Skin Anatomy

The skin is comprised of an outer layer, the epidermis, and an inner layer, the dermis, as well as the underlying subcutaneous layer or hypodermis.

Epidermis: is a stratified squamous epithelium that is continuously renewed. Its thickness depends on how much wear it receives.

Dermis: this layer determines the thickness of the skin. It is composed of collagen bundles and also includes the adnexal structures (hair follicles, sebaceous glands, sweat glands).

Hypodermis: consists of elastin, collagen and fat and closely adheres to the overlying dermis. It contains the panniculus muscle, which is important in reconstructive surgery because it carries the cutaneous arterial supply and is vital for vascular perfusion of the skin.

There are two types of arterial supply to the skin – musculocutaneous arteries and the direct cutaneous arteries.  Musculocutaneous arteries supply skeletal muscle, then terminate perpendicular to the skin, each supplying a small area. Direct cutaneous arteries pass through the skeletal muscle and travel parallel to the skin in the panniculus and supply a specific area of skin that can be anatomically defined.

The skin is not equally pliable in all directions.  Tenison lines are formed by the predominant pull of fibrous tissue within the skin.  They are important in surgical planning, especially where there is minimum skin laxity.  For example, straight wounds or incisions made parallel will gape less, and wounds should generally be closed parallel to tension lines, so there is less tension placed on the sutures and will minimise puckering.

Types of wound healing

  • Primary closure is appropriate for clean wounds that have occurred recently.
  • Delayed primary closure in clean-contaminated wounds is performed 2-3 days after the wound occurs, but before a granulation bed forms, to allow time for elimination of contamination.
  • Secondary closure – used for dirty wounds. Closure is performed after a granulation bed is formed, and it has been managed as opened wounds for five days.
  • Second intention healing is wound healing by contraction and epithelialisation.

Tension relieving techniques

  • Undermining: releases the skin from the underlying fascial attachments. Excessive undermining can cause dead space and seroma formation.
  • Walking sutures – interrupted sutures placed between the dermis and underlying granulation bed or fascia. The skin is stretched and advanced across the defect so that there is less tension on the primary suture wound. Excessive use of walking sutures can compromise blood flow from the dermal plexus.
  • Tension relieving “relaxing” incisions – these can greatly reduce the amount of tension across the wound without having much impact on the final cosmetic appearance of the wound. These may be a single incision made 1-2cm from the primary wound or multiple staggered small stab incisions made parallel to the wound which is left to heal by second intention.
  • Tension relieving suture patterns such as horizontal or vertical mattress sutures that are placed away from the skin margins to decrease tension on the wound edges

Pre-operatively the skin can be pre-stretched prior to surgery. This relies on “mechanical creep”, which allows the collagen fibres to be realigned and the skin to stretch beyond its usual elasticity.

  • Pre-suturing involves placing a row of Lembert sutures in the skin around the mass that is going to be resected.
  • Skin stretching devices can be used such as applying adhesive pads to either side of the wound, which are progressively tightened every 6-8 hours. This may be started 2-3 days prior to surgery and may be left on till 4-5 days after surgery to reduce tension on the sutures.
  • Skin expanders are silicon bladders that are surgically implanted into the subcutaneous space adjacent to the defect that is going to be reconstructed. Once it’s healed, the bag is filled with saline till the skin is taut, and it’s repeated every 4-7 days till it’s expanded enough to cover the defect.

Local recruitment

Advancement flaps rely on the elasticity of the skin around the wound, which is undermined and stretched into the defect.  Two parallel skin incisions are made from the extremities of the wound; the skin is undermined and lengthened, and pulled across the defect. Advancement flaps may be single or bi-pedicle. These are best used in an area where there is lots of loose local skin.

Axial pattern flaps

Axial pattern flaps are skin flaps the incorporate a direct cutaneous artery and vein, so they have better perfusion than random vascular flaps that rely on the subdermal plexus alone for blood supply.

The common direct cutaneous arteries used for axial pattern flaps in the dog include: caudal auricular, omocervical, thoracodorsal, caudal superficial epigastric, medial genicular, deep circumflex iliac and saphenous vessels.

They may be made in the usual ‘peninsula’ conformation or a right-angle, hockey-stock shape. Island flaps are possible, where all four borders of the flap are incised.  These are very versatile, but there is a great risk of damaging the vascular bundle.

Post-op complications include distal flap necrosis and consequent partial incisional dehiscence.  Others include oedema, seroma and haematoma formation and, less commonly, infection.  Necrosis and flap failure is almost always due to inadequate blood supply or excessive tension on the flap margins and usually becomes evident a few days after surgery.

Vacuum assisted closure (VAC)

Vacuum assisted wound closure is commonly used in human wounds and has recently been promoted in animals for degloving wounds, large bite wounds and chronic non-healing wounds.  A local negative pressure is applied to the wound and promotes the removal of exudates and bacteria, as well as increasing local circulation and granulation tissue formation, speeding up wound contraction.

A sterile open-pore foam is cut to the shape of the wound and applied directly onto it.  The suction tubing is then passed into the foam, which is sealed over by adhesive film.  A continuous negative pressure of -125 mmHg is typically recommended during vacuum assisted closure of an open wound.  Bandages should be changed at least every 24 hours initially, then every 2-3 days.

Free skin grafts

Free skin grafts involve transferring variable thicknesses of dermis with the epidermis from a donor site to the recipient wound site that lacks an epithelial surface.  It is usually only used when other techniques are not viable because it does have some limitations and is most often used for distal limb wounds.

Ideally, the recipient site should be clean, free of infection and haemorrhage well controlled.  Direct grafting of a fresh surgical wound is possible, but it is preferable to allow the wound to granulate prior to grafting, so it has an abundant vascular supply in a matrix of collagen with significant growth factor stimulation.

Skin from the donor site should be chosen for hair colour, orientation and texture, and a section large enough to cover the defect should be removed.  All of the hypodermal adipose tissue is excised, then a mesh is created by placing many stab incisions into the graft.  The graft is placed over the defect, which should overlap the wound bed by 2-4mm.  It is then sutured in place with loose simple interrupted sutures, and a non-adherent absorbable dressing placed.  The wound bed should not be disturbed for the first 72 hours.

The graft initially survives by the imbibition of nutrients and oxygen-rich fluid from the recipient.  At this time, the skin appears swollen and even slightly cyanotic.  After 2-3 days, new vessels begin growing into the skin, and it starts to appear pinker and will be more firmly adhered to the recipient bed at this stage.  Hair growth may be evident from 2-3 weeks but may take longer.

Punch Grafts

Punch grafts are small, circular, full-thickness skin grafts that are used to seed a granulation bed with “epithelial islands”.  Their fundamental purpose is to promote wound epithelialisation.

The advantages of using this type of graft are that it is simple to perform and does not require special equipment.  They are useful for high movement areas such as flexor surfaces and will tolerate suboptimal bandage states and high bacterial burdens in the wound.

The primary disadvantage is the final cosmetic appearance.  Hair follicles do not usually survive, so the resultant scar may not be aesthetically pleasing.