Subject-specific Finite Element Analysis Research Articles

ESMAC Best Paper Award 2023: Increased knee flexion in participants with cerebral palsy results in altered stresses at the distal femoral growth plate compared to a typically developing cohort

IntroductionFemoral deformities are highly prevalent in children with cerebral palsy (CP) and can have a severe impact on patients’ gait abilities. While the mechanical stress regime within the distal femoral growth plate remains underexplored, understanding it is crucial given bone's adaptive response to mechanical stimuli. We quantified stresses at the distal femoral growth plate to deepen our understanding of the relationship between healthy and pathological gait patterns, internal loading, and femoral growth patterns. MethodsThis study included three-dimensional motion capture data and magnetic resonance images of 13 typically developing children and twelve participants with cerebral palsy. Employing a multi-scale mechanobiological approach, integrating musculoskeletal simulations and subject-specific finite element analysis, we investigated the orientation of the distal femoral growth plate and the stresses within it. Limbs of participants with CP were grouped depending on their knee flexion kinematics during stance phase as this potentially changes the stresses induced by knee and patellofemoral joint contact forces. ResultsDespite similar growth plate orientation across groups, significant differences were observed in the shape and distribution of growth values. Higher growth rates were noted in the anterior compartment in CP limbs with high knee flexion while CP limbs with normal knee flexion showed high similarity to the group of healthy participants. DiscussionResults indicate that the knee flexion angle during the stance phase is of high relevance for typical bone growth at the distal femur. The evaluated growth rates reveal plausible results, as long-term promoted growth in the anterior compartment leads to anterior bending of the femur which was confirmed for the group with high knee flexion through analyses of the femoral geometry. The framework for these multi-scale simulations has been made accessible on GitHub, empowering peers to conduct similar mechanobiological studies. Advancing our understanding of femoral bone development could ultimately support clinical decision-making.

What makes vertical femoral neck fracture with posterior inferior comminution different? An analysis of biomechanical features and optimal internal fixation strategy

Background and purposeFracture comminution occurs in 83.9%-94% of vertical femoral neck fractures (VFNFs), the majority of which were located in posterior-inferior region, and poses a clinical challenge in fixation stability. We conducted a subject-specific finite element analysis to determine the biomechanical features and optimal fixation selection for treating VFNF with posterior-inferior comminution. Patients and methodsEighteen models with three fracture types (VFNF without comminution [NCOM], with comminution [COM], with comminution + osteoporosis [COMOP]) and six internal fixation types (alpha [G-ALP], buttress [G-BUT], rhomboid [G-RHO], dynamic hip screw [G-DHS], invert triangle [G-ITR], femoral neck system (G-FNS)) were created based on the computed tomography data. By using the subject-specific finite element analysis method, stiffness, implant stress, yielding rate (YR) were compared. Additionally, in order to elucidate distinct biomechanical characters of different fracture types and fixation strategies, we calculated interfragmentary movement (IFM), detached interfragmentary movement (DIM), shear interfragmentary movement (SIM) of all fracture surface nodes. ResultsGenerally, in comparison with NCOM, COM showed a 30.6% reduction of stiffness and 1.46-times higher mean interfragmentary movement. Besides, COM had a 4.66-times (p = 0.002) higher DIM at the superior-middle position, but similar SIM across fracture line, which presented as varus deformation. In COM and COMOP, among all six fixation strategies, G-ALP had significantly the lowest IFM (p<0.001) and SIM (p<0.001). Although G-FNS had significantly highest IFM and SIM (p<0.001), it had the highest stiffness and lowest DIM (p<0.001). In COMOP, YR was the lowest in G-FNS (2.67%). ConclusionsPosterior-inferior comminution primarily increases superior-middle detached interfragmentary movement in VFNF, which results in varus deformation. For comminuted VFNF with or without osteoporosis, alpha fixation has the best interfragmentary stability and anti-shear property among six current mainstream fixation strategies, but a relatively weaker stiffness and anti-varus property compared to fixed-angle devices. FNS is advantageous owing to stiffness, anti-varus property and bone yielding rate in osteoporosis cases, but is insufficient in anti-shear property.

416 A Comparison of Bone Stresses in Transtibial and Transfemoral Osseointegrated Prostheses

OBJECTIVES/GOALS: This investigation aimed to develop and validate a subject-specific finite element analysis (FEA) model with subject-specific mechanical loads during walking and to use this method to compare mechanical stresses between transfemoral and transtibial osseointegrated (OI) implants. METHODS/STUDY POPULATION: One patient with a unilateral transtibial OI prosthesis and one with a unilateral transfemoral OI prosthesis participated in motion analysis to collect kinematics and ground reaction forces during overground walking. Subject-specific musculoskeletal models were created, and static optimization was used to estimate muscle and joint reaction forces throughout walking. 3D FEA models of the tibia, femur, and implants were created using ScanIP and exported into ABAQUS CAE. Muscle forces were applied at corresponding origin/insertion locations, determined from the musculoskeletal models [7]. Fixed boundary conditions were applied at proximal joint centers, and bone stresses throughout gait were calculated. OpenSim and FEA derived estimates of joint reaction forces were compared for validation. RESULTS/ANTICIPATED RESULTS: A maximum stress of 65.53 MPa and 60.70 MPa was observed at the bone-implant interface for the transtibial and transfemoral patients (respectively) in the late stance phase of the walking task, corresponding to terminal stance and heel off. Averaged root mean squared errors of the walking task (in the anterior-posterior, inferior-superior, and medial-lateral directions, respectively) for the transtibial and transfemoral patients were (124, 152, 80) N, (71, 80, 30) N, and (190, 62, 30) N, respectively. DISCUSSION/SIGNIFICANCE: The purpose of the above study was to develop a methodology for determining subject-specific mechanical loads during walking using finite element analysis and compare mechanical stresses in patients with transfemoral and transtibial OI. Similar stresses between the two implant types were found.

Multi-view information fusion using multi-view variational autoencoder to predict proximal femoral fracture load.

Hip fracture occurs when an applied force exceeds the force that the proximal femur can support (the fracture load or "strength") and can have devastating consequences with poor functional outcomes. Proximal femoral strengths for specific loading conditions can be computed by subject-specific finite element analysis (FEA) using quantitative computerized tomography (QCT) images. However, the radiation and availability of QCT limit its clinical usability. Alternative low-dose and widely available measurements, such as dual energy X-ray absorptiometry (DXA) and genetic factors, would be preferable for bone strength assessment. The aim of this paper is to design a deep learning-based model to predict proximal femoral strength using multi-view information fusion. We developed new models using multi-view variational autoencoder (MVAE) for feature representation learning and a product of expert (PoE) model for multi-view information fusion. We applied the proposed models to an in-house Louisiana Osteoporosis Study (LOS) cohort with 931 male subjects, including 345 African Americans and 586 Caucasians. We performed genome-wide association studies (GWAS) to select 256 genetic variants with the lowest p-values for each proximal femoral strength and integrated whole genome sequence (WGS) features and DXA-derived imaging features to predict proximal femoral strength. The best prediction model for fall fracture load was acquired by integrating WGS features and DXA-derived imaging features. The designed models achieved the mean absolute percentage error of 18.04%, 6.84% and 7.95% for predicting proximal femoral fracture loads using linear models of fall loading, nonlinear models of fall loading, and nonlinear models of stance loading, respectively. The proposed models are capable of predicting proximal femoral strength using WGS features and DXA-derived imaging features. Though this tool is not a substitute for predicting FEA using QCT images, it would make improved assessment of hip fracture risk more widely available while avoiding the increased radiation exposure from QCT.

Total hip arthroplasty using a three-dimensional porous titanium acetabular cup: an examination of micromotion using subject-specific finite element analysis

BackgroundWe investigated the mid-term clinical and radiological results of total hip arthroplasty (THA) using a three-dimensional (3D) porous titanium cup and analyzed the micromotion at the interface of the cup using subject-specific finite element (FE) analysis.MethodsWe evaluated 73 hips of 65 patients (6 men and 59 women; mean age at the time of surgery, 62.2 years; range, 45–86 years) who had undergone THA using a 3D porous titanium cup. Clinical evaluations were performed using the Japanese Orthopaedic Association (JOA) hip score system. We assessed the fixation of the acetabular component based on the presence of radiolucent lines and cup migration using anteroposterior radiographs. Subject-specific FE models were constructed from computed tomography data.ResultsThe JOA score improved from a preoperative mean of 52.2 (range, 23–82) to a mean of 87.8 (range, 71–100) at the final follow-up. None of the patients underwent revisions during the follow-up period. Radiolucent lines were observed in 26 cases (35.6%) and frequently appeared at DeLee and Charnley Zone 3. Following the FE analysis, the micromotion at DeLee and Charnley Zone 3 was significantly larger than that at Zone 2. Furthermore, micromotion was large in the groups in which radiolucent lines appeared at Zone 3.ConclusionsThe mid-term clinical outcome of THA using a 3D porous titanium cup was excellent. However, radiolucent lines frequently appeared at DeLee and Charnley Zone 3. FE analysis indicated that micromotion was large at the same site, strongly suggesting that it contributes to the emergence of radiolucent lines. The 3D porous titanium cups are useful in THA, and with improvements focused on micromotion, we anticipate better long-term outcomes.

An instrument for methodological quality assessment of single-subject finite element analysis used in computational orthopaedics

The methodological quality of subject-specific finite element analysis papers depends on the rigor of the study design and detailed description of key elements, while assessment instruments are often confined to clinical trials or quasi-experiments. This study aims to present an instrument for methodological quality assessment of single-subject finite element analysis used in computational orthopaedics (MQSSFE). Based upon existing instruments and relevant review papers, a pilot version was developed consisting of 37 items with 6 domains, including study design and presentation of findings, subject recruitment, model reconstruction and configuration, boundary and loading conditions (simulation), model verification and validation, and model assumption and validity. We interviewed four experts in the field to assess the face validity and refined the instrument. The instrument was tested for interrater reliability among two assessors on nine finite element study papers. Also, the criterion validity was evaluated by comparing the similarity of the MQSSFE and the modified Down and Black instrument. The intraclass correlation coefficient was 0.965, while the MQSSFE was significantly moderately correlated with the modified Down and Black instruments (r ​= ​0.61). We believed that MQSSFE was adequately appropriate, reliable, and valid for assessing the methodological quality for finite element studies used in computational orthopaedics. The instrument could facilitate quality assessment in the systematic reviews of finite element models and checklists for fidelity.

Subject-specific finite element analysis of a lumbar cage produced by electron beam melting.

The aim of this study was the analysis of the mechanical behaviour of a partially porous lumbar custom-made cage by means of a subject-specific finite element analysis (FEA). The cage, made of Ti6Al4V ELI alloy, was produced via electron beam melting (EBM) process and surgically implanted in a female subject, 50 years old. The novelty of this study was the customized design of the cage and of its internal structure, which is impossible to obtain with the traditional production techniques. The 3D model of the spine was obtained from the computed tomography (CT) of the patient. Moreover, high-resolution industrial CT was also used to reconstruct a 3D model of the cage, with its real (as-produced) features, such as superficial roughness, morphology of the bulk and of the porous structure. The workflow was divided in several steps: the main finite element analyses were non-linear and quasi-static regarding: the rhombic dodecahedron (RD) unit cell of the porous structure; the device; the whole L4-L5 motion segment with the implanted cage. Stress distribution was calculated under compression load for all models. For the RD unit cell, the maximum stress appeared at the connected cross nodes, where notch effect was present. For the cage subjected to a load of 1 kN, the porous structure did not present any functional failure. For the whole biomechanical system subjected to a physiological load of 360 N, the calculated stress in the bone was smaller than its yield strength value. On the axial view, a zone with higher compressive stresses was present on the L5 vertebral body. This was due to the contact stress between the cage and the vertebra. From the comparison between FE results and the CT images of the spine, bone remodelling was supposed, with the formation of new bone. Graphical abstract Workflow showing the phases of the research.

Effect of orthopedic implants on canine long bone compression stiffness: a combined experimental and computational approach.

Osteosynthesis for canine long bones is a complex process requiring knowledge of biology, surgical techniques and (bio)mechanical principles. Subject-specific finite element analysis constitutes a promising tool to evaluate the effect of surgical intervention on the global properties of a bone-implant construct, but suffers from a lack of validation. In this study, the biomechanical behavior of 10 canine humeri was compared before and after creation of a 10 mm bone defect stabilized with an eight-hole locking compression plate (Synthes®) and two locking screws on each fragment. The response under compression of both intact and plated samples was measured experimentally and reproduced with a finite element model. The experimental stiffness ratio between plated and intact bone was equal to 0.39 ± 0.06. A subject-specific finite element analysis including density-dependent elasto-plastic material properties for canine bone and automatic generation of orthopedic implants was then conducted to recover these experimental results. The stiffness of intact and plated samples could be predicted, with no significant differences with experimental data. The simulated stiffness ratio between plated and intact canine bone was equal to 0.43 ± 0.03. This study constitutes a first step toward the building of a virtual database of pre-computed cases, aiming at helping the veterinary surgeons to make decisions regarding the most suited orthopedic solution for a given dog and a given fracture.

Influence of altered geometry and material properties on tissue stress distribution under load in tendinopathic Achilles tendons – A subject-specific finite element analysis

Achilles tendon material properties and geometry are altered in Achilles tendinopathy. The purpose of this study was to determine the relative contributions of altered material properties and geometry to free Achilles tendon stress distribution during a sub-maximal contraction in tendinopathic relative to healthy tendons. Tendinopathic (n = 8) and healthy tendons (n = 8) were imaged at rest and during a sub-maximal voluntary isometric contraction using three-dimensional freehand ultrasound. Images were manually segmented and used to create subject-specific finite element models. The resting cross-sectional area of the free tendon was on average 31% greater for the tendinopathic compared to healthy tendons. Material properties for each tendon were determined using a numerical parameter optimisation approach that minimised the difference in experimentally measured longitudinal strain and the strain predicted by the finite element model under submaximal loading conditions for each tendon. The mean Young’s modulus for tendinopathic tendons was 53% lower than the corresponding control value. Finite element analyses revealed that tendinopathic tendons experience 24% less stress under the same submaximal external loading conditions compared to healthy tendons. The lower tendon stress in tendinopathy was due to a greater influence of tendon cross-sectional area, which alone reduced tendon stress by 30%, compared to a lower Young’s modulus, which alone increased tendon stress by 8%. These findings suggest that the greater tendon cross-sectional area observed in tendinopathy compensates for the substantially lower Young’s modulus, thereby protecting pathological tendon against excessive stress.

On the Failure Initiation in the Proximal Human Femur Under Simulated Sideways Fall.

The limitations of areal bone mineral density measurements for identifying at-risk individuals have led to the development of alternative screening methods for hip fracture risk including the use of geometrical measurements from the proximal femur and subject specific finite element analysis (FEA) for predicting femoral strength, based on quantitative CT data (qCT). However, these methods need more development to gain widespread clinical applications. This study had three aims: To investigate whether proximal femur geometrical parameters correlate with obtained femur peak force during the impact testing; to examine whether or not failure of the proximal femur initiates in the cancellous (trabecular) bone; and finally, to examine whether or not surface fracture initiates in the places where holes perforate the cortex of the proximal femur. We found that cortical thickness around the trochanteric-fossa is significantly correlated to the peak force obtained from simulated sideways falling (R 2=0.69) more so than femoral neck cortical thickness (R 2=0.15). Dynamic macro level FE simulations predicted that fracture generally initiates in the cancellous bone compartments. Moreover, our micro level FEA results indicated that surface holes may be involved in primary failure events.

The use of subject-specific Finite Element analysis of L1-L4 vertebra to screening osteoporosis in postmenopausal women.

The aim of this work is to study Computed Tomography (CT)-based Finite Element (FE) modelling of lumbar vertebra to classify osteoporotic patients using dual-energy x-ray absorptiometry (DXA) as reference. Cohort comprised 15 postmenopausal female patients with CT-DXA pairs. Sixty subject-specific CT-based FE models were constructed of L1 to L4 vertebrae. Load-displacement in each node of the models was analyzed. Mean kurtosis values of displacement distributions between osteoporosis and healthy groups at L1 level showed a statistically significant difference (p<;0.005). The CT-based FE model of L1 vertebra with patient specific material properties can be used to identify osteoporosis patients.

Subject-specific finite element analysis of the carpal tunnel cross-sectional to examine tunnel area changes in response to carpal arch loading

BackgroundManipulating the carpal arch width (i.e. distance between hamate and trapezium bones) has been suggested as a means to increase carpal tunnel cross-sectional area and alleviate median nerve compression. The purpose of this study was to develop a finite element model of the carpal tunnel and to determine an optimal force direction to maximize area. MethodsA planar geometric model of carpal bones at hamate level was reconstructed from MRI with inter-carpal joint spaces filled with a linear elastic surrogate tissue. Experimental data with discrete carpal tunnel pressures (50, 100, 150, and 200mmHg) and corresponding carpal bone movements were used to obtain material property of surrogate tissue by inverse finite element analysis. The resulting model was used to simulate changes of carpal arch widths and areas with directional variations of a unit force applied at the hook of hamate. FindingsInverse finite element model predicted the experimental area data within 1.5% error. Simulation of force applications showed that carpal arch width and area were dependent on the direction of force application, and minimal arch width and maximal area occurred at 138° (i.e. volar-radial direction) with respect to the hamate-to-trapezium axis. At this force direction, the width changed to 24.4mm from its initial 25.1mm (3% decrease), and the area changed to 301.6mm2 from 290.3mm2 (4% increase). InterpretationThe findings of the current study guide biomechanical manipulation to gain tunnel area increase, potentially helping reduce carpal tunnel pressure and relieve symptoms of compression median neuropathy.

The Effect of Drop-Landing Height on Tibia Bone Strain

Athletes and military recruits are often afflicted by stress fractures. Rigorous training programs consisting of increased repetitive mechanical loading may contribute to the high incidence of tibia stress fracture in the athletic and army populations. The purpose of this study was to assess the effect of incremented height on tibia bone strains and strain rates during landing. Seven healthy college males performed drop-landing tasks from 26 cm, 39 cm, and 52 cm, respectively. Tibia bone strains and strain rates were obtained through subject-specific multi-body dynamic computer simulations and finite element analyses. One-way repeated measures ANOVAs were conducted. Both 39 cm and 52 cm conditions resulted in larger tibia bone strains and strain rates than the 26 cm condition. The 52 cm condition also resulted in greater bone strains and strain rates than the 39 cm condition. A dose-response relationship exists between incremented landing height and bone strains and strain rates. Activities consisting of high impact landings are associated with increased risk of developing tibia stress fracture. When designing training programs involved high impact activities, athletes and military recruits should consider the effect of impact loading on tibia bone health and giving enough time for bones to adapt to new trainings.

In vivo bone remodeling rates determination and compressive stiffness variations before, during 60 days bed rest and two years follow up: A micro-FE-analysis from HR-pQCT measurements of the berlin Bed Rest Study-2

Bed rest studies are used for simulation and study of physiological changes as observed in unloading/non-gravity environments. Amongst others, bone mass reduction, similar as occurring due to aging osteoporosis, combined with bio-fluids redistribution and muscle atrophy have been observed and analyzed. Advanced radiological methods of high resolution such as HR-pQCT (XtremeCT) allow 3D-visualizing in vivo bone remodeling processes occurring during absence/reduction of mechanical stimuli (0 to <1g) as simulated by bed rest. Induced bone micro-structure (e.g. trabecular number, cortical thickness, porosity) and density variations can be quantified. However, these parameters are average values of each sample and important information regarding bone mass distribution and within bone mechanical behaviour is lost. Finite element models with hexa-elements of identical size as the HR-pQCT measurements (0.082mm×0.082mm×0.082mm, ca. 7E6 elements/sample) can be used for subject-specific in vivo stiffness calculation. This technique also allows quantifying if bone microstructural changes represent a risk of mechanical bone collapse (fracture). Materials and methodsIn the Berlin Bed Rest Study-2, 23 male subjects (20–50 YO) were maintained 60 days under restricted bed rest (6° HDT) aiming to test a - for this study specifically designed - vibration resistive exercise regime for maintenance of bone mass and muscle functionality at normal levels (base line measurements). For comparison a resistive exercise without vibration and a control group were included. Base line HR-pQCT measurements (3 days before bed rest: base line), as well as during 30 days bed rest (BR30 and BR59, 3 days of recovery (R3), R15, R30, R90, R180, R360, and R720 were performed. CT-scan voxels were converted into finite elements (hexa-82µm edge length) for calculating in vivo compressive stiffness during the experiment duration. Histograms of stresses and strains distributions as well as anatomical regions susceptible for mechanical failure were identified and compared. Results:Resistive vibration exercises (RVE) were able to maintain in the majority of the subjects compressive bone strength as determined after modelling a compressive test using finite element models. Compressive bone stiffness using FEA was monitored through analysis of the internal deformation on the trabecular structures and cortical bone, reaction forces, and minimum principal strains on the in vivo CT measured bone regions during the experiment duration. Stress distributions (main stresses) and von Mises stress distribution remained comparable with those determined in the base line measurements for the RVE-group. However, no major differences were found in the group with resistive exercise training alone. Without mechanical stimuli an increment of bone regions with high stress concentration was observed and a reduction of up to 10% of bone compressive stiffness was quantified by using subject-specific finite-element analysis. Anatomically von Mises stress concentrations, thus bone regions susceptible to fail mechanically, were observed at the center of the cancellous bone and at the antero- posterior region of the cortical bone. Conclusions:Finite element simulations from bed rest studies are an invaluable tool to determine subject-specific in vivo compressive stiffness and anatomical mechanically compromised regions under controlled mechanical conditions (unloading) which - until now - are not possible to be determined with any other method. Vibration exercise combined with a resistive compressive force was able to maintain bone structure and density even during 60 days of bed rest.

The mechanics of focal chondral defects in the hip

There is a mean incidence of osteoarthritis (OA) of the hip in 8% of the overall population. In the presence of focal chondral defects, defined as localized damage to the articular cartilage, there is an increased risk of symptomatic progression toward OA. This relationship between chondral defects and subsequent development of OA has led to substantial efforts to develop effective procedures for surgical cartilage repair. This study examined the effects of chondral defects and labral delamination on cartilage mechanics in the dysplastic hip during the gait cycle using subject-specific finite element analysis. Models were generated from volumetric CT data and analyzed with simulated chondral defects at the chondrolabral junction on the posterior acetabulum during five distinct points in the gait cycle. Focal chondral defects increased maximum shear stress on the osteochondral surface of the acetabular cartilage, when compared to the intact case. This effect was amplified with labral delamination. Additionally, chondral defects increased the first principal Lagrange strain on the articular surface of the acetabular cartilage and labrum. Labral delamination relieved some of this tensile strain. As defect size was increased, contact stress increased in the medial zone of the acetabulum, while it decreased anteriorly. The results suggest that in the presence of chondral defects and labral delamination the cartilage experiences elevated tensile strains and shear and contact stress, which could lead to further damage of the cartilage, and subsequent arthritic progression. The framework presented here will serve as the procedure for future finite element studies on cartilage mechanics in hips with varying disease states with simulated chondral defects and labral tears.

On the importance of 3D, geometrically accurate, and subject-specific finite element analysis for evaluation of in-vivo soft tissue loads

Pressure ulcers are a type of local soft tissue injury due to sustained mechanical loading and remain a common issue in patient care. People with spinal cord injury (SCI) are especially at risk of pressure ulcers due to impaired mobility and sensory perception. The development of load improving support structures relies on realistic tissue load evaluation e.g. using finite element analysis (FEA). FEA requires realistic subject-specific mechanical properties and geometries. This study focuses on the effect of geometry. MRI is used for the creation of geometrically accurate models of the human buttock for three able-bodied volunteers and three volunteers with SCI. The effect of geometry on observed internal tissue deformations for each subject is studied by comparing FEA findings for equivalent loading conditions. The large variations found between subjects confirms the importance of subject-specific FEA.

Maximum principal strain as a criterion for prediction of orthodontic mini-implants failure in subject-specific finite element models.

To investigate the most reliable stress or strain parameters in subject-specific finite element (FE) models to predict success or failure of orthodontic mini-implants (OMIs). Subject-specific FE analysis was applied to 28 OMIs used for anchorage. Each model was developed using two computed tomography data sets, the first taken before OMI placement and the second taken immediately after placement. Of the 28 OMIs, 6 failed during the first 5 months, and 22 were successful. The bone compartment was divided into four zones in the FE models, and peak stress and strain parameters were calculated for each. Logistic regression of the failure (vs success) of OMIs on the stress and strain parameters in the models was conducted to verify the ability of these parameters to predict OMI failure. Failure was significantly dependent on principal strain parameters rather than stress parameters. Peak maximum principal strain in the bone 0.5 to 1 mm from the OMI surface was the best predictor of failure (R(2) = 0.8151). We propose the use of the maximum principal strain as a criterion for predicting OMI failure in FE models.

Subject-specific finite element model with an optical tracking system in total hip replacement surgery.

Intra-operative peri-prosthetic femoral fractures are a significant concern in total hip arthroplasty and can occur at any time during surgery, with the highest incidence during implant insertion. This study combines subject-specific finite element analysis modeling with an optical tracking system to characterize the resultant strain in the bone and results of impaction during total hip replacement surgery. The use of ABG II femoral stem (Stryker Orthopaedics, Mahwah, NJ, USA) in the model yielded the following results. Hammer velocity was measured experimentally using a three-dimensional optical tracking system and these data were input into the finite element analysis model so that intra-operative loading scenario could be simulated. A quasi-static explicit simulation and a dynamic loading step using two implant-bone interface friction (0.1 and 0.4 friction coefficients) states were simulated. The maximum swing velocity of a mallet was experimentally measured at 1.5 m/s and occurred just before impaction of the hammer with implant introducer. Two friction states resulted in different results with the lower friction coefficient generating higher strains in the anterior regions of the model and higher displacement of the implant with respect to the femur when compared to the high friction state.

Effects of rotational acetabular osteotomy on the mechanical stress within the hip joint in patients with developmental dysplasia of the hip: a subject-specific finite element analysis.

In this study we used subject-specific finite element analysis to investigate the mechanical effects of rotational acetabular osteotomy (RAO) on the hip joint and analysed the correlation between various radiological measurements and mechanical stress in the hip joint. We evaluated 13 hips in 12 patients (two men and ten women, mean age at surgery 32.0 years; 19 to 46) with developmental dysplasia of the hip (DDH) who were treated by RAO. Subject-specific finite element models were constructed from CT data. The centre-edge (CE) angle, acetabular head index (AHI), acetabular angle and acetabular roof angle (ARA) were measured on anteroposterior pelvic radiographs taken before and after RAO. The relationship between equivalent stress in the hip joint and radiological measurements was analysed. The equivalent stress in the acetabulum decreased from 4.1 MPa (2.7 to 6.5) pre-operatively to 2.8 MPa (1.8 to 3.6) post-operatively (p < 0.01). There was a moderate correlation between equivalent stress in the acetabulum and the radiological measurements: CE angle (R = -0.645, p < 0.01); AHI (R = -0.603, p < 0.01); acetabular angle (R = 0.484, p = 0.02); and ARA (R = 0.572, p < 0.01). The equivalent stress in the acetabulum of patients with DDH decreased after RAO. Correction of the CE angle, AHI and ARA was considered to be important in reducing the mechanical stress in the hip joint.

Subject-specific finite element analysis to characterize the influence of geometry and material properties in Achilles tendon rupture

Achilles tendon injuries including rupture are one of the most frequent musculoskeletal injuries, but the mechanisms for these injuries are still not fully understood. Previous in vivo and experimental studies suggest that tendon rupture mainly occurs in the tendon mid-section and predominantly more in men than women due to reasons yet to be identified. Therefore we aimed to investigate possible mechanisms for tendon rupture using finite element (FE) analysis. Specifically, we have developed a framework for generating subject-specific FE models of human Achilles tendon. A total of ten 3D FE models of human Achilles tendon were generated. Subject-specific geometries were obtained using ultrasound images and a mesh morphing technique called Free Form Deformation. Tendon material properties were obtained by performing material optimization that compared and minimized difference in uniaxial tension experimental results with model predictions. Our results showed that both tendon geometry and material properties are highly subject-specific. This subject-specificity was also evident in our rupture predictions as the locations and loads of tendon ruptures were different in all specimens tested. A parametric study was performed to characterize the influence of geometries and material properties on tendon rupture. Our results showed that tendon rupture locations were dependent largely on geometry while rupture loads were more influenced by tendon material properties. Future work will investigate the role of microstructural properties of the tissue on tendon rupture and degeneration by using advanced material descriptions.