The fidelity of a 3D model created using image segmentation must be precisely quantified and evaluated for the model to be trusted for use in subsequent biomechanical studies such as finite element analysis. The bones within the ankle joint vary significantly in size and shape. The purpose of this study was to test the hypothesis that the accuracy and reliability of a segmented bone geometry is independent of the particular bone being measured. Computed tomography (CT) scan data (slice thickness 1 mm, pixel size 808±7 µm) from three anonymous patients was used for the development of the ankle geometries (consisting of the tibia, fibula, talus, calcaneus, and navicular bones) using Simpleware Scan IP software (Synopsys, Exeter, UK). Each CT scan was segmented 4 times by an inexperienced undergraduate, resulting in a total of 12 geometry assemblies. An experienced researcher segmented each scan once, and this was used as the ‘gold standard’ to quantify the accuracy. The solid bone geometries were imported into CAD software (Inventor 2023, Autodesk, CA, USA) for measurement of the surface area and volume of each bone, and the distances between bones (tibia to talus, talus to navicular, talus to calcaneus, and tibia to fibula) were carried out. The intra-class coefficient (ICC) was used to assess intra-observer reliability. Bland Altman plots were employed as a statistical measure for criteria validity (accuracy) [1].Abstract
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This abstract provides an update on the Open Ankle Models being developed at the University of Bath. The goal of this project is to create three fully open-source finite element (FE) ankle models, including bones, ligaments, and cartilages, appropriate musculoskeletal loading and boundary conditions, and heterogeneous material property distribution for a standardised representation of ankle biomechanics and pre-clinical ankle joint analysis. A computed tomography (CT) scan data (pixel size of 0.815 mm, and slice thickness of 1 mm) was used to develop the 3D geometry of the bones (tibia, talus, calcaneus, fibula, and navicular). Each bone was given the properties of a heterogeneous elastic material based on the CT greyscale. The density values for each bone element were calculated using a linear empirical relation, ρ= 0.0405 + (0.000918) HU and then power law equations were utilised to get the Young's Modulus value for each bone element [1]. At the bone junction, a thickness of cartilage ranging from 0.5–1 mm, and was modelled as a linear material (E=10 MPa, ν=0.4 [2]). All ligament insertions and positions were represented by four parallel spring elements, and the ligament stiffness and material attributes were applied in accordance with the published literature [2]. The ankle model was subjected to static loading (balance standing position). Four noded tetrahedral elements were used for the discretization of bones and cartilages. All degrees of freedom were restricted at the proximal ends of the tibia and fibula. The ground reaction forces were applied at the underneath of the calcaneus bone. The interaction between the cartilages and bones was modelled using an augmented contact algorithm with a sliding elastic contact between each cartilage. A tied elastic contact was used between the cartilages and the bone. FEbio 2.1.0 (University of Utah, USA) was used to construct the open-source ankle model.Abstract
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The aim of this study was to develop an open-source finite element model of the ankle for identification of the best clinical treatment to restore stability to the ankle after injury. The ankle geometry was defined from the Visible Human Project Female CT dataset available from the National Library of Medicine, and segmented using Dragonfly software (Object Research Systems, 2020). The finite element model was created with FEBio (University of Utah, 2021) using the dynamic nonlinear implicit solver. Linear isotropic material properties were assigned to the bones (E=7300MPa, ν=0.3, ρ=1730kg/m3) and cartilage (E=10MPa, ν=0.4, ρ=1100kg/m3). Spring elements were used to represent the ligaments and material properties were taken from Mondal et al. [1]. Lagrangian contact was defined between the cartilaginous surfaces with μ=0.003. A standing load case was modelled, assuming even distribution of load between the feet. A reaction force of 344.3N was applied to the base of the foot, a muscle force of 252.2N, and the proximal ends of the tibia and fibula were fully constrained.Abstract
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Ultra-High Molecular Weight Polyethylene (UHMWPE) can be made radiopaque through the diffusion of an oil-based contrast agent (Lipiodol Ultra-fluid). A similar process is used for Vitamin E incorporated polyethylene, which has a well-established clinical history. This study aimed to quantify the leaching of Lipiodol and compare to vitamin E polyethylene. GUR 1050 polyethylene (4 mm thickness) was cut into squares, 10 mm2. Samples (n=5) were immersed in 25 ml Lipiodol (Guerbet, France), or 15 ml Vitamin E (L-atocopherol, Sigma-Aldrich, UK). To facilitate diffusion, samples were held at 105°C for 18 hours. After treatment, all samples were immersed in DMEM (Sigma-Aldrich, UK) with Penicillin Streptomycin (Sigma-Aldrich, Kent, UK) at 4%v/v and held at 37°C in an incubator. Untreated polyethylene samples were included as controls. Leaching was quantified gravimetrically at weeks 2, 4 and 8. The radiopacity of the Lipiodol-diffused samples was investigated from µCT images (162kV, resolution 0.2 mm, X Tec, XT H 225 ST, Nikon Metrology, UK).Abstract
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Oil-based fluids can be used to enhance the properties of polyethylene materials. For example, vitamin E infused polyethylene has a superior oxidation resistance and Lipiodol infused polyethylene has an enhanced X-ray attenuation. The aim of this study was to evaluate the long-term influence of oily fluid on the chemical, physical and tensile properties of polyethylene. An accelerated ageing procedure (an elevated temperature (80°C) for four weeks in air1) was used to investigate the oxidative stability (ASTM F2012-17)2, tensile (ISO 527)3 and thermal properties4 of oil treated polyethylene (n=5, GUR 1050, Celanese, Germany)and compared with clinically used polyethylene controls (oil-free standard and thermally treated polyethylene). All the experiments were performed on aged and unaged specimens in accordance to international standards and compared to currently available literature. A Kruskal-Wallis test was performed using a custom MATLAB code (R2017a, USA); with p < 0.05 considered statistically significant.Abstract
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Ultra-high molecular weight polyethylene (UHMWPE) is a commonly used as bearing material in joint replacement devices. UHMWPE implants can be hard to see on a standard X-ray because UHMWPE does not readily attenuate X-rays. Radiopaque UHMWPE would enable direct imaging of the bearing both during and after surgery, providing in vivo assessment of bearing position, dislocation or fracture, and potentially a direct measure of wear. The X-ray attenuation of UHMWPE was increased by diffusing an FDA approved contrast agent (Lipiodol) into UHMWPE parts (Zaribaf et al, 2018). The aim of this study was to evaluate the optimal level of radiopacity for a UHMWPE bearing. Samples of un-irradiated medical grade UHMWPE (GUR 1050) were machined into 4mm standard medium Oxford Unicompartmental bearings. Samples were immersed in Lipiodol Ultra Fluid (Guerbert, France) at elevated temperatures (85 °C, 95 °C and 105 °C) for 24 h to achieve three different levels of radiopacity. A phantom set-up was used for X-ray imaging; the phantom contained two perspex rods to represent bone, with the metallic tibial tray and polyethylene bearing fixed to the end of one rod and the metallic femoral component fixed to the other rod. Radiographs of the samples were taken (n=5) with the components positioned in full extension. To ensure consistency, the images of all the samples were taken simultaneously alongside an untreated part. The results of our ongoing study demonstrate that the radiopacity of UHMWPE can be enhanced using Lipiodol and the parts are visible in a clinical radiographs. The identification of the optimal treatment from a clinical perspective is ongoing; we are currently running a survey with clinicians to find the consensus on the optimal radiopacity taking into account the metallic components and alignment. Future work will involve a RSA study to assess the feasibility of measuring wear directly from the bearing.
Femoral head collapse is a possible complication after surgical treatment of femoral neck fractures. The purpose of this study was to examine whether implantation of a Sliding Hip Screw (SHS) or an X-Bolt could increase the risk of femoral head collapse. Similar to traditional hip screws, the X-Bolt is implanted through the femoral neck; however, it uses an expanding cross-shape to improve rotational stability. The risk of collapse was investigated alongside patient factors, such as osteonecrosis. This numerical study assessed the risk of femoral head collapse using linear eigenvalue buckling (an established method [1]), and also from the maximum von Mises stress within the cortical bone. The femoral head was loaded using the pressures reported by Yoshida
Osteonecrosis, modelled as a cone of bone of varying angle, and varying modulus values Cortical thinning Reduced cortical modulus Femoral head size Twenty-two finite element models were run for each implant condition (intact; implanted with the X-Bolt; implanted with a SHS), resulting in a total of 66 models. The finite element models were validated using experimental tests performed on five 4th generation composite Sawbones femurs (Malmö, Sweden), and verified against previously published results [1]. No significant difference was found between the X-Bolt and the SHS, for either critical buckling pressure ( The study by Volokh
Polyethylene wear of joint replacements can cause severe clinical complications, including; osteolysis, implant loosening, inflammation and pain. Wear simulator testing is often used to assess new designs, but it is expensive and time consuming. It is possible to predict the volume of polyethylene implant wear from finite element models using a modification of Archard's classic wear law [1–2]. Typically, linear elastic isotropic, or elasto-plastic material models are used to represent the polyethylene. The purpose of this study was to investigate whether use of a viscoelastic material model would significantly alter the predicted volumetric wear of a mobile-bearing unicompartmental knee replacement. Tensile creep-recovery experiments were performed to characterise the creep and relaxation behaviour of the polyethylene (moulded GUR 4150 samples machined to 180×20×1 mm). Samples were loaded to 3 MPa stress in 4 minutes, and then held for 6 hours, the tensile stress was removed and samples were left to relax for 6 hours. The mechanical test data was used fit to a validated three–dimensional fractional Maxwell viscoelastic constitutive material model [3]. An explicit finite element model of a mobile–bearing unicompartmental knee replacement was created, which has been described previously [4]. The medial knee replacement was loaded to 1200 N over a period of 0.2 s. The bearing was meshed using quadratic tetrahedral elements (1.5 mm seeding size based on results of a mesh convergence study), and the femoral component was represented as an analytical rigid body. Wear predictions were made from the contact stress and sliding distance using Archard's law, as has been described in the literature [1–2]. A wear factor of 5.24×10−11 was used based upon the work by Netter et al. [2]. All models were created and solved using ABAQUS finite element software (version 6.14, Simulia, Dassault Systemes). The fractional viscoelastic material model predicted almost twice as much wear (0.119 mm3/million cycles) compared to the elasto-plastic model (0.069 mm3/million cycles). The higher wear prediction was due to both an increased sliding distance and higher contact pressures in the viscoelastic model. These preliminary findings indicate the simplified elasto-plastic polyethylene material representation can underestimate wear predictions from numerical simulations. Polyethylene is known to be a viscoelastic material which undergoes creep clinically, and it is not surprising that it is necessary to represent that viscoelastic behaviour to accurately predict implant wear. However, it does increase the complexity and run time of such computational studies, which may be prohibitive.
Using Python scripting it is possible to automate the pre-processing, solving and post-processing stages of finite element analysis using ABAQUS software. This is particularly useful when running multiple models parametrically. When the model involves a bony part, it is necessary to assign material properties based on the CT scan to represent bone heterogeneity, and unfortunately this cannot currently be done from within ABAQUS using software such as Bonemat [1]. To address this issue a Python package was written called ‘py_bonemat_abaqus’ to assign material properties from within ABAQUS. The purpose of this study was to compare the material assignments of py_bonemat_abaqus and Bonemat, to compare the processing speed, and to describe the workflow. The software packages were compared using a CT scan of a half pelvis downloaded from the VAKHUM database, and the associated hexahedral finite element mesh of the left half pelvis. To examine different element types, the hexahedral mesh was converted to linear and quadratic tetrahedral elements by dividing each hexahedron into 5 tetrahedral elements. The equations used to convert the Hounsfield Unit ( Equation 1: Equation 2 The time taken to analyse the models by each software was assessed using a Windows 7 PC with a 64-bit operating system, 4 CPUS, 8 GB of RAM and an Intel Core I5-3470 processor. The mean difference between the moduulus assignment made by py_bonemat_abaqus and Bonemat was −0.05 kPa (range −10.19 to 4.50 kPa, standard deviation 0.62 kPa). The Python package took a similar time to run for all element types; this was between 109 and 126 s. Bonemat software was significantly faster, and took between 5 and 20 s. Finally, the Python package was successfully used from within a Python script to perform material assignment from within ABAQUS software in a fully automated manner. Material assignments were almost equivalent between the two software packages, with any differences explainable by rounding effects. To put the differences into context, a difference of −0.05 kPa is 0.00000002% of the typical modulus of cortical bone (20.7 GPa), and 0.00000003% of the modulus of trabecular bone (14.8 GPa) [3]. The Python package was slower to process the models, but was successfully able to assign material properties from within ABAQUS software as part of an automated script.
In pre-operative planning for total hip arthroplasty (THA), femoral offset (FO) is frequently underestimated on AP pelvis radiographs as a result of inaccurate patient positioning, imprecise magnification, and radiographic beam divergence. The aim of the present study was to evaluate the reliability and accuracy of predicting three-dimensional (3-D) FO as measured on computed tomography (CT) from measurements performed on standardised AP pelvis radiographs. In a retrospective cohort study, pre-operative AP pelvis radiographs and corresponding CT scans of a consecutive series of 345 patients (345 hips, 146 males, 199 females, mean age 60 (range: 40-79) years, mean body-mass-index 27 (range: 29-57) kg/m2) with primary end-stage hip osteoarthritis were reviewed. Patients were positioned according to a standardised protocol and all images were calibrated. Using validated custom programmes, FO was measured on corresponding AP pelvis radiographs and CT scans. Inter- and intra-observer reliability of the measurement methods were evaluated using intra-class correlation coefficients (ICC). To predict 3-D FO from AP pelvis measurements, the entire cohort was randomly split in two groups and gender specific linear regression equations were derived from a subgroup of 250 patients (group A). The accuracy of the derived prediction equations was subsequently assessed in a second subgroup of 100 patients (group B). In the entire cohort, mean FO was 39.2mm (95%CI: 38.5-40.0mm) on AP pelvis radiographs and 44.6mm (95%CI: 44.0-45.2mm) on CT scans. FO was underestimated by 14% on AP pelvis radiographs compared to CT (5.4mm, 95%CI: 4.8-6.0mm, p<0.001) and both parameters demonstrated a linear correlation (r=0.642, p<0.001). In group B, we observed no significant difference between gender specific predicted FO (males: 48.0mm, 95%CI: 47.1-48.8mm; females: 42.0mm, 95%CI: 41.1-42.8mm) and FO as measured on CT (males: 47.7mm, 95%CI: 46.1-49.4mm, p=0.689; females: 41.6mm, 95%CI: 40.3-43.0mm, p=0.607). The results of the present study suggest that femoral offset can be accurately and reliably predicted from AP pelvis radiographs in patients with primary end-stage hip osteoarthritis. Our findings support the surgeon in pre-operative templating and may improve offset and limb length restoration in THA without the routine performance of CT.
The viscosity and shear stress versus shear rate relationship for pure bovine serum (BS) and its aqueous solutions with and without carboxymethyl cellulose (CMC) were investigated. BS and diluted BS without CMC showed pseudoplastic flow curves up to a critical shear rate of ∼100 s-1 above which a Newtonian flow with significant rise in shear stress was observed. The viscosity flow curve for the diluted BS+5g CMC showed only shear thinning up to a shear rate of 3000 s-1 whereas diluted BS+1g or +2g CMC showed similar flow curves to pure BS. The shear rate application modified the flow behaviour of BS from a pseudoplastic to a Newtonian flow depending on its purity and CMC content. Friction factor was dependent on viscosity and clearance with mixed lubrication as the dominant mode within the viscosity range 0.001-0.044 Pas. Pure BS and diluted BS are used as in vitro lubricants for tribological studies. Boundary, mixed and fluid film lubrication are the mechanisms involved in the lubrication of both natural and artificial joints. Clearance and lubricant viscosity will influence the nature of contact between the articulating surfaces. The objectives of this work were to study the flow properties of serum-based lubricants with different viscosities and the correlation between the rheological properties and frictional (and lubrication) behaviour of large diameter Biomet ReCaps with various clearances.ABSTRACT
INTRODUCTION
The friction and lubrication behaviour of four Biomet ReCap components with a nominal diameter of 52 mm and diametral clearance ranging from 167-178 μm were investigated using a friction hip simulator. Friction testing was carried out using pure bovine serum and aqueous solutions of bovine serum (BS), with and without carboxymethyl cellulose (CMC), adjusted to a range of viscosities (0.001-0.236 Pas). The Stribeck analyses suggested mixed lubrication as the dominant mode with the lowest friction factor of 0.07 at a viscosity of 0.04 Pas. The femoral resurfacing systems provide an alternative to hemi and total hip arthroplasty and offer several unique advantages including large resurfacing heads (>35–60 mm diameter) allowing increased range of motion (and stability) over the traditional 28 mm artificial hip joints, with excellent tolerances and surface finish leading to a reduction in wear, as well as preserving primary bone with the femoral canal remaining untouched. This work has investigated the friction and lubrication behaviour of four 52 mm metal-on-metal Biomet ReCap components with a clearance of 167-178 μm using serum-based lubricants.ABSTRACT
INTRODUCTION
