THE ANTERIOR CRUCIATE LIGAMENT
The ACL has its origin on the lateral femoral condyle, 20 mm in its greatest length, 10 mm in greatest width. The insertion on the tibial plateau is 10 mm wide and 30 mm long, being slightly medial to the sagittal midline and just anterior to the coronal midline between the tibial eminences. It consists of two major bundles, the anteromedial and the posterolateral (describing the femoral to tibial attachments). The posterolateral bundle is stronger, but the anteromedial is perhaps more ï¿½isometricï¿½. The anteromedial bundle is tight in flexion, while the posterolateral bundle is tight in extension (although in reality, the ligament is a bit of a continuum, with some fibers always being tight during some part of the arc). The blood supply is through the middle geniculate artery.
The ACL, MCL, and PCL are divided into functional bundles:
ACL: Anteromedial / Posterolateral bands
PCL: Anterolateral / Posteromedial bands
MCL: Superficial (tibial collateral ligament) – can be divided into anterior and posterior bundles
Deep (middle capsular ligament) – can be divided into meniscotibial and meniscofemoral
For both anterior and posterior cruciate ligaments, the posterior portion is tight in extension, the anterior portion is tight in flexion.
The anterior superficial MCL tightens with flexion of 70-105o.
In general, the PCL contributes to stability with the knee in FLEXION. It is for this reason that the majority of patients do not feel unstable when they are running and pivoting, unlike the ACL deficient knees where the instability is manifested with the knee in EXTENSION.
In general, the ACL is the primary restraint to anterior tibial translation at all angles of knee flexion, contributing between 80 to 85% of total resistance to this motion. It contributes the most at 30o of knee flexion. The ACL also provides primary resistanceto medial displacement of the tibia at full extension and at 30o of flexion.