The varieties of joints in this class have been determined by the kind of motion permitted in each i.e. number of axes about which the gross visible motion occurs. A further subdivision of the joints is made on the basis of shape and configuration of the ends of the bony components.
Diarthrodial are mainly of three types.
UNIAXIAL JOINT : In this joint the visible motion occurs only in one plane of the body around a single axis. The axis of the motion usually is located near or center of the joint. As the uniaxial joints allow movement only in a single axis they are said to have 1° of freedom of motion.
Two types of uniaxial diarthrodial joints are found in human body. Hinge joint or ginglymus joint and pivot joint or trochoid joint.
Ginglymus or Hinge-joint — This joint is called so because it resembles a door hinge. In this form the articular surfaces are moulded to each other in such a manner as to permit motion only in one plane, forward and backward, the extent of motion at the same time being considerable. The direction which the distal bone takes in this motion is seldom in the same plane as that of the axis of the proximal bone; there is usually a certain amount of deviation from the straight line during flexion. The articular surfaces are connected together by strong collateral ligaments, which form their chief bond of union. The best examples of ginglymus are the interphalangeal joints and the joint between the humerus and ulna. Knee and ankle joints are less typical, as they allow a slight degree of rotation or of side-to-side movement in certain positions of the limb.
Trochoid or Pivot-joint — It is a type of joint where the movement is limited to rotation. The joint is formed by a pivot-like process turning within a ring, or a ring on a pivot, the ring being formed partly of bone, partly of ligament. In the proximal radioulnar articulation, the ring is formed by the radial notch of the ulna and the annular ligament and the head of the radius rotates within the ring. In the articulation of the odontoid process of the axis with the atlas the ring is formed in front by the anterior arch, and behind by the transverse ligament of the atlas; here, the ring rotates around the odontoid process.
BIAXIAL DIARTHRODIAL JOINTS : A biaxial diarthrodial joint permits the motion in two planes around two axes. These joints have 2° degree of freedom. Condyloid and saddle joints represent biaxial joints.
Condyloid Joints—In this form of joint, an ovoid articular surface, or condyle, is received into an elliptical cavity in such a manner as to permit of flexion, extension, adduction, abduction, and circumduction, but no axial rotation. The joint surfaces are shaped in such a manner that the concave surface of one bony component is allowed to slide over the convex surface of another component in two directions. The wrist-joint is an example of this form of articulation. Another example is metacarpophalangeal joint.
Saddle Joints—In this variety the opposing surfaces are reciprocally concavo-convex. The movements are the same as in the preceding form; that is to say, flexion, extension, adduction, abduction, and circumduction are allowed; but no axial rotation. The best example of this form is the carpometacarpal joint of the thumb.
TRIAXIAL OR MULTIAXIAL JOINTS
These joints permit movement in three planes around three axes. Thus these joints have 3° of freedom of motion. Motion at these joints may also occur in oblique planes. The two types of joints in this category are plane joints and ball and socket joints.
Plane or Gliding joints: These joints permit gliding movement only it is formed by the apposition of plane surfaces, or one slightly concave, the other slightly convex, the amount of motion between them being limited by the ligaments or osseous processes surrounding the articulation. It is the form present in the joints between the articular processes of the vertebrae, the carpal joints, except that of the capitate with the navicular and lunate, and the tarsal joints with the exception of that between the talus and the navicular.
Ball and socket joints — These are the joints in which the distal bone is capable of motion around an indefinite number of axes, which have one common center. It is formed by the reception of a globular head into a cup-like cavity, hence the name ball-and-socket. Examples of this form of articulation are found in the hip and shoulder.
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