• Elastic materials (long stretch) contain the ability to apply various amounts of compression using the stretch (elastomer) fiber spandex. In addition, the elastomer allows the fabric to apply therapeutic interface compression levels with changes in local circumference changes due to loss or increases in edema. These fabrics stretch in excess of 100%. When the tension is released the elastic fabric should return to its original length.
• Inelastic materials (short stretch) contain few spandex or elastomeric fibers producing minimal amounts if any elongation or degree of stretch. They therefore require considerable amount of tension to be applied and can only produce minimal if any elongation or degree of stretch.
The distal leg displacement of a wrap system has often been misinterpreted to be reflective only of loss of limb circumference due to edema reduction. But in reality distal displacement of the wrap system is also due to the “creep” of the fabric and the loss of compression related thereto. Wrap systems losing compression during treatment makes them non-effective towards the goal of wound healing.
It’s interesting to note that properly manufactured hosiery maintains more constant levels of compression with less creep for an extended period of time. This is because the compression levels required are engineered into the fabric using the appropriate elastomers and stitch construction allowing precise compression application. In addition, hosiery reacts like a long stretch fabric maintaining compression applied with changes in local circumferences.
This long stretch fabric contains a high amount of spandex which allows for varying amounts of tension to be applied, the continuous application of compression with the reduction of edema and stable compression levels due to minimal creep. This Vascular Wrap is part of a multi-layer wrap system consisting of two layers. The first layer is a terry fabric sleeve contoured to fit the lower leg. The second layer in this new system is the new vascular elastic wrap. In this system the primary compression is applied ONLY through the second elastic wrap layer.
Proposed Interface Pressure Measurement Guidelines: (measuring the sub bandage pressure)
1. During initial application to obtain selected therapeutic pressure
2. During each subsequent visit
3. Prior to removal of bandage dressing for wound inspection
4. During application of a fresh new dressing
5. Prior to removal of wound dressing at the end of treatment
Regimen should become standard of care.
The compression description is based on the range of compression applied to the ankle within a fitting range. For example a 20-30 mmHg hosiery fitting an 8” to 9” ankle will apply no less than 20 mmHg at an 8” ankle and no more than 30 mmHg at 9” ankle. Note typical fitting chart for compression hosiery from one company. Note that for knee length circumferential measurements are measured at the ankle and mid-calf while length is measured from the proximal leg the posterior if you’re aspect of the heel. Measurements were added for full length as noted below. The pressure measurements known in figure 3 refers to the level of the ankle when ordering knee length or thigh length stockings.
The following are the US compression standards for hosiery:
• 15-20 mmHg – Improves circulation, reduce swelling, controversial treatment telangiectasia/varicosities
• 20-30 mmHg – Chronic venous insufficiency, Treatment/prevention of varicosities
• 30-40 mmHg – Treatment of chronic venous insufficiency and ulcers
• 40-50 mmHg – Treatment of chronic venous insufficiency and ulcers
Due to Pascal’s law of physics the interface pressure has direct one-to-one relationship between the pressure applied to the skin and that produced internally within the limb. 40 mm Hg applied to the skin circumferentially delivers 40 mmHg internally. The effects are dramatic. Hugo Partsch in one of his articles demonstrates that only 6 mm of interface pressure applied to the mid-thigh of the patient with an abnormal thigh shape and extremely engorged saphenous vein and deep femoral vein not only reduces the cross-sectional diameter of both of the veins by 50% but also changes the shape of the thigh from an abnormal muscular mass to a rounded functional muscular unit. The effect of an externally applied interface pressure is dramatic and is not appreciated as much as should be. In fact, “when properly prescribed,” it has a significant effect on the internal structures of the limb without any invasive therapeutic aspect. This greatly reduces risk while performing a very functional therapeutic response. Recently an international consensus has as recommended the following system for categorizing Compression bandage systems:
Commentary: Kravitz, S, R, 5 The compression bandage system levels described above bring in many questions. The most important is why these ranges differ from those described for hosiery which have been accepted on a global basis. The ranges of pressure should be the same independent of the device that is providing the therapy. Ideally compression hosiery, multilayer compression wraps and pneumatic compression units should all utilize the same ranges of compression therapy. This allows for easy transfer from one type of device to another. For example a patient with an ulcer using a multilayer wrap at 40 mmHg at the ankle could then be changed to utilizing either a 30-40 or 40-50 compression hosiery. Have different ranges of therapy for different devices is confusing and complicates transferring from one form of therapy to another. All of these devices deliver the compression in the same manner with identical affects and are based on Laplace’s law and Pascal’s Law.
An inelastic bandage has high stiffness when compared to an elastic bandage. In short the term stiffness and the term of elasticity are opposite ends of a continuum. For a number of years these differences have been described using the concept known as “Static Stiffness Index” also known as SSI. The SSI has been used as a method to assess the relative stiffness of multilayer wraps. It is based on the concept that there is an increase in interface pressure when changing position from and non-weigh bearing to a weight bearing stance. This principle has been the basis of many scientific articles and measuring the stiffness or rigidity of many devices. However, University-based research produced by Kravitz et al. demonstrated significant questions and reliability of the SSI and published a well-known article challenging the static stiffness index as not clinically relevant and or inconsistent.4 The in vitro data from this study is based on solid laws of physics and mathematics. The fact that this data conflicts with a large number of in vivo studies raises questions about the previously published studies and the measuring techniques utilized. Measurements for SSI have typically been taken at the location known as “B 1” which is found on the medial aspect of the leg at the junctions of the upper and middle one third’s. The device typically utilized to measure the pressure in these research projects is the Picopress which is a crown-shaped air bladder measuring device. The B1 location overlies significant muscle and tendon tissue which when nonweight bearing is soft and flaccid allowing all the Picopress to sink into the soft tissue when non-weight bearing. However, upon weight-bearing these muscles become active, the subcutaneous tissue becomes rigid and the probe is forced to move and be pushed outward against the overlying bandage or other compressive device. The result is an increase in point pressure due to the prominence of the measuring device but there is not an overall increase in pressure circumferentially. The researchers at North Carolina State University hypothesized that prior in vivo measuring was misinterpreted. They had concluded that the in vivo research had misinterpreted the data as pressure around the limb while they were actually reading an artifact of the measuring device. Factors leading to this conclusion include the relative shape of the Picopress in combination with the change in the relative firmness of the subcutaneous tissue when transposition and from and non-weight-bearing to weight bearing. The SSI principle is not supported by benchmark science and the laws of physics.1,4
Laplace’s law dictates that when a device is applied circumferentially around the limb the only way the interface pressure can increase is if there is an increase in the limb circumference. Thus SSI is based on the assumption that there is an increase in circumference when transpositioning from non-weight-bearing to weight-bearing. Kravitz et al. with University-based data from schools of engineering and textiles demonstrated that there was no consistent increase in circumference at the level of the ankle and that the most subjects measured had a decrease in circumference at the mid-calf when standing versus non-weight-bearing. These results do not only challenge the principle of SSI but also point to discrepancies in compression therapy. For example it has been stated that zinc oxide impregnated gauze bandages, known as the Unna boot in the medical circles, effectively increases venous return in ambulating versus not ambulatory patients. But this assumption is based on the previously accepted principle of SSI meaning that there is an increase in interface pressure with the Unna boot when transpositioning from a non-weight-bearing to weight-bearing and/or walking activity. The research cited in this document however brings that assumption into question. If there is no increase in limb circumference there can be no increase in interface pressure when walking versus non-weight-bearing using the zinc oxide impregnated gauze bandages. Conclusion: Unna boot and similar devices do not increase venous return in ambulatory versus non-weight-bearing patients. To be clear that is not to say that ambulation does not increase venous return. It definitely does have that effect. However, herein described in vitro data suggests the use of an applied Unna boot or similar device does not enhance venous return.
However long stretch fabrics possess a much broader range of therapeutic compression demonstrated by the flatter stretch strain curve as shown above. The fabric used in the new sleeve wrap described in this document possesses the flexibility for a long range therapeutic compression (long stretch or elastic fabric) with a high-end spike acting like a short stretch bandage at the end of the stress strain curve. The flatter portion of the curve demonstrates its ability to expand and thus provide a therapeutic level of compression. Alternatively the flat portion of the curve also demonstrates that as edema reduces the fabric still maintains therapeutic compression allowing it to further reduce edema and enhance venous return. Additionally, as the muscles contract with ambulation there is a resultant increased internal limb pressure directly related to the applied interface pressure from the overlying bandage. This increase in limb internal pressure causes a reduction in the circumference of the deep veins enhancing proximal outflow to further reduce edema.
To reiterate, Pascal’s law demonstrates that externally applied circumferential pressure produces an increase of pressure internally to the limb that is distributed evenly throughout the lower leg. This has a compressive effect reducing the diameter of the veins within the lower leg. If there is not inherent valvular disease, a decrease in deep vein circumference causes the repositioning of the valves which allows them to function normally driving venous return proximally. The rate of blood flow through the veins increases, a proxy for internal pressure and the measurement of the interface pressure applied.
• The circumference of the limb inversely affects the pressure underneath the bandage (interface pressure). The smaller the radius of curvature of the area (bony prominences) the greater the likelihood of pressure damage.
• Applying compression with the bandage system may require adjustment dependent upon the diameter of the leg. While the larger limbs may require increased tension care should be applied when applying multi-layer bandaging to a slim, narrow leg to avoid high levels pressure.
Additionally each layer applied is additive. So if one were applying a two-layer wrap system with 50% overlap using 20 mm of compression will easily achieve 80 mm of compression at the ankle. A four-layer system can double that amount. Measuring pressure prevents excessive amounts of compression while at the same time providing consistency from one application to the next and from one provider to the next. This new technology should become a standard therapy for documentation and to help protect the provider and most importantly protect the patient.
Medical compression hosiery is prescribed according to the pressure it applies to a limb. There are many devices available for measuring this pressure, but differences in the design of the systems used, measurement locations, protocols, and operators result in different pressures being measured for the same garment. This article explores the construction of these compression-measuring devices and the sensing involved in order to highlight the potential causes of these discrepancies. The Tension–Elongation proﬁles of six compression hosiery samples were then measured, and a method of verifying the point pressure measurements from current techniques was proposed and tested. The results of this analysis show that there was an average discrepancy of 1–5 mmHg between point pressure measurements and those predicted from the Tension–Elongation proﬁles. With respect to on-body measurements, this technique predicted a maximum change in pressure of 3 mmHg for the samples tested.
2. Hegarty-Craver, M., Grant, E., Kravitz, S., Reid, L., Kwon, C., Oxenham, W. Research into compression fabrics used in compression therapy and assessment of their impact on treatment regimens, Journal of Wound Care, Academy of physicians in wound healing supplement. 2014. 23: S14-S22
The objective of this paper is to provide a definition for interface compression that uses essential principles of engineering science. This definition discusses factors that influence the amount of applied pressure, including the size of the limb, the amount of fabric tension (graduated pressure profile), the number of layers, and the material creep characteristics. Laplace’s law has been applied to compression therapy. Tension-extension profiles have been derived for different types of compression fabric to demonstrate the effects of resistance and friction. Force-time profiles have been derived for different types of fabric compression systems to demonstrate the effects of creep (creep is defined generally as the irreversible deformation of a material over time in the presence of a constantly applied load). Here, the fabric compression systems were applied to a test-bed that supplied a constant force to the fabric; creep was taken as the
loss of compression (pressure) over time.
Laplace’s law has been interpreted for compression therapy. The amount of fabric tension is determined by the extent to which it is stretched during application, as well as by changes in the size of the limb. The fabric’s relative elasticity, which is quantified by the rigidity index and is related to the slope of the tension-extension profile, dictates the amount that the applied force changes with extension. Compression systems that use multiple layers of fabric are generally more resistive to stretching than single-layer systems. Friction acting between the layers, as well as the added force from each layer, serves to increase the overall compression of these systems. As the applied force rises, the amount of pressure supplied by the fabric increases. However, when the same force (or fabric extension) is used, the applied pressure is less when distributed over a larger surface area. In other words, as the circumference of the limb increases, the pressure decreases. This is the driving principle behind graduated compression. In addition to the changes in fabric tension resulting from extension, the creep characteristics of the material affect the amount of compression provided throughout the wear cycle.
Based upon the interpretation of the material properties of compression fabrics (tension-extension profile, number of fabric layers, and creep), new therapeutic guidelines have been established, and others clarified.
3. Hegarty-Carver, M., Grant, E., Kravitz, S., Kwon, C., Reid, L. Simulated pressure changes in multi-layer, multi-component wrap systems when transitioning from rest to standing, Journal of Wound Care, Academy of physicians in wound healing supplement. 2015 24: S14-S20
The largest changes in circumference were used to simulate the pressure changes under the multi-layer, multi-component products using Laplace’s Law. While the pressure differences were large for the zinc plaster product, pressure changes ranged from 5-I0 mmHg for the other, more elastic products. Additionally it was noted that the leg decreased in circumference at the B1 level and calf for the majority of participants when transitioning from sitting to standing. This decrease in size results in a decrease in bandage tension and applied pressure.
4. Kravitz, S, Hegarty-Carver, M, Reid, L. Challenging current concepts in compression therapy: static stiffness index is not consistent and not clinically relevant, Journal of Wound Care, North American Supplement, 2016 26: S4-S8
Once a circumferential force is delivered to a limb by a compression device, any change in total force is dependent upon a change in circumference, with the rate of change (excluding fabric creep) being dependent on the stress strain curve of the device. Compression is defined as the total amount of circumferential force applied to the surface (interface compression) of a cylinder at a known circumference. This article addresses the pre-conceived and well accepted principle that interface compression delivered by a compression device substantially increases with the position of the limb based on the positions of sitting (non-weight-bearing) to standing and/or during muscle activity (ankle dorsiflexion). Using engineering parameters and clinical measurements, the authors demonstrate that changes in interface pressure are minimal if any, and that the current concept should be modified accordingly.
5. Kravitz, Steven, R. DPM, FAPWHc Commentary provider: Academy of Physicians in Wound Healing, Founder and Executive Director; Council for Medical Education and Testing, (CMET), Executive Director; Physician Certified in Wound Healing-CMET: Temple University School of Podiatric Medicine, Philadelphia, PA; Assistant Professor; Carolon Healthcare Products, Medical Director; International Compression Club, member