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Laboratory-based evaluation of a compression-bandaging system

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The application of sustained graduated compression is a key element in the prevention and treatment of leg ulcers caused by damaged or incompetent veins in the lower leg. While compression hosiery is commonly used to prevent the development or recurrence of venous ulcers, elastic bandages and multilayer bandaging systems are the main forms of treatment for active ulceration (European Wound Management Association, 2003).

Abstract

VOL: 99, ISSUE: 40, PAGE NO: 24

Steve Thomas, PhD, is director at Surgical Materials Testing Laboratory.

Paul Fram is laboratory scientist at Surgical Materials Testing Laboratory.

 

The application of sustained graduated compression is a key element in the prevention and treatment of leg ulcers caused by damaged or incompetent veins in the lower leg. While compression hosiery is commonly used to prevent the development or recurrence of venous ulcers, elastic bandages and multilayer bandaging systems are the main forms of treatment for active ulceration (European Wound Management Association, 2003).

 

 

The optimal degree of pressure required remains a matter of debate. Pressures of about 40mmHg at the ankle are widely quoted in the literature for the prevention or treatment of venous leg ulcers, although some authorities recommend significantly higher pressures (Burnand and Layer, 1986; Stemmer et al, 1980).

 

 

The effectiveness of compression bandaging depends on the degree of pressure provided. This is determined by factors such as the physical and elastic properties of the fabric, size and shape of the limb, technique of the bandager and tension in the bandage fabric during application. The relationship between these variables is described by the Laplace equation (Thomas, 2003).

 

 

Sub-bandage pressure measurement
According to Laplace’s law, sub-bandage pressure is directly proportional to bandage tension, but inversely proportional to the radius of curvature of the limb to which it is applied. The equation may be expressed, where ‘n’ is the number of bandage layers applied and 4,620 is a constant:

 

 

Pressure = Tension (Kgf) x n x 4620

 

 

(mmHg) Circumference (cm) x Bandage width (cm)

 

 

Assuming that a bandage of standard width is applied in the form of a spiral with a 50 per cent overlap to a patient with a leg of relatively fixed dimensions, bandage tension is the only real variable in this equation.

 

 

Limb circumference and bandage tension
Values of sub-bandage pressure have been calculated for various combinations of limb circumference and bandage tension (Table 1). These figures assume that two 10cm-wide layers of bandage have been applied with the stated tension to a leg of a specified diameter with a 50 per cent overlap between turns. The highlighted values represent the target value of 40mmHg +/-10 per cent for each size of ankle.

 

 

Application and bandage tension
Unlike stockings or tubular bandages, where the relationship between extension and fabric tension is ‘preprogrammed’ into the product during manufacturing, the tension developed in most flat bandages during application depends on the bandager. Studies have shown that the tension with which bandages are applied varies significantly from person to person, although the pressure achieved by individuals following repeated application of a bandage is much more consistent (Logan et al, 1992).

 

 

This variation has important implications for the degree of pressure applied and therefore the effectiveness of the treatment. If the pressure is too low, the treatment may be ineffective, but if it is too high there is a possibility of tissue damage and necrosis over vulnerable areas (McCollum, 1992; Moffatt, 1992; Thomas, 1992; Danner et al, 1989; Callam et al, 1987).

 

 

Attempts have been made to reduce the effects of operator variability by marking bandages with geometrical shapes that change from oblongs to squares when a particular level of extension (and therefore tension) has been applied (Thomas, 1996). This has been shown to reduce interoperator variability and produce more consistent levels of compression (Logan et al, 1992).

 

 

Changes in tension and bandage characteristics
The use of application guides, however, fails to address a second problem associated with the use of extensible bandages, specifically the effect of changes in extension on sub-bandage pressure. These changes can occur during or after application - perhaps due to decreased limb size following an elevation or a reduction in oedema. The effects of these changes upon the bandage tension, and therefore the applied pressure, are determined by the physical characteristics of the fabric.

 

 

Hysteresis
This phenomenon may be examined in the laboratory using a constant rate of traverse machine, which records the tension in a test sample while extending it at a predetermined rate. The relationship between extension and bandage tension can be expressed graphically as a load extension curve (Fig 1). The curve has two components: the extension curve (shown in blue), which expresses the relationship between extension and tension as the bandage is stretched; and the regain or retraction curve (shown in red), which records the same parameters as the bandage is allowed to return to its unstretched state. These two curves can rarely be superimposed, and the difference between them is termed ‘hysteresis’.

 

 

For a truly elastic material such as a piece of rubber or a coiled spring, there is a direct relationship between extension and applied force, resulting in a load extension curve that is virtually a straight line for much of its length, with relatively little hysteresis.

 

 

The elastomeric yarns used in some high-compression bandages would, if tested in isolation, tend to perform in this way - although some hysteresis would still be evident. Once incorporated into a bandage fabric, however, their extensibility may be partially inhibited by frictional effects taking place between the non-elastomeric yarns that make up the bandage.

 

 

Depending upon the method of construction, a significant part of the applied force may be dissipated on these textile components, particularly as the bandage is stretched for the first time when all the ‘kinks’ are removed from the textile yarns. Subsequent load extension curves do not show such a marked effect, with a consequent reduction in hysteresis.

 

 

Decay
The tension in elastomeric materials subjected to a sustained extension force gradually reduces over time, typically by 10-20 per cent over a 24-hour period. Although most of this tension is lost in the first couple of hours, the process continues over an extended period at a much slower rate. This is termed ‘decay’ and represents a problem for manufacturers who wish to develop bandages that can deliver high levels of compression in situ over an extended period. Thus, a bandage designed to deliver a specific level of pressure after three days constant use must be applied with higher pressure to allow for this natural decay.

 

 

Bandages that do not contain a significant amount of elastomer, but rely upon heavily twisted textile yarns to impart a degree of elasticity, also exhibit decay. They produce extensibility curves that show a rapid change in tension for relatively small changes in extension, particularly on the important regain portion of the curve. Example of load extensibility curves for two such a bandages - a simple crepe-type bandage and a typical compression bandage - are shown in Fig 2.

 

 

With such bandages, small changes or variations in extension that occur during or after application result in marked changes in tension and therefore sub-bandage pressure. The elastic properties of crepe bandages tend to be extremely poor (Thomas et al, 1986), often requiring frequent reapplication to maintain therapeutic pressure levels (Tennant et al, 1988; Raj et al, 1980).

 

 

Bandages containing significant quantities of elastomer perform better in this respect. These are also better able to maintain applied tension, and ‘follow-in’ as leg circumference is reduced with minimal effect upon sub-bandage pressure.

 

 

A new elastomer
Recently a new type of elastomer was developed, which when incorporated into a bandage, is claimed to impart significant advantages over conventional products. When the bandage is stretched, the tension within it initially increases in a similar way to conventional materials. Reversible changes then take place in the chemical structure of the elastomeric polymer as a result of which further extension can be achieved with relatively little additional tension. This results in a marked decrease in the slope of the load extension curve.

 

 

According to Laplace’s law, limbs of different circumference require bandages to be applied with different levels of tension in order to achieve a consistent level of compression. This means that bandages containing the new elastomer must be produced in a range of sizes. By controlling the number of elastomeric yarns used in their construction, it is possible to produce bandages that can be ‘tuned’ to deliver the required tension, and therefore pressure, on limbs of different dimensions. This study was undertaken to investigate the physical characteristics of the new bandages and determine their projected pressure profiles under simulated ‘real-life’ conditions.

 

 

Study materials
The bandages in the study were made using a new textile technology. They are of woven construction and coated with a pressure-sensitive hot-melt adhesive in a ladder pattern to reduce slippage and keep the system in place for extended periods. This adhesive is latex-free and does not contain any other potential skin irritants. The bandages are available in three sizes, colour-coded as follows:

 

 

- Small (red line) - ankle size 18-22cm;

 

 

- Medium (yellow line) - ankle size 22-28cm;

 

 

- Large (green line) - ankle size 28-32cm.

 

 

Test methods
Load extension characteristics and performance of application guides

 

 

After conducting initial studies to confirm that each type of bandage performed consistently from roll to roll, representative samples of each were placed in turn in the jaws of a constant rate of traverse machine (Instron) in computer-controlled, cyclical mode with a crosshead speed of 200mm per minute and a jaw separation of 200mm. Each sample was cycled three times between this value and a maximum length of 400mm, equivalent to 100 per cent extension.

 

 

The bandages have application aids whereby an oval becomes a circle when the correct extension is achieved. The geometry of the application aids was continuously monitored on both the extension and regain parts of the cycle by one operator and the value of extension at which the oval became a circle was recorded by a second operator from a digital display on the Instron. Judging the exact moment at which this occurs is prone to error in a dynamic situation, so upon completion of each test, the bandage was slowly extended a fourth time using the Instron in manual mode until the application aids indicated that the required extension had been achieved. The bandage was then held at this extension and the tension recorded. Values of displacement and tension recorded for each bandage were downloaded and transferred into a spreadsheet for further examination.

 

 

Decay characteristics
In a second series of tests, samples of each bandage were placed in the Instron and cycled once to a value equivalent to 100 per cent extension then back to their original length. Each bandage was extended a second time to the extension value determined during the static test described above, and held there for 15 hours, during which time the tension was recorded.

 

 

Results
Load extension curves of the new textile technology bandage

 

 

Load extension curves for one of the bandages, together with those of a second compression bandage for comparison purposes are shown in Figs 3 and 4. The blue line represents the first cycle and the yellow and pink lines the second and third cycles respectively.

 

 

Performance of application guides
The tension in each bandage sample corresponding to the extension values dictated by the application guide, determined during each stage of the cycling process, was extracted from each of the various datasets.

 

 

These values, together with those obtained in the more easily controlled static test, revealed obvious differences in the percentage extension and tension values recorded during the extraction and retraction portions of each cycle.

 

 

These differences are most pronounced in the small size bandage and least in the large size bandage. In each case there was also a marked difference between the tensions in the first extension curves and the subsequent values. It is postulated that these differences are due to the effects of the adhesive layer and interactions between the non-elastomeric textile components of the bandage fabric, which are present in higher proportions in the bandages designed for the smaller limbs.

 

 

Within the limits of experimental error, given the problems of assessing the performance of the application guides in a bandage being subjected to continual movement, the tension obtained from the static test was in each case comparable with that obtained from the retraction curves in the dynamic test. However, differences remained between the recorded extension values.

 

 

Predicted sub-bandage pressures
From the various tension values obtained during testing, the pressures that each bandage would apply to limbs of different sizes were calculated (highlighted in blue in Table 2). Several tension and pressure values are quoted for each size of bandage:

 

 

- Maximum tension determines the pressure that would be achieved if the bandage were gently extended to the point at which the oval guide became a circle and then the bandage applied immediately to a limb of the stated circumference;

 

 

- Initial tension is the value obtained during the static test and equates to the tension that would be present in the bandage if it were slightly overstretched and allowed to relax prior to application (i.e. applied on the first regain curve not the first extension curve as above).

 

 

- The two remaining values abstracted from the decay testing, recorded tension and calculated pressure that the bandage could be expected to achieve four and 15 hours after application.

 

 

The effect of small changes in bandage extension on sub-bandage pressure
One of the key features claimed for the new bandage is the shallow extensibility curve of the elastomer over the working range, which should ensure that minor changes in extension have little effect upon applied sub-bandage pressure. In order to determine the significance of this, the effect on bandage tension (and therefore sub-bandage pressure) of a reduction in extension from 65 to 55 per cent was determined from the third retraction curve of each bandage using the datasets that had been collected previously (Table 3).

 

 

Discussion
The literature strongly suggests that although graduated external compression is a key factor in the successful treatment of leg ulcers associated with venous insufficiency, the optimum pressure remains a matter of debate. Many authors have recommended a target of 40mmHg at the ankle, but this is an average and takes no account of the patient’s height or body mass. Although it is generally accepted that pressures significantly below this may be of limited clinical efficacy, virtually no information is available on a safe upper limit for externally applied sustained compression.

 

 

A few references are available describing tissue damage resulting in the formation of ulcers on the dorsum of the foot or tissue necrosis over the tibia caused by badly applied bandages, but these make no mention of the pressures required to cause these adverse effects. The evidence suggests that this damage occurs when bandages are applied with too great an overlap, effectively resulting in three or even four layers of fabric over a particular point. If the bandage is applied with sufficient tension to achieve 40mmHg with two layers of fabric, the additional layers could easily achieve 60mmHg or even 80mmHg overall.

 

 

Not surprisingly, no clinical studies have been undertaken specifically to determine the amount of pressure that must be applied by a bandage to cause tissue damage to human legs. However, some interesting information has been published on the effect of external pressure on tissue viability in the context of pressure ulcer prevention and this may have some relevance.

 

 

Bader (1990) reviewed various studies designed to determine the effects of compressive loading on the viability of human tissue using a variety of techniques. In one study, involving measurement of transcutaneous oxygen tension (TCPO2), considerable variation between subjects was recorded in the pressure required to achieve 50 per cent of the uncompressed value (22-92mmHg). In contrast, 60mmHg was found to produce arterial occlusion and virtually eliminate TCPO2 in all but three of the 17 subjects examined.

 

 

Michel and Gillott (1990) similarly found that 60mmHg arrested microcirculation in an animal model and inhibited clearance of 133Xe from the skin on the volar aspect of the forearm and in the parasacral region. In the absence of conflicting evidence, it therefore seems reasonable to adopt 60mmHg as a maximum upper limit for sub-bandage pressure, particularly over bony prominences.

 

 

Excluding theoretical maximum tension values, which can be avoided by slightly overstretching the bandage prior to application, the pressures the bandages in this study are predicted to achieve on limbs of different sizes range from 59mmHg to 40mmHg immediately following application. They progressively fall to a maximum of 52mmHg after 15 hours. As such, they are below the proposed 60mmHg upper limit (see Table 2).

 

 

As might be expected from Laplace’s law, the highest pressures are associated with the smallest limbs in each of the recommended ranges. Consequently, for this reason, the manufacturer has recommended that for an ankle circumference of around 22cm or 28cm (the point at which there is a crossover between different kits) the bandage system used should be that appropriate to the smaller limb circumference to avoid the possibility of tissue damage. This means the use of a red kit for a 22cm ankle and yellow for a 28cm ankle.

 

 

Conclusions
The new bandage range appears to represent a real advance in technology, but like any powerful therapeutic tool it must be used appropriately by practitioners who have been adequately trained.

 

 

The novel properties of the bandages’ elastomeric component significantly reduce the effects of limited changes in extension upon sub-bandage pressure. Furthermore, the relatively shallow extensibility curve of the fabric, combined with the application guides, will undoubtedly reduce the effects of operator variability. This will ensure that the bandages perform in a consistent fashion, provided they are applied with no more than a 50 per cent overlap.

 

 

The manufacturer aimed to produce a product that delivers clinically effective compression over an extended period. This has inevitably meant that the bandage is designed to deliver pressures initially around the upper limit of the range currently recommended for the treatment of venous disease to allow for the inevitable decay in pressure over time.

 

 

As with all compression bandages, inappropriate or incorrect application can result in pressures exceeding the recommended values, with the potential to cause tissue damage over bony prominences unless these are well protected.

 

 

Whichever bandage is applied, the results of this study suggest that it should be partially prestretched prior to application, particularly when bandaging limbs with a circumference towards the lower end of the recommended range. Provided that potential users receive comprehensive education and training, the new bandage should make an important contribution to the management of venous disease.

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