Mesh morphing strategies for image-based in-silico musculoskeletal biomechanics

Background and benefits
Considerable growth in musculoskeletal intervention is predicted in the next two decades, increasing the need to develop, improve and target interventions. Such developments can be achieved through in-silico models, such as patient-specific finite element models. It is well known that a hexahedral mesh for such models is beneficial on the accuracy of the computational analysis. However, the automatic construction of hexahedral meshes is a research area on its own and therefore the development of image-specific hexahedral meshes is restricted to available tools which require multiple manual steps, making the accuracy of the process is highly user-dependent.

Aims
To facilitate reproducibility and accuracy in the next-generation of in-silico patient-specific musculoskeletal models by developing mesh morphing strategies, applicable to meshes from multiple tools.
The new mesh morphing strategies will allow existing image-based meshes, developed in the current time-consuming fashion, to be automatically adapted to a new subject, significantly speeding up the model development process, without compromising on mesh quality.

Three anatomic tissue sites have been strategically chosen for the development and testing of the tool. Each site provides unique challenges representing milestones in the technical development (detailed below). In addition, the tool can be applied to rapidly develop models from the available image databases (both internal and public) for each anatomic site chosen.

Research project
This PhD project will develop new morphing or warping algorithms. State-of-the-art will be advanced by creating multi-body meshes including image-based information such as grey-scale values.

The project will consist of several aspects:
1/ Building standard hexahedral mesh of the tissues on each anatomic site of interest (hip, ankle, spine) using meshing tools (IA-FEMesh, tools embedded in Finite Element Software's, or tools developed during the PhD).
2/ Development and implementation of relatively generic morphing or warping tools.
3/ Validation of the developed tool using (pre-)clinical image databases from iMBE, or public repositories, on the hip, the ankle and the spine. The superiority of using hexahedral meshes will be demonstrated in accurately modelling incompressible materials and tissues. The new developed meshes will be used to assess the difference in contact mechanics between tetrahedral meshes and hexahedral meshes.

The three applications (spine, hip and ankle) represent areas of existing image databases and modelling expertise within the IMBE. The hexahedral mesh development and morphing methods will be applied to each anatomical area in turn, with additional complexity introduced at each stage:

Spine: The intervertebral disc is responsible for the majority of the movement possible in the human spine and its degeneration is the source of debilitating pain. The relative simple overall shape makes the disc a good subject for initial mesh and morphing development in two and three dimensions. A possible extension is to capture the layered structure within the disc.

Hip: The cartilage covering the pelvic side of the hip joint is one of the sites where damage can occur at an early age, linked with osteoarthritis in later life. The shape of this tissue presents additional challenges. Applying the initial methods here will demonstrate how much shape change the morphing methodology can achieve. A possible extension is to include the 'labral' tissue which surrounds the cartilage at the edge of the cavity.

Ankle: The ankle joint is prone to post-traumatic osteoarthritis, with very small contact areas between bones and small anatomical changes linked to high functional change. Morphing a mesh to fit to the ankle bones will present the additional challenge of resolving multiple meshes at a contact surface, as well as capturing the shape of each bone.