14 Dento-osseous Structures, Blood Vessels, and Nerves
The development of the dentitions has been discussed and a brief review of the development of the neurocranium and splanchnocranium has been presented in Chapter 2. Therefore, in this chapter the focus is on the dentoalveolar and dento-osseous structures of the permanent dentition. The forms of the roots of the teeth and their sizes and angulations govern the shape of the alveoli in the jawbones, and this in turn shapes the contour of the dento-osseous portions facially.
The osseous structures that support the teeth are the maxilla and the mandible. The maxilla, or upper jaw, consists of two bones: a right maxilla and a left maxilla sutured together at the median line. Both maxillae in turn are joined to other bones of the head (Figure 14-1). The mandible, or lower jaw, has no osseous union with the skull and is a movable (ginglymoarthrodial) joint.
Figure 14-1 Representation of an adult skull with permanent dentition. The maxilla consists of a body and four processes (malar, nasal, alveolar, and palatine) that articulate by synarthrosis with cranial and other facial bones, (e.g., frontal, nasal, ethmoid, malar bones). The mandible articulates with the temporal bone by the temporomandibular joint (see Figure 15-6).
The maxillae make up a large part of the bony framework of the facial portion of the skull. They form the major portion of the roof of the mouth, or hard palate, and assist in the formation of the floor of the orbit and the sides and base of the nasal cavity. They support the 16 permanent maxillary teeth.
Each maxilla is an irregular bone, somewhat cuboidal in shape, which consists of a body and four processes: the zygomatic, frontal, palatine, and alveolar processes. The maxilla is hollow and contains the maxillary sinus air space, also called the antrum of Highmore. From the dental viewpoint, in addition to its general shape and the processes mentioned, several landmarks on this bone are among the most important, including the incisive fossa, canine fossa, canine eminence, infraorbital foramen, posterior alveolar foramina, maxillary tuberosity, pterygopalatine fossa, and incisive canal.
The anterior or facial surface (Figures 14-2 and 14-3) is separated above from the orbital aspect by the infraorbital ridge. Medially it is limited by the margin of the nasal notch, and posteriorly, it is separated from the posterior surface by the anterior border of the zygomatic process, which has a confluent ridge directly over the roots of the first molar. The ridge corresponding to the root of the canine tooth is usually the most pronounced and is called the canine eminence.
Anterior to the canine eminence, overlying the roots of the incisor teeth, is a shallow concavity known as the incisive fossa. Posterior to the canine eminence on a higher level is a deeper concavity called the canine fossa. The floor of this canine fossa is formed in part by the projecting zygomatic process. Above this fossa and below the infraorbital ridge is the infraorbital foramen, the external opening of the infraorbital canal. The major portion of the canine fossa is directly above the roots of the premolars.
The posterior or infratemporal surface (Figures 14-3 and 14-4) is bounded above by the posterior edge of the orbital surface. Inferiorly and anteriorly, it is separated from the anterior surface by the zygomatic process and the zygomatic ridge, which runs from the inferior border of the zygomatic process to the alveolus of the maxillary first molar. This surface is more or less convex and is pierced in a downward direction by two or more posterior alveolar foramina. These two canals are on a level with the lower border of the zygomatic process and are somewhat distal to the roots of the third molar.
The inferior portion of this surface is more prominent where it overhangs the root of the third molar and is called the maxillary tuberosity. Medially, this tuberosity is limited by a sharp, irregular margin that articulates with the pyramidal process of the palatine bone and, in some cases, the lateral pterygoid plate of the sphenoid bone. The maxillary tuberosity is the origin for some fibers of the medial pterygoid muscle.
The orbital surface is smooth and together with the orbital surface of the zygomatic bone forms the floor of the orbit. The junction of this surface and the anterior surface forms the infraorbital margin or ridge, which runs superiorly to form part of the nasal process. Its posterior border or edge coincides with the inferior boundary of the inferior orbital fissure.
The thin medial edge of the orbital surface is notched anteriorly, forming the lacrimal groove. Behind this groove, it articulates for a short distance with the lacrimal bone, then for a greater length with a thin portion of the ethmoid bone, and terminates posteriorly in a surface that articulates with the orbital process of the palatal bone. Its lateral area is continuous with the base of the zygomatic process (see Figure 14-3).
Traversing the posterior portion of the orbital surface is the infraorbital groove. This groove begins at the center of the posterior surface and runs anteriorly. The anterior portion of this groove is covered, becoming the infraorbital canal, the anterior opening of which is located directly below the infraorbital ridge on the anterior surface.
If the covered portion of this canal were to be laid open, the orifices of the middle and anterior superior alveolar canal would be seen transmitting the corresponding vessels and nerves to the premolars, canines, and incisor teeth.
The nasal surface (Figures 14-5 and 14-6) is directed medially toward the nasal cavity. It is bordered below by the superior surface of the palatine process. Anteriorly, it is limited by the sharp edge of the nasal notch. Above and anteriorly, it is continuous with the medial surface of the frontal process. Behind this, it is deeply channeled by the lacrimal groove, which is converted into a canal by articulation with the lacrimal and inferior turbinate bones.
Behind this groove the upper edge of the nasal surface corresponds to the medial margin of the orbital surface, and the maxilla articulates in this region with the lacrimal bone, a thin portion of the ethmoid bone, and the orbital process of the palatine bone.
The posterior border of the maxilla, which articulates with the palatine bone, is traversed obliquely from above downward and slightly medially by a groove, which, by articulation with the palate bone, is converted into the greater palatine canal. Toward the posterior and upper part of this nasal surface, a large, irregular opening into the maxillary sinus (antrum of Highmore) may be seen. In an articulated skull, this opening is partially covered by the uncinate process of the ethmoid bone and the inferior nasal concha.
Anterior to the lacrimal groove, the nasal surface is ridged for the attachment of the inferior nasal concha. Below this the bone forms a lateral wall of the inferior nasal meatus. Above the ridge for a small distance on the medial side of the nasal process, the smooth lateral wall of the middle meatus appears.
The zygomatic process may be seen in the lateral views of the maxillary bone as a roughly triangular eminence whose apex is placed inferiorly directly over the first molar roots. The lateral border is rough and spongelike in appearance, where it has been disarticulated from the zygomatic or cheek bone (see Figures 14-1 and 14-3).
Part of this process is formed by the upward continuation of the infraorbital margin medially. Its edge articulates with the nasal bone. Superiorly, the process articulates with the frontal bone. The medial surface of the frontal process forms part of the lateral wall of the nasal cavity. Anteriorly, the frontal process articulates with the nasal bone.
The palatine process (Figures 14-2 through 14-8) is a horizontal ledge extending medially from the nasal surface of the maxilla. Its superior surface forms a major portion of the nasal floor. The inferior surfaces of the combined left and right palatine processes form the hard palate as far posteriorly as the second molar, where they articulate with the horizontal parts of the palatine bone (Figures 14-7 and 14-8) at the transverse palatine suture.
The inferior surface of the palatine process is rough and pitted for the palatine mucous glands in the roof of the mouth and is pierced by numerous small foramina for the passage of blood vessels and nerve fibers. At the posterior border of the process is a groove or canal that passes the greater palatine nerve and vessels to the palatal soft tissues. The posterior edge of the palatine process becomes relatively thin where it joins the palatine bone at the point of the greater palatine foramen. The palatine process becomes progressively thicker anteriorly from the posterior border. Anteriorly, the palatal process is confluent with the alveolar process surrounding the roots of the anterior teeth.
Immediately posterior to the central incisor alveolus, when looking at the medial aspect of the maxilla, one sees a smooth groove that is half of the incisive canal, when the two maxillae are joined together. The incisive fossa into which the canals open may be seen immediately lingual to the central incisors at the median line, or intermaxillary suture where the maxillae are joined. Two canals open laterally into the incisive foramen, the foramina of Stenson, carrying the nasopalatine nerves and vessels. Occasionally, two midline foramina are present, the foramina of Scarpi.
Extending laterally from the incisive foramen to the space between the lateral incisor and canine alveoli are the remnants of the suture between the maxilla and premaxilla. In most mammals the premaxilla remains an independent bone.
The alveolar process makes up the inferior portion of the maxilla; it is that portion of the bone which surrounds the roots of the maxillary teeth and which gives them their osseous support. The process extends from the base of the tuberosity posterior to the last molar to the median line anteriorly, where it articulates with the same process of the opposite maxilla (see Figures 14-7 and 14-8). It merges with the palatine process medially and with the zygomatic process laterally (see Figure 14-8).
When one looks directly at the inferior aspect of the maxilla toward the alveoli with the teeth removed, it is apparent that the alveolar process is curved to conform with the dental arch. It completes, with its fellow of the opposite side, the alveolar arch supporting the roots of the teeth of the maxilla.
The process has a facial (labial and buccal) surface and a lingual surface with ridges corresponding to the surfaces of the roots of the teeth supported by it. It is made up of labiobuccal and lingual plates of very dense but thin cortical bone separated by interdental septa of cancellous bone.
The facial plate is thin, and the positions of the alveoli are well marked on it by visible ridges as far posteriorly as the distobuccal root of the first molar (see Figure 14-2). The margins of these alveoli are frail, and their edges are sharp and thin. The buccal plate over the second and third molars, including the alveolar margins, is thicker. Generally, the lingual plate of the alveolar process is heavier than the facial plate. In addition, the alveolar process is longer where it surrounds the anterior teeth, sometimes extending posteriorly to include the premolars. In short, it extends farther down in covering the lingual portion of the roots.
The bone is very thick lingually over the deeper portions of the alveoli of the anterior teeth and premolars. The merging of the alveolar process with the palatal process brings about this formation. The lingual plate is paper thin over the lingual alveolus of the first molar, however, and rather thin over the lingual alveoli of the second and third molars. This thin lingual plate over the molar roots is part of the formation of the greater palatine canal (see Figure 14-8).
The alveolar process is maintained by the presence of the teeth. Should any tooth be lost, that portion of the alveolar process that supported the missing tooth will be subject to atrophic reduction. Should all of the teeth be lost, the alveolar process will eventually be virtually lost.
The alveolar cavities are formed by the facial and lingual plates of the alveolar process and by connecting septa of bone placed between the two plates. The form and depth of each alveolus are determined by the form and length of the root it supports (see Table 1-1).
The alveolus nearest the median line is that of the central incisor (Figure 14-9; see also Figure 14-8). The periphery is regular and round, and the interior of the alveolus is evenly tapered and triangular in cross section, with the apex toward the lingual.
The second alveolus in line is that of the lateral incisor. It is generally conical and egg-shaped, or ovoid, with the widest portion to the labial. It is narrower mesiodistally than labiolingually and is smaller on cross section, although it is often deeper than the central alveolus. Sometimes, it is curved at the upper extremity (Figure 14-10; see also Figure 14-8).
The canine alveolus is the third from the median line. It is much larger and deeper than those just described. The periphery is oval and regular in outline, with the labial width greater than the lingual. The socket extends distally. It is flattened mesially and somewhat concave distally. The bone is so frail at the canine eminence on the facial surface of the alveolus that the root of the canine is often exposed on the labial surface near the middle third (see Figure 14-2).
The first premolar alveolus (see Figures 14-8 and 14-10) is kidney-shaped in cross section, with the cavity partially divided by a spine of bone that fits into the mesial developmental groove of the root of this tooth. This spine divides the cavity into a buccal and a lingual portion. If the tooth root is bifurcated for part of its length, as is often the case, the terminal portion of the cavity is separated into buccal and lingual alveoli. The socket is flattened distally and much wider buccolingually than mesiodistally (see Table 1-1).
The second premolar alveolus is also kidney-shaped, but the curvatures are in reverse to those of the first premolar alveolus. The proportions and depth are almost the same. The septal spine is located on the distal side instead of the medial side, because the second premolar root is inclined to have a well-defined developmental groove distally. This tooth usually has one broad root with a blunt end, but it is occasionally bifurcated at the apical third.
The first molar alveolus (Figures 14-8 and 14-11) is made up of three distinct alveoli widely separated. The lingual alveolus is the largest; it is round, regular, and deep. The cavity extends in the direction of the hard palate, having a lingual plate over it that is very thin. The lingual periphery of this alveolus is extremely sharp and frail. This condition may contribute to the tissue recession often seen at this site.
Figure 14-11 Alveoli of the molar area. Note the thinness of the buccal plates over the first molar roots compared with those of the second and third maxillary molars. The third molar alveoli are rarely separated as distinctly as in this specimen. Figures 14-9, 14-10, and 14-11 demonstrate a number of significant points concerning the maxillary alveoli. In Figure 14-9 the facial cortical plate of bone is thin over the anterior teeth and is considerably thicker over the posterior teeth, especially the molars. Cancellous bone seems to exist buccal to some of the posterior roots. In Figure 14-10 interradicular septa are thick but with numerous nutrient canals. In Figure 14-11 cancellous bone, furnishing numerous opportunities for blood supply, is evident in the apical portions of the alveoli. The anterior alveoli are lined laterally with a layer of smooth cortical bone. This lining is less prominent in the posterior alveoli.
The mesiobuccal and distobuccal alveoli of the first molar have no outstanding characteristics except that the buccal plates are thin. The bone is somewhat thicker at the peripheries than that found on the lingual alveolus. Nevertheless, it is thinner farther up on the buccal plate. It is not uncommon for one to find the roots uncovered by bone in spots when examining dry specimens.
The forms of the buccal alveoli resemble the forms of the roots they support. The mesiobuccal alveolus is broad buccolingually, with the mesial and distal walls flattened. The distobuccal alveolus is rounder and more conical.
The septa that separate the three alveoli (interradicular septa) are broad at the area that corresponds to the root bifurcation, and they become progressively thicker as the peripheries of the alveoli are approached. The bone septa are very cancellous, which denotes a rich blood supply, as is true of all the septa, including those separating the various teeth as well.
A general description of the alveoli of the second molar would coincide with that of the first molar; these alveoli are closer together, since the roots of this tooth do not spread as much. As a consequence, the septa separating the alveoli are not as heavy.
The third molar alveolus is similar to that of the second molar, except that it is somewhat smaller in all dimensions. Figure 14-11 shows a third molar socket to accommodate a tooth with three well-defined roots, a rare occurrence. Usually, the two buccal (and often all three) roots will be fused. The interradicular septum changes accordingly. If the roots of the tooth are fused, a septal spine will appear in the alveolus at the points of fusion on the roots marked by deep developmental grooves.
The maxillary sinus lies within the body of the bone and is of corresponding pyramidal form; the base is directed toward the nasal cavity. Its summit extends laterally into the root of the zygomatic process. It is closed in laterally and above by the thin walls that form the anterolateral, posterolateral, and orbital surfaces of the body. The sinus overlies the alveolar process in which the molar teeth are implanted, more particularly, the first and second molars, the alveoli of which are separated from the sinus by a thin layer of bone. Occasionally, the maxillary sinus will extend forward far enough to overlie the premolars also. It is not uncommon to find the bone covering the alveoli of some of the posterior teeth extending above the floor of the cavity of the maxillary sinus, forming small hillocks.
Regardless of the irregularity and the extension of the alveoli into the maxillary sinus, a layer of bone always separates the roots of the teeth and the floor of the sinus in the absence of pathological conditions. A layer of sinus mucosa is also always between the root tips and the sinus cavity.