a medical instrument includes a base ; an upper member ; and a linkage mechanism coupled to the base and the upper member and being operable to maintain the upper member in a variable fixed position that is substantially parallel to the base and spaced a predetermined , variable distance therefrom , wherein the linkage mechanism includes at least three linkage elements that are arranged at angles relative to one another such that when one link element opens , the other link elements open with at an equal angle resulting in the upper member being constrained to remain parallel to the base .