Sensitivity of hip tissues contact evaluation to the methods used for estimating the hip joint center of rotation

Computer-based simulations of human hip joints generally include investigating contacts happening among soft or hard tissues during hip movement. In many cases, hip movement is approximated as rotation about an estimated hip center. In this paper, we investigate the effect of different methods used for estimating hip joint center of rotation on the results acquired from hip simulation. For this reason, we use three dimensional models of hip tissues reconstructed from MRI datasets of 10 subjects, and estimate their center of rotation by applying five different methods (including both predictive and functional approaches). Then, we calculate the amount of angular and radial penetrations that happen among three dimensional meshes of cartilages, labrum, and femur bone, when hip is rotating about different estimated centers of rotation. The results indicate that hip simulation can be highly affected by the method used for estimating hip center of rotation. However, under some conditions (e.g. when Adduction or External Rotation are considered) we can expect to have a more robust simulation. In addition, it was observed that applying some methods (e.g. the predictive approach based on acetabulum) may result in less robust simulation, comparing to the other method

Hybrid Hierarchical Collision Detection Based on Data Reuse

To improve the efficiency of collision detection between rigid bodies in complex scenes, this paper proposes a method based on hybrid bounding volume hierarchies for collision detection. In order to improve the simulation performance, the method is based on weighted oriented bounding box and makes dense sampling on the convex hulls of the geometric models. The hierarchical bounding volume tree is composed of many layers. The uppermost layer adopts a cubic bounding box, while lower layers employ weighted oriented bounding box. In the meantime, the data of weighted oriented bounding box is reused for triangle intersection check. We test the method using two scenes. The first scene contains two Buddha models with totally 361,690 triangle facets. The second scene is composed of 200 models with totally 115, 200 triangle facets. The experiments verify the effectiveness of the proposed method

A biomechanics-based articulation model for medical applications

Computer Graphics came into the medical world especially after the arrival of 3D medical imaging. Computer Graphics techniques are already integrated in the diagnosis procedure by means of the visual tridimensional analysis of computer tomography, magnetic resonance and even ultrasound data. The representations they provide, nevertheless, are static pictures of the patients' body, lacking in functional information. We believe that the next step in computer assisted diagnosis and surgery planning depends on the development of functional 3D models of human body. It is in this context that we propose a model of articulations based on biomechanics. Such model is able to simulate the joint functionality in order to allow for a number of medical applications. It was developed focusing on the following requirements: it must be at the same time simple enough to be implemented on computer, and realistic enough to allow for medical applications; it must be visual in order for applications to be able to explore the joint in a 3D simulation environment. Then, we propose to combine kinematical motion for the parts that can be considered as rigid, such as bones, and physical simulation of the soft tissues. We also deal with the interaction between the different elements of the joint, and for that we propose a specific contact management model. Our kinematical skeleton is based on anatomy. Special considerations have been taken to include anatomical features like axis displacements, range of motion control, and joints coupling. Once a 3D model of the skeleton is built, it can be simulated by data coming from motion capture or can be specified by a specialist, a clinician for instance. Our deformation model is an extension of the classical mass-spring systems. A spherical volume is considered around mass points, and mechanical properties of real materials can be used to parameterize the model. Viscoelasticity, anisotropy and non-linearity of the tissues are simulated. We particularly proposed a method to configure the mass-spring matrix such that the objects behave according to a predefined Young's modulus. A contact management model is also proposed to deal with the geometric interactions between the elements inside the joint. After having tested several approaches, we proposed a new method for collision detection which measures in constant time the signed distance to the closest point for each point of two meshes subject to collide. We also proposed a method for collision response which acts directly on the surfaces geometry, in a way that the physical behavior relies on the propagation of reaction forces produced inside the tissue. Finally, we proposed a 3D model of a joint combining the three elements: anatomical skeleton motion, biomechanical soft tissues deformation, and contact management. On the top of that we built a virtual hip joint and implemented a set of medical applications prototypes. Such applications allow for assessment of stress distribution on the articular surfaces, range of motion estimation based on ligament constraint, ligament elasticity estimation from clinically measured range of motion, and pre- and post-operative evaluation of stress distribution. Although our model provides physicians with a number of useful variables for diagnosis and surgery planning, it should be improved for effective clinical use. Validation has been done partially. However, a global clinical validation is necessary. Patient specific data are still difficult to obtain, especially individualized mechanical properties of tissues. The characterization of material properties in our soft tissues model can also be improved by including control over the shear modulus

Contact modeling and collision detection in human joints

Collision detection among virtual objects is one of the main concerns in virtual reality and computer graphics. Usually the methods developed for collision detection are for either very general cases or very specific applications. The first main goal of this thesis is to propose accurate methods for collision detection in computer graphics for rotating or sliding objects. The methods take advantage of the limitation imposed on the rotating/sliding objects in order to ignore unnecessary calculations of the general methods and speed up the processing. In addition to finding the collision, the methods can also return penetration depths in either radial or cylindrical direction, which can be useful for further applications. The second main goal is to apply the proposed collision detection methods in biomedical research related to human hip joints. In fact, during the past few years, femoroacetabular impingement (FAI) was recognized as the leading pathomechanism contributing to a significant number of so-called "primary" hip osteoarthritis. Thus, having medical simulation of hip joint can help both physicians and surgeons for better diagnosis and surgical planning. For diagnosing some of the human joint diseases, it is important to obtain the joint's range of motion. By modifying the pre-processing stage of one of the collision detection methods, a new fast method for finding maximum range of motion in human joint was proposed and tested. The method is working without doing any collision detection tests and its accuracy does not depend on the rotational steps. We also suggested a novel fast strategy for diagnosing hip diseases based on hip contact penetration depths. In this strategy, the contact penetration depths during hip movement are calculated for diagnosing hip impingements, by using the proposed collision detection methods. The strategy has been tested on pathological hip models during a daily activity. The results were found correlated with the contact stresses estimated by finite element method (FEM). By evaluating the results, the strategy proved to be capable for distinguishing among different hip pathologies (e.g. cam and pincer impingements). In orthopedic simulations, the behavior of the bones and the related tissues are usually investigated during their movements about an estimated center of rotation. We also evaluated the importance of the hip joint center of rotation in medical simulations. For this reason, different centers of rotation calculated by five different methods were applied for hip movements about different medical axes of rotation. By calculating the hip contact penetration depths of ten patients during hip movements (using the proposed collision detection methods), the sensitivity of hip simulations to hip center of rotation has been evaluated. Hip contact pressure has been a notable parameter to evaluate the physical conditions inside the hip joint. Many computational approaches estimate the pressure and contact pressures via finite element methods (FEM) by using 3D meshes of the tissues. Although this type of simulation can provide a good evaluation of hip problems, the process may be very time consuming. Also, these mechanical methods strongly depend on the movement details. We proposed and tested a fast statistical model for estimating hip contact pressures during its movement, without performing mechanical simulations and without any need for movement details. The estimation is done by evaluating geometric features extracted from 3D meshes of hip tissues, in order to link an unknown target hip model to some already mechanically evaluated training hip models

The application of three-dimensional mass-spring structures in the real-time simulation of sheet materials for computer generated imagery

Despite the resources devoted to computer graphics technology over the last 40 years, there is still a need to increase the realism with which flexible materials are simulated. However, to date reported methods are restricted in their application by their use of two-dimensional structures and implicit integration methods that lend themselves to modelling cloth-like sheets but not stiffer, thicker materials in which bending moments play a significant role. This thesis presents a real-time, computationally efficient environment for simulations of sheet materials. The approach described differs from other techniques principally through its novel use of multilayer sheet structures. In addition to more accurately modelling bending moment effects, it also allows the effects of increased temperature within the environment to be simulated. Limitations of this approach include the increased difficulties of calibrating a realistic and stable simulation compared to implicit based methods. A series of experiments are conducted to establish the effectiveness of the technique, evaluating the suitability of different integration methods, sheet structures, and simulation parameters, before conducting a Human Computer Interaction (HCI) based evaluation to establish the effectiveness with which the technique can produce credible simulations. These results are also compared against a system that utilises an established method for sheet simulation and a hybrid solution that combines the use of 3D (i.e. multilayer) lattice structures with the recognised sheet simulation approach. The results suggest that the use of a three-dimensional structure does provide a level of enhanced realism when simulating stiff laminar materials although the best overall results were achieved through the use of the hybrid model

Discrete element modelling of the dynamic behaviour of non-spherical particulate materials

PhD ThesisA numerical model based on the discrete element (DE) method, for modelling the flow of irregularly shaped, smooth-surfaced particles in a 3-D system is presented. An existing DE program for modelling the contact between spherical particles in periodic space (without real walls or boundaries) was modified to model non-spherical particles in a system with containing walls. The new model was validated against analytical calculations of single particle movements and also experimentally against data from physical experiments using synthetic non-spherical particles at both a particle and bulk scale. It was then used to study the effect of particle shape on the flow behaviour of assemblies of particles with various aspect ratios discharging from a flat-bottomed hopper. The particles were modelled using the Multi-Sphere Method (MSM) which is based on the CSG (Constructive Solid Geometry) technique for construction of complex solids by combining primitive shapes. In this method particle geometry is approximated using overlapping spheres of arbitrary diameter which are fixed in position relative to each other. The contact mechanics and contact detection method are the same as those used for spheres, except that translation and rotation of element spheres are calculated with respect to the motion of the whole particle....Numerical simulations of packing and flow of particles from a flat-bottomed hopper with a range of aspect ratios were performed to investigate the effect of particle shape on packing and flow behaviour of a particulate assembly. It was found that the particle shape influenced both bed structure and flow characteristics such as flow pattern, shear band strength and the occurrence of bridging. The flow of the bed of spherical particles was smoother than the flow of beds of elongated particles in which flow was fluctuating and there was more resistance to shear.Ministry of Culture and Higher Education of IRAN: University of Mashhad

A virtual environment for the modelling, simulation and manufacturing of orthopaedic devices

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The objective of this work is to investigate whether the game physics based modelling is accurate enough to be used in modelling the motion of the human body, in particular musculoskeletal motion. Hitherto, the implementation of game physics in the medical field focused only on anatomical representation for education and training purposes. Introducing gaming platforms and physics engines into orthopaedics applications will help to overcome several difficulties encountered in the modelling of articular joints. Implementing a physics engine (PhysX), which is mainly designed for video games, handles intensive computations in optimized ways at an interactive speed. In this study, the capabilities of the physics engine (PhysX) and gaming platform for modelling and simulating articular joints are evaluated. First, a preliminary validation is carried out for mechanical systems with analytical solutions, before constructing the musculoskeletal model to evaluate the consistency of gaming platforms. The developed musculoskeletal model deals with the human joint as an unconstrained system with 6 DOF which is not available with other joint modeller. The model articulation is driven by contact surfaces and the stiffness of surrounding tissues. A number of contributions, such as contact modelling and muscle wrapping, have been made in this research to overcome some existing challenges in joint modelling. Using muscle segmentation, the proposed technique effectively handles the problem of muscle wrapping, a major concern for many; thus the shortest path and line of action are no longer problematic. Collision behaviour has also shown a stable response for colliding as well as resting objects, provided that it is based on the principles of surface properties and the conservation of linear and angular momentums. The precision of collision detection and response are within an acceptable tolerance controllable by varying the mesh density. An image based analysis system is developed in this thesis, mainly in order to validate the proposed physics based modelling solution. This minimally invasive method is based on the analysis of marker positions located at bony positions with minimal skin movement. The image based system overcomes several challenges associated with the currently existing methods, such as inaccuracy, complication, impracticability and cost. The analysis part of this research has considered the elbow joint as a case study to investigate and validate the proposed physics based model. Beside the interactive 3D simulation, the obtained results are validated by comparing them with the image based system developed within the current research to investigate joint kinematics and laxity and also with published material, MJM and results from experiments performed at the Brunel Orthopaedic Research and Learning Centre. The proposed modelling shows the advantageous speed, reliability and flexibility of the proposed model. It is shown that the gaming platform and physics engine provide a viable solution to human musculoskeletal modelling. Finally, this thesis considers an extended implementation of the proposed platform for testing and assessing the design of custom-made implants, to enhance joint performance. The developed simulation software is expected to give indicative results as well as testing different types of prosthetic implant. Design parameterization and sensitivity analysis for geometrical features are discussed. Thus, an integrated environment is proposed to link the real-time simulation software with a manufacturing environment so as to assist the production of patient specific implants by rapid manufacturing