The knee joint is the largest joint in the human body, and it is the most complex and most often injured joint. The knee joint is a synovial, single-axis, hinge joint. This means it has a synovial membrane that secretes synovial fluid, lubricating the inner surfaces of the joint; it can only flex and extend in one plane of motion.
Bones
The femur bone of the thigh and the tibia of the leg are connected together to form the knee joint. The ends of these bones are covered with a thin layer of hyaline cartilage, cushioning them. The outer covering of the bones extend together to form a joint capsule, completely encasing and binding the bones together. The fibula of the leg is outside the joint capsule but provides an anchor point for a collateral ligament.
Menisci
Menisci are pads of fibrous cartilage between the ends of bones in a joint. The knee joint has a lateral meniscus on the outer sides of the knee and a medial meniscus on the inner sides of the knee. These are attached to the top of the tibia and form a shallow cavity for the ends, or condyles, of the femur.
Ligaments
Ligaments connect and hold bones together. The fibular collateral ligament connects from the large, lateral condyle of the femur to the head of the fibula. The medial collateral ligament connects from the large, medial condyle of the femur to just below the medial condyle of the tibia. The collateral ligaments protect the knee from side-to-side shifts. The anterior cruciate ligament runs from the back of the femur to the front of the tibia. It prevents the tibia from shifting forward relative to the femur. The posterior cruciate ligament runs from the front of the femur to the back of the tibia. It prevents the tibia from shifting backward relative to the femur. The posterior tibiofibular ligament attaches the lateral meniscus to the femur. The transverse ligament connects the lateral and medial menisci together.
Bursae
Bursae are pillow-like structures which contain synovial or lubricating fluid. They cushion joints and facilitate the movement of tendons across the joint. Each knee joint has 13 bursae surrounding it--four on the lateral side, five on the medial side and four on the front. Inflammation or bursitis of the prepatellar bursae on the front of the knee is known as “housemaid’s knee.”
Function
The structure of the knee joint permits flexion and extension when the quadriceps and hamstrings muscles contract. On a day-to-day basis, optimal functioning of the knee joint enables movement with little to no pain including walking, getting in and out of chairs and climbing stairs. The knees are the main weight-bearing joints of the body, so when they are injured or diseased, it can be debilitating.
References
- “Anatomy & Physiology”; Gary Thibodeau, Ph.D. and Kevin Patton, Ph.D.; 2007
- “Examination of Musculoskeletal Injuries”; Sandra Shultz, Peggy Houglum and David Perrin; 2005
