REIDER PART 271

Chaptek 6 Knee 231

Figurę 6-49. Lachmans test. A, Standard position (arrowshows direction of Córce applied to the tibia). B, Alternative tech-nique with the thigh supported by the exam table.

should feel Iike a dead weight. The fingers of the exam-iner’s upper hand, which are supporting the thigh, are also able to sense any tightening in the hamstrings. If any of the hamstrings are felt to be tight, identifying the tight muscle to the patient and massaging it a bit often allows the patient to relax.

As with the valgus stress test, better rclaxation may sometimes be obtaincd by abducting the lower limb and allowing the thigh to rest on the examination table (see Fig. 6-49B). In this case, the knee is flexed over the side of the table and the foot rests on the lap of the seated exam-iner. Oncc the patient is adeąuately relaxed, the examiner pulls forward on the tibia with one hand while simulta-neously pushing backward on the femur with the other hand in a reciprocating manncr. As in the antcrior drawcr test, the amount of anterior excursion and the quality of the endpoint are assessed.

One of the diffcrcnces that makes the Lachman test casier to assess than the anterior drawer test is that in most normal patients, there is little or no excursion of the tibia when the Lachman test is performed. Eithcr no translation at all or 1 mm to 2 mm of translation with a very firm endpoint is appreciated. In the presence of ACL tear, the translation is increased and the endpoint indef-inite. Often, this increased translation can be visibly appreciated by focusing on the forward movement of the tibia at the tubercle of Gerdy or over the medial tibial plateau. In subtly abnormal cases, the examiner may not be surę that increased excursion is prcsent, but he or she may sense a soft end point that differs from the unin-jured knee.

A special case is the iticomplete ACL injury, in which the ACL is elongated but not totally disrupted. In these cases, increased anterior laxity is present but a firm endpoint is still noted. This endpoint is usually casier to apprcciate after the swclling and stiffness of the acute injury havc subsided.

Theoretically, the Lachman test is morę specific for injury to the posterior portion of the ACL, and the anterior drawer test is morę specific for injury to the anterior fibers. This distinction would only be important, however, in the relatively unusual case in which some of the ACL fibers are tom but the others are relatively intact.

Pivot Shift Phenomenon

The pivot shift test and its variations are dynamie tests that demonstrate the subluxation that can occur when the ACL is nonfunctional. Thus, they are indirect tests of ACL injury. An abnormal result usually indicates complcte rup-ture of the ACL but may also be present when the ligament is cxcessively elongated but still in continuity. In addition, some patients with greater than average anterior knee lax-ity may demonstrate a mild physiologic pivot shift in the absence of any knee injury. Many patients whose knees hyperextend can be expected to demonstrate this physiologic pivot shift. In laboratory studies, sectioning the ACL has been shown to be sufficient to produce a positive pivot shift, although sectioning the lateral ligament complex usually inereases the magnitude of the pivot shift.

Classic Pivot Shift Test. The classic pivot shift test was described by Galway and Mclntosh. With the patient supine and relaxed, the examiner lifts the lower limb off the table by the foot and internally rotates it (Fig. 6-50A).

(Although the classic description of the pivot shift test is performed with internal rotation of the lower limb, some researchers recommend a neutral or even externally rotated position.)

If the patient is properly relaxed, the limb feels like a dead weight and the knee naturally falls into fuli exten-sion. This may be a straight position or hypcrextension, depending on what is normal for that particular patient. If the knee does not fali into fuli extension, whether due to pain, swelling, or a displaced meniscus tear, the pivot shift test may not be accurate.