A critical review on compression textiles for compression therapy: Textile-based compression interventions for chronic venous insufficiency

Abstract

Compression textiles as adjuvant physical interventions are increasingly applied for prophylaxis and treatment of chronic venous insufficiency (CVI), providing benefits of calibrated compression and controlled stretch. Pressure dosage delivered and mechanical properties (stiffness, elasticity and hysteresis) are determined by material nature, stitches structures, fabrication technology and delivery modes. Laplace’s Law and Pascal’s Law contribute to elaborate the static and dynamic working mechanisms behind the interaction between compression interventions and a biological body. However, there is still a lack of sufficient awareness on compression materials, and there is controversy regarding the best solution for clinical application of compression. This study integrates the views from physiology, pathophysiology, biomechanics, material science and textile engineering, to review and clarify physical–mechanical characteristics of compression materials, working mechanisms of textile-based compression interventions and their medical benefits in chronic venous insufficiency treatment. The aim is to enhance understanding of compression textiles applied in compression therapy, and to facilitate cooperation among multiple parties working in related supply chains, thus promoting textile-based compression interventions in chronic venous insufficiency treatment and growth of technical textiles applied in healthcare, medical and rehabilitation fields.

Keywords

chronic venous insufficiency compression textiles compression therapy compression stocking and bandage physical–mechanical characteristics structure and fabrication technology

Chronic venous insufficiency (CVI) is one of the commonest diseases affecting the lower extremities of adults globally. It has been estimated that approximately 2 to 6 million people experience CVI in the United States, and of those nearly 500,000 develop painful venous ulcers.^1,2 In China, around 100 million people (i.e. 8.89%) suffer from venous disorders in lower extremities³ with morbidity rate of 0.5–3.0% per year, of which venous ulcers account for 1.5%.⁴ In Africa, the incidence of varicose veins is around 3–8% among female population.^5–7 Wide prevalence of CVI is also reported in the Europe, e.g. France,⁸ Germany,⁹ Poland¹⁰ and Finland.^11,12 It occurs more frequently in people over age 50, and more in women than in men.^13–16 These abnormalities usually begin with the common symptoms of pain, itching, tiredness and discomfort in the affected limb, and in more complicated cases, progresses to stasis changes (e.g. edema, varicose veins, hyperpigmentation, deep venous thrombosis, phlebetic lymphedema) and ulceration, etc.^17–19 The antecedent factors of CVI are closely linked to aging, sex, family history, obesity, pregnancy, phlebitis and previous leg injury; and they are also association with life style, behavior, occupation and body positions.^8,10,20 Compression therapy is the cornerstone for prophylaxis and treatment of CVI with aims to reduce edema and improvement of venous and lymphatic return from the lower extremity.^21–23

Innovative technical textiles and product design have provided extensive possibilities to improve and optimize compression therapy. Compression textiles, as a basis of physical intervention modalities, are increasingly drawing attention in compression therapy with rising prevalence of CVI.^24,25 Relying on their unique structures and fabrication techniques, compression textiles can flexibly deliver controlled compression with fast molding to cater for different body shapes in an affordable cost. Medical elastic compression stockings (MECS) and compression bandages (CBs) are the representative compression textiles applied in CVI prophylaxis and treatment.^26–30

Compression textiles for compression therapy involves a complex and dynamic supply chain, which constructs a demand network among target users, medical personals, vendors, manufacturers, inspectors and researchers (Figure 1). Each involved party contributes to the medical effectiveness of textile-based compression interventions. The fabrication of compression textiles and their applications relate to multiple disciplines and engineering technologies. Previously, more attention has been placed on the effectiveness of available compression interventions (e.g. MECS, CBs) in clinical treatments and physiological responses;^31–42 and few studies systematically consider how compression materials can be engineered at the fabrication process to impact their physical–mechanical properties and following medical treatments when interacting with user’s body in static and dynamic manners. The awareness and knowledge of compression materials and interventions need to be further elevated, even for the health care providers working in countries with matured compression therapeutic systems (e.g. Germany, France).⁴³

Figure 1.

Complex supply chain of textile-based compression interventions for CVI treatment.

Figure 2.

Vein system of lower extremity.

The present paper attempts to integrate knowledge on textile materials, fabrication technology with mechanism principles of physiology, pathophysiology, biomechanics and clinical practice, to clarify physical–mechanical characteristics and working mechanisms of textile-based compression interventions, as well as factors impacting compression materials properties and their effectiveness in compression therapy. New functional textile materials and innovative product design are introduced. The aim of this review is to enhance understanding of mechanisms of compression for physical therapy, and multiple parties involved in related supply chain of compression textiles, thus promoting application of textile-based compression interventions in CVI treatment.

Anatomy and physiology of veins

The lower limb venous system includes the superficial and deep veins, which are defined by their respective relationships to the muscular fascia (Figure 2). The superficial veins are located between the skin and the muscular fascia, whereas the deep veins are accompanied by the arteries encompassed by muscular fascia. Perforating veins traverse the muscular fascia to connect superficial and deep veins. Communicating veins connect veins within the same venous compartment, either deep to deep or superficial to superficial. The deep veins primarily drain blood flows from muscles. Venous blood flows from the skin drain into superficial veins, and then into the deep veins. There are unidirectional valves that prevent back flow of blood in superficial, perforating, and deep veins.^44,45

Principle forces affecting venous return include blood hydrostatic pressure, the compressive force generated by leg and foot muscles, competence of valves, and respiration.⁴⁶ In a person at rest, venous blood hydrostatic pressure is determined by the vertical distance between the ankle and heart. For a standing person who is 180 cm tall, the venous pressure at ankle level will be approximately 100 mmHg. When a healthy people exercises, the ambulatory venous pressure can drop to below 30 mmHg. This is because approximately 90% of deep venous return from the legs is achieved by the pumping effect of the leg muscles contractions which push blood flow inward and upward in legs. The calf muscle pump contributes 65% of this 90% venous return. Valves function to minimize hydrostatic pressure by separating blood column into lower-pressure segments and to facilitate unidirectional blood flow inward (from superficial to deep veins) and upward (from distal to proximal).^47,48

Chronic venous insufficiency (CVI)

Venous insufficiency is a condition in which the veins have problems sending blood from the legs back to the heart. Chronic venous insufficiency (CVI) is a long-term progressive condition. CVI is most commonly attributable to incompetent valves in the veins, either valvular incompetence in the low-pressure superficial or perforating venous system or valvular incompetence in the high-pressure deep venous system.¹³ In addition, these conditions may result from the congenital absence of venous valves.

One of the risk factors for CVI is sitting or standing for extended periods.¹⁵ The long vertical distance from leg to heart can result in high pressure inside the veins and blood pooling in leg veins. The consequent increase in pressure can stretch the walls of the veins and results in secondary valves failure. If superficial or perforating venous valves become incompetent, blood can flow back from deep veins into the superficial veins, resulting in increased intra-luminal pressure (venous hypertension), which the veins cannot withstand, causing them to become dilated and torturous. This condition is known as varicose veins (Figure 3). In addition, ambulation decreases venous pressures by only 20% in veins with valvular incompetence compared with approximately 90% in the healthy legs. When ambulation ceases, pressure in healthy leg veins returns to its standing pressure in approximately 30 seconds but more quickly in veins with valvular incompetence.

Figure 3.

(a) Normal vein flow (flow from superficial to deep veins); (b) abnormal incompetent valve (flow from deep to superficial veins); (c) varicose veins.

There are various soft tissue changes following the formation of varicose veins owing to chronic venous hypertension. Venous hypertension results in increased permeability of capillaries, which allows fluid, proteins and blood cells to leak into the subcutaneous tissues. Venous hypertension may also lead to an active inflammatory response and reduced oxygenation of skin and subcutaneous tissue. These vascular pathologies cause further complications such as edema, brown pigmentation and ulceration with delayed healing of the skin and subcutaneous tissue.^49,50

The CVI is classified by clinical severity based on CEAP criteria (Clinical, Etiology, Anatomy, and Pathophysiology) into six clinical categories. It provides a basis for uniformity in reporting, diagnosing and treating CVI. The CEAP categories are as follows:

C0: no visible or palpable signs of venous disease;

C1: reticular and spider veins;

C2: varicose veins;

C3: edema/swelling;

C4: evidence of venous stasis skin changes;

C5: healed venous stasis ulceration;

C6: active venous ulceration.

Modalities of textile-based compression interventions

There are different modalities applied in compression therapy. Integrating knowledge from function, configuration and materials, a hierarchical system can be built up as shown in Figure 4, including characteristic layer-I and product layer-II. Layer-I comprises five groups based on elasticity, pressure level, pressure delivery mode, materials and fabrication technology. Layer-II indicates the major product modalities applied in compression therapy with single/hybrid characteristics derived from Layer-I. The adoptions of modalities to great extent depend upon symptoms of end-users and theranostic requirements by medical professionals. Textile-based compression interventions (e.g. MECS, CBs) play a crucial role in CVI treatment.

Figure 4.

Modalities of compression therapy.

Efficacy of textile-based compression interventions in treatment of CVI

Nowadays, the central place for compression therapy in the treatment of CVI is widely accepted. Medical elastic compression stocking (MECS) is one of the compression therapy modalities which is effective in the treatment of CVI at different stages, including varicose veins, reducing edema, preventing ulceration, facilitating healing of ulcers, and preventing recurrence, through improving venous hemodynamics and reducing edema. Table 1 summarizes the effect of compression therapy using MECS on hemodynamics, vascular construction and soft tissue on CVI.^{31,32,35,51–53}

Table 1.

Effect of compression therapy using MECS in CVI treatment

Effectiveness of MECS
•Increased venous flow velocity
•Improved venous return
•Decreased venous blood volume
•Reduced reflux in diseased superficial and/or deep veins
•Reduced diameters of deep calf veins in standing position
•Increased muscle efficiency and beneficial in reducing muscle fatigue
•Improved healing of venous ulcers
•Reduced rates of re-ulceration of venous ulcers
•Improved pain, swelling, activity tolerance and wellbeing

A systematic review on the effect of compression for venous leg ulcers has been conducted based on a total of 47 randomized controlled trials (RCTs) by using different compression modalities.⁵² Overall, venous ulcers heal more rapidly with compression. Eight of the 47 RCTs compared MECS with CBs and the results indicated that there was no significant difference in improving healing between low pressure levels of MECS and CBs.^54–65 Ashby et al.’s recent study⁶⁶ demonstrated that two-layer MECS may be an effective alternative to four-layer bandaging. MECS with higher compression was associated with better ulcer healing, better outcomes for pain and discomfort relieving, and lower costs.

The effects of MECS for treatment of varicose veins without venous ulceration were systematically reviewed by Shingler et al.,⁶⁷ followed by EI-Sheikha et al., who further reviewed the RCTs comparing different compression following the same treatment for symptomatic varicose veins.⁶⁸ There was subjective evidence showing improvement in participants’ symptoms and physiological measurements when MECSs were worn. Regarding the optimum pressure provided by stocking(s) for compression therapy, the conclusions from individual studies were still conflicting. The reasons originate because there are neither identical criteria for compression stocking production nor guidelines on methodological quality of clinical trials.

Currently five major classification systems (i.e. the British Standard, German Standard, French Standard, draft European standard and USA Standard) are applied in compression stockings by different manufacturers globally, who adopt different thresholds of pressure as measured at the ankle for different classes and in a variety of lengths (e.g. knee-length to full tights). Each classification system involves three to four classes in terms of different levels of compression as set by the manufacturers between different countries,⁶⁹ which lead to variations in norms for the interventions of MECS internationally. For example, the British Standard bandage pressure sets Class I between 14 and 17 mmHg, whereas the German Standard can be between 18 and 21 mmHg.

Meanwhile, there are no guidelines on the methodological quality of clinical trials to determine the optimum pressure. For example, it was unclear or not all studies adopted the same subject recruiting criteria, types of interventions and assessed the same outcomes; furthermore, study designs are different and most of the reported studies are not RCTs. For example, one early RCT study suggested that lower pressure (20 mmHg) was as effective as with higher pressure (30–40 mmHg) exerted by knee-length MECS in relieving CVI symptoms during eight-week treatment,⁷⁰ whereas a recent RCT study indicated that thigh-length MECS with Class II (undisclosed pressure values) significantly reduced postoperative pain in treatment of primary varicosis compared with not wearing MECS during two-week wear trials.⁷¹ Optimum pressure needs to be determined in terms of specific application condition and environment based on accurate diagnosis and identification of the source of venous incompetence, which is also affected by many other factors, e.g. dimensional fitting (size and shape), material tactile properties, compliance and activity of users and so on.

In general, there is a basic consensus that compression is more effective than no compression in CVI treatment.^72–76 However, which type of compression interventions should be applied and how much pressure should be delivered remain controversial.

Application of textile-based compression interventions in treatment of CVI

The first medical elastic compression stocking (MECS) was manufactured with rubber threads in 1848 by William Brown in England. Jonathan Sparks further developed rubber threads covered with cotton to improve interface comfortability. Synthetic elastomers have been used for manufacturing MECS since 1960.

The current practice of compression therapy using MECS is to apply constantly graduated compression with the highest pressure exerted at the ankle, with the pressure gradually decreasing up to the top of the calf (or thigh), as the circumference of the limb increases towards the calf (or thigh). The underlying principle of graded compression therapy is based on the fact that a larger radius of curvature in limb results in lower interface pressures according to Laplace’s Law. Applying the highest pressure at the ankle encourages venous return and, therefore, decreases venous hypertension and reduces edema. Compressing the calf enhances the action of the calf muscle pump.

MECS are commonly configured into knee-high, mid-thigh, thigh-high and pantyhose with open-toe or close-toe designs. There is not a consistent recommendation on which types of MECS are more effective in improving venous function, which largely depends on what type of treatment is to be adopted and where the specific locations of the involved veins. Knee-high hosieries are usually used for treatment of CVI, whereas full-length compression stockings (thigh-high or pantyhose) are used for sclerotherapy or compression treatment of a fresh deep venous thrombosis. Open-toe design is often recommended with a “Slippie” to aid for easier donning and doffing.

Up to now, there is no an international standard on compression classes and gradient distributions of MECS. The draft European standard was initiated long ago⁷⁷ to try and ensure consistency within the European Union but this has not occurred. The German RAL-GZ387/1⁷⁸ and British UK BS 6612⁷⁹ models are currently the most widely used guidelines in practical application (Table 2). All MECS classifications describe only the pressure they exert at the ankle B level (the point of ankle’s minimum girth) (Table 3). A large selection of MECS is available with considerable differences between MECS of the same compression class, which may result in different clinical effects.

Table 2.

MECS compression classes^{33–35,78,79,110}

Class		Classification system of MECS pressure (mmHg)*
Class		DE	FR	UK	EU	USA
I	Light	18–21	10–15	14–17	15–21	15–20 (moderate)
II	Medium	23–32	15–20	18-24	23–32	20–30 (firm)
III	Strong	34–46	20–36	25–35	34–46	30–40 (extra firm)
IV	Very strong	>49	>36	N/A	>49	N/A

DE: Germany [RAL-GZ387]; FR: France [ASQUAL]; UK: British [BS:6612]; USA: America;

EU: Europe [ENV12718]; The pressure exerted on the leg in the area of the ankle; 1 mmHg = 133.322 Pa

Table 3.

Pressure gradient distributions by different compression classes.⁷⁸

Gradient level*	Residual pressure ratio in % of pressure at ankle
Gradient level*	Class I	Class II	Class III	Class IV
At F or G	20–60%	20–50%	20–40%	20–40%
At C	50–80%	50–80%	50–70%	50–70%
At B1	70–100%	70–100%	70–100%	70–100%
At B	100%	100%	100%	100%

“

c” means circumference, e.g. cB stands for the circumference of B level. “l” means length from heel level to the specific circumference level, e.g. lC stands for the length from heel to level of cC.

B: ankle with minimum circumference; B1: the level at which the Achilles tendon changes into the calf muscles; C: the calf region at the maximum circumference; D: just below tibial tuberosity; E: around the knee-cap and hollow of the knee; F: the level between E and G (e.g. mid-thigh); G: the level at top of thigh; H: buttocks with maximum circumference.

Inelastic (rigid) bandages appeared in the time of the ancient Egyptians. The first elasticated bandaging containing natural rubber was manufactured in the middle of 19th century.⁸⁰ Bandaging are categorized based on different methods, e.g. compression magnitudes,⁸¹ extensibility,^82–86 static stiffness index^86,87 and numbers of layers.^85–88 The positive performance of integrative multi-component systems (MECS or CBs) in healing leg ulcers has drawn close attention from medical professionals in recent years.⁵¹

The explorations on effects of progressive and degressive pressure delivery patterns along lower extremities are continuing (Figure 5). Recent studies indicate that progressive gradient distribution achieved a significantly higher increase of ejection fraction compared with the traditional recognised degressive gradient pressure.^89,90 Further studies on efficacy of progressive compression are called for with respect to venous ulcers, edema reduction, and post-thrombotic syndrome.^91–93

Figure 5.

Two types of compression delivery patterns: (a) degressive compression (b) progressive compression.

Principles of static and dynamic compression

Compression therapy is to provide treatment for certain diseases with calibrated compression. Compression pressure is defined as a force exerted to an area of body surface. In compression therapy, the unit of mmHg is commonly used as the unit of compression pressure to define pressure dosage (1 mmHg = 133.322 Pascal). All MECS are designed based on the principle of elastic compression. Principles underlying of how compression therapy systems deliver pressure to a limb are known as Laplace’s Law and Pascal’s Law. Laplace’s Law can help us understand how the compression pressures exerted by MECS will vary under static conditions, whereas Pascal’s Law explains the dynamics of compression pressure.

Laplace’s Law

The original theory of Laplace’s Law was developed to relate the wall tension and radius of cylinders (e.g. blood vessels) to the pressure difference that existed between the pressure pushing the two halves of the cylinder apart and the wall tension pulling the two halves together.^94–96 The equation can be shown as

P = \frac{T}{r}

where P denotes pressure (pa), T is the tension in the cylinder wall (Nm^-1) and r is the radius of the cylinder (m). This law is now widely used to explain and assess the pressure delivered to a limb of known radius by a fabric under known tension. Interface pressure (P) on an area of a limb has a positive correlation with fabric tension (T) and a negative correlation with the radius of the limb of curvature (r) at that area. For a bandaging system, the width (b, cm) and number of layer applied (n) should be involved in calculation⁹⁷ (Figure 6). Fabric tensions can be determined by performing Instron test to get the tensile force in KgF or Newton (1 KgF = 9.80665 N) when elastic fabric is stretched to a specific percentage under certain loading and speed.

Figure 6.

Laplace’s law applied in wall tension and blood vessel as well as in the interaction between compression fabrics and a limb.

The Magnetic Resonance Images (MRI) show that the cross-sectional contours along different levels (ankle, calf, knee, thigh) of the leg are irregular, and varying between each other, which affects the corresponding interfacial pressures between skin surface and stocking layer according to the Laplace’s Law^98,99,155 (Figure 7(a) and (b)). The compression pressures at flat regions such as medial and lateral sides of leg are found to be generally less than the mean ankle pressure by 48–75%.⁹⁸ There is very low compression on the retro-malleolar space with significant concave. Insufficient compression may negatively affect venous healing, whereas constant high pressure in anterior and posterior regions may induce local ischemia, artery injury and even ulceration in long-term use. Body postures significantly alter interfacial pressures in dynamic wear, especially in the positions of knee bending and muscular contraction.^99,100 Padding acting as a supplement is inserted between compression intervention layer (s) and body surface to redistribute compression pressure by correcting the radius of curvature at that area. Wool-based padding materials¹⁰¹ and gauze strapping¹⁰² are used at local regions in bandage system, such as at the tibia crest and malleolus positions, to avoid pressure damages at bony areas.

Figure 7.

(a) Cross-sectional contours along different levels of the leg and (b) effects of contours on skin pressure variations.

Pascal’s Law

Blaise Pascal’s research was focused on the properties of fluids in a hydrodynamic system. Pascal’s law states that when there is an increase in pressure at any point in a contained fluid, there is an equal increase in pressure at every other point in the container. It has been proved by ex vivo¹⁰³ and in vivo¹⁰⁴ experiments that soft tissues in the human body act as incompressible fluid. According to Pascal’s law, the MECS forms an enclosed system whereby external pressure applied to the limb is further transmitted equally in all directions within the underlying soft tissues (Figure 8(a)). The internal stresses induced by external pressures redistribute dynamically with time, which has been also demonstrated by related biomechanical simulation studies (Figure 8(b)).^105–109 The compression intervention layers with higher Static Stiffness Index (SSI)¹¹⁰ generate higher recoil forces to squeeze the internal soft tissue and maximize muscle functional activities; the resulting squeezing forces further narrow the diameter of the veins within the superficial and deeper tissues to reduce the volume of blood pooled locally, thus encouraging more fluid to be reabsorbed into the vascular and lymphatic systems and improving hemodynamic efficacy of lower extremities.

Figure 8.

(a) Pressure transmissions governed by Pascal’s Law, (b) internal stress distribution by external compression, (c) internal stress variations at calf cross-section over time.

Compression textiles: Materials, structures and fabrication technology

Textile technology plays a critical role in constructing compression modalities to achieve expected medical efficacy in modern compression therapy. Textile materials, structure and fabrication technology directly affect the physical properties and mechanical behaviors of the compression textiles as well as integral performances of compression products in practical application.

Heterogeneous yarn materials are applied in compression interventions. They can be produced by different covering methods (e.g. single or double covering, core spinning, stitch covering, core twisting and air jet covering) as shown in Figure 9(a)–(c). Core-sheath composite elastomeric yarns are the key elements to ensure fabrics with medical compression function, which are commonly constructed by wrapping natural or synthetic filament fibers (e.g. cotton, polyamide, polyester, viscose, polypropylene) around a stretchable core such as latex or polyurethane (PU) as shown in Figure 9(d), which are commonly applied as inlay threads to knit or weave with ground yarns, to deliver compression fabrics with specific thickness, tension and stiffness.^111–120 The dimensional thickness of inlay yarn is commonly expressed by using linear density with units of denier (den), tex or dtex. Denier is defined as the mass in grams per 9000 meters (1 den = 0.111 tex and 1.110 dtex).^119,121

Figure 9.

Heterogeneous yarn produced by different covering methods applied in textile-based compression interventions. (a) core spinning, (b) core twisting, (c) air-covering, (d) single/double covering.

The linear density, draw ratio, type of covering, covering rate and twist per meter of knitting yarns largely determine the mechanical properties of inlay threads (e.g. elasticity, tenacity, hysteresis and durability, etc.)^122–124 The inlay threads with high linear density may allow the compressive fabrics to be thicker and more stable in shape retention during long-term use, and high power inlay threads with less stretch may generate greater compression dosage in practical use.^113,125

Among the various yarn materials, polyamide-based conventional covered spandex yarns are most frequently used in compression textiles, thanks to polyamide materials’ higher tenacity and extensibility as well as better performances in chemical resistance, abrasion resistance and thermal stability, which provide the formed compression fabrics with more durable mechanical properties in practical use. The selections of yarn materials largely depend upon the pressure dosage required and end use of the products (e.g. MECS, CBs, or other orthopedic supports, etc.). More details on yarn materials with a wide range of specifications and their properties are available in the related studies.^{111,113,115,116,126}

Circular and flat knitting technologies are the major approaches to fabricate custom-made and ready-to-wear MECS. Each has its advantages in production efficiency, knitting flexibility and wearing comfortability.

Seamless circular knitted MECS are commonly produced by circular knitting units with fine machine gauge E16-E41 (E being the number of needles per inch) in high production efficiency (e.g. maximum running speed is up to 500 revolution/minute). The wide range of cylinder diameters (e.g. 3.75″–6″) of knitting units achieves the requirements for different body sizes with comparatively little finishing (Figure 10(a)–(c)).^127,128 The spatial configurations elaborated by nexus of knitting stitches, e.g. plain, tuck, floating (miss), etc., allow MECS fabrics to possess qualities of controlled thickness and stretches. The circular seamless MECS fabrics can be produced as single-faced or double-faced structures depending upon the settings of dial and cylinder needle beds.¹²⁹ Most of them are constructed as single-faced with plain, plating, laid-in and mock rib knitting stitches with thinner and softer handle, whereas double-faced MECS fabrics can produce true rib patterns (e.g. 1 × 1, 3 × 1, 5 × 1, etc.) knitted by different qualities of elastomers. The inlay yarn is alternately tucked into single-faced fabrics but is commonly laid-in between the middle of double-faced fabric layers to produce thicker compression fabrics as shown in Figure 11(a)–(b). By varying loop size, stitch density, cylinder diameters and feed-in tensions, tubular MECS can be fabricated to deliver specific range of compressions with regular sizes.

Figure 10.

Fabrication of MECS: (a) inlay yarn pathway in circular knitting machine in MECS fabrication; (b) compression materials fabricated on circular seamless knitting units; (c) formed seamless circular MECS; (d) knitting yarn pathway in flat-bed knitting units; (e) compression materials fabricated on flat-bed knitting units (f) formed double-faced open-width MECS fabric.

Figure 11.

Stitches structures of compression fabrics.

Flat-knitted MECS are constructed by open-width compression fabrics with relatively coarse and thicker handle with seam, which is commonly knitted by higher linear density yarns on horizontal needle bed with coarser machine gauge, e.g. E12-E18 (e.g. Stoll CMS system), especially producing stiffer compression fabrics with higher compression classes (i.e. Class-III with 34–46 mmHg and Class-IV ≥ 49 mmHg). For example, inlay thread with 350 den (389 dtex) is suggested to be used as elastic core in flat-bed knitted MECS, whereas inlay thread with 279 den (310 dtex) is commonly used for circular seamless MECS fabrics in terms of RAL387/1. The short or long stretch inlay yarn is reciprocatively left–right placed in knitting zones by yarn carrier and is knitted with other ground yarns to deliver controlled stretch and compression as shown in Figure 10(d)–(f). The superiority of flat-bed knitting technology is its patterning versatility and flexible shaping capability by addition or removal needles in operation, thus achieving customized demands for MECS appearance and function as well as fitting various dimensions of lower extremities flexibly.

Compression bandages (CBs) are commonly fabricated by warp knitting (raschel and tricot) and woven/nonwoven technologies. Warp-knitted CBs can produce netted, open or closed loop structures (Figure 11 (d)), to offer compression fabrics with diverse properties varying from highly elastic to rigid in length-wide via the placement of inelastic or elastic inlay threads along the parallel stitches loop chains. Woven CBs with different plain structures (e.g. 2 × 2, 4 × 4) may also achieve the required degree of elasticity and compression by applying short or long stretch covering elastomers as inlaid yarns interwoven with other composite warp yarns (e.g. staple fiber yarn, cotton, textured filament yarn) as shown in Figure 11(e)^130–132 More information on materials and structures of CBs is available in the reports by Kumar et al.^133–135

A number of studies have been conducted to analyze mechanical properties and structures of compression materials and to build up quantitative relationships between materials properties and compression performance.^{133,135–138,141} Figure 12 displayed microscopic structures of formed compression fabrics delivering different levels of compression dosages when worn on the three-dimensional (3D) body. The adoption of yarn materials, knitting machine setting (e.g. needle-bed height, stitch cam setting, running speed of cylinder, etc.), structural configuration, tension control, the way of yarn feeding and placement, as well as manufacturing conditions (e.g. temperatures, humidity)^113,114 determines the global properties of compression fabrics (e.g. thickness, weight, elasticity, compression and stiffness, etc.),^{112,115,116,139–141} thus influencing longevity, comfortability, biocompatibility and medical efficacy of compression interventions and wearer’s compliance in their practical applications.

Figure 12.

Formed compression fabrics viewed under high-power microscope: (a) MECS fabrics with Polyamide and elastomers (light compression 10–14 mmHg); (b) MECS fabrics with Polyamide and elastomers (mild compression 18–25 mmHg); (c) MECS fabrics with Polyamide and elastomers (moderate compression 20–30 mmHg); (d) MECS/CBs fabrics with Polyamide and Lycra (high compression ≥40 mmHg); (e) woven cotton CBs (medium weight); (f) unstretched cotton crepe CBs (heavy weight).

Compression textiles: Physical properties and mechanical behaviors

The interactions between textile-based compression interventions and the human body construct a dynamic and complex biomechanical system. The essential function of compression intervention is to deliver externally a controlled compression to counterbalance or compensate malfunction or insufficiency of the target component parts of body (e.g. derma, soft tissue, skeleton, veins, arterial and lymphatic systems, etc.), thus achieving prophylaxis, treatment or rehabilitation purposes. A number of physical properties and mechanical behaviors are involved in influencing pressure dosage and resulting treatment efficacy in this interactive process.

Stiffness is traditionally used as a term to indicate fabric’s bending resistance quantitatively measured by bending length, flexural rigidity and bending modulus, which is one of the earliest properties to be objectively measured to assess the subjective handling quality of textile materials.^142,143 The knitted fabric’s stiffness is affected by yarn properties (e.g. linear density and elastic modulus), knitting construction and weight, etc., which can be evaluated by multiple ways, such as hanging loop method,¹⁴² Kawabata KES-FB2 pure bending tester,¹⁴⁴ cantilever method¹⁴⁵ and pneumatic stiffness tester described in the ASTM standard.¹⁴⁶ However, how to link the fabric’s stiffness property to its pressure performances is critically important for medical personnel to determine proper textile-based compression interventions in clinical therapy. A special significance of stiffness has been defined for compression textiles in compression therapy. It indicates an increase in interface pressure at B level (i.e. the smallest circumference of the ankle) if the leg circumference increases by 1 cm according to the European Committee for Standardization (CEN).¹⁴⁷ It reflects the ability of the compressive materials to counter the muscle expansion during contraction. Static Stiffness Index (SSI) and Dynamic stiffness index (DSI) are two surrogate indicators to quantitatively assess stiffness of compression materials in clinical practice.^110,148,149 SSI measures the difference in pressure at ankle B1 (where the medial gastrocnemius muscle turns into its tendon) when a patient moves from the supine to the standing position. The unit is in millimeters of mercury per centimeter (mmHg/cm) or hectopascals per centimeter (hPa/cm), which shows the same sensitivity and specificity in distinguishing between elastic and inelastic systems with CEN.¹⁵⁰ DSI reflects the pressure pulsation capability in dynamic wear, and is considered to be a valuable indicator for prescribing the efficacy of compression therapy in clinics. DSI shows a very similar trend with SSI but is in slightly higher values. The mean values of DSI in flat and circular knitted MECS are 23.65 and 17.25 mmHg/cm, respectively, when compressions are delivered by 11 different brands of MECS with range of 23–32 mmHg.¹⁵¹ The compression interventions with high DSI has been demonstrated to have positive benefits in augmenting muscle pumping and reducing venous reflux and hypertension.¹⁵²

The relationships between DSI and pressure performance could be illustrated by four types of situations as shown in Figure 13.¹⁴⁸ Optimizing stiffness and pressure properties may achieve a balance between wearing comfort and medical effectiveness. For example, medium pressure with high DSI is recommended to treat CVI (e.g. edema) owing to its high-pressure amplitudes during walking instead of using strong compression with poor compliance. Compressive fabrics with low compression but with high DSI could be an optimum condition for ambulatory patients (Figure 13(d)). Meanwhile, quality compressive fabrics could maintain constant DSI (i.e. amplitude or pulsations) irrespective of low or high pressure level positioned during daily use, which has been examined by Van der Wegen et al., through testing 12 different commercially available brands of Class I circular knitted and Class II flat-knitted MECS.¹⁴⁹

Figure 13.

Four types of relationships between pressure and DSI (a) high pressure with high DSI, (b) high pressure with low DSI, (c) low pressure with low DSI, (d) low pressure with high DSI.

Apart from compressive materials themselves and wearing time, it’s worth noting that SSI and DSI values may vary in vivo assessment in the presence of other potential factors, e.g., leg position during measurements, the individual muscle strength, skin elasticity and edema reduction. For example, more pronounced skin tissue displacements and greater shear forces during frictional contact with compression textiles may occur in the elderly compared with young people owing to the former’s lower skin elasticity and skin turgor,¹⁵³ which may influence stiffness values in dynamic wear.

Elasticity is the capacity of compressive fabrics to return to their original shape after being deformed. The principle of physics behind it can be explained by Hook’s Law (F = kX), where tensile force F is positively proportional to stretch length X. Owing to internal friction or plasticity existing in the compression materials, Hook’s Law can not be obeyed perfectly, thus an elastic hysteresis happens when the tensile loading is removed, which is the most important indicator reflecting DSI and pressure performance in dynamic long-term wear. The encircled areas by loading and unloading curves indicate the energy dissipated of compression textiles during the stretch–relaxed process, reflecting elastic hysteresis property of compression materials (shading areas shown in Figure 14). With dynamic variations of leg circumference in walking, stretch-relaxation of compression materials are repeatedly acting on the skin and soft tissue. The less the elastic hysteresis, the more sustained and stable the pressure performance in daily wear. Different definitions are applied to determine short or long stretch properties, e.g. the SSI < 10 mmHg/cm signifies elasticity;¹⁵⁴ and based on DIN 61632,¹⁵⁶ stretch ratio greater than 100% is considered to be elastic or long-stretch, 10–100% is short-stretch, less 10% is rigid or inelastic. Figure 14 shows the elasticities and hysteresis of two different compression fabrics knitted by polyamide-based covered Lycra materials (57.72 tex as elastic inlay thread and 4.4 tex as ground knitting yarn) under certain tensile force along fabric wale and course directions. The material I is constructed by alternative 1 × 1 laid-in and 1 × 1 knit-miss stitches with thickness of 0.76 mm; and the material II is constructed by alternative 1 × 1 laid-in and full knit stitches with thickness of 0.69 mm (under compression of 4 gf/cm²). The force–elongation test set the maximum load at 44 N and constant elongation rate at 100 mm/minute under a standard testing condition in terms of ASTM D1776-04 (temperature: 21} 1℃ and relatively humidity: 65} 2%). Gauge length set at 84 mm, and the 3rd cycle was recorded.

Figure 14.

Force–elongation curves of different knitted compression fabrics in wale and course directions (Tan a = △F/△E, indicating stiffness property).

In general, stiffness, elasticity and elastic hysteresis are crucial characteristics affecting pressure performance, longevity, comfortability and medical efficacy in compression therapy. Compression interventions with the same claimed pressure levels may present different dynamic stiffness, elasticity and hysteresis, thus resulting in variations in compression pressure levels. The currently available commercial compression intervention products lack adequate descriptions on these important materials characteristics to assist medical professionals and end users to choose proper compression modalities in CVI treatment.

Compression textiles: Compression assessment system

Compression assessment system is important to monitor and assess pressure dosage in treatment of CVI. Three major methods are used to determine pressure performance, including (a) indirect in vitro method; (b) direct in vivo method; and (c) evidence-based treatment efficacy. The pressure values of compression interventions claimed by manufacturers are commonly measured by using indirect in vitro methods (e.g. HOSY, HATRA, MST,^{79,114,157,158} which is suitable for quality assurance in industrial production. Direct in vivo methods by applying various pressure sensors based on piezo-resistive, air filling or force-dependent resistance (e.g. Pliance X system,¹⁵⁹ Talley Digital Skin Evaluator¹⁶⁰ and Flexiforce⁹⁸) are also most effective in determining interface pressure between intervention and skin, whereas in vivo assessment by clinical wear trials is accepted to be an important method and a common trend for determination of pressure performance in evidence based compression therapy.^152,161 In addition, recently 3D biomechanical mathematical models have been applied as numerical simulation method to investigate pressure performances exerted by compression interventions based on the finite element analysis (FEA),^162,163 by which not only the dynamic interface pressure performances can be quantitatively assessed,^105,107 but also the internal cross-sectional deformations and stress distributions within underlying tissue structures with time can be investigated and visualized.¹⁰⁶ The capability of computational simulation provides a valuable tool to help us further explore the compression systems and their corresponding biomechanical efficacy by compression textile on the human body.

Advanced compression textiles and products design for compression therapy

The performances of compression therapy are continuously improved by applying new materials and innovative product design. Satisfactory compression textiles should possess the characteristics below:

compression comfort;

biocompatibility;

good dimensional stability;

controlled elasticity, stiffness and hysteresis;

dynamic and sustained pressure delivery;

satisfied contactile perception in long-term use;

moisture and thermo-physiological regulation.

The creation of traditional compression can be considered as a type of ‘Passive’ mode, i.e. interfacial pressure variations with fabric stretch induced by changes of leg volume. Innovative biomaterials endow the compression interventions with more ‘Positive’ properties. For example, a new bioactive materials containing shape memory alloys exhibit flexible stretch and recovery with variations of external environment (e.g. temperature or loading changes),¹⁶⁴ which was reported to show a desirable capability of exerting initial mechanical pressure similar to commercial MECS and of improved treatment for leg ulceration.¹⁶⁵ In addition, compression material layer may act as a drug delivery carrier to positively perform multifunctional treatments. For example, MECS fabrics with treated Troxerutin may provide simultaneous benefits of elastic compression and topical sustained hemostatic properties in treatment of topical varicose edema.¹⁶⁶ Wearable electronic textiles may offer compression interventions with capabilities to sense, monitor and measure the compression dosage, proprioception, limb movement and physiological output, by intrinsic and extrinsic modifications to textile substrates. For example, by applying different fabrication technologies (e.g. knitted, woven, non-woven, braided), electronic fibers combined with specific textile structures can control the variations of electrical resistance of fibers, thus forming a reproducible and sensitive sensor network used in chronic leg ulcers treatment.^167–169 Innovative product design involves more considerations on user’s wearing comfort and convenience. For example, 3D knitted spacer bandaging system shows a comparable effect with traditional two-layer CBs in treatment of chronic leg ulcers;¹⁷⁰ silicon paddings and tapes are set in welts to hold interventions staying in place for longer term use; inserted zippers and various stocking “donner” facilitate user to easy donning and doffing;^171–173 Circaid creation allow the users to easily re-adjust compression during the day as needed via relatively simple verification method.^174,175 These are contributing factors in raising the wearer’s compliance and improving efficacy in compression therapy.

Conclusion

Compression textiles are playing a crucial role as structured modalities in CVI treatment owing to the benefits of their calibrated compression and controlled stretch. Interaction between compression textiles and target users forms an integrative compression therapy system (Figure 15). The compression dosage delivered and mechanical properties (stiffness, elasticity and hysteresis) are determined by material nature, stitches structures, fabrication technology and delivery modes. Laplace’s and Pascal’s laws contribute to elaborate the static and dynamic working mechanisms behind of interaction between compression interventions and biological body. Compression standards and assessment methods facilitate compression textiles to achieve the expected properties, while wearing comfort and convenience are vital to realize the medical function in practical use. The compression properties and traditional wearing manners are being reformed by innovative textile materials and product design.

Figure 15.

A complex system of compression textiles for compression therapy.

The development and application of compression textiles for compression therapy is a growing area involving multiple disciplines and industries, which requires collaborations of practitioners engaging in different parties related to the supply chain. The present study attempts to synthesize the views from physiology, pathophysiology, biomechanics, material science and textile engineering, to explore the characteristics of compression materials and their working mechanisms, thus enhancing the awareness and knowledge of practitioners on textile-based compression interventions in CVI treatment, and promoting textile-based compression interventions in physical therapy and growth of technical textiles applied in more extensive fields (e.g. healthcare, medical treatment and rehabilitation, etc.).

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: We would like to thank the Innovation and Technology Fund (ITF) by Hong Kong SAR Government to support this study through research project ITP/049/14TI.

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