The knee: joint structure, movements and muscles

The knee not only helps us bend our legs but it also helps us remain flexible in life. If you feel stuck, there is a high likelihood of imbalance in your knees as well. Learn more about the complexities of this essential joint...

​Joint structure

The knee joint is the meeting place of the condyles of the femur and the superior surface of the head of the tibia. The posterior surface of the patella is also a part of the joint. The articular cartilage of the patella is very thick to protect it from the significant loads experienced during running, jumping and climbing stairs (up to 6 times bodyweight).
 
As a hinge joint, it can move around one axis only – transverse – performing flexion and extension. As we’ll see the knee is remarkably complex for a joint capable of only two movements.
 
The knee is one of the most complex and also the largest synovial joint in the body. There are two articulations in the knee: tibiofemoral and patellofemoral.
 
The tibiofemoral articulation is composed of the two large femoral condyles, which rest on the somewhat flat tibial plateau. This inherently unstable articulation is supported by ligaments (statically) and muscles (dynamically). The concave femoral trochlea and the convex patellar surface also play a role in stabilising the knee with their geometry.
 
The convex patella lying within the trochlear groove of the femur comprise the patellofemoral articulation.
 
The whole joint is surrounded by a capsule. At the front, however, the tendon of the quadriceps takes its place. The joint is secured by distinct bands forming strengthening fibres and expansions from the tendons of adjacent muscles. This thin, strong membrane is augmented by the tendons of the surrounding muscles and by the fascia latae. It attaches to the femur proximally and to the sides of the patella, the margins of the head of the tibia, and to the menisci, distally. 

Ligaments
 
The anterior ligament (ligamentum patellae) is a strong flat band, which is essentially the tendon of the quadriceps. It attaches to the apex and margins of the patella proximally, and to the anterior tubercle of the tibia distally. 

The posterior ligament superiorly attaches to the popliteal surface of the femur just above the intercondyloid notch and the two condyles. Inferiorly it is attached to the posterior border of the head of the tibia. There is also an opening for the tendon of popliteus on the fibular side. An expansion of the tendon of semimembranosus called the oblique popliteal ligament strengthens the posterior ligament.

The tibial collateral ligament, also called the medial collateral ligament, has two parts. These are called the anterior (long) and the posterior (short) portion. They are attached proximally to the medial epicondyle of the femur. Distally, the anterior portion is attached to the proximal part of the medial surface of the shaft of the tibia. The posterior portion is attached to the posterior part of the medial condyle of the tibia, just proximal to the insertion of the tendon of semimembranosus.
 
Proximally, the fibular collateral ligament, also called lateral collateral ligament, is attached to the lateral epicondyle of the femur. Distally, it attaches to the lateral side of the head of the fibula, in front of the styloid process.
 
The short fibular collateral ligament is a fibrous band lying behind. Proximally, it is attached to the lower and posterior part of the lateral epicondyle of the femur, and distally to the apex of the head of the fibula.

There are cruciate ligaments within the joint that cross each other like braces.
 
The anterior cruciate ligament (ACL) attaches distally to the medial side of the rough depression in front of the intercondyloid eminence of the tibia. Proximally, it is attached to the medial and posterior surface of the lateral condyle of the femur. The ACL has two important functions. It prevents excessive internal rotation and helps prevent anterior displacement of the tibia on the femur.
 
Distally, the posterior cruciate ligament (PCL) is attached to the posterior part of the depression behind the intercondyloid eminence of the tibia. Proximally, it attaches to the lateral and anterior part of the medial condyle of the femur. The two main functions of the PCL are to link between the posterior cortex of the femur and the posterior cortex of the tibia and to prevent posterior displacement of the tibia.

There are two crescent-shaped fibro-cartilages called the lateral and medial menisci. They lie between the articular surfaces of the femur and tibia. The menisci attach to the margins and serve to deepen the articular surface of the tibia. The horns of both cartilages attach to the proximal surface of the tibia. The horn of the lateral meniscus lies between those of the medial meniscus.
 
The anterior margins of the lateral and medial menisci are connected by the transverse ligament.
 
The coronary ligaments consist of a number of short fibrous bands which attach the menisci to the margins of the head of the tibia.
 
Once a ligament has been injured, excessive movement and displacement are to be expected.
 
There are deposits of fat in the folds of the synovial membrane. The wedge of fat between the patella, the tibia, and the condyles of the femur is called the infrapatellar pad.

We can find the largest synovial membrane in the knee. It is also very elaborate. It lines the capsule, invests the menisci, and, partially, the cruciate ligaments. The synovial membrane also lines the deep surface of the common extensor tendon. It is prolonged upon the distal part of the front of the shaft of the femur.
 
There is a large bursa, which is usually continuous with the synovial membrane, above the joint on the front of the femur.
 
Bursae are also found between the anterior surface of the patella and the skin over it. There is another one between the ligamentum patellae and the tubercle of the tibia.  In relation to the tendon of semimembranosus and the medial head of gastrocnemius, there is yet another bursa. 
 
Movements and muscles
 
The anterior stability of the knee, mainly a function of the ACL, is supported by the pull of the hamstrings, the menisci and knee flexion. This is counterbalanced by the anterior pull of the quadriceps and the momentum of extending the knee. 

As previously stated, the knee is a hinge joint capable of flexion and extension only.
 
The muscles responsible for flexing the joint are the hamstrings (biceps femoris, semimembranosus and semitendinosus), gastrocnemius, plantaris and popliteus.

​Extension is enabled by the quadriceps (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis). 

Take good care of your knees and you'll never feel stuck.

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