The present invention relates to a medical instrumentation for implanting an acetabular cup, comprising at least one medical guiding instrument for a first reaming tool, wherein the guiding instrument comprises a shaped body, wherein the shaped body has a patient-specific bone abutment surface facing in the distal direction and deviating from a spherical surface section, wherein the bone abutment surface is shaped so as to correspond or substantially correspond to a bone surface of the patient, and wherein a guiding shaft, defining a longitudinal axis, for guiding the first reaming tool protrudes from the shaped body and faces in the proximal direction.