Chronology and morphology of primary and permanent teeth

Outline Tooth development phases Pre-eruptive period Period of eruption of the primary dentition Static period of the primary dentition

Tooth Development Phases 1- Initiation The process of tooth development starts as early as 7 weeks in utero In this phase, the locations of teeth are established with the appearance of tooth germs.

Tooth Development Phases 2- Morphogenesis The shape of the teeth is determined in this phase.

Tooth Development Phases 3- Histogenesis Differentiation of cells takes place to produce the fully formed dental tissues.

Underlying Mesenchyme The primary epithelial bands divides into: Vestibular lamina (buccally). Dental lamina (lingually). Underlying Mesenchyme The dental lamina contributes to the formation of the teeth.

Tooth germ formation Bud stage: (initiation) The enamel organ appears as a simple ovoid epithelial mass Surrounded by mesenchyme. Mesenchyme separated from the epithelium by a basement membrane.

Tooth germ formation Cap stage (morphogenesis) Invagination of the deeper surface of the enamel organ. Peripheral cells start to be arranged as external and internal enamel epithelium.

Cap stage

Tooth germ formation Bell stage The shape of the internal enamel epithelium decides the shape of the crown.

Bell stage

Bell stage Cells of the inner enamel epithelium differentiate into Ameloblasts Adjacent cells from the dental papilla will differentiate into Odontoblasts Odontoblasts produce pre-dentine and dentine. The presence of dentine induces Ameloblasts to form enamel Apposition of enamel and dentin will be followed by calcification

Calcification Starts between 14 and 16 weeks of intrauterine life for primary teeth Begins in cusp tips and incisal edges of teeth and continues cervically Very sensitive process that takes place over a long period of time.

Calcification Any severe systemic event during the development of teeth will result in some dental abnormality Chronological enamel defects.

Calcification Different teeth will show defects at different levels of the crown depending on the stage of crown formation.

Eruption Eruption is the movement of teeth within and through the bone of the jaws and the overlying mucosa to appear in the oral cavity and contact the opposing teeth. Emergence of the tooth is the first sign of appearance in the oral cavity.

Pre-eruptive phase: period during which the tooth root begins its formation and begins to move towards the surface of the oral cavity from its bony vault.

Eruptive phase (prefunctional): period of gingival emergence until contact is achieved with the opposing tooth. Functional eruptive phase: after the tooth meets its antagonist. A dynamic unit throughout life.

Factors influencing tooth formation and eruption Why is it difficult to study and understand the process of eruption? Tooth structure and eruption vary from one species to another Histological studies in humans are rarely possible because of the inaccessibility of tissue for sampling and ethical considerations

Theories of eruption 1- Root formation 2- Hydrostatic pressure 3- Bone remodeling 4- Periodontal ligament

Root formation theory The space for the growing root is accommodated by occlusal movement of the tooth crown However; Some teeth with extensive root development fail to erupt A study using dogs, showed that the tooth itself played no part in the eruptive process by Marks and Cahill2 In this investigation tooth germs were removed and replaced with dead crown shells, synthetic substitutes or given no form of replacement. The follicular changes and the path of eruption were no different from that seen in normally erupting teeth

Hydrostatic pressure theory Studies using dogs demonstrated that the tissue pressure apical to the erupting tooth was greater than occlusally, theoretically generating an eruptive force However; The study only compared pressure differentials but whether this pressure difference actually caused eruption is not proven

Bone remodeling theory Bone remodeling around the tooth causes eruption However; Animal studies showed that bony remodeling occurs around the dental follicle regardless of the presence of a tooth , suggesting that the remodeling process may be under the control of the dental follicle

Periodontal ligament theory Strong evidence exists to show that the periodontal ligament, which is derived from the dental follicle, provides the force required for eruption mainly by fibroblast contraction However; in vitro tissue studies have limitations

In conclusion There is no evidence that one hypothesis fully explains tooth eruption, and that eruption is likely to be a multifactorial process

Basic biology of tooth eruption 1- Bone resorption Resorption appears to be genetically controlled and not mechanically by the eruption of the tooth 2- Role of the dental follicle Removing the follicle means the tooth will not erupt while leaving it and replacing the tooth with an artificial replica means the tooth will erupt 3- Cellular events and molecules Certain molecules will recruit mononuclear cells into the dental follicle Imobilizing the tooth does not stop the formation of the eruption pathway. (Cahill 1969) In succedaneous teeth the eruption pathway follows the gubernacular canal above each tooth. Osteoclast create this pathway even if the tooth is stationary. Thus resorption appears to be genetically controlled and not mechanically by the eruption of the tooth. It has been proven that bone resorption is required for tooth eruption. Osteoclasts appear to arise from an influx of mononuclear cells (osteoclast precursors) into the dental follicle at a specific time prior to eruption.   2) Role of dental follicle: The dental follicle originates form cranial neural crest mesenchyme and is a loose CT sac surrounding the enamel dental organ and develops into the PDL. It is required for eruption as perhaps is the PDL. Shown that removing the follicle means the tooth will not erupt and leaving the follicle and replacing the tooth with an artificial replica will mean this erupts. This disproves such as the pulp theory and the root elongation theory. It is also known that rootless teeth will erupt. Studies have suggested that the force of eruption may come from the PDL and its derivative PDL plays a role in supraossoeus phase of eruption. In mucopolysaccharidoses IV eruption of the permanent molar teeth is retarded and the dental follicles are abnormal as they have successive accumulations of dermatan sulfate. i.e. eruption prevented by abnormal follicles. 3) Cellular events: Prior to the onset of eruption there is an influx of mononuclear cells into the coronal portion of the dental follicle, with a concurrent increase in osteoclasts at the coronal 1/3rd of the alveolar bony crypt. This may be needed so that the osteoclasts resorb bone for the eruption pathways. The dental follicle serves as a target tissue to attract the mononuclear cells and regulate the cellular events of eruption.

Control of eruption 1- Hormonal control 2- Systemic conditions 3- Physical control mechanism

Hormonal control Most eruption occurred in the late evening indicating that eruption was probably under hormonal control Mainly due to the effects of the late evening secretion of growth hormone and thyroid hormone. Children with growth hormone deficiency had delayed tooth eruption Hypopitutarism Hypothyroidism hypoparathyroidism

Systemic conditions Nutritional deficiency (extremes) Preterm and low birth weight infants Cerebral palsy Anemia Renal failure Genetic disorders Apert syndrome Cleidocranial dysostosis Down syndrome Ectodermal dysplasia Gardner syndrome Osteopetrosis

Physical control mechanism According to the equilibrium theory teeth remained in a position within the jaws where forces acting in equal and opposite directions cancelled each other Oral musculature, soft tissue pressures, masticatory forces, and eruptive force.

Pre-eruptive period Upper anterior gum pad (intercanine width) is typically wider than the lower anterior gum pad Upper anterior gum pad protrudes (Overjet) about 5mm Overbite about 0.5mm.

Pre-eruptive period Marked palatal width increase and a decrease in the overjet over the first 6 months of postnatal life.

Pre-eruptive period Labial frenum is usually hypertrophic but it does not hinder suckling Retro incisal papilla is hypertrophic.

Pre-eruptive period Palate is straight at birth. Becomes concave under the effect of growth of the alveolar bone Tongue is relatively large.

Oral mucosa Epstein pearls. Bohn nodules Dental lamina cyst.

Epstein pearls Small white or greyish white lesions Present in about 80% of neonates. Formed along the midpalatine raphe.

Epstein pearls Considered remnants of epithelial tissue trapped along the raphe as the fetus grows. Disappear within a few weeks of life.

Bohn nodules Formed along the buccal and lingual aspects of the dental ridge and on the palate away from the raphe.

Bohn nodules Considered remnants of mucous gland tissue and are histologically different from Epstein pearls. Disappear spontaneously in the early months of life.

Dental lamina cysts Found on the crest of the maxillary and mandibular dental ridges. Are remnants of the dental lamina. Slough within the first few months of life.

Dental lamina cysts Differential diagnosis: natal teeth. No treatment necessary.

Period of eruption of the primary dentition Commences at 6 months and is well established by 30-36 months Maximum growth of the jaws occurs during this period.

Deciduous Teeth 20 in number, 10 in each jaw. There are no premolars in the deciduous dentition. The primary molars are replaced by the permanent premolars. The permanent molars erupt distal to the primary second molars.

Nomenclature Beginning with the midline, the teeth are named as follows: Central incisor. Lateral incisor. Canine. First molar. Second molar.

Tooth numbering Palmer Notation Method. Children’s 20 primary teeth are lettered “A” through "E" in each quadrant. Universal Numbering System. FDI Two-Digit Notation. The currently accepted convention to view the FDI notation chart is from the perspective of the patient's right.

FDI Two-Digit Notation 1s are central incisors, 2s are laterals, 3s are canines, 4s are 1st premolars etc., up through 8s which are 3rd molars The permanent teeth  quadrants are designated 1 to 4 such that 1 is upper right, 2 is upper left, 3 is lower left and 4 is lower right In the deciduous dentition the numbering is correspondingly similar except that the quadrants are designated 5,6,7 and 8.

Chronology of eruption of primary teeth 1st tooth at approximately 6 months Usually the lower central incisor

Chronology of eruption of primary teeth All eruption schedules are estimates No two individuals are alike. Multiple parameters: race, gender, ethnicity, familial environment, heredity.

Chronology of eruption of primary teeth Maxillary Mandibular Central incisors 6-10 months 5-8 months Lateral incisors 8-12 months 7-10 months Canines 16-20 months First molars 11-18 months Second molars 20-30 months

Sequence of eruption A then B then D then C then E Mandibular precede maxillary most of the time.

Rhythm of eruption of primary teeth Symmetrical groups of 4 teeth every 6 months Teeth erupt symmetrically in both jaws, simultaneously and in pairs.

The ‘six/four’ rule for primary tooth emergence Four teeth emerge for each 6 months of age. 6 months: 4 teeth (lower & upper As) 12 months: 8 teeth (1+upper & lower Bs) 18 months: 12 teeth (2+ upper & lower Ds) 24 months: 16 teeth (3+ upper & lower Cs) 30 months: 20 teeth (4+ upper & lower Es)

Static period of the primary dentition Period of stability of the primary teeth 3-6 years of age Child has 20 primary teeth in their final and functional position Occlusion is well established

Static period of the primary dentition Occlusal features Occlusion of the primary second molar Inter-arch relationship of primary teeth

Occlusal features in the established primary dentition Incisor teeth tend to be spaced Primate spaces exist between upper B & C and between lower C and D

Occlusal features in the established primary dentition Upper incisors are upright Incisor relationship is more towards edge to edge.

Occlusal features in the established primary dentition Long axis of primary teeth is parallel Absence of the curve of Spee In general, teeth in the primary dentition tend to be well aligned.

Classification of occlusion of the primary second molar Look at the distal aspect of the primary second molar Flush terminal plane Mesial step Distal step

Flush terminal plane

Mesial step

Distal step

Inter-arch relationship of primary teeth Each tooth occludes with two opposing teeth except for the lower central incisors and the upper second molars. Canine is a key to occlusion in the primary dentition. Look at the long axis of the canine

Canine relationship Long axis of the canine should be placed in the midline between the lower D and C for a class I relationship.

Inter-arch relationship Natural wearing away of the canines is an important physiologic process that facilitates movement of the mandible In children raised on soft food the natural wearing process may be slowed down May have to carry out selective grinding on primary canines, especially in the presence of a unilateral crossbite.