Stress radiographical measurement of the anteroposterior, medial and lateral stability of the knee joint.

A hydraulically operated machine for applying well-defined forces to the knee joint is described. The measurement of anteroposterior as well as lateral and medial stability in the knee joint is based upon roentgenograms exposed while the forces are being exerted on the patient. The advantage of the radiological measuring method over methods using external measurements is the elimination of inaccuracies due to displacements of the soft tissues of the limb. The accuracy was evaluated by test/retest examination of 50 healthy subjects. The radiographs were measured at random after cessation of examinations in the gonylaxometer. The accuracy is +/- 1.2 nm for medial/lateral, +/- 2.4 nm for drawer sign. Diagnostically the difference between the two knees must exceed 2.0 nm (collateral instability) or 3.1 nm (drawer) to exceed standard values. The standard values for the different ligament laxities are given. The applied force to be used for evaluation of medial/lateral stability is recommended to be 9 kg, and for anteroposterior stability between 20 and 30 kg.