The disease often begins in the sacroiliac joints and is symmetrically involved, with progressive upward progression of the lesion to the whole spine, where osteoporosis is present, but does not cause biconcave deformation or wedge deformation of the vertebral body due to spinal ankylosis. Bilateral sacroiliac joints are symmetrically hyperplastic and sclerotic, with blurred and irregular articular surfaces, visible worm-like or brush-like destruction, pronounced iliac surfaces, narrow joint spaces, and even bony ankylosis. The small vertebral joint surfaces are not smooth, hyperplastic and sclerotic, with narrowed gaps or bony ankylosis. Periprosthetic ligaments and cartilage ossification, extensive soft tissue ossification and bone bridges on both sides of the spine occurring in late stages give the spine a bamboo joint shape; a severe hunchback deformity may develop. It can involve the large joints of the extremities, with the hip joint being the most susceptible, with bilateral symmetrical onset, uniform narrowing of the joint space, destruction of the bony articular surfaces, formation of bone redundancy and bony ankylosis.