Rotatory instability of the knee permitting abnormal external rotation
of the tibia on the femur is the result of forced abduction of the
flexed knee and external rotation of the tibia. The basic lesion
present in all patients is a tear of the medial capsular ligament.
Occasionally, this may be present as an isolated lesion but, more
commonly, there are associated lesions of the tibial collateral
and anterior cruciate ligaments. A test for rotatory instability
of the knee, based on the modification of the test for the anterior
drawer sign, has been presented. The essential feature of this test
is the demonstration of a lateral shift of the longitudinal axis
of rotation in the horizontal plane following rupture of the medial
capsular ligament. It is stressed that rupture of the anterior cruciate
ligament need not be present when the test is positive for rotatory instability,
although it is most markedly positive when the anterior cruciate
ligament is torn.