Determinants of pressure exerted by medical compression stockings

Abstract

Summary Objective: The force exerted on a limb by a medical compression stocking (MCS) is given by the properties of the yarn and the technique of knitting. The pressure determined by the manufacturer should be the one acting on the leg provided the stocking fits the leg shape. However, the interface pressure is governed by additional factors which act simultaneously in a poorly defined manner. We sorted out the different material- and patient-related components and assessed their relative role combining in vitro with in situ techniques. Design and method: The force exerted by the stockings was measured with the Zwick and Hosy dynamometers and the pressure exerted on the leg with the SIGaT instrument placed underneath the stockings at various sites. Leg circumference was determined with the Perometer and local radius calculated from computer tomographic slices and assessed with templates. Stockings exerting ankle pressures between 10 and 40 mmHg were tested. Subjects serving in the various experiments were selected from a pool of 28 healthy volunteers. Results: All stockings exerted the predefined pressure when measured at the stocking/leg-interface (in situ) and on the dynamometer (in vitro) provided the local leg curvature was taken into account. The stockings showed slightly different elastic properties (pressure amplitude difference upon leg movement 2–6mmHg). The interface pressure varied significantly when measured at different angular positions on the same circumference (range of change +40%; –20%). The differences were strongly correlated with the local radius (range of variation along the calf circumference 35–125 mm). The differences of local radii got smaller with higher interface pressures. Leg movements provoked simultaneous increases and decreases of interface pressure of up to 15 mmHg depending on the site of measurement. Leg circumference varied only slightly with movements pointing in either direction: 0.25 cm at the ankle (range +0.4; –0.4) and 0.33 cm at the calf (range +1.05; –1.45), respectively. The compressibility of the underlying tissue had no effect on the interface pressure. Conclusions: The interface pressure can be assessed reliably in vitro and in situ. It shows complex, quantitatively important non-uniform local variations both when measured along a given leg circumference at rest and when monitored at one particular site during movements. The changes reflect the discontinuity of the leg configuration and further alterations of the silhouette occurring with every muscle activation. Clearly, a massage effect of stockings is documented which does not necessarily reflect a global effect exerted on the leg.