Ectropion is an X-shaped leg, which is a relatively common deformity of the lower limb. To understand the deformity, it is important to first know what it looks like when it is normal. The normal human lower limb is not completely straight, but the thigh and calf are at an angle, about 7 degrees outward – that is, ectropion, when the angle deviates from the normal value because of congenital developmental deformity or arthritis and other diseases, it produces ectropion and inversion deformity. If the angle is greater than this, it is an ectropion deformity. There is a simple way to self-test, that is, when the knees come together on the inside, the two feet cannot come together; the opposite situation is inversion, when the feet come together on the inside, the knees cannot come together on the inside. Ectropion or valgus deformity can be due to internal or external causes of the knee joint, and there are different treatments for different causes. Knee valgus, commonly known as X-leg, has a variety of causes, most of which are due to congenital dysplasia. In contrast to internal derangement, most patients with internal derangement did not have internal derangement when they were younger, but had internal derangement when they developed osteoarthrosis in their older years, mostly due to wear and tear, if their medical history is followed carefully. In contrast, most patients with valgus knee have valgus in their youth or even adolescence, which is a developmental cause, and wear and tear can gradually worsen with age, and is more common in women. Some of the causes are poor healing of the femoral epicondyle fracture due to trauma, increased lateral wear due to lateral meniscectomy after years of lateral meniscus injury, and overcorrection due to previous osteotomy for valgus deformity. The percentage of knee replacements for valgus deformity is approximately 10% of patients undergoing knee arthroplasty, and this 10% is one of the more difficult knee deformities to manage in an initial replacement. In particular, moderate and severe valgus deformities with a valgus angle of more than 15° are more difficult to operate on than internal knee deformities, and the postoperative results are often inferior to those of patients with internal knee deformities. The main reasons for this are: ① the medial collateral ligament is often elongated or lax due to valgus, and the conventional medial approach tends to aggravate the laxity of the medial structures, and it is more difficult to release the lateral structures with the medial approach; ② there is varying degrees of femoral epicondylar dysplasia in knee valgus, which makes osteotomy positioning difficult; ③ in the medial approach, the joint capsule is poorly sutured and soft tissue covered after prosthesis installation, which easily leads to lateral tension and distress of blood supply; ④ (4) postoperative palsy of the common peroneal nerve is likely to occur. Therefore, there is still no uniform approach and procedure for the surgical approach, osteotomy, soft tissue release and prosthesis selection, and the surgical technique is very demanding.