Fibrous Joints

In a fibrous joint, collagen fibers emerge from the matrix of one bone and penetrate into the matrix of another, spanning the space between them (fig. 9.3). There are three types of fibrous joints: sutures, gomphoses, and syndesmoses. In sutures and gomphoses, the collagen fibers are very short and allow for little movement. In syn-desmoses, the fibers are longer and the attached bones are more movable.

Sutures

Sutures are immovable fibrous joints that closely bind the bones of the skull to each other; they occur nowhere else. In chapter 8, we did not take much notice of the differences between one suture and another, but some differences may have caught your attention as you studied the diagrams in that chapter or examined laboratory specimens. Sutures can be classified as serrate, lap, and plane sutures. Readers with some background in woodworking may recognize that the structures and functional properties of these sutures have something in common with basic types of carpentry joints (fig. 9.4).

Serrate sutures appear as wavy lines along which the adjoining bones firmly interlock with each other by their serrated margins. Serrate sutures are analogous to a dovetail wood joint. Examples include the coronal, sagittal, and lambdoid sutures that border the parietal bones.

Lap (squamous) sutures occur where two bones have overlapping beveled edges, like a miter joint in carpentry. On the surface, a lap suture appears as a relatively smooth (nonserrated) line. An example is the squamous suture between the temporal and parietal bones.

Plane (butt) sutures occur where two bones have straight, nonoverlapping edges. The two bones merely border on each other, like two boards glued together in a butt joint. This type of suture is seen between the palatine processes of the maxillae in the roof of the mouth.

Gomphoses

Even though the teeth are not bones, the attachment of a tooth to its socket is classified as a joint called a gompho-sis (gom-FOE-sis). The term refers to its similarity to a nail hammered into wood.4 The tooth is held firmly in place by a fibrous periodontal ligament, which consists of collagen fibers that extend from the bone matrix of the jaw into the dental tissue (see fig. 9.3b). The periodontal ligament allows the tooth to move or "give" a little under the stress of chewing.

Syndesmoses

Syndesmoses5 (SIN-dez-MO-seez) are joints at which two bones are bound by a ligament only. (Ligaments also bind bones together at synovial joints, but are not the exclusive means of holding those joints together.) Syndesmoses are the most movable of the fibrous joints. The radius and ulna are bound to each other side by side, as are the tibia and fibula, by a syndesmosis in which the ligament forms a broad sheet called an interosseous membrane along the shafts of the two bones (see fig. 9.3c).

4gompho = nail, bolt

Sutures Syndesmosis Gomphosis
Figure 9.3 Types of Fibrous Joints. (a) A suture between the parietal bones; (b) a gomphosis between a tooth and the jaw; (c) a syndesmosis between the tibia and fibula.

Saladin: Anatomy & I 9. Joints I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 9 Joints 297

Chapter 9 Joints 297

Carpentry Joints
Figure 9.4 Types of Sutures. (a) Examples; (b) structure of the adjoining bones; (c) functional analogies to some common wood joints.

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