Knee Joint: Function and Anatomy

What is the knee joint?

The knee joint is one of the largest and most complex joints in the human body. It connects the thigh bone (femur) to the shin bone (tibia). The patella (kneecap) sits in front of the joint, and the smaller fibula bone runs alongside the tibia. The knee is classified as a hinge joint, meaning it primarily allows bending (flexion) and straightening (extension), though it also allows for some rotational movement.

Key structures of the knee joint include:

1. Bones:

● Femur (thigh bone)

● Tibia (shin bone)

● Fibula (a smaller bone next to the tibia)

● Patella (kneecap)

2. Cartilage:

● Articular cartilage covers the ends of the femur and tibia and the back of the patella, helping the bones glide smoothly.

● Menisci are two crescent-shaped pieces of cartilage (medial and lateral menisci) between the femur and tibia, acting as shock absorbers.

3. Ligaments:

● Anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) control the forward and backward motion of the knee.

● Medial collateral ligament (MCL) and lateral collateral ligament (LCL) stabilize the knee from side to side.

4. Tendons: 

Tendons connect muscles to bones, with the quadriceps tendon connecting the thigh muscles to the patella and the patellar tendon connecting the patella to the tibia.

Together, these structures enable the knee to support body weight, provide stability, and facilitate movement such as walking, running, jumping, and kneeling.

What does the knee joint do?

The knee joint performs several essential functions that enable movement and provide stability. Its main roles include:

■ Bending and Straightening (Flexion and Extension):

 The knee acts as a hinge joint, allowing the leg to bend (flex) and straighten (extend). This is crucial for activities like walking, running, sitting, and standing.

■ Weight-bearing:

 The knee supports the body’s weight, especially during standing, walking, and running. It distributes the body’s load between the femur, tibia, and other structures to maintain balance and stability.

■ Shock Absorption: 

The menisci (cartilage) in the knee absorb shocks and reduce impact on the bones during activities like jumping or running, helping to protect the joint from damage.

■ Rotational Movement:

 Although primarily a hinge joint, the knee also allows some degree of rotation, which is important for movements like pivoting or changing direction while walking or running.

■ Stabilization: 

Ligaments (ACL, PCL, MCL, and LCL) and muscles surrounding the knee provide stability, preventing excessive movement and protecting the joint from injury during activities.

In summary, the knee joint enables essential movement, supports weight, absorbs shocks, and maintains stability, playing a key role in most lower body activities.

Where is the knee joint located?

The knee joint is located in the lower part of the leg, between the thigh and shin. Specifically, it connects the femur (thigh bone) at the top to the tibia (shin bone) at the bottom. The patella (kneecap) sits in front of the joint, covering and protecting it. This joint is situated at the midpoint of the leg, enabling movement and support for activities like standing, walking, and bending.

Which type of joint is the knee?

The knee is classified as a hinge joint. This type of joint primarily allows for bending and straightening movements, known as flexion and extension, similar to the motion of a door hinge. While its main function is to move in one plane, the knee also allows for a small amount of rotation, making it slightly more complex than a typical hinge joint. This rotational ability helps with actions like pivoting and changing direction.

What is the structure of the knee joint?

The structure of the knee joint is composed of several key components that work together to enable movement, stability, and weight-bearing. These include bones, cartilage, ligaments, tendons, and other soft tissues. Here's a detailed breakdown:

1. Bones:

● Femur (Thigh Bone):

 The upper part of the knee joint is formed by the lower end of the femur.

● Tibia (Shin Bone):

 The lower part of the joint is made up of the top end of the tibia.

● Patella (Kneecap):

 The small, flat, triangular bone that sits in front of the knee joint, providing protection and improving the leverage of the thigh muscles.

● Fibula: 

The fibula runs parallel to the tibia but is not directly involved in the knee joint; it provides attachment points for ligaments and muscles.

2. Cartilage:

● Articular Cartilage:

 Smooth cartilage covering the ends of the femur, tibia, and the back of the patella, allowing the bones to glide smoothly against each other.

● Menisci: 

Two C-shaped pieces of fibrocartilage, called the medial meniscus (inner side) and the lateral meniscus (outer side), act as shock absorbers between the femur and tibia and help distribute weight across the knee joint.

3. Ligaments (stabilize the knee joint):

● Anterior Cruciate Ligament (ACL):

 Located in the center of the knee, it prevents the tibia from sliding forward relative to the femur and provides rotational stability.

● Posterior Cruciate Ligament (PCL):

 Works with the ACL, preventing the tibia from sliding backward relative to the femur.

● Medial Collateral Ligament (MCL): 

Runs along the inner side of the knee, preventing side-to-side motion of the knee, especially when a force is applied from the outside.

● Lateral Collateral Ligament (LCL):

 Runs along the outer side of the knee, stabilizing the joint from forces that push it inward.

4. Tendons (connect muscles to bones):

● Quadriceps Tendon:

 Connects the quadriceps muscles of the thigh to the patella.

● Patellar Tendon: 

Extends from the patella to the tibia, allowing the quadriceps muscles to extend the leg.

5. Bursae:

These are small fluid-filled sacs that reduce friction between the bones, tendons, and muscles around the knee joint. There are several bursae around the knee, with the prepatellar bursa (in front of the patella) being the most prominent.

6. Muscles:

● Quadriceps Muscles: 

Located in the front of the thigh, these muscles help to extend (straighten) the knee.

● Hamstrings Muscles: 

Located in the back of the thigh, these muscles help to flex (bend) the knee.

These structures work together to ensure the knee can perform complex movements while bearing the weight of the body and maintaining stability.

What are the most common issues or conditions that affect knees?

The knee is a complex joint that is prone to a variety of conditions and injuries. Some of the most common issues that affect the knees include:

1. Osteoarthritis:

A degenerative joint disease where the cartilage that cushions the knee joint wears down over time, leading to pain, stiffness, swelling, and reduced mobility. It is the most common type of arthritis affecting the knees, particularly in older adults.

2. Ligament Injuries:

■ Anterior Cruciate Ligament (ACL) Tear:

 A common sports injury where the ACL, one of the main stabilizing ligaments, is torn due to sudden stops, changes in direction, or impact. It often requires surgery and physical therapy for recovery.

■ Medial Collateral Ligament (MCL) Injury:

 Usually caused by direct impact to the outer side of the knee, leading to pain and swelling on the inner side.

■ Posterior Cruciate Ligament (PCL) Injury: 

Less common than ACL injuries, often caused by a direct blow to the front of the knee, such as during a car accident or a fall.

■ Lateral Collateral Ligament (LCL) Injury:

 Less common than MCL injuries, this ligament injury results from force applied to the inside of the knee, causing the outer side of the knee to stretch.

3. Meniscus Tears:

■  The menisci are two crescent-shaped pieces of cartilage that act as shock absorbers in the knee. A meniscus tear can occur due to twisting or turning quickly, often during sports. Tears can cause pain, swelling, and difficulty extending the knee.

4. Patellar Tendonitis (Jumper’s Knee):

■  Inflammation of the patellar tendon, usually caused by overuse, especially in athletes who engage in jumping or running activities. It leads to pain just below the kneecap.

5. Bursitis:

■  Inflammation of the bursae, small fluid-filled sacs that cushion the knee joint. Knee bursitis can occur from overuse, prolonged kneeling, or injury, causing swelling, warmth, and pain in the affected area.

6. Patellofemoral Pain Syndrome (Runner’s Knee):

■  Characterized by pain around the kneecap, often caused by overuse, misalignment of the kneecap, or muscle imbalances. It is common among runners and athletes but can affect non-athletes as well.

7. Knee Fractures:

■  A break in one of the bones in the knee joint, such as the patella (kneecap), often caused by trauma or falls. Fractures can lead to severe pain, swelling, and inability to move the knee.

8. Knee Dislocation:

■  Occurs when the bones of the knee are forced out of alignment, typically due to trauma or high-impact injury. It can cause severe pain and deformity and is considered a medical emergency.

9. Tendon Tears (Quadriceps or Patellar Tendon):

■  Partial or complete tears of the tendons connecting muscles to the knee, such as the quadriceps tendon or patellar tendon, can occur due to injury, overuse, or aging. Symptoms include pain, swelling, and difficulty walking or extending the leg.

10. Baker’s Cyst:

■  A fluid-filled sac that forms behind the knee, often as a result of arthritis or a knee injury. It can cause swelling and tightness in the back of the knee and may rupture, leading to sharp pain.

11. Gout:

■  A type of inflammatory arthritis caused by the buildup of uric acid crystals in the joint, leading to sudden, severe pain, swelling, and redness, often affecting the knee.

12. Tendinopathy:

■  Chronic degeneration of the tendons, particularly the patellar or quadriceps tendons, often caused by overuse, leading to pain and stiffness.

13. Chondromalacia Patellae:

■  Also known as "runner's knee," this condition involves the softening and breakdown of the cartilage on the underside of the kneecap, leading to pain and discomfort during knee movement.

These conditions can range from mild to severe, and treatment may vary depending on the underlying issue. Common treatments include rest, physical therapy, medications, bracing, and in some cases, surgical intervention.

What are common symptoms that affect knee joints?

Common symptoms that affect knee joints vary depending on the underlying condition or injury, but they typically include the following:

1. Pain:

● Pain in the knee is the most common symptom, and it can range from mild to severe. It may occur suddenly after an injury or develop gradually due to chronic conditions like arthritis. Pain may be localized in different parts of the knee, such as the front, sides, or back.

2. Swelling:

● Swelling in the knee can occur due to fluid buildup inside the joint, often following an injury (like a ligament tear or meniscus tear) or inflammation from conditions like arthritis or bursitis. Swelling may cause the knee to appear enlarged and feel warm to the touch.

3. Stiffness:

● Stiffness, or difficulty moving the knee, is common in conditions like arthritis or after an injury. It can make it hard to fully bend or straighten the knee and may be worse after periods of inactivity.

4. Instability or Weakness:

● A feeling that the knee is unstable or "giving way" is often associated with ligament injuries (like ACL or MCL tears). Muscle weakness around the knee, especially in the quadriceps, may also contribute to instability.

5. Locking or Catching:

● Some conditions, like a torn meniscus, may cause the knee to lock or catch during movement, making it temporarily difficult or impossible to straighten the leg.

6. Clicking or Popping Sounds:

● Clicking, popping, or crunching sounds (crepitus) in the knee may occur when the joint moves. These sounds can be harmless, but if they are accompanied by pain, they may indicate an issue like a torn cartilage or degenerative changes in the joint.

7. Limited Range of Motion:

● Inability to fully extend or flex the knee may be a sign of a mechanical problem like a torn meniscus, swelling, or arthritis. Reduced range of motion can interfere with daily activities like walking or sitting.

8. Tenderness:

● Tenderness around the knee joint, especially when pressing on certain areas, is often a sign of inflammation or injury. This can be seen in conditions like tendinitis, bursitis, or ligament sprains.

9. Redness and Warmth:

● Redness and warmth around the knee joint are usually signs of inflammation or infection. Conditions like bursitis, gout, or septic arthritis may cause these symptoms.

10. Deformity:

● Visible changes in the shape of the knee, such as a noticeable bump or a misshapen knee joint, may result from dislocations, fractures, or advanced arthritis.

11. Difficulty Bearing Weight:

● Difficulty bearing weight on the affected leg, or feeling pain when doing so, is often associated with serious injuries like fractures, ligament tears, or severe arthritis.

12. Bruising:

● Bruising may appear around the knee following trauma or injury, such as in the case of a fracture, ligament tear, or patellar dislocation.

13. Morning Stiffness:

● Stiffness that occurs after waking up or after periods of inactivity is common in arthritis, particularly osteoarthritis. The stiffness typically improves with movement as the day progresses.

These symptoms often indicate underlying issues like injuries (e.g., ligament tears, meniscus tears), inflammation (e.g., bursitis, tendinitis), or degenerative conditions (e.g., osteoarthritis). If knee pain or other symptoms persist, worsen, or interfere with daily activities, it's important to seek medical evaluation for diagnosis and appropriate treatment.

Which tests are done to knees?

When assessing knee conditions or injuries, doctors use a combination of physical examinations, imaging tests, and laboratory tests to diagnose the problem. Here are the most common tests done to evaluate the knee joint:

1. Physical Examination:

A doctor will perform a series of manual tests to evaluate the movement, stability, and pain in the knee. These include:

■ Palpation:

 The doctor feels around the knee to identify areas of tenderness, swelling, or warmth.

■ Range of Motion Test: 

The patient is asked to bend and straighten the knee to evaluate any limitations in movement.

■ Lachman Test:

 Used to assess for an ACL injury by checking the forward movement of the tibia relative to the femur.

■ Anterior Drawer Test: 

Another test to check for ACL tears, involving pulling the tibia forward while the knee is bent.

■ Posterior Drawer Test: 

Checks for PCL injury by pushing the tibia backward with the knee flexed.

■Valgus and Varus Stress Tests:

 These tests are used to assess the MCL and LCL by applying pressure to the sides of the knee.

■ McMurray Test: 

Used to check for meniscus tears by rotating the knee while applying pressure.

2. Imaging Tests:

Imaging is essential to visualize internal structures of the knee, such as bones, cartilage, ligaments, and soft tissues. Common imaging tests include:

■ X-ray:

     ▪︎ X-rays are often the first imaging test used. They are useful for detecting bone fractures, arthritis, and bone deformities.

■ Magnetic Resonance Imaging (MRI):

     ▪︎ An MRI provides detailed images of soft tissues such as cartilage, tendons, ligaments, and menisci. It’s particularly useful for diagnosing ligament tears (e.g., ACL, PCL), meniscal injuries, and cartilage damage.

■ Ultrasound:

     ▪︎ Ultrasound can help visualize soft tissues, including tendons, ligaments, and fluid buildup in the knee (e.g., bursitis). It is non-invasive and often used to guide injections.

■ Computed Tomography (CT) Scan:

     ▪︎ CT scans provide detailed cross-sectional images of bones and are helpful for diagnosing complex fractures or issues not visible on X-rays.

■ Bone Scan:

     ▪︎ This imaging technique is used less frequently but can detect stress fractures, infections, and bone disorders.

3. Laboratory Tests:

■ Blood Tests:

 Blood work can help diagnose systemic conditions that affect the knee, such as rheumatoid arthritis, gout, or infection. Common blood tests include:

     ▪︎ Erythrocyte Sedimentation Rate (ESR):

 Measures inflammation in the body.

     ▪︎ C-Reactive Protein (CRP): 

Another marker of inflammation, often elevated in cases of infection or inflammatory arthritis.

     ▪︎ Rheumatoid Factor (RF):

 Used to diagnose rheumatoid arthritis.

     ▪︎ Uric Acid:

 To check for gout, a form of arthritis caused by uric acid crystal buildup.

■ Joint Aspiration (Arthrocentesis): 

A sample of fluid is drawn from the knee joint using a needle. The fluid is analyzed for infection, gout (uric acid crystals), or inflammatory conditions (e.g., rheumatoid arthritis). This test is particularly helpful in diagnosing septic arthritis or gout.

4. Arthroscopy:

■ Knee Arthroscopy is a minimally invasive surgical procedure in which a small camera (arthroscope) is inserted into the knee joint through small incisions. This allows doctors to view the inside of the knee and diagnose issues like cartilage damage, ligament tears, or meniscus injuries. Sometimes, minor repairs can be performed during the procedure.

5. Gait Analysis:

■ This test evaluates how a person walks and assesses abnormalities in knee movement or alignment. It is often used for chronic knee conditions or post-surgical evaluation.

6. Stress X-rays:

■ Stress X-rays are performed with the knee under specific conditions to check for ligament stability. Pressure is applied to the knee in different directions while an X-ray is taken, providing insight into any ligament damage or joint instability.

These tests help doctors identify the underlying cause of knee pain or dysfunction, allowing for appropriate treatment plans such as physical therapy, medications, or surgery.

How can I take care of my knees?

Taking care of your knees is important for maintaining mobility and preventing injuries or conditions that could lead to pain and reduced function. Here are several strategies to help you protect and maintain healthy knees:

1. Maintain a Healthy Weight:

● Excess body weight puts added pressure on your knees, especially during activities like walking, running, or climbing stairs. Maintaining a healthy weight reduces stress on the knee joints and lowers the risk of conditions like osteoarthritis.

2. Exercise Regularly:

● Strengthen the Muscles: 

Strong muscles around the knee, especially the quadriceps, hamstrings, and calves, help support the knee joint and reduce strain on the ligaments.

● Low-Impact Activities: 

Engage in exercises that minimize joint stress, such as swimming, cycling, walking, or using an elliptical machine. These activities help improve knee function without causing too much wear and tear.

● Flexibility and Stretching:

 Regularly stretch your muscles, particularly your quadriceps, hamstrings, and calves, to maintain flexibility and prevent stiffness. Yoga and Pilates can also help improve flexibility and joint health.

● Balance and Stability Exercises:

 These exercises help prevent falls and improve coordination. Simple balance exercises, like standing on one foot, can strengthen muscles around the knee and improve stability.

3. Use Proper Form During Exercise:

● Avoid Overuse: 

Repeated stress from overtraining or improper technique can cause knee injuries. Gradually increase the intensity of your workouts and incorporate rest days to allow your knees to recover.

● Warm Up and Cool Down: 

Always warm up before exercise to increase blood flow to the muscles and reduce the risk of injury. Cooling down with light stretching afterward helps prevent stiffness.

● Wear Proper Footwear: 

Choose shoes that provide adequate cushioning, arch support, and shock absorption to reduce stress on your knees. Replace worn-out shoes regularly, especially if you’re a runner or engage in high-impact activities.

4. Avoid Prolonged Sitting or Kneeling:

● Sitting or kneeling for long periods can stiffen the knee joints and weaken muscles. Take regular breaks to stretch and move around, especially if you have a desk job or spend a lot of time sitting.

5. Protect Your Knees During Physical Activity:

● Wear Knee Pads:

 If your activities or job involve frequent kneeling (e.g., gardening, cleaning, or construction), wear knee pads to protect your knees from excessive pressure and impact.

● Use Supportive Gear:

 If you’re prone to knee issues or engage in sports, consider wearing knee braces or supports to provide additional stability and protection.

● Modify High-Impact Activities:

 If you participate in high-impact sports like running, basketball, or soccer, mix in low-impact activities to reduce strain on your knees.

6. Maintain a Healthy Posture:

● Posture Matters: 

Poor posture, especially when walking or standing for long periods, can place undue stress on your knees. Focus on maintaining proper alignment by keeping your head up, shoulders back, and knees slightly bent.

7. Avoid Sudden Changes in Activity Levels:

● If you’ve been inactive, don’t suddenly engage in intense activities without gradually building up your strength and stamina. A sudden increase in activity can lead to injuries like ligament tears or tendinitis.

8. Stay Hydrated:

● Drinking enough water helps maintain joint lubrication, which is essential for reducing friction in the knees and preventing cartilage breakdown.

9. Listen to Your Body:

● If you experience knee pain, don’t ignore it. Continuing to exercise or participate in activities through pain can worsen injuries. Rest, apply ice, and consider seeing a healthcare provider if the pain persists.

10. Focus on Core Strength:

● Strengthening your core muscles (abs, back, and hips) helps improve balance and stability, which in turn supports your knees. A strong core reduces the risk of knee injuries, especially during activities that involve twisting or sudden direction changes.

11. Treat Injuries Promptly:

● If you injure your knee, follow the RICE method (Rest, Ice, Compression, Elevation) to reduce swelling and pain. Seek medical attention for severe injuries to avoid long-term damage.

12. Take Supplements if Necessary:

● Some people take supplements like glucosamine and chondroitin for joint health, though the evidence for their effectiveness is mixed. Omega-3 fatty acids, found in fish oil, may help reduce inflammation in joints.

13. Consult a Doctor for Knee Pain:

● If you experience persistent or severe knee pain, swelling, or instability, consult a healthcare provider. Early diagnosis and treatment of knee problems (such as arthritis or ligament tears) can help prevent further damage and improve your long-term knee health.

By incorporating these habits into your daily routine, you can protect your knees and ensure their long-term health, reducing the risk of injury and joint problems.