Unicompartmental knee replacements are performed to treat osteoarthritis or osteonecrosis of the knee especially when it affects only the medial femoro-tibial compartment. To place the prosthesis, bone removal is necessary. The orientations of the bony cuts directly influence the position of the different elements of the prosthesis and the limb alignment. It is known that good positioning of the prosthesis is the key of a good survival of the implants. To guide the blade of the saw, cutting guides are used. The position of these guides gives the final position of the cuts. At present, two main techniques are used to position the guides: the conventional one, not very costly but not very accurate, and the navigated procedure, which is more accurate but also more expensive and more invasive. Since few years, patient specific cutting guides are used in total knee replacement with encouraging results and more recently in uni compartmental knee replacement with no scientific proof of efficiency.