A medical apparatus has a data processing device for storing an image dataset of the body of a subject and a navigation system for determining the position of a medical instrument relative to the body of the subject. At least one bone fragment is shown in the image represented by the image dataset, the image of the one bone fragment being segmented with a computer. The bone fragment is to be grasped and set with the medical instrument. The data processing device also determines the modified position of the bone fragment on the basis of the position of the medical instrument relative to the body of the subject determined with the navigation system, and displays the modified position in the image.