SURGICAL ANATOMY OF THE MAXILLA AS AN ORGAN

The maxilla is pyramidal in shape with the root of the zygoma as its apex (Figures 22-1 and 22-2). The latter can be palpated in the buccal vestibule of the oral cavity. It divides the facial surface of the maxilla into anterolateral and posterolateral surfaces of the pyramid. The third surface of the pyramid is the orbital plate of the maxilla. The base of the pyramid is the lateral wall of the nose or the medial wall of the maxillary sinus. The alveolar process of the maxilla related to the anterolateral surface carries the incisors, the canines, and the premolars, whereas that of the posterolateral surface carries the molars and ends as the maxillary tuberosity. The intraoral part of the maxilla is limited by the mucobuccal fold and the orbicularis oris muscle anteriorly and by the buccinator muscle posteriorly. The posterolateral surface of the maxilla above the mucobuccal fold forms the anterior wall of the infratemporal fossa and is difficult to palpate. However, the anterolateral surface of the maxilla beyond the mucobuccal fold can be palpated easily under the skin, as can the anterior nasal spine, the anterior nasal aperture, and the frontal process of the maxilla. Intraorally, it is possible to palpate the canine eminence, the canine fossa (distal to the canine eminence and a common site for facial access to the maxillary sinus), the maxillary tuberosity, and the hamular notch. The maxilla extends as a horizontal plate medially to form the anterior two thirds of hard palate. The horizontal plate of the palatine bone forms the posterior one third of hard palate. The palatine bone has a vertical plate that articulates with the base of the maxilla; it also has a pyramidal process that interposes between the maxillary tuberosity and the pterygoid processes of the sphenoid bone. Mucosal incision at the maxillary tuberosity that extends into the hamular notch may expose the pyramidal process of the palatine bone. Distal to this point, one may expose the medial pterygoid muscle, which takes origin from the tuberosity and the lateral pterygoid plate of the sphenoid. The medial wall of the maxilla begins at the sharp edge of the anterior nasal aperture and extends posteriorly, with a concavity that bounds the nasal fossa and continues distal to the canine. Once there, it forms the medial wall of the maxillary sinus and continues all the way back to the maxillary tuberosity. The medial wall of the maxilla provides attachment to the inferior nasal concha and to the vertical plate of the palatine bone. The opening of the maxillary sinus is found in the medial wall of the maxilla, close to the floor of the orbit. The opening is reduced in diameter by the uncinate process of the ethmoid bone. The latter provides the superior and middle conchae of the lateral nasal wall. The orbital plate of the maxilla forms the floor of the orbit and also the roof of the maxillary sinus. The infraorbital canal carries the infraorbital nerve and vessels, and it forms a ridge that can be seen in the sinus cavity.

Figure 22-1 Anatomical features of the maxilla and mandible that are of clinical importance.

Figure 22-2 Anatomical features of the maxillary sinus.

Muscles Attached to the Maxilla

As the maxillary alveolar bone resorbs, the crest of the residual ridge migrates toward the muscles that take their origin from the basal bone of the maxilla. Descriptions of muscles of surgical importance to oral implantologists follow (Figures 22-3 to 22-5).

Figure 22-3 A, Origin of depressor septi muscle; B, origin of superior incisivus muscle; C, origin of nasalis muscle; D, origin of levator labii superioris muscle; E, infraorbital foramen; F, origin of levator anguli oris (caninus) muscle; G, origin of buccinator muscle; H, insertion of lateral tendon of temporalis muscle; I, insertion of masseter muscle; J, origin of depressor anguli oris (triangularis) muscle; K, insertion of platysma muscle; L, mental foramen; M, origin of inferior incisivus muscle; N, origin of depressor labii inferioris muscle; O, origin of mentalis muscle.

Figure 22-4 A, Genial tubercles, site of origin of genioglossus (superior tubercle) and geniohyoid (inferior tubercle) muscles; B, digastric fossa, site of origin of anterior belly of digastric muscle; C, sublingual fossa, location of sublingual gland; D, mylohyoid line, site of origin of mylohyoid muscle; E, insertion of medial tendon of temporalis muscle; F, mandibular foramen; G, mylohyoid groove, formed by mylohyoid nerve; H, site of insertion of medial pterygoid muscle.

Figure 22-5 Muscles attached to the maxilla and mandible.

Orbicularis Oris Muscle

The orbicularis oris muscle originates from the modiolus at each corner of the mouth. The muscle fibers fan out into the upper and lower lips, where they form upper and lower peripheral portions under the skin and marginal portions under the vermilion zone of the lips. Some of the orbicularis oris fibers attach to the ala of the nose and to the nasal septum. In the midline of the upper lip, the peripheral portions from both sides interdigitate to create the philtrum. The marginal portions interdigitate and create the labial tubercle. Although unattached to the bone of the maxilla, the muscle limits the depth of the upper and lower facial vestibule. The orbicularis oris receives innervation from the buccal and mandibular branches of the facial nerve.

Incisivus Labii Superioris Muscle

The incisivus labii superioris muscle originates from the floor of the incisive fossa of the maxilla above the eminence of the lateral incisor and deep to the orbicularis oris. To expose the bone of the premaxilla between the canines, a mucoperiosteal flap reflection may detach the incisivus labii superioris. It may also detach the septalis and oblique fibers of nasalis muscle. The first is attached to the skin of the nasal septum and the latter to the ala of the nose. These small muscles will reattach after placement of the flap. However, if the muscles were damaged, then drooping of the septum and flaring of the ala of the nose may result.

Buccinator Muscle

The buccinator muscle originates from the base of the alveolar process opposite to the first, second, and third molar of both jaws. This muscle also takes origin from the pterygoid hamulus of the medial pterygoid plate of the sphenoid bone and therefore bridges the gap between the maxillary tuberosity anteriorly and the hamulus posteriorly. Extension of a subperiosteal frame design into the pterygoid plates may interfere with the fibers of these muscles without adding too much to the retention of the implant. When incising and reflecting the mucosa overlying the areas of the maxillary tuberosity and hamular notch before taking impressions for maxillary subperiosteal implants, avoid injuring the tendon of the tensor veli transact muscle, which passes around the pterygoid hamulus. The tendon moves on an underlying bursa whenever the soft palate moves; therefore it may become irritated by the subperiosteal frame and result in inflammation and pain. Fibers of the buccinator and medial pterygoid muscles are also found in the area of reflection. The majority of the fibers of the medial pterygoid muscle originate from the medial surface of the lateral pterygoid plate of the sphenoid bone, whereas the rest of the fibers form the tuberal head, which takes origin from the maxillary tuberosity. Near the pterygoid hamulus, a fibrous tissue raphe or, in some cases, a broad fascialike structure is found between the transaction and the superior pharyngeal constrictor muscles. In some cases, no raphe or fascia is found. Injury to the latter muscle should be avoided during reflection of the mucosa, particularly on the palatal aspect of the area of the hamulus.

Levator Labii Superioris Muscle

The levator labii superioris muscle takes origin from the infraorbital margin above the infraorbital foramen and therefore is rarely of concern to the implant surgeon. The zygomatic branch of the facial nerve innervates this muscle.

Levator Anguli Oris (Caninus) Muscle

The levator anguli oris muscle originates in the maxilla below the infraorbital foramen. The infraorbital nerve and vessels arise between this muscle and the levator labii superioris. In the severe atrophic division D maxilla, the infraorbital foramen is relatively close to the crest of the ridge. Reflection of the tissues for autogenous grafts and implant placement into sinus grafts may approximate this region and cause parasthesia. In subperiosteal implant cases that require extensive framework extension for retention, the operator should be aware of the location of the infraorbital neurovascular bundle in relation to the caninus and levator labii superioris muscles. The zygomatic branch of the facial nerve innervates the caninus muscle.

Sensory Innervation of the Maxilla

The maxillary nerve (V2) innervates the maxilla (Figure 22-6). The nerve leaves the middle cranial fossa by passing through the foramen rotundum and appears in the pterygopalatine fossa. It exits the fossa and passes briefly into the infratemporal fossa; from there it enters the floor of the orbit or the roof of the maxillary sinus by passing through the infraorbital fissure. It then exits the orbit via the infraorbital foramen. The pterygopalatine portion of the maxillary nerve provides the descending palatine and sphenopalatine branches. The sphenopalatine nerve enters the nasal cavity from the pterygopalatine fossa by passing through the sphenopalatine foramen. It supplies the nasal cavity and becomes the incisive nerve that supplies the palatine mucosa opposite to the upper six anterior teeth. The descending palatine nerve terminates as the great palatine nerve, which supplies the mucosa of the hard palate, and the lesser palatine nerves, which supply the mucosa of soft palate. These sensory nerves also carry parasympathetic fibers from the sphenopalatine ganglion that innervate the mucous glands of the palate. The infratemporal portion of V2 branches into the posterior alveolar nerve and zygomatic nerve. The latter divides into the zygomaticofacial and zygomaticotemporal cutaneous nerves. The posterior superior alveolar nerve supplies the buccal gingiva, buccal alveolar bone, second and third molars, and two roots of first molar. The infraorbital portion of V2 gives rise to anterior superior alveolar and occasionally middle superior alveolar nerves. These nerves run in bony grooves in the facial wall of the maxillary sinus under the Schneiderian membrane. The nerves supply the sinus wall and the premolars; the canine, lateral, and central incisor on the same side; and also the central incisor at the contralateral side. The infraorbital nerve exits the maxilla at the infraorbital foramen and supplies cutaneous branches to the lower eyelid, side of the nose, and upper lip. Implantologists often need to block V2 or several of its branches. Luckily these can be achieved by an intraoral route. V2 can be reached via the great palatine foramen and descending palatine canal, or via the pterygomaxillary fissure by following the slope of the posterolateral surface of the maxilla into the pterygopalatine fossa.

Figure 22-6 Sensory innervation of the maxilla and mandible.

Posterior Superior Alveolar (Dental) Nerve

The nerve arises within the pterygopalatine fossa, courses downward and forward, passing through the pterygomaxillary fissure, and enters the posterior aspect of the maxilla. It runs between the bone and the lining of the maxillary sinus. This nerve supplies the sinus, the molars, the buccal gingiva, and the adjoining portion of the cheek; it may be injured during a sinus augmentation with a lateral approach. Clinically this does not appear to be of major consequence.

Infraorbital Nerve

This nerve is a continuation of the main trunk of the maxillary division. It leaves the pterygopalatine fossa by passing through the inferior orbital fissure to enter the floor of the orbit. It runs in the infraorbital groove and then in the infraorbital canal. The nerve exits the orbit through the infraorbital foramen to give cutaneous branches to the lower eyelid, the ala of the nose and the skin, and the mucous membrane of the lip and cheek. The infraorbital foramen is located between the levator labii superioris muscle, which takes origin above the foramen, and the levator anguli oris (caninus) muscle, which takes origin below the foramen. This foramen and neurovascular contents are within 5 to 10 mm of an extremely resorbed maxilla. When applying onlay grafts, which expose the entire maxilla, the implant dentist must be very aware of this situation. Fixation screws or implants may cause paresthesia when inserted through the graft and into this structure. Subperiosteal implants designed for an atrophied maxilla should not extend into the site of the infraorbital nerve and vessels. In some cases of maxillary sinus disorder, the site of the infraorbital foramen becomes tender, probably as a result of inflammation of the infraorbital nerve. This is an important diagnostic test for possible postoperative involvement after sinus augmentation procedures.

Middle Superior Alveolar (Dental) Nerve

This branch of the infraorbital nerve is given off as the infraorbital nerve passes through the infraorbital groove. The middle superior alveolar nerve runs downward and forward in the lateral wall of the sinus to supply the maxillary premolars. This region is routinely violated for the lateral approach to sinus grafts, with apparently no consequence.

Anterior Superior Alveolar (Dental) Nerve

This branch of the infraorbital nerve arises within theinfraorbital canal. It initially runs laterally within the sinus wall and then curves medially to pass beneath the infraorbital foramen. It turns downward to supply the maxillary anterior teeth. A nasal branch passes into the nasal cavity to supply the mucosal lining of a portion of the nasal cavity. Before elevation of nasal mucosa and placement of grafts, this nerve must be anesthetized. The infraorbital nerve block or V2 block anesthesia is suggested. Implant dentists must also anesthetize this branch before placement of implants in the incisor region. The anterior, middle, and posterior superior alveolar nerves intermingle to form the superior dental plexus. The posterior, middle, and anterior superior alveolar nerves run in the facial wall of the maxillary sinus between its lining membrane and the bone. During antrostomy procedures to augment the floor of the sinus, the operator should be aware of these structures, which are present even in the absence of teeth.

Palatine Nerve

The greater (anterior) and lesser (posterior) palatine nerves supply the hard and soft palate, respectively. They exit the pterygopalatine fossa through the superior opening of the descending palatine canal, travel downward, and enter the oral cavity by way of the greater and lesser palatine foramina. The greater palatine nerve runs forward in a groove on the inferior surface of the hard palate to supply the palatal mucosa as far forward as the incisor teeth. Here the nerve communicates with the nasopalatine nerve. The nerve supplies the gingiva, mucous membrane, and glands of the hard palate. The greater palatine artery and vein accompany the nerve during its course in the hard palate. As the maxillary alveolar process atrophies, it shifts to the palate and brings the crest of the ridge closer to the groove where the greater palatine neurovascular bundle is found. The restoring dentist should be aware that an incision too palatal to the crest of the ridge in the atrophied maxilla might injure these vital structures. This foramen is entered for a V2 block anesthesia. One may find it by taking a blunt instrument and pressing firmly along the alveolar palatal bone angle. The instrument will depress over the foramen when in the correct position.

Nasopalatine (Sphenopalatine) Nerve

The nasopalatine nerve leaves the pterygopalatine fossa through the sphenopalatine foramen located in the medial wall of the fossa. It enters the nasal cavity and supplies portions of the lateral and superior aspects of the nasal cavity. The longest branch reaches the nasal septum, where it turns downward and forward, traveling on the surface of the septum. While on the septum it forms a groove on the vomer bone. The nerve supplies the nasal mucosa, descends to the floor of the nose near the septum, passes through the nasopalatine canal, and then exits onto the hard palate through the incisive foramen. The latter opening is deep to the incisive papilla. The nerve communicates with the greater palatine nerve. The incisive nerve should be anesthetized before elevation of the mucosa of the floor of the nose for subnasal grafts or implants that engage the nasal floor in the incisor region.

Arterial Supply to the Maxilla

The majority of arterial blood supply (Figure 22-7) comes from the maxillary artery, which is one of the terminal branches of the external carotid artery. The artery starts deep to the neck of the mandibular condyle (mandibular portion) and then proceeds either superficial or deep to the lateral pterygoid muscle (pterygoid portion). It then branches close to the pterygomaxillary fissure, where one branch enters the fossa (pterygopalatine portion). The other branch, called the infraorbital artery, enters the floor of the orbit via the infraorbital fissure; it proceeds in the infraorbital canal and exits on the face by passing through the infraorbital foramen. Branches of the maxillary artery are as follows:

1. Mandibular portion: deep auricular, tympanic, middle meningeal, and inferior alveolar arteries

2. Pterygoid portion: deep temporal, lateral pterygoid, medial pterygoid, and masseteric arteries

3. Pterygopalatine portion: posterior superior alveolar, descending palatine, and sphenopalatine arteries

4. Infraorbital portion: anterior and middle superior alveolar, palpebral, nasal, and labial arteries

Figure 22-7 Arterial supply of the maxilla and mandible.

Supplemental arterial blood supply reaches the maxilla via two branches from the cervical portion of the facial artery (ascending palatine and tonsillar arteries), two dorsolingual arteries from the lingual artery, and the ascending pharyngeal branch of external carotid artery. All the collateral circulation reaches the maxilla from the area of the soft palate. During orthognathic surgery to correct maxillary prognathism, the surgeon often cuts the posterior, middle, and anterior superior alveolar arteries, as well as the descending palatine arteries, without compromising the blood supply to the maxilla because of the presence of supplemental blood supply from the branches mentioned previously. I/>

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