Knee Joint Composition and Mechanics
You may be surprised to learn that the knee is the largest joint in the human body. The knee is a hinge joint and moves in two directions: flexion and extension. The femur, tibia, and patella are the bones directly involved in the knee joint. The tibia and femur roll on each other to create this motion into bending and straightening.
The femur is the long bone in your thigh. The patella, more commonly known as the ‘knee cap’, is a small bone in the knee joint, and the tibia joins from the bottom (shin bone). The patella acts as a pulley for your thigh muscles (quadriceps), to direct force to the rest of the lower extremity. The fibula is a bone that runs along-side the tibia, but does not bear weight from the body; it is a site of ligament attachments for knee stability.
The ends of the femur and tibia that connect are covered with cartilage to allow smooth movement during weight-bearing activities (running, walking, weight lifting), and non-weight bearing activities (cycling and swimming). This cartilage is known as your meniscus. It is shaped like a crescent moon and lines both the inner (medial) and outside (lateral) articulating bony surfaces. The condition of degenerating knee cartilage over time is called osteoarthritis.
The quadriceps act to straighten the knee, and protect the posterior cruciate ligament or PCL. The hamstrings, in turn, bend the knee, and protect the integrity of the anterior cruciate ligament of ACL.
Ligaments provide stability with all knee and lower-extremity movements such as walking up and down stairs, walking or running on uneven surfaces, and squatting. In addition to the PCL and ACL ligaments which prevent over-bending and over-straightening of the knee, there are two other ligaments that protect the knee. The medial collateral ligament or MCL protects the knee from falling inward past mid-line, and the lateral collateral ligament or LCL prevents excessive knee movement away from the body.
The meniscus provides the knee with cushioning during high impact activities. Some weight bearing activities are beneficial to the knee joint, as this weight bearing pushes around and lubricates the knee joint with a nutrient filled fluid called synovial fluid.
Lastly, fluid-filled sacs called bursa sit around the knee to protect and cushion ligaments and tendons. These sacs can become inflamed with injury or with chronic over-use creating bursitis. RICE can help decrease symptoms of acute bursitis.