Engineering in Medicine

Hydraulic strengthening, viscous interaction between marrow and trabecular bone, and viscous resistance by the marrow to shear, appear not to occur in long bones when subjected to non-destructive axially applied compressive loads. These conclusions are based on the following observations.

Firstly, volumetric measurements suggest that long bones deform in a way that maintains a near constant volume. Hence the mechanism by which ‘hydraulic strengthening’ could operate seems not to exist.

Secondly, with little or no volume change, no significant movement between the marrow and the adjacent trabecular bone is likely to occur. The only mechanism by which the marrow could function mechanically would be by its own viscous resistance to shear deformation. As marrow is near liquid at 37°C (with a measured viscosity of 67 × 10^-3 Pa s) this ‘resistance’ is shown to be negligible, even when calculated using values most favourable to the production of large shear forces.

Early detection of mechanical weakening of the femoral neck would help to prevent fractures at this site which are the most serious clinical problem in osteoporosis. In the vicinity of the femoral neck, the greater trochanter is a superficial bone structure more easily accessible to in vivo measurements. The purpose of the present study was to evaluate the relation between the strength of the femoral neck and the composition of the trabecular tissue in the greater trochanter. Twenty-nine fresh human intact femora were examined. The relationship between the average shear stress in failure of the femoral neck (ASS) and the Ca concentration (CC), mineral density (BMD), and overall mass density (BD) of the greater trochanter was investigated. Linear stepwise regression analysis showed that the ASS of the femoral neck can be predicted from the BD, CC, and BMD of the trochanteric tissue, and their contribution to the estimation of the ASS was ranked in the above order. The linear function relating the ASS to these variables showed a highly significant multiple correlation (R = 0.87, p < 0.001). Therefore in vivo measurements of the density and mineral content in the greater trochanter should be considered useful in assessing the risk of fracture of the femoral neck.

This paper reports on a comparative investigation into the mechanical and handling properties of a selection of currently marketed bandage-form casting materials. The aim of the investigation was to develop a quantitative evaluation of available materials for use in the particular application of femoral fracture bracing. A field survey of Northern Ireland hospitals was conducted which showed that certain handling properties were important to those actually using the materials. A series of engineering and handling tests were developed on the basis of the replies received and executed on each of the materials. It was found that all the modern synthetic casting materials performed better than plaster of Paris. The findings of the investigation and the results of the field survey will be of interest to anyone involved in the application of the new materials and also to those manufacturing them.

The strength pattern of cancellous bone was examined in 17 ankles by a cylinder compression test and in eight distal tibial specimens by an osteopenetration test.

With the compression test, 6 mm levels were examined, five levels at the tibia and three levels at the talus. Cylinders were obtained from all levels from corresponding locations. The cylinders were tested at a constant deformation rate of 5 mm/min until failure. From the load deformation curves ultimate strength, yield strength, yield strain, ultimate strain and elastic modulus were calculated.

The penetration test involves resection of the joint surface just above the subchondral bone plate at the distal tibia. Measuring force and displacement, a 2.5 mm conical needle was pushed into the bone at 25 locations in a 5 × 5 matrix. The force-displacement curves were integrated at intervals of 2 mm. Three levels were evaluated, corresponding approximately to the first level in the compression test.

The study indicates the presence of a strong antero-lateral part and a slightly weaker postero-medial part at all levels in the tibial and talar bone. The compression test at level 1 and the penetration test at levels 1, 2, and 3 in the distal tibia revealed a high strength area crossing the central part, in contrast to the proximal levels of the tibia where the central parts were very weak. At the talus, a high strength area ran along the lateral and posterior border. There were no significant differences in the profiles of ultimate strength, yield strength, and elastic modulus.

The development of a simple and efficacious ophthalmic applicator for the irradiation of choroidal melanoma and other intra-ocular tumours is described, together with early surgical experience.

Papers were invited on the above theme for a Meeting which took place at the Belfast City Hospital on Friday, 27 March 1987.

The aim was to provide an opportunity for presentation and discussion of work in rehabilitation and orthopaedic engineering from units throughout the British Isles.

The Abstracts below reflect the success of the Meeting and the dedication of the scientific contributors to their topic.

The Meeting was jointly organised by The Ulster Biomedical Engineering Society (TUBES) and the Northern Ireland Rehabilitation Engineering Centre and sponsored by Cardiac Services Limited, Belfast.