MOIST WOUND HEALING
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There are two papers frequently cited as evidence that wound healing is better in a moist environment. Both papers were published in the early 1960s in the scientific journal Nature. The first was published in 1962 by George Winter working in London, UK. In the report Winter describes how partial thickness skin wounds were created using a scalpel in the skin of Large White pigs. The wounds were then divided into two groups. Control wounds were left to heal by direct exposure to the air and the formation of a scab. Experimental wounds were kept moist by covering them in polythene film. Six control wounds and 6 experimental wounds were created in the pig and the wounds examined under the microscope (presumably after wound excision) at days 1, 3, 5, 7, 9 and 11. At 3 days there was more healing (epithelialisation) in the moist group when compared with the control group. By 5 days the moist wounds were all healed and by 7 days all the wounds including the control wounds were healed.
The second study was published in 1963 by Hinman and Maibach from California. In their study healthy adult male volunteers were used and small wounds were created on the inner forearm. An antibiotic (neomycin) was applied to all wounds and then control wounds were left exposed to the air to heal as a scab. In the experimental wounds polythene was used to cover the wounds. At days 3, 5, 7 and 9 wounds were excised with an 8mm punch and examined under the microscope. By 7 days all wounds were healed whether control (dry) or experimental (moist). At 5 days there was more healing in the experimental group but only 3 comparisons in 3 volunteers were made and there was significant variation between the volunteers in the amount of healing.
At the time these experiments were performed experimental rigour was not as well developed as it is today. There are a number of problems with these experiments and their interpretation. Firstly only a limited number of comparisons are made. Possibly only two pigs were used and the positive findings in the human subjects only relate to 3 volunteers. In biology there is considerable natural variation between subjects and it is important that these comparisons are made many more times to ensure that the findings are not merely random variation. Secondly the observers were not blinded as to which wounds were experimental and which were control. In unblinded studies like these there is real potential for the observer's expectations to influence the results. This is usually an unintentional error where the observer will tend to err by reporting the results they expect. Today blinding the observer as to which wound was which would be considered essential.
If we accept the results as genuine then the only (limited) conclusion we can make is that partial thickness wounds in Large White pigs and the inner forearm of human volunteers heal marginally faster when covered with polythene and this may be assisted if topical antibiotics are applied first. This is a long way from the current situation where any and all wounds are considered for moist wound healing whether partial or full thickness, whatever their cause, whatever the patient's other illnesses. There is clearly a huge gulf between these experimental wounds in pigs and healthy young men and the conditions of healing in a lower leg ulcer with venous insufficiency and arterial disease complicated by diabetes and smoking. Ulcers are also by definition full thickness epithelial loss and totally different wounds in terms of healing when compared with partial thickness wounds describer in these experiments. Finally even if we accept that moist wound healing has made a significant difference in these experiments does this translate into a real world difference when dealing with wounds in patients. In these experiments all wounds healed whether moist or dry and the only advantage was about 2 days in the moist group. With natural variation between patients 48 hours improvement is unlikely to be clinically important. As an interesting aside the "human volunteers" appear to have been prisoners in San Quentin prison as the medical director is acknowledged at the end of the paper.
A huge industry has grown up around the concept of moist wound healing. Sometimes it can feel from the literature and sales representatives as though there is no other way a wound can heal when in fact the evidence underlying such an approach is poor. Neither of these frequently cited studies proves that moist wound healing is a useful tool in wound healing in patients, although it is unlikely to be detrimental. If polythene was the moist dressing that was in common use this would not be such a problem as it is cheap and freely available, but costs associated with wound care are very significant (http://www.todayswoundclinic.com/the-cost-outpatient-wound-care). There are hundreds of expensive dressings in current use with poor evidence to support their use.
Winter GD. Formation of the scab and the rate of epithelization of superficial wounds in the skin of the young domestic pig. Nature 1962; 193: 293-294.
Hinman CD, Maibach H. Effect of air exposure and occlusion on experimental human skin wounds. Nature 1963; 200: 377-378.
Last updated> 7 November, 2010
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