The knee: ligaments, menisci and the popliteal fossa

1. Bones

The femoral condyles roll and glide on the flat tibial plateaus; the patella runs in the trochlear groove at the front of the femur. The fibula does not take part in the knee joint.

Surgeon’s corner: In a knee replacement you cut the distal femur at 5–7° of valgus to the anatomical axis and the tibia at 90° to its mechanical axis, so the leg ends up straight.

2. The menisci

Two crescent-shaped fibrocartilage pads deepen the plateaus. The medial meniscus is C-shaped and fixed to the capsule and the medial collateral ligament; the lateral one is almost circular and more mobile.

Surgeon’s corner: Because it is less mobile, the medial meniscus tears more often. Only the outer third has a blood supply: tears there can be repaired, inner tears are trimmed (partial meniscectomy).

3. The cruciate ligaments

Inside the notch the anterior cruciate ligament runs from the front of the tibia up and back to the lateral condyle; it stops the tibia sliding forwards. The posterior cruciate runs from the back of the tibia to the medial condyle and stops it sliding backwards.

Surgeon’s corner: Lachman test for the ACL; posterior drawer and the sag sign for the PCL. In a cruciate-retaining knee replacement the PCL is kept; in a posterior-stabilised design it is removed.

4. The collateral ligaments

The tibial (medial) collateral ligament is broad and blends with the capsule and medial meniscus; the fibular (lateral) collateral ligament is a cord running to the fibular head.

Surgeon’s corner: The medial collateral ligament heals well without surgery in most isolated injuries. In knee replacement, ligament balancing means releasing the tight side until the gaps are equal in flexion and extension.

5. The popliteal fossa

Behind the knee, from superficial to deep: tibial nerve, popliteal vein, popliteal artery lying directly on the joint capsule. The common fibular nerve follows biceps femoris to the outer side and winds around the fibular neck.

Surgeon’s corner: A knee dislocation can tear the popliteal artery: check pulses and the ankle–brachial index in every case. In knee replacement the artery is only millimetres behind the tibia when you make the tibial cut.