Medical Blog

SKIN GRAFT TAKE

By definition a graft is completely removed from its donor site, loses its blood

supply and requires revascularization when applied to the wound bed. Split skin

grafts take on wound beds by adherence, plasmatic imbibition and revascularization.

When a split skin graft is applied to a wound bed, rapid adherence is a good

sign implying that the graft will take. Adherence is due to fibrin bonds, which are

weak at first and can be disrupted by shear, hematoma or seroma. If serous fluid

starts to leak through mesh interstices or fenestration’s immediately after graft

application and the graft is adherent, the good take is ensured.

Over the first 48 h the graft survives by plasmatic imbibition, that is, absorption

of fluid into the graft due to accumulation of osmotically active metabolites

and denatured matrix proteins. This fluid may contribute to cell nutrition and

may keep vascular channels within the graft open until it is revascularized. Thin

grafts survive this process better than thicker ones.

The graft is revascularized over a period of 3-4 days with vessel anastomoses

between the wound bed vasculature and existing vessels within the graft (inosculation)

and by direct fibrovascular ingrowth from the wound bed into the graft

matrix forming new vascular channels.

Full-thickness grafts require a rich blood supply from the wound bed to reestablish

blood flow within the dermal plexus of vessels. They get the majority of

their revascularization from wound edges and are best on freshly excised wounds.

They do not take well on contaminated or granulating wound beds. In the initial

stages graft revascularization can be prevented or disrupted by graft shear, hematoma

or seroma formation. Graft shear can be minimized by nonshear dressings,

pressure dressings, exposure or graft quilting. Hematoma can be minimized

by meticulous hemostasis following wound excision. Mesh grafting or fenestration

will allow drainage of hematoma or seroma.

Adherence and subsequent take depend on the wound bed, thus freshly excised

wounds take graft well with fascia better than fat. Fresh granulating wounds

also take grafts well but chronic granulating wounds and contaminated wounds

have poorer graft takes due to proteolytic enzymes in the wound that can be produced

by both bacteria and cells within the wound itself. In chronic granulating

wounds it is not uncommon to see ‘ghosting’ of skin where initial graft take is

good but then the grafts slowly ‘dissolve’ over a period of days. They can be salvaged

by wound care with topical antimicrobials.

GRAFT MATURATION

Following take the graft goes through a number of stages to achieve maturation.

Initially there is epithelial hyperplasia and thickening which leads to scaling

and desquamation. The epithelial appendages such as sweat and sebaceous glands

do not survive grafting but can regenerate in thicker grafts. The grafts are dry and

require moisturizing until these functions return. Hair follicles lie in deeper parts

of the dermis and in the hyperdermis and are not transplanted in most split skin

grafts. They are transplanted in full-thickness grafts and care must be taken in

selecting donor sites so as not to transplant hair to a nonhair bearing area. Grafts

tend to be re-innervated over a period of time with sensation developing within a

month but continued improvement can occur for several years.

Pigmentation of grafts can be troublesome, particularly inpatients with dark

skin. Grafts can be hypo- or hyperpigmented and it is difficult to predict which. In

general, grafts harvested from the lower half of the body tend to be paler and can

become yellow if placed above the clavicle.

The main problem with grafts is that of hypertrophic scarring and contraction.

All grafts will be surrounded by a marginal hypertrophic scar. Interstices in

mesh grafts will develop hypertrophic scarring and in widely meshed graft will

give a ‘crocodile skin’ appearance.

The wound bed contracts and this is inhibited to a certain degree by the skin

graft. The greatest amount of inhibition of wound contraction occurs with grafts

in the following order: full-thickness, thick split thickness, thin split thickness,

meshed. The amount of wound contraction depends on the proportion of dermal

thickness within the graft. Thus areas where there is no dermis, such as at the

margin of grafts and in grafts interstices, hypertrophic scarring and wound contraction

are inevitable.

Wound contraction around mobile areas and anatomical landmarks can lead

to contractures resulting in deformity and impairment of function.

SPECIAL GRAFTING TECHNIQUES

In general most grafts are applied and secured in the straightforward fashion

given above. However in certain instances special grafting techniques are required

to graft difficult or large areas.

MESHING

Grafts are meshed using machines, which essentially cut holes in the skin allowing

it to be stretched to cover a larger area than that harvested. The grafts can

be meshed at varying ratios, 1:1, 1.5:1, 2:1, 3:1, 4:1, 6:1, 9:1 using different machines.

In practice, grafts meshed with an expansion ration more than 3:1 need

protection as the wound bed that lies in the interstices (holes) of the graft is prone

to desiccation. The wound bed can undergo desiccation and necrosis with subsequent

graft loss and deepening of the underlying wound. Therefore special techniques

are used to protect widely meshed grafts. Once the graft has taken and the

interstices re-epithelialize, hypertrophic scarring in the interstices can be significant

with a ‘crocodile skin’ appearance. This is more pronounced with widely

meshed grafts; thus they are reserved for large burns. Mesh ratios of greater than

4:1 are not frequently used. In addition to expanding the area covered by a graft,

meshing allows drainage of any underlying hematoma or seroma. Sheet grafts

give a better cosmetic appearance but are more susceptible to loss secondary to

hematoma or seroma. Sheet grafts are used in cosmetically and functionally important

areas such as the face and hands.

FENESTRATION AND QUILTING

Fenestration (cutting multiple small holes in the sheet graft) allows drainage

of any hematoma or seroma. The fenestrated areas can leave scars and this technique

is best used for areas such as hands that are more functionally than cosmetically

important. Quilting of sheet grafts (suturing the graft at multiple sites to

resemble a quilt) prevents accumulation of hematoma or seroma under large areas

of graft. Quilting is also useful in securing grafts in cosmetically important

areas such as the face that are difficult to dress.

BOLSTERS

Tie over bolster dressings are used to secure grafts to difficult or mobile areas.

In the acute phase of the burn illness they are used for securing grafts to the back,

buttocks, shoulders, axillae or any other similar areas to prevent movement and

shearing of the graft. In these cases large bolsters are usually required using gauze

dressings for the bolster to apply pressure and 0 silk sutures through the surrounding

skin or tissue to tie the dressing on with.

In reconstructive cases smaller bolsters are used to secure full-thickness or thick

split skin grafts into areas that have undergone surgical release. In these cases,

provaflavine wool or sterile foam sponge can be used as the bolster with 2/0 or 4/

0 silk tie over sutures.

OVERLAY TECHNIQUE

This technique is the mainstay for covering large areas of open wound with

small amounts of autologous split skin graft. It is useful inpatients with large burns

and scarce donor sites. Described by Alexander in 1981, it involves harvesting autograft

from available areas and meshing it 4:1. It is placed on the wound and

maximally expanded to cover as large an area as possible. This is then overlaid

with unexpanded 2:1 meshed fresh or cryopreserved allograft applied at 90° to the

autograft in a sandwich pattern (Fig. 4.6). It is important not to expand the allograft

as this protects the underlying autograft interstices from desiccation and

infection.

The grafted areas are dressed and looked after with standard graft care protocols.

Both autograft and allograft adhere and take to the wound. As epithelial migration

across the autograft interstices occurs, the overlying autograft becomes

loose and detaches leaving a re-epithelialized area underneath. This is known as creeping substitution. This process can take 2-3 weeks before the whole grafted

area has epithelialized. Greater expansion ratios for the mesh autograft can be

used (6:1, 9:1), but re-epithelialization takes longer and the resultant scarring and

appearance tend to be worse.

GRAFT DRESSINGS

The successful take of skin grafts does not solely rely on their application. As

already mentioned, wound preparation and hemostasis are vital. The positioning

of grafts is also of vital importance in minimizing functional and cosmetic deformity

and is discussed below.

Securing the graft and dressing it are important in minimizing loss through

shear, hematoma, seroma and infection.

Our traditional preferred graft dressing is gauze impregnated with a Polysporin

and mycostatin ointment. The dressings are applied so that the grafts are compressed

against the wound. They are then covered with a rolled gauze dressing

followed by a bulky gauze dressing to soak up any wound exudate. An elasticized

crepe bandage is then applied. Care must be taken not to put circumferential dressings

on too tightly on limbs as they can cause distal ischemia, particularly if they

dry out and cause constriction. In the same way dressings on the trunk can impair

chest wall excursion and inhibit respiration. In children under two who use the

diaphragm to breathe, circumferential dressings around the abdomen must not

be too tight.