UNIT 9: Joints

Chapter 9: Joints Key Words and Topics Make certain that you can define, and use in context, each of the terms listed below, and that you understand the significance of each of the concepts. 1. Define the term “articulation” (joint).  articulation or arthrosis 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. 2. Classify joints on the basis of structure and function.  structural classification -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.  functional classification  synarthrosis- An immovable joint. The plural is synarthroses.  Amphiarthrosis-A slightly movable joint. The plural is amphiarthroses.  Diarthrosis- A freely movable joint. The plural is diarthroses. All diarthroses are synovial joints. They have a variety of shapes and permit several different types of movements. 3. Describe the structure and functions of the three types of fibrous joints.  fibrous joints (see above for definition)  suture A suture (SOO‐chur; sutur = seam) 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 (Figure 9.1a). The irregular, interlocking edges of sutures give them added strength and decrease their chance of fracturing. Sutures are joints that form as the numerous bones of the skull come in contact during development. They are immovable or slightly movable. In older individuals, sutures are immovable (synarthroses), but in infants and children they are slightly movable (amphiarthroses) (Figure 9.1b). Sutures play important roles in shock absorption in the skull.  Some sutures, although present during growth of the skull, are replaced by bone in the adult. Such a suture is called a synostosis (sin′‐os‐TŌ‐sis; os‐ = bone), or bony joint—a joint in which there is a complete fusion of two separate bones into one. 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 frontal or metopic suture (me‐TŌ‐ pik; metopon = forehead). A synostosis is classified as a synarthrosis because it is immovable.  Syndesmosis A syndesmosis (sin′‐dez‐MŌ‐sis; syndesmo‐ = band or ligament) is a fibrous joint in which there is a greater distance between the articulating surfaces and more dense irregular connective tissue than in a suture. The dense irregular connective tissue is typically arranged as a bundle (ligament), allowing the joint to permit limited movement. One example of a syndesmosis is the distal tibiofibular joint, where the anterior tibiofibular ligament connects the tibia and fibula (Figure 9.1c, left). It permits slight movement (amphiarthrosis).  Gomphosis Another example of a syndesmosis is called a gomphosis (gom‐FŌ‐sis; gompbo‐ = bolt or nail) 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 (Figure 9.1c, right). The dense irregular connective tissue between a tooth and its socket is the thin periodontal ligament (membrane). A healthy 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 (in′‐ter‐OS‐ē‐us), which is 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 (see Figure 8.6) and the other occurs between the tibia and fibula in the leg (Figure 9.1d). 4. Describe the structure and functions of the two types of cartilaginous joints.  cartilaginous joints Like a fibrous joint, a cartilaginous joint (kar′‐ti‐LAJ‐i‐nus) lacks a synovial cavity and allows little or no movement. Here the articulating bones are tightly connected by either hyaline cartilage or fibrocartilage (see Table 4.6). The two types of cartilaginous joints are synchondroses and symphyses.  Synchondrosis A synchondrosis (sin′‐kon‐DRŌ‐sis; chondro‐ = cartilage) 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 (Figure 9.2a). A photomicrograph of the epiphyseal plate is shown in Figure 6.7b. Functionally, a synchondrosis is immovable (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 (synarthrosis) synostosis, or bony joint (see Figure 7.22b)  Symphysis A symphysis (SIM‐fi‐sis = 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 (plural) occur in the midline of the body. The pubic symphysis between the anterior surfaces of the hip bones is one example of a symphysis (Figure 9.2b). This type of joint is also found at the junction of the manubrium and body of the sternum (see Figure 7.22) and at the intervertebral joints between the bodies of vertebrae (see Figure 7.20a). A portion of the intervertebral disc is composed of fibrocartilage. A symphysis is a slightly movable joint (amphiarthrosis).