Normal Anatomy of the Knee Joint
The knee is made up of four bones. The femur or thighbone is the bone connecting the hip to the knee. The tibia or shinbone connects the knee to the ankle. The patella (kneecap) is the small bone in front of the knee and rides on the front of the knee joint as the knee bends. The fibula is a shorter and thinner bone running parallel to the tibia on its outside. The joint acts like a hinge but with some rotation.
The knee is a synovial joint, which means it is lined by synovium. The synovium produces fluid lubricating and nourishing the inside of the joint and the articular cartilage is the smooth shock absorbing surfaces at the end of the femur and tibia. It is the damage to this surface which causes arthritis. This surface is like the firm rubber of a car tyre.
Bands of tissue, including the cruciate and collateral ligaments, keep the different bones of the knee joint together and provide stabilization to the joint. Surrounding muscles are connected to the knee bones by tendons. The bones work together with the muscles and tendons to provide mobility to the knee joint. The whole knee joint is covered by a ligamentous capsule, which further stabilizes the joint. This ligamentous capsule is also lined with the synovial membrane that secretes synovial fluid for lubrication.
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.
The two femoral condyles make up for the rounded lower end of the femur. Its smooth articular surface allows the femur to move easily over the tibial (shinbone) surface and meniscus.
The tibia (shinbone), the second largest bone in the body, is the weight bearing bone of the leg. The menisci (medial and lateral meniscus: there are 2 of them) incompletely cover the superior surface of the tibia where it articulates with the femur. The menisci also act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee.
The fibula, although not a weight bearing bone, provides attachment sites for the Lateral Collateral Ligament (LCL) and the biceps femoris tendon.
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 and taking part in ankle movements further down the leg.
The patella (kneecap), attached to the quadriceps tendon above and the patellar tendon 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. It increases the power of the leg for all bending movements.
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 (this varies considerably with age) 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 especially for high speed rotational movements). The ACL is located in the centre of the knee joint and runs from the femur (thigh bone) to the tibia (shin bone), through the centre 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. Thus, 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)
Much less research has been done on the posterior cruciate ligament (PCL) because it is injured far less often than the ACL.
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.
Collateral Ligaments prevent hyperextension, adduction, and abduction
- Superficial MCL (Medial Collateral Ligament) connects the medial epicondyle of the femur to the medial condyle of the tibia below the joint line, and resists valgus force
- Deep MCL (Medial Collateral Ligament) connects the medial epicondyle of the femur with the medial meniscus and immediately adjacent tibia.
- LCL (Lateral Collateral Ligament) entirely separate from the articular capsule, connects the lateral epicondyle of the femur to the head of the fibula and resists varus force.