Partial Rupture of the Anterior Cruciate Ligament

The anterior cruciate ligament (ACL) consists of two major fiber bundles, namely the anteromedial (AM) and posterolateral (PL) bundles. Although disagreement exists among arthroscopic surgeons about the occurrence of isolated ruptures of the AM or PL bundle, there are reports of partial ruptures of the ACL in the literature. A potential reason for controversy could be that with conventional magnetic resonance imaging, isolated PL ruptures are difficult to diagnose because of the oblique course of this bundle. Another reason could be that isolated ruptures of the AM or PL bundle are difficult to diagnose during arthroscopy. During arthroscopy, an isolated PL bundle rupture can easily be missed when viewing from the standard anterolateral portal. The AM bundle overlies the PL bundle, and the PL bundle can only be seen by retraction of the AM bundle with a probe. When the knee is extended, the PL bundle is tight and the AM bundle is moderately lax. As the knee is flexed, the femoral attachment of the ACL becomes horizontally oriented, causing the AM bundle to tighten and the PL bundle to relax. Whereas the AM bundle is the primary restraint against anterior tibial translation in flexion, the PL bundle tends to stabilize the knee near full extension, particularly against rotatory loads. The different bundle contributions to knee stability in the flexed or extended positions can aid in the diagnosis of partial ACL ruptures. Isolated rupture of the AM bundle has more effect on the anterior drawer sign than on the Lachman test, whereas the converse is true for isolated rupture of the PL bundle. Rotational instability as a result of PL bundle rupture can be tested with the pivot-shift test. Pivot shift can be negative in cases with isolated AM bundle rupture. If only one bundle of the ACL is torn, isolated AM or PL bundle reconstruction should be considered. Although potentially difficult, a careful diagnostic evaluation is necessary before ACL surgery.