Patient specific instrumentation (PSI) for orthopedic surgery

The patient specific instrument (PSI) assembly for guiding a surgical tool on a tibia during a knee replacement surgery includes an extra-medullary portion at a distal end, an elongated rod connected at a distal end to the extra-medullary portion and extending in a direction along a mechanical axis of the tibia, and an upper mounting portion at a proximal end. The extra-medullary portion has a pair of spaced apart malleoli clamping elements with a size and/or relative position that is patient-specific for engaging respective ones of the lateral and medial malleoli. The upper mounting portion including a base body connected to a proximal end of the elongated rod and a surgical guide having guide openings extending therethrough and anchor element(s) on the upper mounting portion in locations adapted to overlay a peripheral contour of the tibia. The peripheral contour of the tibia including an anterior surface of the proximal tibia.