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Background: 2 mm wide Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) tape improves contact pressure at root repair sites compared to high strength suture and provides stronger repair construct. UHMWPE tape is commonly used in rotator cuff repair and fixation is often achieved with knotless suture anchors. For meniscal root repair the optimal method for tape fixation has not been established.Purpose: We hypothesised that the use of suture anchors for the tibial fixation of 2 mm UHMWPE tape trans-osseous root repairs would lead to better biomechanical performance compared to other fixation methods.Methods: In 25 porcine knees the medial meniscus posterior root attachment was divided, a standardised trans-tibial root repair performed with 2 mm UHMWPE tape and testing performed by cyclic loading followed by load to failure. Tibial fixation was randomised to 5 tibial fixation types: i) Cortical Fixation Button, ii) Pound-in Suture Anchor with screw down interference suture locking iii) Tap-in Suture Anchor with inner locking plug iv) Post Screw v) Post Screw and Washer. Results: There was not difference in displacement during cyclic loading between tibial fixation groups, but a highly significant difference in the max. load at failure. Repairs in both suture anchor fixation groups all failed by tape slippage at relatively low loads (median 145 N and 116 N) respectively. Repairs tied over a cortical button, post screw or screw and washer failed by tape breakage at loads of 421 N, 405 N and 531 N. Conclusion: For meniscal root repairs with 2 mm UHWMPE tape, use of suture anchors offers weaker fixation compared to tying over a button or post screw/washer. Whilst suture anchor fixation may be adequate for non-weight bearing post-operative protocols it may not allow for more accelerated weight bearing.Keywords: Tibial fixation; Knotless anchors; Mechanical testing; Failure load; Meniscus root repair

INTRODUCTIONMusculoskeletal models of the cervical spine are valuable tools to assess the intervertebral loads and injury mechanisms of sporting head impacts and provide an intermediate step between in-vivo experimental trials and detailed finite element analysis. A previous musculoskeletal model created for estimating intervertebral joint loading during rugby contact [1] events included kinematic constraints, which neglected individual joint translation and therefore may oversimplify the behavior of the cervical spine during impacts. In order to both confidently estimate the internal loading and resulting vertebral kinematics, intervertebral viscoelastic (“bushing”) elements should be included in the model. Such viscoelastic elements are used to describe the behaviour of the joints’ passive structures [2] and allow for resulting motions to be calculated compared to kinematic constraints. A key challenge is to identify the optimal bushing element that can generate reliable kinematics of the model’s cervical spine with respect to the load applied. Therefore, the aim of this study was to assess the performance of anteroposterior shear viscoelastic elements, that were validated together with compressive elements for impulsive axial loading conditions, under loads containing shear components. METHODSAxial and anteroposterior viscoelastic bushing parameters were estimated via an optimisation procedure in a combined in-vitro and in-silico study. The optimisation procedure that identified the parameters included forward dynamic simulations of five specimen specific models that were driven by the experimental axial loads in order to replicate the in-vitro experiment. We carried out a 1000 sample Monte-Carlo analysis that perturbed the viscoelastic parameters by ±50% from their optimised values to evaluate the models’ sensitivity to a range of parameter values under the same axial loads. This method classified which parameters had the greatest effect on the models’ performance. The initial parameters estimated by the optimisation procedure were then implemented in the C2-C3, C3-C4, C4-C5 and C5-C6 joints of the population specific musculoskeletal model [1]. Forward dynamic simulations were executed with the updated musculoskeletal model of the cervical spine and head positioned parallel to the horizontal. An initial simulation with a constant anterior shear load of 250 N applied to the base of the head was completed to compare the model’s anteroposterior joint displacement against available experimental data. A second simulation investigated a possible injurious loading scenario that could be experienced in a head first impact in rugby. Experimental loads were collected from an instrumented ATD headform during live scrummaging trials. The compression flexion load was scaled to included maximum values of 1100 N and 700 N for anterior shear and compression respectively. A final simulation applied impulses with higher maximal values of 2500 N and 1500 N for shear and compressive forces respectively after the experimental load was scaled further. For all simulations, peak values of the applied loads were reached in 60 ms after an initial 40 ms latent period. No muscle activation was prescribed to reduce the active muscle force effects but their passive contribution remained.RESULTS AND DISCUSSIONThe values of the bushing parameters identified by the optimisation increased from their respective initialised values and were 23.1 MN/m for axial stiffness, 4300 Ns/m for axial damping, 75.9 kN/m for shear stiffness and 1400 Ns/m for shear damping. Perturbations in shear damping and axial stiffness had the most effect on model performance. Decreasing values of shear damping resulted in larger error in the sensitivity analysis (Figure 1). Axial stiffness also showed an effect of increasing or decreasing values however this effect was smaller than the one observed in shear damping. This is possibly caused by the considerably higher values of axial stiffness (23.1 MN/m) compared to shear damping (1400 Ns/m), which, even when perturbed by – 50 %, remain in the same order of magnitude. On the contrary, the same proportional perturbation of the optimised shear damping reduces the value by an order of magnitude. During these impulsive axial loads applied to the cervical spines in the experiments a buckling response was observed as previously observed by [3]. The anterior shear motion of the vertebrae caused by the loading of the specimens in the current study, however, did not lead to injuries because the applied load was chosen to be sub-catastrophic. The sensitivity analysis thus highlighted the importance of correct choice for anteroposterior joint damping parameters used in musculoskeletal model of the cervical as lower values chosen for axial impacts may result in misrepresentative kinematic responses of the model cervical joints in anteroposterior translation. The three forward dynamic simulations resulted in anterior cervical spine joint displacements representative of the loads applied to the base of the head segment (Figure 2). Anterior displacements of the C2-C3, C3-C4, C4-C5 and C5-C6 joints ranged from 3 to 3.5 mm under the 250 N pure shear load applied to the base of the head. These values are in line with the 2 mm anterior joint displacements of three-segment specimens produced under non-catastrophic quasi-static experimental loads of 220 N recorded by [4]. During the second impact simulation, the cervical joints displaced anteriorly by 9 to 10 mm. The simulation that reached 2500 N anterior shear load resulted in maximal anterior displacements of 13 to 19 mm. Anterior shear failure tests on porcine cervical specimens have shown maximal displacements of 16 to 22 mm under loads of 2300 to 3500 N [5]. It should be noted that both studies used ex-vivo specimens with musculature removed, which is a simplification of the complex mechanism of the cervical spine. Also, the clinical presentation of cervical spine injuries sustained in-vivo by similar loading patterns are heterogeneous [6], making the identification of injury mechanism challenging.CONCLUSIONSCervical spine musculoskeletal models used in the analysis of axial impacts are sensitive to anteroposterior joint shear damping parameter values and that shear values estimated under such axial impacts provide a viable approach to analyse loads representative of sport collisions.

Objectives Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure. Materials and Methods A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix). Results For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy. Conclusion We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure.

Agri-Food is the largest manufacturing sector in the UK. It supports a food chain that generates over £108bn p.a., with 3.9m employees in a truly international industry and exports £20bn of UK manufactured goods. However, the global food chain is under pressure from population growth, climate change, political pressures affecting migration, population drift from rural to urban regions and the demographics of an aging global population. These challenges are recognised in the UK Industrial Strategy white paper and backed by significant investment via a Wave 2 Industrial Challenge Fund Investment (“Transforming Food Production: from Farm to Fork”). Robotics and Autonomous Systems (RAS) and associated digital technologies are now seen as enablers of this critical food chain transformation. To meet these challenges, this white paper reviews the state of the art in the application of RAS in Agri-Food production and explores research and innovation needs to ensure these technologies reach their full potential and deliver the necessary impacts in theAgri-Food sector.The opportunities for RAS range include; the development of field robots that canassist workers by carrying payloads and conduct agricultural operations such as crop and animal sensing, weeding and drilling; integration of autonomous systems technologies into existing farmoperational equipment such as tractors; robotic systems to harvest crops and conduct complex dextrous operations; the use of collaborative and “human in the loop” robotic applications to augment worker productivity; advanced robotic applications, including the use of soft robotics, to drive productivity beyond the farm gate into the factory and retail environment; and increasing the levels of automation and reducing the reliance on human labour and skill sets, for example,in farming management, planning and decision making. RAS technology has the potential totransform food production and the UK has an opportunity to establish global leadership within the domain. However, there are particular barriers to overcome to secure this vision:1. The UK RAS community with an interest in Agri-Food is small and highly dispersed. There is an urgent need to defragment and then expand the community.2. The UK RAS community has no specific training paths or Centres for Doctoral Training to provide trained human resource capacity within Agri-Food.3. While there has been substantial government investment in translational activities at high Technology Readiness Levels (TRLs), there is insufficient ongoing basic research in Agri-FoodRAS at low TRLs to underpin onward innovation delivery for industry.4. There is a concern that RAS for Agri-Food is not realising its full potential, as the projects being commissioned currently are too few and too small-scale. RAS challenges often involve the complex integration of multiple discrete technologies (e.g. navigation, safe operation, grasping and manipulation, perception). There is a need to further develop these discrete technologies but also to deliver large-scale industrial applications that resolve integration and interoperability issues. The UKcommunity needs to undertake a few well-chosen large-scale and collaborative “moon shot” projects.5. The successful delivery of RAS projects within Agri-Food requires close collaboration between the RAS community and with academic and industry practitioners. For example, the breeding of crops with novel phenotypes, such as fruits which are easy to see and pick by robots, may simplify and accelerate the application of RAS technologies. Therefore, there is an urgent need to seek new ways to create RAS and Agri-Food domain networks that can work collaboratively to addresskey challenges. This is especially important for Agri-Food since success in the sector requires highly complex cross-disciplinary activity. Furthermore, within UKRI many of the Research Councils and Innovate UK directly fund different aspects of Agri-Food, but as yet there is no coordinated and integrated Agri-Food research policy per se.Our vision is a new generation of smart, flexible, robust, compliant, interconnected robotic and autonomous systems working seamlessly alongside their human co-workers in farms and food factories. Teams of multi-modal, interoperable robotic systems will self-organise and coordinatetheir activities with the “human in the loop”. Electric farm and factory robots with interchangeable tools, including low-tillage solutions, soft robotic grasping technologies and sensors, will support the sustainable intensification of agriculture, drive manufacturing productivity and underpin future food security. To deliver this vision the research and innovation needs include the development of robust robotic platforms, suited to agricultural environments, and improved capabilities for sensing and perception, planning and coordination, manipulation and grasping, learning and adaptation, interoperability between robots and existing machinery, and human-robot collaboration, including the key issues of safety and user acceptance.Technology adoption is likely to occur in measured steps. Most farmers and food producers will need technologies that can be introduced gradually, alongside and within their existing production systems. Thus, for the foreseeable future, humans and robots will frequently operate collaboratively to perform tasks, and that collaboration must be safe. There will be a transition period in which humans and robots work together as first simple and then more complex parts of work are conducted by robots, driving productivity and enabling human jobs to move up the value chain.

Background Despite the evolution of Ventricular Assist Devices (VADs), VAD patients still suffer from complications, often due to damage to the blood by fluid dynamic stress. To date, most numerical models for blood damage are functions of the scalar shear stress (SSS), the second invariant of the strain rate. Since rotary VADs are assumed to exert mainly shear stress, the measurements of blood damage for these models are obtained from shear flow experiments. However, measurements of cell deformation show elongational and shear stress deform cells differently, and then potentially damage cells differently. Aim The aim of this work was to use computational fluid dynamics (CFD) to assess the significance of elongational stress, in comparison with shear stress, in rotary VADs.MethodsCFD was used to calculate flow fields in a centrifugal and an axial VAD. The velocity of the blood defined the reference frame, with both stresses computed from the transformed strain rate. Firstly, volumes of the VADs experiencing shear or elongational stress above threshold values were found. And secondly, the cell deformation index (DI= (L-W)/ (L+W); given the cell’s length, L, and width, W) was set to 0.5, and the regions of the VADs producing DI > 0.5 due to shear or elongation stress were compared.Results Compared with elongation stress, blood in the VADs experiences higher shear stress over a larger volume. However, when comparing the stress using a threshold value for cell deformation, elongational stress occurs in a smaller but significant volume of both VADs (significant elongational stress volumes: centrifugal 0.2 cm3, axial 0.15 cm3, compared with significant shear stress volumes: centrifugal 0.3 cm3, axial 0.5 cm3). Conclusion Although shear stress volumes are larger than elongational volumes for both VADs, the latter are still significant in size. Crucially, given that the axial design reduces the significant elongational stress volume, but increases that for shear stress, more experimental data is needed on elongational stress-induced damage, with which to inform the design of new VADs.

IntroductionOsteoporosis is the most common bone disease, contributing to over 3.5 million fragility fractures each year in the EU [1]. Managing these fractures costs €37 billion each year with a 25% increase predicted by 2025. Fracture fixation in reduced density osteoporotic bone is a surgical challenge and failures are common, occurring in 15-40% of cases [2]. It is hypothesised that the torque used to fasten bone screws during fracture fixation significantly influences the holding strength of screws and thus the likelihood of fixation failure. The aim of this study was to develop a validated finite element model capable of predicting the effect of different insertion torques on the holding strength of cortical bone screws for a range of bone densities and cortical thicknesses.MethodsExperimental tests (n=100) were conducted to evaluate cortical screw pull-out strength using bovine tibiae (4-5 months). The bone density (assessed using CT-images) and cortical thickness of each specimen were recorded. Screws were inserted using a range of insertion torques, and the pullout strength was measured for each (Instron 5967, High Widcomb, UK). Additionally, the effect of stress-relaxation was investigated by imaging the surface of the bone surrounding the screw immediately after screw fastening at logarithmic intervals up till 17 hours post-fixation. Strains were evaluated using digital image correlation (Ncorr v1.2, G.I.T., USA). An axisymmetric finite element (FE) model was developed based on the experimental tests. An idealized screw geometry incorporating screw threads was based on CT-images (Simpleware ScanIP, Exeter, UK). Using a maximum tensile strain failure criteria, (considering the tensile yield strain of cortical bone to be 1.0%), the FE model was used to predict the pull-out strength for the variables considered (Ansys 15.0, USA).ResultsSignificant stress relaxation was found to occur post-fixation, with the strain distribution appearing to be related to the proximity of the thread to the surface. There was good agreement in predicted and measured pullout force versus mean cortical thickness.DiscussionAlthough relatively simple, the developed validated model is capable of predicting cortical bone screw pullout strength for a range of bone densities and cortical thicknesses. This study found that significant stress-relaxation occurs post-fixation, in line with previous literature [3]. Currently, the effect of screw insertion and stress-relaxation is not included in the models. The relationship between fastening torque and axial screw preload is well defined [4], thus screw insertion torque can be reasonably approximated in this way. We aim to incorporate both of these effects to understand the post-fixation screw mechanics and further improve our modelling predictions. Our intention is to develop this model into a patient-specific tool providing surgeons with guidance regarding appropriate screw fastening torques for different bone qualities. This tool has the potential to enhance construct stability and reduce the incidence of failures. The developed model could also be used as a design tool for screws.References1. Hernlund et al, 2013, Arch Osteoporosis 8:136.2. Broderick et al., 2013, The Scientific World Jn. 5151973. Iyo et al. 2004. Journal of Biomechanics 37:1433–14374. Exp Meth in Ortho Biomech. Ch.8. Ed. Zdero, 2017.

IntroductionThe total hip replacement (THR) is the second most successful and cost-effective surgical procedure of all time. Over the last few decades patients have developed considerably higher expectations after joint replacement and are demanding earlier joint replacement surgery. The registry data from both Sweden and the UK show that cemented hip cups have a superior survival compared to cementless hip sockets. Much research has focused on the stem design and fixation, but the data shows that hip cup failure is the greater problem. The focus of this study is to improve methods of cemented cup fixation through validation experiments and finite element analysis (FEA).MethodsExperimental testing was conducted on five Sawbones composite pelves (4th generation, model 3405). A custom 3D printed jig was used to drill four 8 mm bolt holes at the superior part of the sacroiliac joint, used later for clamping. The pelves were reamed to a diameter of 60 mm in a CNC machine. The cups were machined out of a bar of UHMWPE (TIVAR 1000) to inner and outer diameters of 28 mm and 54 mm respectively. Surgical Simplex P cement (Stryker) was vacuum mixed at room temperature, to fix the cup to the acetabulum. A custom jig ensured a mantle thickness of 3 mm and cup positioning of 45° abduction and 10° anteversion. [1] Each composite pelvis was instrumented with triaxial strain gauges (SGD-2/350-RY63, Omega Engineering Limited) at four different locations of high predicted strain. The samples were bolted securely in the uniaxial testing machine (Series 5965 with a 5 kN load cell; Instron) with the loading vector F in the direction of the peak force during normal walking.After preconditioning load was applied to the each construct through a 28 mm metal head up to 500 N in increments of 50 N, for a total of five repetitions. The directional surface strains were used to evaluate the equivalent strain. Specimen specific finite element models were developed based on CT scan data using ScanIP (2016.09 Synopsis Inc.). The images were obtained using a micro CT scanner (XTH225ST, Nikon Metrology UK). The mesh (Figure 2) consisted of an average of 760 thousand linear tetrahedral elements and was solved in ANSYS APDL 15.0.Results and DiscussionThe equivalent strain at each location was repeatable for each pelvis but varied within the group. This could be caused either by the variation in mechanical properties between the pelves, or the sensitivity of the gauges to positioning. The agreement between the predicted and experimental equivalent strains was good. Hence, validating this specific methodology of conducting finite element analyses of the pelvis based on CT image data. The next stage is to use the same methodology to develop a patient specific FEA model, including a bone remodeling algorithm and muscle forces, based on the CT images already obtained (VHP). This model will be used to optimize the cemented fixation and would be verified experimentally using composite pelves. This research is aimed at informing clinical practice and enhancing longterm cemented fixation. Reducing the need for revision surgery will greatly improve patient quality of life, whilst also reducing the burden on the healthcare delivery system.References1. Zhang QH et al, J Biomech 43(14):2722-2727, 20102. Heller M et al, J Biomech 34(7):883-893, 2001

IntroductionThe aim of this study was to assess the effect of assembly force on the strength of head-trunnion interface and to measure the initial displacement of the head on the trunnion with different assembly forces.MethodsThree assembly load levels were assessed (A:2kN, B:4kN, C:6kN). The stems were mounted in a custom rig and the respective assembly loads were applied to the head at a constant rate of 0.05kN/s (ISO7260-10:2003). Load levels were recorded during assembly. Head displacement was measured with a laser sensor. The disassembly force was determined by a standard pull-off test.ResultsThe maximum head displacement on the trunnion was significantly different between the 2 kN group and the other two groups (4kN, 6kN, p=0.029), but not between the 4kN and 6kN groups (p=0.89). The disassembly forces between the three groups were significantly different (mean ± SD, A:1316±223kN; B:2224±151kN; C:3965±344kN; p= 0.007), with increasing assembly load leading to a higher pull-off force. For the 4kN and 6kN groups, a first peak of approximately 2.5kN was observed on the load recordings during assembly before the required assembly load was eventually reached corresponding to sudden increase in head displacement to approximately 150µm. DiscussionAn assembly force of 2kN may be too low to overcome the frictional forces needed to engage the head and achieve maximum displacement on the trunnion and thus an assembly load of greater than 2.5kN is recommended.

Introduction: 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.Materials & Methods: 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 et al. for a patient sitting down (reported to put the femoral head at greatest risk of collapse [2]), with a peak pressure of 9.4 MPa and an average pressure of 1.59 MPa. The femur was fixed in all degrees of freedom at a plane through the femoral neck. The X-Bolt and SHS were implanted in accordance with the operative techniques. The femoral head and implants were meshed with quadratic tetrahedral elements, and cortical bone was meshed with triangular thin shell elements. A converged mesh seeding density of 1.2 mm was used. All models were create and solved using ABAQUS finite element software (version 6.12, Simulia, Dassault Systèmes, France). The influence of implant type and presence was examined alongside a variety of patient factors: osteonecrosis (modelled as a cone of bone of varying angle, and varying modulus values), cortical thinning, reduced cortical modulus, and 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].Results: No significant difference was found between the X-Bolt and the SHS, for either critical buckling pressure (p=0.964), or the maximum von Mises stress (p=0.274), indicating no difference in the risk of femoral head collapse. The maximum von Mises stress (and therefore the risk of collapse) within the cortical bone was significantly higher for the intact femoral head compared to both implants (X-Bolt: p=0.048, SHS: p=0.002). Of the factors examined, necrosis of the femoral head caused the greatest increase in risk.Discussion: The study by Volokh et al. [1] concluded that deterioration of the cancellous bone underneath the cortical shell can greatly increase the risk of femoral head collapse, and the results of the present study support this finding. Interestingly the presence of either an X-Bolt or SHS implant appeared to reduce the risk of femoral head collapse.References: [1] Volokh, K. Y. et al. “Prediction of femoral head collapse in osteonecrosis.” (2006) J Biomech Eng 128, p467-470.[2] Yoshida, H. et al. “Three-dimensional hip contact area and pressure distribution during activities of daily living” (2005) J Biomech 39, p1996-2004.

Introduction: 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.Materials & Methods: 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 (HU) values to apparent density (papp), and to convert the apparent density to elastic modulus (E) are shown in Equations 1&2 [2]. Equation 1: papp = -0.021075 + 0.000786 HU Equation 2: E = 2.0173 papp 2.46The 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.Results: 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.Discussion: 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. References: [1] Taddei, F. et al. “The material mapping strategy influences the accuracy of CT-based finite element models of bones: An evaluation against experimental measurements” (2007) Med Eng Phys 29, p973-979.[2] Anderson, A.E. et al. “Subject-Specific Finite Element Model of the Pelvis: Development, Validation and Sensitivity Studies” (2005) J Biomech Eng 127, p364-373.[3] Rho, J.Y. et al. “Young's modulus of trabecular and cortical bone material: ultrasonic and microtensile measurements.” (1993) J Biomech 26 p111-119

This study investigated whether increased loading (representing obesity) in the extended knee and flexed knee led to increased stresses in areas of typical medial and lateral osteoarthritis cartilage lesions, respectively. We created two paired sets of subject-specific finite element models; both sets included models of extended knees and of flexed knees. The first set represented normal loading; the second set represented increased loading. All other variables were held constant. The von Mises stresses and contact areas calculated on the tibial cartilage surfaces of the paired models were then compared.In the extended knee models, applying a larger load led to increased stress in the anterior and central regions of the medial tibial cartilage. These are the typical locations of medial osteoarthritis cartilage lesions. Therefore, the results support that increased loading in the extended knee may result in medial osteoarthritis.In the flexed knee models, applying a larger load increased stress in the anterior and central regions of the lateral tibial cartilage. Lateral osteoarthritis cartilage lesions typically occur centrally and posteriorly. Therefore, these results do not support our hypothesis. Shear stress was increased in areas of typical lateral lesions, however, and should be investigated in future studies.

Introduction The majority of experimental biomechanical studies relating to the proximal femur use synthetic composite test specimens because of the advantages in terms of cost, availability and preservation. Studies report significantly (p

Femoral acetabular impingement (FAI) is thought to be a key underlying reason for the development of osteoarthritis of the hip. There are two main types of FAI, cam-type and pincer-type. The cam-type FAI gives riseto cartilage delamination initially thought to occur on the acetabular side of the joint. The purpose of the current study was to look at the effects of cam-type impingement on the generation of shear strains at the bone/cartilage interface, using both experimental and finite element simulation methods. Sagittal slices (n=9) of femoral porcine cartilage-bone, 10 mm thick, were loaded using a five-axis custom test machine with a curved (radius 90 mm) steel indenter. The five-axis test machine allowed the samples to be subjected to compression and mixed compression/shearloading regimens. The specimen strains were measured using twodimensional digital image correlation (DIC). Each test was also simulated using finite element analysis, and the results compared with the DIC data.The specimens were then cyclically loaded either with or without damage to the cartilage layers; damage simulated clinically reported lesions.Maximal shear strain was found at the cartilage-bone interface, and was a function of compressive loading level. The finite element predictions matched the DIC measurements. The two parameters that were mostimportant in terms of shear strain were the cartilage thickness and contact area radius. It was found that increased cartilage thickness and increased contact radius gave rise to higher shear strains. Cyclically loading the damaged specimens produced features of cartilage delaminationconsistent with clinical observations. The results of this study indicate high shear strain at the bone/cartilage interface is a possible mechanism leading to cartilage delamination, and may be the mechanism behind cartilage degradation in patients with cam-type FAI.

Introduction and Objectives: Tibial fracture is a possible complication after unicompartmental knee replacement (UKR) which can have severe consequences for patient recovery and outcome [1]. It appears that the issue is not product specific, as peri-prosthetic fractures have been reported in numerous designs, both mobile and fixed. However, it has been suggested that cementless components might be at greater risk than cemented [2]. The exact causes of tibial fracture are unknown, although surgical factors are most commonly proposed in the literature [1,3]. The objectives of the study were to; (1) determine the range of positions and depths of the surgical cuts required to prepare the tibial plateau for a UKR, (2) use the measured parameters to create a representative range of finite element models, (3) statistically assess the influence of each surgical parameter on the risk of fracture. Methods: Tibial plastic Sawbones (n=23) were prepared for mobile UKR during an instructional course. The parametersmeasured from the sawbones were: (a) the resection depth, (b) the angle between the horizontal and vertical cuts, (c) the distance between the vertical wall and the keel slot, how excessively deep the vertical cut and horizontal cuts were anteriorly (d and e, respectively), and posteriorly (f and g, respectively), and (h) the depth of the pin hole (Figure 1). A parametric finite element model was created in ABAQUS software (v6.12, Dassault Systèmes) with an automated python script to create the surgical cuts. One hundred models were created, where the surgical cut parameters were varied within the distributions measured from the Sawbones. A mesh element size of 2.4 mm was used, selected as a result of a mesh convergence study. The tibia was modelled as a heterogeneous linear elastic material, with a Poisson's ratio of 0.3. The modulus of each element was assigned based upon the corresponding position of that element in the CT scan of the tibia. The equations used for this have been previously defined and the tibial model validated [4]. Muscle and joint loading of a tibia at 15% of the gait cycle was applied, corresponding to maximal medial contact force, and the distal portion of the tibial constrained in all degrees of freedom. The risk of fracture was quantified based upon the Maximum Principal Stress criterion equations defined by Schileo et al. [5]. The influence of each surgical parameter on the risk of fracture was assessed using linear regression with R (r-project). Results: In the tibial Sawbone measurements, the greatest surgical variation was observed in the depth of the posterior vertical cut and the pin hole, which had standard deviations of 3.9 and 6.8 mm respectively (Table 1). The only surgical cut parameters which were found to significantly affect the risk of fracture were the resection depth, and the posterior depth of the vertical cut (p=0.009, and p=0.000001, respectively). Some finite element models demonstrated a noticeable region at high risk of fracture, which extended diagonally from the vertical saw cut, past the base of the keel slot to the tibial cortex. This matched well with typical fracture paths observed clinically [1]. Conclusion: This study has shown accuracy in the depth of the vertical cut made to prepare the tibial plateau for UKR, has the greatest clinical variation and has the greatest influence on the risk of fracture out of all the parameters assessed in this study. It is therefore important that instrumentation be designed to improve surgical accuracy for this part of the operative technique. References: [1] Pandit, H., et al., Orthopedics, 30: 28-31, 2007.[2] Seeger, J.B., et al., Knee Surg Sports Traumatol Arthrosc, 20: 1087-1091, 2012.[3] Clarius, M., et al., The Knee 16: 314-316, 2009.[4] Gray, H.A., et al., J Biomech Eng, 130: 031016, 2008.[5] Schileo, E., et al., J Biomech, 41: 356-367, 2008.