The maxilla, also known as the upper jaw, is a vital structure of the viscerocranium. It is involved in the formation of the orbit, nose and palate, holds the upper teeth and plays an important role for mastication and communication.
The maxilla consists of the body and its four projections:
- frontal process
- zygomatic process
- palatine process
- alveolar process
The body of the maxilla is the largest part of the bone and shaped like a pyramid. It contributes to the anterior margin and floor of the bony orbit, the anterior wall of the nasal cavity and the inferior part of the infratemporal fossa. It contains the maxillary sinuses which extend from the orbital ridge to the alveolar process and drain to the middle meatus of the nose. The infraorbital foramen is located underneath the orbital ridge and serves as a pathway for the infraorbital nerve and vessels.
The alveolar process is an inferior extension of the maxilla with a rather porous structure. It forms the maxillary dental arch containing eight cavities where the upper teeth are held.
The frontal process has a vertical ridge which constitutes the medial border of the orbit (anterior lacrimal crest). Posteriorly it forms the lacrimal groove together with the lacrimal bone. Superomedially it is in close contact with the anterior ethmoidal sinuses.
The zygomatic process of the maxilla grows laterally and meets the zygomatic bone.
Lastly, the palatine process is a horizontal extension on the medial side of the bone constituting the roof of the mouth and the floor of the nasal cavity. Together with the palatine bone it forms the hard palate. Anteriorly it features a small process, the anterior nasal spine. The incisive foramen can be found on the median line just posteriorly to the incisor teeth where the nasopalatine nerve and greater palatine vessels pass through.
The maxilla articulates with numerous bones: superiorly with the frontal bone, posteriorly with the sphenoid bone, palatine and lacrimal bones and ethmoid bone, medially with the nasal bone, vomer, inferior nasal concha and laterally with the zygomatic bone.
Note that the maxilla may look like a single bone but is truly paired forming a delicate suture in the middle line known as the median palatine (or intermaxillary) suture. Furthermore the bone comes in contact with the septal and nasal cartilages.
All five parts of the maxilla undergo intramembranous ossification through two ossification centers. In the 7th week of fetal life one differentiates between the maxilla and premaxilla (or incisive bone).
In the third month both parts fuse around the area of the alveolar process after which the premaxilla becomes the anterior part of the maxilla. In newborns the maxilla is horizontally much longer than vertically compared to adults. Furthermore their teeth sockets extend almost far up until the orbital ridge.
As all paranasal sinuses the maxillary sinuses are relatively small and become larger during the development of the maxilla and the other skull bones. In old age the alveolar process is increasingly absorbed and the teeth fall out. Since the maxilla becomes smaller it seems to come 'forward' in elderly people.
Periodontal disease is a common cause for bone resorption within the alveolar process which may result after a severe inflammation of the gums (gingivitis). Children, older people and people with poor oral hygiene are particularly affected. Certain bacteria or immunosuppression may also contribute to the progress of this disease. Another causes for alveolar ridge resorption can be an aplastic tooth or missing tooth (e.g. after extraction).
As the maxilla is the central bone of the midface it can fracture through various accidents, most commonly the Le Fort fractures which are subclassified into three types:
- Le Fort I fracture: detachment of the alveolar process from the maxilla in a rectangular form, with the center being at the inferior border of the bony nasal cavity. This leaves the patient with a mobile upper jaw.
- Le Fort II fracture: pyramidal in shape, involving the alveolar process, midface and nasal bones. The midface is mobile.
- Le Fort III fracture: separation of the viscerocranium from the neurocranium. The entire maxilla and nasal bones detach from the skull, leaving the face in its entirety to hang at the discretion of the facial tissues.