One of the key features of psoriasis in many patients is a predilection for sites prone to chronic low-grade trauma such as elbows and knees. Certainly more direct trauma such as a cut or wound can Koebnerize or turn into psoriasis. For this reason, trauma to the skin needs to be avoided or minimized in psoriasis.
The Koebner phenomenon (Koebnerization, isomorphic response) occurs when a new area of psoriasis develops in injured skin. For example, after a surgery, psoriasis may develop around the surgical scar. This phenomenon may also help explain why psoriasis tends to occur on areas of constant low-intensity trauma such as elbows and knees. Koebnerization can occur after non-traumatic skin injury such as a sunburn, or an allergic reaction to a medication. In patients who suffer from dandruff or seborrheic dermatitis of the face and scalp, psoriasis can superimpose itself due to irritation and scratching and a crossover or combination dermatitis known as "sebopsoriasis" develops. Koebnerization is not specific to psoriasis.
Recently a patient of mine who was quite stable and completely clear of psoriasis while under treatment with Efalizumab came to the office with a severe outbreak of psoriasis on the fingers. After a bit a questioning, the reason became clear. He had installed a fence a few days earlier and sustained repeated trauma to his fingers in the process. Over the next month or so, his regular treatment cleared the psoriasis but he'll be much more careful next time!
Koebnerization can occur after indirect or even non-physical trauma as well. I've seen sunburns and allergic drug reactions turn into psoriasis. It's heartbreaking to see when someone who was trying to clear their psoriasis with sunlight ends up even worse due to a sunburn induced whole-body flare up.
Baby your skin to keep it calm. Minimize all sorts of trauma and you'll be one step closer to preventing outbreaks of psoriasis.