In the case of AS, this procedure is done to improve vision, hugiene, and function. With severe kyphosis the field of vision can be decreased considerably. Patients may not be able to lift the head enough to see in front of them. It can be difficult to open the mouth, brush the teeth, and for men, to shave.
Patients with this problem may be able to drive longer than they can safely walk. Sitting down helps bring them into a more upright posture. But when driving is no longer possible, osteotomy and fusion can help restore field of vision and function.
The surgeon will correct the deformity in a position that will allow the person to look ahead when standing and walking. Overcorrection is avoided so that the person can still work at a desk or drive a car.