Bone Modelling

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This was a joint project involving Medical Engineering, Computer Science and Medical Physics.

The group has an interest in the structural changes that occur in bone with diseases such as osteoporosis and arthritis. In particular it is interested in cancellous bone, and has developed a powerful bone modelling environment (BMU-SIM) that can simulate the changes that occur in the bone and can quantify the effects on its structural properties.

Background

Most bones are made up of a dense outer shell of cortical bone enclosing a central porous region of trabecular (or cancellous) bone. This consists of an interconnecting three dimensional network of thin bars or plates of bone (known as trabeculae) interspersed with marrow. This gives the bone strength whilst minimising weight. However, any thinning or loss of this network can dramatically decrease the strength of the bone, since the internal architecture of the bone plays a crucial role in its stiffness and strength. This network of bone is continually being renewed by a process known as remodelling, that replaces 10% of the bone in a healthy skeleton every year. This occurs by small amounts of bone being resorbed and then replaced with new bone. If there is any imbalance in the process, then a net loss of bone results, which may lead to osteoporosis. As the remodelling process only occurs at bone surfaces, trabecular bone with its large surface area undergoes the majority of remodelling activity and is therefore the most affected by osteoporosis

Simulation of bone remodelling

A simulation of cancellous bone remodelling has been developed which is based upon the concept of a basic multicellular unit (BMU) where net resorption (-ΔB.BMU) or formation (+ΔB.BMU) is considered at bone/marrow surfaces. The simulation considers both the probability that any surface pixel will be activated into a BMU and, if activated, the length of the resorption/formation activity. [Bone, 22:4, 1998].

This has now been implemented in a 3D bone modelling simulator and environment BMU-SIM. The program can simulate the net effect of complex osteoblast and osteoclast activity on biopsy and computer-generated bone samples and, with an integrated finite element solver, predict the resultant effects on the elasticity, strength and integrity of the structures. Integrated histomorphometry tools allow the relationship between structure and mechanical properties to be fully elucidated, and a sophisticated graphical interface allows 3D stereoscopic visualization of the bone structures at every stage of an analysis.

Strain based adaptation

BMU-SIM can simulate deterministic bone remodelling, based for example on strain or strain energy density. This allows the bone to adapt to the external load applied to it. Various adaptation algorithms are possible. In fact the simulator has an equation parser, so that any function can be specified for the osteoblast and osteoclast activities in the adaptation cycle. For example, these can be a function of bone age, stress and strain, strain energy density or position in the model.
 

Validation and visualisation of the models has been examined through the use of stereolithography (STL) models, and the facility exists to create STL models of any structure developed in the simulator.