# Skin Grafts: Biology and Technique

## Overview
Skin grafting is one of the most commonly performed procedures in plastic surgery. A skin graft is a segment of dermis and epidermis that is completely detached from its blood supply and transferred to a recipient wound bed, where it must establish a new vascular supply to survive. Understanding graft biology, donor site selection, and causes of graft failure is essential for predictable outcomes.

## Types of Skin Grafts

### Split-Thickness Skin Grafts (STSG)
Contain epidermis and a variable portion of dermis. Thickness classification: **Thin** (0.005-0.012 inches / 0.12-0.30 mm) -- Thiersch graft. **Intermediate** (0.012-0.018 inches / 0.30-0.45 mm) -- most commonly used.

**Thick** (0.018-0.028 inches / 0.45-0.70 mm). Donor site re-epithelializes from retained dermal appendages (hair follicles, sweat glands). Greater graft take rate than FTSG due to thinner tissue requiring less revascularization. Greater secondary contraction than FTSG. Poorer color and texture match compared to FTSG.

### Full-Thickness Skin Grafts (FTSG)
Contain epidermis and entire dermis. Donor site must be closed primarily or with another graft. Better color match, texture, and less secondary contraction than STSG. Lower take rate -- requires well-vascularized wound bed. Common donor sites: postauricular, preauricular, supraclavicular, groin crease, hypothenar eminence, inner arm.

### Composite Grafts
Contain skin plus an additional tissue type (cartilage, fat, perichondrium). Examples: auricular composite graft for nasal alar defects. Limited size (usually <1.5 cm) due to need for diffusion-based nutrition. Highest failure rate of all graft types.

## Graft Physiology and Healing

| Phase | Timeframe | Key Events |
|-------|-----------|------------|
| 1. Plasmatic Imbibition | 0-48 hours | Graft absorbs plasma by capillary action; survives on diffusion alone |
| 2. Inosculation | 48-72 hours | Donor and recipient capillaries align and form direct anastomoses |
| 3. Revascularization | Day 3-7 | New vessel ingrowth from wound bed; graft turns pink by day 4-5 |
| 4. Remodeling | Weeks-months | Nerve regeneration (4-5 weeks); sensation returns (pain first, then touch, then temperature) |

### Phase 1: Plasmatic Imbibition (0-48 hours)
Graft absorbs plasma from the wound bed by capillary action. Provides nutrients and maintains graft viability. Graft is edematous and appears pale/cyanotic. Graft survives on diffusion alone during this phase.

### Phase 2: Inosculation (48-72 hours)
Donor and recipient capillaries align and form direct anastomoses. "Kissing" of cut vessel ends -- creates a connection between graft and recipient vasculature. This is somewhat controversial; may occur simultaneously with neovascularization.

### Phase 3: Revascularization / Neovascularization (Day 3-7)
Ingrowth of new blood vessels from the wound bed into the graft. Vessels grow along existing graft vascular channels. By day 4-5, flow is established; graft turns pink. Lymphatic drainage restored by day 4-5 (resolution of edema).

### Phase 4: Remodeling
Nerve regeneration begins at 4-5 weeks; recovery of sensation over months. Pain sensation returns first, then touch, and temperature last. Sebaceous gland function may not return in STSG (thinner grafts lose appendages). Contraction occurs at graft-wound interface (myofibroblasts).

## Primary vs. Secondary Contraction

| Feature | Primary Contraction | Secondary Contraction |
|---------|--------------------|-----------------------|
| Timing | Immediate (upon harvest) | Weeks to months (during healing) |
| Mechanism | Elastic fiber recoil in dermis | Myofibroblasts in wound bed |
| Greater in | FTSG > STSG (more dermis = more recoil) | STSG > FTSG (less dermis to resist) |
| Clinical implication | Harvest 10-20% larger than defect | Avoid thin STSG across joints/hand |

### Primary Contraction
Immediate shrinkage of the graft upon harvest due to elastic fibers in the dermis. **FTSG > STSG** (more dermis = more elastic recoil). Clinically: harvest a graft 10-20% larger than the defect to account for this.

### Secondary Contraction
Contraction of the grafted wound during healing (weeks to months). Mediated by myofibroblasts in the wound bed. **STSG > FTSG** (thin grafts contract more; less dermis to resist contraction). More dermis = greater resistance to secondary contraction. Clinically significant: avoid thin STSG across joints or in the hand where contraction limits function.

## Graft Harvest Technique

### STSG Harvest
**Dermatome types**: powered (Zimmer, Padgett), hand-held (Weck, Goulian), drum (Reese). Common donor sites: anterolateral thigh, buttock, upper arm (concealable). Apply mineral oil to donor site for smooth dermatome glide. Set dermatome width and thickness before use.

Apply even pressure and maintain consistent speed during pass. Meshing: 1:1 (no expansion), 1.5:1, 2:1, 3:1, or wider ratios. Meshing allows conformity to irregular surfaces, drainage of fluid, and expansion of coverage area. Unmeshed (sheet) grafts provide superior cosmetic outcome -- preferred for face and hands.

### FTSG Harvest
Elliptical excision at selected donor site. Remove all subcutaneous fat from the graft undersurface (fat is avascular and impedes revascularization). Donor site closed primarily. Pattern a template of the defect and transfer to the donor site with ~10-20% oversizing.

## Graft Application and Fixation

### Wound Bed Preparation
Must have adequate vascularity -- granulation tissue, periosteum with intact periosteal vessels, perichondrium, paratenon. **Avascular surfaces that will NOT support a graft**: bare cortical bone, bare cartilage, bare tendon without paratenon, irradiated tissue (compromised vascularity). Wound bed must be free of infection, necrotic tissue, and excessive bioburden. Hemostasis is critical -- hematoma beneath graft is the most common cause of graft failure.

### Fixation Methods
**Bolster (tie-over) dressing** -- sutures placed around graft perimeter, long tails tied over a cotton/foam bolster; prevents shear and provides gentle compression. **Negative pressure wound therapy (NPWT/VAC)** -- excellent for STSG; uniform pressure distribution, removes fluid. **Staples** -- rapid fixation, especially for large STSG. **Fibrin sealant** -- alternative to sutures/staples; reduces seroma, useful for difficult areas. **Quilting sutures** -- absorbable sutures through the graft into the wound bed to eliminate dead space.

### Postoperative Care
Immobilize grafted area for 5-7 days. First dressing change: bolster removed at day 5-7. STSG donor site: heals by re-epithelialization in 10-14 days (covered with transparent film, foam, or alginate). Protect grafted skin from sun exposure for 6-12 months to prevent hyperpigmentation.

## Causes of Graft Failure
1. **Hematoma** -- most common cause; separates graft from wound bed, preventing imbibition and revascularization. 2. **Seroma** -- fluid collection beneath graft; similar mechanism to hematoma. 3. **Infection** -- bacterial proliferation (especially Pseudomonas, beta-hemolytic Streptococcus) destroys graft vasculature. Beta-hemolytic Streptococcus produces fibrinolysins that prevent fibrin attachment.

Bacterial count >10^5 organisms per gram of tissue significantly reduces graft take. 4. **Shear/movement** -- disrupts fragile vascular connections during inosculation/neovascularization. 5. **Poor wound bed** -- avascular surface (bone, cartilage, tendon without paratenon). 6. **Technical errors** -- fat left on undersurface of FTSG, graft placed upside down, inadequate contact. 7. **Patient factors** -- smoking, malnutrition, immunosuppression, peripheral vascular disease.

<image>
Side-by-side cross-sectional comparison of split-thickness skin graft (STSG) and full-thickness skin graft (FTSG). The STSG shows epidermis and partial dermis being harvested with a dermatome, leaving behind deep dermis with hair follicles and sweat glands for donor site re-epithelialization. The FTSG shows the full thickness of epidermis and dermis being excised with a scalpel. Labeled layers include epidermis, papillary dermis, reticular dermis, subcutaneous fat, and skin appendages. Clean medical illustration with color-coded layers.
</image>

<image>
Sequential illustration of the three phases of skin graft revascularization: (1) Plasmatic imbibition showing the graft absorbing plasma from the wound bed via capillary action with a thin fibrin layer between graft and wound bed, (2) Inosculation showing alignment and connection of donor and recipient capillary buds, (3) Neovascularization showing new vessel ingrowth from wound bed into the graft with established blood flow. Each phase labeled with timeframe. Medical illustration style with cross-sectional view.
</image>

## Key Clinical Pearls
Hematoma is the most common cause of graft failure -- meticulous hemostasis of the wound bed is paramount. Always defat FTSG completely -- residual subcutaneous fat acts as a barrier to revascularization. Beta-hemolytic Streptococcus is uniquely destructive to grafts due to fibrinolysin production. FTSG undergoes greater primary contraction but less secondary contraction than STSG -- use FTSG across joints and on the face.

Sheet (unmeshed) STSG should be used on the face and hands for optimal cosmetic results. Meshed grafts allow drainage and expansion but leave a permanent mesh pattern. NPWT (wound VAC) bolsters improve STSG take rates by providing uniform compression and fluid removal. Always account for primary contraction by harvesting a graft 10-20% larger than the defect. The wound bed, not the graft, determines the degree of secondary contraction (more dermis in the graft resists it).

## References
- Ratner D. Skin grafting. *Semin Cutan Med Surg*. 2003;22(4):295-305.
- Freshwater MF, Krizek TJ, Cariddi A, Porque AV. Skin graft survival in a wound with bacterial contamination. *Plast Reconstr Surg*. 1975;55(1):77-80.
- Rudolph R, Ballantyne DL Jr. Skin grafts. In: McCarthy JG, ed. *Plastic Surgery*. W.B. Saunders; 1990.
- Nguyen DQ, Potokar TS, Price P. An objective long-term evaluation of Integra (a dermal skin substitute) and split thickness skin grafts, in acute burns and reconstructive surgery. *Burns*. 2010;36(1):23-28.