The development of plastic strains during fatigue damage accumulation of ageing human cortical bone

Winwood, K., Zioupos, P., Cotton, J.R. and Taylor, M. (2001) The development of plastic strains during fatigue damage accumulation of ageing human cortical bone. International Society of Biomechanics XVIIIth Congress, Zurich, Switzerland. 07 - 12 Jul 2001. p.721 .

Abstract

The migration, subsidence and loosening of prosthetic implants are a major concern for both the clinical surgeon and for the morbidity and mortality of the patient. Such effects are frequently reported post-operatively within the first six months and progressively later on. Subsidence can be due to a number of factors including the quality of the cancellous/cortical bone, the method of fixation and design of the implant. In recent studies of ours we have utilised FEA codes and considered the build up of yield and plastic strains around the prostheses. For this we made use of the very limited bone data available in the literature. To the best of our knowledge there have been no studies linking the development of 'plastic' (showing as an irrecoverable translation on the strain axis at zero stress) strains during fatigue cycling of cortical bone to the accumulation of damage (in material terms a modulus reduction), the loading mode, and in particular with relation to ageing. Microdamage has been implicated previously in aseptic loosening, however, the new factor we examine here is the 'plastic' strains that may be coupled to this damage-related behaviour of bone. Cortical bone specimens were obtained from the femurs of four healthy donors of an age range 27 to 69y. The specimens were subjected to fatigue cycling (2 Hz, 37oC, Ringer's solution) in both tension and compression in a Dartec-HC10 servo-hydraulic machine. Strains were recorded by use of a waterproof extensometer. The data was analysed in terms of reduction in modulus and the development of 'plastic' strain Our results showed effects for both the loading mode (tension vs. compression) and the age. In tension both 1)the overall damage, 2)the course of damage with the cycle number and 3)the amount and course of the 'plastic' strains were higher in the older individuals. In compression 1)the development of damage was not different in the various individuals, but 2)we have found a significant increase in the magnitude of 'plastic' strain with age and stress in older individuals. While cycling below the yield stress in compression and tension was age independent, cycling above the yield stress showed rapid accumulation of 'plastic' strains in both tension and compression and also rapid modulus reductions in simple tension. These results may be very important in the understanding of implant failure as seen in clinical situations and around prostheses where high stress concentrations may occur locally. The age, stress, and loading mode 'plastic' strain dependency of bone may explain why some prostheses are more prone to failure than others.

This record has no associated files available for download.

Contributors

Download statistics