Adjacent level ossification is a complication of anterior cervical discectomy and fusion (ACDF) with anterior plate stabilization. It represents pathological heterotopic ossification of the soft tissues above or below the ends of the plate, contiguous with the adjacent vertebral body. It occurs particularly if the end of the plate is within 5 mm the adjacent unfused disc 1. Generally, adjacent level ossification occurs far more frequently at the cranial adjacent segment, rather than the caudal one 1.
Although it can co-exist with adjacent segment degeneration, it is believed to be a separate process as prominent ossification can be seen in cases where the adjacent disc is normal 1.