These are the places where bones meet. Another name for articulation is joint. Arthrology is the study of joints.
A. Functions - The most obvious function of a joint is to permit movement. Some joints do not permit movement and therefore function to rigidly joint two bones.
l. Fibrous - These joints are held together by tough, nonelastic tissue. They lack a joint cavity. There are three specific types.
a. Suture - These are found only between membranous bones and hence only in the skull. The bones have jagged irregular edges that interlock, leaving only a very small space between the bones which is spanned by the connective tissue, which blends into the periosteum. They are immovable (synarthrotic)
c. Gompphoses - This is the joint that connects the tooth to the bony alveolar socket. Immovable.
2. Cartilaginous joints – The bones are connected by cartilage. There is no joint cavity. There are two types of joints.
a. Synchondroses – The bones are united by hyaline cartilage. These are almost always synarthrotic. The epiphyseal plates are the most widespread example.
b. Symphysis - Here the bones are joined by a pad of fibrocartilage. These joints are usually reinforced significantly by ligaments yielding a very strong joint, but one that is slightly movable (amphiarthrotic). These joints also cushion against impact as well as connect. Examples include the intervertebral disks and the pubic symphysis.
3. Synovial –diarthrotic - These are the most widely distributed and the most complex of joints. Characteristically, the bones meet inside of a fluid filled capsule. The basic components of a synovial joint are as follows.
a. Articular capsule - This is a capsule of dense connective tissue that surrounds the joint. It is continuous with the periosteum of the bones and is reinforced by ligaments which may be either separate bands or thickenings of the capsule itself. This capsule contains the fluid and physically holds the bones together.
b. Articular cartilage - This is a thin layer of hyaline cartilage that covers the ends of the articulating bones. It makes the ends of the bones very smooth and thereby greatly reduces friction during movement.
c. Synovial membrane - This membrane lines the articular capsule. It produces synovial fluid which fills the joint cavity. This fluid functions to protect and lubricate the joint, as well as distribute nutrients. Besides its role in the joint cavity, synovial membrane forms two other structures which are not part of the joint, but are frequently associated with synovial joints and movement. These are as follows.
(1) Synovial bursae - These are connective tissue sacs which are lined by synovial membrane. The membrane produces the fluid which fills the sacs. Bursae are usually found between structures which rub one another. They function to reduce friction. Bursitis is a condition whereby the bursae become inflamed and swell up.
(2) Tendon sheaths - These resemble tubular synovial bursae. They surround large tendons that must move long distances or which are subject to pressure. Again, the function is friction reduction.
4. Types of synovial joints - Synovial joints are classified based upon the type of movement which they permit.
a. Uniaxial joints - These permit movement in one plane only. There are two basic types.
(1) Hinge joint - These have a single transverse axis much like a door. Examples include the elbow, knee, and interphalangeal joints.
(2) Pivot joints - These have a single axis which is the long axis of the bones involved. The only example is the radio-ulnar joint where the head of the radius pivots without displacement while the distal end of the radius rotates around the ulna.
b. Biaxial joints - These joints permit movement in two planes. There are two types of this joint.
(1) Condyloid joint - The articular surface of one bone is concave and the other is convex. Such joints not only have a transverse axis, but an anterior-posterior axis as well. Examples include the radio-carpal joint and the metacarpal-phalangeal joint.
(2) Saddle joint - This is a unique joint found only in at the carpo-metacarpo joint of the thumb. The articular surface is saddle shaped. Movement is the essentially the same as the condyloid joint. It is this joint that results in the opposable thumb which makes the human hand such a unique manipulating tool.
c. Multiaxial joints - These joints might be called universal joints because they permit movement in many planes. There is but one type.
(l) Ball and socket - The name tells the structure. One bone has a ball at one end which fits into a cup shaped depression on the second bone. There are two of these in the body. There is the hip joint (femur-os coxa) and the shoulder joint (humerus-scapula).
d. Nonaxial (gliding) joints - This type of joint permits a slight sliding movement in several different directions. They are sometimes considered multiaxial, but the amount of movement they permit is so slight it seems that to consider them non-axial is more appropriate. These are the joints found between the carpals and the tarsals.
C. Types of movement - The various synovial joints previously described permit several different varieties of movement. Usually there are two movements which are the opposite of one another. The reasons for this will become clear later when the actions of skeletal muscles are discussed. The major types of movement are listed below. The type of joint which each movement is characteristic of is included in parentheses.
l. Flexion - This decreases the angle between two bones (hinge joints).
2. Extension - This increases the angle between two bones (hinge joints).
3. Supination - Rotation of the hand so that the palm is up (pivot joint).
4. Pronation - Rotation of the hand so that the palm is down (pivot joint).
5. Abduction - Movement away from the midline (condyloid joint).
6. Adduction - Movement toward the midline (condyloid joint).
7. Circumduction - Rotation of a limb so that a cone is circumscribed (ball and socket joint).
8. Inversion - The medial movement of the foot (gliding joint).
D. Special joints - Below are listed several joints of the body that have unique or special characteristics.
l. Tempro-mandibular joint - This joint is unique due to the fact that there are two joint cavities. This allows for typical hinge type operations, opening and closing (depression-elevation), and for two other kinds of movement, protraction-retraction, and lateral movement.
2. Elbow joint – This joint forms a hinge with the humerus by means of the troclea for the ulna and capitulum for the radius. The radius also participates in a pivot joint with the ulna.
3. Gleno-humeral joint - This is a ball and socket joint with a very shallow socket and a slightly loose connection. This permits great mobility in the shoulder, but the shallowness of the socket results in easy displacement.
4. Hip joint - This ball and socket joint has an extremely deep socket and is reinforced by internal ligaments that connect the head of the femur to the os coxa. As a result the joint is extremely strong, the strongest of the synovial joints.
5. Knee joint - This is a hinge joint with relatively flat articulating cartilages. Because of the flatness of the joining bones the joint depends entirely upon ligaments to prevent side (lateral) movement. For these reasons, and because of its location and constant use, it is the most vulnerable joint in the body as far as damage is concerned.
E. Anatomy of a representative joint - The knee joint has been chosen for a detailed examination. This is because in may respects it is a typical synovial joint, but also because it is probably the most frequently damaged joint in the body.
1. The joint occurs between the femur and the tibia. The other lower leg bone, the fibula, joints directly to the tibia and does not articulate with the femur.
2. Two semilunar cartilages, the medial and lateral meniscus, lie on the medial and lateral condyles of the tibia. These are the cartilages which are frequently damaged in athletic injuries.
3. The articular capsule is reinforced by lateral and medial collateral ligaments, a posterior ligament, and an anterior patellar ligament.
4. Inside of the articular capsule are two other ligaments, the cruciate ligaments, which connect the femur to the tibia. They are arranged in a crossing fashion. This means that one will be taut when the leg is extended and its opposite slack. When the leg is flexed the previously slack one becomes taut and its opposite goes slack.
5. The patella protects the joint anteriorly. It is inserted in the broad patellar tendon which inserts on the tibia right below the knee joint. The origin of this tendon is the large anterior thigh muscles which extend the leg.
6. Several synovial bursae lie between the skin and the patellar tendon and several more lie between the tendon and the joint itself. Frequently blows to the knee cause these bursae to swell and accumulate fluid. This is sometimes referred to as water on the knee.
F. Disorders - The joints of the body are subject to a number of problems. Some of these are due to disease, others to injuries, and still others due to the aging process. Three of the more common are listed below.
1. Dislocation - This involves displacement of the articular surfaces. This is more common where the articular surfaces do not interlock in some fashion. Usually there is damage to the surrounding tissues as well.
2. Sprains - This involves damaged ligaments, frequently torn. It results from the twisting or wrenching of a joint.
3. Arthritis - This term can include a number of different diseases, all of which involve inflammation of joints. The result is swelling and pain with movement. In rheumatoid arthritis degeneration of the joints often results in fusion of the bones together and a complete loss of mobility.