There are several varieties. In the _congenital form_, which is apparently due to a faulty attitude of the lower extremities _in utero_, the patella may be imperfectly developed or absent; the knee is convex backwards, and attempts to flex the joint cause pain. Other deformities frequently coexist. The treatment consists in flexing the joint to a right angle under an anæsthetic, and maintaining this attitude by means of plaster-of-Paris or splints until the growth of parts overcomes any tendency to relapse. _Acquired Forms._--The most common acquired form is the result of anterior poliomyelitis, and is described in the next section. The deformity may also be due to rickets which has caused a backward bend of the tibia immediately below its upper epiphysis--sometimes combined with an exaggerated forward curve of the femur. If there is no prospect of spontaneous rectification, the upper end of the tibia should be divided with the osteotome, and the limb straightened. It may result also from fracture or from separation of one of the epiphyses in the region of the knee, or from cicatricial contraction of the quadriceps. As a result of bone and joint disease, it is met with chiefly in neuro-arthropathies when the knee has become disorganised and flail-like. #Deformities of the Knee resulting from Anterior Poliomyelitis and from Spastic Paralysis.#--When there is paralysis of all the muscles acting on the knee, the joint may be so flail-like that the patient is unable to stand without the aid of a crutch, or when weight is put on the limb, it assumes the attitude of genu recurvatum. The usefulness of the limb may be improved by the application of a rigid apparatus with a lock at the joint so that it can be used in the extended position for walking or in the flexed position for sitting. The rigid knee produced by arthrodesis affords good support but is inconvenient in sitting. When the _quadriceps alone_ is paralysed, the patient is obliged to maintain the joint in the position of extreme extension, because the least degree of flexion results in the limb giving way under him. In course of time the posterior ligament is stretched, and the joint becomes hyper-extended, acquiring the attitude of _genu recurvatum_. When it is bilateral the gait is seriously impaired. The treatment consists in applying an apparatus which prevents hyper-extension, in improving the condition of the thigh muscles, and in wearing a splint at night which secures the flexed position. Recourse may be had to operative measures, such as transplanting one of the hamstrings into the patella, so as to compensate for the loss of power in the quadriceps, arthrodesis, or supra-condylar osteotomy of the femur.