The cause of the abnormal mobility of the patella varies in different cases; in some there is congenital laxity of the ligaments, in others a faulty formation of the lower end of the femur. Bade has observed families in which several children were affected, and although there was nothing abnormal in the shape of the bones, the knee was slender and delicately formed.
The use of a strong knee-cap may prevent falling, but as a rule an operation is required, and there is quite a number to choose from, the principle of them all being to prevent displacement of the bone without unduly restricting flexion of the joint. That devised by Goldthwait consists in exposing, by means of a vertical incision, the whole length of the patellar ligament, splitting it longitudinally, separating the lateral half from the tibia, passing it under the medial portion and suturing it to the periosteum; this gives the quadriceps a straight line of pull. We have achieved the same result by dividing the lax capsule and synovial membrane on the medial side of the patella, and overlapping the edges with a double line of catgut sutures.
Lateral dislocation of the patella is met with in extreme forms of _knock-knee_, and after correction of this deformity by osteotomy, and its possible occurrence should be guarded against at the time of the operation.
#Genu Recurvatum.#--In this deformity the knee is hyper-extended, the thigh and leg forming an angle which is open forwards; the attitude may be permanent or may only appear on walking. It is an extremely disabling and unsightly deformity.
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.