Introduction
The upper and lower jaws belong to the human jaw. While the lower jaw is a single bone, the upper jaw belongs to the bony facial skull.
Bony part
The jaw is formed from the lower jaw bone (mandible) and the upper jaw bone (maxilla). The lower jaw bone (mandible) consists of a body (corpus mandibulae) and a frame (ramus mandibularis). The body is divided into a base and an alveolar part (pars alveolaris), which contains the 18 teeth of the lower jaw.
The frame (ramus mandibularis) is divided into two extensions, the coronoid process and the condylar process, which merges into the temporomandibular joint (Art. temporomandibulris). The body and the frame of the mandible together form the mandibular angle (Angulus mandibularis).
This angle changes from 150 degrees in the newborn during the course of life and then becomes smaller and smaller. In adulthood it is 120-130 degrees and in old age it rises again to about 140 degrees. This change is due to a change in the size of the body (corpus mandibulae) and is dependent on the number or shape and presence of teeth. The upper jaw bone (maxilla) is divided into the body (corpus maxillae), the frontal process (frontal process), the zygomatic process, the palatal process (palatine process) and the alveolar process (alveolar process) in which the 16 teeth of the upper jaw are located.
The temporomandibular joint
The temporomandibular joint (Articulatio temporomandibularis) is formed by the articular process (Processus condylaris) of the mandible (Mandibula), on which the head of the joint (Caput mandibularis) lies, and the socket (Fossa mandibularis) of the temporal bone (Os temporalis), which lies directly in front of the external auditory canal (Meatus acusticus externus). The joint head is cylindrical in shape and covered with articular cartilage at the front. Due to its shape, it is often referred to as the jaw roll.
Between these two joint partners lies a 3-4 mm thick, concave, fibrocartilaginous joint disc (Discus articularis), which is quite thin in the middle and increases in thickness towards the edges. It is fused on all sides with the outer layer of the joint capsule (Membrana fibrosa) and divides the temporomandibular joint into two superimposed chambers. It serves as a sliding socket, sits on the head of the temporomandibular joint (caput mandibularis) and shifts forward with it when the mouth is opened.
The joint capsule (Capsula articularis) of the temporomandibular joint consists of two layers. The membrana synovialis as the inner layer, which lines all inner surfaces except the articular cartilage and the discus, and the membrana synovialis as the outer layer, which is located at the anterior edge of the joint hump (tuberculum articulare), is applied internally and externally to the edges of the joint socket (articular fossa), at the back in the area of the suture of the temporal bone and occipital bone (sutura tympanosquamosa) and below the upper part of the neck of the jaw (collum mandibulare). Three ligaments outside the joint capsule strengthen the temporomandibular joint.
The outer and inner ligaments (lateral and medial ligaments), which run along the joint capsule from the outer and inner side of the zygomatic arch (Arcus zygomaticus) backwards at an angle to the temporomandibular neck (Collum mandibulae). The sphenomandibular ligament runs from the sphenoidal spine (Spina ossis sphenoidalis) to the inner surface of the mandibular bone (Lingula mandibulae). The last of the three bands is the stylus process band (Ligamentum stylomandibulare), which runs from the stylus process (Processus styloideus) to the rear edge of the mandibular angle (Angulus mandibulae).
The temporomandibular joint is a special joint because the right and left sides always work together. The two sides are connected via the mandible and always exert forces simultaneously. The function of the temporomandibular joint is primarily to connect the lower jaw with the temporal bone of the skull.
This enables the chewing movement. There are 3 main movements. When moving the lower jaw back and forth, both joints act congruently.
So also when opening and closing the mouth. However, if grinding movements are performed, i.e. circular movements in all dimensions (right and left, back and forth, up and down), the joints move asymmetrically. This is only possible thanks to the joint anatomy.
The joint consists of a socket, a joint head and the joint capsule. The fossa mandibularis is the socket (a cavity into which the head protrudes). It is located on the skull bone.
It is preceded by a small hump (articulated tubercle).This prevents the joint head from jumping out of the pit too quickly and causing a lockjaw (mouth can no longer be closed). Between the condyle, which is located at the articular process of the lower jaw, and the acetabulum, there is the articular disc, a cartilage that facilitates the sliding of the condyle in the acetabulum. If this cartilage is worn out with age, pain can occur, as with other joints. Together with three large ligaments, the joint capsule ensures the stability of the joints.
