Dental occlusion in primary dentition is established by two and a half to three years of age. The dental arches are half-round or ovoid. The curve of Spee is almost flat, and overjet overbite are minimal. The deciduous incisors are more upright compared to their permanent successors.

Between the ages of 4 and 5 years, the interdental spaces appear due to the rapid development of jaws. The spaces between incisors in the primary dentition are secondary or developmental. A spaced primary dentition is good because it accommodates permanent successors without crowding. The distinct spaces between the mandibular canine, the first primary molar, between the maxillary lateral incisor and the primary canine are called primate spaces (simian spaces/anthropoid spaces) ( Fig. 15.1 A and B).

(A and B) Primate space: these are often found distal to deciduous lateral incisor in the maxilla, deciduous canine in the mandible.

From 4 years of age to the eruption of the permanent molars, the sagittal dimensions of the dental arches remain nearly unchanged. Only minor changes occur in primary dentition arches from 3.5 to 6 years, mainly in transverse dimensions. ^,

The deciduous maxillary inter-canine distance is 1.7 mm greater in spaced dentition than in closed/non-spaced dental arch. The mandibular inter-canine distance is 1.5 mm higher in spaced dentitions. A high prevalence of spacing is seen in primary dentition. It is more frequent in males than in females. The sagittal occlusion relationship has no direct bearing on spacing occlusion. Spaced dentition will likely have a less-frequent posterior crossbite and more-frequent open bites.

The absence of spacing in the deciduous dentition and the presence of overjet, a straight terminal plane molar relationship, suggest the presence of incipient malocclusion.

During the growth of the jaws, the canines and molars remain in a positive contact relationship. During this period, all mandibular teeth occlude slightly lingually to maxillary teeth. The mandibular deciduous molars occlude slightly mesially to their antagonist due to the difference in mesiodistal crown dimension of the maxillary mandibular central incisors. For normal occlusion in both jaws, the part forming the permanent teeth should be distally located in the jaw compared to their predecessor, except for the canines, which are at the same level. The terminal plane of the molars is one of the significant indicators of the possible future sagittal occlusion in the permanent dentition.

Shedding or exfoliation of each primary tooth should occur at the predetermined age to allow eruption of the permanent successor and establishment of the adult occlusion. The events are genetically controlled and influenced by the local environment and other systemic conditions, including growth and nutrition. An orchestrated physiologic sequence of eruption of the successor’s teeth and several adjustments in dental arch dimensions lead to normal occlusion. Careful monitoring of these events, a thorough clinical examination and an appropriate radiographic surveillance review should assist dental surgeons in monitoring the normal or any disturbances associated with dental eruption.

Variations in the eruption of permanent teeth associated with exfoliation of primary teeth are not uncommon. A 6- to 10-month range for early or delayed exfoliation is generally considered within the normal range.

Apart from idiopathic aetiology, extensive dental caries and trauma are common reasons for the early loss of primary teeth. The systemic factors associated with premature loss are hypophosphatasia, rickets, acrodynia, leukaemia, juvenile periodontitis and Papillon–Lefèvre syndrome.

The consequences of premature exfoliation of deciduous teeth are multifactorial and can manifest as poor articulation and orthodontic and psychosocial disorders. The loss of primary teeth can have an adverse effect on children’s phonation, resulting in speech distortion. The maintenance of deciduous arch integrity is a crucial factor to consider, as it significantly influences the formation and development of permanent successors.

Malocclusions that result from early tooth loss often require orthodontic intervention to maintain the integrity of dental arch length with a space maintainer appliance. The deciduous teeth exhibiting unusual mobility and premature loss should always prompt clinicians to perform a thorough investigation. Proper diagnosis to exclude a systemic disease or local pathology requires help from the medical team and oral surgeon to rule out any systemic condition of bone metabolism, endocrine disorder or local pathologies of bone or odontogenic origin. A collaborative and communicative practice that involves shared roles and responsibilities among different disciplines is necessary for achieving long-term therapeutic success.

Premature exfoliation of primary teeth can have a negative impact on the eruption pattern of permanent teeth and arch length. This can affect the establishment of normal occlusion and causes space loss, with the most significant loss occurring within the first year, particularly within the first six months after exfoliation. The quantum of arch length loss depends on the tooth segment, age of premature exfoliation and jaw type. The deciduous molar region is the most adversely affected area, with the maxillary arch buccal region experiencing faster arch length loss than the mandible.

The early loss of mandibular deciduous first molar affects arch length loss by allowing a drift mesial and distal to space, impacting space for erupting first premolar, while early loss of second deciduous molar leads to a mesial shift of permanent first molar and impacting space of second premolar. ^, Loss of deciduous canines leads to anterior arch collapse. The erupting incisors, under the influence of lip pressure, undergo lingual tilt, leading to the deepening of the bite.

The maxillary arch length is more susceptible to early loss of the second deciduous molar, sometimes under the influence of an erupting permanent first molar leading to class II malocclusion.

Delayed exfoliation of primary teeth can also pose problems for the eruption of their successors, consequently adversely affecting the development of normal occlusion. Vigilant clinical examination and regular monitoring supplemented with appropriate radiographs can help in the early detection of intervention of retained teeth. Delayed exfoliation or retention of deciduous teeth beyond their age of exfoliation can result from the congenital absence of the permanent follicle, ankylosis or trauma. The systemic factors include familial patterns, endocrine disturbances (hypothyroid, pituitary hormone disorder) and syndromic or congenital defects such as cleidocranial dysostosis.

A tooth not exfoliated by a certain age of its shedding is called a retained tooth. A primary central incisor is labelled as retained if not exfoliated by 8.2 years, and a primary lateral incisor by 8.4 years. The retained teeth would require extraction to facilitate the normal eruption of the successor’s tooth. A firm primary incisor whose root had failed to resorb past the shedding schedule would also require extraction.

The decision to extract a deciduous incisor to accommodate an erupting permanent successor is controversial. The permanent mandibular incisors typically erupt lingual to primary dentition. With time, due to the influence of the tongue and continued alveolar growth, permanent incisors move into their designated normal labial position. If exfoliation of the maxillary primary incisor is delayed or deciduous tooth/teeth are retained, the labial positioning of permanent incisors is adversely affected, causing an anterior crossbite.

The erupting permanent mandibular first molars tend to push the primary molars mesially, closing the primate space distal to the primary canines, thereby converting the straight terminal plane to a mesial step relationship. Consequently, the first permanent molars emerge to establish a class I molar occlusion relationship. This phenomenon is termed an ‘early mesial shift’, which in part is also contributed by utilising primate spaces by deciduous mandibular first molar and growth of the mandible ( Fig. 15.3 ).

• Incisor liability: The erupting permanent incisors are larger in mesiodistal dimensions than their deciduous predecessor; therefore, they need more arch length to accommodate the alveolar arches. The additional space required for erupting permanent incisors amounts to ‘incisor liability’. Incisor liability is 7.6 mm in maxillary and 5 mm in mandibular arch. Incisor liability is adjusted through the spaced primary dentition and continued growth of the bony alveolar segments.

• The mechanism of anterior adjustments : The erupting permanent mandibular incisors are housed lingual to the roots of the deciduous incisor. The eruption is accompanied by root resorption and mobility of the deciduous incisors and exfoliation. When the primary central incisors have exfoliated, the incisors slowly take a position in the neutral zone between the tongue and the lip musculature. The equilibrium between lingual and labial musculature determines the labiolingual position of erupting anterior teeth.

  • The lower lateral incisors often erupt in a transitory crowding state at ages 8–9. This minor crowding is relieved by arch development during the canine eruption. If crowding in lower incisors is moderate, then the extra space is achieved by the slight increase in the width of the dental arch across the canines, by the labial position of the permanent incisors relative to primary incisors and movement of mandibular canine into primate space.

  • The additional arch length requirements for the erupting incisors are made available through the growth of the jaws, more so in the transverse dimensions, availability of labial inclination, spaced primary dentition and primate spaces in the mandible.

  • Typically, the average additional arch length provided by the spaced dentition is approximately 3.8 mm in the maxillary and 2.7 mm in the mandibular arch. The transverse dimensions of jaws, as indicated by the inter-canine arch width, are an increase of about 3.0 mm in each arch. Lack of interdental spacing in primary dentition is a predictor of crowded permanent teeth, which is a possibility in about 40% of cases. A lack of space in the arch to accommodate erupting lateral incisors in the lower arch is likely to push the deciduous canine laterally and cause premature loss, leading to a midline shit.

  • The third mechanism of natural space availability for erupting maxillary and mandibular incisors is due to the more labial positioning of the erupting teeth, the quantum of which is around 2.2 mm in the maxillary dentition of about 1.3 mm in the mandibular arch.

  • A collective arch length gain with growth, incessant arch length and labial positions accounts for around 9.0-mm arch length in the maxillary dentition and 7.0 mm in the mandibular dentition. ^,

• Inter transitional period of two years : During this period, four permanent incisors, the first permanent molars, the deciduous canines and the second deciduous molars, constitute the dental arches. During this period, significant changes do not happen in the dentition.

Early mesial shift.

The permanent mandibular molars emerge into class I molar relation on eruption in dentition. Such an occlusion develops either in a pre-existing mesial step deciduous molar relation or in a spaced primary dentition. Lower primary molars move mesially, which allows the lower permanent molars to erupt in class I relationship, called early shift.

At 8–10 years, the maxillary permanent canines start erupting. The maxillary canine moves from the high position in the maxilla somewhat from near the lateral wall of the pyriform fossa towards the occlusion plane glided by the distal surface of roots of the maxillary lateral incisors. The erupting crown of the permanent cuspid exerts pressure on the roots of the permanent lateral incisors. Thus, the permanent lateral incisors crowns are displayed distally from the midline; this creates a space between the crowns of the permanent central lateral incisors. At this age of transitional dentition, the child does not look aesthetic but unesthetic. Broadbent called this transitional spacing of teeth during the late mixed dentition stage the ‘Ugly Duckling Stage’ ( Fig. 15.4 ).

(A, B, C) Transition spacing of teeth during the canine eruption. Broadbent called it the ugly duckling stage because the child doesn’t look aesthetic in this stage.

As the permanent cuspid continues to erupt at approximately the level of the crowns of the lateral incisors, it exerts pressure against the distal crown of the permanent lateral incisor. It has an uprighting effect, which moves the permanent lateral crown towards the midline, closing the diastema. However, an initial diastema of the ugly duckling stage of more than 2 mm is unlikely to close fully. ^,

The permanent canine eruption is also essential for the transition of juvenile chewing pattern to adult chewing pattern at the age of 12 years.

During the second transitional period of dental development, two significant events occur. First, the lower canines and upper first premolars erupt around the age of ten. The upper lower second premolars and maxillary canines erupt by 12–13 years of age.

There is a significant growth of the maxilla and mandible and transverse dimension of the dental arches, which accommodate erupting canines. The sexually dimorphic characteristics of dental eruption are evident at this stage, with girls achieving eruption and establishment of occlusion earlier than boys.

Late mesial shift . The second transitional period is marked by the eruption of second molars at the age of 12 years and the establishment of occlusion with full permanent dentition (excluding third molars). The available leeway space is utilised.

Leeway space refers to the difference between the mesiodistal widths of the primary canine, first and second molars and the sum of the mesiodistal widths of the permanent canine, first and second premolars in each quadrant. On average, the unerupted canine premolars are 1.7 mm smaller mesiodistally per quadrant in the lower arch and 0.9 mm per quadrant in the upper arch ( Fig. 15.5 ).

Leeway space.

Primary dentition preserves space for the permanent dentition to allow class I molar relationship, greater mesial shift of permanent molars in the lower arch compared to the upper arch

A late mesial shift of the mandibular permanent first molars leads to establishing a class I molar relationship of the occlusion. Due to the difference in mesiodistal dimensions of the primary second molars and the permanent second premolar teeth, the permanent mandibular first molars migrate mesially into leeway space following the shedding of the primary mandibular second molars around 11 years of age. These movements transform the straight terminal plane relationship of primary molars into a mesial step relationship. This phenomenon occurs mainly with closed primary dentition and primary second molars in a straight terminal plane relationship. In this condition, an early mesial shift is not possible; permanent maxillary, mandibular first molars emerge into a cusp-to-cusp relationship. This change has been referred to as the late mesial shift. The late mesial shift is facilitated by utilising leeway space, which is slightly more in the mandible ( Fig. 15.6 A and B).

Late mesial shift to class I occlusion.

(A) The permanent first molars emerge into straight terminal plane relationship. (B) With the shedding of deciduous molars, both upper lower first molars move mesially into leeway space. The lower molar moves mesially more than the upper molar. Class I molar relationship is established much late hence, the name late mesial shift.

The physiologic mesial shift of the molars shows much more variability in addition to the utilisation of the Leeway space. The average mesial shift of the maxillary and mandibular first molars on each side is usually 2.2 and 2.3 mm, respectively. These values are larger than the average leeway space. These observations signify that the molar relationship is significantly influenced by the growth difference between the jaws during the transitional dentition period.

The sagittal skeletal growth of the maxilla and mandible shows varying growth differences. Overall, the mandibular sagittal growth exceeds that of the maxilla by 1.79 ± 3.45 mm. This difference significantly influences the change in a molar relationship during the transitional dentition. Thus, the leeway space and growth of the craniofacial skeleton influence physiologic mesial lower molar shift. The maxillary first molars may be under a more significant influence on the growth than the mandibular first molars.