Paul De Meester

The mechanical behaviour of intracondylar cancellous bone of the femur at different loading rates

The compressive properties of human cancellous bone of the distal intracondylar femur in its wet condition were determined. Specimens were obtained from six cadaveric femora and were tested at a strain rate of 0.002, 0.10 and 9.16 sec−1. It was found that the compressive strength decreases with an increasing vertical distance from the joint. The highest compressive strength level was recorded in the posterior medial condyle. Correlations among the mechanical properties, the bulk specimen density and the bone mineral content yield (i) highly significant correlations between the compressive strength and the elastic modulus (ii) highly significant correlations between the compressive strength or the modulus of elasticity and the bulk specimen density (iii) a doubtful correlation between the compressive strength and the bone mineral content. All recorded graphs of the impact loaded specimens displayed several well defined stress peaks, unlike the graphs recorded at low loading rates. It can be concluded that upon impact loading the localized trabecular failure which is associated with each peak, does not affect the spongy bones stress capacity in a detrimental way.

The mechanical behaviour of porous austenitic stainless steel fibre structures

The mechanical properties of metal fibre porous structures were studied in the light of their potential application as surface coatings of implants. Stainless steel AISI 316 L fibres with diameters of 50 and 100μm were compacted and sintered. The variation of the modulus of elasticity with density, as obtained in tension, corresponds closely with theoretical models. The ultimate failure of the tensile specimens proceeds through the fibres, and not through the sinter bonds, except at lower densities. Differences in yield strength between 50 and 100 μm fibre tensile specimens are explained on the basis of the onset of plastic deformation of the individual fibres. Upon compression the modulus of elasticity is nearly 10 times smaller than in tension. This result is due to the different deformation patterns of the fibres in compression and tension.

Factors governing the mechanical behavior of the implant-porous coating--trabecular bone interface.

Abstract Mechanical advantages can arise from the use of implants with a porous surface layer that allows bone ingrowth. This study first outlines the factors which can influence the mechanics at the interface between the porous coating and the surrounding bone. The influence of some of these factors is assessed quantitatively by finite element stress analysis. The basic assumption made prior to the stress analyses is that bone ingrowth has occurred and thus that skeletal fixation has been established. A mechanically successful fixation by ingrowth implies that a critical value for the shear stress at the porous coating-bone interface is not exceeded. The stresses have been calculated for a modelled prosthesis with a porous coating, surrounded by trabecular bone and subjected to either a compression or a bending loading mode. The results show that the value of the elastic modulus of the porous coating has no major influence on the porous coating spongy bone interfacial shear stress upon compression and bending. This finding is explained by the deformation and stress pattern of the whole structure. It is pointed out that load uptake by the cortex is essential and that the underlying trabecular bone where the stresses in the bone are the highest, will probably build up a gradient in elasticity in order to minimize the stresses at the interface and in the underlying bone.