Clinical Success in Surgical and Orthodontic Treatment of Impacted Teeth (eBook)

Components of Eruption

At the embryonic bell stage, the dental bud is made up of three parts: the dental organ, the dental papilla, and the dental follicle (Fig 1-1). Embryonic tissue derived from the dental lamina organizes itself around this group, adapting to growth of the tooth bud; lamellar bone then gradually begins to replace it, forming an encasement of bone known as the bony crypt. Inside this casement, calcification of the crown and formation of the root both begin.

Generally, when one quarter of the root has developed, the tooth will start to erupt (van der Linden 1983). This is a complex process of many stages, none of which has been completely elucidated. However, it is clear that this mechanism ensures that root formation will be coordinated with development of the tooth’s periodontal environment, the dental bud’s osseous journey, the emergence of the crown into the oral cavity, and its eventual arrival at the occlusal plane.

Bony crypts

As this pre-eruptive tooth bud choreography proceeds, the bony crypts enact parallel movements; their ceilings, or cell walls, oriented toward the occlusal plane, form an opening that provides access to the gubernacular canal. This tunnel through bone, which contains a fibrous cord where epithelial remnants of the dental lamina persist, connects the crypt to the cortical plate and sometimes to the alveolar wall of the primary tooth. This collective voyage makes a profound impression on the tooth as it erupts (Fig 1-2).

Dental follicle

When the crown is completely calcified, cells of the follicular envelope exert primary control on the two active poles of the dental bud (Cahill and Marks 1980) (see Figs 1-1 and 1-3).

The apical pole serves as the site for the formation of the supporting dental tissues, cementum, periodontal ligament, and surrounding alveolus. Bone formed by apposition organizes around the roof of the crypt in relation to the walls of the developing root, which partially invests the space that the crown had occupied (Figs 1-4a and 1-4b). As a result, the architecture of the crypt changes as it keeps pace with the progress of tooth eruption. The coronary pole of the follicle continues to adhere to the crown until it fuses with the buccal epithelium before the tooth emerges into the arch.

It is this portion of the follicle that provides the osseous cleft through which the tooth passes along the gubernacular canal. Mononuclear cells that are the precursors of osteoclasts and osteoblasts are stored within the follicle before eruption begins (Craddock and Youngson 2004). The osteoclasts are then liberated to assist in preparing a path for migration of the permanent tooth’s crown by creating an opening in the vault of the crypt and by initiating resorption of the primary tooth’s root (Kawakami et al 2000).

At the same time, a cleft in the bone, several millimeters deep at the alveolar border of the single-rooted primary teeth, will ease the eruption of their successors (Fig 1-5). But the extent of this resorption varies according to the type of tooth. Premolars migrate into substantial alveolar craters, but canines, whose coronal dimensions are substantially larger than those of their primary predecessors, require even larger openings through which to move successfully. These two resorption processes, of bone and primary tooth roots, join to create space for passage of the permanent teeth.

The location of the opening of the gubernacular canal indicates the site in the lingual cortical plate where the permanent successor will emerge behind the primary tooth, although sometimes it will emerge directly into the primary tooth’s pulp cavity. In the first instance the gubernacular fibers bind into the gingival lamina propria; in the second they intersperse with the primary tooth’s periodontal membrane (Figs 1-6 and 1-7).

Not all of the mechanisms that govern the initiation of the eruption of permanent teeth are as yet understood, but many experiments have shown the important role that the dental follicle plays in this process. Accordingly, dental practitioners should adhere to well-established anatomic and physiologic operative protocols for freeing, or extricating, permanent teeth whose eruption has been delayed.

Localization of the Bony Crypts of the Maxillary Permanent Teeth

Incisors

While the bony crypts that surround most of the buds of the permanent teeth are interconnected, the intermaxillary suture separates the two maxillary permanent central incisors (as it did their primary predecessors), usually generating a characteristic posteruptive diastema (see Figs 1-5 and 1-11).

The crowns of the central incisors calcify under the floor of the nasal cavity and stimulate resorption, with a lingual bevel, of their primary predecessors, behind which they are forming. The crypts of the maxillary lateral incisors are located more lingually behind those of the central incisors so that, viewed frontally in 4-year-olds, half of their crowns would be masked by the central incisors. This backward positioning of the lateral incisor places it in the same plane as the canine, for whose eruption it will later serve as a guide. At this stage of calcification, which occurs in a constricted space, the developing permanent teeth are normally quite crowded.

The lateral incisor crypts are closer to the occlusal plane than those of the other teeth in the arch; calcification proceeds at varying rates in the maxilla as a function of the length of the tooth roots (van der Linden 1976).

Canines

The bony crypts of the canines are located near the external border of the nasal fossae anterior to the sinuses, from which they are separated only by a thin lamina of bone (Figs 1-8 and 1-9).

Taken together, the maxillary teeth have a conical appearance. As a result, the bony crypts of the canines, which are placed higher than the others, are the most internal. Their buds develop behind the roots of the primary teeth and behind the buds of the other permanent teeth.