G. C. Weatherly

Osteogenic Phenomena Across Endosteal Bone-Implant Spaces with Porous Surfaced Intramedullary Implants

Porous surfaced femoral components of hip prostheses stabilized by tissue ingrowth are often situated a certain distance away from the endosteal cortex in the diaphysis. The purpose of this study was to examine the significance of this space between an implant and the cortex on bone growth into the porous surface of the implant. Intramedullary rods of different diameters with porous surface regions made of powder metal were inserted into the femurs of adult beagles. The rods had outside diameters of 2.5, 3.2, 4.5, and 5.5 millimeters; this variation produced endosteal bone-implant surface spaces ranging from 0 to 4 millimeters. The animals were sacrificed at 4, 8, 12, and 16 weeks. Histological sections revealed that by 12 weeks the implants became generally surrounded by a thin shell of spongy bone which was joined to the endosteal cortex by bony trabeculae. This feature was most prominent for implants which were approximately 2 millimeters or less from the endosteum. Denser, more haversian-like bone developed up to and within those areas of implants which were in contact with the cortex. The development of this intramedullary type of bone could significantly contribute to the fixation strength of clinical porous surfaced prostheses whose stems do not completely fill the medulla.

The effects of nitrogen additions to a cobalt-chromium surgical implant alloy

The determination of the feasibility of adding nitrogen to a cobalt-chromium implant alloy was undertaken with the ultimate goal of the work being the improvement of the static and fatigue properties of the alloy. Nitrogen additions were made using high-temperature heat treatments in a nitrogen-containing gas atmosphere. The effects of the nitrogen additions were characterized in this study using several techniques. The maximum solid solubility of nitrogen in the alloy at 1200° C (the heat-treatment temperature) was found to be approximately 0.35 wt % N. X-ray diffraction using nitrogen heat-treated powder samples indicated that the addition of nitrogen in solution resulted in a lattice dilation lying in the range of 0.0021 to 0.0035 nm per wt % N. Above the solubility limit, Cr2N and Cr2(CN) were present at the nitrogen heat-treatment temperatures in the form of large second-phase particles. Ageing of the alloy containing approximately 0.35 wt % N at 400° C resulted in the precipitation of CrN. A study of the nitrogen distribution suggested that the diffusion of nitrogen ms affected by the carbon content of the cobalt-base alloy.

Static mechanical properties of cast and sinter-annealed cobalt-chromium surgical implants

Porous-surface-layered surgical implants may be produced by sintering at elevated temperatures. An investigation was undertaken to determine the effect of these sintering heat treatments on the tensile properties of the cobalt-chromium casting alloy specified by ASTM F75-76. Specimens which were given a sintering treatment and then rapidly cooled from elevated temperature were found to lack ductility. This was due to the incipient melting of an interdendritic material which was subsequently retained in the grain boundaries as a brittle solid after quenching. Two methods were found which would reduce the amount of this brittle solid: (i) modify the heat treatment to include a slow cooling step to temperatures below that at which incipient melting first occurred; and (ii) reduce the carbon content of the alloy. Reduced-carbon alloys gave the greatest post-sintering ductility, but showed a lower 0.2% yield stress. The techniques of thermal activation analysis were used to investigate the effect of second phases upon the initial low-strain work-hardening rates and the 0.2% yield stress. It was found that the work-hardening rate from the elastic limit to a total strain of about 0.01 to 0.02 depends, in part, directly on the volume fraction of second phase.

The effects of nitrogen additions to a cobalt-chromium surgical implant alloy: Part 2 Mechanical properties

The efficacy of adding nitrogen to a Co-Cr surgical implant alloy in order to improve tensile and fatigue properties has been investigated. Using the heat treatments described in Part 1 of this study, the tensile properties of specimens with nominally 0.14, 0.19, 0.21 and 0.33 wt% carbon were evaluated in air at room temperature. The fatigue testing consisted of a rotating beam fatigue test at room temperature in air, at a frequency of 10 Hz. The results of the mechanical tests indicated that interstitial nitrogen additions to low carbon alloys (nominally 0.14 wt % carbon) increased the yield strength while maintaining good ductility. However, this beneficial effect was not apparent for the material tested in fatigue, possibly because of the overriding influence of inherent flaws within the heat treated testpieces.

A STEM study of grain-boundary segregation in Al-6.5 wt% Mg alloy

A STEM-EDX analysis study of grain-boundary segregation in an AI-6.5 wt% Mg alloy is presented. STEM-EDX analysis using an electron probe size of nm is shown to provide statistically significant compositional data on grain-boundary segregation in Al-Mg alloys. Solute profiles taken normal to grain boundaries show both non-equilibrium segregation and precipitation phenomena in Al-6.5 wt% Mg alloys water-quenched from 350, 400 and 570° C.