The difference between dense osteitis and ankylosing spondylitis is as follows: 1. Dense osteitis: most occur in the iliac bone, also known as iliac dense osteitis. It is usually the result of trauma, chronic strain, chronic hypotoxic infection of the genitourinary system, impaired circulation, and spinal and hip lesions. The bone density at the site of the lesion increases significantly, while the bone structure is normal. 2, ankylosing spondylitis: most of the simultaneous onset of bilateral, sacroiliac joint space is the most common, can also involve the spine. There is significant narrowing or fusion of the joint space, with surrounding osteoporosis and bamboo-like changes in the spine, limited spinal motion, and in severe cases, disability. Therefore, the difference between dense osteitis and ankylosing spondylitis is significant.