An intervertebral spacer device having a pair of opposing plates for seating against opposing vertebral bone surfaces, coupled to one another by a convex element mounted to one of the plates and maintained against the other plate by a retaining wall and a retaining ring....http://www.google.com.au/patents/US7208014?utm_source=gb-gplus-sharePatent US7208014 - Intervertebral spacer device utilizing a belleville washer having radially extending grooves
