Preferably the body 5 is constructed of a bio-compatible metal or the like such as stainless steel or titanium and the rear wall acts as a spring to try to maintain the upper wall and lower wall 15 and 16 in parallel relationship, while also functioning as a hinge, when the two walls 15 and 16 are forced apart opposite the rear wall 17 by application of spreading force to the walls 15 and 16.