The two maxillary bones which combined are often just referred to as the maxilla is a complex bone that not only, as mentioned previously, forms most of the palate, but houses the upper teeth, contributes to the floor of the orbit, and forms much of the mid face. The union of the left and right maxillary bones occurs by ossified sutures in the midline. Between the plates of bone forming the palate and the plate forming the mid face is the largest sinus cavity in the skull. This sinus extends over the premolars and molars in the adult and superiorly to the base of the orbit. Sinus congestion is common in this sinus as it drains into the middle meatus which is superior to the majority of the cavity. Therefore, unlike the other paranasal sinuses gravity works against drainage of fluid from the cavity.
Parts – The central portion of the maxilla that makes up the mid face is referred to as the body. The body has several extensions referred to as processes. The vertical extension that passes between the nasal and lacrimal bones articulates with the frontal bone also and is named the frontal process. The extension to articulate with the zygomatic bone is known logically as the zygomatic process of the maxilla. Very logically the processes forming the palate are known as the palatal processes. These processes meet at the palatine suture. The maxilla also has a process that houses the roots of the upper teeth called the alveolar processes. The alveolar process only forms when the teeth do and will resorb in areas where the teeth are removed. There are prominences in the bone where the tooth roots are. These form fossa between them. The most important of these are the incisive fossa between the roots of the central and lateral incisors and the canine fossa between the roots of the lateral incisor and the canine.
Openings – The maxilla forms the anterior boundary of the inferior orbital fissurewith the sphenoid forming the posterior boundary and the zygomatic bone forming the lateral boundary. Leading from the inferior orbital foramen anteriorly there is a groove carrying blood vessels and nerves which eventually enter the infraorbital canal that runs through the maxilla in the floor of the orbit. The canal terminates at the face as the infraorbital foramen. Anteriorly on the palate, immediately posterior to the central incisors, is another opening called the incisive foramen or alternatively the nasopalatine foramen. This is actually a single opening at the confluence of the two incisive canals which are also referred to as the nasopalatine canals.
Embryologic formation – The histologic formation of the maxillary bones are notable in that they form not from two primordial processes but three. The premaxilla forms a triangular shaped section centrally from canine to canine with the apex a short distance into the palate. It arises from the same fetal tissue that forms the nose while the rest of the maxilla is formed by the fetal maxillary processes that form the rest of the bone. The palate is formed when the two palatal shelves which form on either side of the developing nasal cavity join the premaxilla. This occurs at about 8 weeks in utero at a stage when bones have not yet developed. Failure of these processes to join properly is the cause of cleft lips and palates depending on where the failure occurs. This explains why cleft lips and clefts of the anterior palate are in the area of the canines rather than in the midline while clefts in the more posterior sections of the palate are in the midline.