## 3 Systems and Methods of Compression Measurement  
Orthopaedic compression is the direct application of pressure to a limb, which is usually expressed in mmHg or Pa. Currently, there is no uniform classification of compression classes. Furthermore, there is no uniform standard for measuring product compression. Some countries have national standards or use other countries' national standards; for example, the United States Health Services and other pharmaceutical retailers in Europe use British standards [59]: [32; 40; 60; 61]. There are three different ways to determine the compression: (a) direct in vivo method, (b) indirect in vitro method and (c) control of treatment therapies [62]. The essential principle of the direct in vivo measurement method is that the pressure measurement is carried out directly on the affected limb. During this kind of test, a person wears gradual elastic compression stockings, and the pressure sensor measures the contact pressure along a leg [27; 31; 61; 63]. The SIGG-test (SIGaT-Sigvaris, Germany) and PicoPress (MediGroup, Stafford, Australia) are two of the well-known devices used for in vivo measurement of compression. There are a number of drawbacks to the direct in vivo measurement method--it is quite difficult to achieve appropriate precision, especially when a person breathes, moves or is tired. This measurement method is not suitable for the repeatable evaluation of product parameters, because the same person may not be able to test the product for a long time. In addition, dimensions of the same human limb change over time [64]. The basic principle of the indirect in vitro method is the determination of the stretching behaviour of a knit or a woven fabric. The compression product is fastened on a special mannequin with sensors that measure compression. These measurement methods are applied to the German standard RAL-GZ 387/1:2008 and British standard BS 6612:1985 [59]. The indirect (theoretical) method of determining compression is the compression calculation according to Laplace's Law [65; 66]:  
\[P=\frac{2\cdot\pi\cdot F}{S} \tag{1}\]  
where \(P\) is the pressure in Pa, \(F\) is the tensile force in N, \(S\) is the area of the specimen in m\({}^{2}\). In this case, compression is evaluated as a function of the tensile force and the area of the product. There are many devices used for measuring the pressure to a limb, i.e., for extrapolation of compression from tensile force measurement. The two most known devices for compression testing of stockings are HATRA (Hosiery and Allied Trades Research Association, UK), which is required for validation under the British standards (BS 6612:1985, BS 7563:1999, BS 7672:1993) [59], and HOSY (Forschungsinstitut Hohenstein, Hohenstein, Germany), which is required for validation under the German standard RAL-GZ-387/1 [38; 60]. However, this method of compression measurement fails to capture compression changes resulting from complex geometry and differences in the shape of the limb. During the design and production processes of compression products, the modelling of the product must be based on the analysis of the compression phenomenon [66; 67; 68]. The mechanical properties of the materials used and, especially, the relaxation phenomenon that occurs during long-term wear must be evaluated, too. Due to the stress relaxation, compression values, if compared the newly produced product and the same product after several hours of wearing, can differ significantly [66]. Currently, the design and production of compression products are based on an identical percentage decrease between the dimensions of the main structural perimeter compared with the corresponding values of the patient's body dimensions [19]. Moreover, it is highly important to combine appropriate compression and individual consumer parameters. Therefore, measurements of the volumes of the limb usually are executed in collaboration with doctors and manufacturers.