Wound healing is a complex dynamic process that is influenced by numerous intrinsic and extrinsic factors. Tissue that is deprived of an adequate blood supply and nutrients will die, resulting in necrotic tissue.
The literature suggests that the presence of devitalised tissue on the wound bed can delay or prevent healing (Bergstrom et al, 1994; Goode, 1995). Although this has not yet been tested in a randomised controlled trial, it is based upon clinical observation and the belief that dead tissue provides a focus for, and encourages the growth of, pathogenic organisms (Haury et al, 1980; Mulder, 1995). It is generally accepted, therefore, that healing will not take place until necrotic material is removed (Razor and Martin, 1991).
Necrosis can be caused by pressure damage, thermal injury, surgical complications and diminished blood flow. It can involve both epidermis and dermis, and necrotic tissue may range in colour and consistency from hard, black leathery eschar to soft, almost liquid, pale yellow slough (Figures 1 and 2).
Slough consists of a mixture of leucocytes, fibrin, cell debris and bacteria (Thomas, 1990). The colour and consistency are dependent on the duration of exposure to air, which causes the wound to dry out, and the depth of tissue involved (Thomas, 1990; Bale, 1997).
The presence of devitalised tissue in diabetic foot ulcers is attributed to a number of factors:
- Diminished or absent blood supply: This is caused by macro- or microvascular disease, which results in occlusion of blood vessels and tissue death.
- Pressure damage: Hyperglycaemia produces non-enzymatic glycosylation of collagen and keratin, making these tissues more rigid, inflexible and resistant to enzymatic digestion by collagenases. This results in hyperkeratinosis, which intensifies the forces on the dermis and subcutaneous tissue (Elkeles and Wolfe, 1991).
- Lowered immunity to infection: Hyperglycaemia also inhibits the activity of leucocytes, thus decreasing their ability to phagocytose bacteria (Elkeles and Wolfe, 1991) and dead cells. In addition, cellular hypoxia reduces the normal ability of neutrophils to phagocytose bacteria. The result is a build-up of dead cells and the formation of slough.
It is essential for the practitioner to undertake a thorough assessment of the patient and his/her wound to determine the cause of the devitalised tissue, as this will generally dictate the method of debridement.
Natural debridement process
Autolysis – the spontaneous separation of devitalised tissue from healthy tissue – occurs naturally in a wound that is not traumatised or compromised. It is brought about by macrophages which are responsible for clearing the wound of debris and release proteolytic enzymes such as collagenases that degrade unwanted materials (Clark, 1993).
Autolysis is enhanced in the presence of moisture, and can therefore be promoted in most wounds by the use of dressings that maintain a moist environment (Mulder, 1995).
However, caution is advised when using a colour classification system such as the red, yellow, and black (Cuzzell,1988; Stotts, 1990). It should not be assumed that all black tissue can be debrided using dressings that promote autolysis via hydration; where devitalised tissue is the result of peripheral vascular disease, it should be left dry and debridement should not be attempted until a further vascular assessment is made.
In patients with diabetes, however, the normal autolytic process is often impaired or absent for the reasons discussed above.
Reasons for debridement
Debridement of a diabetic lesion is carried out to achieve one or more of the following aims:
- Visualisation of the wound bed: to enable accurate measurement of the wound and identification of tracts and sinuses
- Drainage of exudate and pus: if exudate becomes trapped by callus, excessive tissue destruction may occur
- Decrease pressure on the capillary bed and wound edge: removal of excess callus will decrease the pressure on the wound and improve the blood supply to the area
- Increase antibiotic penetration: slough and necrotic tissue often prevent antibiotic diffusing into the infected tissue beneath.
Methods of debridement
The various methods of debridement are shown in Table 1. The method chosen will depend on:
- Systemic and local factors affecting the wound and patient
- Availability of materials and resources
- Expertise of the practitioner
Surgical (sharp) debridement
Sharp or surgical debridement is the dissection of devitalised tissue from the wound bed using a scalpel or scissors. It is reported to be the fastest, most efficient method of wound debridement (Fowler and Van Rijswijk 1995; Bale and Jones 1997). Where large amounts of dead tissue are present, debridement should be performed – the patient’s condition permitting – in theatre by a surgeon, under general or local anaesthetic (Figure 3). The costs associated with using an operating theatre for this procedure are high, and often it is difficult to find space in an already overburdened theatre list. However, the risk of an infection spreading cannot be over-emphasised, and debridement should be performed before it becomes a surgical emergency.
Conservative sharp debridement is an essential part of the management of any patient with a diabetic foot ulcer, to remove not only necrotic tissue and/or slough in the wound bed but also surrounding callus (Figure 4). This is an advanced practice skill which should not be performed by an inexperienced practitioner. Debridement should not cause any further problems and should ideally be performed when there is a clear distinction between healthy and devitalised tissue (Bale, 1997).
Traditionally, debridement of this kind has been carried out by a chiropodist or podiatrist, but with the expansion of specialist diabetic foot clinics it is now also performed by nurses (Poston, 1996; Bale, 1997).
Sharp debridement is not without side-effects, notably pain (except in the insensate foot), bleeding, further tissue loss and risk of sepsis.
When large amounts of tissue or hard, thick eschar are present, and theatre is not an option, removal may be carried out in stages with the aid of hydrocolloids or hydrogels.
To allay alarm, the patient should be informed that the wound will appear larger following sharp debridement (Figure 5) and that the process may cause bleeding. Pain control with an anaesthetic should always be considered.
Sterile maggots are considered to be ideal debriders, having a ferocious appetite for necrotic material while avoiding healthy tissue (Sherman et al, 1995) (Figures 6a and b). As yet there have been no controlled clinical trials of the use of maggots in debridement, but their use for this purpose has been reported in small numbers of patients with devitalised tissue (Thomas et al, 1996).
The larvae from adult flies can be harvested and sterilised for use on patients, the most commonly used fly being Lucilia sericata. The maggots are placed directly onto the wound surface and the surrounding skin is protected with a hydrocolloid dressing to avoid excoriation from proteolytic enzymes secreted by the maggots. The maggots are held in place with a gauze mesh, which is secured firmly using a waterproof adhesive tape to prevent the live maggots escaping from the wound (Bale, 1997).
The number of larvae applied will depend on the size of the wound. The maggots should be left on the wound for 3 days (Thomas et al, 1996).
Larvae are suitable for debriding dry gangrenous areas and avoid the problems of maceration that occur with autolytic debridement (Rayman et al, 1998).
Wet-to-dry debridement is infrequently used in the UK, but is common in many European countries and the USA. This technique involves the application of moistened saline to devitalised tissue before the application of gauze. As the devitalised tissue dries, it rehardens and becomes attached to the gauze. When the dressing is changed, the dead tissue (and presumably healthy tissue also) is removed (Poston, 1996). The technique is both painful for the patient and traumatic to the wound (Baxter and Rodeheaver, 1990).
Irrigation and hydrotherapy
High pressure irrigation and whirlpool hydrotherapy are other methods in common use in the USA. Like wet-to-dry methods, they are not recommended for debridement of diabetic foot ulcers.
Several different enzyme preparations are available for digesting slough and necrotic tissue. In the UK, Varidase, a formulation containing streptokinase and streptodornase, is licensed for use as a debriding agent (Thomas, 1990). Enzymatic debridement is quick and effective and may be a useful alternative to surgery (Berger, 1993; Bale, 1997).
If enzymes are to be used on hard necrotic tissue, cross-hatching and the use of an occlusive dressing are advisable (Marquez, 1995). It could be argued that it is the moist environment, rather than the enzyme, that is achieving debridement.
Other proteolytic enzymatic debriding agents agents include collagenases derived from crab (Glyvanstev et al, 1997) and from krill (Mekkes et al, 1998). However, at present, the use of these agents is confined mainly to selected patients in specialist units.
Many chemical agents have been used to effect debridement, including eusol, hydrogen peroxide and Aserbine, but these are no longer popular in modern practice.
Dextranomer (Debrisan) is presented as a paste or anhydrous porous beads. It is capable of rapidly absorbing exudate from a necrotic sloughy wound, which is thus removed when the wound is cleansed.
The most commonly used cadexomer dressing is cadexomer iodine (Iodosorb) (Thomas, 1990). This dressing consists of a three-dimensional network of cadexomer (modified starch gel) and tiny beads (microspheres) containing 0.9% iodine, which is released only in a moist wound environment. This dressing has the ability to remove debris and bacteria from a wound bed and to reduce the number of viable organisms, although the major indication for the use of this product is to effect debridement (Thomas, 1990).
Hydrocolloids and hydrogels are frequently used as a means of stimulating autolytic debridement. Their mode of action and the results of clinical trials will be covered in a subsequent article in this series.
There are many methods of debriding necrotic or sloughy tissue. The choice is often dictated by the availability of resources and products, but is greatly influenced by the skill of the practitioner.
Sharp debridement is generally the method of choice for the removal of callus, but the inexperienced practitioner may cause more harm than good if he/she attempts to remove too much tissue with a scalpel.
As always, assessment of the vascular and neurological status of the patient’s foot is essential, and will often clarify the amount of tissue that is to be removed and the method of debridement to use.
Figures 1–6 are not available in the online version.