Patent ID: 8231631

Claim:
A medical instrument comprising: a base; an upper member; and a linkage mechanism that is a separate structure relative to the base and, upper member and is 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 two link pairs, each link pair having a first link that is pivotally coupled to the upper member and a second link that is pivotally coupled to the base, the first and second links being pivotally coupled to one another about a pivot axis, the at least two link pairs being arranged at an angle relative to one another such that the pivot axes of the at least two link pairs are oriented in a non-parallel manner resulting in that when one link pair opens, the other link pair opens at an equal angle resulting in the upper member being constrained to remain parallel to the base.