The invention relates to a medical instrumentation with a navigation system which comprises an optical detection unit comprising a camera, and a data processing unit coupled to the detection unit, with a medical article and with a marking device which is held directly or indirectly on the article and is positioned in a defined spatial arrangement in relation thereto, or which marking device is comprised by or formed by the article, the location and orientation of the marking device being determinable with the navigation system, it being possible for at least one image of the marking device and the article to be taken by means of the detection unit and for the position and shape of the article to be determined by the data processing unit on the basis of an image. The invention also relates to a method for determining the shape of a medical article.
