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Other Terms: Upper jaw bone, Maxillaire
The maxillae are large, paired bones that when united form the upper jaw. They also contribute to the walls of the nasal cavity, orbit, oral cavity, and maxillary sinus. The maxilla has a central body, hollowed out as the maxillary sinus, and four processes of variable shape. The processes are the posterolateral projecting zygomatic, the horizontal, medially projecting palatine, the arched, inferiorly projecting alveolar, and the superiorly projecting frontal. Like the other bones of the skull, the maxilla consists of outer lamellae of compact bone with cancellous centers.
The term maxilla may be a corrupted form of the Latin term mala meaning cheek. Another possibility is that it comes from the Latin verb macerare meaning to chew. Originally the Romans used the word to define both upper and lower jaw bones.
The maxilla articulates with nine, sometimes ten, bones: the frontal, ethmoid, nasal, lacrimal, zygomatic, inferior nasal concha, vomer, palatine, and the other maxilla. It will sometimes form an articulation with the orbital surface or lateral pterygoid plate of the sphenoid bone. In addition, it forms gomphotic joints with each tooth. The superior border of the frontal process forms a serrate suture with the frontal bone. It articulates with the ethmoid along the medial aspect of its orbital plate. The anterior border of the frontal process articulates with the posterior border of the nasal bone, while the posterior border of the frontal process articulates with the anterior border of the lacrimal bone. The lateral surface of the zygomatic process forms a rough articular surface with the zygomatic bone. Inside the nasal cavity, the maxilla forms the lateral wall of the cavity and joints with the inferior nasal concha. The two maxillae form junctions with each other throughout the midline and are joined by the vomer superiorly at the palatine junction. The posterior margin of the palatal process articulates with the anterior margin of the palatines horizontal plate.
The ossification of the maxilla is much debated. There is no argument to the fact that this is a bone that forms in connective tissue anterior to the cartilaginous nasal capsule. The debate is over the number of centers involved in maxillary ossification and whether a separate element, the os incisivum - the homologue to the premaxilla, is incorporated into the development. The simplest scenario involves a single ossification center arising in the body of the maxilla and spreading to form the entire bone. Other workers suggest that additional centers, up to two, form in the incisor region corresponding to the premaxillary bone of other mammals. These centers appear during the sixth embryonic week and join by the third month. By late fetal life there exists no clear indication of separate centers on the facial side of the bone however, on the palate approximately 25 percent of skulls retain a suture line at the junction of the two centers. This suture is retained in adult skulls.