Unless efficiently treated, a sprain of the knee is liable to result in weakness and instability of the joint from stretching of the ligaments, and this is often associated with effusion of fluid in the synovial cavity (_traumatic hydrops_). This is more likely to occur if the joint is repeatedly subjected to slight degrees of violence, such as are liable to occur in football or other athletic exercises--hence the name "footballer's knee" sometimes applied to the condition.
A further cause of disability, following upon sprains of the knee, is _wasting of the quadriceps muscle_. The stability of the joint, whenever the position of full extension has been departed from, is largely dependent upon its capacity of controlling the amount of flexion, notably in descending a stair or in walking on uneven ground, hence it is that with a wasted quadriceps there is increasing liability to a repetition of the sprain. With each repetition of the sprain, there is an addition to the fluid in the joint, stretching of ligaments, and further wasting of the quadriceps. A form of vicious circle is established in which there is at the same time increased liability to sprain and diminished capacity of recovering from it. Even after the repair of the damaged ligament or the removal of the mobile or torn meniscus, wasting of the quadriceps remains a source of weakness and disability and calls for treatment by massage and electricity.
_Treatment._--In recent and severe cases the patient must be confined to bed, and firm pressure applied over the joint by means of cotton wool and a bandage. This may be removed once or twice a day to admit of the joint being douched, and at the same time it should be massaged and moved to promote absorption of the effusion and prevent the formation of adhesions.
Chronic effusion into the joint is most rapidly got rid of by rest and blistering. If the patient is unable to lie up, massage should be systematically employed, and a firm elastic bandage worn. A patient who has once had a severe sprain of the knee, or who has developed the condition of "footballer's knee," must give up violent forms of exercise which expose him to further injuries, otherwise the condition is liable to be aggravated and to result in permanent impairment of the stability of the joint.