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

Skin Graft Take: Epidermis DayHistochemical changes 4 Increased RNA in basal cells, indicating protein synthesis 10 RNA returns to normal

Skin Graft Take: Dermis Fibrous component: Collagen Hyalinized early and progressively replaced with new fibres by 6 weeks; Turned over 3-4X faster than normal skin. Elastin 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: – FTSG 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

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