Aging wear and tear of the knee joint, also known as osteoarthritis, generally begins in the patella. The first manifestation of aging patellar cartilage is patellar tenderness, and the earliest manifestation of the patient is pain in the front of the knee, or pain behind the patella or around the patella, with pain in going down stairs at the earliest, and then pain in going up stairs, and then in severe cases, when they are afraid to squat, and when they are not careful when walking, they hit their weak legs or even fall down accidentally. Some people also experience medial knee pain first, especially those with mild internal knee valgus (O-leg). Excessive medial pressure causes wear and tear on the medial knee condyle, and the more wear and tear the more pronation forms a vicious cycle. When the patellar cartilage and medial knee condyle are worn out in about ten years, it is recommended that the patient have a total knee replacement according to the current consensus of our joint surgeons. However, many patients do not receive a total knee replacement, or only have patellar wear or wear on the medial knee condyle, so what should be done? For those with small cartilage defects, cartilage grafts can be done, and for large areas, microfractures (allowing bone marrow stem cells to form repair tissue) can be done. If both sides of the cartilage that rubs against each other are worn away, cartilage replacement of the talus and patella can be done in the patella, and unicondylar replacement can be done in the medial and lateral condyles. This preserves the normal structures of the knee joint, such as the cruciate ligament and the meniscus on the non-operated side, to the greatest extent possible. In layman’s terms, this is like replacing a few dentures versus a full denture, where a full denture is not as good as a small one. Because unicondylar replacements are more natural and work better for the patient, some doctors are now doing two unicondylar replacements for patients who already need a total knee replacement (to preserve the cruciate ligament and make the knee more stable). So, for those patients who don’t want to do a total knee replacement, I would do a glide replacement or a unicondylar replacement. This type of partial articular surface replacement is less invasive, has a shorter operative time, and has a faster recovery and better function.