In patients with inversion of the knee combined with arthritis, the main cause of knee pain is excessive pressure in the medial compartment. By correcting the deformity, the body’s weight is passed over the outer side of the knee with normal cartilage, relieving the pressure on the worn out medial cartilage, thus reducing knee pain and restoring knee function. Two things need to be understood.
Firstly, the pain is caused by medial arthritis. The patient’s pain is mainly associated with weight bearing, which is aggravated by walking, and this weight bearing is mainly in the medial compartment. A significant reduction in knee pain in the non-weight bearing state suggests that decompression of the medial compartment of the knee will relieve the symptoms. It is important to note to patients in particular that medial knee pain in this group of patients is not directly related to meniscal damage, synovitis, effusion, osteophytes, etc. as commonly understood by patients.
Secondly, medial arthritis is caused by an inversion deformity of the knee joint, also known as rotundity. The knee joint is divided into inner and outer sides. A normal leg is straight, then the forces are evenly distributed between the inner and outer sides. When an inversion deformity of the knee occurs, it is mainly the medial side that wears down when walking. As a result, the cartilage on the inside wears thinner and thinner, until the bone wears the bone. This is like wearing shoes. When the foot is out of alignment, the shoe always wears on one side. The inversion of the knee leads to increased stress on the medial compartment, increased wear on the medial side and increased inversion of the knee. Osteotomy correction surgery will break this vicious circle by correcting the lines of force, shifting most of the stress to the lateral side and reducing the medial stress, resulting in significant pain relief. When the line of force is shifted from 0% to 62% of the target line of force on the medial side of the knee, the lateral interval pressure is 70% higher than the medial side. With correction the medial pressure is reduced, the stress is shifted to the lateral side and the pain disappears.
The surgery then is to correct the inversion of the knee by making the leg straight and allowing the lateral side of the knee to bear the weight so that walking is painless. Just as with shoes, this side wears out and is replaced by wearing the other side so that the joint will last for many more years.