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Knee Joint Anatomy, Function and Problems

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Knee Joint Anatomy, Function and Problems

Knee anatomy is about the structure of the knee – that is the parts that makeup the knee. This article also tells you how a normal knee works and provides resources for problem of the knee joint or it’s parts including knee injuries.

Our knee is the most complicated and largest joint in our body. It’s also the most vulnerable because it bears enormous weight and pressure loads while providing flexible movement. When we walk, our knees support 1.5 times our body weight; climbing stairs is about 3-4 times our body weight and squatting about 8 times.

The knee joint is a synovial joint which connects the femur, our thigh bone and longest bone in the body, to the tibia, our shinbone and second longest bone. There are two joints in the knee—the tibiofemoral joint, which joins the tibia to the femur and the patellofemoral joint which joins the kneecap to the femur. These two joints work together to form a modified hinge joint that allows the knee to bend and straighten, but also to rotate slightly and from side to side.

The knee is part of a chain that includes the pelvis, hip, and upper leg above, and the lower leg, ankle and foot below. All of these work together and depend on each other for function and movement.

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The knee joint bears most of the weight of the body. When we’re sitting, the tibia and femur barely touch; standing they lock together to form a stable unit. Let’s look at a normal knee joint to understand how the parts (anatomy) work together (function) and how knee problems can occur.

Knee Solution Program

Anatomical terms allow us to describe the body clearly and precisely using planes, areas and lines. Instead of your doctor saying “his knee hurts” she can say “his knee hurts in the anterolateral region” and another doctor will know exactly what is meant. Below are some anatomic terms surgeons use as these terms apply to the knee:

Structures often have their anatomical reference as part of their name, such as the medial meniscus or anterior cruciate ligament. The medial meniscus would refer to the meniscus on the inside of the knee, the anterior crucial ligament would be on the anterior side (front) of the knee.

Structures of the Knee

Bones of the knee joint

The main parts of the knee joint are bones, ligaments, tendons, cartilages and a joint capsule, all of which are made of collagen. Collagen is a fibrous tissue present throughout our body. As we age, collagen breaks down.

The adult skeleton is mainly made of bone and a little cartilage in places. Bone and cartilage are both connective tissues, with specialized cells called chondrocytes embedded in a gel-like matrix of collagen and elastin fibers. Cartilage can be hyaline, fibrocartilage and elastic and differ based on the proportions of collagen and elastin. Cartilage is a stiff but flexible tissue that is good with weight bearing which is why it is found in our joints. Cartilage has almost no blood vessels and is very bad at repairing itself. Bone is full of blood vessels and is very good at self repair. It is the high water content that makes cartilage flexible.

Bones of the Knee
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The bones give strength, stability and flexibility in the knee. Four bones make up the knee (see above image):

Ligaments of the knee
Ligaments in the knee

The knee works similarly to a rounded surface sitting atop a flat surface. The function of ligaments is to attach bones to bones and give strength and stability to the knee as the knee has very little stability. Ligaments are strong, tough bands that are not particularly flexible. Once stretched, they tend to stay stretched and if stretched too far, they snap.

The pair of collateral ligaments keep the knee from moving too far side-to-side. The cruciate ligaments crisscross each other in the center of the knee. They allow the tibia to “swing” back and forth under the femur without the tibia sliding too far forward or backward under the femur. Working together, the 4 ligaments are the most important in structures in controlling stability of the knee. There is also a patellar ligament that attaches the kneecap to the tibia and aids in stability. A belt of fascia called the iliotibial band runs along the outside of the leg from the hip down to the knee and helps limit the lateral movement of the knee.

Tendons in the Knee

Tendons are elastic tissues that technically part of the muscle and connect muscles to bones. Many of the tendons serve to stabilize the knee. There are two major tendons in the knee—the quadriceps and patellar. The quadriceps tendon connects the quadriceps muscles of the thigh to the kneecap and provides the power for straightening the knee. It also helps hold the patella in the patellofemoral groove in the femur. The patellar tendon connects the kneecap to the shinbone (tibia)—which means it’s really a ligament.

Cartilage of the knee
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Cartilage of the knee

The ends of bones that touch other bones—a joint—are covered with articular cartilage. It’s gets its name “articular” because when bones move against each other they are said to “articulate.” Articular cartilage is a white, smooth, fibrous connective tissue that covers the ends of bones and protects the bones as the joint moves. It also allows the bones to move more freely against each other. The articular cartilages of the knee cover the ends of the femur, the top of the tibia and the back of the patella. In the middle of the knee are menisci—disc shaped cushions that act as shock absorbers.

In a healthy knee, the rubbery meniscus cartilage absorbs shock and the side forces placed on the knee. Together, the menisci sit on top of the tibia and help spread the weight bearing force over a larger area. Because the menisci are shaped like a shallow socket to accommodate the end of the femur, they help the ligaments in making the knee stable. Because the menisci help spread out the weight bearing across the joint, they keep the articular cartilage from wearing away at friction points.

The weight bearing bones in our body are usually protected with articular cartilage, which is a thin, tough, flexible, slippery surface which is lubricated by synovial fluid. The synovial fluid is both viscous and sticky lubricant. Synovial fluid and articular cartilage are a very slippery combination—3 times more slippery than skating on ice, 4 to 10 times more slippery than a metal on plastic knee replacement. Synovial fluid is what allows us to flex our joints under great pressure without wear.

Muscles Around the Knee
Muscles of the knee
Muscles Around the Knee (anterior view)

The muscles in the leg keep the knee stable, well aligned and moving—the quadriceps (thigh) and hamstrings. There are two main muscle groups—the quadriceps and hamstrings. The quadriceps are a collection of 4 muscles on the front of the thigh and are responsible for straightening the knee by bringing a bent knee to a straight position. The hamstrings is a group of 3 muscles on the back of the thigh and control the knee moving from a straight position to a bent position.

The Joint Capsule

The capsule is a thick, fibrous structure that wraps around the knee joint. Inside the capsule is the synovial membrane which is lined by the synovium, a soft tissue that secretes synovial fluid when it gets inflamed and provides lubrication for the knee.

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Bursae

There are up to 13 bursa of various sizes in and around the knee. These fluid filled sacs cushion the joint and reduce friction between muscles, bones, tendons and ligaments. There are bursa located underneath the tendons and ligaments on both the lateral and medial sides of the knee. The prepatellar bursa is one of the most significant bursa and is located on the front of the knee just under the skin. It protects the kneecap. In addition to bursae, there is a infra patellar fat pad that helps cushion the kneecap.

Plicae

Plicae are folds in the synovium. Plicae rarely cause problems but sometimes they can get caught between the femur and kneecap and cause pain.

Knee Arteries and Veins

Knee arteries and veins
Knee arteries and veins

Knee Function

So now we have all the parts, let’s see how the knee moves (articulates)—which is how we walk, stoop, jump, etc.  The knee has limited movement and is designed to move like a hinge.

The Quadriceps Mechanism is made up of the patella (kneecap), patellar tendon, and the quadriceps muscles (thigh) on the front of the upper leg. The patella fits into the patellofemoral groove on the front of the femur and acts like a fulcrum to give the leg its power. The patella slides up an down the groove as the knee bends. When the quadriceps muscles contract they cause the knee to straighten. When they relax, the knee bends.

In addition the hamstring and calf muscles help flex and support the knee.

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Problems in the Knee

The knee doesn’t have much protection from trauma or stress (pressure or force). In addition to wear and tear on the knee, sports injuries are the source of many knee problems.

Symptoms

Knee symptoms come in many varieties. Pain can be dull, sharp, constant or off-and-on. Pain can also be mild to agonizing. The range of motion in the knee can be too much or too little. You may hear grinding or popping, the muscles may feel weak or the knee can lock. Some knee problems only need rest and ice, others need physical therapy (knee rehab exercises) or even surgery.

Pathological Conditions and Syndromes in the Knee

Traumatic Knee Injuries

Repetitive Knee Injuries

Types of Knee Surgery

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