography (CT) examinations are being developed to provide quantitative information to assist the differentiation between accidental and inflicted injury, and unsuspected underlying disease. As the growing skeleton contains non-ossified cartilaginous regions at the epiphyses, which are not well characterised on CT examinations, it is difficult to evaluate the mechanical behaviour of the developing whole bone. This study made use of paired paediatric post mortem femoral CT and magnetic resonance imaging (MRI) examinations at two different stages of development (4 and 7 months) to provide anatomical and constitutive information for both hard and soft tissues. The work aimed to evaluate the effect of epiphyseal ossification on the propensity to shaft fractures in infants. The outcomes suggest that the failure load of the femoral diaphysis in the models incorporating the non-ossified epiphysis is within the range of bone-only FE models. There may however be an effect on the metaphysis. Confirmation of these findings is required in a larger cohort of children.

Evaluation of the capability of the simulated dual energy X-ray absorptiometry-based two-dimensional finite element models for predicting vertebral failure loads

Prediction of the vertebral failure load is of great importance for the prevention and early treatment of bone fracture. However, an efficient and effective method for accurately predicting the failure load of vertebral bones is still lacking. The aim of the present study was to evaluate the capability of the simulated dual energy X-ray absorptiometry (DXA)-based finite element (FE) model for predicting vertebral failure loads. Thirteen dissected spinal segments (T11/T12/L1) were scanned using a HR-pQCT scanner and then were mechanically tested until failure. The subject-specific three-dimensional (3D) and two-dimensional (2D) FE models of T12 were generated from the HR-pQCT scanner and the simulated DXA images, respectively. Additionally, the areal bone mineral density (aBMD) and areal bone mineral content (aBMC) of T12 were calculated. The failure loads predicted by the simulated DXA-based 2D FE models were more moderately correlated with the experimental failure loads (R² = 0.66) than the aBMC (R² = 0.61) and aBMD (R² = 0.56). The 2D FE models were slightly outperformed by the HR-pQCT-based 3D FE models (R² = 0.71). The present study demonstrated that the simulated DXA-based 2D FE model has better capability for predicting the vertebral failure loads than the densitometric measurements but is outperformed by the 3D FE model. The 2D FE model is more suitable for clinical use due to the low radiation dose and low cost, but it remains to be validated by further in vitro and in vivo studies.

Material and Structural Modeling Aspects of Brain Tissue Deformation under Dynamic Loads

The aim of this work was to assess the numerous approaches to structural and material modeling of brain tissue under dynamic loading conditions. The current technological improvements in material modeling have led to various approaches described in the literature. However, the methods used for the determination of the brain’s characteristics have not always been stated or clearly defined and material data are even more scattered. Thus, the research described in this paper explicitly underlines directions for the development of numerical brain models. An important element of this research is the development of a numerical model of the brain based on medical imaging methods. This approach allowed the authors to assess the changes in the mechanical and geometrical parameters of brain tissue caused by the impact of mechanical loads. The developed model was verified through comparison with experimental studies on post-mortem human subjects described in the literature, as well as through numerical tests. Based on the current research, the authors identified important aspects of the modeling of brain tissue that influence the assessment of the actual biomechanical response of the brain for dynamic analyses.

Validation of Material Algorithms for Femur Remodelling Using Medical Image Data

The aim of this study is the utilization of human medical CT images to quantitatively evaluate two sorts of "error-driven" material algorithms, that is, the isotropic and orthotropic algorithms, for bone remodelling. The bone remodelling simulations were implemented by a combination of the finite element (FE) method and the material algorithms, in which the bone material properties and element axes are determined by both loading amplitudes and daily cycles with different weight factor. The simulation results showed that both algorithms produced realistic distribution in bone amount, when compared with the standard from CT data. Moreover, the simulated L-T ratios (the ratio of longitude modulus to transverse modulus) by the orthotropic algorithm were close to the reported results. This study suggests a role for "error-driven" algorithm in bone material prediction in abnormal mechanical environment and holds promise for optimizing implant design as well as developing countermeasures against bone loss due to weightlessness. Furthermore, the quantified methods used in this study can enhance bone remodelling model by optimizing model parameters to gap the discrepancy between the simulation and real data.

Biomechanics of the classic metaphyseal lesion: finite element analysis

BACKGROUND

The classic metaphyseal lesion (CML) is strongly associated with infant abuse, but the biomechanics responsible for this injury have not been rigorously studied. Radiologic and CT-pathological correlates show that the distal tibial CML always involves the cortex near the subperiosteal bone collar, with variable extension of the fracture into the medullary cavity. Therefore, it is reasonable to assume that the primary site of bone failure is cortical, rather than intramedullary.

OBJECTIVE

This study focuses on the strain patterns generated from finite element modeling to identify loading scenarios and regions of the cortex that are susceptible to bone failure.

MATERIALS AND METHODS

A geometric model was constructed from a normal 3-month-old infant's distal tibia and fibula. The model's boundary conditions were set to mimic forceful manipulation of the ankle with eight load modalities (tension, compression, internal rotation, external rotation, dorsiflexion, plantar flexion, valgus bending and varus bending).

RESULTS

For all modalities except internal and external rotation, simulations showed increased cortical strains near the subperiosteal bone collar. Tension generated the largest magnitude of cortical strain (24%) that was uniformly distributed near the subperiosteal bone collar. Compression generated the same distribution of strain but to a lesser magnitude overall (15%). Dorsiflexion and plantar flexion generated high (22%) and moderate (14%) localized cortical strains, respectively, near the subperiosteal bone collar. Lower cortical strains resulted from valgus bending, varus bending, internal rotation and external rotation (8-10%). The highest valgus and varus bending cortical strains occurred medially.

CONCLUSION

These simulations suggest that the likelihood of the initial cortical bone failure of the CML is higher along the margin of the subperiosteal bone collar when the ankle is under tension, compression, valgus bending, varus bending, dorsiflexion and plantar flexion, but not under internal and external rotation. Focal cortical strains along the medial margins of the subperiosteal bone collar with varus and valgus bending may explain the known tendency for focal distal tibial CMLs to occur medially. Further research is needed to determine the threshold of applied forces required to produce this strong indicator of infant abuse.

In Vivo Assessment of Elasticity of Child Rib Cortical Bone Using Quantitative Computed Tomography

Elasticity of the child rib cortical bone is poorly known due to the difficulties in obtaining specimens to perform conventional tests. It was shown on the femoral cortical bone that elasticity is strongly correlated with density for both children and adults through a unique relationship. Thus, it is assumed that the relationships between the elasticity and density of adult rib cortical bones could be expanded to include that of children. This study estimated in vivo the elasticity of the child rib cortical bone using quantitative computed tomography (QCT). Twenty-eight children (from 1 to 18 y.o.) were considered. Calibrated QCT images were prescribed for various thoracic pathologies. The Hounsfield units were converted to bone mineral density (BMD). A relationship between the BMD and the elasticity of the rib cortical bone was applied to estimate the elasticity of children's ribs in vivo. The estimated elasticity increases with growth (7.1 ± 2.5 GPa at 1 y.o. up to 11.6 ± 1.9 GPa at 18 y.o.). This data is in agreement with the few previous values obtained using direct measurements. This methodology paves the way for in vivo assessment of the elasticity of the child cortical bone based on calibrated QCT images.

Fixation Release and the Bone Bandaid: A New Bone Fixation Device Paradigm

The current gold standard of care for mandibular segmental defeat reconstruction is the use of Ti-6Al-4V immobilization hardware and fibular double barrel graft. This method is often successful immediately at restoring mandible function, however the highly stiff fixation hardware causes stress shielding of the grafted bone and stress concentration in the fixation device over time which can lead to fixation device failure and revision surgery. The purpose of reconstructive surgery could be to create normal stress trajectories in the mandible following engraftment. We investigate the use of a two stage mechanism which separates the immobilization/healing and regenerative phases of mandibular segmental defect treatment. The device includes the use of a very stiff, Ti-6Al-4V, releasable mechanism which assures bone healing. Therefore it could be released once the reconstructed boney tissue and any of its ligamentous attachments have completely healed. Underneath the released Ti-6Al-4V plate would be a pre-loaded nitinol (NiTi) wire-frame apparatus that facilitates the normal stress-strain trajectory through the engrafted bone after the graft is healed in place and the Ti-6Al-4V fixation device has been released. Due to the use of NiTi wires forming a netting that connects vascularized bone and possibly bone chips, bone grafts are also more likely to be incorporate rather than to resorb. We first evaluated a healthy adult mandible during normal mastication to obtain the normal stress-strain distribution. Then, we developed the finite element (FE) model of the mandibular reconstruction (in the M1-3 region) with the proposed fixation device during the healing (locked state) and post-healing (released state) periods. To recreate normal stress trajectory in the reconstructed mandible, we applied the Response Surface Methodology (RMS) to optimize the Bone Bandaid geometry (i.e., wire diameters and location). The results demonstrate that the proposed mechanism immobilizes the grafted bone in the locked state properly since the maximum resultant gap (21.54 micron) between the graft and host mandible surfaces are in the safe region (less than 300 micron). By considering the von Mises criteria for failure, FE analysis together with experimental studies (i.e., compressive and tensile testing on the inferior and superior fixation devices, respectively) confirm that the proposed fixation devices do not fail, showing safety factor of at least 10.3. Based on the Response Surface Methodology (RSM) technique, the optimal parameter values for the wires are achieved (0.65 mm and 1 mm for the superior and inferior wires, respectively) and the required level of preload on each wire are calculated (369.8 N and 229 N for the inferior and superior wires, respectively). The FE results for stress distribution on the reconstructed mandible during the released state closely match that of a healthy mandible.

Does Orthodontic Treatment Affect the Alveolar Bone Density?

Few studies involving human participants have been conducted to investigate the effect of orthodontic treatment on alveolar bone density around the teeth. Our previous study revealed that patients who received 6 months of active orthodontic treatment exhibited an ∼24% decrease in alveolar bone density around the teeth. However, after an extensive retention period following orthodontic treatment, whether the bone density around the teeth can recover to its original state from before the treatment remains unclear, thus warranting further investigation.The purpose of this study was to assess the bone density changes around the teeth before, during, and after orthodontic treatment.Dental cone-beam computed tomography (CBCT) was used to measure the changes in bone density around 6 teeth in the anterior maxilla (maxilla central incisors, lateral incisors, and canines) of 8 patients before and after orthodontic treatment. Each patient underwent 3 dental CBCT scans: before treatment (T0); at the end of 7 months of active orthodontic treatment (T1); after several months (20-22 months) of retention (T2). The Friedman test was applied to evaluate the changes in the alveolar bone density around the teeth according to the 3 dental CBCT scans.From T0 to T1, a significant reduction in bone density was observed around the teeth (23.36 ± 10.33%); by contrast, a significant increase was observed from T1 to T2 (31.81 ± 23.80%). From the perspective of the overall orthodontic treatment, comparing the T0 and T2 scans revealed that the bone density around the teeth was relatively constant (a reduction of only 0.75 ± 19.85%). The results of the statistical test also confirmed that the difference in bone density between T0 and T2 was nonsignificant.During orthodontic tooth movement, the alveolar bone density around the teeth was reduced. However, after a period of bone recovery, the reduced bone density recovered to its previous state from before the orthodontic treatment. However, the bone density around ∼10% of the teeth in this region could not recover to 80% of its state from before the orthodontic treatment.

A method of determining bending properties of poultry long bones using beam analysis and micro-CT data

While conventional mechanical testing has been regarded as a gold standard for the evaluation of bone heath in numerous studies, with recent advances in medical imaging, virtual methods of biomechanics are rapidly evolving in the human literature. The objective of the current study was to evaluate the feasibility of determining the elastic and failure properties of poultry long bones using established methods of analysis from the human literature. In order to incorporate a large range of bone sizes and densities, a small number of specimens were utilized from an ongoing study of Regmi et al. (2016) that involved humeri and tibiae from 3 groups of animals (10 from each) including aviary, enriched, and conventional housing systems. Half the animals from each group were used for 'training' that involved the development of a regression equation relating bone density and geometry to bending properties from conventional mechanical tests. The remaining specimens from each group were used for 'testing' in which the mechanical properties from conventional tests were compared to those predicted by the regression equations. Based on the regression equations, the coefficients of determination for the 'test' set of data were 0.798 for bending bone stiffness and 0.901 for the yield (or failure) moment of the bones. All regression slopes and intercepts values for the tests versus predicted plots were not significantly different from 1 and 0, respectively. The study showed the feasibility of developing future methods of virtual biomechanics for the evaluation of poultry long bones. With further development, virtual biomechanics may have utility in future in vivo studies to assess laying hen bone health over time without the need to sacrifice large groups of animals at each time point.