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THE KNEE JOINT

The knee joint is a compound, condylar synovial joint, in which more than two

bones are involved in its formation. Three bones make up the knee joint, namely

(i) patella (ii) distal end of femur and (iii) proximal end of tibia. The fibula doesnot contribute to the knee joint. The knee joint is the largest and most elaborate in

the body. It is primarily a hinge joint, allowing only flexion and extension. [It can

allow some degree of rotation when flexed, but not when fully extended].

Articular surfaces

These are formed by the femoral condyles, the patella and the tibial condyles. All

articular surfaces are covered by articular cartilage. The patella articulates only

with the femur. The femoral condyles articulate with the tibial condyles. However,

these surfaces are not congruent. Thus the presence of two menisci renders thesesurfaces congruent.

Two major components can be recognized within the knee joint. These are:

(A) The femoropatellar joint

(B) The femorotibial joint

However, these two joints are continuous with each other.

(A) Femoropatellar joint

This is formed between the femoral trochlear and the patella. The latter is the

largest sesamoid bone in the body. The patellas anterior surface is roughened for

the attachment of the quadriceps femoris tendon and patella ligament inferiorly.

The posterior surface is smooth, being covered by hyaline cartilage, and exhibits a

vertical midline ridge. The patella runs along the trochlear groove during flexion

and extension of the knee joint. Hence the patella acts as a pulley, increasing the

mechanical advantage of the quadriceps femoris muscle during extension of the

knee joint. The patella ligament is considered to be the tendon of insertion of thequadriceps femoris muscle. This ligament attaches to the tibial tuberosity of the

tibia.

(B) The femorotibial joint

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This is constituted by the condyles of the femur and the condyles of the tibia.

Interposed between the condyles are two fibrocartilage discs, called the medial and

lateral menisci. Each meniscus is C-shaped, having a thick convex peripheral

border, and a thin concave central border. [The outer curved border of eachmeniscus is attached to the joint capsule by the loose coronary ligament]. The

lateral meniscus is a more sharply curved C than the medial meniscus. The thin

tips of each meniscus are attached to the intercondylar area of the tibia. The

anterior convex margins of both menisci are attached to each other by a transverse

ligament. [In addition, the outer margin of the medial meniscus is firmly attached

to the deep surface of the tibial collateral ligament]. The menisci may contribute

to weight-bearing, but their main function is to increase congruity of the apposing

articular surfaces. [The inner concave margins and both the upper and lower

surfaces of the menisci project freely into the joint cavity]. A narrow posterior

meniscofemoral ligament connects the posterior convex surface of the lateralmeniscus to the medial femoral condyle. It runs just posterolateral to the posterior

cruciate ligament.

The Joint cavity and joint capsule

The joint capsule is attached along the articular margins of the femoral condyles

medially and laterally. It also attaches just above the intercondylar notch

posteriorly. Anteriorly, the joint cavity communicates with the suprapatellar

bursa. The latter lies between the distal end of the femur and the quadricepsfemoris muscle and tendon. The suprapatellar bursa is a good site for obtaining

synovial fluid samples or relieving severe swelling of the joint (say, when

inflamed). This archieved by inserting a needle along either the lateral or medial

margin of the patella.

Two other bursae may exist. These are the prepatellar and infrapatellar bursae.

The prepatellar bursa is subcutaneous, lying just anterior to the patella. When this

bursa is inflamed, there is pain and reduced mobility of the joint. The condition is

known as the housemaids knee. The infrapatellar bursa lies between the

patellar ligament and the proximal end of tibia. Both the prepatellar andinfrapatellar bursae do not communicate with the joint cavity. Also, distal to the

patella, the fibrous and synovial layers of the capsule are separated by the

intracapsular fat body.

Other ligaments

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(a) Collateral ligaments

There are collateral ligaments namely, the fibular (lateral) and the tibial (medial)

collateral ligaments.

(i) The fibular collateral ligament is a strong, rounded cord. It extends from the

lateral femoral epicondyle to the head of the fibula. It is separated fro the lateral

meniscus by the tendon of the popliteus muscle. The fibular collateral ligament is

rarely damaged.

(ii) The tibial collateral ligament is a broad, flattened ligament. Hence it is much

weaker than the fibular collateral ligament and so it is more often damaged. It

extends from the femoral epicondyle to just below the medial condyle of tibia.

Collateral ligaments reinforce the medial and lateral aspects of the knee joint,helping to prevent side movements.

(b) Cruciate ligaments

These are (i) the anterior and (ii) posterior cruciate ligaments. They are

centrally-placed ligaments and are covered by a synovial membrane. They are two

strong bands which cross each other as they extend between the femur and tibia,

hence the name cruciate (Crux = Cross). These ligaments are named according to

their attachment to tibia. The anterior cruciate ligament extends from the anteriortibial intercondylar area to the medial surface of the lateral condyle of femur. Thus

it runs posterosuperiorly. The posterior cruciate ligament extends from the

posterior tibial intercondylar area to the lateral surface of the medial condyle of

femur. Thus it runs anterosuperiorly. The anterior cruciate ligament is always

lateral to the posterior one. The two cruciate ligaments provide stability to the knee

joint. The anterior cruciate ligament prevents forward sliding of the tibia on the

femur. Likewise, the posterior cruciate ligament prevents backward sliding of the

tibia on the femur. Rupture of these ligaments can be diagnosed by testing for these

movements when the knee is flexed. In this position, when rupture has occurred,

the leg will be able to slide excessively forward or backward with respect to thefemur. The condition is known as the drawer sign.

Movements

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The overall movements of the knee joint are flexion and extension. In addition,

there is slight medial rotation of the femur with respect to the tibia. This occurs as

the joint becomes fully extended. In a relaxed standing position, the knee joint is

actually slightly hyper-extended, and the knee is said to be in a locked position.

Under these circumstances, the muscles of the thigh and leg can relax, so that thisposition can be maintained with minimum expenditure of energy.

Blood and nerve supply to the knee

Blood supply is derived from the superior, middle and inferior genicular branches

of the femoral and popliteal arteries. These branches form a rich anastomosis

around the knee joint. This anastomosis forms an important collateral circulation

that may serve to bypass the main vessels when these become narrowed or

occluded.

Innervation of the knee joint is from the nearby somatic nerves. These are the

sciatic, femoral and obturator nerves.

The popliteal fossa

This is a depression situated just posterior to the knee. It is a diamond-shaped area

through which important nerves and vessels pass from the thigh to the leg.

Boundaries of the popliteal fossa

The fossa is bounded inferiorly by the lateral and medial heads of the

gastrocnemius muscles. Superiorly, it is bounded by the biceps femoris muscle on

the lateral side, and the margins of the semimembranosus and semitendinosus

muscles on the medial side. The roof is formed by the deep fascia of the thigh

(fascia lata). The floor is formed by the posterior aspect of the distal end of the

femur and the popliteus muscle.

Contents of the popliteal fossa

(a) Vessels of the popliteal fossa

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The popliteal artery is the continuation of the femoral artery. The name changes

after the femoral artery emerges from the adductor hiatus (of the adductor canal)

enter the fossa. The popliteal artery terminates by dividing into the anterior and

posterior tibial arteries. Within the fossa, it gives off two superior, one middleand two inferior genicular arteries. A popliteal pulse can be felt within the fossa.

The popliteal vein runs upward through the popliteal fossa and then enters the

adductor hiatus. Within the popliteal fossa, it receives the small saphenous vein

and the veins corresponding to the branches of the popliteal artery.

(b) Nerves

The sciatic nerve may divide into its two major branches anywhere in the posterior

thigh. However, this division often occurs as the sciatic nerve enters the poplitealfossa. The two branches are the tibial nerve and the common fibular (peroneal)

nerve. Within the fossa the tibial nerve nerve gives off muscular branches and a

medial sural cutaneous branch. The muscular branches supply the gastrocnemius,

popliteus and soleus muscles.

The common fibular (peroneal) nerve also gives off the lateral sural cutaneous

nerve. A communicating branch from the lateral sural cutaneous nerve joins the

medial sural cutaneous nerve to form the definitive sural nerve.

Other contents of the popliteal fossa

The remaining space of the fossa is filled with fat. Several lymph nodes are

embedded in this fat.