Ankylosing spondylitis is one of many forms of inflammatory arthritis, the most common of which is rheumatoid arthritis. Ankylosing spondylitis primarily causes inflammation of the joints between the vertebrae of your spine and the joints between your spine and pelvis (sacroiliac joints). However, ankylosing spondylitis may also cause inflammation and pain in other parts of your body like where your tendons and ligaments attach to bone, joints between your ribs and your spine, hips, shoulders, knees, feet, and even eyes. As the condition worsens and the inflammation persists, new bone forms as a part of the healing process. Your vertebrae begin to grow together, forming vertical bony outgrowths (syndesmophytes) and becoming stiff and inflexible. Fusion can also stiffen your rib cage, restricting lung capacity and function. Ankylosing spondylitis is a chronic condition. Also called spondylitis or rheumatoid spondylitis, ankylosing spondylitis affects about 129 of every 100,000 people in the United States.