Computational Techniques to Predict Orthopaedic Implant Alignment and Fit in Bone

Among the broad palette of surgical techniques employed in the current orthopaedic practice, joint replacement represents one of the most difficult and costliest surgical procedures. While numerous recent advances suggest that computer assistance can dramatically improve the precision and long term outcomes of joint arthroplasty even in the hands of experienced surgeons, many of the joint replacement protocols continue to rely almost exclusively on an empirical basis that often entail a succession of trial and error maneuvers that can only be performed intraoperatively. Although the surgeon is generally unable to accurately and reliably predict a priori what the final malalignment will be or even what implant size should be used for a certain patient, the overarching goal of all arthroplastic procedures is to ensure that an appropriate match exists between the native and prosthetic axes of the articulation. To address this relative lack of knowledge, the main objective of this thesis was to develop a comprehensive library of numerical techniques capable to: 1) accurately reconstruct the outer and inner geometry of the bone to be implanted; 2) determine the location of the native articular axis to be replicated by the implant; 3) assess the insertability of a certain implant within the endosteal canal of the bone to be implanted; 4) propose customized implant geometries capable to ensure minimal malalignments between native and prosthetic axes. The accuracy of the developed algorithms was validated through comparisons performed against conventional methods involving either contact-acquired data or navigated implantation approaches, while various customized implant designs proposed were tested with an original numerical implantation method. It is anticipated that the proposed computer-based approaches will eliminate or at least diminish the need for undesirable trial and error implantation procedures in a sense that present error-prone intraoperative implant insertion decisions will be at least augmented if not even replaced by optimal computer-based solutions to offer reliable virtual “previews” of the future surgical procedure. While the entire thesis is focused on the elbow as the most challenging joint replacement surgery, many of the developed approaches are equally applicable to other upper or lower limb articulations

Rock-shape and its role in rockfall dynamics

Rockfall threaten infrastructure and people throughout the world. Estimating the runout dynamics of rockfall is commonly performed using models, providing fundamental data for hazard management and mitigation design. Modelling rockfall is made challenging by the complexity of rock-ground impacts. Much research has focused on empirical impact laws that bundle the rock-ground impact into a single parameter, but this approach fails to capture characteristics associated with the impact configuration and, in particular, the effects of rock-shape. While it is apparent that particular geological settings produce characteristic rock-shapes, and that different rock-shapes may produce characteristic runout dynamics, these aspects of rockfall are poorly understood. This study has focused on investigating the mechanics behind the notion that different rock-shapes produce characteristic runout dynamics and trajectories. The study combines field data on rockfall runout, trajectory and dynamics, laboratory analogue testing in controlled conditions, and numerical modelling of the influence of rock-shape. Initially rock-shape, deposition patterns and rockfall dynamics were documented at rockfall sites in Switzerland and New Zealand. This informed a detailed study of individual rock-ground impacts on planar slopes in which laboratory-scale and numerical rockfall experiments were combined to isolate the role of rock-shape on runout. Innovatively, the physical experiments captured the dynamics of impacts and runout paths using high speed video tracking and a sensor bundle with accelerometers and gyroscopes. Numerical experiments were performed using a 3-D rigid-body rockfall model that considers rock-shape, and has allowed the variability of rockfall behaviour to be explored beyond the limitations of physical experimentation. The main findings of the study were on understanding rockfall-ground impacts, the influence of rock-shape on rockfall dynamics, and influence of rock sphericity. By measuring velocity, rotational speed, impact and runout character, it has been possible to quantify the variability of individual rock-ground impacts as a function of rock-shape. Investigation of single rebounds reveals that if classical restitution coefficients are applied, $R_n$ values greater than unity are common and rebounds are highly variable regardless of constant contact parameters. It is shown that this variability is rooted in the inherent differences in the magnitudes of the principal moment of inertia of a rock body brought about by rock-shape. Any departure from a perfect sphere induces increased range and variability in rock-ground rebound characteristics. In addition to the popular description of a rock bouncing down slope, rebounds involve the pinning of an exterior edge point on the rock, creating a moment arm which effectively levers the rock into ballistic trajectory as it rotates. Observations reveal that the angle of the impact configuration plays a key role in the resulting rebound, whereby low angles produce highly arched rebounds, while large impact angles produce low flat rebounds. The type of rebound produced has a strong bearing on the mobility of the rocks and their ability to maintain motion over a long runout. The mobility of rocks is also shown to be related to rotation, which is governed by the differences in the principal inertial axes as a function of rock-shape. Angular velocity measurements about each principal inertial axis indicate that rocks have a tendency to seek rotation about the axis of largest inertia, as the most stable state. Rotations about intermediate and small axes of inertia and transitions between rotational axes are shown to be unstable and responsible for the dispersive nature of runout trajectories, which are inherent characteristics of different rock-shapes. The findings of this research demonstrate the importance of rock-shape in rockfall runout dynamics and illustrate how it is essential that the rock-shape is included in rockfall modelling approaches if the variability of rockfall behaviour is to be simulated

Advanced modelling and design of a tennis ball

Modern tennis has been played for over a hundred years, but despite significant improvements in the design and manufacture of tennis balls to produce a long-lasting and consistent product, the design of a tennis ball has barely changed in the last century. While some work has been done to better understand the dynamic behaviour of a tennis ball, no structured analysis has been reported assessing how the typical constructions of the inner rubber core and cloth panels affect its behaviour and performance. This research describes the development of an advanced and validated finite element (FE) tennis ball model which illustrates the effects of the viscoelastic and anisotropic materials of a tennis ball on ball deformation and bounce during impacts with the ground and the racket,representative of real play conditions. The non-linear strain rate properties exhibited by the materials of a tennis ball during high velocity impacts were characterised using a series of experiments including tensile and compressive tests as well as low and high velocity impact tests. The impacts were recorded using a high speed video (HSV) camera to determine deformation, impact time, coefficient of restitution (COR) and spin rate. The ball material properties were tuned to match the HSV results, and the ball s model parameters were in good agreement with experimental data for both normal and oblique impacts at velocities up to 50 m/s and 35 m/s, respectively. A time sequenced comparison of HSV ball motion and FE model confirmed the accuracy of the model, and showed significant improvement on previous models. Although the existing construction of tennis ball cores was found to provide a sufficiently uniform internal structure to base competition standard tennis balls, the anisotropic nature of the cloth panels resulted in deviation angles as high as 1.5 degrees in ball bounce. Therefore, new cloth panel configurations were modelled which allowed the cloth fibre orientations around the ball to be adjusted resulting in better bounce consistency. The effect of cloth seam length on ball flight was explored through wind tunnel tests performed on solid balls made by additive manufacturing (AM) and on actual pressurised tennis ball prototypes. A reverse Magnus effect was observed on the AM balls, however, this phenomenon was overcome by the rough nature of the cloth cover on the real tennis ball prototypes. A ball trajectory simulation showed that there was no obvious dependence between seam length and shot length or ball velocity. Finally, a basic panel flattening method was used to determine the 2Dsize of the cloth panel patterns corresponding to the new configurations, and tiling methods were designed to estimate cloth wastage. The traditional dumbbell design appeared to result in the minimum amount of waste. The work reported in this thesis represents a significant improvement in the modelling of tennis ball core, cloth and seams, as well as the ball s interaction with the ground and racket strings. While this research focused on woven cloth, needle cloth is also widely used in the manufacture of balls in the US. The modelling of needle cloth could therefore be part of a future study. Additionally, details such as the depth and roughness of the cloth seam could be included in the model to study their effect on spin generation. Also, including cloth anisotropy in the flattening method would allow a better prediction of cloth wastage which could then have an influence on the configuration of the cloth panels

Multiple Representation Approach to Geometric Model Construction From Range Data

A method is presented for constructing geometric design data from noisy 3-D sensor measurements of physical parts. In early processing phase, RLTS regression filters stemming from robust estimation theory are used for separating the desired part of the signal in contaminated sensor data from undesired part. Strategies for producing a complete 3-D data set from partial views are studied. Multiple representations are used in model construction because there is no single representation that would be most appropriate in all situations. In particular, surface triangulation, NURBS, and super-ellipsoids are employed in order to represent efficiently polygonal and irregular shapes, free form surfaces and standard primitive solids. The size of the required control point mesh for spline description is estimated using a surface characterization process. Surfaces of arbitrary topology are modeled using triangulation and trimmed NURBS. A user given tolerance value is driving refinement of the obtained surface model. The resulting model description is a procedural CAD model which can convey structural information in addition to low level geometric primitives. The model is translated to IGES standard product data exchange format to enable data sharing with other processes in concurrent engineering environment. Preliminary results on view registration using simulated data are shown. Examples of model construction using both real and simulated data are also given

Combined numerical and experimental investigation of transmission idle gear rattle

Gear rattle is caused by engine torsional vibration (engine order response) imparted to the transmission components, further causing the gears to oscillate within their functional backlashes. These oscillations lead to the repetitive impact of gear teeth, which lead to noisy responses, referred to as gear rattle. The lack of in-depth research into the effect of lubricant on gear rattle has been identified as a deficiency in the previous research in rattle. The aim ofthe current work is to address this shortcoming. The thesis outlines a new approach in investigating the problem of idle gear rattle. The approach is based on the assumption that under idling condition the teeth-pair impact loads are sufficiently low and the gear speeds are sufficiently high to permit the formation of a hydrodynamic lubricant film between the mating gear teeth. This film acts as a non-linear spring-damper that couples the driver and the driven gears. A torsional single-degree of freedom model is used in the development of the theory. The model is then expanded into a seven-degree of freedom torsional model and finally into an Il-degree of freedom model that also includes the lateral vibrations of the supporting shafts. The Il-degree of freedom model is based on a real life transmission that is also used in experimental studies to validate the model. It is found that lubricant viscosity and bearing clearance (lubricant resistance in squeeze) play important roles in determining the dynamics of the system and its propensity to rattle. At low temperatures, the lateral vibrations of the shafts, carrying the gears interfere with the gear teeth impact action. The severity of rattle is determined by the relationship between the entraining and squeeze film actions of the hydrodynamic film. When the latter dominates, the system can rattle more severely. The numerical results are found to correlate well with the experimental findings obtained from vehicle tests in a semi-anechoic chamber and also with those from a transmission test rig in the powertrain laboratory

Experimental investigation of collisional properties of spheres

We present experimental results on the collisional properties of spheres obtained through high-speed video analysis. An apparatus is built that produces collisions of spheres of various sizes with a wide range of impact, velocities and incidence angles. Edge detection techniques are implemented to track the position of the spheres from frame to frame whereby the translational velocities may be computed. In order to determine the rotational velocities, small markers are imprinted on the surfaces of the spheres and also tracked and matched from one frame to the next.. From the pre and post collision kinematic data, three collisional properties are directly extracted: the coefficient of restitution in the normal direction of impact, the coefficient. of friction and the coefficient of restitution of the relative tangential velocity. These measurements substantiate an existing impact model predicting exclusively rolling and sliding collisions. Finally the dependence of the coefficient of restitution on the magnitude of the normal impact velocity is studied for two different, materials which both exhibit different behaviors from what available theoretical results predict. We could not observe any size dependence of the coefficient, of restitution. This is due to the limited accuracy of our measurements but also to the possible sensitivity of the coefficient of restitution to the angle of incidence. However softer materials should provide more conclusive results

Brain injury mitigation effects of novel helmet technologies in oblique impacts

Cyclists are a rapidly growing group of the world population, particularly after the COVID-19 pandemic which made cycling an attractive form of active mobility for commuters. Yet, cyclists are among the most vulnerable road users. Their severe injury and fatality rate per passenger mile are several folds larger than car occupants and bus passengers. Analysis of accident data shows that impacts to a cyclist’s head occur at an angle in vast majority of real-world head collisions. This produces large rotational head motion. There is significant body of research that shows rotational head motion is the key determinant of brain deformation and subsequent damage to the brain tissue. Hence, novel helmet designs adopt shear-compliant layers within a helmet with the aim of reducing the rotational head acceleration and velocity during an impact, hence reducing risk of brain injury. Cellular materials can be engineered to have interesting mechanical properties such as negative Poisson ratio or anisotropy. Their cellular structure gives rise to a unique combination of properties which are exploited in engineering design: their low density makes them ideal for light-weight design, and their ability to undergo large deformations at relatively low stresses make them ideal for dissipating kinetic energy with near-optimal deceleration. As revealed in this thesis, it also is possible to engineer cellular structures to have high or low shear stiffness with minimal change to their axial stiffness, and vice versa. This has the potential to be very beneficial for cases that require oblique impact management where both axial and shear stiffnesses play a role. However, this domain has seldom been explored, let alone applied to a use case which may result in improved performance that saves lives such as helmets. The main question this thesis aims to address is: Can helmets be improved to reduce the risk of cyclist brain injury in oblique impacts? To answer this question, it was necessary to first assess conventional helmets and emerging technologies aiming to improve helmets in oblique impacts. Hence, 27 bicycle helmets with various technologies were assessed in three different oblique impact conditions. The outcome of studying this proved that helmets may be improved with shear compliant mechanisms between the head and helmet. However, the improvements were marginal and highly dependent on impact site. This is hypothesised to be due to the presence of expanded polystyrene (EPS) foam alongside these shear-compliant mechanisms which hinders their performance. We found that one of the best performing helmets in oblique impacts was one that utilises air and entirely replaces EPS foam yet had some drawbacks such as lack of reusability and shell structure. This encouraged the work that followed which aimed to replace the EPS foam layer in helmets with an air-filled rate-sensitive cellular structure. This work leveraged finite element modelling which employed visco-hyperelastic material models which were validated with axial and oblique impact tests of the bulk material and cellular array samples different speeds. The novelty is that the axial and shear stiffness of the cells could be tailored independently with simple changes to the geometry of the cells. This led to an exciting investigation to determine whether shear-compliant cells outperformed their shear-noncompliant counterparts, which exhibit similar axial stiffness, with respect to brain injury metrics in a helmet. The results showed that, although this may be the case, often the shear-compliant cells dissipated less energy during impact and bottomed-out as a result, leading to adverse effects. Hence, introduction of shear-complaint structures in helmets should be done with care as the energy is dissipated in shear with such cellular structures during oblique impacts which needs to be properly managed. In future, the performance improvements may be implemented for different impact speeds utilising the viscoelastic nature of the cells and inflation of the cells to change their shape.Open Acces

Dynamic loads analysis system (DYLOFLEX) summary. Volume 1: Engineering formulation

The DYLOFLEX computer program system expands the aeroelastic cycle from that in the FLEXSTAB computer program system to include dynamic loads analyses involving active controls. Two aerodynamic options exist within DYLOFLEX. The analyst can formulate the problem with unsteady aerodynamics calculated using the doublet lattice method or with quasi-steady aerodynamics formulated from either FLEXSTAB or doublet lattice steady state aerodynamics with unsteady effects approximated by indicial lift growth functions. The equations of motion are formulated assuming straight and level flight and small motions. Loads are calculated using the force summation technique. DYLOFLEX consists of nine standalone programs which can be linked with each other by magnetic files used to transmit the required data between programs