Joints of the Skull and Trunk
Joints of the Skull
The joints of the skull include synchondroses, osseus sutures, the intermandibular joint, the temporohyoid joint, and the temporomandibular joint.
Synchondroses are cartilaginous junctions found in young animals. Some 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. They are synarthrosis (immovable) joints.
The intermandibular joint is a median osseous junction that unites the right and left mandibular bodies. A small articular area remains cartilaginous, forming a synchondrosis (hyaline cartilage joint). In pigs and horses, this joint exhibit 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. The suspensory apparatus includes the epihyoid, stylohyoid, and tympanohyoid, which articulate with the temporal bone. The tympanohyoid articulates with the styloid process in ruminants and horses. In carnivores it articulates with the mastoid process, and in pigs, with the nuchal process of the temporal bone.
The temporomandibular joint in cats and dogs is nearly congruent. It is formed by the head of the condyloid process of the mandible and the articular area of the temporal bone. The articular area of the temporal bone consists of the articular tubercle rostrally, the mandibular fossa with its transverse articular surface in the middle, and the retroarticular process caudally. The joint capsule extends from the free margins of the articular surfaces and attaches to the entire edge of the disc. The inner synovial layer divides it into two compartments: a larger dorsal and a smaller ventral compartment. The outer fibrous layer is strengthened by tight lateral and caudal ligaments. Carnivores and pigs lack a caudal ligament.
Joints of the Vertebral Column, Thorax, and Skull
Joints between the skull and vertebral column, adjacent vertebrae, thoracic vertebrae, and ribs.
Atlanto-occipital joint: Located between the skull and C1 (atlas). It is formed between the occipital condyles and concavities of the atlas. It consists of two ellipsoidal joints that allow flexion and extension in the sagittal plane only. Each joint has its own joint capsule, which attaches around articular surfaces. Carnivores, ruminants, aged pigs, and horses have two joint cavities that communicate ventrally. Carnivores have an atlanto-occipital joint that shares a common joint cavity with the atlantoaxial joint. Ligaments include lateral ligaments, and dorsal and ventral atlanto-occipital membranes. Dorsal and ventral atlanto-occipital membranes are individual, expansive sheets of fibrous tissue. They reinforce the dorsal and ventral sides of the joint capsule and cover the extensive joint space between the occipital and the atlas.
Atlanto-axial joint: Located between C1 (atlas) and C2 (axis). It is formed by the dens of C2 and the cavity of C1 (axis). It is a trochoid or pivot joint that allows rotation around the longitudinal axis. It has a common joint capsule that 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. Ligamenta alaria strengthens the dens of C2 and arise from the dens and attach to the inner surface of the ventral arch of C1 in ruminants and horses, to the medial surface of the condyles in carnivores, and to the rim of the foramen magnum in pigs. The ventral atlantoaxial ligament reinforces the joint capsule ventrally in ruminants and horses, extending between the ventral tubercle of C1 and the ventral spine of C2. Transverse ligaments of the atlas strap the dens to the atlas in carnivores and pigs, preventing undue movement of the dens in relation to the vertebral canal and protecting the medulla oblongata from fatal mechanical insults.
Joints between Adjacent Vertebrae: Intervertebral discs connect adjacent vertebrae. The intervertebral discs include a fibrous ring and nucleus pulposus. The thickness of the intervertebral discs is largely responsible for the flexibility of the spine. Cervical intervertebral discs are thinner dorsally than ventrally and decrease in thickness throughout the thoracic and lumbar regions. The lumbar region has the thinnest discs. Advanced age can cause degeneration of the nucleus pulposus, resulting in protrusion or herniation of the disc towards the vertebral canal. Short ligaments bridge successive vertebrae and include interarcuate, intertransverse, and interspinous ligaments. Long ligaments forming functional units include the dorsal longitudinal ligament, ventral longitudinal ligament, nuchal ligament, and supraspinous ligament. The nuchal ligament consists of the nuchal funiculus and nuchal lamina. Dogs have paired nuchae, ruminants have paired funiculus and paired cranial lamellar part and horses have paired nuchal funiculus and a lamellar part.
Joints between Thoracic Vertebrae and Ribs: The head of the ribs articulates with the appropriate vertebra via the costovertebral joint, and the tubercle of the ribs articulates with the appropriate vertebra via the costotransverse joint.
Articulation of the Thorax: The ribs articulate with costal cartilages via costochondral joints. In carnivores and horses, these are symphyses, while in pigs and ruminants, they are firm joints. The sternum articulates with costal cartilages via sternocostal joints. In pigs and horses, the first rib of both sides shares a common articular cavity on the manubrium of the sternum, and the articular surfaces of the other sternal ribs are placed laterally at the junction of the sternebrae, enclosed in a tight joint capsule. The costal cartilages form the costal arch via elastic soft tissue that joins the costal cartilages of the asternal ribs together. The individual sternebrae are connected by intersternebral cartilages, sternal cartilaginous joints, and the sternal ligament.