Deciduous teeth
Deciduous teeth, also known as primary or milk teeth, are the first set of teeth that erupt in human infants and young children, serving as temporary dentition before being replaced by permanent teeth.[1] There are twenty deciduous teeth in total, with ten in the maxillary (upper) arch and ten in the mandibular (lower) arch, comprising eight incisors for cutting, four canines for tearing, and eight molars for grinding, but no premolars.[1] These teeth are smaller and whiter than permanent teeth, with thinner enamel and dentin layers, larger pulp chambers, and broader contact points between adjacent teeth to facilitate their role in early oral function.[1] The eruption of deciduous teeth follows a predictable sequence, beginning with the mandibular central incisors around 6 to 10 months of age, followed by maxillary central incisors at 8 to 12 months, and completing with the second molars by 23 to 33 months, resulting in a full set by approximately 27 to 36 months.[1] Exfoliation, or shedding, starts around 6 years with the central incisors and progresses posteriorly, with the last deciduous teeth typically lost between 10 and 12 years as the 32 permanent teeth emerge.[1] Anatomically, each deciduous tooth features a crown covered in enamel, roots encased in cementum, and internal structures including dentin and pulp, connected to the jaw via the periodontal ligament; the roots are shorter and more divergent than those of permanent teeth to allow for resorption during replacement.[1] Beyond mastication, deciduous teeth are essential for proper speech articulation, facial aesthetics, and guiding the alignment of permanent successors by preserving arch space and influencing jaw development.[2] Loss of deciduous teeth due to decay or trauma can lead to malocclusion or delayed permanent tooth eruption if not managed, underscoring their clinical importance in pediatric dentistry.[1] Early childhood caries remains a prevalent issue affecting these teeth, highlighting the need for preventive care such as fluoridation and regular dental visits from the first year of life.[1]Anatomy and Morphology
Number and Types
Humans possess a total of 20 deciduous teeth, consisting of 10 teeth in the maxillary arch and 10 in the mandibular arch.[1] These teeth are classified into three main types: incisors, canines, and molars, with no premolars present in the primary dentition.[3] Specifically, there are 8 incisors (4 central incisors and 4 lateral incisors), 4 canines, and 8 molars.[1] In terms of arrangement, each of the four quadrants of the mouth contains 2 incisors (one central and one lateral), 1 canine, and 2 molars, ensuring symmetrical distribution across the dental arches.[1] Deciduous teeth are identified using specialized notation systems to facilitate clinical communication; the Universal Numbering System assigns letters A through T, starting from the upper right second molar (A) and proceeding clockwise to the lower right second molar (T).[4] The FDI World Dental Federation system employs a two-digit format for primary teeth, with the first digit indicating the quadrant (5 for upper right, 6 for upper left, 7 for lower left, 8 for lower right) and the second digit specifying the tooth position (1 for central incisor, 2 for lateral incisor, 3 for canine, 4 for first molar, 5 for second molar), resulting in codes such as 51–55 for the upper right quadrant.[5] Compared to the permanent dentition, which comprises 32 teeth including premolars, the deciduous set is fewer in number and lacks these intermediate posterior teeth, reflecting its role as a temporary structure.[1]Physical Characteristics
Deciduous teeth, also known as primary teeth, possess smaller crowns and shorter roots compared to permanent teeth, facilitating their temporary role in the oral cavity. The crowns are typically bulbous in shape with distinct cervical constrictions at the neck, contributing to their overall compact morphology. Enamel thickness in deciduous teeth ranges from approximately 0.6 to 1.0 mm, significantly thinner than the 2.0-2.5 mm observed in permanent teeth, which influences their durability and susceptibility to wear. Dentin is also thinner, and the pulp chambers occupy a larger proportion of the tooth volume relative to the crown, making the teeth more prone to pulp exposure during decay or trauma. In terms of specific morphology, the incisors feature broader and more rounded crowns than permanent incisors, with sharp incisal edges adapted for initial food incision. Canines exhibit shorter stature and a more incisal-like form, with a single prominent cusp and a cone-shaped crown, differing from the more pointed permanent canines. Molars display fewer cusps overall: the first molars generally have four cusps arranged around a central Y-shaped groove pattern, while the second molars have five cusps, including occasional accessory structures like the tubercle of Carabelli in maxillary examples. Roots are shorter and more widely divergent, particularly in molars, to accommodate the developing permanent dentition beneath. Average crown heights provide insight into their scaled dimensions; for instance, the maxillary central incisor measures about 5.3 mm, the lateral incisor around 4.7 mm, and the first molar approximately 4.5-5.5 mm. Root lengths vary by tooth type but are generally 7-10 mm for anterior teeth and 8-9 mm for molar roots, with greater splaying to promote spacing for successors. The higher pulp-to-tooth volume ratio, often substantially larger than in permanent teeth, results in wider pulp chambers that enhance vascularity but increase vulnerability to resorption during exfoliation. Deciduous teeth appear whiter than permanent ones due to the translucent thinner enamel overlying the underlying dentin. Key structural differences, such as the thinner hard tissues and expansive pulp spaces, render deciduous teeth more readily resorbable by odontoclasts, ensuring orderly replacement by permanent teeth without excessive force. These traits collectively support their function in guiding arch development and maintaining space.Development and Eruption
Embryological Formation
The embryological formation of deciduous teeth initiates during the sixth week of embryonic development through the formation of the dental lamina, an ectodermal thickening in the developing jaws that serves as the primordium for all teeth. This process begins earlier for deciduous teeth compared to permanent ones, with the dental lamina extending into the underlying mesenchyme to outline the positions of the 20 primary teeth.[6] Tooth development progresses through a series of morphological stages: the bud stage (8 weeks), where epithelial buds protrude into the mesenchyme; the cap stage (around 12 weeks), characterized by the invagination of the enamel organ to enclose mesenchymal cells; and the bell stage (following cap stage), during which the enamel organ fully differentiates and the dental lamina begins to disintegrate. These stages reflect dynamic epithelial-mesenchymal interactions, with the oral ectoderm-derived epithelium inducing the neural crest-derived mesenchyme to condense and form the dental papilla and follicle. The enamel organ comprises the inner and outer enamel epithelium, stellate reticulum, and stratum intermedium, while the dental papilla will develop into the dental pulp and odontoblasts, and the follicle contributes to the periodontal ligament and cementum.[6] Histogenesis follows, with the inner enamel epithelium differentiating into ameloblasts that secrete the enamel matrix, and mesenchymal cells in the dental papilla becoming odontoblasts that produce predentin. Mineralization commences around the 14th week of gestation, as odontoblasts deposit hydroxyapatite crystals to form dentin, followed by ameloblasts mineralizing the enamel matrix into the hardest tissue in the body. Genetic regulation is critical, with transcription factors such as PAX9 and MSX1 playing key roles in epithelial-mesenchymal signaling; mutations in these genes disrupt bud formation and lead to non-syndromic tooth agenesis, often affecting multiple teeth.[6][7] By the fourth month of gestation, calcification is active across the deciduous dentition, beginning around the fourth month for incisors and canines, and at birth for first molars, with crown formation completing postnatally: incisors by 1.5-2.5 months, canines by 9 months, first molars by 5.5-6.5 months, providing a foundation for postnatal root development and eruption. However, the second molars lag, with their crown calcification initiating later and completing in the first few years after birth.[6][8]Eruption Timeline
The eruption of deciduous teeth, also known as primary or baby teeth, typically begins around 6 to 10 months of age and is completed by 2.5 to 3 years, following a predictable sequence that maintains the heterodont arrangement of incisors, canines, and molars.[9][10] The process starts with the mandibular central incisors, progresses to the maxillary incisors and lateral incisors, then to the first molars and canines, and concludes with the second molars, with anterior teeth generally erupting before posterior ones.[11][12] The typical ages for eruption vary slightly by arch and tooth type, as outlined in the following table based on established clinical guidelines:| Tooth Type | Typical Eruption Age (Months) |
|---|---|
| Mandibular central incisors | 6–10 |
| Maxillary central incisors | 8–12 |
| Maxillary lateral incisors | 9–13 |
| Mandibular lateral incisors | 10–16 |
| Maxillary and mandibular first molars | 13–19 |
| Maxillary canines | 16–22 |
| Mandibular canines | 16–22 |
| Maxillary second molars | 25–33 |
| Mandibular second molars | 23–33 |