Patent ID: 8548726

Claim:
A marker system for determining a shape and an axial location of a rotationally symmetrical hole in an instrument, said instrument including a plurality of instrument markers attached thereto, comprising: a body including a plurality of rotationally symmetrical extensions, wherein at least two extensions of the plurality of extensions are dimensionally different from one another; a plurality of body markers attached to said body; a detection device operative to detect signals emitted by said body markers and by said instrument markers and, based on said detected signals, to output data signals representative of a) a location of the plurality of body markers relative to the detection device and b) at least three different locations of the instrument markers relative to the detection device; and a data processing device operatively coupled to said detection device and configured to to receive said data signals, to store an association between each axis of a respective extension with at least one characteristic that defines a shape of the respective extension, to calculate, based on the assumption that the at least three different locations of the instrument markers lie on a circular trajectory, a location of a center point of the circular trajectory relative to the detection device, said calculation further based on the data signals corresponding to the at least three different locations of the instrument markers relative to the detection device, to determine which of the axes of the rotationally symmetrical extensions the calculated location of the center point lies on, and to determine, based on the stored association, the at least one characteristic that is associated with the determined axis.