Eruption

Eruption refers to when the developing tooth breaks through the epithelial covering of the alveolar process in an occlusal direction. The development and the phase of eruption of the primary and permanent dentitions are linked to the growth in size of the skull and the jaws and hence to a person's general physical development. A distinction is made between the primary dentition and permanent dentition. The teeth of the permanent dentition comprise 20 successional and 12 accessional teeth; the latter originate from the same dental lamina as the 20 primary teeth and are the 12 molars of the permanent dentition.

The teeth of the permanent dentition are already developing during the primary dentition stage.The primary teeth require about 2 to 4 years for their development from tooth germs, from eruption through root formation; thereafter, they bear full masticatory function for approximately 4 years. Permanent teeth require approximately 12 years from tooth germ formation until the roots are completely formed: exfoliation is finished by age 10 to 13 years (Fig 4-64).

Movement phases in dental eruption occur as three changes in position:

1. Changes in the position of the tooth germs during development (preeruptive movement)
2. Occlusal eruption of teeth as far as the occlusal plane (prefunctional eruptive movements)
3. Changes of position in the dentition (eg, mesial or occlusal migration)

The actual eruption of teeth in an occlusal direction starts as soon as the first edges of the roots are formed. Root growth and tooth eruption are synchronous, the length of eruption roughly corresponding to the length of root growth. The eruption length is only greater than root length in the case of the incisors and canines, because these tooth germs lie more deeply in the jaw. During occlusally directed growth, the alveolar crests grow in an occlusal direction as a result of bone formation.

In the primary teeth, there is dense connective tissue over the occlusal surfaces and incisal margins before eruption. The eruption of the primary teeth (ie, teething) can be associated with pain and fever because sharp edges cause injuries and inflammation in the maxillomandibular mucosa.

The germs and crowns of the accessional and successional teeth are covered with bone, which must be broken down during eruption. If the tooth is pushed through the connective tissue as far as the mucosal epithelium, slight inflammatory processes may occur.

During root formation of the successional teeth, the roots of the primary teeth are resorbed and their crowns exfoliated (Fig 4-65). Eruption of the successional teeth also starts with early root growth, when root dentin and cementum are formed and the bone tissue is remodeled. Both processes are associated with the alveolar bone and the roots of the primary teeth being resorbed. The resorption starts a few years before a primary tooth is shed.

The posterior successional teeth, the premolars, are located between the flared roots of the primary teeth, whereas the anterior successional teeth are located anterior to the axis of the primary teeth.The primary tooth root is therefore resorbed at an angle from the lingual aspect. Later the successional teeth move directly inferior to the primary teeth, so that there is more space available for the successional teeth to grow. Growth and resorption processes are interrupted by resting phases.

The pulp tissue of the primary teeth is not dissolved but remains functional until the teeth are shed.The coronal pulp retains its normal structure and remains fully functional until shortly before the crown is shed. In an inflammatory process, the pulp is transformed into granulation tissue at that time. As a result, the shedding of primary teeth happens without purulent inflammation or swelling. If coronal pulp has died, the resorption process may be slightly delayed.

The driving force for tooth eruption is mechanical growth pressure from root growth and the tensile force of the periodontal fibers already formed at the cervical root stump. The tooth is lifted out of the alveolar bone by the pull of bundles of fibers running in a coronal direction.

Eruption does not start until the periodontal ligament has developed at the coronal parts of the root. Later, physiologic tooth movements also take place as a result of the force of the periodontal fibers. Defective developments occur as a result of natural events, accidents, or dental interventions.