Ankylosing spondylitis (AS) is a chronic systemic inflammatory disease that primarily involves the spine, the medial joints, and eventually leads to bony ankylosis of the spine. The disease usually first affects the sacroiliac joints and then progresses up the spine, gradually involving the lumbar, thoracic, and even cervical spine. If the disease is not controlled, the intervertebral discs, interarticular joints and intervertebral ligaments will ossify, resulting in a characteristic bamboo-like bony ankylosis of the spine, often accompanied by varying degrees of hunchback deformity. Therefore, sacroiliac arthritis, bunion-like spine and hunchback deformity are often important factors in the diagnosis of the disease, and the presence of these manifestations in the late stage of AS is not difficult to diagnose. However, it is important not to make a hasty conclusion based on only one of them without differentiation. 1. Diseases that can occur as “sacroiliac arthritis” Dense iliopsoas This disease is commonly seen in female patients, mostly with postpartum onset, and the main clinical manifestation is lumbosacral pain. X-rays show sclerosis of the bone on the iliac side of the sacroiliac joint, and the sclerotic area is inverted triangular. Since the X-ray is angled through the compound and sclerotic bone tissue, the joint gap is usually not clear enough and is easily misdiagnosed as sacroiliac arthritis of AS. However, CT examination can clearly show that the sclerotic area is confined to the iliac side, with slight changes on the sacral side and no bone erosion on the articular surface. There was no involvement of the spine. Laboratory examinations such as blood sedimentation, CRP and HLA-B27 are usually not abnormal. 2.Diseases that can present with hunchback deformity Osteochondritis of the spine Also known as youth hunchback. Patients have a history of overwork during the adolescent bone development period, resulting in secondary vertebral dyskinesis. Like AS, this disease also occurs in young males and is associated with low back pain and a rounded humpback deformity very similar to AS. However, physical examination shows no significant restriction of spinal movement. X-rays show multiple vertebral wedges, and in some patients, Hugh Morse nodes can be seen. The sacroiliac joint is normal. 3, can make the spine activity is limited, and even the occurrence of vertebral bony ankylosis The disease patients are also mostly young men, and due to pain and lumbar spine activity is limited, severe cases can also be manifested as plate waist, modest posture. However, there is often an obvious history of trauma, and pain in the lower back and lower limbs is accompanied by changes in sensation, muscle strength, and knee and ankle reflexes in the affected innervation zone. imaging examinations of the sacroiliac joint are unchanged, and laboratory tests are unremarkable. CT and MRI may show compression of nerve roots or cauda equina by the nucleus pulposus of the herniated disc. Proliferative spondylitis The degenerated vertebrae of patients with this disease usually have osteophytes generated at the edges of the vertebral body, and the osteophytes of adjacent vertebrae can grow in opposite directions and eventually fuse to form a bone bridge, which can easily be misdiagnosed as AS bamboo-like spine. The differentiation points are: 1, occurring in middle-aged and elderly people over 40 years old. 2, column activity restriction and hunchback deformity is relatively mild. 3, iliac joint imaging shows degenerative changes, no bone erosion or bony fusion. 4, column X-ray can be seen narrowing of the intervertebral space, vertebral body edge osteophytes, hyperplasia and bone bridges usually beyond the intervertebral disc fiber ring range. 5.Changed intervertebral disc and hyperplastic bone superfluous compression of nerve roots or cauda equina nerve can lead to radicular neuralgia and abnormal skin sensation, muscle strength and physiological reflexes, and combined lumbar spinal stenosis can show intermittent claudication. 6, sink and other laboratory tests are not abnormal. Diffuse idiopathic bone hypertrophy (DISH) The etiology of this disease is unknown, and epidemiological studies have shown an association with obesity, abnormal glucose tolerance, and adult-onset diabetes mellitus, commonly in middle-aged and elderly men, with a male to female ratio of approximately 2:1, and the incidence increases with age and weight. The disease is characterized by hypertrophy of the anterolateral laminae of the vertebral body and is easily confused with advanced AS in the x-ray presentation. However, the sacroiliac joint, vertebral space and small vertebral joints are mostly normal or show degenerative changes, which can be used to differentiate it from AS. Brownish yellow disease (urinic aciduria) Brownish yellow disease is a rare genetic metabolic disease. It is due to a congenital deficiency of the enzyme urea oxidase, and thus urea cannot be further broken down. Excess uronic acid is excreted by urine and oxidized to black in the air. After birth, there are no symptoms except that the urine becomes brownish-black after oxidation and the diapers are often dyed black. After 30 to 40 years of age, due to the excessive deposition of uronic acid, a series of symptoms are produced, and the main symptom is the pigmentation of the skin all over the body, which is brownish yellow, hence the name brownish yellow disease. The cartilage of the ear shell becomes thickened and hardened, and it has a stone blue color. The ear wax is also black, the tympanic membrane is gray-black at the edges, hearing is often diminished, and the sclera also has melanin spots. Uric acid is deposited on the aortic and mitral valves, causing them to harden and cause a murmur. Male patients often have a combination of black prostate stones. The spondylolisthesis is more common in males than females, with 10%-15% of patients complaining of lumbar pain and restricted movement with mild hunchback deformity, similar in appearance to AS. X-rays show a change in the physiological curvature of the spine, narrowing of the intervertebral space, marked marginal osteophytes, and thin lamellar calcifications of the intervertebral discs. There is no obvious osteoporosis.