Abstract: With the invention a modular instrument system for knee joint prostheses is shown. A shaft (1), which can be provisorily but rotationally fixedly anchored along the anatomical axis (4) of the distal femur bone (2), terminates in the direction towards the joint with a collar (6). A parallel guide (7) is provided in the direction of a mechanical axis (9) from the collar (6) towards the interior of the shaft (1), which stands off by a guide angle a towards the medial to the shaft axis, which coincides with the anatomical axis (4). A coupling piece (10) with a projecting guide part (8) which is displaceably journalled in the parallel guide (7) serves as a reception for manipulation condyles (23) in order to test and determine the ideal position with respect to a tibia platform in the end positions of the articulation before resection blocks are positioned with the fixed coupling piece (10) for the final resection. The collar (6) is particularly advantageous in re-operations as a reference abutment (30).

Abstract: The invention is shown by a tibia platform comprising a metallic lower part and different bearing parts made of plastic which can be anchored to one another at a posterior position via an open hinge joint. The bearing part has a guide rib for guiding femur condyles which projects upwards by a height of at least 15 mm above the lowest point of the sliding surfaces. Large lateral forces such as occur in high guide ribs and in varus and valgus positions of the knee can be taken up because of a separate metallic pawls which engages at a horizontal projection of the lower part at a distance "1" away from the hinge joint. The pawl is displaceably journalled with respect to the bearing part and is permanently positionable by means of auxiliary means by an enforced movement into a latched position at the projection.