Pathogenesis of square bodies in ankylosing spondylitis.

The pathogenesis of squaring of the vertebral bodies is analysed in a 25 year old patient with ankylosing spondylitis. The squaring derives from a straightening of the anterior contour. The disease was diagnosed five years before the patient's death. The remodelling of bone that explains squaring of the vertebral bodies is described for the first time. Histologically, an acute and chronic spondylitis with destruction and simultaneous rebuilding of the cortex and the spongiosa of the vertebral bodies has been found. The development of square vertebral bodies is shown to be based on a combination of a destructive osteitis and repair.