1 At what ages do most children enter adolescence, and when do they attain PHV?

The adolescence growth spurt starts when decelerating childhood growth rates change to accelerating rates. During the first part of the growth spurt, statural growth velocities increase steadily until peak height velocity (PHV) is attained. Longitudinal assessments provide the best indicators of when adolescence is initiated and PHV is attained. Longitudinal studies pertaining to North American and European children< ?xml:namespace prefix = "mbp" />¹ show that girls are advanced by approximately 2 years compared with boys in the age of initiation of adolescence and age of PHV. Based on the 26 independent samples of girls and 23 samples of boys, the average ages of PHV are 11.9 and 14.0 years, respectively. Girls and boys initiate adolescence at 9.4 years and 11.2 years, respectively. Maximum adolescent growth velocity in body weight usually occurs 0.3 to 0.5 year after PHV (Fig. 1-1).

FIG 1-1 Frequency distribution of 26 sample ages of PHV for boys (A) and girls (B).

(From Malina RM, Bouchard C, Beunen G. Ann Rev Anthropol 1988;17:187-219.)

2 What is the mid-growth spurt, and how does it apply to craniofacial growth?

The mid-growth spurt refers to the increase in growth velocity that occurs in some, but not all, children several years before the initiation of the adolescent growth spurt. Mid-growth spurts in stature and weight have been reported to occur between 6.5 and 8.5 years of age; they tend to occur more frequently in boys than girls.^2,3 Based on yearly velocities, mid-growth spurts have been demonstrated for a variety of craniofacial dimensions—also between 6.5 and 8.5 years of age—occurring simultaneously or slightly earlier for girls than boys.^4–7 Applying mathematic models to large longitudinal samples, Buschang and colleagues⁸ reported mid-growth spurts in mandibular growth for subjects with Class I and Class II molar relationships at approximately 7.7 years and 8.7 years of age for girls and boys, respectively.

3 Which skeletal indicators are most closely associated with PHV?

According to Grave and Brown,⁹ PHV in males and females occurs slightly after the appearance of the ulnar sesamoid and the hooking of the hamate, and slightly before capping of the third middle phalanx, the capping of the first proximal phalanx, and the capping of the radius. According to Fishman’s¹⁰ skeletal maturity indicators, capping of the distal phalanx of the 3^rd finger occurs less than 1 year before PHV, capping of the middle phalanx of the 3^rd finger occurs just after PHV, and capping of the middle phalanx of the 5^th finger occurs less than one half year after PHV. Based on the cervical vertebrae, PHV occurs between the development of the concavity on the inferior borders of the 2^nd and 3^rd vertebrae (CVMS II) and development of a concavity on the inferior borders of the 2^nd, 3^rd, and 4^th vertebrae (CVMS III).¹¹

4 What is the equilibrium theory of tooth position?

Although Brodie¹² was among the first to identify the relationship between muscles and tooth position, it was Weinstein and colleagues¹³ who experimentally established that the teeth are maintained in a state of equilibrium between the soft tissue forces. Based on a series of experiments, they concluded that:

Proffit,¹⁴ who revisited the equilibrium theory 15 years later, noted that the primary factors involved were:

He further noted that extrinsic pressures, such as habits and orthodontic forces, can alter equilibrium, provided that they are sustained for at least 6 hours each day. Proffit¹⁴ also identified head posture and growth displacements/rotations as secondary factors determining equilibrium. As the mandible rotates, the incisors move as dental equilibrium is reestablished. Björk and Skieller,¹⁵ for example, have shown an association between changes in lower incisor angulation and true mandibular rotation.

5 What is the prevalence of Class II dental malocclusion among adolescents and young adults living in the United States?

The best direct epidemiologic evidence comes from the National Health Survey,^16,17 which evaluated approximately 7400 children between 6 and 11 years of age and over 22,000 youths 12 to 17 years of age. Unilateral and bilateral distoclusion occurs approximately 16.1% and 22.7% of the time among Caucasian children and 7.6% and 6.0% of the time among African-American children, respectively. Comparable incidences among Caucasian youths were 17.8% and 15.8% and 12.0% and 6.0% among African-American youths. Based on overjet provided by the NHANES III, Proffit and associates¹⁸ estimated that the prevalence of Class II malocclusion (overjet ≥ 5 mm) decreases from over 15.6% between 12 and 17 years of age to 13.4% for adults. They also showed that Class II malocclusion is more prevalent among African-Americans (16.5%) than Caucasians (14.2%) and Hispanics (9.1%).

6 What is the prevalence of incisor crowding among individuals living in the United States, and how does it change with age?

According to the initial NHANES III data,¹⁹ incisor irregularities increase from an average of 1.6 mm for children 8 to 11 years, to 2.5 mm for youths 12 to 17 years, to 2.8 mm for adults 18 to 50 years of age. Although incidences are similar at the youngest age, African-American youths and adults show significantly less crowding than Caucasians and Hispanics. Based on the complete NHANES data set, including 9044 individuals between 15 and 50 years of age, approximately 39.5% of U.S. adults have mandibular incisor irregularities ≥ 4 mm and 16.8% have irregularities ≥ 7.²⁰ Adult males tend to show greater crowding than females; Hispanics show greater crowding than Caucasians, who in turn display greater crowding than African-Americans. Based on the available data for untreated subjects followed longitudinally, rates of crowding increase precipitously between 15 and 50 years of age, especially during the late teens and early 20s (Fig. 1-2).²⁰

FIG 1-2 Average mandibular alignment scores, U.S. persons, 1988-1991.

(Adapted from Brunelle JA, Bhat M, Lipton JA. J Dental Res 1996;75[special issue]:706-713.)

7 Do the third molars play a role in determining crowding?

Although third molars have been related with crowding,^21–24 most contemporary studies show little or no relationship. A NIH conference in 1979 came to the consensus that there is little or no justification for extracting third molars solely to minimize present or future crowding of the lower anterior teeth.²⁵ Ades and co-workers²⁶ found no difference in subjects whose third molars were impacted, erupted in function, congenitally absent, or extracted at least 10 years before post-retention records were taken. Sampson and colleagues²⁷ also showed no difference in crowding between subjects whose third molars have erupted completely or partially, remained impacted, or were missing. A randomized controlled trial based on 77 patients followed for 66 months showed a 1.0 mm difference in anterior crowding between patients whose third molars had and had not been removed; the authors concluded that removal of third molars to reduce or prevent late crowding cannot be justified.²⁸ Based on the NHANES data, individuals who had erupted third molars displayed significantly less crowding than those who did not have erupted third molars.²⁰

8 How does horizontal and vertical mandibular growth affect crowding?

Vertical growth makes the maintenance of lower incisor alignment after orthodontic treatment more problematic. Based on the notion that the lower incisors are carried into the lower lip as the mandibula grows anteriorly or rotates downwards, late mandibular growth has been suggested as a major contributor to post-retention crowding.²⁹ Although incisor compensation to backward mandibular rotation has been demonstrated,¹⁵ crowding as a result of anterior growth displacements remains to be established. However, changes in lower incisor crowding have been shown to be related to vertical growth. Both treated and untreated patients who undergo greater inferior growth displacements of the mandible and associated greater eruption of the lower incisors show greater crowding than those who undergo less vertical growth and less eruption.^30,31 Since vertical mandibular growth continues well beyond the teen years, patients would be well advised to wear their retainers into their early and mid-20s.

9 How much should the mandibular incisors and molars be expected to erupt during adolescence?

Based on natural structure superimpositions of the mandible performed between 10 and 15 years of age, McWhorter³² showed that the mandibular central incisors and first molars erupt approximately 4.3 and 2.5 mm in males and females, respectively. Also using natural structure superimpositions, Watanabe et al.³³ demonstrated that the mandibular molars and incisors erupt at rates ranging from 0.4 to 1.2 mm/yr and 0.3 to 0.9 mm/yr, respectively. Rates of eruption were greater in males than females, attaining peak velocities at approximately 12 and 14 years of age for females and males, respectively.

10 How does untreated arch perimeter change between the late primary dentition and the permanent dentition?

Computed based on a centenary curve extending from the mesial of the first molar to mesial of first molar,³⁴ arch perimeter increases during the early mixed dentition and decreases during and after the transition to the permanent dentition. Maxillary perimeter increases 4 to 5 mm between 6 and 11 years of age and decreases 3 to 4 mm between 11 and 16 years. In contrast, mandibular arch perimeter increases approximately 2 to 3 mm initially and then decreases 4 to 7 mm, with greater decreases in females than males (Fig. 1-3).

FIG 1-3 Maxillary (A) and mandibular (B) arch perimeters between 6 and 16 years of age.

(Adapted from Moyers RE, van der Linden FPGM, Riolo ML, McNamara JA Jr. Standards of human occlusal development. Monograph #5, Craniofacial Growth Series, Center for Human Growth and Development, University of Michigan, Ann Arbor, Michigan, 1976.)

11 How do untreated maxillary and mandibular intermolar widths change during childhood and adolescence?

Bishara and colleagues³⁵ reported that intermolar widths increase 7 to 8 mm between the deciduous dentition (5.0 yrs of age) and the early mixed (8.0 yrs of age) dentitions and an additional 1 to 2 mm between the early mixed and early permanent (12.5 yrs of age) dentitions. Between 6 (first molar fully erupted) and 16 years of age, Moyers and colleagues³⁴ showed greater increases for males than females for both maxillary (4.1 versus 3.7 mm) and mandibular (2.6 versus 1.5 mm) intermolar widths. Based on a sample of 26 subjects followed longitudinally between 12 and 26 years of age, DeKock³⁶ reported no significant change for females and only slight increases (1.4 and 0.9 mm for maxilla and mandible, respectively) in intermolar width for males (Fig 1-4).

FIG 1-4 Maxillary (A) and mandibular (B) intermolar widths between 6 and 16 years of age.

(Adapted from Moyers RE, van der Linden FPGM, Riolo ML, McNamara JA Jr. Standards of human occlusal development. Monograph #5, Craniofacial Growth Series, Center for Human Growth and Development, University of Michigan, Ann Arbor, Michigan, 1976.)

12 Without treatment, how do maxillary and mandibular arch depths change during childhood and adolescence?

Maxillary and mandibular arch depths, midline distances between a line tangent to the incisors, and a line drawn tangent to the distal crown of the deciduous second molars or their permanent successors show different patterns of growth changes. Maxillary arch depth increases 1.4 and 0.9 mm in males and females, respectively, during the eruption of the permanent incisors.³⁷ Mandibular arch depth shows little change over the same period. With the loss of the deciduous molars, maxillary arch depth decreases 1.5 and 1.9 mm, while mandibular arch depth decreases 1.8 and 1.7 mm in males and females, respectively.³⁷ DeKock³⁶ reported decreases (approximately 3.0 mm) in arch depth between 12 and 26 years of age, />