Viscoelastic properties of passive skeletal muscle in compression-cyclic behaviour.

Skeletal muscle relaxation behaviour in compression has been previously reported, but the anisotropic behaviour at higher loading rates remains poorly understood. In this paper, uniaxial unconfined cyclic compression tests were performed on fresh porcine muscle samples at various fibre orientations to determine muscle viscoelastic behaviour. Mean compression level of 25% was applied and cycles of 2% and 10% amplitude were performed at 0.2-80Hz. Under cycles of low frequency and amplitude, linear viscoelastic cyclic relaxation was observed. Fibre/cross-fibre results were qualitatively similar, but the cross-fibre direction was stiffer (ratio of 1.2). In higher amplitude tests nonlinear viscoelastic behaviour with a frequency dependent increase in the stress cycles amplitude was found (factor of 4.1 from 0.2 to 80Hz). The predictive capability of an anisotropic quasi-linear viscoelastic model previously fitted to stress-relaxation data from similar tissue samples was investigated. Good qualitative results were obtained for low amplitude cycles but differences were observed in the stress cycle amplitudes (errors of 7.5% and 31.8%, respectively, in the fibre/cross-fibre directions). At higher amplitudes significant qualitative and quantitative differences were evident. A nonlinear model formulation was therefore developed which provided a good fit and predictions to high amplitude low frequency cyclic tests performed in the fibre/cross-fibre directions. However, this model gave a poorer fit to high frequency cyclic tests and to relaxation tests. Neither model adequately predicts the stiffness increase observed at frequencies above 5Hz. Together with data previously presented, the experimental data presented here provide a unique dataset for validation of future constitutive models for skeletal muscle in compression.

Viscoelastic properties of passive skeletal muscle in compression: stress-relaxation behaviour and constitutive modelling.

The compressive properties of skeletal muscle are important in impact biomechanics, rehabilitation engineering and surgical simulation. However, the mechanical behaviour of muscle tissue in compression remains poorly characterised. In this paper, the time-dependent properties of passive skeletal muscle were investigated using a combined experimental and theoretical approach. Uniaxial ramp and hold compression tests were performed in vitro on fresh porcine skeletal muscle at various rates and orientations of the tissue fibres. Results show that above a very small compression rate, the viscoelastic component plays a significant role in muscle mechanical properties; it represents approximately 50% of the total stress reached at a compression rate of 0.5% s(-1). A stiffening effect with compression rate is observed especially in directions closer to the muscle fibres. Skeletal muscle viscoelastic behaviour is thus dependent on compression rate and fibre orientation. A model is proposed to represent the observed experimental behaviour, which is based on the quasi-linear viscoelasticity framework. A previously developed strain-dependent Young's Moduli formulation was extended with Prony series to account for the tissue viscoelastic properties. Parameters of the model were obtained by fitting to stress-relaxation data obtained in the muscle fibre, cross-fibre and 45 degrees directions. The model then successfully predicted stress-relaxation behaviour at 60 degrees from the fibre direction (errors <25%). Simultaneous fitting to data obtained at compression rates of 0.5% s(-1), 1%s(-1) and 10% s(-1) was performed and the model provided a good fit to the data as well as good predictions of muscle behaviour at rates of 0.05% s(-1) and 5% s(-1) (errors <25%).

Strength of cancellous bone trabecular tissue from normal, ovariectomized and drug-treated rats over the course of ageing.

Hormone therapy (HT) drugs and bisphosphonates prevent osteoporosis by inhibiting osteoclastic bone resorption. However, the effects of osteoporosis and anti-resorptive drugs on the mechanical behavior of the bone tissue constituting individual trabeculae have not yet been quantified. In this study, we test the hypothesis that the mechanical properties of bone trabecular tissue will differ for normal, ovariectomized and drug-treated rat bones over the course of ageing. Microtensile testing is carried on individual trabeculae from tibial bone of ovariectomized (OVX) rats, OVX rats treated with tibolone and placebo-treated controls. The method developed minimizes errors due to misalignment and stress concentrations at the grips. The local mineralization of single trabeculae is compared using micro-CT images calibrated for bone mineral content assessment. Our results indicate that ovariectomy in rats increases the stiffness, yield strength, yield strain and ultimate stress of the mineralized tissue constituting trabecular bone relative to normal; we found significant differences (P < 0.05) at 14, 34 and 54 weeks of treatment. These increases are complemented by a significant increase in the mineral content at the tissue level, although overall bone mineral density and mass are reduced. With drug treatment, the properties remain at, or slightly below, the placebo-treated controls levels for 54 weeks. The higher bone strength in the OVX group may cause the trabecular architecture to adapt as seen during osteopenia/osteoporosis, or alternately it may compensate for loss of trabecular architecture. These findings suggest that, in addition to the effects of osteoporosis and subsequent treatment on bone architecture, there are also more subtle processes ongoing to alter bone strength at the tissue level.

A validated model of passive muscle in compression.

A better characterisation of soft tissues is required to improve the accuracy of human body models used, amongst other applications, for virtual crash modelling. This paper presents a theoretical model and the results of an experimental procedure to characterise the quasi-static, compressive behaviour of skeletal muscle in three dimensions. Uniaxial, unconstrained compression experiments have been conducted on aged and fresh animal muscle samples oriented at various angles from the fibre direction. A transversely isotropic hyperelastic model and a model using the theory of transverse isotropy and strain dependent Young's moduli (SYM) have been fitted to the experimental data. Results show that the hyperelastic model does not adequately fit the data in all directions of testing. In contrast, the SYM gives a good fit to the experimental data in both the fibre and cross-fibre direction, up to 30% strain for aged samples. The model also yields good prediction of muscle behaviour at 45 degrees from the fibre direction. Fresh samples show a different behaviour than aged tissues at 45 degrees from the fibre direction. However, the SYM is able to capture this difference and gives a good fit to the experimental data in the fibre, the cross-fibre and at 45 degrees from the fibre direction. The model also yields good prediction of muscle behaviour when compressed at 30 degrees and 60 degrees from the fibre direction. The effect of the time of test after death has also been investigated. Significant stiffening of muscle behaviour is noted a few hours after death of the subject.

Measurement of the migration of a cemented hip prosthesis in an in vitro test.

OBJECTIVE: To develop a method to measure the migration of a cemented hip prosthesis in an in vitro experimental test. DESIGN: A device to measure prosthesis movement relative to bone was designed and fabricated. It was tested using a Lubinus prosthesis (W. Link, Germany) implanted in a composite femur. BACKGROUND: Clinical studies using radiostereophotogrammetry have shown that those cemented hip prosthesis that migrate rapidly in the first two post-operative years are the ones that require early revision. If migration be used as a basis for a pre-clinical test, then it should be possible to screen-out inferior designs before implantation in animal or clinical trials. METHODS: The micromotion measurement device consisted of a 'target' of three spheres arranged in a cruciform structure. Six linear variable displacement transducers were aligned with the spheres so that motion of the prosthesis relative to the bone could be measured. RESULTS: The displacement and rotation of the prosthesis relative to the composite femur was recorded for two million cycles. Relative rapid initial migration was found, followed by a period of steady-state migration. Distal migration (called 'subsidence' in this paper) of up to 0.1 mm was measured; however the variability in absolute prosthesis migration was very high despite efforts to ensure that all extraneous factors were minimised. In the majority of cases, the prostheses migrated medially, distally and anteriorly. The absolute subsidence, and its variability, were similar to that recorded clinically. CONCLUSIONS: A method has been designed and tested which measures prosthesis migration in an experimental test. It provides a basis for a pre-clinical testing standard. Relevance. Hip prostheses need to be tested experimentally before implantation. However, no reliable test exists for such experimental tests. Rapid migration of a cemented prosthesis relative to bone has been shown in vivo to correlate with early failure, and in this paper a method to make such migration measurements in vitro is described and tested.