Hrev_master Veins and Lymphatics 2013; volume 2:e4 [Veins and Lymphatics 2013; 2:e4] [page 11] Terminology: resistance or stiffness for medical compression stockings? André Cornu-Thenard, Jean-Patrick Benigni, Jean-François Uhl French University Group for Medical Compression, Saints Peres University, Paris, France Abstract Based on previous experimental work with medical compression stockings it is proposed to restrict the term stiffness to measurements on the human leg and rather to speak about resistance when it comes to characterize the elastic property of compression hosiery in the textile laboratory. Introduction Pressure and stiffness are the two items which characterize a medical compression stocking (MCS).1-6 The meaning of pressure is easy to understand for a health care profes- sional. The correct meaning of stiffness is less easy to explain, especially since this word can relate to two different concepts. Laboratory pressure and interface pressure: definition Pressure is defined as a force per unit of surface area, for example Newton/m² or cN/cm². For many reasons medical compres- sion manufacturers and doctors prefer using mmHg.7,8 Two different pressures should be differentiated: laboratory and in vivo pressures The laboratory (lab) pressure is determined by manufacturers using a dynamometer, a special device made only for these measure- ments (Figures 1 and 2).8 Several brands of dynamometers exist and all give measure- ments in cN/cm² (force/cm²) easily trans- formed in mmHg.8,9 The stocking to be measured is placed on a model leg so as to locate and mark the differ- ent points along the leg (B, C, D, etc.). The B point (ankle region of the stocking) is marked first and then the B-segment is placed in the dynamometer jaws. Force is measured during stretch and also in the relaxed phase. Results are printed on a rolling chart. Hysteresis curves obtained: on the x-axis the circumference of the MCS is plotted in centimeter (which simulates the leg’s perimeter) and on the y-axis the correspon- ding pressure in mmHg (Figure 3). Therefore it is easy to identify the MCS pressure depending on its size. This permits to declare the lab pressure in mmHg (or the compression class) on the box of the garment. The pressure on the human leg is measured in clinical studies (or due to personal inter- est) by using special pressure probes as Kikuhïme (TT MediTrade, Sore, Denmark) or Picopress® [(Microlab Elettronica Sas, Roncaglia di Ponte San Nicolò (PD), Italy]. The sensor is placed on the B1 point where the medial gastrocnemius muscle turns into its tendinous part and the MCS is applied.10 The pressure measured on the leg in mmHg is called the interface pressure.1-5 This method allows the pressure measure- ment at several levels along a leg. Resistance and stiffness: definition In the European Prestandard for medical compression hosiery stiffness is defined as the increase in compression per centimeter increase in the circumference of the leg.6 Two different types of Stiffness exist: the stiffness on the human leg following the above definition and the corresponding parameter derived from the hysteresis curve. In fact the same word is used in two situa- tions: for the lab measurement of stiffness used by the manufacturers and the stiffness measurements on human legs made by inves- tigators in the course of their assessment of the quality of MCS. Such a distinction should be made by presenters and authors when dis- cussing this topic. Therefore in an oral presentation or publi- cation there may be some confusion: Do the author mean lab or in vivo stiffness? Proposition Pressure is measured in two different situ- ations: in lab and in vivo. The same two situ- ations exist for the measurement of stiffness. The word used by industry to characterize the hardness or rigidity of numerous materials, for example in physics or aeronautics, is the word resistance. The authors and some International Compression Club (ICC) mem- bers propose that this word should be used in our Medical Compression vocabulary which means inelasticity.11 Perhaps words similar to resistance or resistance coefficient could be used such as hardness, rigidity, firmness, inelasticity and others. Correspondence: André Cornu-Thenard, French University Group for Medical Compression, Saints Peres University, 45 rue des Saints Pères 75005 Paris, France. E-mail: andre.cornuthenard@wanadoo.fr Key words: resistance, stiffness, compression stockings. 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/). Contributions: AC-T, manuscript writing; J-PB, J- F, ideas and experience providing. Received for publication: 2 November 2012. Revision received: 18 February 2013. Accepted for publication: 18 February 2013. This work is licensed under a Creative Commons Attribution 3.0 License (by-nc 3.0). ©Copyright A. Cornu-Thenard et al., 2013 Licensee PAGEPress, Italy Veins and Lymphatics 2013; 2:e4 doi:10.4081/vl.2013.e4 Figure 1. The IFTH dynamometer (Paris, France). No n- co mm er cia l u se on ly Conference presentation [page 12] [Veins and Lymphatics 2013; 2:e4] Definition and measurement The resistance (laboratory meas- urement) The authors suggest the definition of resist- ance in medical compression as the stiffness measurement performed by a dynamometer. The value should be declared on the packaging for individual compression garments. At present this value is not shown, perhaps to avoid confu- sion or questions from interested users. The resistance coefficient (RC) number will reflect the hysteresis curve at the MCS size point. In the curve shown in Figure 3 the RC is +/-1 mmHg/cm. This means that this MCS is more rigid, firm or resistant than a 0.5 mmHg/cm and less resistant than a 2 mmHg/cm. The stiffness (measurement on the leg) At the B1 point two measurements of the interface pressure are done during two succes- sive different positions of the leg, at rest and during a significant muscle contraction (e.g. dor- siflexion, standing). This will create two differ- ent but similar circumferences, one maximum the other minimum. The difference between the two values characterizes the stiffness of the MCS.1,11 The properties of any MCS can there- fore be more completely described using the fol- lowing measurements: the pressure and the RC measured in the lab, and interface pressure and stiffness measured on the leg. Arguments to differentiate resistance of a medical compression stocking and its stiffness In summary arguments to differentiate resistance of a MCS and its stiffness are: i) the two measured points are different: B point for resistance and B1 point for stiffness; ii) the two values cannot be compared (for the moment): - the resistance results are obtained in mmHg/cm corresponding to the steepness of hysteresis curves using a dynamometer; - for stiffness only pressure increase is meas- ured as a routine but not the change of leg circumference. To consider these parameters could yield much useful information: i) MCS characteris- tics should be completed and recorded on the box; ii) this would allow a useful comparison between different brands of MCS. Conclusions To avoid confusions it could be extremely useful if ICC members, companies and doctors agree with this proposed terminology: resistance instead of stiffness measured in lab and stiff- ness measured on the leg. References 1. Partsch H, Clark M, Bassez S, et al. Measurement of lower leg compression in vivo. Recommendations for the performance of measurements of interface pressure and stiffness. A consensus statement. J Dermatol Surg 2006;32:224-33. 2. Rabe E, Partsch H, Jünger M, et al. Guidelines for clinical studies with compression devices in patients with venous disorders. Eur J Vasc Endovasc Surg 2008;35:494-500. 3. Partsch H, Flour M, Coleridge Smith P, et al. Indications for compression therapy in venous and lymphatic disease - a consensus. Int Angiol 2008;27:193-219. 4. Khaburi JA, Nelson EA, Hutchinson J, Dehghani-Sanji AA. Impact of variation in limb shape on sub-bandage interface pres- sure. Phlebology 2011;26:20-8. 5. Hirai M, Niimi K, Iwata H, et al. Comparaison of stiffness and interface pressure during rest and exrecice among various arms sleeves. Phlebology 2010;25:196-200. 6. European Committee for Standardization (CEN). Non active medical devices. Working Group 2 ENV 12718: European Pre-standard ‘Medical Compression Hosiery.’ CEN TC 205. Brussels: CEN; 2001. 7. Cornu-Thenard A. Measuring units for elastic stockings: priority to mmHg rather than class- es. Phlébologie 1992;45: 457-8. 8. Partsch H. Evidence based compression-ther- apy. An initiative of the International Union of Phlebology (IUP). VASA 2004;34:3-37. 9. Stolk R. Quick pressure determining device for Medical Stockings. Swiss Med 1988;10:91- 6. 10. Stout N, Partsch H, Szolnoky G, et al. Chronic edema of the lower extremities: international consensus recommendations for compres- sion therapy clinical research trials. Int Angiol 2012;31:316-29. 11. Cornu-Thenard A. Reduction of a venous edema by elastic stockings, unique or super- imposed. Resistance coefficient notion. Phlébologie 1985;38:159-68. [Abstract in English]. Figure 3. Hysteresis curve of a 25 mmHg medical compression stocking (MCS) with a 23- 24 cm size. The resistance coefficient equals the tangent at the MCS size point. On this hysteresis curve the pressure increases in 1 mmHg between 23 and 24 cm. So the resist- ance coefficient equals 1 mmHg on 1 cm, equals 1. Figure 2. The HOSY dynamometer (Germany). No n- co mm er cia l u se on ly