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Maybe you can find some useful ifo here: http://www.clinexprheumatol.org/article.asp?a=1455
and from the MD guidelines: "The prognosis is variable but generally favorable, with a tendency for the condition to be self-limiting with either spontaneous remission or a mild course. The disease may also be active over a period of years with life expectancy reduced after 10 years of active disease. The outcome of medical or surgical intervention varies in each individual, although some lessening of symptoms can be expected in all individuals.
Comparatively few individuals will develop severe functional disability. Disability correlates with duration of disease, disease activity, fixed spinal deformity, and spinal mobility. Severe hip involvement also results in greater functional impairment." (source http://www.mdguidelines.com/ankylosing-spondylitis)