Development of Dentition and Development of Dentition and Dental ArchesDental Arches

Presented by:

Dr. Sandhya Anand

Under the guidance of : Prof. Ashima Valiathan Director of postgraduate studies Head of Department Dept. of Orthodontics and Dentofacial

Orthopaedics Manipal College of Dental Sciences, Manipal..

ContentsContents

Evolution of human dentition Prenatal dental development The predental period The deciduous dentition The mixed dentition The permanent dentition Dental arch development References

EVOLUTION OF HUMAN EVOLUTION OF HUMAN DENTITIONDENTITION

During evolution several changes took place in jaws and teeth. When the reptilian dentition evolved to mammalian, dentition went from “polyphyodont” (many sets of teeth), to “diphyodont” (two sets of teeth), and then to “heterodont” (different types of teeth).

There are four stages of tooth evolution::

1.Reptilia stage (Haplodont). This type of dentition is depicted by the

simplest form of teeth – a single cone. It usually includes many teeth in both jaws. Jaw movements are limited to opening and closing only. No occlusion of teeth is seen in this class.

2.Early mammalian stage (Triconodont).This exhibits three teeth in line in the posterior teeth. Anthropologically, the largest cusp is centered, with a smaller cusp anteriorly and another posteriorly.

3.Triangular stage (Tritubercular stage).

The three triconodont lines are changed to three cone-shaped structures, with the teeth more or less by-passing each other when the jaws are opened or closed.

4.Quadritubercular stage. The next stage of development created

a projection on the triangular form that finally occluded with the antagonist of the opposing jaw.During this time, as an accommodation to changes in tooth form and anatomy, the articulation of jaws changed accordingly.

Human tooth size has undergone a clear cut reduction during the Upper Paleolithic Age, and the rate of that reduction has accelerated since the end of the last Ice Age. Beginning about 10,000 years ago,

the rate of reduction seems to have doubled to about 1% every 1,000 years.Associated with the overall dental reduction is a trend for substantial decrease in sexual dimorphism in tooth size.

Common evolutionary trends in primates.

1. There was shortening of jaws due to decrease in size of olfactory organs, upright body position and wide angle of head to body.

2. Decrease in tooth size occurred, so as to accommodate the teeth into the smaller jaws, with subsequent elimination of some teeth from the dentition.

3. There was progressive shortening and relative widening of the dental arches.

4. The canines reduced in size.5. The lower premolars became more symmetrical

from oval.6. The first molars became the dominant cheek teeth.7. The third molars, which were larger than the first

molars, were reduced in size and often eliminated.

Characteristics of the human dentition.

• Acrodont – teeth attached to the jaws by a connective tissue.

• Pleurodont – teeth set inside the jaws.• Thecodont – teeth inserted inside a

bony socket.• Diphyodont – two sets of teeth.

PRENATAL DENTAL PRENATAL DENTAL DEVELOPMENTDEVELOPMENT

The embryonic development of both deciduous and permanent teeth proceeds in four stages:

1. Initiation2. Bud stage3. Cap stage4. Bell stage.

a.) INITIATION OF ODONTOGENESIS. The first sign of tooth development appears

late in the third embryonic week when the epithelial lining of the oral cavity begins to thicken in broad zones. The epithelial thickenings occur on the inferolateral borders of the maxillary and on the superolateral borders of the mandibular arches where the two join to form the lateral borders of the mouth. At 6 weeks, the four maxillary odontogenic zones coalesce to form a continuous dental lamina, and the two mandibular odontogenic zones fuse at the midline.

The teeth begin with invagination of the dental lamina into the underlying mesenchyme at specific locations along the free border of each arch.

Morphologic changes in the dental lamina begin at about 6 weeks in utero and continue beyond birth to the fourth or fifth year. This occurs in three main phases:

1. Initiation of the entire deciduous dentition occurs during the 2nd month in utero.

2. Initiation of the permanent teeth occurs by the growth of the free distal end of the dental lamina into the surrounding connective tissues, giving rise to the successional lamina.

3. The dental lamina elongates distal to the second deciduous molar and gives rise to the permanent molar tooth germs.

b.) BUD STAGE..

Soon after dental lamina formation, a vestibular furrow divides the cheeks and lips from the dental arches. Subsequently, the dental lamina shows specific sites of increased mitotic activity which produce knob-like tooth buds corresponding to the ten deciduous teeth in each jaw. The first buds to form are the mandibular anterior teeth, at about the 7th week. By the 8th week, all maxillary and mandibular deciduous tooth buds are present.

cc.)CAP STAGE. The growth rate at the periphery of the

bud is greater. By the end of the 8th week, there appears a concavity on the deep surface of the bud. The tooth is now in its cap stage. As the epithelium of the cap-shaped organ enlarges and proliferates into the deeper ectomesenchyme, there is increased activity in cells contiguous with the ectodermal tooth bud. At this time, the essential parts of the tooth – enamel organ, dental papilla and dental follicle – are identifiable. Collectively they are called the tooth germ.

d.)BELL STAGE.d.)BELL STAGE. Enlargement of the overall size of the tooth germ

and deepening of its undersurface occurs. Epithelial cells next to the papilla develop into an enamel-producing layer of cells, the inner enamel epithelium; epithelial cells along the leading edge of the germ form the outer enamel epithelium, which eventually gives rise to the dental cuticle.

The differentiation of dentin producing odontoblasts in the dental papilla is initiated by the neighboring cells of the inner dental epithelium. Neighboring cells of the two epithelia progressively constrict around the dental papilla to leave only a small opening, which will become the apical foramen. At this time, the dentin which forms the tooth root is first laid down. The germ loses its connection with the oral epithelium and the inner dental epithelium begins to fold, making it possible to recognize the crown shape of specific morphologic classes of teeth. teeth.

PRENATAL ARCH SHAPE.

The prenatal dental arch progressively changes shape; at 6 to 8 weeks it is anteroposteriorly flattened. By the bell stage of the tooth germs, the anterior segment of the dental arch has elongated and approaches the form of a catenary curve by the beginning of the fourth month.

THE PREDENTAL PERIOD.THE PREDENTAL PERIOD. This period lasts from birth to the eruption of the first

deciduous tooth into the oral cavity, at about 6 months of age.

The Gum Pads. The alveolar arches at the time of birth are termed

gum pads, and they are firm and pink. They develop in two distinct parts, a labio-buccal and a lingual portion. The labio-buccal portion is differentiated first and grows more rapidly. It is divided by transverse grooves into ten segments, each corresponding to a deciduous tooth germ. Of these grooves, those between the canines and first deciduous molar segments are called the lateral sulci , and they extend to the buccal side. The lingual portion remains smooth. These portions are separated by the dental groove , which is the site of origin of the dental lamina. The lingual portion is limited lingually by the gingival groove. In the upper jaw, the gingival groove separates the gum pad from the palate.

Relationship of gum pads. At rest the gum pads are separated by

the tongue, which protrudes over the lower gum pad to lie immediately behind the lower lip, and may even protrude a little between the lips. The anteroposterior movement vary greatly, while the lateral movements are very much limited. The upper gum pad is wider than the lower, and when the two are approximated there is complete overjet of the upper over the lower gum pad, with a considerable anterior overjet. The lateral sulcus of the lower gum pad is usually posterior to that of the upper.

At birth, the gum pads are not sufficiently wide to accommodate the developing incisors, which are crowded and rotated in their crypts. During the first year of life the pads grow rapidly. The growth is most marked in the lateral direction. This increase in width permits the incisors to erupt in good alignment and to be spaced. Also during this period, there is a rapid increase in the labio-lingual dimensions of the gum pads. The length of the gum pads increases more moderately.

The size of the gum pads at birth may be determined by any one of the following factors, according to Leighton :

• the state of maturity of the infant at birth.

• the size at birth as expressed by birth weight.

• the size of the developing primary teeth.

• genetic factors.

THE DECIDUOUS THE DECIDUOUS DENTITIONDENTITION

The deciduous dentition period begins with the eruption of the first primary tooth at about 6 months of age, and lasts till the eruption of the first permanent molars.

DEVELOPMENT OF THE PRIMARY TEETH1.Calcification. The sequence of initial calcification of the

primary teeth is central incisors (14 weeks), first molars (15 ½ weeks), lateral incisors (16 weeks), canines (17 weeks) and second molars (18 weeks). The crowns of the teeth continue to grow in width until there is coalescence of the calcifying cusps, at which time most of the crown diameter has been determined. The crown morphology, rate and sequence of growth, pattern of calcification and mineral content are under genetic control.

2. Eruption.

Eruption of the primary teeth begins in a variable fashion but not until root formation has begun. Occasionally, a “natal tooth” is present at the time of birth. The natal tooth may be a supernumerary one but usually is a very early erupted normal primary central incisor. For this reason such a tooth should not be extracted casually.

Chronology of tooth development, Primary Dentition.

Tooth

Calcification begins

Crown completed

Eruption Root completed

Max. Mand.

Max. Mand.

Max. Mand

Max. Mand

Central

14 wk 14 wkI.U. I.U.

1 ½ 2 ½ Mo. Mo.

10 8Mo.

Mo.

1 ½ 1 ½ Yr. Yr.

Lateral

16 wk 16 wkI.U. I.U.

2 ½ 3 Mo. Mo.

11 13Mo. Mo.

2 yr. 1 ½ yr.

Canine

17 wk 17wk I.U. I.U.

9 Mo. 9Mo.

19 20Mo. Mo.

3 ½ 3 1/4Yr. Yr.

1st Molar

15wk 15wk I.U. I.U.

6 5 ½ Mo. Mo.

16 16Mo. Mo.

2 ½ 2 ¼Yr. Yr.

2nd Molar

19wk 19wkI.U. I.U.

11 10Mo. Mo.

29 27Mo. Mo.

3 Yr. 3 Yr.

FEATURES OF PRIMARY DENTITION.

Spacing.Spacing. In the primary dentition stage a child may

have generalized spaces between the teeth, localized spaces, no spaces, or a crowded dentition. The presence of spacing in the deciduous dentition is a common occurrence.

Generalized spacing occurs in nearly 2/3 of the individuals in the primary dentition stage and is a requirement for the proper alignment of the permanent incisors.

In addition to the generalized spacing, localized spacing are often present and are referred to as ‘primate spaces’. Such spaces are present in 87% of the maxillary arches between the lateral incisor and canine. In the mandibular arch, their incidence is 78% and they occur between the canines and first molars. The primate spaces are normally present from the time the teeth erupt.

Spacing is normal throughout the anterior part of the primary dentition also. Spaces develop between the deciduous incisors subsequent to their eruption, but become somewhat larger as the child grows and the alveolar processes expand. Failure of incisor spacing to appear before 5 years of age occurs in about 20% of cases and usually indicates crowding in the permanent dentition.

Overbite. Overbite is the vertical overlap between

the maxillary and mandibular central incisors. The overbite in the deciduous dentition varies between 10% and 40%. Foster in a study of 100 British children between the ages of 2 and 3 years described the overbite relationship as ideal (19%), reduced (24%), and excessive overbite (20%). The fact that more than 60% of the children in this population have a reduced overbite or an open bite is attributed to the effects of the various oral habits (finger or pacifier sucking) that are common in this age group.

Overjet. Overjet is the horizontal

relationship or the distance between the most protruded maxillary central incisor and the opposing mandibular central incisor. The normal range of overjet in the primary dentition varies between 0 and 4 mm. In the same study by Foster, the overjet was ideal in 28% of the cases and excessive in 72% of the cases. Again, this feature was attributed to the effects of the oral habits

Molar relationship. The anteroposterior molar relationship in

the primary dentition is described in terms of the terminal planes. The terminal planes are the distal surfaces of the maxillary and mandibular second primary molars. The two terminal planes can be related to each other in one of three ways.

In the flush terminal plane relationship, both the maxillary and mandibular planes are at the same level anteroposteriorly.

In the mesial step relationship, the maxillary terminal plane is relatively more posterior than the mandibular terminal plane.

In the distal step relationship, the maxillary terminal plane is relatively more anterior than the mandibular terminal plane.

In a study of 121 Iowa children at age 5 years, the distribution of the terminal plane relationships of the primary second molars were found to be as follows:

Distal step 10% Flush terminal plane 29% Mesial step of 1.0 mm 42% Mesial step >1.0 mm 19% Determining the terminal plane relationships in

the primary dentition is clinically important because the erupting first permanent molars are guided by the distal surfaces of the second primary molars as they erupt into occlusion.

At the late primary dentition stage of development, the maxilla and mandible are housing the greatest number of teeth ever, including 20 erupted primary teeth and at least 28 unerupted but partially forming permanent teeth.

Anomalies of primary dentition

Anomalies of crown development are seen less frequently in primary than in permanent dentition. Primary teeth are rarely congenitally missing, the incidence being 1%.Most frequently missing teeth are the maxillary lateral incisors, maxillary central incisors and the first primary molars in that order.

Ankylosis of primary teeth Primary teeth are more likely to be

ankylosed than permanent teeth and lower teeth twice as often as upper. Ankylosis occurs during the normal physiological resorption of teeth. The majority of ankylosed primary teeth are observed in the late primary and the mixed dentitions. The condition is often bilateral and a posterior open bite appears as the occlusal level of the ankylosed fails to keep up with the vertical development of adjacent teeth. Ankylosed teeth often are referred to as “submerged teeth”.

THE MIXED THE MIXED DENTITIONDENTITION

The mixed dentition period begins at approximately 6 years of age with the eruption of the first permanent molars, and is normally completed at the time the last primary tooth is shed. During the mixed dentition period, the deciduous teeth along with some permanent teeth are present in the oral cavity.

DEVELOPMENT OF PERMANENT DENTITION

I. Calcification Nolla arbitrarily divided the development

of each tooth into ten stages:1. Absence of crypt2. Presence of crypt3. Initial calcification4. One third of crown completed5. Two thirds of crown completed6. Crown almost completed

7. One third of root completed8. Two thirds of root completed9. Root almost complete, open apex10. Apical end of root completed

Girls are more advanced in calcification of permanent teeth than are boys at each stage and more so in the later stages.

II. Eruption.II. Eruption.

(a) Interrelationships between calcification and eruption.

Eruption is the developmental process that moves a tooth from its crypt position through the alveolar process into the oral cavity and to occlusion with its antagonist.

Those permanent teeth that follow into a place in the arch once held by a primary tooth are called successional teeth (e.g. incisors, cuspids and bicuspids). Those permanent teeth that erupt posteriorly to the primary teeth are termed accessional teeth.

During eruption of succedanous teeth, many activities occur simultaneously: the primary tooth resorbs, the root of the permanent tooth lengthens, the alveolar process increases in height, and the permanent tooth moves through the bone.

Permanent teeth do not begin eruptive movements until after the crown is completed. They usually emerge when three-fourths of their roots are completed. They pass through the crest of the alveolar process at varying stages of root development. It takes from two to five years for the posterior teeth to reach the alveolar crest following completion of their crowns and from 12-20 months to reach occlusion after reaching the alveolar margin. It takes about 2-3 years for the roots to be completed after the tooth has erupted into occlusion.

Developmental processes during eruption of succadaneous teeth. A-elongation of permanent root. B-

resorption of primary predecessor. C-movement of permanent tooth occlusally. D-growth of alveolar

process. E-inferior border of mandible, which shows much less growth activity than the other four processes.

Chronology of Tooth Development, Permanent Dentition

Tooth

Calcification begins

Crowncompleted

Eruption Root completed

Max.

Mand.

Max

Mand

Max

Mand

Max Mand

Central

3 mo.

3 mo.

4 ½ Yr.

3 ½Yr.

7 ¼Yr.

6 ¼Yr.

10 ½Yr.

9 ½Yr.

Lateral

11 mo.

3 mo.

5 ½Yr.

4 yr.

8 ¼Yr.

7 ½Yr.

11 yr.

10 yr.

Canine

4 mo.

4 mo.

6 yr.

5 ¼Yr.

11 ½Yr.

10 ½ Yr.

13 ½Yr.

12 ¾Yr.

1st

Premolar

20Mo.

22Mo.

7 yr.

6 ¾ Yr.

10 ¼Yr.

10 ½Yr.

13 ½Yr.

13 ½Yr.

2nd Premolar

27 Mo.

28Mo.

7 ¾Yr.

7 ½Yr.

11 yr.

11 ¼Yr.

14 ½Yr.

15 Yr.

1st Molar

32WkI.U.

32WkI.U.

4 ¼Yr.

3 ¾Yr.

6 ¼Yr.

6 yr.

10 ½Yr.

10 ¾Yr.

2nd

Molar27Mo.

27 Mo.

7 ¾ yr.

7 ½Yr.

12 ½Yr.

12Yr.

15 ¼Yr.

16 Yr.

3rd

Molar8 yr.

9 yr.

14 yr.

14 yr.

20 yr.

20 yr.

22 yr.

22 yr.

The physiologic principles underlying tooth eruption are the same for both primary and permanent teeth.

Pre-emergent Eruption. Eruptive movement of the tooth follicle begins

soon after the root begins to form. Two processes are necessary for pre-emergent eruption. First, there must be resorption of the bone and primary roots overlying the crown of the erupting tooth. Second, the eruption mechanism itself must then move the tooth in the direction where the path has been cleared.

Failure of tooth eruption due to failure of bone resorption occurs in the case of cleidocranial dysplasia.

The precise mechanism through which eruptive force is generated is still not entirely understood. Various theories have been put forward over the years:

• Lengthening of the root within its crypt was initially considered to be the mechanism which caused the tooth to erupt. However, eruption of teeth even after removal of their apical area rejected this hypothesis.

• Localized variations in blood pressure or flow in the vessels surrounding the developing tooth was another theory.

• Forces derived from contraction of fibroblasts were thought to constitute the eruptive force.

• Alterations in the extracellular ground substance of the periodontal ligament similar to those that occur in thixotropic gels were thought to be the driving force behind eruption of teeth.

From animal studies, it presently seems clear that the major eruption mechanism is localized within the periodontal ligament. It is theorized that the cross-linking of maturing collagen in the periodontal ligament provides the eruption force. This is supported by the fact that eruptive movements begin when root formation starts and a periodontal ligament begins to develop.

Post-emergent Eruption. Once a tooth erupts into the mouth, it erupts rapidly

until it approaches the occlusal level and is subjected to the forces of mastication. At that point, its eruption slows and then as it reaches the occlusal level of other teeth and is in complete function, eruption all but halts. The stage of relatively rapid eruption from the time a tooth first penetrates the gingiva until it reaches the occlusal level is called the post-emergent spurt, in contrast to the following phase of very slow eruption, termed the juvenile occlusal equilibrium. During the juvenile occlusal equilibrium, teeth that are in function erupt at a rate that is parallel to the rate of vertical growth of the mandibular ramus. As the mandible continues to grow, it moves away from the maxilla, creating a space into which the teeth erupt. Due to this, a pubertal spurt in the eruption of teeth accompanies the pubertal spurt in jaw growth. When the pubertal growth spurt ends, a final phase in tooth eruption called the adult occlusal equilibrium is achieved.

The amount of tooth eruption after the teeth have come into occlusion equals the vertical growth of the ramus. Vertical growth increases the space between the jaws, into which the upper and lower teeth

erupt.

During adult life, teeth continue to erupt at an extremely slow rate. If its antagonist is lost at any age, a tooth can again erupt more rapidly, a condition called supraeruption. b.) Variability in eruption timing.Eruption timing varies with racial differences, lineages and within a dentition, that is, those children who erupt any tooth early or late tend to acquire other teeth similarly early or late. Sex differences are also seen in eruption. Except for third molars girls erupt their permanent an average of approximately five months earlier than boys.

c.) Sequence of eruption In the maxilla the sequences 6-1-2-4-3-5-7 and 6-1-2-

4-5-3-7 account for almost half of the cases, whereas in the mandible the sequences (6-1)-2-3-4-5-7 and (6-1)-2-4-3-5-7 include more than 40% of all children.

Normal variations in eruption sequence having clinical significance include

Eruption of second molars ahead of premolars in mandibular arch – this tends to shorten the arch perimeter and may create space difficulties for the second premolar and may lead to its being partially blocked out of the arch.

Eruption of canines ahead of premolars in maxillary arch – this forces the canine labially out the arch especially when there is an overall lack of space in the arch.

Asymmetries in eruption between right and left sides

Favorable eruption sequence

d) Factors regulating and affecting eruption

Both the sequence and timing of eruption seemed to largely genetically determined. Racial differences, socioeconomic status, nutritional influences, mechanical disturbances and localized pathosis all influence eruption of teeth. If the primary tooth is extracted after the permanent successor has begun active eruptive movements, the permanent tooth will erupt earlier. If the primary is extracted prior to the onset of permanent eruptive movements, the permanent tooth maybe delayed in its eruption. Crowding of permanent teeth has been shown to affect their rate of calcification and eruption.

e.) Ectopic development Ectopic teeth are teeth developing

away from their normal position. The most common found in ectopy are the maxillary first permanent molar and the maxillary cuspid followed by the mandibular cuspid, maxillary second molar, other premolars and maxillary lateral incisors. Girls show more tooth germs in ectopy than boys.

Ectopic eruption of maxillary first molars is associated with (1) large primary and permanent teeth, (2) a diminished maxillary length, (3) posterior positioning of the maxilla and (4) an atypical angle of eruption of the first molar. The treatment for this problem is best begun early in dental development in order to utilize the natural forces of eruption. Surgical uncovering and repositioning are required before orthodontic treatment.

Impacted teeth are ones that cannot erupt because of impingement. Third molars and maxillary cuspids are most commonly impacted. Transposition is a very rare form of ectopy, which involves exchanged positions between cuspids and first premolars or cuspids and lateral incisors.

f.) Factors determining the tooth’s position during eruption.

During eruption, the tooth passes through four distinct phases of development – preeruptive, intraalveolar, intraoral and occlusal.

During intraalveolar eruption, the tooth’s position is affected by the presence or absence of adjacent teeth, rate of resorption of the primary teeth, early loss of primary teeth, localized pathologic conditions and any factors that alter the growth or conformation of the alveolar processes. There is a strong tendency of the teeth to drift mesially even before they appear in the oral cavity. This phenomenon is called mesial drifting tendency.

Stages of eruption. 1. Pre-eruptive 2. Intra-alveolar 3. Intra-oral

4.Occlusal

Once the oral cavity has been entered, the tooth can be moved by the lip, cheek and tongue muscles, or by extraneous objects brought into the mouth(e.g. thumb, fingers, pencils).

In the occlusal stage of eruption, the muscles of mastication exert an influence through the interdigitation of cusps. The upward forces of eruption and alveolar growth are countered by the opposition of the apically directed force of occlusion. The periodontal ligament disseminates the forces of chewing to the alveolar bone. The axial inclination of the permanent teeth is such that some of the forces of chewing produce a mesial resultant through the

contact points of the teeth, the “anterior component of force”. This is countered by the approximal contacts of the teeth and by the musculature of the lips and cheeks.

The anterior component of force

SPACE RELATIONSHIPS IN REPLACEMENT OF INCISORS. .

The permanent incisors are considerably larger than the primary incisors they replace. Due to this, spacing between the primary incisors is critical to accommodate the former. Spacing in the primary dentition is distributed among all incisors, in addition to the primate spaces present in both arches.

When the central incisors erupt, they use up almost the entire space available in the primary dentition. With the eruption of the lateral incisors, the space situation in both arches becomes tight. The maxillary arch usually has just enough space to accommodate the permanent lateral incisors when they erupt. However, in the mandibular arch, when the lateral incisors erupt, there is an average deficit of 1.6mm space to align the four permanent incisors. This difference between the amount of space needed for the incisors and the amount available for them is called the “incisor liability”. Due to this, a child goes through a transitory stage of mandibular incisor crowding at age 8 to 9. Continued development of the arches improves the spacing situation, and by the time the canines erupt, space is once again adequate.

The extra space to overcome the incisor liability and to accommodate the incisors comes from three sources:

1. A slight increase in the inter-canine width of the dental arch. As growth continues, the teeth erupt upward and slightly outward. This increase is only about 2mm. but it contributes to the resolution of early incisor crowding. More width is gained in the maxillary arch than in the mandible, and more is gained by boys than by girls.

2. Labial positioning of the permanent incisors relative to the primary incisors. This contributes 1 to 2mm. of additional space in the arch, and thus helps resolve crowding.

3. Repositioning of the canines in the mandibular arch. As the permanent incisors erupt, the canine teeth widen out slightly, and also move slightly back into the primate spaces.

These changes occur without significant skeletal growth in the anterior part of the jaws.

Mandibular anterior crowding is identified as the discrepancy between the mesio-distal tooth widths of the four permanent incisors and the available space in the alveolar process. However, incisor crowding is not merely a tooth-arch size discrepancy but a discrepancy among many variables. Several factors can be assumed to affect the development and severity of crowding, such as the direction of mandibular growth, early loss of deciduous molars, mesio-distal tooth widths and arch dimensions, oral and perioral musculature and incisor and molar inclination.

In a study by Turkkahraman & Sayin (Angle Orthod., 2004) , it was determined that patients with crowding had smaller lower incisor to NB angles, maxillary skeletal length, mandibular skeletal length and mandibular dental measurements. They also had greater interincisal angles, overjet, overbite and Wits appraisal measurements and FMIA. Thus the study concluded that crowding of mandibular incisors is not only a tooth size- arch length discrepancy. Dentofacial characteristics also contribute to this malalignment.

Another study by Sayin & Turkkahraman (Angle Orthod.. 2000) showed that crowded dentitions had significantly smaller mandibular deciduous inter-canine width, mandibular deciduous inter-molar width, mandibular permanent inter-molar width and mandibular inter-alveolar width. The space available for the mandibular permanent incisors was also less in crowded dentitions, as was the total arch length.

However, the total width of the four permanent incisors did not vary greatly between crowded and non-crowded dentitions.

Ugly Duckling Stage Sometimes a transient malocclusion is seen in the

maxillary incisor region between 8-9 years of age. This is particularly seen during the eruption of the canines. As the developing permanent canines erupt, they displace the roots of the lateral incisors mesially. This transmits the force to the roots of the central incisors, which also get displaced mesially. A resultant distal divergence of the two central incisors causes midline spacing. This condition has been described by Broadbent as the ugly duckling stage, as the appearance of the teeth is not very aesthetic.

The spaces tend to close as the canines erupt. The greater the amount of spacing, the less the likelihood that a maxillary central diastema will totally close on its own. Generally, a diastema of 2mm. or less will probably close spontaneously, while total closure of a diastema greater than 2mm. is unlikely.

SPACE RELATIONSHIPS IN REPLACEMENT OF CANINES AND PRIMARY MOLARS.

The permanent premolars are smaller than the primary teeth they replace. The combined mesio-distal widths of the permanent canines and premolars are usually less than that of the deciduous canines and molars. The surplus space is called the leeway space. It amounts to a total of about 1.8mm. in the maxillary arch and 3.4mm. in the mandibular arch.

At the time the primary second molars are lost, both the maxillary and mandibular molars tend to shift mesially into the leeway space, but the mandibular molar normally moves mesially more than the maxillary molar. This differential movement contributes to the normal transition from a flush terminal plane relationship in the mixed dentition to a Class I relationship in the permanent dentition. Also, differential growth of the mandible more than the maxilla carries the lower molar more mesial than the upper molar and helps to establish a Class I relationship in the permanent dentition.

MOLAR RELATION IN PERMANENT DENTITION.

The occlusal relationships in the mixed dentitions determine the molar relation in the permanent dentition. The transition in molar relation from the mixed dentition to the early permanent dentition is usually accompanied by a one-half cusp (3 to 4mm.) relative forward movement of the lower molar, accomplished by a combination of differential growth and tooth movement

• A child’s distal step relation may change to an end-to-end relationship in the permanent dentition, but it is not likely to change all the way to a Class I relation. It is also possible that the pattern of growth may not lead to greater prominence of the mandible, in which case the molar relation in the permanent dentition will remain a full cusp Class II.

• Similarly, a flush terminal plane relation in the mixed dentition can change to a Class I relation in the permanent dentition or can remain end-to-end if the growth pattern is not favorable.

• A mesial step relation in the primary molars may produce a Class I permanent molar relation at an early age. It can proceed to a half-cusp Class III during the molar transition and progress further to a full Class III relationship with continued mandibular growth.

THE PERMANENT THE PERMANENT DENTITIONDENTITION..

Characteristics of the “normal” occlusion in the permanent dentition stage include the following:

• Overlap: in a normally occluding dentition, the maxillary teeth are labial/buccal to the mandibular teeth.

• Angulations: in the primary dentition stage, the teeth are vertically positioned in the alveolar bone. In contrast, the teeth in the permanent dentition stage have buccolingual and mesiodistal angulations.

• Occlusion: with the exception of the mandibular central incisors and the maxillary second molars, each permanent tooth occludes with two teeth from the opposite arch.

• Arch Curvatures: the anteroposterior curvature in the mandibular arch is called the curve of Spee. The corresponding curve in the maxillary arch is called the compensating curve. The buccolingual curvature from one side to the other is called the Monson curve or Wilson curve.

• Overbite and overjet: the overbite often ranges between 10 % and 50%, and the overjet ranges between 1 mm and 3 mm.

• Posterior relationships: the maxillary and mandibular molars are in Class I occlusion( i.e. the mesiobuccal cusp of the maxillary first molar is in the buccal groove of the mandibular first molar). In addition the whole posterior segment needs to be well interdigitated.

LATE CHANGES IN THE PERMANENT DENTITION STAGE

After the eruption of the permanent teeth, the dentition is relatively stable when compared with the cascade of changes observed in the mixed dentition stage.

Changes considered to be of clinical importance are : • In both males and females the lips become more

retruded relative to the nose and chin between 25 and 45 years of age. The implication is that orthodontic treatment at earlier ages should not result in an overly straight soft tissue profile and overly retrusive lips because the expected changes in the relative positions of the nose, lips and chin may exaggerate these characteristics.

• In both males and females, interincisor and intercanine arch widths decreased. Also, total arch lengths decreased and , as a result, anterior crowding increased.

Richardson (Dent Update, 2002) reviewed the causes of crowding that commonly occurs in the lower arch after the eruption of the second permanent molars. It was concluded that the factors responsible for late lower arch crowding included:

a.) Mesially directed forces – the mesial migration of teeth may be due to the physiological mesial drift, the anterior component of force of occlusion on the mesially inclined teeth, the mesial vectors of muscular contraction, the contraction of the trans-septal fibres of the periodontal ligament and the presence of a developing third molar.

b.) Distally directed forces – which may cause retroclination of the lower incisors, with reduction in arch length and consequent crowding. These forces may be due to incisor uprighting, growth patterns, skeletal structure or soft tissue maturation.

c.) Occlusal factors – which may produce a different pattern of masticatory forces or an occlusion with premature contacts. Such occlusal changes may be due to tooth loss, restorations, development of parafunctional habits or orthodontic treatment.

d.) d.) Direction of eruptionDirection of eruption – mesially inclined – mesially inclined molars and distally inclined incisors that molars and distally inclined incisors that continue to erupt in the same direction would continue to erupt in the same direction would result in reduction in arch depth and result in reduction in arch depth and increased crowding.increased crowding.

e.) e.) Tooth morphologyTooth morphology – well aligned lower – well aligned lower incisors are smaller mesiodistally and larger incisors are smaller mesiodistally and larger labiolingually.labiolingually.

f.) f.) Degenerative tissue changesDegenerative tissue changes – gingival – gingival recession and bone loss are likely causes of recession and bone loss are likely causes of late crowding.late crowding.

g.) g.) Orthodontic treatmentOrthodontic treatment – teeth that have – teeth that have been moved orthodontically have a tendency been moved orthodontically have a tendency to return to their original (crowded) positions.to return to their original (crowded) positions.

• Both overbite and overjet decrease throughout the second decade of life, due to relatively greater forward growth of the mandible.

• Third molar development: third molars show more variability in calcification and eruption than do any other teeth. Impaction of third molars is a frequent and serious problem in modern man. Mandibular third molar impactions, which are usually more serious, are seen more often with skeletal Class II particularly when the body of the mandible is short and acutely angled.

• By the end of the second decade most persons display idiopathic resorption of one or more teeth. Nearly 90% of all teeth show some evidence of resorption by the time a person is 19 years of age.

DENTAL AGE On the basis of dentition, 3 related

estimates of dental age can be made:1. From the number and type of teeth

visible in the oral cavity.2. Based on the schedule of calcification of

permanent mandibular first molar. However, this is a limited estimate.

3. Based on schedule of calcification of the dentition as a whole.

Dental age is highly correlated with body height and chronologic age.

DENTAL ARCH DENTAL ARCH DEVELOPMENTDEVELOPMENT

DIMENSIONAL CHANGES IN THE DENTAL ARCH

Three sets of measurements are taken for the dental arches:

(1)The combined widths of teeth (2) the dimensions of the dental arch in which the teeth are arrayed (3) the dimensions of the mandible or maxilla proper i.e. the so called basal bone..

The usual arch dimensions measured are: (1) widths at the canines, primary molars (premolars), and first permanent molars; (2) length (or depth) and; (3) circumference.

Arch dimensions. A, arch length. B1, bicanine diameter. B2,bimolar

diameter. C-C, arch perimeter/circumference

I. Width. Dental arch width increases correlate

highly with vertical alveolar process growth, whose direction is different in the two arches. Maxillary alveolar processes diverge, due to vertical growth of the alveolar processes (which also coincides with eruption of teeth), while the mandibular processes are more parallel. As a result maxillary width increases are much greater and can be more easily altered in treatment.

The crowns of the first molars erupt tipped somewhat lingually and do not upright fully until the time of the eruption of the second molars. As the first molars upright, they cause an increase in the bimolar width. Furthermore, both first molars move forward at the time of the late mesial shift to use up any remaining leeway space and thus assume a narrower diameter along the convergent dental arch.

The only postnatal mechanism for widening the basal bony width of the mandible is deposition on the lateral borders of the corpus mandibularis. Such deposition occurs only in small amounts. The maxilla, in contrast, widens with vertical growth because the alveolar processes diverge; therefore, more width increase is seen and more can be procured during treatment. Furthermore, the midpalatal suture can be reopened with “rapid palatal expansion” to acquire large amounts of actual widening of the maxilla.

A study by Marshall, Dawson, Southard et. al (AJO DO, 2003) showed that (1) maxillary molars erupt with buccal crown torque and upright with age, whereas mandibular molars erupt with lingual crown torque and upright with age, and (2) molar crown torque changes are accompanied by concurrent increases in maxillary and mandibular inter-molar width.

II. Length or Depth Dental arch length or arch depth is

measured at the midline from a point midway between the central incisors to a tangent touching the distal surfaces of the second primary molars or second premolars. It does not have the clinical importance of the circumference. Any changes in arch length are but coarse reflections of changes in perimeter.

III. Circumference or Perimeter It is measured from the distal surface of the

second primary molar (or mesial surface of the first permanent molar) around the arch over the contact points and incisal edges in a smoothed curve to the distal surface of the second primary molar (or first permanent molar) of the opposite side.

The reduction in mandibular arch circumference during the transitional and early adolescent dentition is the result of (1) the late mesial shift of the first permanent molars as the “leeway space” is preempted, (2) the mesial drifting tendency of the posterior teeth throughout all of life, (3) slight amounts of interproximal wear of the teeth, (4) the lingual positioning of the incisors as a result of the differential mandibulomaxillary growth, and (5) the original tipped positions of the incisors and molars.

• Northway in 1977 reported that moderate caries, severe caries, and early loss of primary molars caused dramatic increases in the amount of perimeter loss.

• Hunter and Smith in 1972 noted that children with crowded arches in the early mixed dentition showed less arch perimeter loss by the time of the completed permanent dentition and more continued crowding.

• Moorrees in 1969 reported that the arch length and width affect arch circumference, or the space available for the alignment of teeth. Between the ages of 5 and 18 years, maxillary arch circumference increases slightly in the average boy (1.3mm.) and in the average girl (0.5mm.), while in the mandible, a mean decrease of 3.4 and 4.5mm. occurs in boys and girls, respectively. The individual variations in the changes in arch circumference are considerable and are explained by differences in the amount of interdental space in the deciduous dentition, the changes in arch breadth and arch length, the ratio of mesiodistal crown diameters of deciduous teeth and their permanent successors, and the sequence of shedding and emergence of the permanent posterior teeth. .

• A study by Slaj et. al (Angle Orthod., 2003) suggests that dental arch dimensions are more defined by tooth eruption and less so by the growth of the supporting bone during the mixed dentition. In the early mixed dentition, inter-canine relations are primarily defined by the early onset of mandibular growth. However, the skeletal growth of the maxillofacial complex in the late mixed dentition is not always predictable. The period between the early and late mixed dentition is suitable for environmental factors to disrupt the pattern of ideal symmetrical development of ideal arch form. Since a number of orthodontic treatments may be planned or applied in the period of early or late mixed dentition, this factor should be kept in mind for deciding upon and administering the appropriate orthodontic therapy.

IV. Overbite and Overjet. Overbite (vertical overlap of the incisors) and

overjet (horizontal overlap) undergo significant changes during the primary and transitional dentitions. During the primary dentition, the overbite normally decreases a slight amount, and the overjet often is reduced to zero. From the early mixed dentition to the completion of the permanent occlusion the average overbite increases slightly and then decreases, but there is great variability in its behavior. Overbite is correlated with a number of vertical facial dimensions (e.g., ramus height), whereas overjet usually is a reflection of the anteroposterior skeletal relationship. Overjet is also sensitive to abnormal lip and tongue function.

REFERENCESREFERENCES1. . Gardiner J.H., Leighton B.C., Luffingham

J.K, Valiathan, A.: Orthodontics for Dental Students. Oxford University Press, 1998; 4th Edition.

2. Moyers Robert E.: Hand book of Orthodontics. Year book Medical publishers, Inc, 1988; 4th Edition.

3. Bishara Samir E.: Text book of Orthodontics. Saunders 2003.

4. Nanda Surender K.: The developmental basis of occlusion and malocclusion. Quintessence Publishing Co. Inc. 1983.

5. Proffit W.R.: Contemporary Orthodontics. Mosby 2000. 3rd Edition.

6. Salzmann J.A.: Practice of Orthodontics Vol. 1. J.B. Lippincott Co. 1966.

7. Orban’s Oral Histology and Embryology. Harcourt Asia Pvt. Ltd. 11th Edition.

8. Tencate R.: Oral histology – development, structure and function. Mosby 2001. 5th Edition.

9. Tibana H.W., Palagi L.M., Miguel J.A.M.: Changes in dental arch measurements of young adults with normal occlusion-a longitudinal study. Angle Orthod. 2004;74:618-623.

10. Sayin, Turkkahraman: Factors contributing to mandibular anterior crowding in the early mixed dentition. Angle Orthod. 2004; 74:754-758.

11. Turkkahraman, Sayin: Relationship between mandibular anterior crowding and lateral dentofacial morphology in the early mixed dentition. Angle Orthod. 2004; 74:759-764.

12. Slaj M., Jezina M.A., Lauc R., Rajic-Mestrovic S., Miksic M.: Longitudnal dental arch changes in the mixed dentition. Angle Orthod. 2003;73:509-514

13. Marshall S., Dawson D., Southard K.A., Lee A.N., Casko J.S., Southard T.E.: Transverse molar movements during growth. Am J Orthod Dentofacial Orthop 2003; 124:615-24.

14. Richardson M.E.: Late lower arch crowding: the aetiology reviewed. Dent Update 2002; 29: 234-238.

15. Moorrees C.F.A.: Growth studies of the dentition-a review. Am J Orthod Dentofacial Orthop 1969 55:600-616.