Joints and Ligaments

Joints of the Lower Extremity.

The sacro-innominate articulation consists of the sacroiliac joint and the sacro-sciatic ligaments. The former is one of the amphiarthroses or half-joints by which the sacrum is bound to the ilium. The mechanism of the human sacrum is that of a suspension bridge slung between the two pillars or ilia by the very strong posterior sacroiliac ligaments which represent the chains. The axis of the joint passes through the second sacral vertebra, but the sacrum is so nearly horizontal that the weight of the body, which is transmitted to the first sacral vertebra, tends to tilt that part down. This tendency is corrected by the sacro-sciatic ligaments, which fasten the lower part of the sacrum to the tuberosity and spine of the ischium, so that, although the sacrum is a suspension bridge when looked at from behind, it is a lever of the first kind when seen from the side or in sagittal section.

The pubic symphysis is the union between the two pubic bones. It has all the characteristics of a symphysis, already described, and may have a small median cavity.

Hip Joint.

This, like the shoulder, is a ball and socket, but does not allow such free movement because the socket or acetabulum is deeper than the glenoid cavity and the capsule is not so lax. At the same time the loss of mobility is made up for by increased strength. The capsule has three thickened bands of which the most important is the ilio-femoral or Y-shaped liga ment of Bigelow. The stalk of the Y is attached to the anterior inferior spine of the ilium, while the two limbs are fastened to the upper and lower parts of the spiral line of the femur. The ligament is so strong that it hardly ever ruptures in a dislocation of the hip. As a plumb-line, dropped from the centre of gravity of the body, passes behind the centre of the hip joint, this ligament, lying as it does in front of the joint, takes the strain in Man's erect position. The other two thickened parts of the capsule are known as pubo-femoral and ischio-femoral, from their attach ments. Inside the capsule, and deepening the margin of the acetabulum, is a fibrous rim known as the cotyloid ligament, which grips the spherical head of the femur and is continued across the cotyloid notch as the transverse ligament. The floor of the acetabulum has a horseshoe-shaped surface of articular cartilage, concave downward, and, occupying the "frog" of the horse's hoof, is a mass of fat called the Haversian pad. Attached to the inner margin of the horseshoe, and to the transverse ligament where that is deficient, is a reflexion of synovial membrane which forms a covering for the pad and is continued as a tube to the de pression on the head of the femur. This reflexion carries blood vessels and nerves to the femur, and also contains fibrous tissue from outside the joint. It is known as the ligamentum teres.

Comparative Anatomy. Bland Sutton regards the ilio-femoral ligament as an altered muscle, the scansorius, and the ligamentum teres as the divorced tendon of the pectineus muscle. The subject

requires much more investigation. In many Mammals, and among them the Orang, there is no ligamentum teres. In others, such as the Armadillo, the structure has not sunk right into the joint, but is connected with the pubo-femoral part of the capsule.

Knee Joint.

This is a hinge formed by the condyles and trochlea of the femur, the patella, and the head of the tibia. The capsule is formed in front by the ligamentum patellae, and on each side special bands form the lateral ligaments. On the outer side there are two of these : the anterior or long external lateral liga ment is a round cord running from the external condyle to the head of the fibula, while the posterior is slighter and passes from the same place to the styloid process of the fibula. The internal lateral ligament is a flat band which runs from the inner condyle of the femur to the internal surface of the tibia some two inches below the level of the knee joint. The posterior part of the cap sule is strengthened by an oblique bundle of fibres running upward and outward from the semimembranosus tendon (posterior liga ment of Winslow).

The intra-articular structures are numerous and interesting. Passing from the head of the tibia, in front and behind the spine, are the anterior and posterior crucial ligaments. These cross like an X. The semilunar fibro-cartilages---external and internal—are partial menisci, each of which has an anterior and a posterior cornu by which they are attached to the head of the tibia in front and behind the spine. They are also attached round the margin of the tibial head by a coronary ligament, but the external one is more movable than the internal, and this perhaps accounts for its coronary ligament being less often ruptured and the cartilage displaced than the inner one is. The external cartilage is broader, and forms more of a circle than the internal. The synovial cavity of the knee runs up, deep to the extensor muscles of the thigh, for about two inches above the top of the patella, forming the bursa suprapatellaris. At the lower part of the patella it covers a pad of fat, which lies between the ligamentum patellae and the front of the head of the tibia, and is carried up as a narrow tube to the lower margin of the trochlear surface of the femur. The tendon of the popliteus muscle is an intracapsular structure, and is therefore covered with a synovial sheath. There are a large number of bursae near the knee joint, one of which, common to the inner head of the gastrocnemius and the semimembranosus, often communicates with the joint. The hinge movement of the knee is accompanied by a small amount of external rotation at the end of extension, and a compensatory internal rotation during flexion. This slight twist is enough to tighten up almost all the ligaments so that they may take a share in resisting over-exten sion, because, in the erect position, a vertical line from the centre of gravity of the body passes in front of the knee.