SILICONE KELOID & PROTEASE - MODULATING MATRIX DRESSINGS
Overview
Silicone elastomer sheeting is used to prevent the development and improve the appearance and feel of hypertrophic and keloid scars. Examples: Silicone elastomer sheet and cica-care Silicone gel sheet. Indications: Scar management, painful wounds, where there is macerated or fragile skin. Pros: Easy to apply and remove. Cons: transient rashes, skin irritation at the scar site, pruritus of scar and foul smell underneath sheet. Notes: shouldn’t be used on open wounds (77).
Description of System
Many prescription and OTC silicone products are available for the treatment and prevention of hypertrophic and keloid scars. The differences among the products lie in their physical characteristics. Available products include silicone elastomer sheeting with and without a fabric backing, polyurethane foam, pressure garments, splints, fabric bandages that have one surface coated with silicone gel, silicone gels, ointments, sprays that dry to form a thin coat of silicone over the skin surface, and silicone liquid and strips enhanced with vitamin E and/or steroids.
Silicone bonds with oxygen to form a siloxane molecule (77). Repeating siloxane units form polysiloxanes, which methyl groups attach to. The resulting molecules, polydimethylsiloxanes or silicone polymers, may cross-linked to form gels and elastomers. A high degree of cross-linking yields a rubbery, solid elastomer. Less extensive cross-linking creates a silicone gel. As the degree of cross-linking increases, the silicone becomes more durable, but less adherent. To maximize the durability and adhesiveness of silicone elastomer sheeting, some products combine a silicone elastomer that has a relatively low degree of cross-linking with an expanded polytetrafluoroethylene membrane to create an interpenetrating polymer network. The IPN has greater durability than the silicone elastomer alone but still retains good adhesion to the skin (77).
Mechanisms Involved
The precise mechanism of action of silicone elastomer sheeting is unknown. Possible mechanisms include increased temperature, hydration caused by occlusion of the underlying skin, increased oxygen tension, direct action of the silicone oil and polarization of the scar tissue caused by the negative static electric charge generated by movement of the silicone. Silicone sheeting may also correct aberrant immunologic processes, which if left, may alter the tissue repair process and result in the formation of hypertrophic and keloid scars (77).
Costs and Evidence
Clinical trials report that this device it is safe and effective for the treatment and prevention of hypertrophic and keloid scars if worn over the scar for 12 - 24 hours per day for at least 2 - 3 months. Some of the silicone elastomer sheeting products currently available are durable and adhere well to the skin. They are an attractive treatment option because of their ease of use and low risk of adverse effects compared to other treatments, e.g. surgical excision, intralesional corticosteroid injections, pressure therapy, radiation, laser treatment, and cryotherapy (77). CICA-CARE is medically proven to be up to 90% effective in the improvement of red, dark or raised scars. It has been used by the medical profession to treat over one million people worldwide (78).
Image 51 – cica-care gel sheet (78).
Overview
Promogran protease modulating matrix is an advanced topical treatment for chronic wounds that may alter the wound environment positively, to promote healing. This can help to improve outcomes for patients with static or hard-to-heal wounds.
Promogran is an absorbent open-pored, sterile, freeze-dried matrix composed of 55% collagen and 45% oxidised regenerated cellulose (ORC). These are both natural materials readily broken down or reabsorbed when placed in the wound. When the collagen/ORC matrix comes into contact with exudate, it absorbs the liquid to form a soft gel (79). This allows the dressing to conform to the shape of the wound and contact with all areas of the wound. The gel physically binds to and inactivates damaging proteases, matrix metalloproteases (MMPs) and elastase present. It also binds with natural growth factors and prevents them from being broken down by proteases.
As the matrix slowly breaks down, the growth factors are released back into the wound in an active form, whilst damaging proteases remain inactive. Examples: Promogran and Promogran Prisma. Indications: Exuding wounds including venous leg ulcers, diabetic foot ulcers and pressure ulcers, traumatic and surgical wounds. Pros: Interactive, cost effective. Cons: Clinical significance yet to be demonstrated. Notes: Avoid in full-thickness burn injuries and active vasculitis (79).
Description of System
It’s formed by preparing medical-grade collagen and fibres of oxidised regenerated cellulose in a liquid suspension. In the case of Promogran Prisma, silver-ORC fibres are added at this stage. The suspension is then frozen and placed under a vacuum. In this frozen form the water in the formulation exists as ice crystals, which sublimes (turns directly from a solid to a gas) under the high vacuum, and is gradually removed from the frozen material. When all of the water has been removed, the remaining collagen/ORC is left as a 3D structure. This dehydration process (known as lyophilisation or freeze-drying) allows for the structural properties of these natural materials to be preserved. The products are manufactured as a 3mm-thick sheet, which is cut into hexagonal pieces (79).
Wounds are generally more circular in shape, but producing a circular-shaped dressing leads to a lot of waste during production. The product was therefore shaped as hexagonal pieces; providing a repeating pattern, minimising waste and closely resembling a circular shape (79).
Mechanisms Involved
Chronic wounds contain elevated levels of inflammatory cytokines, free radicals and proteases, creating a hostile wound environment that is detrimental to healing. This perpetuates wound chronicity as it causes further tissue damage and degradation of key functional molecules like growth factors, which are required to stimulate cell growth and production of new tissue. The presence of bacteria amplifies an already hostile environment, increasing the inflammatory response with increased levels of bacterial proteases. Promogran can be used to modify the wound environment by reducing harmful factors such as free radicals and excess metal ions, whilst protecting positive factors e.g. matrix proteins and growth factors, leading to an overall increase in new tissue formation and progression towards healing (79).
Effect on proteases: Many studies have shown that MMPs and serine proteases are elevated in chronic wounds. In particular, MMP-8 and 9 and human neutrophil elastase, which are inflammatory-derived proteases, have been shown to be the most predominant in the chronic wound. This reduction in protease activity is rapid and sustained, even when the material breaks down. Clinical studies have shown that the level of inflammatory cytokines and proteases are reduced in the presence of collagen/ORC. Its ability to scavenge free radicals, be an end-product of inflammation, and bind to endotoxins and excess metal ions, provides indirect support for its favourable effect on the inflammatory process (79).
Effect on bioburden: Collagen/ORC may help to control bacterial levels through its ability to lower pH, due to the ORC component. The addition of silver-ORC to the formulation has been shown to be non-cytotoxic and may help to reduce the number of pathogens in the wound, irrespective of bacterial bioburden (79).
Costs and Evidence
Collagen/ORC dressings have been evaluated in several randomised controlled clinical trials to measure their performance in diabetic foot ulcers, pressure and venous leg ulcers. Whilst these studies demonstrate the clinical effectiveness of Promogran and Promogran Prisma, they do not address the interactive nature of these matrices. However, several investigators have performed clinical research studies to examine the mechanism by which collagen/ORC can modify the wound environment. Lobmann et al concluded that wounds treated with Promogran showed a greater reduction in wound size compared to the control dressing (16% vs 1.65%) (79).
Biochemically, they found that Promogran-treated wounds showed a reduction in the MMP9:MMP2 ratio. Further analysis demonstrated that this reduction in protease levels was not due to an alteration in the production of MMPs but was more likely to be due to the binding of the MMPs to the matrix (79). Promogran Prisma Matrix (Collagen, silver and oxidised regenerated cellulose matrix, applied directly to wound and covered with suitable dressing) by Systagenix Wound Management Ltd costs £6.31 or £17.98 (54), which is relatively expensive. However, the sheets are reusable.
