An incomplete or partial dislocation of two adjacent vertebrae. Normally the vertebral bodies are squarely situated atop one another; however, when such things as trauma or certain forms of arthritis intervene, one vertebral body may shift with respect to its neighbor. When the shift does not completely abolish contact between the two normally adjacent surfaces but does alter their position with respect to one another, the abnormally positioned vertebra is said to be subluxed or partially dislocated. Services of chiropractors are covered under Medicare only when such subluxation is demonstrated on an X-ray.