What Are Ligaments?

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1:00 What Ligaments Do
1:46 Structure of a Joint
2:23 Extracapsular Ligaments
2:53 Intracapsular Ligaments
3:50 Torn Ligaments
4:23 Lesson Summary

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Lesson Transcript

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Your bones are held together by ligaments. In this lesson, you will learn about the structure of ligaments and discover the very important role they play in the body.

Think about your knees and elbows for a minute. First, bend your arms and legs as far as you can. Now, straighten your limbs. Your muscles are able to freely move your legs and arms using the knee and elbow joints.

Now, bend your elbows and knees again, only this time try to bend them in the other direction. Can't do it? Don't try too hard. Your joints have evolved structures called ligaments that specifically prevent this unwanted and potentially damaging movement.

Ligaments are bundles of connective tissue that connect one bone to an adjacent bone. The basic building blocks of a ligament are collagen fibers. These fibers are very strong, flexible, and resistant to damage from pulling or compressing stresses. Collagen fibers are usually arranged in parallel bundles, which help multiply the strength of the individual fibers. The bundles of collagen are attached to the outer covering that surrounds all bones, the periosteum.

What Do Ligaments Do?

We have already stated that ligaments connect bones to other bones. It helps to imagine the bones of your leg as a chain. Your thigh bone, the femur, is one chain link. The lower leg bone, the tibia, is another link. The ligaments of your knee joint form the chain link that attaches the femur and the tibia together. Just like in a chain, the individual links can move freely, but they must remain in sequence and can't move apart. This allows for movement of the joint while preventing it from dislocating.

Bones and ligaments form a sort of chain in your leg

Ligaments can also prevent movement. Recall trying to bend your knee in the opposite direction? Your knee also contains ligaments that ensure your leg can't bend backwards. These ligaments are important for maintaining the stability of the joint while it is in motion.

The Structure of a Joint

In order to understand the types of ligaments required for each function, we need to examine the structure of a joint. Let's take a closer look at the knee joint.

Your knee joint is classified as a hinge and a synovial joint. In a synovial joint, the bones, in this case, the femur and tibia, are separated by a joint cavity filled with fluid. Synovial fluid is a thick, slippery liquid that lubricates the joint, supplies nutrients, and removes waste products. The articular capsule is two layers of connective tissue that hold the bones in place and prevent the fluid from leaking out.

The knee joint

Extracapsular Ligaments

The knee has two extracapsular ligaments. The medial collateral ligament (MCL) is located on the inside, or medial, surface of the knee. The lateral collateral ligament (LCL) runs along the outside, or lateral, knee surface.

Intracapsular Ligaments

Some ligaments are located inside the articular capsule and are referred to as intracapsular ligaments. The intracapsular ligaments of the knee still connect the femur and the tibia, and they perform two tasks. First, they help the extracapsular MCL and LCL stabilize the knee joint. Secondly, these ligaments prevent your knee from bending in the opposite direction.

There are two intracapsular ligaments, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Both lie in the middle of the capsule, together forming an X-shape, described by the term 'cruciate,' which means 'crossed.'

It's important to note that not all joints are synovial joints, so the specific orientation of ligaments varies. The functions, however, remain the same: connect bones to bones, stabilize joints, and restrict unwanted joint movement.

Torn Ligaments

Have any of your favorite athletes ever torn an ACL? Torn ligaments are common injuries for athletes. While the collagen that forms the ligaments is both strong and flexible, it does have limits. Any abnormally strong force or awkward angle can stretch or tear a ligament. You can probably guess that it is important to allow these injuries to heal completely to avoid compromising the stability of the joint in question. Try bending and straightening your knees again. That's your ligaments in action!

Lesson Summary

Let's review. Ligaments are bundles of connective tissue that connect one bone to an adjacent bone. The basic building blocks of a ligament are collagen fibers. These fibers are usually arranged in parallel bundles and attached to the outer covering that surrounds all bones, the periosteum.

In a synovial joint, such as the knee, the bones, in this case, the femur and tibia, are separated by a joint cavity filled with fluid known as synovial fluid. The articular capsule is two layers of connective tissue that hold the bones in place and prevent the fluid from leaking out.

Outside of the articular capsule are ligaments called extracapsular ligaments. They function to hold the bones in place, provide stability, and prevent dislocation injuries. In the knee, the medial collateral ligament (MCL) is located on the inside, or medial, surface of the knee, and the lateral collateral ligament (LCL) runs along the outside, or lateral, knee surface.

Intracapsular ligaments are located inside the articular capsule and include the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Both lie in the middle of the capsule, together forming an X-shape.

Functions of Ligaments

Ligaments connect one bone to an adjacent bone
They prevent damage and stress to joints through their strong fiber bundles
Ligaments prevent joints from moving in the wrong direction
They provide stability to joints

Learning Outcomes

When you are done, you should be able to:

Explain what ligaments are and describe their functions
Recall the structure of ligaments
Describe the ligament structures of the knee

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Video: What Are Ligaments? - Definition & Types

What Are Ligaments? - Definition & Types

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