Arthrology (Joints)
The study of joints or articulations is called arthrology. It is important to understand that movement does not occur in all joints. The first two types of joints to be described are immovable joints and only slightly movable joints, which are held together by several fibrous layers, or cartilage. These joints are adapted for growth rather than for movement.
Classification of Joints
Functional
Joints may be classified according to their function in relation to their mobility or lack of mobility as follows:
Structural
The primary classification system of joints, described in Gray’s Anatomy
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and used in this textbook, is a structural classification based on the three types of tissue that separate the ends of bones in the different joints. These three classifications by tissue type, along with their subclasses, are as follows:
Fibrous Joints
Fibrous joints lack a joint cavity. The adjoining bones, which are nearly in direct contact with each other, are held together by fibrous connective tissue. Three types of fibrous joints are syndesmoses, which are slightly movable; sutures, which are immovable; and gomphoses, a unique type of joint with only very limited movement (Fig. 1.23).
Syndesmoses 1
Syndesmoses are fibrous types of articulations that are held together by interosseous ligaments and slender fibrous cords that allow slight movement at these joints. Some earlier references restricted the fibrous syndesmosis classification to the inferior tibiofibular joint. However, fibrous-type connections also may occur in other joints, such as the sacroiliac junction with its massive interosseous ligaments that in later life become almost totally fibrous articulations. The carpal and tarsal joints of the wrist and foot also include interosseous membranes that can be classified as syndesmosis-type joints that are only slightly movable, or amphiarthrodial.
Sutures
Sutures are found only between bones in the skull. These bones make contact with one another along interlocking or serrated edges and are held together by layers of fibrous tissue, or sutural ligaments. Movement is very limited at these articulations; in adults, these are considered immovable, or synarthrodial, joints.
Limited expansion- or compression-type movement at these sutures can occur in the infant skull (e.g., during the birth process). However, by adulthood, active bone deposition partially or completely obliterates these suture lines.
Gomphoses
A gomphosis joint is the third unique type of fibrous joint, in which a conical process is inserted into a socket-like portion of bone. This joint or fibrous union—which, strictly speaking, does not occur between bones but between the roots of the teeth and the alveolar sockets of the mandible and the maxillae—is a specialized type of articulation that allows only very limited movement.
Cartilaginous Joints
Cartilaginous joints also lack a joint cavity, and the articulating bones are held together tightly by cartilage. Similar to fibrous joints, cartilaginous joints allow little or no movement. These joints are synarthrodial or amphiarthrodial and are held together by two types of cartilage: symphyses and synchondroses (Fig. 1.24).
Symphyses
The essential feature of a symphysis is the presence of a broad, flattened disk of fibrocartilage between two contiguous bony surfaces. These fibrocartilage disks form relatively thick pads that are capable of being compressed or displaced, allowing some movement of these bones, which makes these joints amphiarthrodial (slightly movable).
Examples of such symphyses are the intervertebral disks (between bodies of the vertebrae), between the manubrium (upper portion) and body of the sternum, and the symphysis pubis (between the two pubic bones of the pelvis).
Synchondroses
A typical synchondrosis is a temporary form of joint wherein the connecting hyaline cartilage (which on long bones is called an epiphyseal plate) is converted into bone at adulthood. These temporary types of growth joints are considered synarthrodial or immovable.
Examples of such joints are the epiphyseal plates between the epiphyses and the metaphysis of long bones and at the three-part union of the pelvis, which forms a cup-shaped acetabulum for the hip joint.
Synovial Joints
Synovial joints are freely movable joints, most often found in the upper and lower limbs, which are characterized by a fibrous capsule that contains synovial fluid (Fig. 1.25). The ends of the bones that make up a synovial joint may make contact but are completely separate and contain a joint space or cavity, which allows for a wide range of movement at these joints. Synovial joints are generally diarthrodial, or freely movable. (Exceptions include the sacroiliac joints of the pelvis, which are amphiarthrodial, or slightly movable.)
The exposed ends of these bones contain thin protective coverings of articular cartilage. The joint cavity, which contains a viscous lubricating synovial fluid, is enclosed and surrounded by a fibrous capsule that is reinforced by strengthening accessory ligaments. These ligaments limit motion in undesirable directions. The inner surface of this fibrous capsule is thought to secrete the lubricating synovial fluid.
Movement Types of Synovial Joints
There are a considerable number and variety of synovial joints, and they are grouped according to the seven types of movement that they permit. These are listed in order from the least to the greatest permitted movement.
NOTE: The preferred name is listed first, followed by a synonym in parentheses. (This practice is followed throughout this textbook.)
Plane (gliding) joints
This type of synovial joint permits the least movement, which, as the name implies, is a sliding or gliding motion between the articulating surfaces.
Examples of plane joints are the intermetacarpal, carpometacarpal, and intercarpal joints of the hand and wrist (Fig. 1.26). The right and left lateral atlantoaxial joints between C1 and C2 vertebrae are also classified as plane, or gliding, joints; they permit some rotational movement between these vertebrae, as is described in Chapter 8.
Ginglymus (hinge) joints
The articular surfaces of ginglymi, or ginglymus (jin′-gli-mus) joints, are molded to each other in such a way that they permit flexion and extension movements only. The articular fibrous capsule on this type of joint is thin on surfaces where bending takes place, but strong collateral ligaments firmly secure the bones at the lateral margins of the fibrous capsule.
Examples of ginglymi include the interphalangeal joints of fingers and toes and the elbow joint (Fig. 1.27).
Pivot (trochoid) joints
The pivot or trochoid (tro′-koid) joint is formed by a bony, pivot-like process that is surrounded by a ring of ligaments or a bony structure or both. This type of joint allows rotational movement around a single axis.
Examples of pivot joints are the proximal and distal radioulnar joints of the forearm, which demonstrate this pivot movement during rotation of the hand and wrist.
Another example is the joint between the first and second cervical vertebrae. The odontoid process (dens) of the axis (C2) forms the pivot, and the anterior arch of the atlas (C1), combined with ligaments, forms the ring (Fig. 1.28).
Ellipsoid (condylar) joints
In the ellipsoid (e-lip′-soid) joint, movement occurs primarily in one plane and is combined with a slight degree of rotation at an axis at right angles to the primary plane of movement. The rotational movement is limited by associated ligaments and tendons.
This type of joint allows primarily four directional movements: flexion and extension and abduction and adduction. Circumduction movement also occurs; this results from conelike sequential movements of flexion, abduction, extension, and adduction.
Examples of ellipsoidal joints include the metacarpophalangeal joints of the fingers, the radiocarpal (wrist joint), and the metatarsophalangeal joints of the toes (Fig. 1.29).
Saddle (sellar) joints
The term saddle, or sellar (sel′-ar), describes this joint structure well in that the ends of the bones are shaped concave-convex and are positioned opposite each other (Fig. 1.30). (Two saddle-like structures fit into each other.)
Movements of this biaxial type of saddle joint are the same as for ellipsoidal joints—flexion, extension, adduction, abduction, and circumduction.
The best example of a true saddle joint is the first carpometacarpal joint of the thumb. Other sellar joints include the ankle and the calcaneocuboid joints. Although the ankle joint was classified as a ginglymus in earlier references, current references classify it as a saddle joint.
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Ball-and-socket (spheroidal) joints
The ball-and-socket or spheroidal (sfe′-roid-el) joint allows for the greatest freedom of motion. The distal bone (humerus) that makes up the joint is capable of motion around an almost indefinite number of axes, with one common center.
The greater the depth of the socket, the more limited is the movement. However, the deeper joint is stronger and more stable. For example, the hip joint is a much stronger and more stable joint than the shoulder joint, but the range of movement is more limited in the hip.
Movements of ball-and-socket joints include flexion, extension, abduction, adduction, circumduction, and medial and lateral rotation.
Bicondylar joints 3
Bicondylar joints usually provide movement in a single axis, such as flexion and extension. They can permit limited rotation. Bicondylar joints are formed by two convex condyles, which may be encased by a fibrous capsule.
Two examples of bicondylar joints are the knee (formerly classified as ginglymus) and the temporomandibular joint (TMJ) (Fig. 1.32).
See Table 1.3 for a summary of joint classification.
TABLE 1.3
