Simulation of soft tissues deformation

		The hip joint model. Showing bones, cartilages, the acetabular rim and the pubofemoral ligament. Stress is colour mapped. Modeling pipeline. MRI and topological information are the main inputs. Geometrical models are reconstructed and soft parts are discredited. Then, tissues behaviour is configured, and finally the joint is simulated.

A computer graphics model of the human hip joint reconstructed from magnetic resonance images (MRI) is simulated in motion. Such model is built combining a conceptual anatomy-based kinematical model of the joint and a conceptual model of soft tissues deformation, and is configured from several sources of information. It is then the result of a hybrid approach, being partially kinematical and partially biomechanical. The kinematical component defines the bony rigid motion from measures on the static and dynamic magnetic resonance images. Then, the biomechanical component computes soft connective tissues deformation, and allows estimating force exchange and consequent stress on those soft structures.

Soft and rigid organs are geometrically represented by b-rep in the form of 3D meshes generated from MRI analysis, which are used for visualisation and collision detection. In addition, soft ones are discredited such that a generalised mass-spring model can process and propagate the deformations. Mass-spring systems are widely used in Computer Graphics for animation of deformable objects. However, special considerations had to be taken into account to adapt to medical applications. Though other extensions have been proposed, like dynamic volumes associated to the masses that are used for displacement of liquid inside the tissue, the most important consideration is the correct biomechanical behaviour of the biomaterials. A method has been developed to configure the springs lattice such that our virtual ligaments and cartilages have a predictable elasticity. Elasticity is defined by the Young's Modulus (E) of the material; the rheology standard. Other material properties are also considered, like anisotropy, viscoelasticity, permeability and so on.

The hip model is then used in simulations where the physical status of the joint can be assessed. Stress and strain along the tissue are calculated and can be inspected at any simulation time. An example is mapping a colour associated to stress values on the cartilage surface during motion. Another is limiting the joint range of motion to a stress threshold on specific ligaments. Such simulations allow understanding possible malfunction of a patient's articulation, and aid in planning disease treatment.