Hrev_master Veins and Lymphatics 2013; volume 2:e6 [Veins and Lymphatics 2013; 2:e6] [page 17] Elasticity, hysteresis and stiffness: the magic triangle H.A. Martino Neumann Department of Dermatology, Erasmus MC University Medical Center, Rotterdam, The Netherlands Abstract The use of external compression on the human leg is still a cornerstone in the treat- ment of venous diseases. The most important question to answer is: how will compression perform on the human leg? Introduction First of all the applied compression generated by a device as a medical elastic compression stocking (MECS), bandage or whatever is used, exerts its pressure on the surface of the leg, e.g. the skin. Normally this is expressed as interface pressure and the shape of the leg pressure dif- ferences are depending on Laplace low. Second step is the transmission of this interface pres- sure into the tissue as the subcutaneous fat, muscles, and veins. This process depends on Pascal law. The third item is the pressure changes during walking depending on the cir- cumference chances of the leg (Laplace law) and, fourth, the durability of the pressure in time, which depends on the quality of the device. Brief Report For a long time research was focused on interface pressure, usually under static condi- tions and pressure course in time.1 The quality of compression capacity of a given device is depending on the characteris- tics of the used materials. All used materials for medical compression therapy have three major characteristics: i) elasticity, which is the capacity to return to the original shape and size after the material has been stretched. The pressure/elasticity relation under static condition on the leg is influenced by Laplace law; ii) stiffness or elasticity coeffi- cient; this term is defined as the increase in pressure after a certain given elongation. For MECS the Centre Européenne de Normali - sation uses the increase of the normal ten- sion at the B1 level with 1 cm expressed in hpa. Stiffness is depending on elasticity in static condition; iii) hysteresis, which reflects the inborn resistance of material as result of internal friction hysteresis, can be visualized in a force/elongation curve (Figure 1A). By increasing the speed to perform such a fair elongation curve the angle towards the x-axis will move. So hysteresis is influenced by the speed of movements (Figure 1B).2 These three characteristics works all togeth- er in compression therapy (Figure 2). As nor- mally compression is only expressed as inter- face pressure we are not informed about the contribution of stiffness, hysteresis and changes during walking. In fact we only know the resting pressure which is far away from the reality of a walking patient with a compres- sion device as a MECS. To overcome this prob- lem we defined the dynamic stiffness index (DSI).3 Analyzing the differences between static and dynamic compression it turns out that hysteresis is the most important factor. Our triangle can be changed from static (Figure 2) into dynamic (Figure 3) where hys- teresis plays the main role. As pressure diminishes in time static com- pression will become ineffective during the day. However DSI remains in the same time (Figure 4).4 For MECS, DSI is independent from compression pressure (class) and manu- facturing differences as round- and flat knit- ted.5 The clinical implication of DSI is that low Correspondence: H.A. Martino Neumann, Dermatologie en Venereologie, Erasmus MC University Medical Center, Rotterdam, The Netherlands. E-mail: h.neumann@erasmusmc.nl Key words: compression, elasticity, hysteresis, stiffness. Conference presentation: part of this paper was presented at the International Compression Club (ICC) Meeting on Stiffness of Compression Devices, 2012 May 25, Vienna, Austria (http:// www.icc-compressionclub.com/). Received for publication: 9 October 2012. Revision received: 15 November 2012. Accepted for publication: 29 November 2012. This work is licensed under a Creative Commons Attribution 3.0 License (by-nc 3.0). ©Copyright H.A.M. Neumann, 2013 Licensee PAGEPress, Italy Veins and Lymphatics 2013; 2:e6 doi:10.4081/vl.2013.e6 Figure 1. A) Hysteresis curve of an elastic fabric: X-axis represents stretch, y-axis the applied force; B) force elongation curve of elastic knitwear. The elongation increments are progressively made larger. The steepness of the initially small cycle is diminished with the increased amplitude (modified from Stolk and Salz, 19882 with permission). Figure 2. The magic triangle, static. No n c om me rci al us e o nly Conference presentation [page 18] [Veins and Lymphatics 2013; 2:e6] compression and high DSI can be very effi- cient for ambulatory patients and have the same effect as high compression with low DSI. To combine compression and DSI the physi- cian can prescribe the optimal device, e.g. MECS for the patient. As logical consequence the higher changes of interface pressure dur- ing walking will be transferred to the tissue resulting in a high massage effect and by this the effects of Laplace and Pascal law comes together. Conclusions In order to optimize venous function with compression therapy, three key-points should be considered: i) hysteresis, mainly influenced by the type of knitwear determines the efficacy of compression force elongation relation; ii) the quality of compression (Laplace law) defined by DSI; iii) the final effect of compres- sion (Pascal law) defined by the composition of the subcutaneous tissue. For daily practice: DSI is the most important characteristic of compression. References 1. Veraart JCJM, Daamen E, Neumann HAM. Short stretch versus elastic bandages: effect of time and walking. Phlebology 1997;26:19-24. 2. Stolk R, Salz P. A quick pressure-determi- ning device for medical stockings based on the determination of the counter-pressure of air- filled leg-segments. Swiss Med 1988;10:91-6. 3. Stolk R, van der Wegen-Franken CPM, Neumann HAM. A method for measuring the dynamic behaviour of medical com- pression hosiery during walking. Dermatol Surg 2004;30:729-73. 4. Van der Wegen CPM, Tank B, Nijsten T, Neumann HAM. Changes in the pressure and the dynamic stiffness index of medical elastic compression stockings after having been worn for eight hours: a pilot study. Phlebology 2009;24:31-7. 5. Van der Wegen-Franken CPM, Tank B, Neumann HAM. Correlation between the static and dynamic stiffness indices of medical elastic compression stockings. Dermatol Surg 2008;34:1477-85. Figure 3. Dynamic relation between pres- sure, stiffness and hysteresis. Figure 4. In spite of decreasing pressure of a stocking, stiffness is maintained (modi- fied from Van der Wegen et al., 20094 with permission). No n c om me rci al us e o nly