Eruption is a lifelong activity, which begins with the emergence of a tooth from the alveolar bone, and then proceeds to the more gradual process, which keeps the tooth in contact with its opponent in order to compensate for tooth wear. The stability of the dentition is influenced by the nature of forces transmitted to teeth. The effect these forces have is partly determined by the direction, magnitude, and frequency of the force and also the angles of the occlusal surfaces of opposing teeth. Forces generated by the tongue are also of great influence to the st1.0″ of the dentition. There is considerable variation in the size, and relationship, between the maxillary and mandibular dental arches, which, while they may not be considered ideal, are nevertheless quite normal and functional. Tooth wear is also normal, in fact a requirement of efficient masticatory function, though it is seen by many clinicians as undesirable and requiring repair or reconstruction.
Eruption is a lifelong process, but it can be conveniently described in three main phases (▶ Fig. 8.1).
Fig. 8.1 A diagrammatic representation of the three phases of tooth eruption. During the preeruptive phase, bone deposition at the base of the follicle and resorption of the overlying alveolar bone cause the tooth bud to migrate toward the surface. During the eruptive stage, root growth and continued bone deposition at the base of the root allow the tooth to emerge into the oral cavity. During the posteruptive stage, periodontal ligament tension provides the continued eruption against the opposing tooth as wear occurs. The slow apposition of cementum and further alveolar bone growth compensate for tooth wear.
The preeruptive phase describes the passage of a tooth from its crypt in the alveolar bone, through the bone between it and the oral cavity, through the oral mucosa and into the oral cavity. The tooth takes with it the remains of the enamel epithelium which becomes the future epithelial attachment.
The eruptive phase describes the migration of the tooth into the oral cavity until it makes contact with the tooth in the opposing arch. The deciduous teeth make this journey uninterrupted, but some of the permanent teeth which succeed a deciduous tooth are only able to erupt after the roots of the deciduous tooth have been resorbed by osteoclasts.
A permanent tooth may take 2 to 4 years to move through the alveolar bone and into occlusion. Its most rapid progress is seen just after breaking through the oral mucosa, when its root would be two-thirds complete. The maxillary incisors show the most rapid movement, about 1 mm/mo. Molar teeth achieve only half this rate, and if crowded move at 1 mm/6 mo.
When enamel formation is complete, the internal and external enamel epithelium fuse to form the reduced enamel epithelium (REE). The cells still in contact with the enamel develop hemidesmosome, which attaches to the enamel surface. As the tooth erupts, the REE cells mingle with the oral epithelium and hence form a “junction” between the tooth and the future gingiva (▶ Fig. 8.2). At no time during eruption, there is a break in this epithelium, and hence there is no bleeding or risk of infection as the tooth emerges through the oral mucosa. During eruption of the first teeth, they may provoke a mild immune response comprising swelling around the erupting tooth and a slight fever. After the first few teeth have erupted, the process generally occurs without the child noticing that a new tooth has arrived. The erupting permanent tooth passes through a band of fibrous connective tissue, the gubernaculum, which connects the tooth germ with the oral mucosa. So, like the deciduous tooth, the permanent tooth does not cause a break in the epithelium as it erupts into the mouth.
Fig. 8.2 A diagrammatic representation of the formation of the epithelial attachment and the gingival sulcus. The reduced enamel epithelium fuses with the oral epithelium before the tooth erupts. The tooth moves through a core of epithelium, so that connective tissue and the dental follicle are never exposed to the oral cavity. The is converted to the epithelium of the gingival sulcus, but where it remains attached to enamel it becomes the junctional epithelium.
For the unerupted tooth to emerge from its bony surrounds, it is clear that two processes have to occur. The first is the removal of bone above the tooth to create a pathway for the tooth to emerge. The second is the deposition of bone under the developing root of the tooth to help move the unerupted tooth out of the alveolar bone. These two processes are achieved by bone remodeling, which we have noted is an event which is controlled by osteoblasts, which mediate the removal of bone over the tooth by osteoclasts. Osteoblasts lay down new bone under the growing root. The molecular events which control bone deposition and removal around the erupting tooth and its follicle are similar to those described in bone remodeling (see Chapter 7.7 Bone Remodeling). Just before eruption begins, there is a burst of osteoclast activity above the follicle. This is initiated by the protein colony-stimulating factor 1 (CSF-1). This factor attracts monocytes from the blood and also blocks osteoprotegerin (OPG), an inhibitor of the differentiation of monocytes into osteoclasts. The influences of CSF-1 are augmented by raised levels of vascular endothelial growth factor (VEGF). VEGF initiates angiogenesis (new blood vessel formation), and receptor activator of nuclear factor kappa-B ligand (RANKL), which stimulates the differentiation of osteoclasts. At the same time, bone is being deposited at the base of the follicle which is reflected by raised levels of the bone morphogenic protein (BMP). The importance of the osteoclasts and osteoblasts in eruption is confirmed by several studies which show that a tooth will not erupt if the dental follicle is removed. These two mechanisms, bone deposition and bone removal, explain the preeruptive phase of eruption in which the tooth emerges from the alveolar bone. There have, however, been some alternative explanations which have been popular:
Root elongation occurs as the tooth emerges until the root is fully formed. There is no evidence that root growth is an eruptive force in the preeruptive phase as rootless teeth can erupt, and teeth may fail to erupt, though they have fully developed roots.
During the eruptive and posteruptive phase, alveolar bone growth continues although once the preeruptive phase is over, there is no bone to impede the tooth’s progress and no further need for any osteoclastic activity. If bone growth at the base of the tooth continued at the preeruptive rate, it would soon exfoliate the tooth. Bone deposition around the emerging root is, however, essential in providing the full development of the tooth socket. Periodontal ligament tension was thought to be able to pull the tooth out of its socket. This does not seem possible during the preeruptive phases, as inert replicas are able to erupt without any ligament.
There is evidence that the periodontal ligament fibroblasts may exert some tensional force on the tooth during its eruptive and posteruptive phases. During the posteruptive phase, both continued active eruption and bone growth may be mechanisms of compensating for tooth wear. No theory of tooth eruption mechanism can claim to account for all aspects of eruption. A view has emerged that eruption might be brought about by different mechanisms at different stages, which at the present state of our knowledge is probably the best explanation. Deposition and removal of alveolar bone may be the driving mechanism at the preeruptive stage while periodontal ligament forces may be mostly responsible for posteruptive and continued eruption.
A biting force directed down the long axis of a tooth toward the root apex, which would balance the eruptive force. If the bite force were directed at an angle to the long axis of a tooth, it would cause the tooth to rotate, and reposition laterally. When a cusp articulates well into the opposing tooth fossa, such laterally displacing forces are reduced. Cusps may serve to guide the erupting tooth into a stable position within the dental arch.
The lateral forces encountered by adjacent teeth (see Chapter 1.2.3 Tooth Wear in Man).
The magnitude, direction, duration, and frequency of all these forces make impossible any simple equation which might determine the equilibrium position of a tooth. However, when one of the above components is completely absent from the equation, the consequences may be predictable. If there is no opposing tooth (most commonly due to extraction), the eruptive force of a tooth is unopposed, and it may continue to erupt. If this does happen, the tooth appears to bring its gingival attachment and the alveolar bone with it at first. This has been termed periodontal growth, and may occur quite naturally in subjects whose teeth have been worn by a course diet (see Chapter 8.3.2 Consequences of Tooth Wear). If the tooth continues to erupt without the alveolar bone, the distance from the cementoenamel junction (CEJ) to the alveolar bone crest increases. This process has been termed active eruption. Later, the gingival and periodontal attachment may move apically and the root surface may be exposed. This process is termed passive eruption. These observations suggest that continued eruption occurs both due to alveolar bone growth and the active eruption of the tooth.
Overeruption due to loss of opposing teeth may destabilize the entire arch of teeth and in order to prevent this, especially in young children, appliances may be fabricated which exert a light but sufficient force to prevent overeruption.
The complete eruption of a tooth may be blocked by other teeth whose eruption is more advance, and already have occupied a place in the dental arch. One of the most common sights for this incomplete eruption is the mandibular third molar. The bud of this tooth may be orientated in a mesial direction, and unless there is sufficient space for it to erupt mesially and coronally, its eruption is blocked by the second molar. Less common is for the maxillary canine tooth bud to be orientated in a horizontal direction, and this position may make eruption impossible. More commonly, the maxillary canine does erupt into the oral cavity, but there is insufficient space, and it is blocked out of the arch in a labial position. This problem occurs as the maxillary canine is the last tooth to erupt, and there may not be sufficient space after all the other teeth have erupted.
Overcrowding is a common problem in the modern human dentition. Human teeth vary considerably in size; the teeth of males are slightly larger than the teeth of females. Anthropological studies reveal a normal distribution of tooth sizes throughout a population. A normal distribution means that if the frequency of the data is plotted on a curve, it is evenly distributed and the curve looks like a bell. there will be a similar proportion of individuals at one end of the curve, who have unusually small teeth, and an equally similar number of individuals at the opposite end of the curve, who have unusually large teeth. One of our ancestors (or cousins), Australopithecus robustus, had huge teeth and an unusually large jaws to accommodate them. If we should inherit a tendency for larger teeth and a smaller face, the problems of overcrowding are likely.
It is common to assume that the contact of a tooth in the opposing arch is required to prevent continued or overeruption. An uncommon growth disorder causes a failure in alignment of the maxillary and mandibular dental arch. The result is called an anterior open bite. It can be so severe that only the third molars meet when the jaw is closed. It is surprising therefore to find that the teeth do not overerupt. This condition may require surgical repositioning of entire sections of the teeth and alveolar bone. It is not certain what would be the long-term stability of the dentition if it were not treated.
The deciduous dentition consists of five teeth in each quadrant, two incisors (1 and 2), a canine (3), and two molars (4 and 5). The purpose of the deciduous dentition is to provide the growing child with effective chewing ability consistent with the size of the jaws. It allows time for the development and growth of larger and more durable teeth. The first deciduous teeth begin to calcify in the fourth month of pregnancy and erupt 6 months after birth (▶ Fig. 8.3). The mandibular incisors are the first to erupt, followed by the maxillary incisors. The next teeth are not the canines but the first deciduous molars. Then come the canines and lastly the second molar. In most cases, the mandibular teeth erupt before their maxillary counterparts. Root formation continues to occur from 12 to 18 months after a tooth has erupted. By the age of 2 to 4 years, the entire deciduous dentition has erupted (▶ Fig. 8.4).
No permanent teeth will erupt until the age of 6 or 7, but the formation of the permanent teeth is already advanced. The permanent incisors and first molars begin to calcify at birth. At the age of 4, the crowns are fully formed.
Dental pulp: The pulp chambers in deciduous teeth are larger, but the pulp canals are narrower than permanent teeth. In deciduous teeth, there is no clear distinction between the pulp chamber and the root canal, whereas in permanent teeth, there tend to be clearly defined entrances to the root canal. This is particularly true of multirooted teeth where there is a well-defined floor of the pulp chamber with separate openings for each root canal.
The changeover period between the deciduous and permanent dentition happens gradually, so that there is a period, the mixed dentition, when there are both deciduous and permanent teeth. This stage begins with the eruption of the permanent maxillary incisors at 6 years and ends with the loss of the last deciduous tooth, the canines. During the mixed dentition period, the permanent teeth erupt in a fairly predictable order although there is some variation seen (▶ Fig. 8.5). Eruption of the permanent teeth may be earlier than normal if the overlying deciduous tooth is extracted early. If the deciduous second molar is lost early due to caries, the permanent first molar may drift forward as it erupts, thereby reducing the space for the second premolar which may then be crowded out.
As a tooth erupts into the oral cavity, it becomes subject to a variety of forces. The final position occupied by the tooth is not just dependent on the magnitude of these forces, but also the duration, frequency, and direction of these forces. Light continuous forces may have a greater impact than infrequent, short-acting, high forces.
Occlusion is a topic which has exercised the minds of dentists for the last 100 years. There have been entire schools of occlusal philosophy, most of which have been directed toward the anatomy of occlusion or a mechanical interpretation of how the teeth work. The solution to a reliable, evidence-based understanding of occlusion is to focus on the basic physiology, with a little help from comparative evolution. This process should provide some stable ground, on which to choose from the many prescribed clinical practices in occlusion, with some expectation that the choices are based on sound evidence and not on dogma.
Mammals experience relatively little trouble from their teeth. Their gnawing/nibbling/crushing food processors last lifelong without needing repair. If we look at a mammalian skull, the teeth appear to be indestructible pegs fixed into the jaw bone, but of course they are not static. They are responsive to change, such as physical wear and demineralization, and they are not fixed permanently in one position. In comparison, dental implants are completely static. The long life of mammalian teeth is partly due to characteristics acquired over millions of years of evolutionary selection and also due to their interaction with other teeth which bring stability to the whole and its parts. Modern human dentitions are not as stable as those of our ancient human relatives. We experience caries and periodontal disease which reduce the life of individual teeth and eventually the entire dentition. Loss of even one or more teeth may cause instability in the teeth remaining. This instability may be due to overeruption when an opposing tooth is lost, or tilting and drifting into a space left by the loss of a neighboring tooth in the arch.
As the dentition is reduced by tooth loss, the load on the remaining teeth becomes excessive, and they become even more unstable and collapse. With dental treatment we try to reduce the disease processes and to restore the stability of individual teeth and the dentition as a unit. It is useful to understand the factors which maintain the stability of the dentition, so as to ensure our restorations contribute to long-term occlusal stability.
It has been noted above that after eruption teeth stabilize in a position where forces acting on them are in equilibrium. Thus, a tooth would occupy a position in the arch, where the pressures of the tongue and lips were equalized. This theory is supported by the observation that tongue-thrusting habits in children tend to create a longer arch and splay the anterior teeth forward. However, Proffit has shown that even in children without a thrusting tongue, the forward/outward pressures of the tongue during swallowing are considerably greater than the inward forces of lips and cheeks.1. He therefore doubts that a position of equilibrium exists due to a balance of soft tissue pressures alone. There are other factors which determine the stability of the arch and these include the way the teeth meet each other, occlusion. We have seen that teeth are able to reposition in the bony socket as a result of forces acting on them. For example, teeth drift in a mesial direction as a result of approximal tooth wear and a mesial component of the bite force. This mesial component might be quite a small force. What about the much larger bite forces which would tend to intrude the teeth? When cracking a nut, we may be able to exert a bite force on a single tooth of up to 100 kg. Why does this tooth not become intruded by remodeling of the socket? Proffit reminds us that the magnitude of the force is not the only factor which could cause the tooth to reposition. Duration, frequency, and direction of the force are as important as magnitude in determining how living tissue will respond to force. When cracking a nut, let us consider these other factors:
The direction of the bite force is more or less down the long axis of the tooth, so we are spreading the load over the entire periodontal ligament. In spite of the high magnitude of force used in cracking a nut, the supporting tissues are not damaged and do not remodel.
Let us now return to explain the apparent paradox of Proffit’s findings. During swallowing, the tongue pushes harder against the mandibular teeth than the lip does. However, the duration of the swallow is short, and the frequency is relatively low. The lip, on the other hand, exerts a light force against the teeth, but its duration is very long and the frequency is continuous. It is the light continual forces which orthodontists find move teeth most effectively, not intermittent high forces. So, continual light muscle forces of the lip may well balance out the infrequent high forces of the tongue, leaving the tooth at the end of the day, balanced by the influences of the lip and the tongue. If the duration and forces generated by the tongue are abnormally high or the lip muscles are flaccid and incompetent, the balance may be disturbed and the teeth may become displaced.
We have noted that there is a mesial direction of force generated during biting which tends to push the teeth forward, keeping the teeth together and the arch intact. If a tooth is lost and the support it gave to its neighbors reduced, the neighboring teeth may rotate into the space it occupied. If the posterior sections of the arch are lost, the maxillary anterior teeth may splay out labially, as they are readily rotated, by having to withstand the full impact of the bite forces which are not directed down the long axis of the tooth (▶ Fig. 8.6). These are the factors which tend to destabilize the arch:
Fig. 8.6 A clinical example showing the collapse of the anterior teeth due to loss of posterior support. Loss of posterior teeth may place excessive loads on the anterior teeth. The inclined contacts are unstable and cause the maxillary anterior teeth to splay outward. Occlusal vertical height is reduced.