Patient-specific surgical instrumentation for preparing the knee of the patient comprising: • a patient-specific block that has two opposing faces respectively delimiting: a fixed support surface on a first bone, either the femur or the tibia of the patient, wherein the fixed support surface is specifically shaped to fit a non-resected end of the first bone and is turned towards the second bone in order to cooperate with at least one of the internal and external compartments of the end of the first bone, and a free surface, and • a spacer element, which is designed to be detachably interposed, when in use, between the second bone and the patient-specific block fixed to the first bone and which has two opposing faces respectively delimiting: a fixed support surface on a resected end of the second bone and turned towards the first bone, and a surface for cooperating with the free surface of the patient-specific block, characterised in that the free surface of the patient-specific block is curved in such a manner that this free surface and the cooperating surface of the spacer element are arranged to rest against one another in a substantially fitted manner when the ligaments of the knee are balanced while the knee moves from one flexion-extension configuration to another flexion-extension configuration. • a patient-specific block that has two opposing faces respectively delimiting: a fixed support surface on a first bone, either the femur or the tibia of the patient, wherein the fixed support surface is specifically shaped to fit a non-resected end of the first bone and is turned towards the second bone in order to cooperate with at least one of the internal and external compartments of the end of the first bone, and a free surface, and a fixed support surface on a first bone, either the femur or the tibia of the patient, wherein the fixed support surface is specifically shaped to fit a non-resected end of the first bone and is turned towards the second bone in order to cooperate with at least one of the internal and external compartments of the end of the first bone, and a free surface, and • a spacer element, which is designed to be detachably interposed, when in use, between the second bone and the patient-specific block fixed to the first bone and which has two opposing faces respectively delimiting: a fixed support surface on a resected end of the second bone and turned towards the first bone, and a surface for cooperating with the free surface of the patient-specific block, characterised in that the free surface of the patient-specific block is curved in such a manner that this free surface and the cooperating surface of the spacer element are arranged to rest against one another in a substantially fitted manner when the ligaments of the knee are balanced while the knee moves from one flexion-extension configuration to another flexion-extension configuration. a fixed support surface on a resected end of the second bone and turned towards the first bone, and a surface for cooperating with the free surface of the patient-specific block, characterised in that the free surface of the patient-specific block is curved in such a manner that this free surface and the cooperating surface of the spacer element are arranged to rest against one another in a substantially fitted manner when the ligaments of the knee are balanced while the knee moves from one flexion-extension configuration to another flexion-extension configuration.