Biomechanics and biology: bridging the gap Sam Evans School of Engineering

Bone strength and GH How do changes in growth hormone and other endocrine factors affect bone strength? How to test the strength of the bones?

Problems Many groups of bones to test Considerable variability Small, weak bones

How to do it? Impractical to machine tensile test specimens from bones Need to test whole bones 3 point bend tests simulate a typical fracture scenario Need to measure cross- section to determine geometric contribution to strength (2 nd moment of area) ‏

Second moment of area Irregular shape- elliptical assumption gives a reasonable approximation for most bones

Examples Femoral bending strength Squares: male Circles: female White:WT Black: TGR Evans et al, Journal of Bone and Mineral Research 18(7) (2003):

Challenges Understandable results Geometric and material variability Many variables and specimens ? Engineering rigour

Future challenges? High throughput automated testing Specimen- specific measurements and models using imaging Fast, reliable computer models needed to analyse the results Better standards for measurement and modelling

Conclusions Working with biologists is fun! Many opportunities for interesting research Possibilities for big advances using new technology

Examples Effect of growth hormone deficiency on bone strength

Variability Need detailed, subject specific measurements and models for accurate results Many tests needed to achieve statistical significance Sophisticated material models required

Many variables and specimens Many groups often need to be tested Many biological questions Complex problems require many tests to investigate interaction of multiple variables High throughput testing is needed

Understandable results Results need to be understood by biologists Even expert engineers struggle with complex models of soft tissues etc Requires models that are no more complex than necessary Good explanations needed too

Methods Standard testing machine with 100N load cell A range of 3pb fixtures in various sizes Eight measurements of cross section using travelling microscope to determine second moment of area

Stress and strain

What do we want to do? Often need to predict or measure the mechanical behaviour of biological materials eg implant design, development of surgical procedures Measuring effects of biological changes

What do we need? Constitutive model  =E  =-  T /  L Stress analysis  =F/A  =  L/L Behaviour of material Measurements

So what’s the problem? Standard tests assume a simple stress analysis which in turn assumes a simple constitutive model Not valid for inhomogenous, anisotropic materials! More tests needed, and more complex analysis

Any other problems? Most biological materials change irretrievably in vitro Cutting specimens disrupts their structure and alters their behaviour Human tissue often different from animals Need clinical measurements Ideally want to test in vivo

What do we need? Complex constitutive model, many parameters Numerical simulation Behaviour of material 3D, time dependent measurements

Testing and simulation Can’t simulate tissue behaviour without measuring parameters Can’t test tissue without some sort of model or simulation Need to validate simulations Testing & simulation are linked Has testing been neglected?

Cells with various aspect ratios. Load more or less independent of length

Fatigue of bone cement

Disc replacement

Periodontal ligament In vivo testing of human periodontal ligament (PDL) using small scale motion analysis Development of sophisticated computational models at UWCM Funded by EPSRC

Actual Measured (  m) ‏

Actual

Conclusions Simulation and testing must go hand in hand Many standard methods assume a model that may be invalid We need to work together!