Healing by first intention or Primary intention healing happens when the wound edges are approximated e.g. by sutures, staples or glue.
Healing by second intention or Secondary intention healing takes place when the wound edges cannot be approximated and the wound needs to heal from the bottom.
Tertiary intention healing is a combination of both of the above, i.e. the wound cannot be stitched up immediately but it can after a while.
Healing by primary intention
The condition for enabling primary healing is that the wound edges are sharp and completely clean and free of microbes as is the case with a wound produced via surgical incision (in a sterile environment). It is also possible to close some cuts caused by trauma via primary intention but they need to be sutured within 4 to 6 hours after the incident in order for the wound edges not to have become too inflamed, colonised or necrotic. The advantage of primary healing is that the time to closure is short which reduces the risk of infection and, furthermore, the scarring is limited. If the wound edges cannot be approximated, the wound will need to heal by second intention.
Healing by secondary intention
Secondary intention healing implicates that the wound edges cannot be approximated. This can be the case if there is not enough skin in order to pull the edges together without causing stasis in the area, as is often the case in venous leg ulcers, or if the tissue loss is extensive with a need for considerable new tissue generation as e.g. in fourth degree burns. It is also the case if the wound area is dirty or colonised or the wound edges are not sharp and clean as both would be the case in blast trauma wounds. Secondary intention healing is almost always necessary in dehisced (sprung open) surgical wounds as the tensile strength of the peri-wound tissue (the wound edges and the tissue just next to them) has proven too weak to sustain the tension of e.g. sutures or staples.
Secondary healing will typically be characterised by visible granulation tissue and the scar will be bigger than in wounds healed by first intention. The simple fact alone that wounds healing by second intention will be open for longer will render them at high risk of infection. Furthermore, as they will typically be colonised and often dirty and infected they will be prone to complications.
Healing by tertiary intention
In tertiary intention healing, there is a need for the wound to be open for a period of time before it can be sutured. Examples can be a wound left open to allow drainage and later is closed or a wound that is left to heal by secondary intention but encounters complications, where after a very thorough debridement is performed followed by an approximation of the wound edges.
MPPT in wounds to heal by non-primary intention
MPPT is intended for use in secondary and the first part of tertiary intention healing. It supports infection removal and prevention, autolytic debridement as well as granulation and epithelialisation, i.e. the wound healing process itself.