On-board tool tracking system and methods of computer assisted surgery

A method for computer assisted surgery (CAS) using a freehand surgical tool, the method comprising:

• creating a three dimensional representation of a portion of a patient to which a bone or tissue cutting procedure is to be performed;
• identifying an area of the three dimensional representation corresponding to the portion of bone or tissue for which the procedure is to be performed using an active element of the freehand surgical tool;
• creating a surgical plan for the area of the three dimensional representation corresponding to the portion of bone or tissue;
• determining a position of the portion of bone or tissue for which the procedure is to be performed;
• determining a position of the freehand surgical tool;
• calculating a distance between the position of the portion of bone or tissue and the position of the hand held surgical tool;
• setting a mode of the hand held surgical tool to a normal tracking mode if the distance between the portion of bone or tissue and the hand held surgical tool is greater than a first threshold distance;
• setting the mode of the hand held surgical tool to an enhanced tracking mode if the distance between the portion of bone or tissue and the hand held surgical tool is less than the first threshold distance and is greater than a second threshold distance;
• and
• setting the mode of the hand held surgical tool to a cutting mode if the distance between the portion of bone or tissue and the hand held surgical tool is less than the second threshold distance.