Statistics very. One estimate is that 7-26% of psoriasis patients progress to psoriatic arthritis. So guessing conservatively think 15-20% of the 5 million psoriasis patients, so 750k-1million patients.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206220/
Arthritis Foundation estimates 17-42% of 7.5 million people with psoriasis get psoriatic arthritis, so that would be much more.
http://blog.arthritis.org/psoriatic-arthritis...psoriasis/

and this:
https://www.psoriasis.org/about-psoriatic-arthritis

Quote:

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tudies show that delaying treatment for psoriatic arthritis as little as six months can result in permanent joint damage. Early recognition, diagnosis and treatment of psoriatic arthritis are critical to relieve pain and inflammation and help prevent joint damage.

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