The knee is a synovial joint, which means that the capsule is lined by synovium. The synovium produces fluid lubricating and nourishing the inside of the joint. Articular cartilage is the smooth surfaces at the end of the femur and tibia. It is the damage to this surface which causes arthritis.
The femur (thighbone) is the largest and the strongest bone in the body. It is the weight bearing bone of the thigh. It provides attachment to most of the muscles of the knee including Quadriceps, which is the largest muscle of the body. It articulates with the pelvis socket to make the hip joint superiorly and with the shin bone below to make the knee joint below.
The two femoral condyles make up for the rounded end of the femur. Its smooth articular surface allows the femur to move easily over the tibial (shinbone) meniscus. The groove between the two acts as the surface on which the knee cap glides during the movement of the knee. The ACL and PCL ligaments are attached to the femur in the area between the 2 condyles inferiorly and posteriorly.
The tibia (shinbone), the second largest bone in the body, is the weight bearing bone of the leg. The menisci incompletely cover the superior surface of the tibia where it articulates with the femur. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation and gliding movement of the knee.
The fibula, although not a weight bearing bone, provides attachment sites for the Lateral collateral ligaments (LCL) and the biceps femoris tendon along with tendons that go into the foot. The articulation of the tibia and fibula also allows a slight degree of movement, providing an element of flexibility in response to the actions of muscles attaching to the fibula.
The patella (kneecap), attached to the quadriceps tendon above and the patellar ligament below, rests against the anterior articular surface of the lower end of the femur and protects the knee joint. The patella acts as a fulcrum for the quadriceps by holding the quadriceps tendon off the lower end of the femur. This helps in optimizing the force during straightening the leg at the knee joint.
The medial and the lateral meniscus are thin C-shaped layers of fibrocartilage, incompletely covering the surface of the tibia where it articulates with the femur. The majority of the meniscus has no blood supply and for that reason, when damaged, the meniscus is unable to undergo the normal healing process that occurs in the rest of the body. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee. As secondary stabilizers, the intact menisci interact with the stabilizing function of the ligaments and are most effective when the surrounding ligaments are intact.
Anterior Cruciate Ligament (ACL)
The anterior cruciate ligament (ACL) is the major stabilizing ligament of the knee. The ACL is located in the center of the knee joint and runs from the femur (thigh bone) to the tibia (shin bone), through the center of the knee. The ACL prevents the femur from sliding backwards on the tibia (or the tibia sliding forwards on the femur). Together with the posterior cruciate ligament (PCL), ACL stabilizes the knee in a rotational fashion. If one of these ligaments is significantly damaged, the knee will be unstable when planting the foot of the injured extremity and pivoting, causing the knee to buckle and give way.
Posterior Cruciate Ligament (PCL)
The PCL prevents the femur from moving too far forward over the tibia. The PCL is the knee’s basic stabilizer and is almost twice as strong as the ACL. It provides a central axis about which the knee rotates.
Medial Collateral Ligament (MCL)
MCL connects the medial epicondyle of the femur to the medial condyle of the tibia and is present on the inner side of the knee joint. It resists valgus (outward) force on the knee. It can be injured in sporting activities or accidents if something hits the knee from outer aspect.
Lateral Collateral Ligament (LCL)
LCL connects the lateral epicondyle of the femur to the head of the fibula tibia and is present on the outer side of the knee joint. It resists varus (inward) force on the knee. It can be involved in high energy accidents and usually in association with ACL injuries.
Knee joint has a large surface area covered with articular cartilage – under surface of femur, upper surface of tibia and back of patella. The cartilage is supplied nutrition from the underlying bone as well as the synovial (joint) fluid. Th smoothness and congruity of the cartilage helps in smooth movements of the knee joint. The cartilage can be injured in a traumatic event especially in young patients or may be involved with degeneration process of the knee. Cartilage can also be involved in necrosis due to poor blood supply (Avascular Necrosis).