Ligaments

For Young Learners

Structure and Location

Ligaments are tough cords that join bone to bone. The knee has four main ones.

Two sit inside the joint and cross over each other, which is why they are called cruciate, from the Latin for cross. The front one is the anterior cruciate ligament, or ACL. The back one is the posterior cruciate ligament, or PCL.

Two sit on the sides. The medial collateral ligament runs down the inner side. The lateral collateral ligament runs down the outer side.

Function

Think of the knee as a tent. The poles are the bones. The guy ropes are the ligaments, and without them the tent falls over.

The two crossed ligaments inside stop the shin sliding forwards and backwards under the thigh. The ACL stops it sliding forwards, the PCL stops it sliding backwards.

The two at the sides stop the knee bending sideways. Together, all four allow bending and straightening while blocking the movements the knee is not meant to make.

For Patients

Structure and Location

The anterior cruciate ligament runs from the back of the outer femoral condyle, forwards and inwards, to the front of the tibia. The posterior cruciate ligament runs in roughly the opposite direction and is the thicker and stronger of the two.

The medial collateral ligament is a broad band on the inner side, attached to the joint capsule and to the inner meniscus. The lateral collateral ligament is a distinct cord on the outer side, running to the head of the fibula, and is separate from the capsule.

Function

Ligaments are passive restraints. They do not contract. They simply resist movements beyond a certain point, like a seatbelt that is loose until you need it.

Each has a primary job. The ACL resists the shin sliding forwards and limits rotation. The PCL resists the shin sliding backwards. The medial ligament resists the knee being pushed inwards; the lateral resists it being pushed outwards.

They also carry sensors that tell your brain where your knee is in space. Losing a ligament loses some of that sense, which is part of why a knee can feel unreliable even when it is not visibly unstable.

Clinical Relevance

Ligaments have a poor blood supply compared with muscle, and the two inside the joint are bathed in joint fluid, which hinders healing further. The collateral ligaments on the outside heal considerably better than the cruciates inside.

This difference explains much of how knee injuries are managed. A torn medial collateral ligament often heals with time and rehabilitation. A fully torn ACL usually does not heal back to its original strength.

The detail of individual ligament injuries, and the operations used for them, belongs to the conditions and surgery sections.

For Medical Students

Structure and Location

Four ligaments dominate. The anterior and posterior cruciate ligaments lie in the intercondylar region and cross one another. They are intracapsular but extrasynovial, which is to say they sit inside the joint capsule and outside the fluid filled space.

The medial collateral ligament is a broad structure blended with the capsule on the inner side. The lateral collateral ligament is a discrete cord on the outer side and, unlike its medial counterpart, is separate from the capsule.

Precise attachments, bundle anatomy and the named vessels supplying each ligament belong to the Anatomy Academy.

Function

The cruciates are the primary restraints to anteroposterior tibial translation, the anterior cruciate resisting forward translation and the posterior cruciate resisting backward translation. Both contribute secondarily to rotational control. The collaterals are the primary restraints to valgus and varus angulation respectively.

Each ligament contains fibres that tighten at different flexion angles, so some portion remains under tension through the arc rather than the whole structure loading at once.

Ligaments also carry mechanoreceptors, providing proprioceptive feedback about joint position.

Clinical Relevance

The intra-articular position of the cruciates, bathed in synovial fluid, is associated with poor intrinsic healing. The collaterals, particularly the medial, heal considerably better, and this difference shapes management more than any other single fact about these structures.

Loss of proprioceptive input contributes to the functional deficit after rupture, independently of measured laxity, which is why rehabilitation addresses neuromuscular control rather than strength alone.

Ligament reconstruction is covered in the surgery section.