Physiology of Skin Grafts

SKIN:

Physiology & Function

• Epidermis: – protective barrier (against mechanical damage, microbe invasion, & water loss) – high regenerative capacity – Producer of skin appendages (hair, nails, sweat & sebaceous glands)

SKIN:

Physiology & Function

• Dermis: – mechanical strength (collagen & elastin) – Barrier to microbe invasion – Sensation (point, temp, pressure, proprioception) – Thermoregulation (vasomotor activity of blood vessels and sweat gland activity)

SKIN:

Physiology & Function

• • • • Immunological surveillance Most skin is thin, hair-bearing, has sebaceous glands Skin of palms/soles/flexor surface of digits is thick, not hair-bearing, no sebaceous glands Vascular supply confined to dermis

SKIN:

Anatomy

SKIN:

Anatomy

Skin Grafts:

Classification

• Full thickness skin grafts: • - epidermis & full thickness of dermis Split skin graft: - epidermis & a variable proportion of dermis - thin, intermediate or thick

Skin Grafts: SSG

Skin Grafts:

Classification



Autografts



Isografts



Allografts



Xenografts

Skin Grafts:

“Process of Take”

• • • Vascularity of donor site Tolerance to ischaemia Metabolic activity of the graft

Skin Grafts:

“Process of Take”

• 4 Phases: – Fibrin adhesion – Plasmatic imbibition – Revascularization: Inosculation & capillary ingrowth – Remodelling: Revascularization & fibrous attachment in restoring normal histological architecture

Skin Grafts:

“Process of Take”

• Plasmatic Imbibition: – Initially graft ischaemic (24 – 48 hrs) – Fibrin adhesion – Imbibition allows the graft to survive this period – ? Important for nutrition of graft – ? Stops drying out

Skin Grafts:

“Process of Take”

• Inosculation & capillary ingrowth: – At 48 hrs – Through fibrin layer – Capillary buds from recipient bed contact graft vessels – Open channels (neo-vascularization)  pink graft

Skin Grafts:

“Process of Take”

• Revascularization & fibrous attachment: – Connection of graft & host vessels via anastomoses (inosculation) – Formation of new vascular channels by invasion of graft (neovascularisation) – – Combination of old & new vessels (revascularisation) Fibroblast proliferation: conversion of fibrin adhesion  fibrous tissue attachment (anchorage within 4 days)

Skin Grafts:

“Process of Take”

Skin Graft Take: Epidermis

Days 0 – 4 Histological changes Epithelium doubles; crusting, scaling of epidermis; swelling of nuclei & cytoplasm; epithelial cell migration to surface; mitosis of follicular & granular cells ++ mitotic activity in SSG not FTSG 3 4 – 8 Week 4 Proliferation & thickening of epithelium (up to 7x) desquamation Epidermis returned to normal thickness

Skin Graft Take: Epidermis

Day 4 10 Histochemical changes Increased RNA in basal cells, indicating protein synthesis RNA returns to normal

Skin Graft Take: Dermis

• Fibrous component:

Collagen Elastin Hyalinized early and progressively replaced with new fibres by 6 weeks; Turned over 3-4X faster than normal skin.

Accounts for resilience; Days 3-7 fragment; Replaced 4-6 weeks.

Extracellular matrix Proteins direct the behaviour of keratinocytes; Communication between keratinocytes & fibroblasts.

Skin Graft Take: Dermis

• Appendages: - sweating dependent on no. of transplanted sweat glands & degree of sympathetic reinnervation; will sweat like recipient site in FTSG only - sebaceous gland activity mostly in thicker grafts: SSG usually dry & shiny - hair grows from FTSG if well taken with no complications

Skin Graft Healing

• • • • Initially white then pinkens with new blood supply Lymphatic drainage by day 6 Collagen replacement from day 7 to week 6 Vascular remodelling for months

Skin Graft Healing

• Contraction: - shrinks immediately due to elastic recoil: 40%; medium SSG 20%; thin SSG 10%.

- secondary contracture as heals: - FTSG remains same size after above shrinkage; - SSG will contract as much as possible; - more dermis = less contraction - ? Due to myofibroblasts – FTSG

Skin Graft Healing

• Reinnervation: – from margins to bed; – 4/52 to 2 years; – Depends on graft thickness and bed; – Uneventful healing leads to near normal 2PD; – Cold sensitivity can be a problem.

Skin Graft Expansion

• • • Based on principle that wounds reepithelialized from the periphery Expansion provides larger areas from which epithelium can grow Larger areas can be covered with less skin

Skin Graft Expansion

• Meshing - covers large area - easier to contour - fluid can drain through holes - cosmetic results less than ideal - various mesh ratio

Skin Graft Survival

• • • • • • Meticulous technique Atraumatic graft handling Well vascularized bed Haemostasis Immobilization No proximal constricting bandages

Skin Graft Failure

• • • • • • • • • • Haematoma Infection Seroma Mobility Inappropriate bed Dependency Arterial insufficiency Venous congestion Lymphatic stasis Technical – upside-down