Intervertebral spacer device having a radially thinning belleville spring

An intervertebral spacer device comprising:

• first and second plate members, each having first and second plate surfaces thereof, said plate members being disposed in a spaced apart relationship such that said first plate surfaces oppose one another, and said second plate surfaces face in opposite directions;
• and
• at least one subassembly disposed between said first plate surfaces of said first and second plate members, and disposed such that at least a portion of a compressive load applied to said second plate surfaces of said plate members is transmitted to said at least one subassembly,
• said subassembly including a ball captured in a curvate volume, at least a portion of said curvate volume being formed in a convex element that is maintained against said first plate member by a retaining wall and a retaining ring,
• the retaining ring being circumferentially adjacent a large end of said convex element,
• the retaining wall being on the first plate member and engaged with the retaining ring to hold said convex element against said first plate member,
• said subassembly establishing a center of rotation between said first and second plate members and about which said first and second plate members rotate relative to one another.