The knee is a large hinge joint between the femur and tibia, with the patella gliding in front. Because its bones fit loosely, it depends on ligaments and menisci for stability.
Bones and menisci
The rounded femoral condyles rest on the flat tibial plateau, cushioned by two C-shaped menisci, medial and lateral. The menisci absorb shock and can tear when the knee twists with the foot planted.
Cruciate ligaments
The anterior cruciate ligament (ACL) stops the tibia sliding forward and the posterior cruciate ligament (PCL) stops it sliding back. They cross each other inside the joint, which is why they are called cruciate.
Collateral ligaments
The medial collateral ligament (MCL) resists a force from the outside that pushes the knee inward (valgus). The lateral collateral ligament (LCL) resists the opposite force (varus).
Patella and extensor mechanism
The patella sits inside the quadriceps tendon and improves the leverage of the quadriceps. The patellar tendon continues from its lower edge to the tibial tuberosity.
| Ligament | Prevents | Bedside test |
|---|---|---|
| ACL | Tibia sliding forward | Lachman, anterior drawer |
| PCL | Tibia sliding back | Posterior drawer |
| MCL | Knee bending inward (valgus) | Valgus stress test |
| LCL | Knee bending outward (varus) | Varus stress test |
At the bedside
A pop, rapid swelling and giving way after a twisting injury suggest an ACL tear with bleeding into the joint. Compare each stress test with the uninjured knee to judge laxity.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).