Unicondylar knee replacements are non-constrained knee replacements. The motion of the knee joint is multifaceted, with flexion and extension in the sagittal plane, as well as adduction and abduction in the coronal plane and rotation in the transverse plane. The axis of rotation for flexion and extension is not fixed, but varies along a “J” shaped trajectory on the femoral condyle. Therefore, these physiological knee movements should also be accomplished after an ideal prosthetic knee replacement. The unicondylar and unicompartmental prostheses, Marmor’s modular prosthesis, and Gunston’s multicentric prosthesis are all non-constrained artificial knee joints, whose constraint is almost entirely dependent on friction. These prostheses can be used in cases with relatively limited articular surface destruction, destruction of only one side (medial or lateral) of the femoral or tibial articular surface, instability due to ligamentous laxity, and minimal deformity. Of course, there may be other options for this type of lesion, such as fresh allograft osteochondral grafts. Indications for unicondylar knee arthroplasty: – unicondylar defect; – intact ligaments (stable knee); – axial deviation: max 10° – extension retardation < 5° - maximum weight not exceeding 90 kg As shown in the figure: diagram of sled unicondylar knee arthroplasty in operation (this is a standard incision, a minimally invasive incision is also available) sled unicondylar knee arthroplasty trial and anterior and posterior X-rays RAY film Before unicondylar knee arthroplasty, if the indications are selected appropriately and the surgeon operates well, the prosthesis is generally used for 7-10 years after surgery, and there are reports of 18 years of recorded prosthesis preservation with good joint space maintenance. The diverse morphology of the metal lamellar replacement can meet the high demand, high motion, and rapid recovery of young patients, pictured here with a cartilage replacement. Currently, there are new improvements in the technique of unicondylar replacement, so its indications have changed. If necessary, unicondylar replacement can also be done simultaneously with corrective surgery of the force line, such as osteotomy orthopedics. In some cases, the indications may be relaxed, depending on the physician's understanding and the instruments at hand and the technical platform of the medical unit. For example, the picture above shows a unicondylar and unilateral replacement, which is smaller and simpler than the traditional unicondylar replacement, but with a narrower and stricter indication (the name is Hemi Cap of the femoral condyle, according to the latest literature reviewed by Shu Chai Xu), and to some extent it can be called a cartilage replacement. In fact, for osteochondral defects such as these, plastic rubber pads/allograft osteochondral replacement has been used with some success. However, most knee unicondylar prosthesis replacements are mostly transitional procedures, hopefully delaying total knee replacement with revision.