Both psoriasis and psoriatic arthritis are part of a family of conditions called psoriatic disease, and both of them cause inflammation. In the case of psoriasis, this inflammation happens on the skin, with red, itchy and scaly patches (known as plaques) appearing most commonly on the elbows, knees, scalp, chest and lower back. In psoriatic arthritis, however, this inflammation is not as obvious to the eye; it results in swollen and painful joints and tendons, and can occur in any area of the body.
Nearly all individuals with psoriatic arthritis also have psoriasis, and for around 80% of them, the skin condition appears before the joint condition. People may also be at increased risk for developing psoriatic arthritis if their psoriasis symptoms involve scalp lesions; misshapen, damaged or infected nails; or inflammation of the skin in the buttock area.
Something to keep in mind is that there is no clear link between the severity of your psoriasis and the severity of the symptoms you might have if you develop psoriatic arthritis.