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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
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.
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.
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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.
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.
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Outside of the articular capsule are ligaments called extracapsular ligaments. They function to hold the bones in place, provide stability, and prevent dislocation injuries. For the knee, the extracapsular ligaments would connect the femur and the tibia.
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.
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.
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!
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.
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When you are done, you should be able to:
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