Computational modelling of diastole for human ventricle

Diastolic heart failure (DHF) with normal systolic pump function has been typically observed in the majority of HF patients. DHF changes regular diastolic behaviour of left-ventricle (LV), and increases the ventricular wall stress. Therefore, normalisation of increased LV wall stress is the cornerstone of many existing and new therapeutic treatments. However, information regarding such regional stress-strain distribution for human LV is extremely limited in the literature. Thus, the study aimed at estimating the normal range and regional variation of diastolic stress-strain field in healthy human LVs, and exploring the infl uence of fibre structure, geometrical heterogeneity and material properties on passive infl ation of LV. It is envisaged that such information could be used as targets for future in-silico studies to design optimised HF treatments. FE modelling of passive diastolic mechanics was carried out using personalised ventricular geometry, that was constructed from magnetic resonance imaging (MRI), and structure-based orthotropic constitutive law. Laplace-Dirichlet-Region growing-Finite element (LDRF) algorithm was developed in order to assign the myocardium fibre map on ventricular geometry. The effect of right ventricle (RV) deformation, that has not been taken into account by the majority of researchers due to modelling simplification, was investigated for the first time by comparing the results predicted by bi-ventricle (BV) and single LV models, constructed from the aforementioned MRI data. In addition, personalised in-vivo measurement of fibre structure, that might be different in individual subjects and diseased conditions, is still an open question. Therefore, the sensitivity of LV diastolic mechanics to the details of the fibre structure was accomplished for the first time using eight different fibre orientations. In-vivo passive orthotropic myocardium properties for healthy human myocardium, indispensable for personalised LV wall stress estimation, was identified, and subsequently, the regional variations of LV wall stress-strain were investigated by incorporating geometrical heterogeneity, personalised myocardium properties and LV base movements in the FE models. RV deformation increased average fibre and sheet stress-strain in LV wall during diastole, and therefore, the effect should always be included in cardiac biomechanics study. Any pathological remodelling, that increased the amount of transmural fibre angle, led to an additional LV infl ation. The study indicates that a change in fibre orientation may contribute to the heart failure with preserved ejection fraction (HFpEF) development. Future therapeutic intervention should consider the effect of altered fibre orientation for better outcome. Due to the ill-posed nature of the inverse optimisation problem, the average myocardial stiffness was extracted by identifying the normal ranges of the parameters. A novel method was developed by combining FE modelling, response surface method (RSM) and genetic algorithm (GA) to identify the passive orthotropic myocardium properties for healthy human myocardium using routinely used clinical data. These myocardium properties can directly be utilised in future computational studies. Although the regional stress-strain distribution of the LV wall was highly heterogeneous amongst the individuals, it was observed that the inner wall of the LV experienced higher fibre stress compared to the outer wall. The LV wall near the base and the lateral region received greater stress-strain compared to the other regions. The incorporation of LV base movement (not addressed in the literature) improved the FE model predictions, and therefore, it is recommended to be considered in later studies. In addition, normal ranges of various stress-strain components in different regions of LV wall were reported for five healthy human ventricles considering RV deformation, LV base movement, and subject-specific myocardium properties. This information could be used as a reference map for future studies. The study revealed that the FE modelling can be employed to analyse the effect of geometry, fibre-structure and material properties on normal ventricular mechanics, and therefore, can provide a greater insight into the underlying mechanics of failing heart and plan for optimised surgical intervention. Hence, the research has impacts on computational cardiac biomechanics as well as clinical cardiac physiology fields

A novel hybrid shell element formulation (QUAD+ and TRIA+) : a benchmarking and comparative study

This paper introduces a novel hybrid finite element (FE) formulation of shell element to enable assembly process simulation of compliant sheet-metal parts with higher efficiency and flexibility. Efficiency was achieved by developing both new hybrid quadrilateral and triangular elements. Quadrilateral element (QUAD+) was formulated by combining area geometric quadrilateral 6 (AGQ6) nodes and mixed interpolated tensorial components (MITC) to model membrane and bending/shear component respectively. Triangular element (TRIA+) was formulated by merging assumed natural deviatoric strain (ANDES) for membrane and MITC for bending/shear component. Flexibility was addressed by developing an open-source C++ code, enhanced by the OpenMP interface for multiprocessing programming. Tests and benchmarks were compiled and executed within Matlab using the MEX API interface. Extensive benchmark studies were accomplished to evaluate the performance of the proposed hybrid formulation and the shell formulations used in three FEM packages - ABAQUS, ANSYS and COMSOL- under static linear elastic condition with small strain assumption. It was observed that the proposed QUAD+ and TRIA+ elements performed better amongst the FE packages, especially when there was in-plane mesh distortion, with errors below 3%. It was also identified that the best efficiency is obtained by adopting dominant QUAD+ elements compared to the TRIA+ when working on complex geometries. This paper also contributes to present a wide set of benchmark studies required to verify new release of FE packages using shell element or evaluate the performance of new shell formulations

Bone-to-bone and implant-to-bone impingement : a novel graphical representation for hip replacement planning

Bone-to-bone impingement (BTBI) and implant-to-bone impingement (ITBI) risk assessment is generally performed intra-operatively by surgeons, which is entirely subjective and qualitative, and therefore, lead to sub-optimal results and recurrent dislocation in some cases. Therefore, a method was developed for identifying subject-specific BTBI and ITBI, and subsequently, visualising the impingement area on native bone anatomy to highlight where prominent bone should be resected. Activity definitions and subject-specific bone geometries, with planned implants were used as inputs for the method. The ITBI and BTBI boundary and area were automatically identified using ray intersection and region growing algorithm respectively to retain the same ‘conical clearance angle’ obtained to avoid prosthetic impingement (PI). The ITBI and BTBI area was then presented with different colours to highlight the risk of impingement, and importance of resection. A clinical study with five patients after 2 years of THA was performed to validate the method. The results supported the study hypothesis, in that the predicted highest risk area (red coloured zone) was completely/majorly resected during the surgery. Therefore, this method could potentially be used to examine the effect of different pre-operative plans and hip motions on BTBI, ITBI, and PI, and to guide bony resection during THA surgery

Femur first navigation can reduce impingement severity compared to traditional free hand total hip arthroplasty.

Impingement is a major source of dislocation and aseptic loosening in total hip arthroplasty (THA). We compared impingement free range of motion (ROM) using a novel computer navigated femur first approach to conventional THA. In addition, impingement between genders was also explored. In a retrospective analysis of 121 THA patients, subject-specific post-operative ROM was simulated using post-operative 3D-CT data, and compared with the benchmark ROM, essential for activities of daily living. Three parameters were defined to express both implant-to-implant (ITI) and bone-to-bone (BTB) impingement - coverage percentage, third angle, and impingement severity. Although coverage percentage was similar between the navigated and conventional group for both ITI (p = 0.69) and BTB (p = 0.82) impingement, third angle was significantly reduced in the navigation group for both ITI (p = 0.02) and BTB (p = 0.05) impingement. Impingement severity for both ITI (p = 0.01) and BTB (p = 0.05) was significantly decreased in the navigation group compared to the conventional. Impingement severity in men was considerably higher compared to women for both ITI (p = 0.002) and BTB (p = 0.02). Navigation guided femur first THA is able to improve alignment of ROM axis, and consequently, to reduce impingement in THA. Men seem to be more prone to impingement than women

Subject-specific surgical planning for hip replacement : a novel 2D graphical representation of 3D hip motion and prosthetic impingement information

Prosthetic impingement (PI) following total hip arthroplasty (THA), which arises due to the undesirable relative motion of the implants, results in adverse outcomes. Predicting PI through 3D graphical representation is difficult to comprehend when all activities are combined for different implant positions. Therefore, the aim of the paper was to translate this 3D information into a 2D graphical representation for improved understanding of the patient’s hip motion. The method used planned implanted geometry, positioned onto native bone anatomy, and activity definitions as inputs to construct the 2D polar plot from 3D hip motion in four steps. Three case studies were performed to highlight its potential use in (a) combining different activities in a single plot, (b) visualising the effect of different cup positions and (c) pelvic tilt on PI. A clinical study with 20 ‘Non-Dislocators’ and 20 ‘Dislocators’ patients after 2 years of THA was performed to validate the method. The results supported the study hypothesis, in that the incidence of PI was always higher in the ‘Dislocators’ compared to the ‘Non-Dislocators’ group. The proposed 2D graphical representation could assist in subject-specific THA planning by visualising the effect of different activities, implant positions, pelvic tilt and related aspects on PI

The effect of clamping sequence on dimensional variability of a manufactured automotive sheet metal sub-assembly

The use of holding and inspection fixtures is common across many sectors of manufacturing. The concept of a fixture for holding a component, for the purposes of assembly or inspection, is straightforward. The fixture design and the associated clamping strategy can have a significant impact on the process. This paper presents a methodology for investigating the effects of clamping sequence for a production inspection fixture, on the dimensional variability of an automotive production-representative sheet metal sub-assembly, along with experimental findings and analysis of measurement data. The study utilises both a coordinate measuring machine and laser tracker to capture a range of features and surface points, and compares four different clamping sequences, including the manufacturer’s defined sequence, to evaluate their effect on the dimensional results from a predetermined measurement plan. The results from the study show that there was significant variation in measurements taken from the four different clamping sequences, and these variations can show the same points and/or features being within tolerance for one clamping sequence and out of tolerance for another. This clearly has major implications for product development and subsequent volume manufacture, so needs to be considered and optimised in the measurement planning process

Passive diastolic modelling of human ventricles : effects of base movement and geometrical heterogeneity

Left-ventricular (LV) remodelling, associated with diastolic heart failure, is driven by an increase in myocardial stress. Therefore, normalisation of LV wall stress is the cornerstone of many therapeutic treatments. However, information regarding such regional stress–strain for human LV is still limited. Thus, the objectives of our study were to determine local diastolic stress–strain field in healthy LVs, and consequently, to identify the regional variations amongst them due to geometric heterogeneity. Effects of LV base movement on diastolic model predictions, which were ignored in the literature, were further explored. Personalised finite-element modelling of five normal human bi-ventricles was carried out using subject-specific myocardium properties. Model prediction was validated individually through comparison with end-diastolic volume and a new shape-volume based measurement of LV cavity, extracted from magnetic resonance imaging. Results indicated that incorporation of LV base movement improved the model predictions (shape-volume relevancy of LV cavity), and therefore, it should be considered in future studies. The LV endocardium always experienced higher fibre stress compared to the epicardium for all five subjects. The LV wall near base experienced higher stress compared to equatorial and apical locations. The lateral LV wall underwent greater stress distribution (fibre and sheet stress) compared to other three regions. In addition, normal ranges of different stress–strain components in different regions of LV wall were reported for five healthy ventricles. This information could be used as targets for future computational studies to optimise diastolic heart failure treatments or design new therapeutic interventions/devices

Prediction and visualisation of bony impingement for subject specific total hip arthroplasty

Bony impingement (BI) may contribute to restricted hip joint motion, and recurrent dislocation after total hip arthroplasty (THA), and therefore, should be avoided where possible. However, BI risk assessment is generally performed intra-operatively by surgeons, which is partially subjective and qualitative. Therefore, the aim of the study was to develop a method for identifying subject-specific BI, and subsequently, visualising BI area on native bone anatomy to highlight the amount of bone should be resected. Activity definitions and subject-specific bone geometries, constructed from CT scans, with planned implants were used as inputs for the method. For each activity, a conical clearance angle (CCA) was checked between femur and pelvis through simulation. Simultaneously, BI boundary and area were automatically calculated using ray intersection and region growing algorithm respectively. The potential use of the developed method was explained through a case study using an anonymised pre-THA patient data. Two pure (flexion, and extension) and two combined hip joint motions (internal and external rotation at flexion and extension respectively) were considered as activities. BI area were represented in two ways: (a) CCA specific where BI area for each activity with different CCAs was highlighted, (b) activity specific where BI area for all activities with a particular CCA was presented. Result showed that BI area between the femoral and pelvic parts was clearly identified so that the pre-operative surgical plan could be adjusted to minimise impingement. Therefore, this method could potentially be used to examine the effect of different preoperative plans and hip motion on BI, and to guide bony resection during THA surgery

A sensor-based screening tool for identifying high pelvic mobility in patients due to undergo total hip arthroplasty

There is increasing evidence that pelvic mobility is a critical factor to consider in implant alignment during total hip arthroplasty (THA). Here, we test the feasibility of using an inertial sensor fitted across the sacrum to measure change in pelvic tilt, and hence screen for patients with high pelvic mobility. Patients (n = 32, mean age: 57.4 years) due to receive THA surgery participated in the study. Measures of pelvic tilt were captured simultaneously using the device and radiograph in three functional positions: Standing, flexed-seated, and step-up. We found a strong correlation between the device and radiograph measures for the change in pelvic tilt measure from standing to flexed-seated position (R2 = 0.911); 75% of absolute errors were under 5 degrees. We demonstrated that the device can be used as a screening tool to rapidly identify patients who would benefit from more detailed surgical planning of implant positioning to reduce future risks of impingement and dislocation