Continuing Education Article R6 Refereed Peer Revlew

Equine Grafting: Principles

can lessen the healing time of I and Field Applications* I( and improve~cosmetic outcome. Auburn University SmnA. Carson-Dunkerley, DVM R. Reid Hanson, DVM - Skin grafts are classitied according to their source, thickness, and blood supply, hen horses incur cutaneous wounds that cannot be sutured because p. 872. of significant cutaneous damage or loss of deeper tissue, healing by second intention must occur. However, large wounds that heal by -E The success of a is based second intention heal slowly, and the scars that form are often cosmetidy un- on its fibrinous adherence to appealing.'~~ may reduce healing time, improve cosmesis, and the bed, plasmatic imbibition, allow a horse to return to function sooner than if the were allowed to revascularization of the graft, and heal by second intenti~n.~With the proper equipment and expertise, many the organization of the grafted skin grafting techniques can be performed on the farm.' This article describes wound with collagen-secreting grafting procedures that can be conveniently performed with the horse stand- fibroblasts, p. 873. ing or anesthetized with a short-acting general anesthetic. +

After , fmtion of a CLASSIFICATION healthy bed of granulation tissue Skin grafts are classified on the basis of three characteristics: source, thick- takes at least 5 days, p. 873. ness, and blood supply." Gr& that are classified according to the source include autogenous gra'cs, homografts, and xenografts. Autogenous grafts Chlorhexidine diacetate, at a (autografts) are taken from one site and moved to another site on the same an- imal.L" Homografts (allogratis) are transferred between genetically different useful antimicrobial agent in the individuals of the same specie^.^ Xenografts (heterografts) are transferred be- postoperative care of skin grafts, tween individuals of different specie^.^ W~thautogenous grafting, an immuno- logic reaction against the donor skin does not occur. Grafts that are classified by thickness include split-thickness grafis (which are composed of the and a portion of the ) and full-thickness grafts (which include the epidermis and all of the dermi~).~Both types are readily accepted by horses?." Because MI-thickness grafts are easier to obtain and because harvesting them does not require specialized equipment (e-g., a ), full-thickness grafting is easier to perform in the field.6 *Publication Series No. 2541, College of Veterinary Medicine, Auburn University, Auburn, AL 36849. Eauine 873

Grafts that are classified by blood supply are the blasts. These cells secrete collagen and ground sub- free graft and the pedicle graft. A free graft is devoid stance into the wound, increasing the tensile strength of blood supply; a pedicle graft maintains its own of the attachment of the graft.'0.'5 blood s~ppl~.~.~Because pedicle grafting requires general anesthesia to isolate the required nutrient ar- PREPARATION OF THE RECIPIENT SITE terial vessel, free grafting is more practical for use in Sheet grafts are best applied to fresh wounds that the field.' have no granulation tissue, but such grafts also can be Free gafts are further classified as island grafts (e.g., applied to granulating wounds. Island grafts, however, pinch, punch, and tunnel grafts) and sheet grafts. Sheet can be applied only to granulating wounds. The granu- gafts can be split-thickness or full-thickness and can be lation tissue to which grafts are applied should be new- applied as solid or meshed sheets. Pinch, punch, tun- ly formed, richly vascular, firm, and free of purulent nel, and sheet grafting can be applied in the field, with di~charge."~'~~"The presence of advancing the horse standing or recumbent, using short-acting at the margin of the wound is generally a good indica- !general anesthesia. Sheet grafts provide more complete tor of the bed's health and capacity to accept a dermal coverage of the wound; island grafts provide Formation of a healthy bed of granulation tissue re- dermal tissue only at the graft site and provide epithe- quires at least 5 days after injury." lial coverage- in between. Grafts help to control infec- Because wounds in horses are frequently treated for tion, provide dermal coverage of the wound, and stim- weeks before the decision to graft is made, the granula- ulate epithelialization from the epithelial edges of the tion tissue is often mature and contains more fibrous wound and grafL2s9 tissue and a poorer blood supply than would a fresh bed of granulation tissue. Consequently, the ability to HEALING OF GRAFTS accept a graft is less than optimum.' Before the graft is The success of grafting is based on four events: (1) applied, the mature granulation tissue must be excised fibrinous adherence of the graft to its bed, (2) plasmatic to below the skin surface to allow development of a imbibition (passive absorption of nutrients by the ves- healthier, more vascular granulation tissue bed. Because sels of the (3) revicularization of the graft, and of the lack of innervation, granulation tissue can be (4) organization of the grafted wound with collagen- excised with the horse standing." Deep crevices and secreting fibroblast^.^ The graft adheres within minutes depressions in the tissue should be excised so that of implantation.2.6Fibrin, formed from fibrinogen, is the wound is smooth, allowing better contact with and brought into the recipient site by the surrounding ves- thus better acceptance of the graft.3 sels and attaches the graft to its bed." For the first few Studies indicate that trimming granulation tissue as days after grafting, the delicate fibrinous attachment is much as 4 days before skin grafting may increase accep- the sole method of adhe~ion.~Early immobilization to tance of Because of the significant amount of protect this delicate attachment thus is critical to the hemorrhage that results from such trimming, the bed acceptance of the graft." should be trimmed no less than 24 hours before graft- Before the graft is revascularized, it must survive by ing. Wounds that are apparently infected can be cul- plasmatic imbibition. This is the process by which the tured for bacterial identification and antimicrobial sen- grafi bed provides an oxygen-rich, plasmalike nutrient sitivity testing.'' fluid for absorption into the dilated vessels of the After , the wound should be covered graft."-'* The duration of plasmatic imbibition depends with a nonadhesive pad and a well-padded bandage. If on the quality of the recipient site bed. The more vas- the wound is at a highly mobile site (e.g., the metacar- cular the recipient bed, the faster the graft is revascular- pophalangeal joint), the site should be stabilized by a ized and the less it must rely on plasmatic imbibi- splint incorporated into the bandage before grafting to tion.'O"* accustom the horse to the decreased range of motion of The graft revascularizes by two processes: (1) vascular the joint.17 The acceptance of the graft is positively in- anastomoses through a direct connection of grafi and fluenced by the use of appropriate bandages that pre- host vessels (inosculation) and (2) ingrowth of host ves- vent movement between the graft and the recipient bed sels into the dermis of the graft.".I3 Inosculation usually and that help protect the delicate granulation tissue begins during the first 2 days, restoring circulation to from the graft by the fourth to seventh day.10.'3,14Revascular- For grafts to be accepted, the concentration of bac- ization by invasion of new vessels begins at 4 to 12 teria in the wound must be minimized." P-hemolytic days.2 When revascularization is well established, the Streptococcus, Protezls, and Psezldmnonas species are capa- fibrinous attachment of the =aft is infiltrated with fibro- ble of producing destructive proteolytic enzymes that

degrade fibrinous attachn~ents PREPARATION OF between the "graft and reci~ient THE DONOR SITE bed.?.3,(~,18,1'1These bacteria also -- produce copious p~~rulentdis- charge that can separate the nique to be used. A donor site graft from its bed.'.'.".'"'" ' should be easily accessible for procuring the graft or grafts. The Topical treatment of infected . rn granulating wounds with ap- most cosmetic sites for graft pro- propriatc anti~nicrobialdrugs is curement may be regions of the more effective than systemic neck covered by the mane, the treatment.'-.'" Systemically ad- ventral abdol-nen, or the ventral ministered antirl~icrobialdrugs pectoral region. Other donor are often incapable of achieving sltes Include the sternal region, therapeutic concentrations in the region caudal to the girth the granulation tissue because path, and the perineum.'-IS fibrin in the wound prevents Cosmesis of the wound is best penetration of the drug to the when the donor (color and surface of the wound.".'" Sys- texture) matches that adjacent to teniic antibiotic. therapy- nlay Figure 1-Harvesting pinch gmfts from a donor site. the graft site. Local anesthesia of be indicated if ot -1-he split-thickness graft 3..is harvested by ,evering the the skin and possibly sedation of the deep tissues is cone ofshn [he needle the horse are necessary for safe Dilute chlorhexidine solution ------retrieval of the fi~ll-thickness (0.05%) can be used daily to .' :'!. ; , . . . . : donor skin. cleanse wounds; it is niinimally ; , U,..,'l''.<.' + Skin for island grafting can be cytotoxic and more bactericidal obtained from the perineun~after in the presence of cellular de- caudal epidural anestI~esia.~This bris than is povidone-iodine donor site is particularly conve- solution." Prolonged contact of nienr for grafting a wound on a a stronger solution of chlorhex- rear limb. Split-thickness grafts idine (0.5%) has been denion- can only be harvested safely when strated to slow formation of the horse is anesthetized. granulation tissue and to hinder Regardless of the donor sire tissue healing." chosen, it should be clipped Before grafting, bone bc- (rather than shaved) to prevent neath deep wou~idsshould be darnage to the epithelium. The examined radiographically for site should be prepared with an the presence of osteitis (which antimicrobial scrub.'" must be resolved) or sequestra (which n~ustbe removed). A TECHNIQUES large wound often develops Pinch Grafting healthy granulation tissue Pinch grafting is the technique along its periphery before a of implanting small disks of skin seqilestrum is visible radio- (3 mm in diameter) into the graphically. In our experience, granulation tissue of a wound.' '"' grafting can be performed Pinch grafiing is easily performed successfi~llyalong the periph- in rhc field. Grafts are procured by ery of such wounds. A second elevating the skin \vith forceps or a grafting procedure can then needle with a bent point and sev- be performed in the center ering the elevated conc of skin at of the wound after the its base (Figure 1). 'The center seqllcsrrum is removed and Figure 2-Iniplantarion of pinch grafts into pockets of such a graft is ,,Carly full- healthy granulation has (crcatcd hv a No. 15 blade) in [he gran~llariontissue thickness and contains sornc hair developed. ------of the recipient bed. follicles. The graft is implanted into prevent loss of grafts. I a pocket created in the granu- For pinch grafts, a 75% ac- lation tissue by means of a ceptance rate can be expect- i No. 15 or No. 1 1 scalpel ed." This percentage is good 1 blade (Figure 2). The gran- and may be partially related ulation tissue is stabbed to the pockets, which hold 1 obliquely with the blade to the graft in contact with the i create a pocket 2 mm below granulation tissue. The sur- I the surface such that the face of the granulation pock- opening of the pocket is prox- et normally necroses during imodorsal. The pockets are healing; epithelialization then usually created in parallel proceeds circumferentially rows at I-cm intervals. Digi- from the graft. Because pinch tal pressure should be applied grafts are small, complete to the pocket for 15 to 30 epithelialization may take seconds to control hemor- a long time, especially if rhage, which may dislodge few grafts were implanted the grafts. I or there is a high percentage Insertion of grafts into the of graft loss.' pockets can be tedious. A An advantage of this graft- small thumb forceps, a blunt- ing technique is that the ed needle, or a No. 15 blade donor-site defects are small helps in implanting the grafts and not completely full- into the pockets. Grafts can Figure *Advanced stage of epithelializaaon afcer pinch-grafi . thickness. They are thus usu- be placed on a saline-soaked implantation. The cobblestone appearance of divergent, sparse: ally left open to heal by sec- sterile sponge, or four to five hair tztowth is cosmetically unacceptable to same clients. ond intention. grafts can be placed on the One disadvantage of pinch .-..,. .. . -. . wound (where ;hey are main- grafting is the difficulty in tained by hydoscopic pres- orienting the direction of sure) as the pockets are being hair growth because of the created. This speeds implan- small donor-graft size. Be- tation by minimizing repeti- r-r cause woundsheal primarily tive steps. Immediately after I by epithelialization and the insertion, moderate digital amount of hair follicles that pressure for 15 to 30 seconds survive is small, it may not may be necessary to control be worthwhile to try to hemorrhage and maintain maintain the orientation of the grafts in the pockets. I hair. Nevertheless, systematic Because of hemorrhage, grasping of the graft and pocket creation and grafting maintaining it in this orien- of a wound should com- tation while it is placed in its mence distally and progress Fioure 4-Yunch grafts obmned directly from the donor site pockets may allow appropri- proximally. Large wounds of (with the horse standing). Because this is a full-thickness ate direction of . If the the limb are best divided graft, the should be sharply excised be- hairs are direaed pro~~P into proximal to distal thirds. for:!:* f?Iantation. .------they help to stabilize the Starting with the most distal & graft and make it easier to gY.3. 2;. section, pockets are created and grA are ~planted.Af- insert into tne recipient bed. Despite carem orientation, ter the distal section of the wound is grafied, that section hair growth may occur in multiple directions. This di- is bandaged with a sterile nonadherent pad and sterile vergence of hair can help to cover the epithelial scar that gauze roll before the next (more proximal) section is develops benveen the islands ofskin. treated. Staged bandaging prevents loss of grafts if the Once they are healed, wounds often have a cobble- horse moves or there is excessive hemorrhage. Although stone appearance (Figure 3). For owners of show hors- the bandage becomes blood-soaked, this technique helps es, this may not be cosmetically acceptable."' - Punch Grafting for the punch biopsies.' We Punch !grafting is similar to I use 8-mm biopsy punches for pinch grafting and can be harvesting and 6-mm punch- readily performed in the field es to create the recipient sites with the horse standing. Us- (Figure 6). Biopsy holes in ing a punch, the granulation tissue are circular plugs of skin are placed at 5- to 15-mm inter- removed directlv from the vals. Cotton-tipped applica- anesthetized donor site (Fig- tors can be placed in a hole ure 4) or punched from a to aid in identifying the hole piece of skin that has been in the granulation tissue, removed from the donor site which rapidly becomes ob- (Figure 5). During harvest- scured by hemorrhageVFig- ing" from the ventral pectoral ure7). region, grafts are best Figure 5-An elliptic piece of skin removed from the chest to A second approach to create punch or tunnel grafts. ,,laced on a saline-soaked ...... ------.-----controlling hemorrhage after sponge.'"ecause p~lnch placement of punch sites is !grafis are full-thickness, sub- to bandage the recipient site cutaneous tissue and fascia must be removed with a for several hours. The grafts can be applied directly from blade before the grafts are implanted."' It is easier to re- the donor site or can be stored in a refrigerator on move subcutaneous tissue from a section of skin before saline-soaked gauzes during this ~eriod.Because some punches are harvested because removal from individual hair follicles are present, it may be beneficial to orient plugs is tedious. the gaft so that hair gowth is in a proper direction. If A biopsy punch, iisually 2 mm smaller than that used the wound is large, staged bandaging during the im- to obtain the donor sgrafi, is ~isedto create recipient sites plantation process (as described for pinch grafting) may prevent loss of grafts by pre- eluding- excessive hemor- rhage or movement of the graft. The donor graft sites can be closed with sutures or sta- ples or can be left open to heal by second intention. If an elliptic piece of skin is re- moved to obtain the punch sgrafts, the donor site should be sutured." If punches are removed directly, the donor sites heal with multiple stel- late scars." As with pinch grafting, the recipient site may develop an uncosmetic cobblestone appearance. Nev- ertheless, hair coverage is more prominent with this technique.'-'~"'"tudies have reported grafi survival as high as 95% and epirhelialization of the entire wound within Figure 6-Creation of the recipient sires for Figure 7-Sterile applicators placed in the re- 47 days.l.7.1%.l&l(, donor punch grafts using a 6-mm skin biopsy cipient-site pouches to facilitate hemostasis punch. and identification of the site (which is ob- *I~oI)*.-*mc)aL--- Tunnel Grafting ------scured by blood) in rhe gran~~lationtissue. . . Tunnel grafting requires urn Julv 1997 harvesting of full-thickness cutting needle.4 The nee- dle and graft are inserted 0, split-thickness strips of 1 skin that are 2 to 5 mm wide through the granulation tis- and slightly longer than the sue 5 rnm below the sur- WOUnd3.4.6,17.18,27(Figure 8). face. The haired side should Full-thickness skin is often be facing outward, and the used in field situations be- hair should be oriented in cause split-thickness skin the proper dire~tion.~ grafcs are difficult to obtain Because the needle is of- without special equipment ten smaller than the wound (e.g., free-hand knives or length, multiple small strips motorized dermatomes) and of grahs are used or a longer !general anesthesia." To im- graft is placed over the prove acceptance of the width of the wound by a graft, subcutaneous fascia Figure 8-Creation of a tunnel graft from skin along the neck. two- or three-step passage and fat should be sharply ...... of the needle.'' The needle dissected from full-thickness reenters the point of exit for donor skin." Ofren, the easi- the second and third bite. est method of obtaining tun- Tunnel grafts inserted into a nel grafts is to remove a sheet wound on a limb should be of skin from the donor site. placed circumferentially be- Strips of skin are then easily cause of the increased diffi- cut from the stretched and culty in placing the grafts in stabilized sheet'' (Figure 5). the proximodistal direction. To minimize scarring, the When tape is used to fa- donor site is sutured by pri- cilitate implantation of the mary clo~ure.'~ graft, the graft may become The grafts are implanted dislodged, especially if the in tunnels created in the tape is removed immediate- granulation tissue, which ly after insertion or later has been allowed to develop with excision of overlying 4 to 8 mm above skin level4 granulation tiss~e.~*'~At- These tunnels are created via taching the tape to the graft a cutting needle, a flattened can be tedious and can in- Kirschner wire with a trocar crease surgical time.28 Such point, a straight teat blade, attachment is unnecessary if or a malleable alligator for- an alligator forceps is used." ceps.3.4.~4,27.2splfl all igator for- In some cases, tunnels can ceps or a small tendon for- be created with the horse ceps is then used to grasp standing; however, because and pull the skin strip into slight movement may de- the bed4 (Figure 9). To pre- stroy the tunnel, short-term vent vascular compromise of general anesthesia may be the granulation tissue, tun- required.ls In our opinion, nel grafts should be placed general anesthesia facilitates no less than 1 cm apart.4 placement of grafts. Figure %--Creation of a tunnel in the recipient granulation In one popular method tissue and implantation of a tunnel graft using a tendon for- minimize used to implant tunnel ceps to pull the grafted tissue through the wound bed. time, a full-thickness sheet grafcs, adhesive tape (slightly ------we.).)e------of tissue can be harvested longer than the graft) is while the horse is standing. placed on the haired side of tl 1e gafc to minimize graft The sheet is cut into strips, stored on sponges soaked The graft and tape are then threaded through with physiologic saline, and then placed into the granu- the eye of a 10- to 12-cm, half-curved or straight lation bed after the horse is anesthetized. The ends of the graft are then sutured, rained from the recipient stapled, or glued to the edge site and trimmed to fit the of the wound for additional wound. Alternatively, a tem- security." Graft survival is plate of the wound can be enhanced by exposing the made from sterilized paper tunnel grafts away from the and used to trace the recipi- overlying granulation tissue ent bed to ensure that the 6 to 10 days after implanta- !graft is the correct size." ti~n.~"ecause most failures Once excised, full-thick- can be attributed to acci- ness grafts undergo primary dental removal of the graft shrinkage due to recoil of or failure to expose the elastic fibers in the deep graft, the surgeon who dermal layers." To allow for placed the gafts should re- primary shrinkage, the graft move the overlying granula- should be cut slightly larger tion tissue.I8 than the template.30,3'If the Tunnel grafts are useful section of donor skin re- for coverage of sites that are quired to cover the wound hard to bandage or highly would be so large that the mobile (e.g., the dorsal sur- donor site could not be su- face of the hock) because I tured, it may be necessary to the graft is restrained by the transport the horse to a fully overlying granulation tis- equipped practice; a split- sue2," (Figure 10). Survi- thickness graft (which does val rates of 80% have been not require closure of the reported for tunnel grafts, donor site) can be obtained and cosmesis of the healed via dermatome equipment wound is generally better with the horse anesthetized. than that of similar wounds To improve acceptance of that are left ungrafted.zx B~-Figure 10-Epithelialization after tunnel grafting of a wound.I a fl,ll-thickness sheet graft cause a tunnel graft is a type (Courtesy of Dr. D. G. Wilson, Department of Surgical Sci- by the recipient bed, it is ences, College of Veterinary Medicine, University of Wiscon- of island graft, the wound generally necessary to (1) re- sin) heals largely by epithelializa- ,,,,, -. -.- . - .. RkkT _.. . _ move the subcutaneous tis- tion. s.;:: =.,, ..-...t sue from the graft, (2) es- tablish hemostasis of the Sheet Grafting wound, and (3) stabilize the graft. The subcutaneous Sheet grafting produces better cosmesis at the recipi- tissue should be carefully excised with a scalpel to pre- ent site than does island grafting.18 Because a sheet graft vent crushing or scraping of the dermis. Hemostasis is covers the entire wound, it is less effective in allowing critical to prevent the graft from separating from the the exudate to escape; this can interfere with the ability recipient bed.3 of the graft to revascularize.'%cceptance is good only if To prevent contraction, the graft should be fixed to the recipient bed is uncontaminated, at an immobile the edge of the wound by means of sutures, staples, or site, and fresh or composed of immature granulation ~~anoacrylateglue." Caution is necessary to avoid oc- tissue.' Horses with wounds that contain mature granu- cluding the small dermal vessels by overstretching the lation tissue are thus poor candidates for this type of skin.32It may be necessary to inject local anesthetic into skin grafting.' the edge of the wound if the graft is to be sutured or sta- Full-thickness sheet grafting is easy to perform, re- pled with the horse standing. Interrupted sutures may quires slight expertise, and does not require expensive be required to attach the center of the graft to the recipi- equipment. Most grafts can be obtained from a stand- ent bed. If possible, the grafted site should be bandaged ing, sedated horse via regional or local anesthesia. The to maintain the graft against the recipient bed. most suitable area for obtaining full-thickness grafts is Full-thickness gafts have a relatively low rate of ac- the cranial pectoral region, where loose skin is most ceptance because revascularization of these grafts is more abundant.1',2%n elliptic section of skin can be ob- difficult and exudate may become trapped beneath the