Morphology, histophysiology and pathophysiology of immature temporary and permanent teeth
Introduction :
Like all mammals, man has two successive sets of teeth: he is “diphyodont”.
The first set of teeth is therefore made up of 20 temporary or primary teeth (in English primary teeth ), also called deciduous or milk teeth (improper term) commonly called “baby teeth”, which will be replaced by 32 successive teeth, definitive or permanent: the difference is explained by the absence of premolars and the growth of a third molar.
1. Morphology, histophysiology and pathophysiology of temporary teeth
Introduction
While waiting for the full development of the maxillae, temporary teeth precede the permanent teeth during the first years of life. There are twenty of them: 10 in the maxilla and 10 in the mandible. These so-called baby teeth are considered unstable.
They have a lifespan determined in time because rhizalysis, which is a physiological phenomenon, intervenes to allow the replacement tooth to take its place on the arch.
Compared to permanent teeth, they have particular anatomical and histophysiological characteristics which must be taken into account in order to fully understand their specificity at the pathological and therapeutic level.
1-1-Morphological characteristics of the temporary tooth
The difference with the temporary tooth is not only in size because the morphology influences the periodontal context and the specificities must be analyzed at the coronal and radicular level
1.1.1. At the coronary level
– The very bulbous and round shape is due to significant cervical constriction.
– Their globular appearance is linked to the fact that the mesio-distal diameter is generally greater than their cervico-occlusal height.
– Size: it is a third smaller than the permanent teeth except at the level of the upper temporary molars where the mesio-distal diameter is greater than that of the premolars which they will replace.
– The color is milky white; the thickness of the enamel layer is thinner than on permanent teeth.
– The dentin layer is about half that of permanent teeth.
– The pulp horns are more tapered and high and prominent
– The pulp volume is greater than that of permanent teeth.
– The cameral floor is crossed by numerous pulpo-periodontal canals, the number of which increases with the physiological resorption of the roots.
1.1.2. At the root level
The roots are:
· Thinner and slimmer
· Relatively longer
· Much more curved and especially divergent because the germ of the permanent tooth is between them.
· They diverge from the crown and do not have a common trunk, their separation occurs near the cervical line.
· The channels have flattened shapes, their section resembles a bean
There are numerous pulpo-periodontal canals, especially at the inter-radicular and apical level
1-2- histological characteristics of temporary teeth:
Histological specificity concerns both mineralized tissues and pulp.
1.2.1. Email :
There are two types of enamel:
One formed in utero, the most internal and the other formed from birth.
They are histologically clearly distinct and separated from each other by a neonatal line of Orban. This neonatal line is only the optical reflection of the accentuation of a dense Retzius streak that is present postnatally.
These streaks are less significant in number and pigmentation than those observed on the permanent tooth. Their presence, however, gives the postnatal enamel a darker appearance which distinguishes it, in section, from the prenatal enamel.
A peculiarity of enamel prisms in the gingival third. In the temporary tooth, these prisms tend to remain inclined slightly towards the occlusal surface or the free edge, unlike the permanent tooth where these prisms take, as they tilt, a direction perpendicular to the axis of the tooth
The interprismatic spaces are more numerous and more important; but the superficial layer is often aprismatic.
At this stage, it can already be seen that the fragility and receptivity to caries of baby teeth are largely due to the poor distribution of enamel formations in the proximal areas; moreover, if we consider the structure of these teeth, we see that this fragility seems to be mainly due to the postnatal peripheral enamel, which is less dense than the underlying layers.
1.2.2. Dentin
The same distinction of origin is found at the level of dentin between a prenatal dentin, denser and more homogeneous, and a postnatal dentin, less compact and more porous.
1.2.3. The pulp
It has a structure comparable to that of the permanent tooth.
It is richly vascularized. The cellular and fibrous constituents are represented by fibroblasts, histiocytes, endothelial cells, mesenchymal cells and collagen fibers.
1.3. Physiological characteristics of temporary teeth:
1.3.1. Physiological characteristics of the pulp of temporary teeth:
The pulp tissue of primary teeth is similar to that of permanent teeth.
According to Kopel, the pulp of temporary teeth develops more easily reaction dentin, in response to pulpal aggression than permanent teeth. This defense reaction is however only observed for temporary teeth in stages 1 and 2.
The innervation is not as dense as for permanent teeth, explaining why temporary teeth appear less sensitive.
The reparative potential of the pulp observed in stage 2 temporary teeth is most often manifested by the significant apposition of reactional dentin, which constitutes a barrier preventing the spread of the aggression towards the pulp.
1.3.2. Physiological characteristics of the pulp of temporary teeth:
Temporary teeth are ephemeral, deciduous organs on the dental arch, intended to be replaced during the growth of children. Their life cycle necessarily ends with their exfoliation after the progressive resorption of their roots.
They are formed during the intrauterine period. They erupt between the ages of 6 months and 3 years. In fact, a 3-year-old child normally has all his baby teeth on the arches and his baby molars are in occlusion.
The average value of the functional period of a temporary tooth in the oral cavity is considered to be about 8 years ± 3 months. This characteristic evolutionary physiology can be divided into three phases of unequal duration.
Morphology, histophysiology and pathophysiology of immature temporary and permanent teeth
3.1. Growth phase
It is understood between the moment of the eruption of the temporary tooth in the oral cavity and the end of its root edification. This phase lasts between 1 and 1 and a half years. This is stage I also called the formation or immaturity stage.
Stage I or immaturity (Stage M):
Anatomically, there are no major peculiarities apart from the fact that the roots have not yet completed their development.
The ability to react to external stimuli (caries, trauma) varies at the pulp level and results in:
Either a repair with preservation of pulp vitality,
Either abnormal stimulation of dentinogenesis with closure of the pulp chamber, then of the pulp canal,
This is pulp necrosis, often asymptomatic, which manifests itself by discoloration of the tooth and then by periodontal damage.
3.2. Maturation phase
It is included between the end of root formation and the beginning of physiological resorption.
At the end of this period the temporary tooth is mature.
This phase lasts about 3 years and 9 months. It is stage II, also called the stability stage.
Stage II or stability (Stage S)
The tooth is fully formed and we have the outline of the replacement tooth germ
In its stable phase, the temporary tooth has a physiology quite comparable to that of the permanent tooth.
It has the ability to react to physiological stimuli; dentinogenesis can be particularly intense and more significant than in permanent teeth.
Its significant cellular potential allows it to fulfill its multiple functions of induction, formation, nutrition, protection, defense and repair.
The pulp also has a structure comparable to that of the permanent tooth.
Its central region is richly vascularized, its innervation seems to be less than that of the permanent tooth
Its relations with the periodontium are made by the apical zone as for the permanent teeth, as well as by numerous accessory pulpo-periodontal canals which constitute, even in periods of stability, multiple exchange pathways.
Physiopathological characteristics:
The stable temporary tooth can respond to an aggression by concentrating its defense and repair activities and therefore react actively like the permanent tooth.
Pulp damage can occur rapidly despite a slight loss of substance and a small cavitation. This rapid development is also caused by the lower mineralization of the enamel and by the fact that young dentine is all the more permeable, bacterial penetration into the dentinal tubules being facilitated, pulp inflammation will be generated more quickly than on a permanent tooth.
3.3. Regression phase, Stage III or resorption (Stage R)
It is understood between the beginning of root resorption and its exfoliation.
It lasts about 3 years and 6 months. This stage is characterized by modifications of the root and periodontal structures and by its proximity to the germ of the permanent tooth.
Physiological resorption results in the exfoliation of the temporary tooth and its replacement by the permanent tooth. There is a close relationship between the degree of maturation of the permanent tooth and the degree of resorption of the temporary tooth.
Several changes due to resorption are noted:
Migration of the epithelial attachment to the regions undergoing resorption,
Modification of the root structures: with widening of the apical orifices and multiplication of the secondary orifices which constitutes numerous pulpo-periodontal communications.
These modifications are observed both in the external root morphology and also in the internal anatomy of the canal network by apposition of secondary dentin.
Modification of the inter-radicular zone: inflammatory state and transformation of the connective tissue into hyperemic granulation tissue. This tissue has a strong osteoclastic activity which will lead to the destruction of the cementum and dentin to result in the exfoliation of the deciduous tooth.
Pulp modification: presence of multinucleated inflammatory cells and odontoclasts.
All these changes show us that the temporary tooth is an organ whose capacity for defense and reaction will decrease as resorption progresses.
The temporary tooth in stage III is a widely open entity, in direct contact with the periodontal areas, in continual remodeling
2. Morphology, histophysiology and pathophysiology of immature permanent teeth
Introduction :
A permanent tooth, present in an arch, is said to be immature as long as the apical cemento-dentinal junction is not in place. Immature permanent teeth are present from the establishment of the mixed dentition (6 years) until the beginning of the young adult dentition phase (15 years).
An immature permanent tooth presents certain histological, anatomical and physiological particularities, making it very unique to treat.
2.1. Morphological characteristics
The immature tooth is characterized by:
* by an apical region not completely formed.
*The vascular-nervous bundle is voluminous in a flared endodontic canal, thus revealing a widely open apex.
With thin and fragile dentin walls, the root appears thin and more or less short depending on the degree of evolution.
2.2. Histological characteristics:
2.2.1- Enamel immaturity:
When the tooth erupts, the enamel is immature and then undergoes a process of post-eruptive maturation (secondary mineralization) which will make it mature.
This is a complex mechanism in which water and proteins are gradually replaced by calcium and phosphate, called demineralization-remineralization cycles.
In its mature form, enamel consists of numerous hydroxyapatite crystals and contains more than 96% mineral phase, little water (3.2%) and only traces of organic matrix (0.4%).
While its immature form consists of a reduced mineral phase (37%) and a much larger aqueous phase (44%), and a larger organic matrix (19%),
Thus, the enamel of immature teeth contains numerous structural defects and therefore a porous surface promoting the retention of bacterial plaque conducive to the development of carious lesions.
2.2.2- Dentin immaturity
The immature permanent tooth also has immaturity at the level of dentin. Dentinogenesis continues throughout the formation of the tooth.
During tooth development, three types of dentin are produced:
- Primary dentin is normal dentin, composed of regularly spaced tubules.
- Secondary dentin is deposited in response to biomechanical factors, such as temperature changes, mechanical shocks or chemical irritations.
- Tertiary dentin, also called reactional or irregular dentin, is deposited after major pulp irritation such as attrition, caries, etc. It forms exclusively in areas adjacent to the irritation.
From a morphological point of view, the immature permanent tooth presents:
An absence of secondary dentin, which implies a large pulp volume, without retraction of the walls of the pulp chamber and pulp horns.
In addition, immature permanent teeth do not yet benefit from the deposition of pericanalicular dentin, which gradually allows the obliteration of the tubules. These widely open tubules make the dentine very permeable.
Clinically, this results in the rapid proliferation of caries once the enamel-dentin junction is reached.
2.2.3. Pulp:
Due to its significant vascularization and the, DPI has a greater repair potential than DP
The innervation is also immature, which contributes to making it less sensitive to various external stimuli. The absence of apical constriction means that the nerve pathways are not compressed during possible inflammation, which explains a reduction or even absence of pain.
2.3. Physiological characteristics:
The tooth erupts into the oral cavity when the root has reached two-thirds of its total length (Nolla stage 8). It will take between three and four years to reach stage 10; between these two stages, the tooth will be considered immature.
Root formation occurs by epithelial proliferation in the connective tissue called HERTWIG’s sheath.
It is this sheath which allows the differentiation of cells of the young radicular pulp into odontoblasts ensuring dentinogenesis of the apical end of the root.
Once the definitive length of the root has been acquired, the disintegration is observed of the HERTWIG sheath is observed, thus exposing the dentin in direct contact with the surrounding connective tissue.
This process then leads to the formation of cementoblasts which will produce the primary cementum (which participates in the closing of the apical orifices) and the secondary cementum (which covers the apical part of the root), the latter predominating at the level of the apical third of the roots and thus contributing to the formation of the apex.