nd occurs as a result of
extreme degrees of violence, especially of a wrenching or twisting
character.

Rupture of the popliteal vessels, or pressure exerted on them by the
displaced bones, may lead to gangrene of the limb, and necessitate
amputation. The common peroneal nerve is frequently damaged. When the
lesion is compound, also, amputation may become necessary on account
of infective complications.

The varieties of dislocation are named in terms of the direction in
which the tibia passes: forward, backward, medial, and lateral.

#Dislocation forward# is the most common variety, and results from
sudden hyper-extension of the knee, tearing the collateral and
cruciate ligaments. The leg remains fully extended, and lies on a
plane anterior to that of the thigh. The condyles of the femur are
palpable posteriorly, and the skin is tightly stretched over them, or
may even be torn, rendering the dislocation compound. The patella is
projected forward, the quadriceps tendon is lax, and the skin over it
is thrown into transverse folds. The limb is shortened by two or three
inches.

#Dislocation backward# is usually due to a direct blow driving one of
the bones past the other. The leg remains hyper-extended, the head of
the tibia occupies the popliteal space, while the lower end of the
femur projects forward with the patella either in front or to one side
of it.

The #medial and lateral dislocations# are generally incomplete, and
are liable to be mistaken for separation of the lower epiphysis of the
femur. When the tibia passes _medially_, the lateral condyle of the
femur forms a prominence, and there is a depression below it. The head
of the tibia projects on the medial side, and the medial condyle is in
a depression.

When the tibia is displaced _laterally_, the relative position of the
prominences and depressions is reversed.

_Treatment._--In dislocations of the knee no special manipulations are
necessary to restore the displaced bone to its place, and reduction is
not accompanied by a distinct snap.

If, while the patient is fully anaesthetised, traction is made on the
leg and counter-traction on the thigh with the knee in the flexed
position, the bones can usually be replaced by manipulation.

After reduction has been effected, in antero-posterior dislocations,
the limb should be flexed and placed on a pillow, massage and movement
being employed from the first. The patient is usually able to walk
within a mo