Joints of the Axial Skeleton

The joints of the skull are chiefly sutures, with adjacent bones united by fibrous tissue. In old age these typically ossify, becoming synostoses. in addition to the sutures associated with the braincase, the skull also features the symphysis of the mandible, the synchondrosis at the junc­tion of the sphenoid bone and occipital bone at the base of the skull, and the temporoman­dibular joint.

The temporomandibular joint between the mandible and the temporal bone of the skull consists of an articular surface on the skull and one on the mandible, with a plate of cartilage (articular disk or meniscus) between. The tem­poromandibular joint acts as a ginglymus joint when the mouth opens and closes and as a plane joint when the jaw moves from side to side and forward and back, as in grinding food. The temporomandibular joint may be described as a condylar joint.

The atlanto-occipital joint between the occipital bone of the skull and first cervical vertebra (the atlas) is strictly a ginglymus joint. Two condyles on the occipital bone fit into corresponding depressions in the atlas. The only movements possible are flexion and exten­sion in the sagittal plane, as in nodding the head “yes.”

Rotation of the head occurs between the atlas and axis, at the atlantoaxial joint. The dens, a toothlike projection from the cranial extremity of the axis, projects into the vertebral foramen of the atlas, where it is held by a group of strong ligaments that permit considerable rotary movement. The atlantoaxial joint is the best example of a pivot joint, in which one segment rotates around the long axis of another.

The symphyseal (fibrocartilaginous) joints between adjacent vertebrae throughout the rest of the vertebral column exhibit relatively little motion. The bodies of adjacent vertebrae are united by a heavy disk of fibrocartilage, the intervertebral disk (IVD), that is flexible enough to permit some bending in any direc­tion, even twisting.

The articular processes of adjacent vertebrae have flat surfaces that are apposed to form plane joints with limited gliding movements. These surfaces are larger and the movements more extensive near the head, decrease in the thoracic region, and are again more extensive in the lumbar region. The joints between sacral vertebrae fuse completely, and the sacrum becomes a single bone with the segments joined by synostoses.

The ribs are attached to the vertebral column by two separate joints. one is between the head of the rib and the costal fovea formed by the bodies of two adjacent thoracic vertebrae; the other is between the tubercle of the rib and a facet on the transverse process of the vertebra of the same number as the rib. The heads of the paired ribs on each side of an intervertebral space are joined by an intercapital ligament, which forms a reinforcing band across the dorsal side of the intervertebral disk. The pres­ence of the intercapital ligament helps explain why disk protrusions are rare in the thoracic region of the vertebral column.

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