Joint Classification

Original Editor - Yasmina Hossam

Top Contributors - Yasmina Hossam, Lucinda hampton, Kim Jackson, Claire Knott and Alexandra Kopelovich

Introduction

A joint or articulation /articular surface is the connection made between bones in the body which link the skeletal system into a functional whole .They are constructed to allow for different degrees and types of movement and are classified by both structural design and functionailty.

Joints can be classified:

1. Histologically, on the dominant type of connective tissue. ie fibrous, cartilaginous, and synovial.
2. Functionally, based on the amount of movement permitted. ie synarthrosis (immovable), amphiarthrosis (slightly moveable), and diarthrosis (freely moveable).

The two classification schemes correlate:

1. Synarthroses are fibrous joints
2. Amphiarthroses are cartilaginous joints
3. Diarthroses are synovial joints

The 5 minute video outlines the basics.

Fibrous Joints

In fibrous joints (synarthrodial joint) the bones are joined by fibrous tissue, namely dense fibrous connective tissue, and no joint cavity is present. The amount of movement allowed depends on the length of the connective tissue fibers uniting the bones. Although a few are slightly movable, most fibrous joints are immovable.

The three types of fibrous joints are sutures, syndesmoses, and gomphoses.

1. Sutures are immobile joints in the cranium. The plate-like bones of the skull are slightly mobile at birth because of the connective tissue between them, termed fontanelles. This initial flexibility allows the infant’s head to get through the birth canal at delivery and permits the enlargement of the brain after birth. As the skull enlarges, the fontanelles reduce to a narrow layer of fibrous connective tissue that suture the bony plates together. Eventually, cranial sutures ossify- the two adjacent plates fuse to form one bone (termed synostosis).
2. Gomphoses are the immobile joints between the teeth and their sockets in the mandible and maxillae. The periodontal ligament is the fibrous tissue that connects the tooth to the socket.
3. Syndesmoses are slightly movable joints (amphiarthroses). In syndesmosis joints, the two bones are held together by an interosseous membrane. Eg Middle Tibiofibular Joint, a fibrous joint formed by the interosseus membrane connecting the shafts of the tibia and the fibula.

Cartilaginous Joints

Cartilaginous joints are a type of joint where the bones are entirely joined by cartilage, either hyaline cartilage or fibrocartilage. These joints generally allow more movement than fibrous joints but less movement than synovial joints.

• Primary cartilaginous joints: These cartilaginous joints are composed entirely of hyaline cartilage and are known as synchondroses. Most exist between ossification centres of developing bones and are absent in the mature skeleton, but a few persist in adults. eg First Sternocostal Joint, between first rib and manubrium (all other sternocostal joints are plane synovial joints); Growth plates.
• The secondary cartilaginous joint, also known as symphysis, may involve either hyaline or fibrocartilage. These joints are slightly mobile (amphiarthroses). eg The pubic symphysis: Intervertebral discs.

Synovial Joints

The primary purpose of the synovial joint is to prevent friction between the articulating bones of the joint cavity. While all synovial joints are diarthroses, the extent of movement varies among different subtypes and is often limited by the ligaments that connect the bones. Nearly all joints of the limbs and most joints of the body fall into this class.

See Synovial Joint Link for full explaination.

General Anatomy of Synovial Joints

• Articular cartilage. Glassy-smooth hyaline cartilage covers the opposing bone surfaces as articular cartilage. These thin (1 mm or less) but spongy cushions absorb compression placed on the joint and thereby keep the bone ends from being crushed
• Joint (synovial) cavity. A feature unique to synovial joints, the joint cavity is really just a potential space that contains a small amount of synovial fluid
• Articular capsule. The joint cavity is enclosed by a twolayered articular capsule, or joint capsule. The external layer is a tough fibrous capsule, composed of dense irregular connective tissue, that is continuous with the periostea of the articulating bones. It strengthens the joint so that the bones are not pulled apart. The inner layer of the joint capsule is a synovial membrane composed of loose connective tissue. Besides lining the fibrous capsule internally, it covers all internal joint surfaces that are not hyaline cartilage
• Synovial fluid. A small amount of slippery synovial fluid occupies all free spaces within the joint capsule. This fluid is derived largely by filtration from blood flowing through the capillaries in the synovial membrane. Synovial fluid has a viscous, egg-white consistency (ovum = egg) due to hyaluronic acid secreted by cells in the synovial membrane, but it thins and becomes less viscous, as it warms during joint activity
• Reinforcing ligaments. Synovial joints are reinforced and strengthened by a number of bandlike ligaments. Most often, these are capsular, or intrinsic, ligaments, which are thickened parts of the fibrous capsule. In other cases, they remain distinct and are found outside the capsule (as extracapsular ligaments) or deep to it (as intracapsular ligaments). Since intracapsular ligaments are covered with synovial membrane, they do not actually lie within the joint cavity
• Nerves and blood vessels. Synovial joints are richly supplied with sensory nerve fibers that innervate the capsule. Some of these fibers detect pain, as anyone who has suffered joint injury is aware, but most monitor joint position and stretch, thus helping to maintain muscle tone. Stretching these structures sends nerve impulses to the central nervous system, resulting in reflexive contraction of muscles surrounding the joint. Synovial joints are also richly supplied with blood vessels, most of which supply the synovial membrane. There, extensive capillary beds produce the blood filtrate that is the basis of synovial fluid

Besides the basic components described above, certain synovial joints have other structural features. Some, such as the hip and knee joints, have cushioning fatty pads between the fibrous capsule and the synovial membrane or bone. Others have discs or wedges of fibrocartilage separating the articular surfaces. Where present, these so-called articular discs, or menisci , extend inward from the articular capsule and partially or completely divide the synovial cavity in two . Articular discs improve the fit between articulating bone ends, making the joint more stable and minimizing wear and tear on the joint surfaces. Besides the knees, articular discs occur in the jaw, and a few other joints

Types of Synovial Joints

Although all synovial joints have structural features in common, they do not have a common structural plan. Based on the shape of their articular surfaces, which in turn determine the movements allowed, synovial joints can be classified further into six major categories ( plane, hinge, pivot, condyloid, saddle, and ball-and-socket joints ).

A - Plane Joints

Multiaxial joint , the articular surfaces are essentially flat, and they allow only short nonaxial gliding movements. Examples are the gliding joints introduced earlier—the intercarpal and intertarsal joints, and the joints between vertebral articular processes. Gliding does not involve rotation around any axis, and gliding joints are the only examples of nonaxial plane joints

B -Hinge Joints

Uniaxial Joint, the cylindrical end of one bone conforms to a trough-shaped surface on another. Motion is along a single plane and resembles that of a mechanical hinge. Uniaxial hinge joints permit flexion and extension only, typified by bending and straightening the elbow and interphalangeal joints.

C - Pivot Joints

Uniaxial Joint , the rounded end of one bone conforms to a “sleeve” r ring composed of bone (and possibly ligaments) of another. The only movement allowed is uniaxial rotation of one bone around its own long axis. An example is the joint between the atlas and dens of the axis, which allows you to move your head from side to side to indicate “no.” Another is the proximal radioulnar joint, where the head of the radius rotates within a ringlike ligament secured to the ulna.

D -Condyloid ( ellipsoidal ) Joints

Biaxial joints , The oval articular surface of one bone fits into a complementary depression in another . The important characteristic is that both articulating surfaces are oval. The biaxial condyloid joints permit all angular motions, that is, flexion and extension, abduction and adduction, and circumduction. The radiocarpal (wrist) joints and the metacarpophalangeal (knuckle) joints are typical condyloid joints.

E - Saddle Joints

Biaxial Joints , resemble condyloid joints, but they allow greater freedom of movement. Each articular surface has both concave and convex areas; that is, it is shaped like a saddle. The articular surfaces then fit together, concave to convex surfaces. The most clear-cut examples of saddle joints in the body are the carpometacarpal joints of the thumbs, and the movements allowed by these joints are clearly demonstrated by twiddling your thumbs

F - Ball-and-Socket Joints

Multiaxial joint , In ball-and-socket joints , the sherical or hemispherical head of one bone articulates with the cuplike socket of another. These joints are multiaxial and the most freely moving synovial joints. Universal movement is allowed (that is, in all axes and planes, including rotation). The shoulder and hip joints are the only examples .