JOINTS OF HUMAN SKELETAL SYSTEM: Features of joints, components, classification based on structure & function, fibrous joints, synovial joints, cartilaginous joints (structure, components are explained with suitable diagram).

The joints of the skeletal system contribute to homeostasis by holding bones together in ways that allow for movement and ﬂexibility. Bones are too rigid to bend without being damaged. Fortunately, ﬂexible connective tissues form joints that hold bones together while still permitting, in most cases, some degree of movement. A joint, also called an articulation or arthrosis, is a point of contact between two bones, between bone and cartilage, or between bone and teeth. When we say one bone articulates with another bone, we mean that the bones form a joint.

You can appreciate the importance of joints if you have ever had a cast over your knee joint, which makes walking difﬁcult or a splint on your ﬁnger, which limits your ability to manipulate small objects. The scientiﬁc study of joints is termed arthrology. The study of motion of the human body is called kinesiology.

This article describe the structural and functional classifications of joints.

Joints are classiﬁed structurally, based on their anatomical characteristics, and functionally, based on the type of movement they permit.

The structural classiﬁcation of joints is based on two criteria:

(1) The presence or absence of a space between the articulating bones, called a synovial cavity, and

(2) The type of connective tissue that binds the bones together.

Structurally, joints are classiﬁed as one of the following types:

• Fibrous joints: There is no synovial cavity, and the bones are held together by dense irregular connective tissue that is rich in collagen ﬁbers.

• Cartilaginous joints: There is no synovial cavity and the bones are held together by cartilage.

• Synovial joints: The bones forming the joint have a synovial cavity and are united by the dense irregular connective tissue of an articular capsule, and often by accessory ligaments.

Fibrous joints

As previously noted, ﬁbrous joints lack a synovial cavity, and the articulating bones are held very closely together by dense irregular connective tissue. Fibrous joints permit little or no movement. The three types of ﬁbrous joints are sutures, syndesmoses, and interosseous membranes.

Sutures

A suture is a ﬁbrous joint composed of a thin layer of dense irregular connective tissue; sutures occur only between bones of the skull. An example is the coronal suture between the parietal and frontal bones. The irregular, interlocking edges of sutures give them added strength and decrease their chance of fracturing. Because a suture is immovable, it is classiﬁed functionally as a synarthrosis. Some sutures that are present during childhood are replaced by bone in the adult. Such a suture is an example of a synostosis, or bony joint a joint in which there is a complete fusion of two separate bones into one bone. For example, the frontal bone grows in halves that join together across a suture line. Usually they are completely fused by age 6 and the suture becomes obscure. If the suture persists beyond age 6, it is called a metopic suture. A synostosis is also classiﬁed functionally as a synarthrosis.

Syndesmosis

A syndesmosis is a ﬁbrous joint in which there is a greater distance between the articulating surfaces and denser irregular connective tissue than in a suture. The dense irregular connective tissue is typically arranged as a bundle (ligament) and the joint permits limited movement. One example of a syndesmosis is the distal tibioﬁbular joint, where the anterior tibioﬁbular ligament connects the tibia and ﬁbula. It permits slight movement (amphiarthrosis). Another example of a syndesmosis is called a gomphosis or dentoalveolar joint, in which a cone-shaped peg ﬁts into a socket. The only examples of gomphoses in the human body are the articulations between the roots of the teeth and their sockets (alveoli) in the maxillae and mandible. The dense irregular connective tissue between a tooth and its socket is the thin periodontal ligament (membrane). A gomphosis permits no movement (synarthrosis). Inﬂammation and degeneration of the gums, periodontal ligament, and bone is called periodontal disease.

Interosseous membrane

The ﬁnal category of ﬁbrous joint is the interosseous membrane, a substantial sheet of dense irregular connective tissue that binds neighboring long bones and permits slight movement (amphiarthrosis). There are two principal interosseous membrane joints in the human body. One occurs between the radius and ulna in the forearm and the other occurs between the tibia and ﬁbula in the leg.

Cartilaginous joints

Like a ﬁbrous joint, a cartilaginous joint lacks a synovial cavity and allows little or no movement. Here the articulating bones are tightly connected by either hyaline cartilage or ﬁbrocartilage. The two types of cartilaginous joints are synchondroses and symphyses.

Synchondrosis

A synchondrosis is a cartilaginous joint in which the connecting material is hyaline cartilage. An example of a synchondrosis is the epiphyseal (growth) plate that connects the epiphysis and diaphysis of a growing bone. Functionally, a synchondrosis is a synarthrosis. When bone elongation ceases, bone replaces the hyaline cartilage, and the synchondrosis becomes a synostosis, a bony joint. Another example of a synchondrosis is the joint between the ﬁrst rib and the manubrium of the sternum, which also ossiﬁes during adult life and becomes an immovable synostosis, or bony joint.

Symphysis

A symphysis (growing together) is a cartilaginous joint in which the ends of the articulating bones are covered with hyaline cartilage, but a broad, ﬂat disc of ﬁbrocartilage connects the bones. All symphyses occur in the midline of the body. The pubic symphysis between the anterior surfaces of the hip bones is one example of a symphysis. This type of joint is also found at the junction of the manubrium and body of the sternum and at the intervertebral joints between the bodies of vertebrae. A portion of the intervertebral disc is composed of ﬁbrocartilage. A symphysis is an amphiarthrosis, a slightly movable joint.

Synovial joints

Structure of Synovial Joints

Synovial joints have certain characteristics that distinguish them from other joints. The unique characteristic of a synovial joint is the presence of a space called a synovial (joint) cavity between the articulating bones. Because the synovial cavity allows a joint to be freely movable, all synovial joints are classiﬁed functionally as diarthroses. The bones at a synovial joint are covered by a layer of hyaline cartilage called articular cartilage. The cartilage covers the articulating surface of the bones with a smooth, slippery surface but does not bind them together. Articular cartilage reduces friction between bones in the joint during movement and helps to absorb shock.

Articular Capsule

A sleeve like articular (joint) capsule surrounds a synovial joint, encloses the synovial cavity, and unites the articulating bones. The articular capsule is composed of two layers, an outer ﬁbrous membrane and an inner synovial membrane. The ﬁbrous membrane usually consists of dense irregular connective tissue (mostly collagen ﬁbers) that attaches to the periosteum of the articulating bones. In fact, the ﬁbrous membrane is literally a thickened continuation of the periosteum between the bones. The ﬂexibility of the ﬁbrous membrane permits considerable movement at a joint, while its great tensile strength (resistance to stretching) helps prevent the bones from dislocating. The ﬁbers of some ﬁbrous membranes are arranged as parallel bundles of dense regular connective tissue that are highly adapted for resisting strains. The strength of these ﬁber bundles, called ligaments, is one of the principal mechanical factors that hold bones close together in a synovial joint. The inner layer of the articular capsule, the synovial membrane, is composed of areolar connective tissue with elastic ﬁbers. At many synovial joints the synovial membrane includes accumulations of adipose tissue, called articular fat pads. An example is the infrapatellar fat pad in the knee.

A “double-jointed” person does not really have extra joints. Individuals who are “double-jointed” have greater ﬂexibility in their articular capsules and ligaments; the resulting increase in range of motion allows them to entertain fellow partygoers with activities such as touching their thumbs to their wrists and putting their ankles or elbows behind their necks. Unfortunately, such ﬂexible joints are less structurally stable and are more easily dislocated.

Synovial Fluid The synovial membrane secretes synovial ﬂuid, a viscous, clear or pale yellow ﬂuid named for its similarity in appearance and consistency to uncooked egg white. Synovial ﬂuid consists of hyaluronic acid secreted by ﬁbroblast-like cells in the synovial membrane and interstitial ﬂuid ﬁltered from blood plasma. It forms a thin ﬁlm over the surfaces within the articular capsule. Its functions include reducing friction by lubricating the joint, absorbing shocks, and supplying oxygen and nutrients to and removing carbon dioxide and metabolic wastes from the chondrocytes within articular cartilage. (Recall that cartilage is an avascular tissue, so it does not have blood vessels to perform the latter function.) Synovial ﬂuid also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint.

When a synovial joint is immobile for a time, the ﬂuid becomes quite viscous (gel-like), but as joint movement increases, the ﬂuid becomes less viscous. One of the beneﬁts of warming up before exercise is that it stimulates the production and secretion of synovial ﬂuid; more ﬂuid means less stress on the joints during exercise. We are all familiar with the cracking sounds heard as certain joints move, or the popping sounds that arise when people crack their knuckles. According to one theory, when the synovial cavity expands, the pressure of the synovial ﬂuid decreases, creating a partial vacuum. The suction draws carbon dioxide and oxygen out of blood vessels in the synovial membrane, forming bubbles in the ﬂuid. When the bubbles burst, as when the ﬁngers are ﬂexed (bent), the cracking or popping sound is heard.