Abstract
Compression is a common therapy for management of chronic disease, including oedema of the lower limb. Modern compression interventions exert pressure on the lower limb through use of one or more materials which exert pressure against the limb over time. Where these materials are textiles, they range from elastic to inelastic, and are produced using knitting, weaving, or other textile technologies which can be manipulated to control performance properties. The radius of curvature at any cross-section of the lower limb (i.e. calf (gastrocnemius), tibial crest (anterior edge of the tibia bone)) is irregular, and differs among individuals and populations. Thus, understanding of both the materials/textiles and the human body is needed if the most appropriate compression device and treatment strategy is to be used. Neither is independent of the other.
Current understanding of pressure generated by conventional lower limb compression products such as bandages and stockings is based on Laplace’s law. According to this law, the larger the radius of a cylinder, the larger is the wall tension required to withstand a given internal fluid pressure. This theory has been interpreted mathematically with several researchers modifying the Laplace model to better estimate the interface pressure on skin of a lower limb. These estimates have been checked against conventional compression products but not air pneumatic compression devices (PCD), nor have they been checked against models of varying radii.
An air pneumatic compression device consists of an inflatable bladder/s that encircles the lower limb. Investigation into effects of air pneumatic devices as part of sustained compression therapy has led to the conclusion that compression treatment does indeed, offer benefits. However, participants in human trials have been shown to resist or abandon compression treatments due to discomfort and low sensorial acceptability of the treatment involved. These findings suggest pneumatic compression devices need to create a satisfactory environment if the treatment is to be effectively employed. Thus, the aim of this investigation was to develop a theoretical model to predict the pressure exerted by an air pneumatic compression device on an irregularly shaped lower limb, and design a compression device which can create a satisfactory environment on the oedematous lower leg.
Two theoretical models, first considering the lower leg as circular in shape, and second as elliptic in shape were developed to predict the pressure generation by an air pneumatic compression device. In order to validate the model, compression sleeves for three positions on a lower leg manikin were fabricated. A pressure instrument for measuring the effect of the compression sleeves on this leg manikin was developed. The theoretical model developed for an elliptic shaped lower leg predicted the pressure more accurately than that predicted by the circular shape.
Two approaches to development of the textile-based compression device were taken: creation of a tubular braided structure, and creation of a multi-layered textile/silicone structure.
The experimental set-up for characterising the materials included (a) single layers of two next-to-skin knit fabrics in both relaxed and extended conditions, (b) two layers of silicone, and (c) a multi-layered assembly with and without vents (each next-to-skin extended fabric in combination with two layers of silicone). Key performance properties in question included thermal and water-vapour resistance, water-vapour and air permeability, and drying rate. Next-to-skin fabrics were examined in a relaxed form and also in arrangements simulating use: that is, fabric extension during use was measured in the length and width directions for three different locations on a lower leg manikin. Specimens in these locations were cut, stretched, and fitted to a pre-prepared frame simulating arrangement during use. When testing multi-layer ensembles, one layer of extended next-to-skin fabric (technical face uppermost), and two layers of silicone were stacked on each other, with silicone layers placed uppermost. To determine the effect of vent variables i.e. shape, size, and number of vents on fabric properties, vent variables were optimised. Results for single-layer fabric (relaxed and extended arrangements), and multi-layer assembly with and without vents were examined.
As expected structural properties of the knits (thickness, mass) dominated thermal resistance in the multi-layered assembly and the silicone layers rendered the multi-layered assembly impermeable to water-vapour. Results confirmed the need for some form of ventilation to manage water-vapour from a user’s skin to the environment. By creating the eighteen circular vents with 314 mm2 area of each vent across the silicone layers, the water-vapour resistance of the multi-layer assembly dropped significantly from non-detectable values (very high) to below ~300 m2Pa/W for the multi-layer assembly with the heavy weight fabric and ~225 m2Pa/W for multi-layer assembly with the light weight fabric.
Recommendations are made: i.e. examine the effect of different thickness of silicone on performance properties of multi-layer assemblies, examine the friction between human skin and layers of multi-layer assemblies, human trials of sensory and thermo-physiological performance properties of newly developed pneumatic compression device, and re-consider the tubular braids with different compositions of yarns and structures.