Skin Grafts

Some wounds are not amenable to management by primary, delayed primary, or secondary closure. In such cases skin grafts can be used for secure wound coverage

Definition - A skin graft is a piece of skin taken from one part of the body, detached from its blood supply, and then transferred to another part of the body that allows its re-incorporation.

Early adhesion of the graft is due to fibrin clot that forms within minutes after skin transfer

The process of a graft being accepted at the recipient site is called ‘Take’.

Steps of Graft uptake-

1. Imbibition (24–48 h)—During this stage, the graft relies on nutrients reaching it by passive means from the recipient bed. The graft increases in weight and volume. 
2. Inosculation—Some vascularity is re-established by blood vessels in the recipient bed connecting to vessels in the graft dermis.
3. Revascularization—after 4–7 days, new vessels form due to host endothelium ingrowth into the graft. 

  

TYPES OF SKIN GRAFT

1) Partial-Thickness Grafts = Split Skin Grafts (SSGs, Thiersch Graft)

• A STSG  consists of the entire epidermis and some of the dermis 

• The donor site will usually heal spontaneously, since the epidermal regenerative capacity resides in the dermal appendages (the sebaceous glands, sweat glands and hair follicles) left behind.
  
• The graft is taken with a Watson skin graft knife or by a powered dermatome.
  
• Decisions that pertain to the thickness of a particular STSG are made based on
  
  1. The demands of the wound,
     
  2. A vailability of donor sites, and
     
  3. Training of the surgeon.
     
• The thinner STSGs (0.008 to 0.012 in.) tend to take more easily than thicker grafts.
  
• Thicker grafts provide more durable wound coverage and contract less.
• Appropriate donor sites for STSGs include the
  
  1. Lower extremities,
     
  2. Buttock,
     
  3. Trunk, and
     
  4. Occasionally the upper extremities.
     
• The scalp provides an excellent donor site for STSGs to the face, although raising a thick graft may result in alopecia.
  

     Selection of the donor site - 

• Place the donor site in an area on which the patient does not lie.
  
• Avoid selecting a donor site immediately adjacent to the recipient wound, so that differing site- dressing requirements do not interfere with one another.
  
• Select a donor site that is aesthetically acceptable and easily concealed.

• As soon as it is raised, the STSG undergoes primary contraction.(recoil of dermal elastic fibers) -> approximately 20% of the graft's surface area is lost .

Meshing of an STSG is a process in which multiple staggered rows of full-thickness incisions are placed in the graft.

• Advantages of Meshing - 

1. The meshed graft can be expanded to a surface area of 1.5 or more times its original surface area. 
2. Fluid accumulating beneath the graft can escape through the interstices, discouraging hematoma or seroma formation. 
3. A meshed graft conforms more accurately to an irregular wound bed.

• Disadvantage of the meshed graft is its inferior esthetic appearance.

• STSG is trimmed to fit the wound precisely and should be secured to the recipient site using sutures.
  
• Avoid small areas of graft overlapping intact skin, as the STSG will not take and may become a nidus for infection. 

A suitable dressing consists of

1. One layer of nonadherent gauze
2. Covered by a soft, bulky, pliable, absorbent material held in place by tie-over sutures or, in the case of an extremity, by a circumferential wrap.
3. This is reinforced with a splint or cast to limit joint mobility above and below the grafted site. 

• After 3 to 7 days, remove the dressing and inspect the wound, perform a dressing change every 24 h for an additional 7 to 14 days 
• As healing progresses, the STSG undergoes color changes from an early pink hue to a more waxy yellowish color, often lighter than the surrounding skin 
  

2) Full-Thickness Grafts (FTSGs, Wolfe Graft)

• A full-thickness skin graft (FTSG) consists of the entire dermis and epidermis.
  

• FTSGs shrink by up to 40% in surface area immediately on being raised as a result of elastic fiber recoil within their substance (primary contraction).
  
• When sutured in place, however, they tend to contract less than STSGs during the fibroblastic and remodeling phases of wound healing (secondary contraction). 
  

• Therefore, SSGs will have less primary contraction (10% vs 40%) but more secondary contraction than FTSGs, but overall, SSGs contract more than FTSGs.

• This feature is particularly important when skin grafts are planned for the hands and feet.
  
• An FTSG is often selected over an STSG when wound contracture would result in a loss of maximum joint function.

• A large ellipse of lower abdominal or groin skin for grafting can be removed and the resultant defect sutured under little or no tension with the patient in a sitting position.

• Other FTSG donor sites include the
  
1. Medial Arm,
   
2. Retroauricular Region,
   
3. Inferior Gluteal Fold, And
   
4. Supraclavicular Area. 

• The principles of bed preparation are the same as those for STSGs.
  
• After the FTSG has been raised, all fat and loose connective tissue are removed from the deep surface, exposing the dermis
  
• The FTSG is sutured in place and covered with a compression dressing, and the area is immobilized
  
• Wounds are usually inspected in 5 days at the time of the first dressing change.
  
• A bluish hue is typical for the FTSG when initially exposed; this changes to a more normal skin tone over 3 to 7 days.
  

• Full-thickness skin grafts are ‘superior’ to SSGs because 

1. They have the full thickness of the skin so the appearance is better.
2. The donor can be closed to a linear scar.
   
3. The graft is more durable and functionality is more likely to be preserved.
4. FTSGs are less dry as the sweat glands are maintained, and when re-innervated, their function approaches that of the recipient. (SSGs are typically very dry and may require emollients.)

• Re-innervation leads to sensory return that begins after around 4 weeks, with pain recovering before light touch or temperature.

Storage 

• Excess skin graft may be stored in a fridge at 4°C for up to a week (or kept in specific medium for several weeks, or in liquid nitrogen for much longer, but these latter options are impractical).

Causes Of Skin Graft Failure

In the presence of an adequately vascular bed, the commonest reasons for skin graft failure are:

1. Haematoma/Seroma 
2. Shearing
3. Infection

1) Haematoma/Seroma

• The fluid raises the graft away from the vascular bed, limiting the transfer of nutrients.
• This can be reduced by careful haemostasis.
  
• Making fenestrations in the graft either by hand or by machine meshing will allow drainage of potential collections but will leave unattractive marks.
  
• A pressure dressing or a tie-over dressing may also help; small collections may be carefully ‘rolled out’. 

2) Shearing 

• Shearing will disrupt vascular connections.
  
• This problem is reduced by immobilizing the graft: securing the periphery with sutures, glue or staples, and fixing the graft to the bed (quilting sutures).
  
• A ‘tie over’ dressing may be used and a splint reduces movement near joints.
  
• A suction dressing may be used for grafts in difficult areas such as the perineum.
  
• With a cooperative patient, an exposed graft may be an option and has the benefits of being able to inspect the graft continuously.
  
  

3) Infection 

• A clean bed is needed; small amounts of colonization of most bacteria are generally not important, but heavy growth of bacteria will lead to graft failure.
• There is a case for swabbing the wounds prior to elective surgery and administering prophylactic antibiotics. 

4) Total failure 

• It is usually due to the graft being placed upside down on the recipient bed  (the waterproof stratum corneum will not allow nutrients to pass to reach the graft cells.)