The process of skin Scar formation is dependent on the type of injury inflicted as well as the type of healing. For this reason it is important to understand how the healing process occurs. For this reason we will focus on the type on the healing classification.
A skin wound can heal in 4 ways:
1) Healing by First or Primary Intention
- Primary wound healing or primary closure
- Describes a wound closed by approximation of wound margins or by placement of a graft or flap, or wounds created and closed in the operating room.
- Best choice for clean, fresh wounds in well-vascularised areas
- Indications include recent (<24h old), clean wounds where viable tissue is tension-free and approximation and eversion of skin edges is achievable.
- Wound is treated with irrigation and débribement and the tissue margins are approximated using simple methods or with sutures, grafts or flaps.
- Wound is treated within 24 h following injury, prior to development of granulation tissue.
- Final appearance of scar depends on: initial injury, amount of contamination and ischemia, as well as method and accuracy of wound closure, however they are often the fastest and most cosmetically pleasing method of healing.
To summarise, generally healing by Primary intention means closing the wound with stitches and allow healing while the wound is stitched up. This type of healing allows quick recovery, however the risks of infections in the inner part of the wound could cause unwanted complications (i.e. cysts). This is why is is used in very well vascularised areas and in clean areas of the body. This type of healing is not always advisable, especially if the wound is located on a part of the body which could be highly prone to infections.
2) Healing by second or secondary intention
- Secondary wound healing or spontaneous healing
- Describes a wound left open and allowed to close by epithelialization and contraction.
- Commonly used in the management of contaminated or infected wounds.
- Wound is left open to heal without surgical intervention.
- Indicated in infected or severely contaminated wounds.
- Unlike primary wounds, approximation of wound margins occurs via reepithelialization and wound contraction by myofibroblasts.
- Presence of granulation tissue.
- Complications include late wound contracture and hypertrophic scarring.
In essence the wound may either inflicted by surgery or caused by an accident. Due to the high likelihood of infection it is left open and packed regularly to avoid bridging. Healing by secondary intention is normally used in Pilonidal sinus interventions to minimise complications during the healing process and future recurrence.
3) Healing by Third or Tertiary intention
- Tertiary wound healing or delayed primary closure
- Useful for managing wounds that are too heavily contaminated for primary closure but appear clean and well vascularized after 4-5 days of open observation. Over this time, the inflammatory process has reduced the bacterial concentration of the wound to allow safe closure.
- Subsequent repair of a wound initially left open or not previously treated.
- Indicated for infected or unhealthy wounds with high bacterial content, wounds with a long time lapse since injury, or wounds with a severe crush component with significant tissue devitalisation.
- Often used for infected wounds where bacterial count contraindicates primary closure and the inflammatory process can be left to débribe the wound.
- Wound edges are approximated within 3-4 days and tensile strength develops as with primary closure.
Put more simply, healing by tertiary intention means starting healing by secondary intention and subsequently stitching up the wound in order to accelerate the closure of the wound minimising the risks of complications.
4) Partial Thickness Wounds
- Wound is superficial, not penetrating the entire dermis.
- Type of healing seen with 1st degree burns and abrasions.
- Healing occurs mainly by epithelialization from remaining dermal elements.
- Less contraction than secondary healing in full-thickness wounds
- Minimal collagen production and scar formation.
The above three modality of wound healing are normally associated to healing following minor skin injury, excision. However for most common wounds partial thickness healing process model is applicable.