Bones are joined by articulations. These come in different types and have different functions. In the head and neck, we find a number of different types of joints each of which is adapted for the function of that joint. The following is a description of the types of joints found in the area along with where they are found in the head and neck.
Synostoses – These joints are immovable bony unions. They seem not to be joints at all as they look like one continuous piece of bone. These are found between a number of bones in the skull. One example is the union of the two maxillary bones, the junction of which is not discernable in many areas.
Sutures – These joints are characterized by a complex interlocking of bone but with a small space that is secured by connective tissue. These joints are also common in the skull and the important ones will be discussed later. The parietal bones are joined in the midline by a suture.
Symphysis – These slightly movable joints have fibrocartilage between the bone surfaces. They exhibit slight movement and are found between the cervical vertebrae except for the atlanto occipital and atlantoaxial joints to be discussed in the next section.
Synovial Joints – Synovial joints are freely moving joints that are held together with connective tissue. This tissue will form a capsule that is filled with a lubricating fluid called synovial fluid. To further lessen the friction within the joint the joint surfaces will be covered in hyaline cartilage which is very smooth compared to bone. The movement allowed at each joint is a function of the shape of the articular surfaces and any ligaments that act to limit the motion in a particular direction. Synovial joints are classified by the types of movements they allow. There are four of these in the skull. They are described below:
Temporomandibular Joints – These account for half of the synovial joints in the skull as there is both one on the right and one on the left although as they are both attached to the mandible, they are not completely independent of one another. They are classified as ginglymoarthrodial which is a big, hard to pronounce word, that describes a joint that has both gliding and hinge components. While the TMJ is a course by itself it basically has two compartments separated by a piece of fibrocartilage called the meniscus. The upper compartment allows for the gliding motion and the lower compartment for the rotational movement.
Atlanto Occipital Joint – This is the joint between the atlas (the first cervical vertebra) and the occipital bone which as we will see is at the base of the skull. It is a condyloid joint which allows for movement in two directions, rotation in the sagittal and coronal (frontal) planes. This allows one to nod and tilt the head from side to side.
Atlantoaxial Joint – This joint is found between the atlas (C1) and the axis (C2) and is a pivot joint which allows only rotational movement. This allows the rotation of the head around the neck allowing one to say no without words.