Interface pressures
A bandage is also defined by the interface pressures that it exerts on the limbs, i.e. the pressure measured in vivo between the bandage and the skin (read more...)
The interface pressure is the pressure measured in vivo between the compression system applied to a limb and the skin.
This measurement is performed using sensors applied directly to the patient’s limb, before application of the compression system being assessed, at standardised points (for example, at the base of the calf muscle at point B1, i.e. 10-15 cm above the ankle bone, for assessment of pressure at the ankle).
This measurement still requires international standardisation of the method since it depends primarily on the type of the sensor used and the anatomical zones at which the measurements are taken.
This pressure is expressed in mmHg.
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In the past, only technical data produced using laboratory measurements were taken into account to compare bandages. However, in the last few years, the position of international experts has shifted and new international recommendations tend towards in vivo measurement of interface pressures.
Largely overlooked until recently, the interface pressure is now recognised as being the most important biophysical property of compression systems applied to patients.
This pressure varies depending on the type of bandage and whether the limb on which the measurement is being taken is :
- immobile, giving rise to a resting pressure (RP) produced directly by the passive tension exerted by the compression system
- mobile, giving rise to a working pressure (WP) produced intermittently on exertion when the muscle is exercised.
The difference between these 2 pressures corresponds to the static stiffness index of the bandage.
The higher this index is, the more rigid the bandage and the more it fits into the short-stretch (low-elasticity) category.
The lower this index is, the more elastic the bandage it and the more it fits into the long-stretch category.
(Read more...)
Resting pressure, working pressure, static stiffness index
Since measurements of the pressure exerted by a compression system are performed in vivo in healthy volunteers or patients, it is necessary to take into account the mobility of the limb on which the measurement is being taken, and hence the status of the muscle pump.
It is therefore necessary to measure the interface pressures at rest and at work.
The resting pressure (RP) is the pressure exerted by the compression system in the absence of any muscle contraction. It is therefore directly dependent on the tension of the system applied to the limb, corresponding to the minimum passive force exerted continuously on the leg by the compression system.
The working pressure (WP) is the pressure exerted during muscle contraction, on exertion. It corresponds to the intermittent active force produced by the compression system exerting a resistance pressure when the muscle contracts and increases in volume.
The stiffness index is the difference between the working pressure and the resting pressure.
Today, a principal method has been defined to measure this index: the Static Stiffness Index (SSI), which is the difference between the pressures measured standing up and lying down.
Other methods are also suggested in the literature today for dynamic pressure measurement (e.g.: ankles flexed and extended or measured on a treadmill).
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Since the interface pressures depend on the extensibility of a product, short and long-stretch bandages do not therefore offer the same in vivo interface pressure profiles.
These characteristics will have a direct influence on the expected result on the veno-lymphatic system.
(see chapter on compressive effects as a function of different bandage types)
