ostosis, is an immobile joint formed when the gap between two bones ossifies and they become, in effect, a single bone. Bony joints can form by ossification of either fibrous or cartilaginous joints. An infant is born with right and left frontal and mandibular bones, for example, but these soon fuse seamlessly into a single frontal bone and mandible. In old age, some cranial sutures become obliterated by ossification and the adjacent cranial bones, such as the parietal bones, fuse. The epiphyses and diaphyses of the long bones are joined by cartilaginous joints in childhood and adolescence, and these become bony joints in early adulthood. The attachment of the first rib to the sternum also becomes a bony joint with age.
A fibrous joint is also called a synarthrosis. It is a point at which adjacent bones are bound by collagen fibers that emerge from one bone, cross the space between them, and penetrate into the other. There are three kinds of fibrous joints: sutures, gomphoses, and syndesmoses. In sutures and gomphoses, the fibers are very short and allow for little or no movement. In syndesmoses, the fibers are longer and the attached bones are more mobile.
Figure 1. Fibrous joints
Note: (a) A suture between the parietal bones. (b) A gomphosis between a tooth and the jaw. (c) A syndesmosis between the tibia and fibula.
Sutures are immobile or only slightly mobile fibrous joints that closely bind the bones of the skull to each other; they occur nowhere else. Sutures can be classified as serrate, lap, and plane sutures. Readers with some knowledge of woodworking may recognize that the structures and functional properties of these sutures have something in common with basic types of carpentry joints.
Serrate sutures appear as wavy lines along which the adjoining bones firmly interlock with each other by their serrated margins, like pieces of a jigsaw puzzle. 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 where the temporal bone meets the sphenoid and parietal bones. The beveled edge of the temporal bone.
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 joint is represented by the intermaxillary suture in the roof of the mouth.
Figure 2. Sutures joint
Even though the teeth are not bones, the attachment of a tooth to its socket is classified as a joint called a gomphosis. The term refers to its similarity to a nail hammered into wood. 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. The periodontal ligament allows the tooth to move or give a little under the stress of chewing. Along with associated nerve endings, this slight tooth movement allows you to sense how hard you are biting and to sense a particle of food stuck between the teeth.
A syndesmosis is a fibrous joint at which two bones are bound by relatively long collagenous fibers. The separation between the bones and length of the fibers give these joints more mobility than a suture or gomphosis has. An especially mobile syndesmosis exists between the shafts of the radius and ulna, which are joined by a broad fibrous interosseous membrane. This permits such movements as pronation and supination of the forearm. A less mobile syndesmosis is the one that binds the distal ends of the tibia and fibula together, side by side.
A cartilaginous joint is also called an amphiarthrosis. In these joints, two bones are linked by cartilage. The two types of cartilaginous joints are synchondroses and symphyses.
Figure 3. Cartilaginous joints
Note: a) A synchondrosis, represented by the costal cartilage joining rib