Anatomical Concepts (UK)
Masters of Rehabilitation Engineering
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The PRAFO and preventing pressure sores

The PRAFO designs of ankle foot orthoses are an effective way of preventing or healing pressure sores at the heel. So why is it successful in achieving this in both immobile and ambulant patients? The answer to the PRAFO’s effectiveness is very simple - even if understanding how pressure ulcers can develop is complex.

Pressure ulcers remain a problem but are preventable

The term “Pressure Ulcer” suggests the long standing belief that “high pressure” at the interface between body tissues and a support surface is the most important extrinsic factor in pressure ulcer development. This suggests logically that high pressure applied locally to tissue is bad - and low pressure is good. However things are not quite that simple as we shall see.

Recently, mechanical terms such as shear and friction have become part of the discussion but are usually poorly understood by clinicians. To be fair, these concepts are complex and interact with each other. Nor can they be measured within tissues where the damage is actually done. The mechanical environment described via pressure, shear and friction is certainly important in the development of pressure sores along with the microclimate of temperature and moisture.

Let’s start with the concept of pressure. Imagine a person lying on a bed and we focus just on the area of their heels in contact with the bed surface. Pressure is defined as the amount of force applied perpendicular to a surface per unit area of application.

So the force applied over a small area of contact such as the heel (where there is a bony prominence with little tissue covering) will produce greater local pressure than the same force applied over a much larger area.

The illusion of “safe” combinations of pressure and time

Since the 1930’s the primary mechanism of pressure-induced tissue damage has been thought to be localised blood flow reduction.

In the late 1950’s the duration of pressure applied to an area became implicated in pressure ulcer development. In the 1970’s Reswick and Rogers published work that, on the face of it, mapped out combinations of pressure and time of application that defined injurious and non-injurious situations. The notion being that small amounts of pressure could be tolerated for longer than high pressures. This led to the characteristic curves depicted here.

Whilst the general characteristic is intuitively appealing in the sense that it is likely that low pressures can be tolerated for longer than high pressures, in fact the data at the extremes of the timescales were based on extrapolation and so subject to error. We really have no idea in an individual situation what a “safe” level of pressure and time might be.

Let’s take a look at what happens under the surface when pressure is applied to our patients heel.

Comparing the loaded and unloaded situations we can imagine the soft tissues being deformed internally due to the surface pressure. In fact, some areas of tissue directly under the bone will be subject to compressive stress whilst areas on either side will be subject to high shear and tensile stresses.

Since the 1980’s we have known that internal tissue stresses cannot be predicted by means of interface pressure measurements. Earlier work had suggested that tissue damage was caused when blood flow was restricted and prolonged by compressing capillaries in the tissue. The figure of 32 mmHg of pressure derived from work in the 30’s is still sometimes quoted as that necessary to prevent blood flow through a capillary. However, subsequent work has shown this figure to vary widely in practice.

Pressure applied to tissue causes complex patterns of shear effects

We think that the reason shear effects are so powerful is that they are very efficient at shutting off capillary blood flow.

Reducing risk of pressure sores at the heel seems an overwhelming problem once we start to look at the tissue mechanics - and we haven’t even delved into the powerful effects of the intrinsic factors such as poor nutrition, dry skin and concomitant disease.

Despite the complexity we can still rely on the fact that reducing pressure applied to an area at risk such as the heel is important. We have seen that even if the total force being applied does not and cannot change (we cannot eliminate gravity) we can reduce the pressure in practice by applying the force over a larger area.

650SKG completely offloads the heel area whether the patient is ambulant or recumbent

The reason the PRAFO range is so effective, to respond to our statement at the start of this article, is that it can totally offload the heel area at risk of pressure ulcers and safely apply these loads to areas of the body which are better able to tolerate them safely. This sidesteps the problem of trying to estimate what is a safe level of pressure. The at risk heel area is totally offloaded from pressure and shear as there is no contact at the areas of greatest risk..