Knee Joint

Knee joint is the largest and most complicated joint of human body. Basically, it consists of three bones and two joints. The bones are Femur, Tibia and Patella while the joints are tibiofemoral and patellofemoral joints. The tibiofemoral joint is a complex hinge joint between tibia and femur. The patellofemoral joint between femur and patella is a gliding synovial joint.

Articular surfaces of knee joint:

Superiorly, there are rounded condyles of femur and below are the condyles of tibia and their cartilaginous menisci. In the front, the articulation takes place between more or less flat surface of patella and lower end of femur on the anterior side. The articular surfaces of femur, tibia and patella are all covered with hyaline cartilage.

Capsule of knee joint:

The capsule of knee joint is attached to the margins of articular surfaces. It surrounds the sides and the posterior aspect of the joint. Anteriorly, the capsule is absent, thus allowing the synovial membrane to pouch upward beneath the quadriceps tendon, forming the suprapatellar bursa.
The capsule is strengthened on either side and towards the back by corresponding muscles and ligaments. On the lateral side, there is tendon of vastus lateralis muscle to strengthen the joint. On medial side, there is tendon of vastus medialia. On the posterior side, the joint gains additional strength by an expansion of the semimembranosus muscle called the oblique popliteal ligament.

Ligaments of knee joint:

Because knee is a complex joint consisting of a number of important ligaments, the same are divided into two categories: Extracapsular (those that lie outside the capsule) and Intracapsular (those that lie within the capsule).

Extracapsular ligaments:

  • Ligamentum Patellae: It is attached above to the lower border of patella and below to the tuberosity of tibia. In fact, this ligament is a continuation of the common tendon of the quadriceps femoris muscle containing the patella as a sesamoid bone.
  • Medial Collateral Ligament: It is flat in form, attached above to the medial condyle of femur and below to the shaft of tibia. It is firmly attached to the edge of medial meniscus.
  • Lateral Collateral Ligament: It is cord-like in form, attached above to the lateral condyle of femur and below to the head of fibula. Unlike the medial collateral ligament, it does not attach to the lateral meniscus. The tendon of popliteus muscle intervenes between the two.
  • Oblique Popliteal Ligament: It lies towards the posterior aspect of the joint. It is actually a tendinous expansion derived from the semimembranosus muscle. It strengthens the posterior aspect of the capsule of knee joint.

Intracapsular ligaments:

Inside the capsule, there are two strong ligaments that cross each other within the joint cavity. They are named according to their attachment to tibia as anterior cruciate and posterior cruciate ligaments. They are very important ligaments of the knee joint because they form the primary bond between tibia and femur through the entire range of movement.

  • Anterior Cruciate Ligament: It is attached to the anterior intercondylar area of tibial head, from where it passes upward, backward and laterally to be attached to the medial surface of lateral condyle of femur. In an extended knee joint, it prevents posterior displacement of the femur on tibia. With the knee joint flexed, it prevents the tibia from being pulled anteriorly.
  • Posterior Cruciate Ligament: It is attached to the posterior intercondylar area of tibial head, from where it passes upward, forward and medially to be attached to the anterior part of lateral surface of medial femoral condyle. With the knee joint extended, it prevents anterior displacement of femur on tibia, while in the flexed condition; it prevents the tibia from being pulled posteriorly.

Menisci of knee joint:

Knee joint contains two menisci: lateral and medial. These are C-shaped sheets of cartilage presents between the articular surfaces of tibia and femur. Their peripheral border is thick and attached to the capsule of joint. The inner border is thin and concave, forming a free edge. The upper surfaces of these menisci are in contact with the femoral condyles while the lower surfaces are in contact with tibial condyles. Each meniscus is attached to the upper surface of tibia by two horns: anterior and posterior. In contrast to the lateral meniscus, medial meniscus is also attached to the medial collateral ligament and is therefore relatively immobile. The menisci perform two important functions;

  • They deepen the articular surfaces of tibia so that the femoral condyles (that are convex in form) can fit in better.
  • They serve as cushions between the two bones and protect them from any type of damage during forces generated by activities like jumping or kicking etc.

Synovial membrane of knee joint:

The synovial membrane lines the capsule of the joint and is attached to the margins of articular surfaces. At numerous places, the membrane protrudes from the joint capsule forming numerous bursae including suprapatellar, popliteal and semimembranosus bursae.
From the posterior part of the capsule, the synovial membrane is reflected forward. Consequently, the cruciate ligaments lie behind the synovial cavity and are not bathed in synovial fluid.

Bursae related to knee joint:

Numerous bursae are related to knee joint. They are found in places where skin, tendon or muscle rubs against the bone. Four of these bursae are situated in front while six of them are behind the joint.

Bursae Anterior to the joint:

  • Suprapatellar bursa: It lies beneath the quadriceps muscle and communicates with the joint cavity. It extends upward beneath the quadriceps femoris muscle. It is held in its position by a small portion of the vastus intermedius muscle known as articularis genus.
  • Prepatellar bursa: It lies in the subcutaneous tissue between the skin and the front of the patella and upper part of ligamentum patellae. It reduces the friction when the skin is rubbed against this ligament.
  • Superficial Infrapatellar bursa: It lies in the subcutaneous tissue between the skin and lower part of ligamentum patellae.
  • Deep Infrapatellar bursa: It lies between the ligamentum patellae and the tibia.

Bursae Posterior to the joint:

  • Popliteal bursa: It is found in association with the tendon of the popliteus. It communicates with the joint cavity.
  • Semimembranosus bursa: It is found related to the insertion of semimembranosus muscle. It is interposed between the medial head of the gastrocnemius and the medial condyle of femur and semimembranosus muscle. It may or may not communicate with the joint cavity.
  • The remaining four bursae are found related to the tendon of insertion of the biceps femoris, Sartorius, gracilis and semitendinosus muscles as they pass to their insertion on tibia.

Blood supply of knee joint:

There is a complex anastomosis around the patella and condyles of tibia and femur. This anastomosis consists of a superficial network and a deep network. The superficial network spreads between the fascia and the skin around patella. It also supplies the fat deep to patellar tendon. The deep network lies on the femur and tibia near the adjoining articular surfaces, and supplies the bone, the joint capsule, synovial membrane and the cruciate ligaments.
The arteries involved are superior, middle and inferior genicular branches of popliteal artery, descending genicular branches of femoral artery, lateral circumflex femoral artery and the circumflex fibular artery.
The venous drainage corresponds in name to the arterial supply and runs with it. The smaller veins eventually drain into the femoral and popliteal veins.

Lymphatic drainage of Knee joint:

The lymphatic drainage of knee joint is to popliteal nodes. Most of the lymph vessels accompany the genicular arteries. The popliteal nodes drain into the inguinal group of lymph nodes.

Nerve Supply to knee joint:

The knee joint receives its nerve supply from branches of the obturator, femoral, tibial and common peroneal nerves.

Locking and unlocking of knee joint:

In the position of full extension, the femur is medially rotated on the tibia to lock and stabilize the joint. This locking and stabilization result from twisting and tightening of all the major ligaments of the joint. Thus the extended knee is said to be in locked position.
Before flexion can occur, it is essential that the major ligaments be untwisted and slackened to permit movement between the joint surfaces. This important action of unlocking or untwisting is performed by the popliteus muscle, which laterally rotates femur on tibia.

Movements of knee joint:

The knee joint is basically a hinge joint thus the primary movements are flexion and extension. However, it also allows a certain degree of rotation. The range of rotation is considerable when the knee joint is flexed to a right angle. It this position, the tibia can also be moved forward and backward passively. It is possible because all the major ligaments, especially the cruciate ligaments are loose in this position.
Muscles producing movements:

  • Flexion: Biceps femoris, Semitendinosus and Semimembranosus. These are assisted by Gracilis, Sartorius, and Popliteus.
  • Extension: Quadriceps femoris.
  • Medial Rotation: Sartorius, Gracilis and Semitendinosus.
  • Lateral Rotation: Biceps femoris.

Stability of knee joint:

Stability of this joint primarily depends on the tone of the muscles acting on it. The strength of the ligaments around the joint is also an important factor.