is certainly a joint disease
as it involves mainly the axial skeleton
meaning the entire spine and hips. The patient usually male ( also seen in females but not common) develops buttock pain , worse in the mornings, in his twenties. Alternating buttock pain lasting months is classical. This ascends up till entire spine , neck downwards, is stiff and inflexible( 'bamboo spine'). Mobility is severely restricted when hips get involved and hip replacements can make a great difference.