Hrev_master Veins and Lymphatics 2013; volume 2:e3 [Veins and Lymphatics 2013; 2:e3] [page 7] The Mannequin-leg: a new instrument to assess stiffness of compression materials Masafumi Hirai,1 Hugo Partsch2 1Department of Vascular Surgery, Tohkai Hospital, Nagoya, Japan; 2Private prac- tice, Vienna, Austria Abstract Stiffness of compression material, which has major impact on the performance of the used product, has mainly been investigated by clini- cal in vivo experiments up to now. Experimental two-centre study has been per- formed in Japan and in Austria. Results are presented using a novel leg model, whose cir- cumference can mechanically be extended by 1 cm. The change of the interface pressure meas- ured under a compression device corresponds to its stiffness. Inelastic and multi-component bandages show stiffness values which are more than three times higher than those of elastic bandages and of compression stockings. There is a significant correlation between the stiff- ness values measured with the simple man- nequin-leg and those obtained from exten- someter measurements (Hohen stein-method) on one hand, and also with data on the human leg (static stiffness index) on the other hand. The average variation coefficient with repeated measurements is 5.4%. The absolute values dif- fer with the used pressure probes. The newly developed mannequin-leg offers a simple method to measure and to compare the stiff- ness of compression stockings and bandages, including the combination of such devices. Introduction In the last years several experimental stud- ies have clearly shown that stiffness is an important parameter determining the perform- ance and efficiency of a compression product. In patients with chronic venous insufficiency higher stiffness is associated with a stronger effect concerning reduction of venous reflux,1 improved venous pumping function2,3 and edema reduction.4 Measurements of the inter- face pressure of compression products on the leg in the lying and standing position allowed us to assess stiffness of a specific device in vivo and to correlate the so-called static stiff- ness index, which is the difference of standing minus lying pressure with the efficacy of the venous calf pump.5,6 Laboratory tests using dif- ferent extensometers are used by compression hosiery manufacturers mainly to check the pressure range of the products in relation to the leg size. However, the relationship between stretch and force (the slope of the hysteresis curve), characterizing the elastic property of the product, is not declared to the consumer. The used methodologies (Hosy, Hatra, Instron, ITF, MST-Professional),7 are elaborate, which may be the reason why up to now the stiffness of a specific compression stocking is not declared by the producers. Also the air-filled drum device developed by R. Stolk8 is too sophisticated to be widely used.9 A report will be given on first experiences com- ing from Japan (M.H.) and from Europe (H.P.) achieved with a newly developed leg-model, specifically designed to assess stiffness in an easy manner.10 Materials and Methods This report combines results obtained in the laboratory of the inventor in Japan (M.H.) with data measured in Austria (H.P.). Pressure was measured by air-filled transducers, 1 cm diame- ter, in Japan (air-pack type analyzer, Model AMI- 3037®, AMI Co., Tokyo, Japan), and by Picopress® probes, 4.5 cm diameter [Microlab Elettronica Sas, Roncaglia di Ponte San Nicolò (PD), Italy], in Austria. Following the definition in the European Committee for Standardization document11 stiffness may be defined by the increase of the interface pressure of a compres- sion device on the leg when the circumference increases by 1 cm. This induced Hirai and co- workers to develop an artificial model, the so- called mannequin-leg, whose circumference can be enlarged by 1 cm (Figure 1).10 Flat, air-filled pressure probes are attached to measuring points marked on the model (points B1 and C). (Point B1 on the human leg is characterized by the transition of the medial gastrocnemius mus- cle into the tendon; point C corresponds to a medial point at the level of the largest calf cir- cumference). The pressure is registered imme- diately after application of the compression device and the model is enlarged by pushing down the lever three times. The difference between the highest-pressure increase after the third extension of the model and the following resting pressure is defined as the static stiffness index (SI) (Figure 2). Results Comparison compression stockings versus bandages Compression stockings and elastic bandag- es show significantly reduced stiffness values compared to inelastic bandages (Figure 3).10 As can be seen from Figure 4 compression stockings differ from multi-component band- ages more concerning the stiffness than the exerted pressure. All stockings tested were in a pressure range between 10 and 40 mmHg at B1 (Picopress®), double stockings achieved pressures between 40 and 50 mmHg. Their stiffness (SI) did not exceed 10 mmHg. The tested bandages were in a comparable pres- sure range, but their stiffness values were all higher than 30 mmHg. Elastic tubes wrapped over by elastic bandages (T+E in Figure 4) showed SI values between 10 and 15 mmHg, which were slightly higher than the corre- sponding values of the stockings. Reproducibility Thirteen different compression stockings were applied three times to the mannequin leg and pressure and stiffness were meas- ured. Figure 5 shows that the variation coeffi- cients (VC) were small (3.9-5.4% in average), only applying double stockings over each other resulted in an increase of the VC to Correspondence: Hugo Partsch, Steinhäusl 126, 3033 Altlengbach, Austria. Tel. +436641437274. E-mail: Hugo.Partsch@meduniwien.ac.at Key words: compression therapy, stockings, band- ages, stiffness, leg-model. 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: MH, instrument (mannequin-leg) design, results providing; HP, personal experi- ences reporting, manuscript writing. This work is dedicated to Dr. Masafumi Hirai who started the research but unfortunately passed away. Dr. Hugo Partsch concluded the project which is published in his honor and memory. Conflict of interests: the authors declare no potential conflict of interests. Received for publication: 25 August 2012. Revision received: 15 October 2012. Accepted for publication: 29 November 2012. This work is licensed under a Creative Commons Attribution 3.0 License (by-nc 3.0). ©Copyright M. Hirai and H. Partsch, 2013 Licensee PAGEPress, Italy Veins and Lymphatics 2013; 2:e3 doi:10.4081/vl.2013.e3 No n- co mm er cia l u se on ly Conference presentation [page 8] [Veins and Lymphatics 2013; 2:e3] more than 20%. This shows clearly that the main cause for the variability is the change- able pressure distribution along the leg by donning the stockings several times. Correlation with other in vitro measuring devices A comparison of stiffness values measured by the mannequin-leg and the Hohenstein method performed in Japan gave a significant correla- tion between the two methods10 (Figure 6). Correlation with in vivo assessment of stiffness Forty custom made, small sized compression stockings between compression classes I and III tested on the mannequin leg were applied to one and the same human leg (ankle circumfer- ence 22 cm) in which the pressure was meas- ured in the lying and standing position at B1 by the same Picopress® probe, and the static stiff- ness index was calculated by subtracting lying pressure from standing pressure.5 The same procedure was performed by applying elastic and then an inelastic bandage over a class II stocking. Figure 7 shows an excellent correla- tion between the pressures measured at B1 at the mannequin leg and the corresponding measuring point on the human leg in the lying position (r=0.91). There was also a statistical- ly significant correlation for the stiffness val- ues (r=0.75). Discussion The clinical efficacy of compression devices depend mainly on the interface pressure and the stiffness of the product in use.1-4 For com- pression hosiery we rely on the pressure range in relation to the prescribed stocking-size given by the producers who, up to now, do not give us any information on the stiffness of their products. The pressure exerted by a band- age depends on the strength of application and the amount of layers. The stiffness of bandag- es is a rather complex parameter, relating mainly to the elasticity of the textile and to internal and external friction of the fibers. By adding several elastic layers over each other the final bandage is getting stiffer, mainly due to an increase of friction between the layers.12 These characteristics of different types of bandages could only be elucidated by examina- tions performed on human legs during the last few years.13,14 In vivo assessment of stiffness is based on the changes of interface pressure induced by changes of the circumference of the leg by standing up (static stiffness index)13 or by exer- cise (dynamic stiffness index).15 The preferred measuring point is B1corresponding to the site where the medial gastrocnemius muscle turns into the tendious part6 because this leg seg- ment shows the biggest increase of circumfer- ence by standing up and by walking.8 In addi- tion at this point the gastrocnemius tendon will protrude by contraction of the muscle so that the radius at the corresponding leg seg- ment will get smaller contributing to an increase of local pressure due to Laplace’s law. It is very obvious that such changes of the leg configuration will vary between single individ- uals being less pronounced especially in patho- logical cases like lymphoedema, or lipoder- matosclerosis compared to normal legs. This explains the high variability of the reported stiffness values, so that comparisons of com- pression devices by in-vivo testing only may be problematic.16 In contrast the mannequin leg offers a well-standardized procedure for com- paring different compression products always under the same anatomical condition in a rest- ing position and after stretch of the textile by an increase of the leg circumference by 1 cm. The dimension of the air-filled pressure probes and its deformation under a compression device has an important impact on the numer- ic outcome. This fact explains the differences between the results achieved with the AMI® transducer and the Picopress® device. As a consequence one should be careful by Figure. It shows a picture of the model, which is commercially available (AMI Techno, Tokyo, Japan). The model, made of plastic material has an ankle circumfer- ence of 20.5 cm and a calf circumference of 34.5 cm. There is a lengthwise transversal cut, which can be extended medially and laterally by 5 mm by pushing down a lever so that the circumference of the model will increases by 1 cm at each level. Figure 4. Characterization of several com- pression stockings and multi-component bandages concerning pressure (x-axis) and stiffness values (y-axis). The application of a second stocking over the first in 6 cases increases the stocking pressure to values over 40 mmHg. [T+E=tubular device (Tubulcus®) + elastic bandage wrapped over]. All multi-component bandages (in the upper rectangle) showed stiffness indices over 30 mmHg (Picopress®). Figure 2. A ready made compression stock- ing, size small, achieves a pressure of 33 mmHg at the B1 point of the model. This pressure drops to 30 mmHg after stretch- ing the model by 1 cm three times. SI=3 mmHg (Picopress® probe). Figure 3. Comparison of stiffness values (mean+standard deviation) between elastic stockings (left), long stretch bandages (middle) and short stretch bandages (right), resting pressures 23-46 mmHg (AMI-3037®). The difference between elas- tic and inelastic material is significant (P<0.001). No n- co mm er cia l u se on ly Conference presentation [Veins and Lymphatics 2013; 2:e3] [page 9] comparing absolute values. Based on the expe- riences by measuring the static stiffness index on the human leg it has been proposed to take the value of 10 as a reasonable borderline to differentiate elastic (<10) from inelastic material (>10).12 This same cut-off could also be accepted for the mannequin-leg when a Picopress® sensor is used (Figure 4). Using the AMI transducer® the cut-off value is lower and comes closer to the results of the tests performed with the Hohenstein-method which may be considered as the gold-standard method (Figure 6). However, in contrast to the Picopress® probe17 accuracy and variability of the AMI® probe has not yet been clearly estab- lished in clinical studies. Preliminary compar- isons of custom-made stockings between man- nequin- results using Picopress® and different kinds of extensometers (Hosy, Instron) showed also excellent correlations. Previous investigations had also shown a good correla- tion between pressure and stiffness values on human legs with extensometer data.18 Methodological flaws of the mannequin leg compared to the in vivo situation are the rigid consistency of the model leading to slightly higher pressure values than those measured over soft, yielding tissue and the relatively flat local radius at B1 which does not change when the model is extended. Another draw-back is the fact that up to now only one small sized model is available. Larger models or even forms containing a thigh part could be useful in order to obtain stiffness - data also from usual European sized and thigh high stock- ings. As shown in this report the obtained data will depend on the dimensions of the pressure probes so that comparisons of absolute data between will only be possible when the same kind of pressure monitoring system is used. Conclusions The presented concept of the extensible mannequin leg is a practically important step forward to assess the stiffness of different compression products and their combinations by a simple and reproducible technique. References 1. Partsch H, Menzinger G, Mostbeck A. Inelastic leg compression is more effective to reduce deep venous refluxes than elas- tic bandages. Dermatol Surg 1999;25:695- 700. 2. Partsch H. Improving the venous pumping function in chronic venous insufficiency by compression as dependent on pressure and material. Vasa 1984;13:58-64. Figure 6. Correlation of stiffness measured by the Hohenstein method (x-axis) and by the mannequin-leg (y-axis) in 17 stockings (AMI-3037®). Figure 7. Correlation for pressure at B1 (left) and stiffness (right) between the Mannequin leg (x-axis) and a human leg (y-axis). Encircled are the values obtained after wrapping elastic and inelastic bandages over a class II stocking. Figure 5. Measurement of pressure at B1 (left) and of stiffness (right) on the mannequin leg of 13 different compression stockings, three times repeated (Picopress® probe). No n- co mm er cia l u se on ly Conference presentation [page 10] [Veins and Lymphatics 2013; 2:e3] 3. Mosti G, Mattaliano V, Partsch H. Inelastic compression increases venous ejection fraction more than elastic bandages in patients with superficial venous reflux. Phlebology 2008;23:287-94. 4. van Geest AJ, Veraart JC, Nelemans P, Neumann HA. The effect of medical elastic compression stockings with different slope values on edema. Measurements underneath three different types of stock- ings. Dermatol Surg. 2000;26:244-7. 5. Partsch H. The use of pressure change on standing as a surrogate measure of the stiffness of a compression bandage. Eur J Vasc Endovasc Surg 2005;30:415-21. 6. Partsch H, Clark M, Bassez S, et al. Measurement of lower leg compression in vivo: recommendations for the perform- ance of measurements of interface pres- sure and stiffness: consensus statement. Dermatol Surg 2006;32:224-32. 7. Partsch H, Rabe E, Stemmer R. Compression therapy of the extremities. Paris: Editions Phlebologiques Francaises; 2000. 8. Stolk R, Wegen van der-Franken CP, Neumann HA. A method for measuring the dynamic behavior of medical compression hosiery during walking. Dermatol Surg 2004;30:729-36. 9. van der Wegen Franken K. Medical elastic compression stockings. Thesis, University of Rotterdam, The Netherlands; 2009. 10. Hirai M, Niimi K, Miyazaki K, et al. Development of a device to determine the stiffness of elastic garments and bandag- es. Phlebology 2011;26:285-91. 11. European Committee for Standardization (CEN). Non-active medical devices. Working group 2 ENV 12718: European Prestandard “Medical compression hosiery” CEN/TC 205. Brussels, CEN; 2001. 12. Mosti G, Mattaliano V, Partsch H. Influence of different materials in multicomponent bandages on pressure and stiffness of the final bandage. Dermatol Surg 2008;34:631- 9. 13. Partsch H, Clark M, Mosti G, et al. Classifi - cation of compression bandages: practical aspects. Dermatol Surg 2008;34:600-9. 14. Hirai M, Niimi K, Iwata H, et al. A compar- ison of interface pressure and stiffness between elastic stockings and bandages. Phlebology 2009;24:120-4. 15. van der Wegen-Franken K, Tank B, Neumann M. Correlation between the stat- ic and dynamic stiffness indices of med- ical elastic compression stockings. Dermatol Surg 2008;34:1477-85. 16. Schuren J. Compression unravelled. Thesis, University of Rotterdam, the Netherlands; 2011. 17. Partsch H, Mosti G. Comparison of three portable instruments to measure compres- sion pressure. Int Angiol 2010;29:426-30. 18. Partsch H, Partsch B, Braun W. Interface pressure and stiffness of ready made com- pression stockings: comparison of in vivo and in vitro measurements. J Vasc Surg 2006;44:809-14. No n- co mm er cia l u se on ly