THE POSTERIOR COMPARTMENT
The Posterior Cruciate Ligament
The posterior cruciate ligament originates on the medial femoral condyle. The origin is oval shaped, lying more vertical with the knee at 90o and horizontal in full extension. It spirals distally and becomes a flatter structure that inserts into the back of the tibia over an area about 13mm in diameter, approximately 10 mm distal to the tibial articular surface. The intra-articular length is approximately 32-38 mm in length. The ligament is covered in synovium and is intimately associated with the posterior capsule.
The PCL has two main components: the anterolateral component (femoral – tibial insertion) which is two times larger in cross sectional area, and has 150% stiffness and ultimate strength of the posteromedial component.
The anterolateral and posteromedial bands have different tensioning patterns depending on knee flexion:
the anterolateral band is increasingly tight with flexion
the posteromedial band is increasingly tight with extension
The Meniscofemoral Ligaments of Humphrey and Wrisberg
Humphrey’s ligament – anterior meniscofemoral ligament
Wrisberg’s ligament – posterior meniscofemoral ligament
These ligaments have significant stiffness and ultimate strength – in some cases is greater than that of the posteromedial component. However, their role in knee stability is poorly defined. One of the two is present in approximately 79% of knees. Rarely are they both present.
The Posterior extension.
The anterior and posterior capsule resists 25%
Sectioning of the MCL increases valgus instability markedly. Isolated sectioning of the ACL, PCL, LCL, or posterolateral structures does not cause large increases in valgus angulation
The largest increase in valgus laxity occurs with combined sectioning of the MCL and PCL
The only major restraints are the MCL, followed by the ACL. Internal rotation laxity is maximal around 20-40o of knee flexion.
Loss of the ACL in combination with the LCL or posterolateral structures also results in increased rotatory laxity
The PCL does not have much of a role in internal rotation stability
The LCL and posterolateral capsule are the primary restraints to external rotation. External rotation laxity is maximal at 30-40o of knee flexion.
Only markedly increased when the posterolateral capsule, LCL or both are cut. Concomitant loss of the PCL and results in greater increase in rotation laxity, but the PCL sectioned alone does not contribute much to rotatory instability.
THE ROLE OF SPECIFIC LIGAMENTS
primary restraint to anterior tibial translation, especially at 30o of knee flexion.
secondary restraint to valgus rotation
secondary restraint to both internal and external rotation (more internal).
primary restraint to posterior translation, especially at 90o of knee flexion.
secondary restraint to external tibial rotation.
secondary restraint to valgus rotation.
primary restraint to valgus rotation.
primary restraint to internal tibial rotation.
secondary restraint to anterior tibial translation.
primary restraint to varus rotation.
primary restraint to external tibial rotation.
Normal mechanical axis
– center of hip, through the central third of the knee, through the center of the ankle – should all be a straight line. This mechanical axis is in fact in 3o of varus.
– Why do we cut the femur at 7o of valgus?
– Why do we externally rotate the femoral component?
Normal anatomic axis – the distal femur is in 9o of valgus, the proximal tibia in about 3o of varus. This amounts to a joint in about 6o of anatomic valgus.