What deformity is most likely to result from ankylosing spondylitis?

Ankylosing spondylitis is a chronic inflammatory disease that primarily affects the spine and sacroiliac joints, leading to pain and stiffness. Over time, the progressive inflammation can result in the fusion of vertebrae, which may create a fixed flexion deformity of the spine.

As the condition advances, patients often adopt a forward-stooped posture characterized by limited lumbar extension and increased thoracic kyphosis. This posture is a result of both muscle contractures and joint involvement, leading to a significant loss of the normal spinal curvature and movement. This fixed flexion position is a hallmark of ankylosing spondylitis, and it is the reason why flexion deformity is the most likely outcome associated with this condition.

In contrast, deformities such as extension, rotation, or lateral bending are not characteristic of ankylosing spondylitis due to the nature of the disease process, which promotes fusion and restricts spinal mobility primarily in the flexed position.