Computational and experimental investigation of the femur to allow accurate assessment of the effects of damage to the musculo-skeletal construct

In the UK, the number of hip and knee replacements has risen rapidly, with 65,000 of each being carried out per year. In the US these figures rise to 240,000 hip replacements, and 420,000 knee replacements. However knowledge of the manner in which the natural femur, as well as the femur following either hip or knee surgery, behaves is still relatively limited. Previous attempts to model the femur, through experimental or numerical methods have not examined the femur as a complete musculoskeletal construct. That is to say that the role of the muscles and ligaments, with insertion points on the femur, crossing both the hip and knee joints, has not been represented fully. Thus it is not possible to translate the results of in vitro laboratory experiments, and corresponding numerical models, to the in vivo situation. This project will develop a biofidelic finite element model of the femur, spanning between the hip and knee joints, in which the mechanical effects of muscles and ligaments, interacting with the hard tissue structure of the femur will be included. In addition the interaction of the femur, with other bones, at the patella-femoral, tibial-femoral, and acetabular-femoral joints will be investigated. This will allow the more accurate assessment of the behaviour of the natural femur, as well as the femur following muscle or ligament damage, and hip or knee arthroplasty.The benefits of such a model will be in allowing a more realistic assessment of the natural femur, and the changes in the stresses placed on the femur, due to damage to muscles and ligaments, either through disease, injury or surgery. For example it will allow surgeons performing hip arthroplasty to assess if particular exposure approaches (causing damage to different muscle groups) are more likely to lead to loss of function, or long-term osteolysis of parts of the femur. The model will also allow the more accurate assessment of the potential effects of using different types of hip and knee implants. Previous finite element models of the femur, assessing, for example the effects of hip arthroplasty have not treated the femur, and surrounding muscles and ligaments as a complete system. Significant assumptions have been made with regard to boundary conditions in particular, with rigid constraints being applied to the distal femur, and with the role of muscles or ligaments ignored, or limited. The inaccuracies introduced by making these assumptions are not known.Some validation of current finite element models, has been achieved through in vitro experimental testing, with the use of strain gauges attached to the surface of the femur. However this only allows comparison at as many points as there are strain gauges, and does not assess whether the numerical model captures the overall behaviour of the femur. In addition problems still exist regarding bonding of strain gauges to the sample being tested. A significant part of the project will investigate the use of speckle stereo-photogrammetry (SSP) to give a more complete view of the behaviour of the femur during in vitro. The technique provides a non-contact method, that will allow a complete contour map of the surface strains to be produced, for comparison with that predicted by the numerical model. Fixed boundary condition numerical models of the femur will be developed, and compared with the results found using SSP, prior to creating more advanced models of the femur, in which the effects of muscles and ligaments will be included.