Utilizing a new testing method, we are reporting for the first time on the ranked order of importance of each ligament and capsular structure in resisting the clinical anterior and posterior drawer tests. The ranked importance is based on the force provided by each ligament in resisting the drawer tests. Tests were conducted with the knee at 90 and 30 degrees of fiexion up to five millimeters of anterior and posterior drawer in the intact knee. A second series of tests was also performed at larger drawer displacements to determine the back-up restraints to drawer motions when the cruciate ligaments have been cut.

CLINICAL RELEVANCE:​ In this report we introduce the concept of primary and secondary ligament restraints to the clinical examination. In anterior drawer the antenor cruciate ligament is the primary restraint. It provides an average of 86 per cent of the total resisting force. All other ligaments and capsular structures provide the remaining secondary restraint, each typically less than 3 per cent. These results explain many clinical paradoxes of function of the anterior cruciate ligament. The secondary restraints, although small, may block the clinical drawer test despite rupture of the anterior cruciate hgament. This is because the clinical drawer test is performed with small manual forces. However, with loss of the primary restraint (the anterior cruciate higament), anterior stability under higher forces of functional activity is markedly affected, although un- derestimated by the clinical drawer test. Over time, the weak secondary restraints stretch and the laxity in- creases. In the absence of the anterior cruciate ligament, the restraints to anterior drawer are the ihiotibial tract and band, the middle one-third of the medial and lateral capsules, and the medial and lateral collateral ligaments. These are the structures that are tested in clinical cases of chronic laxity of the anterior cruciate ligament, assuming that they undergo no concomitant injury.

For straight posterior drawer, the posterior cruciate ligament provides a mean of 95 per cent of the total restraining force. After loss of the posterior cruciate ligament, the secondary restraints to the postenor drawer test are the posterior lateral capsule and pophiteus complex combined (58 per cent), the medial collateral ligament ( 16 per cent), and to a lesser extent many other structures. The anterior cruciate ligament did not resist posterior drawer, nor did the posterior cruciate ligament resist anterior drawer. A false positive anterior drawer test after rupture of the postenor cruciate ligament occurs due to a posterior shift in the starting position of the tibia.

We concluded that:​ (1) Knee stability and proposed surgical procedures must be analyzed in terms of all ligament restraints. However, special attention must be given to the primary restraints that cannot be substituted for by only the secondary ones. (2) Secondary restraints may block clinical laxity tests, but often stretch out and do not provide knee stability under higher functional forces of activity. (3) The anterior and posterior cruciate ligaments provide the over- whelming resistance to these respective tibial displacements. After the cruciate ligaments are torn, minimum back-up ligament support exists. This explains the high risk of altered joint function after cruciate injury, when functional stability is dependent on muscle re- straints and joint geometry.