Joints of the Skull and Trunk

Joint Classifications

• Joints are classified as simple or compound based on the number of articulating bones.

• Structural classifications include fibrous, cartilaginous, and synovial joints. Fibrous joints are immovable, cartilaginous joints are slightly movable, and synovial joints are freely movable.

• Functional classifications include synarthrosis (immovable), amphiarthrosis (slightly movable), and diarthrosis (freely movable) joints.

Joints of the Skull

• The presentation identifies the following joints of the skull:

  • Synchondroses

  • Osseus sutures

  • Intermandibular joint

  • Temporohyoid joint

  • Temporomandibular joint

• Synchondroses are cartilaginous junctions found in young animals, some of which remain cartilaginous and radiographically visible, such as the synchondrosis spheno-occipitalis, synchondrosis sphenopetrosa, synchondrosis intersphenoidalis, and synchondrosis petro-occipitalis.

• Osseus sutures are ossified cartilaginous junctions and fibrous joints between skull bones and are synarthrosis (immovable) joints.

• The intermandibular joint is a median osseous junction that unites the right and left mandibular bodies. In pigs and horses, this joint becomes a synostosis.

• The temporohyoid joint joins the suspensory part of the hyoid apparatus at the base of the skull. The hyoid apparatus consists of the basihyoid, thyrohyoid, and ceratohyoid, which connect to the tongue and larynx, and the suspensory apparatus, which includes the epihyoid, stylohyoid, and tympanohyoid, which articulate with the temporal bone.

• The temporomandibular joint is between the mandibular ramus and squamous part of the temporal bone. It is a synovial joint that facilitates up and down movement, with limited lateral grinding and forward and backward movement. A fibrocartilaginous disc compensates for incongruency between articulating surfaces. In cats and dogs, this joint is nearly congruent. The joint capsule extends from the free margins of the articular surfaces and is strengthened by tight lateral and caudal ligaments. Carnivores and pigs lack a caudal ligament.

Joints of the Vertebral Column, Thorax, and Skull

• These include joints between the skull and vertebral column, adjacent vertebrae, thoracic vertebrae and ribs, and articulations of the thorax.

• The joint between the skull and vertebral column is the atlanto-occipital joint, located between the occipital condyles and the concavities of the atlas. It consists of two ellipsoidal joints that primarily allow flexion and extension in the sagittal plane. Each joint has its own joint capsule, and in carnivores, ruminants, aged pigs, and horses, the joint cavities communicate ventrally. The joint is supported by lateral ligaments and dorsal and ventral atlanto-occipital membranes.

• Also included in the joints between the skull and vertebral column is the atlanto-axial joint, between C1 (atlas) and C2 (axis). It is a trochoid or pivot joint that allows rotation along the longitudinal axis. A common joint capsule forms a single synovial cavity. Ligaments include the dorsal atlantoaxial membrane, elastic dorsal axial ligament, ligamenta alaria, ventral atlantoaxial ligament, longitudinal ligaments, and transverse ligaments of the atlas.

• Joints between adjacent vertebrae include intervertebral discs, which have a fibrous ring and nucleus pulposus. These joints connect the cranial and caudal extremities of adjacent vertebrae. Cervical intervertebral discs are thinner dorsally than ventrally, and thickness decreases throughout the thoracic and lumbar regions. With advanced age, degeneration of the nucleus pulposus can lead to protrusion or herniation of the disc towards the vertebral canal.

• Ligaments between adjacent vertebrae include short ligaments that bridge successive vertebrae (interarcuate, intertransverse, and interspinous ligaments) and long ligaments that form functional units. Examples of long ligaments are the dorsal and ventral longitudinal ligaments, nuchal ligament, and supraspinous ligament.

• Joints between thoracic vertebrae and ribs include the costovertebral and costotransverse joints.

• Articulations of the thorax include joints between ribs and costal cartilages (costochondral joints), between the sternum and costal cartilages (sternocostal joints), between costal cartilages (costal arch), and between individual sternebrae. The costochondral joints are symphyses in carnivores and horses, and firm joints in pigs and ruminants. The sternocostal joints are formed by the condylar sternal extremity of the sternal costal cartilage and the articular cavities of the sternum. The costal arch is formed by elastic soft tissue that joins the costal cartilages of the asternal ribs together. Joints between individual sternebrae include intersternebral cartilages, sternal cartilaginous joints, and the sternal ligament.