An optical tracking system comprises a reference marker unit a sticker marker unit a shape measurement unit a tracking sensor unit and a processing unit. The reference marker unit is fixed relatively to a first part of a patient. The sticker marker unit is attached in the form of a sticker on a second part which is in a position that enables rigid registration with respect to the first part. The shape measurement unit measures the three dimensional shape of the first part and the second part. The tracking sensor unit senses the reference marker unit and the shape measurement unit. The processing unit obtains a first coordinate conversion of the reference marker unit the tracking sensor unit the shape measurement unit and the first part of the patient and a second coordinate conversion of the reference marker unit the tracking sensor unit the shape measurement unit and the second part of the patient extracts a third coordinate conversion of the first part and the second part and thus tracks the first part.
