Engineering in Medicine

This review considers the following areas of seating research: anthropometrics (the use of body measurements to determine chair dimensions); anatomy (sitting posture and spinal curvature); muscle activity (the effect of different sitting positions on the muscles of the back); comfort; weight-bearing (prevention of pressure sores); ease of rising; and seating for the disabled (including contoured seating and armchairs). The major contributions in each area will be outlined in turn. If the total requirements for a chair are to be met, aspects of all of these factors may need to be considered in chair design. Despite this body of research many seating problems have not been solved. It is suggested that a bio-engineering approach may be more successful in providing answers than many of the subjective and comparative trials of the past.

Pressure between the ischium and the flat surface of a pressure sensitive device (pedobarograph) are reported. There are no consistent patterns of the pressures detected this way with age, weight, sex or gross anatomy. When sitting on a gel cushion or 3 in foam cushion, the centre of load moves anteriorly and the pressures between the under surface of the cushion and the flat surface of the pedobarograph appear to be both similar and considerably lower than those pressures detected between the ischium and the flat surface.

In spinal cord injured patients with pressure sores, the device indicated areas of high pressures (greater than 300 mm Hg) in the region of the pressure sores as well as a very much reduced contact area (usually less than a third) compared with normal subjects of similar weight.

A pedobarograph that measures and records plantar pressures has been used to evaluate the possibility of using ‘Shutrak’ as a system to measure pressures under the foot clinically. The Shutrak system is sensitive to pressures of approximately 2.5 kPa, however because of the method of loading, the record cannot be used reliably for standing pressures but can show areas of cumulative contact pressures above 2.5 kPa. Individual areas of high pressure cannot be reliably detected by this system, differentiation between light and dark areas of contact are not discernible, except where the pressures exceed 100 kPa.

Plastazote depth impressions have a better sensitivity but care in heating and applying the load is necessary because of the mechanical and thermal characteristics of the material. If these are standardized, Plastozote could be used to quantify areas of high pressure as well as foot shape, but care has to be taken in interpreting data by this method.

Both devices used in conjunction with a clinical examination would provide dynamic and static plantar pressure data for diagnostic use and plantar pathology investigation.

A method of displaying discrete areas of pressure beneath the foot has been produced. The device employs a pressure sensitive elastomer which gives quantitative readings of the pressure developed. The 512 load cells are enclosed in a low profile platform only one inch in depth which provides a low-cost system that can be employed in clinical situations.

To answer a clinical need, a simple goniometry system for the measurement of the lower limb joints has been devised. By means of a low-cost microprocessor the results are provided in printout, in a clinically useful form.

The physical and mechanical properties of three new bandage-form splinting materials (Baycast, Crystona and Hexcelite) have been compared with those of plaster of Paris. Properties evaluated including working strength, rate of strength development, elastic modulus, exothermic heat and X-ray absorption coefficient. All mechanical testing was carried out on rectangular cross-section specimens in three-point bending. Whilst the three newer materials differed widely they all showed some advantages over plaster of Paris although none could be definitely stated as being superior in all respects. As regards mechanical properties, Baycast was found to be significantly stronger than the other materials and to have a higher strength to weight ratio. The difficulties in formulating laboratory tests which are clinically relevant are discussed and the need to consider other factors, such as ease of application and advantages to the patient, is emphasized.

Although problems with fixation of cemented components have been more common with tibial components than with femoral, there is clinical evidence that femoral loosening may occur but after a longer interval of time. Experimental and theoretical studies were carried out to examine the motions and stresses in the implant-bone assembly. The effect of a central intramedullary stem, and of unbonding the interfaces of the patella flange and the posterior condyles, were particularly examined. Perfect bonding led to severe stress protection of the distal layer of bone. A central stem needed to project considerably above the flange to be effective and was recommended for special indications only.

The characteristics of impact forces on the legs during vertical landing of human vertical free fall in different falling conditions were studied to reveal the parameters which take part in the attenuation of these impact forces. The following parameters were investigated: body position during landing, range of flexion of the joints of the legs at impact, usage of ground-roll immediately after impact and softness of the ground.

The results indicate that joint movements and muscle action play a major role in reducing peak forces during landing. This emphasizes the importance of adequate training to improve the pre-programmed non-reflex muscle action, necessary in the early phase of impact.