Medical apparatus for tracking movement of a bone fragment in a displayed image

A medical apparatus comprising:

• a data processor in which an image dataset of a body of a subject is stored, said subject containing a bone fragment and said bone fragment being represented in said image dataset;
• a display connected to said data processor for displaying an image, obtained from said image dataset, said image containing a visualization of said bone fragment;
• an image computer connected to said data processor for segmenting said visualization of said bone fragment in said image on said display;
• a medical instrument adapted for introduction into the body of the subject and adapted to interact with said bone fragment in the body of the subject to move said bone fragment;
• a navigation system which determines a position of the medical instrument relative to the body of the subject while said bone fragment is being moved with said medical instrument, and generating medical instrument position data representing said position;
• and
• said data processor being connected to said navigation system and being supplied with said medical instrument position data and determining, from said medical instrument position data, a modified position of the bone fragment relative to the body of the subject, and causing a segmented visualization of said bone fragment to be displayed in said image on said display in conformity with said modified position.