This surgical apparatus comprises, on the one hand, means for spatial marking-out of the patella and femur of a patient to be operated on, and, on the other hand, means for simultaneous and relative representation of this patella provided virtually with a patellar implant of the prosthesis, and of this femur provided virtually with a femoral implant of the prosthesis, in a transverse plane passing both through the patella and through the trochlea of the aforementioned femoral implant. Using this apparatus, the surgeon can easily and quickly check the congruent position of the patellar implant with respect to the prosthetic femoral trochlea in order to reduce the risks of subsequent dislocations of the patella fitted with the prosthesis.