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Knee pain may have many possible causes.

Injury and arthritis are chief among these potential sources of knee pain.

The location of the pain and what brought on the knee pain may provide clues to what is going on.

Knowing how the knee functions and local anatomy are important to help identify the root cause and understand how to help alleviate symptoms in some cases.

Anatomy of the Knee

The Bones
Femur
Tibia
Fibula
Patella (knee cap)

The Knee Joint
Synovium: the lining of the knee joint.
The synovium allows nutrients to diffuse into the knee joint, as well as acts as a active tissue secreting both inflammatory and anti-inflammatory signals.

Cartilage
There are two discoid menisci. The lateral meniscus is the outer meniscus.

The medial meniscus is on the inside of the knee. The meniscus functions to allow gliding of the femur on the tibia, provide joint stabilization, and acts as a shock absorber.

Both the femur and tibia are covered with a cartilage layer to separate the bones, this allows frictionless motion across the menisci during motion. In abnormal conditions of trauma or arthritis, defects in the cartilage covering the femur and tibia occur. These are known as osteochondral defects.

Tears in the meniscus or meniscal degeneration also impair normal knee joint functioning.

Ligaments
Medial and Lateral collateral ligaments: connect the femur and tibia as well as the meniscus.

Anterior cruciate ligament (ACL) : connects the femur and tibia in the middle of the knee joint. The ACL functions to stabilize the knee against primarily anterior translation motion.

Posterior cruciate ligament (PCL) : connects the femur and tibia in the middle of the knee joint in an opposing direction to the ACL forming a cross pattern. The PCL functions to stabilize the knee primarily against posterior translational motion.

Muscles
Primarily the large quadriceps in the front and the hamstrings in the back.

Additional muscle groups include the adductor muscles insertion on the inner aspect of the knee. The outer aspect also includes the tensor fascia lata (TFL).

The gastrocsoleus or calf muscles insert behind the knee joint.

Smaller muscles that lie deeper to these muscles include the plantaris and popliteus.

Bursae
Pes Anserine: is located on the inside of the knee
Popliteal fossa: behind the knee
Suprapatellar: above the knee cap
Infrapatellar: below the knee cap

Knee Arthritis
Arthritis may occur in the synovium, cartilage, meniscus, and the underlying bone may be affected with advancing arthritis.