Computer-navigated surgery has become widely used in orthopedics in recent decades, thanks to its proven effectiveness in knee, hip and shoulder arthroplasty. This solution provides real-time assistance to the surgeon in the operating room, optimizing implant sizing and positioning. During the surgery, a specific module localizes the patient's anatomical structures thanks to markers located on the bone structures and the surgeon's instruments. Navigation software displays relevant clinical information to assist the surgeon. Existing solutions are mainly based on bi-ocular optical cameras that localize markers fixed to the areas of interest. The size and weight of the markers make these solutions unsuitable for extremity surgery, in particular trapeziometacarpal arthroplasty, where the size of the incision and bones is very small (of the order of a cm). In this context, the XtremLoc project will offer the first integrated solution to assist the surgeon and to accurately guide the fitting of a metacarpal prosthesis. The pathology targeted here is rhizarthrosis, that is osteoarthritis affecting the trapeziometacarpal joint at the base of the thumb. It is the most common form of osteoarthritis in the hand and has increased significantly in recent years, due to the more regular use of keyboards and smartphones. The main objective of the XtremLoc project is to develop a complete navigation system to repair these small joints. This device will guide the surgeon during the implantation of a trapeziometacarpal prosthesis and contribute to optimize its positioning, enabling the patient to regain better mobility and limit post-operative complications. The solution proposed in this project is based on three innovative aspects. The first involves the design of non-invasive three-dimensional optical localization system, having a micrometric accurate and compatible with the operating room environment. It incorporates mini optical retroreflectors (volume in the order of mm3) attached to bones and surgical instruments, to localize them using high-speed (kHz) laser beam scanning technology. This scanning is performed by MEMS mirrors that can rotate around two orthogonal axes. By exploiting the reflections of the laser beams on these retroreflectors, it is possible to localize them and the structures that carry them in real time (i.e. the patient's anatomical features and the surgeon's probing instruments), in terms of both position and orientation. The second component is a software suite orchestrating planning and guidance. It is based on automatic segmentation and modelling of bone structures from scanner images, enabling detection of patient-specific anatomical features and calculation of optimal implant positioning. A precise registration method between intraoperative information from optical sensors and planning data enables intraoperative guidance of the surgeon. The third part of the project involves the integration of a navigated surgery prototype combining the above hardware and software bricks. The physical implementation of the localization module will be brought into line with the rules governing housing watertightness, instrument sterilization and ocular safety, so that it can be integrated into the surgical workflow. Navigation tests will be carried out on the surgical platform PLaTIMed, enabling a complete surgery to be performed on anatomical specimens, and the final demonstrator to be validated in a realistic environment.