You are here: Home > Guides > Knee Pain Guide: Physiology, Common Knee Injuries & Diseases

Knee Pain Guide: Physiology, Common Knee Injuries & Diseases

If you are a healthy, active individual, there is a high probability that at some point in your life you will have a knee injury. If you play sports that require quick pivots such as basketball, soccer, football, gymnastics, dance, tennis and skiing, you are particularly at risk for different types of knee injuries.

While ankle injuries are slightly more prevalent, a knee injury is much more likely to be debilitating. 50% of sports knee injuries will require medical attention and more than 400,000 individuals have full-on knee replacement surgery each year. Ask any sports medicine professional, and they will tell you that knee injuries are the most common reason a player is sidelined for an entire season.

In light of how prevalent knee injuries are and how drastically a knee injury can affect one’s mobility and lifestyle, we are putting together a definitive guide to common knee injuries, the physiology of the knee, and how to prevent knee injuries in the future.

First, let’s look at the overall physiology of the knee. An understanding of the knee and how it works is essential for effective knee injury prevention and knee injury treatment.

Knee Anatomy & Physiology

The knee is a remarkable hinge joint between the femur (thigh bone), the tibia (shin bone), fibula (long slender bone next to the tibia) and the patella (knee cap). It has considerable stability thanks to a network of ligaments, cartilage and muscles, but an injury can destabilize the patella, forcing an athlete to wear a patella stabilizer during training. The Q-angle, or the joint area between the tibia and femur, is a portion of the knee that is prone to overuse injury and osteoarthritis.

Critical knee-related muscles include the quadriceps and hamstrings. The quadriceps (quads) are the longest, leanest muscle group in the human body. They connect to the top part of the patella / kneecap and when the quads contract, they pull laterally, extending the leg. Your quads are made up of four muscle groups, the vastus medialis, the vastus lateralis, the vastus intermedius and the rectus femoris. When individuals experience a knee injury in this area, they often lose significant strength in the vastus medialis, located at the lower inside potion of the quad. This muscle is critical to patellar stability, so an effective rehabilitation regimen should pay specific attention to strengthening that area in order to restore knee tracking alignment. Conversely, the hamstring muscle group provides medial knee stability and controls the knee flexion (i.e. bending your leg at the knee.)

When it comes to common knee injuries, tendons are particularly at risk. The knee relies on four key tendons for stability, specifically the MCL, LCL, ACL & PCL. The medial collateral ligament (MCL) is part of the medial stabilizers and is located deep within the knee below the knee. It connects to the lateral meniscus, so it is not uncommon to have other injuries, such as a meniscus, ACL or cartilage tear when you injure the MCL. For this reason, many MCL tears are effectively treated with an ACL brace. The anterior cruciate ligament (ACL) connects to the femur to the lateral meniscus. Like the MCL, an ACL injury is frequently accompanied by other ligament or cartilage injuries. The lateral collateral ligament (LCL) runs along the side of the knee, connecting the femur to the tibula. This ligament is not attached to the lateral meniscus, making it less likely to be injured. Lastly, the posterior cruciate ligament (PCL) also connect the femur to the tibia, holding the knee into place and preventing the tibia from moving posterior to the femur.

Types of Minor Knee Injuries

The knee is a complex part of the body and bares a lot of weight, making it more prone to injury than most joints. It is estimated that every year approximately 5.5 million Americans visit the doctor for knee issues and injuries. Most injuries are the result of a sudden twist or turn, but there are also degenerative knee injuries that arise over time due to wear-and-tear.

We will begin with the lesser injuries. While these injuries tend to be less severe, they are still inconvenient and at times painful.

Patellofemoral Pain Syndrome (aka PFPS or Runner’s Knee)

PFPS is a good example of a wear-and-tear, degenerative injury. Often found in runners – hence the nickname “Runner’s Knee” – PFPS usually arises when the cartilage around the knee is worn down after repeated movement, allowing the kneecap to track out of place and irritate the femoral groove. Individuals with high arches or flat feet may be particularly prone for this type of injury, and women are more likely than men to be diagnosed as well.


Tendonitis is the inflammation, swelling and irritation of a tendon or tendons, with tendons in the hand and knee being particularly prone to this ailment. Tendonitis creates a deep aching in the knee area. Knee tendonitis is the most common knee injury, but often goes undiagnosed and untreated. While tendonitis can occur all around the knee, the most common area of pain is just below the knee cap.

Iliotibial Band Syndrome

While iliotibial band syndrome (aka ITBS) is actually a thigh injury, it does result in pain on the outer side of the knee and is easily the most common cause for lateral knee pain in athletes. This injury is commonly associated with running, weight-lifting and cycling and it involves the inflation or swelling of the iliotibal band as moves over the femur. There are a series of stretches that can help to prevent or reduce ITBS.

While tendonitis, runner’s knee and ITBS are all very unpleasant, they are rarely debilitating. More severe knee injuries will leave you in significant pain and compromise your mobility for weeks to months. These injuries include tears in the major supporting ligaments.

The 4 main ligaments that support the knee include the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and lateral collateral ligament (LCL) Many ligament tears require surgery, physical rehabilitation and a proper hinged knee brace for a full recovery.

Types of Major Knee Injuries

Serious knee injuries almost inevitably result in acute pain and debilitation, but if diagnosed accurately early on, treatment can minimize discomfort and get the individual back on their feet. The primary serious knee injuries include tendinitis, fractures, ligament injuries and meniscus tears.

Chronic Tendonitis of the Knee

Tendons are durable, yet flexible fibrous bands of tissue that connect muscles to bones allowing the body to move. You can find tendons just about anywhere your body moves, from your fingers to your toes. There are literally 100’s of tendons throughout the human body, but there are a select few tendons that appear to be more prone to injury. One of the most common tendon injuries is tendonitis. Tendinitis is the chronic inflammation of the tendon typically as the result of overuse, though there are occasionally genetic root causes for tendonitis. Tendonitis leads to swelling, tenderness and pain during movement.

If you develop tendonitis of the knee (aka "jumper’s knee"), you experience inflammation of the patellar (below the kneecap) or popliteal tendons (located at the back of the knee). Most cases of knee-area tendonitis can be treated with reduced use, continuous cold therapy devices and anti-inflammatory medication. In some instances a medical knee brace or strap can mitigate pain and allow for increased activity.

Knee Fractures

The knee is a very solid and sturdy bone, but fractures occasionally occur. Typically the result of massive trauma such as a high impact injury or automobile accident, a knee fracture requires immediate medical attention. Treating a fracture may involve surgery, a cast or bracing.

Knee Ligament Injuries

Perhaps one of the most common-place sports-related injuries, a knee ligament injury can be quite mild like a sprain or very severe as is the case with a full blown ligament tear. There are three possible areas the ligament can tear: the inside of the knee (medial collateral ligament), the outside of the knee (lateral collateral ligament) or inside the knee (cruciate ligaments). You will know immediately if you tear a ligament as it can be quite painful, especially when bending the knee, and many report a loud popping noise upon occurrence. It is not unusual, however, to have difficulty pin-pointing exactly where the pain is coming from and the ligaments and their respective nerves are all located within close proximity. Other symptoms of a torn ligament include swelling and localized temperature increase. Depending on the severity of your injury, ligament injuries are treated with continuous cold therapy, immobilization and elevation to decrease swelling. The individual should avoid placing weight on the injured knee, and crutches or a hinged knee brace may be required. In very severe cases surgery and long-term rehabilitation may be required.

Torn Meniscus

The meniscus is a flexible c-shaped disk that cushions the knee cap. If the knee is forcefully rotated suddenly, the meniscus can tear. We see this most often in contact sports and activities that required sudden pivoting. Likewise, the risk increases for older sports participants due to degenerated cartilage. In some instances, you can have more than a single tear. These tears are characterized by a locking sensation, instability and can lead to swelling and localized warmth. In most cases, however, it takes a doctor’s expertise and an MRI to accurately diagnose a torn meniscus.

None of these knee injuries are pleasant, but they can be successfully treated with minimal discomfort if diagnosed properly early on. You should always consult a physician when you experience a traumatic knee injury, but specifically when:

  • The pain is acute, surpassing a mild throbbing sprain sensation
  • Swelling occurs instantaneously and persists
  • You cannot bear weight on your knee
  • The knee appears deformed
  • You cannot feel below the injury site
  • The lower leg, foot or ankle turn cold post injury
  • No pulse can be detected in ankle

Painful Conditions & Diseases of the Knee

There are several knee diseases or conditions that can also contribute to chronic knee discomfort. These diseases often affect the soft tissues, joints, nerves and/or bones within the knee area. The most common knee diseases and conditions include arthritis, arthritis-related cysts, infections, calcification and softening of vital cartilage. An overview of each of these ailments can be found below.

Knee Arthritis

There are several types of arthritis, including genetic, degenerative and inflammatory. Genetic arthritis includes both rheumatoid arthritis and gout. An individual with these conditions is genetically predisposed to have chronic inflammation of the joint, which can be painful as well as limit movement. Osteoarthritis is the most common form of arthritis and is degenerative in nature. The cartilage gets worn down over time due to repeat usage of the knee joint. Knee arthritis can be treated with an arthritis knee brace or, in very severe cases, knee replacement surgery. In some arthritis cases, the knee’s swelling can lead to fluid building up in a cyst typically located in the back of the knee. These are often referred to ask Baker cysts. If the cyst becomes large and painful, you can have a physician drain the fluid, as well as use physical therapy and medication.

Infection on the Knee Joint or Bone

Following a puncture wound, a bacterial pathogen can penetrate the knee area through the joint, bursa, bone or skin and lead to an internal infection in the knee joint or bone. While the knee may not experience explicit pain as the result of an infection, the individual may develop a fever, experience chills in their body and a localized heat around the knee. Once diagnosed, these infections are typically treated with a round of antibiotics.


Knee tumors are rare, but when they do appear they can impede walking and cause pain. The best treatment for tumor is surgical removal, but in a few extreme cases, it may also require lower leg amputation.


The collateral ligament located on the inside of the knee can calcify. This is usually seen in people who have had a significant MCL ligament injury in their past and is known as Pellegrini-Stieda syndrome. The calcification leads to inflammation, but it can be treated effectively with continuous cold therapy, immobilization and rest. In some instances a cortisone shot can help.


Chrondromalacia is a condition characterized by the softening of cartilage under the patella (knee cap). It is most often seen in women, likely due to how their hips distribute weight to their knees. When the cartilage softens, it can cause significant deep knee pain as well as stiffness. Most cases of Chrondromalacia are effectively treated with anti-inflammatory medication, strengthening exercises for the quads, rest and cold therapy.

If you’re suffering from knee pain, we highly recommend you consult a doctor. For less severe knee injuries, the application of compression sleeves or knee braces can often times help alleviate knee pain and/or promote healing. However, with more serious knee injuries, you may be required to get medical assistance in healing.