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.

This study aims to validate the procedure of patient specific cutting guides.

30 patients will be included in a mono centric, single-arm study with evaluation of the primary outcome by an independent expert comity. Each participant will have an MRI which will be sent to the society in charge of creating the patient specific cutting guides. The intervention will be performed using the patient specific cutting guides and in parallel, a station of navigation will be used to provide a safety control of the orientation of the cutting guide and to collect per operative data.

Evaluation visit is performed 6 months after surgery.

Inclusion Criteria:

• patients aged 18 or older;
• indication of medial unicompartmental knee replacement (validated by the orthopedic surgeons staff) ;
• written informed consent.

Exclusion Criteria:

• MRI contraindication ;
• Absence of the main investigator or the associated scientist during the surgical procedure ;
• Adults under legal protective regimen or deprived of liberty.