Wound healing in dogs and cats

17 Wound Healing in Dogs and Cats

Dudley E. Johnston, B.V.Sc., M.V.Sc., A.M.

From the Department Dogs and eats suffer accidental wounds and surgical incisions in of Clinical Studies, much the same manner as do humans. Ideally, the surgical incision University of and most accidental wounds are closed by sutures so that primary Pennsylvania School healing results. There are probably few differences in the healing of Veterinary Medi- processes in these sutured wounds in dogs and cats as compared tine, Philadelphia, with similar wounds in experimental animals such as rats, rabbits, Pennsylvania and guinea pigs or in humans. Some surgical incisions and many

accidental wounds must be left open, for various reasons, to heal as open wounds by contraction and epithelialization. It is in this mode of wound healing that similarities are seen between dogs and cats and the experimental animals such as rats, rabbits, and guinea pigs; however, major differences are seen between dogs and eats and humans.

Healing in a Sutured Wound

The known basic facts concerning wound healing can be applied to a clean, sutured wound in dogs and eats. In wounds caused by a surgeon, minimal tissue damage is done, minimal dead tissue remains in the wound, minimal contamination exists, and the wound edges are approximated under physiologic degrees of tension.

Healing in the clean, controlled wound occurs rapidly. The early productive or substrate phase of healing is brief, as minimal debridement by body cells is required. The peri-incisional epithelium, on either side of the incision, reacts within hours of the injury by thickening and by beginning to migrate across the intra- incisional clot beneath the scab. The advancing new epithelial sheaths, usually within 48 hours, unite with one another across the incision. After the second postoperative day, the new epithelium thickens and, in most instances, some further downward epithelial growth occurs into the dermal breech, thus forming epithelial spurs. The epithelium bridging the gap gradually acquires some of the structural features of the adjacent uninjured epidermis. The development of keratin in the uppermost scar epithelial cells pre- cedes the loosening of the overlying scab, which can usually be dislodged after the fifth day (Figs. l-4). The similarities between

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f 3~. 1. (above). An incised wound in dog skm, 12 hours after surgery. In spite of careful suturing wit1

silk (, the wound edges are united at a step (A) and the suture wound is obvious (6). (Reproduced,

per missioff, from Jobnsto~,‘O)

F :IG. 2. (below). An incised wound in dog skin, six days after surgery. The advancing new epithelial sh

are thickened and have united beneath the scab. Regenerative epithelium from a hair follicle has joined the surface epithelium bridging the incision. (Reproduced, with permission, from Johnsto~.?~)

7 4/o

with

ieets with

July-September 1984 Volume 2 Number 3 Wound Healing in Dogs and Cats 145

FIG. 3. (above). An incised wound in dog skin, eight days after surgery. Epithelium has united firmly over the incision site (A), and healing is occurring in the dermis (B). Obvious reactions are seen at the site of

insertion of 4/O silk sutures which were removed on day 7 (C). (Reproduced, with permission, from ~ohnsfOn.‘O)

FIG. 4. (below). An incised wound in dog skin, nine days after surgery. The cross section has been made through a suture tract. Epithelium is growing along the entire suture tract (A). Epithelium has joined across the incision site (8). Epithelium from a hair follicle has contributed to the surface layer of bridging epithelium.

(Reproduced, with permission, from Johnston.‘O)

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FIG. 5. This incision in a dog has

been closed with largesutures (above).

which leave obvious suture scars (be-

low). Subcutaneous sutures and fine

skin sutures would have minimized su-

ture scar formation. (Reproduced, with

permission, from Johnston.‘O)

nound healing in dog skin and that described in experimental animals by Ordman and Gillman’ is obvious . .

One of the interesting applications of the knowledge of basic wound healing relates to the method of suturing skin wounds so that scarring is minimized. It is known that suture tracts are areas of injury and epithelium grows down these tracts to produce unsightly scars (Figs. 1,3,4). This scarring is reduced if nonirritating fine suture material is used and the sutures are removed as early as possible. Unfortunately, if skin sutures are removed before the wound edges are sta- bilized by collagen, the scar tends to become widened, up to approximately the width that the skin edges were separated before the skin

sutures were inserted. In some instances, the tensile strength in the scar is insufficient to meet the normal stresses in skin and the wound opens. The suture scars and the wide skin scar are usually of minor importance in animals: however, in some reconstructive procedures they are significant (Fig. 5).

There are basically two methods for reducing scars in skin wounds in dogs and cats. First, a subcuticular suture of nonabsorb- able material is placed in the dermis and left in place permanently, or at least for 20 days when the wound edges are held together by collagen. Some problems with this technique are that there is little basic strength in the suture itself, and the edges of the epidermis may not be coapted accurately, thus produc-


ing a step deformity. In dogs and cats place- ment of a subcuticular suture is difficult because the dermis is relatively thin.

A more common method for reducing scars in skin in dogs and cats is to place a row of fine sutures in the deepest layer of the dermis and subcutaneous fascia to accurately coapt the skin edges, then to place fine sutures to coapt the epidermis and to remove these latter sutures in five to six days. The fine subcutaneous sutures hold the skin edges together until collagen production has occurred. Sur- geons will usually select 5/O sutures of synthetic materials such as nylon, chromicized surgical gut, or synthetic absorbable material for these subcutaneous sutures. The absorbable sutures seem to retain strength for sufficient time. The use of the row of simple interrupted absorbable sutures in dogs and cats is an excellent way to provide tensile strength to a healing wound in a patient that does not always cooperate with the surgeon during the period of wound healing. In gen- eral, the use of adhesive strips across healing wounds, as is done in human surgery, to provide additional tensile strength and stability to the wound margins is not satisfactory in dogs and cats. The skin is covered with a fine hair stubble and the adhesive strips adhere loosely.

Healing in Open Wounds

As in incised wounds, epithelial regenera- tion in open wounds commences by cell mobil- ization and migration and by cell division at the wound edges. In large open wounds, all stages of epithelial repair are present. At the margin of the wound, hyperplastic thickening of the original epithelium is present. Adjacent to this is the oldest part of the regenerated epithelium and several layers of cells are present, with cell differentiation and keratinization and mitosis in deep layers. Beyond this, the advancingepithelium is eventually reduced to a single layer of flat- tened cells moving across the wound. In most wounds in dogs and cats some degree of muti- lation is present, usually produced by abra-

sion, scratching, licking by the animal’s tongue, or trauma from an ill-fitting and loose bandage. This interferes with the orderly pattern of cell migration.

The open wound is initially covered by a fibrin clot, then, following evaporation of fluid from the clot, a scab. Within four to five days after wounding, invasion of the wound bed by fibroblastsoccurs and new capillaries are formed. This produces the typical granulating surface, which in the healthy state is flat, not exuberant, and is red and firm. The granulating bed is important in the healing of open wounds for several reasons. The bed is extremely resistant to infection, the epithelium is able to migrate across its surface, and it is most likely that the mechanism of wound contraction is centered in cells (myofibroblasts) in the granulation tissue.

Ehrlich and Needle,” in 1983, described the healing of open wounds in normal mice and in mutant mice, known as “tight-skin mice.” Significant differences were noted in the rate of wound closure between normal and tight-skin mice, with retarded wound closure in the latter. The hypothesis that the dermal fibroblasts were defective in the mutant mice was not supported by in L&-O studies. McGrath3 suggested that the cause of the delayed contraction in the tight-skin mice could lie in the abnormal mechanical properties of the skin.

In humans, healing of open wounds is often associated with deposition of excessive scar tissue, the process known as hypertrophic sear formation. In the slowly healing wounds in the tight-skin mice, the formation of hyperplastic granulation tissue was noted.” It is interesting to compare the healing of open wounds in dogs and cats with open wounds in laboratory animals and in humans.

Open Wound in Loose Body Skin of Dogs and Cats

Wound contraction is defined by Van Win- kle as the “diminution in size of an open wound which is the result of centripetal movement of the whole thickness of the surrounding skin.“4+ The process has been well

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described in the loose skin on the bocl~~ of rabbi&Ii These authors described the process as follows: Immediately after excision of the skin the area of the wound cavity expands to 1219,. At day 3, a scab forms on the surface and the area is reduced to 53’~,i. The scab is fragmented and cast off by day 7. revealing a whitish-pink layer of granulation tissue on the wound floor. Contraction begins on da>, 7 and on day 10 the ivound area is reduced to 26%. on day 16 to lX.:<‘ti, and on day 18 t(J

62.5%. It is important to note that in these experiments no dressings were applied and wound scabs were allowed to form and to be discarded without interference. The authors suggest that the diminution in size of the wound size to day 7 is caused probably b> shrinkage of the scab resulting from de- hydration, and not wound contraction. In similar experiments in rabbits, Russell and Billingham; showed that the unit area of a central wound, covered with dressings of Vaseline gauze, was reduced by about 7’:) on each day during all but the first three days and the last five days of a 45day healing period.

In accidental wounds in dogs involving the freely movable body skin, wound contrar- tion. similar to that described by Russell and Billingham in rabbits, is seen. The open wound in thedogin FigureGwastheresuItof a dog fight and was not sutured. The wound was washed daily with warm saline solution and, 60 days later, the wound had almost closed by contraction. At no time was the granulation tissue proliferative. and hypertrophic scar formation was not seen. It is interesting to note that, at 60 days, the resid- ual narrow zone of granulation tissue is not proliferative and that the zone of epithelialization varies in width around the wound. Dorsally, the zone of epithelium regrowth is only 1 to 2 mm wide. Ventrally it is approximately twice this width. Caudally, the width of the zone of epithelial regrowth is 2 to 3 cm. One can probably presume that the anatomy of the skin is similar 360” around the open wound. The significant difference between the various skin areas surrounding the wound is the degree of mobility in the skin edges.

Dorsally there is extreme skin mobility and skin can be drawn from across the dorsal midline of the dog, if needed. Ventrally there is much less mobile skin and caudally, where the skin merges into perineum, there is vir- tually no freely movable skin. It seems highly likely that if the skin can move freely the wound closes predominantly by contraction. If there is no freely movable skin, even though the skin type is similar, the wound closes by epithelialization.

A boxer dog sustained a large open wound as a result of a burn from a heating pad. All necrotic tissue was eventually removed and the large open granulating wound shown in Figure 5 resulted. A small island of viable skin \vas present in the bed of granulating tissue. This wound was treated by regular cleansing with warm saline solution. It is interesting to note that, in comparison with the lvound shown in Figure 6. this wound healed by an approximately equal contribu- tion from contraction and epithelialization. Again, hypertrophic scar formation was not noted. In this author’s opinion, slower healing, hyperplastic granulating tissue, and increased scar formation can occur if anti- septic and antibiotics are applied to the granulating surface.

Russell and Billingham’ reported that, in rabbits, contraction of a rectangular wound culminated in a graceful line of opposition of the skin edges, the corners undergoing prac- tically no displacement. The end result was a four-pointed stellate scar. They also reported that a circular wound healed in a crumpled, unpredictable pattern, with a reduction of almost 40% in perimeter. Also, in circular wounds, the movement of the skin across the wound bed was impeded and there was approximately a 30%slower rate of contraction.

Square and rectangular wounds in dogs and cats seem to contract more readily than circular wounds. The large wound on the


FIG. 6. (top). A large open wound on the side of a dog. A (left). The wound was debrided and allowed to

heal by contraction and epithelialization. B (right). In 60 days, the size of the wound has been considerably

reduced by contraction. In the scar is a zone of older and darker regenerated epithelium; a zone of recently

formed, not yet pigmented, epithelium; and a zone of granulation tissue. The latter zone will be reduced

further by contraction. (Reproduced, with permission, from Johnston.lo)

FIG. 7. (center). A large open wound in a boxercaused by a burn from a heating pad. A (left). The wound is

20 days old and epithelial growth is occurring. B (right). After 60 days, the wound has contracted signifi- cantly; however, epithelialization is equally important in wound closure. The remaining granulation tissue is

not proliferative. (Fig. 7A reproduced, with permission, fromJohnston.“)

FIG. 6. (bottom). Large wound on the body. A (left). The wound was allowed to heal by contraction and epithelialization. B (right). In 60 days, contraction has reduced theareaof thewound. Atthis time contraction

ceased, probably due to the accumulation of collagen in the granulation tissue and reduction in myofibro- blast population. The remaining area healed by epithelialization. (Fig. 8A reproduced, with permission, from

Johnston.~l)

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Dermatology

FIG. 9. (above). A (left). A large rectangular wound on the leg of a dog. The wound was covered by Vaseline

gauze and a bandage. B (right). After 17 days, there is marked epithelialization, with minimal contraction

resulting from centripetal movement of the sides, not the corners.

FIG. 10. (below). Acral lick dermatitis In dogs: A (left). Chronic irritation leads to a thickened, firm plaque.

B (center). A massive, ulcerated lesion bordered by irregular epidermal hyperplasia on the latter aspect of the

metatarsal region. The lesion was originally a callus caused by pressure necrosis from the leg pressing

against the floor. The chronic irritation that prevents healing is due to licking and pressure. C (right).

Hypertrophic scar formation in an old accidental wound on the leg. Licking by the dog is one of the main irritants.

body in a loose-skinned dog shown in Figure and less efficient contraction process is seen 8 was allowed to heal by contraction and epi- typically in circular accidental wounds in thelization. After 60 days, contraction re- dogs and cats. duced the areaof the wound; however, at this time contraction ceased. It seems that the contraction process exists for a finite time,

Open Wounds on the Legs of

then ceases, probably as a result of excessive Dogs and Cats

collagen production and reduction in the It can be predicted that these open wounds population of myofibroblasts. The skin sur- heal largely by epithelialization because of rounding the incompletely healed wound is the presence of relatively tight skin sur- still highly mobile at 60 days. This slower rounding the wound. The large rectangular

July-September 1964 Volume 2 Number 3 Wound Healing in Dcas and Cats 151

FIG. 11. Wound contraction after grafting: A (left). This open wound contracted for approximately 75 days.

Further closure did not occur, probably because of the presence of the pale fibrous granulation tissue lacking

in myofibroblasts. B (center). Successful cover by split-thickness skin grafts of the wound. C (right).

Considerable contraction has occurred after skin grafting.

wound in Figure 9 was allowed to heal as an open wound and was covered by Vaseline gauze and a soft bandage. It is interesting to note that the wound contracted in a manner similar to the open wound on the side of a rabbit, as described by Luccioli and others,6 in that the four corners of the wound did not move; however, the four sides of the wound did. The very efficient epithelializatipn process and lack of hypertrophic scar formation can be seen.

A common lesion seen on the legs of dogs is known as acral lick dermatitis (Fig. 10) or “lick granuloma.” It appears that the condi- tion is a psychogenically caused urge to lick a portion of a leg, producing a thickened, firm, oval plaque.8 The licking causes alopecia; then ‘the epidermis erodes, and an ulcer develops. Histologically, the ulcerated surface is bordered by irregular epidermal hyperplasia, there is an accumulation of neu- trophils and mononuclear cells, and the dermis shows varying degrees of fibroplasia.8 Similar lesions occur on the legs of dogs as a result of chronic irritation from sources other than licking.

Wound Contraction after Epithelialixation and

Split-thickness Skin Grafts

Russell and Billingham reported that, in rabbits, the outgrowth of epidermis from the wound margins in a healing open wound has little or no influence on the course of the contraction of the wound. They stated that the “latter process proceeds inexorably beneath the newly formed epidermis which is gradually obliterated as the full-thickness margins draw toward one another.”

This phenomenon is seen clearly in dogs and cats in healingof accidental wounds. The open wound shown in Figure 11 was allowed to heal as an open wound, then the granulating surface and fibrous tissue were excised and the area covered by a thick split-thickness skin graft. After three months, a significant reduction in the size of the skin graft is seen. In open wounds on the feet in dogs when healing has occurred by contraction and epithelialization, complete epithelialization can occur, to be followed by an amazing degree of contraction of the scar.

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152 Johnston Dermatology

FIG. 12. Contracturedeformityfollowingskin loss

on the leg resulting from infection. A (above, left).

The wound is approximately 30 days old. The wound

was allowed to contract. B (above, right). A serious

contracture deformity is preventing extension of the

stifle. An area of granulation tissue remains distally.

C (left). The deformity was corrected by Z-plasty,

and the granulating bed was covered by a split-

thickness skin graft. (Reproduced, with permission,

from John~ton.~~)

FIG. 13. A typical avulsion injury to the forearm of a dog that was hit by a car: A (left). The ulna is exposed. B (right). After 60 days of treatment, during which wound contraction can be monitored, the resulting defect is skin grafted or closed by a reconstructive procedure before deformity occurs. Note absence of hypertrophic scar formation in this controlled wound.


Deformities Caused by Wound Contraction

In humans, the ill effects of wound contraction are stressed as much as, if not more than, the beneficial effects, and the research efforts of workers such as Gabbianig have been directed to understanding and control- ling, or eliminating, the contraction process. In dogs and cats the beneficial effects pre- dominate; however, some serious side effects have been observed.

Two types of contraction deformities occur in dogs and cats in association with contraction of open wounds near the flexor aspect of joints. In one type, a web of scar tissue forms, limiting extension of the joint (Fig. 12). This has been seen in dogs and cats and can be treated by a reconstructive procedure such as Z-plasty.

A second type of deformity is not associated with formation of scar tissue, butwith excessive tightness of normal skin produced by contraction of an open wound adjacent to the joint. In these cases, the skin tightness could be alleviated by physiotherapy involving forced extension of the joint, or even normal use of the joint. The animal often car- ries its leg, further augmenting the problem because the joint is held in flexion, or it seriously resents the pain or discomfort of physiotherapy. Accordingly, reconstructive surgical procedures may be needed.

An important principle in the treatment of many large open wounds in dogs and cats is to allow contraction under careful monitor- ing (Fig. 13). Then a reconstructive procedure is done before deformities occur and when the contraction process slows or ceases.

Conclusion

The healing of incised and sutured wounds in dogs and eats probably does not differ appreciably from that seen in experimental

animals and in humans. Open wounds heal by a combination of contraction and epithelialization as seen in experimental animals, and the predominance of contraction or epithelialization seems to depend on the degree of mobility of surrounding skin. The problems of hypertrophic sear formation are relatively rare in dogs and cats, and are seen only in the presence of chronic trauma.

References

1. Ordman LJ, Gillman T. Studies in the healing of cutaneous wounds. Arch Surg. 1966;93:

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

857-928.

Ehrlich PJ, Needle AL. Wound healing in tight- skin mice: delayed closure of excised wounds. Plast Reconstr Surg. 1982;72:190-8.

McGrath MH. Wound healing in tight skin mice: delayed closure of excised wounds. Plast Reconstr Surg. 1982;72:197-8.

Van Winkle W Jr. Wound contraction. Surg Gynecol Obstet. 1967;125:131-42.

Peacock EE Jr, Van Winkle W Jr. Wound repair. 2nd ed. Philadelphia: WB Saunders, 1976.

Luccioli GM, Robertson HR, Kahn DS. The pattern of constraction during the healing of excised skin wounds in the rabbit. Can J Surg 1963; 6,449-510.

Russell PS, Billingham RE. Some aspects of the repair process in mammals; Progress in Surgery, vol. 2. Basel/New York: Karger, 1962:1-72.

Muller GH, Kirk RW, Scott DW. Small animal dermatology. 3rd ed. Philadelphia: WB Saun- ders, 1983.

Gabbiani G, Hirschel BJ, Ryan GB, Statkov PR, Majno G. Granulation tissue as a contrac- tile organ: a study of structure and function. J Exp Med. 1972;135:719-34.

Johnston DE. The processes in wound healing. J Am Animal Hosp Assoc 1977;182-96.

Johnston DE. The healing processes in open skin wounds. Compendium Cont Ed. 1979;l: 789-95.

Address for correspondence: Dudley E. Johnston, B.V.Sc., M.V. SC., A.M., Department of Clinical Studies, Veterinary Hospital of the University of Pennsylvania, 3850 Spruce Street, Philadelphia, PA 19104.

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Wound healing in dogs and cats