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As severe ankylosing spondylitis advances:

• The head moves forward and the neck becomes stiff.
• The upper back curves forward (kyphosis).
• The normal curve in the low back flattens.
• The hips may become stiff and not straighten completely.
• People often keep their knees bent a little to make up for the changed curves in their back and stiffness in their hips. This bending of the knees causes the body to shift back a little so it's easier to balance and to see straight ahead.