Guidance system for rotary surgical instrument

A surgical instrument for use in a computer assisted navigation system, said instrument comprising:

• a rotary member having first and second opposed ends;
• a mounting assembly operably coupled to said rotary member wherein said rotary member and said mounting assembly are relatively rotatable about an axis;
• and
• at least one reference element registerable in the computer assisted navigation system disposed on said mounting assembly at a predetermined location, said mounting assembly defining a center of gravity spaced radially outwardly from said axis wherein said predetermined location defines a first angular position relative to said axis and said center of gravity defines a second angular position relative to said axis, said first and second angular positions separated by at least 90 degrees.

A surgical instrument for use in a computer assisted navigation system, said instrument comprising:

• a rotary member;
• a mounting assembly operably coupled to said rotary member wherein said rotary member and said mounting assembly are relatively rotatable about an axis;
• at least one reference element registerable in the computer assisted navigation system, said at least one reference element disposed on said mounting assembly at a predetermined location;
• and
• an anti-rotation feature disposed on said mounting assembly biasing said mounting assembly toward an orientation wherein said reference element is disposed vertically above said axis during relative rotation of said rotary member and said mounting assembly about said axis when said axis is generally horizontally disposed.