Artificial knee replacements can resolve the vast majority of end-stage joint pathologies and have been a boon to many patients. The knee is anatomically composed of three compartments: the medial compartment, the lateral compartment, and the patellofemoral compartment, which together form the articular surface required for knee motion, and total knee replacement can address all three compartments. However, not all knees requiring surgical treatment have lesions in all three compartments, and studies have shown that approximately one-third of knee lesions are confined to one compartment, with no significant lesions in the other two compartments. So, in this case, is total knee replacement necessary? Will replacement of only the diseased compartment solve the problem? It is like a car, one wheel is broken, so do we need to replace the whole car? Can we solve the problem by replacing just one wheel? A unicondylar replacement, where only the diseased part is replaced, is like replacing only the wheel, not the whole car. Because patients who need joint surgery are often elderly and many also have a combination of many chronic diseases such as hypertension and diabetes, the impact of the surgical blow on the patient and post-operative rehabilitation is a concern for many patients. Total knee arthroplasty is a very traumatic surgery with a lot of bleeding, which poses a significant challenge for the patient’s post-operative recovery. The unicondylar replacement has significant advantages in terms of surgical trauma and bleeding due to the small extent of the surgery, and the surgical incision is significantly smaller than that of a total knee arthroplasty. In addition, unicondylar replacement preserves as much of the joint’s own structure and intra-articular ligaments as possible, and has certain functional and sensory advantages. Also, unicondylar replacements are less expensive in terms of cost and long-term results, and available studies have shown that unicondylar replacements are no worse than total knee replacements in terms of longevity. However, the benefits of unicondylar replacement should not be taken for granted. If the lesion is not confined to a single compartment and other compartments are involved at the same time, the results of unicondylar replacement can be greatly reduced or even fail. This requires careful examination and preoperative testing to be rigorous and to select patients with truly unilateral interventricular lesions for surgical treatment. Overall, unicondylar replacement has the advantages of less surgical injury, less bleeding, faster recovery, better function, and lower cost, but requires strict screening of patients.