This study involves investigating the in vitro degradation of a poly(propylene fumarate) (PPF) based composite material for orthopaedic applications. The effects of PPF molecular weight, PPF to vinyl monomer ratio, and solid phase content were studied. Mechanical properties, pore morphology, and sample mass loss were analyzed over a 12-week period of degradation. An initial increase in both compressive modulus and strength was seen for all formulations incorporating high molecular weight PPF. The PPF/monomer ratio was not seen to have a significant effect on any observations. Incorporation of /8-tricalcium phosphate (/3-TCP) resulted in an increase in mechanical properties and had no effect on weight loss. A composite formulation with an initial PPF//3-TCP ratio of 1.0 g/0.66 g exhibited an initial compressive strength of 2.60 MPa, which rose to 9.38 MPa at 3 weeks, and fell to 3.24 MPa at 7 weeks into the study. The initial modulus of 62.0 MPa for the same formulation increased to 250 MPa at 3 weeks, and fell to 63.7 MPa at 7 weeks. These studies further show that PPF//J-TCP composite scaffolds can be fabricated exhibiting initial mechanical properties similar to human trabecular bone and maintain these properties over several weeks of degradation.

When two metal cylinders roll together under a contact pressure sufficient to cause yielding, a surprising mode of plastic deformation occurs. The surface of each cylinder is progressively displaced in the forward direction of rotation relative to the core by plastic shearing in a thin subsurface layer. This phenomenon was first observed by Crook (6) in 1957 and the results of a more complete experimental investigation are reported by Hamilton (9) in an accompanying paper. In this paper an attempt is made to explain the mechanism of ‘forward flow’ by an approximate numerical analysis of the elastic-plastic stress cycles to which the material is subject in repeated rolling contact. On the basis of an idealized material which is elasticperfectly plastic and isotropic, it is shown that a forward displacement of the surface would be expected as a result of the complex cycle of stress and strain encountered in rolling. It is also shown that residual compressive stresses are introduced into the immediately subsurface layers during the first few cycles of the load. If the load does not exceed the elastic limiting load by more than 66 per cent further plastic deformation would then cease. At higher loads a steady pattern of plastic deformation is predicted which in its principal features is consistent with the observed behaviour.

No abstract

No abstract

No abstract