Knee Joint Structure, Function and Problems

The knee joint connects the femur, the longest bone in the body, to the tibia, the second longest bone. The knee joint is a hinge joint and bears most of the weight of the body. It is capable of both flexion (bending) and extension (straightening). Sitting, the two bones barely touch, standing they lock together to form a stable unit. Let’s look at a normal knee joint in order to understand how the parts—anatomy—work together—function—and how knee problems can occur. Let’s define some common anatomic terms as they relate to the knee.


• Anterior — facing the knee, this is the front of the knee
• Posterior — facing the knee, this is the back of the knee, also used to describe the back of the kneecap, that is the side of the kneecap that is next to the femur
• Medial — the side of the knee that is closest to the other knee, if you put your knees together, the media side of each knee would touch
• Lateral — the side of the knee that is farthest from the other knee (opposite of the medial side)

Structures often have their anatomical reference as part of their name, such as the medial meniscus or anterior cruciate ligament.

Structures of the Knee

(click for larger image)

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.

Bones of the knee

The bones provide strength, stability and flexibility in the knee. Four bones make up the knee (see above image):
• Tibia —commonly called the shin bone, runs from the knee to the ankle.
• Patella—the kneecap is a flat, triangular bone; the patella moves when the leg moves. It’s function is to relieve friction between the bones and muscles when the knee is bent or straightened and to protect the knee joint.
• Femur—commonly called the thigh bone; it’s the largest, longest and strongest bone in the body. The round knobs at the end of the bone are called condyles.
• Fibula—long, thin bone in the lower leg on the lateral side, and runs along side the tibia from the knee to the ankle.

(click for larger image)

Ligaments of 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 provide 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.
• Medial Collateral Ligament (tibial collateral ligament) – attaches the medial side of the femur to the medial side of the tibia and limits sideways motion of your knee.
• Lateral Collateral Ligament (fibular collateral ligament) – attaches the lateral side of the femur to the lateral side of the fibula and limits sideways motion of your knee.
• Anterior cruciate ligament – attaches the tibia and the femur in the center of your knee; it’s located deep inside the knee and in front of the posterior cruciate ligament. It limits rotation and forward motion of the tibia.
• Posterior cruciate ligament – is the strongest ligament and attaches the tibia and the femur; it’s also deep inside the knee behind the anterior cruciate ligament. It limits the backwards motion of the knee.
• Patellar ligament – attaches the kneecap to the tibia

The collateral ligaments keep the knee from moving too far side-to-side. The cruciate ligaments 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.

Tendons of the knee

Tendons are elastic tissues that connect muscles to bones. 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.

(click for larger image)

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.
• medial meniscus—made of fibrous, crescent shaped cartilage and attached to the tibia
• lateral meniscus—made of fibrous, crescent shaped cartilage and attached to the tibia
• articular cartilage is on the ends of all bones in any joint—in the knee joint it covers the ends of the femur and tibia and the back of the patella. The articular cartilage is kept slippery by synovial fluid (which looks like egg white) made by the synovial membrane (joint lining). Since the cartilage is smooth and slippery, the bones move against each other easily and without pain.

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 assist 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.

Muscles

There are two muscle groups that control the knee—the quadriceps (thigh) 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 are on the back of the thigh and control the knee moving from a straight position to a bent position.

The Capsule

The capsule is a thick, fibrous structure that wraps around the knee joint. Inside the capsule is the synovium, soft tissue that secretes synovial fluid when it gets inflamed.

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 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.

Problems in the Knee

The knee doesn’t have much protection from trauma or stress.

Symptoms

• Swelling: One of the most common symptoms is local swelling. There are two types of swelling. One is caused by the knee producing too much synovial fluid and the other is caused by bleeding into the joint (hemarthrosis). Swelling within the first hour of an injury is usually from bleeding. Swelling from 2-24 hours is more likely to be from the joint producing large amounts of synovial fluid trying to lubricate an abnormality inside the knee.  The best home treatment for swelling is R.I.C.E. therapy. Chronic swelling can distend the knee, prohibit full range of motion and the muscles can atrophy from non-use. Also, if the cause of the swelling is blood, the blood can be destructive to the joint.

• Locking. Locking is when something is keeping the knee from fully straightening out. This is usually a loose body in the knee. The loose body can be as small as a grain of sand or as big as a quarter. The best treatment is removal of the loose body by arthroscopy. Another type of locking is when the knee hurts so bad that you just won’t use it. the best treatment here is rest and maybe some ice, although swelling is not usually present.

• Giving Way. If your kneecap slips out of is track for an instant, it causes your thigh muscles to loose control causing the feeling of instability—that is, you don’t feel like  your knee is stable—and you usually try to grab hold of something for support. Giving way can also be caused by weak leg muscles or an old ligament injury.

• Snaps, Crackles and Pops. Noises coming from  your knee without pain are likely nothing to worry about. Sometimes the noise is caused by loose bodies that just float around and are not causing pain or injury to the knee. However, If you have pain, swelling or loss of knee function, you should see an orthopedist. The most common cause—chondromalacia patella—is caused by an injury. Another common cause is a dislocating kneecap—that is, a kneecap that keeps slipping out of its groove. Pops without trauma are not worrisome, pops with trauma can mean ligament tears. Crackling, grinding or grating (crepitus) means there is a roughness to the bone surfaces and likely from degenerative disease or wear and tear arthritis.

• Pain and Tenderness. Where and how bad the pain is will help determine the underlying cause. It also helps to know what caused it and what makes it hurt. Pain that gets worse with activity is often tendinitis or stress fractures. Pain and tenderness accompanied by swelling can be more serious such as a tear or sprain. Some pain can be caused by muscles spasms associated with trauma.

Pathological Conditions and Syndromes

• Osteochondritis Dissecans
• Osteoarthritis (Degenerative Arthritis)
• Infectious Arthritis
• Chondromalacia Patellae
• Gout
• Plica Syndrome
• Rheumatoid Arthritis
• chondromalacia
• osteoarthritis

Traumatic Knee Injuries

• Anterior cruciate ligament (ACL) Injury
• Meniscus tear
• Medial Collateral Ligament (MCL) Injury
• Lateral Collateral Ligament (LCL) Injury
• Posterior Cruciate Ligament (PCL) Injury
• Dislocation of the Patella
• Rupture of the Patellar Tendon
• Fracture and Stress Fracture

Repetitive Knee Injuries

• Patellofemoral Syndrome (Runner’s Knee)
• Tendonitis
• Bursitis (Housemaid’s Knee)
• Illiotibial Band Syndrome
• Osgood-Schlatter Disease

Knee Surgery

• Knee Replacement
• Knee Arthroscopy

Related posts:

1. Knee Replacement Surgery
2. Knee Replacement (Total)
3. Knee Arthroscopy
4. Knee Replacement – Regaining Strength and Motion
5. Osteoarthritis (Degenerative Joint Disease)