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A review of changes in lower archalignment from seven to fifty years
A Review of Changes in Lower Arch Alignment From Seven to Fifty Years Margaret E. Richardson Changes in alignment in the untreated lower arch were studied at various developmental stages: 7 to 10 years, 10 to 12 years, 12 to 15 years, 13 to 18 years, 18 to 21 years, 21 to 28 years, and 18 to 50 years. On average, crowding decreased between 7 and 12 years and increased thereafter. The maximum increase occurred in the teenage years between 13 and 18, little or no change occurred in the third decade, and small increases occurred later in life. The possible cause of these changes is discussed in relation to the deterioration in alignment reported in orthodontically treated patients after retention. (Semin Orthod 1999;5:151-159.) Copyright © 1999 by W.B. Saunders Company
he stability of orthodontic t r e a t m e n t must be j u d g e d against the b a c k g r o u n d of naturally occurring changes in the untreated dentition. It is generally recognized that the h u m a n dentition is in a dynamic state, continually changing t h r o u g h o u t life. Particularly noticeable in this respect is the change in alignment of the lower arch which may be crowded in the early mixed dentition, less so in the years between 8 and 12, and b e c o m e m o r e crowded after eruption of the second p e r m a n e n t molars, a process which is said to continue well into the third decade and beyond.1 T h e decrease in lower incisor crowding in the years immediately after their eruption has b e e n well d o c u m e n t e d . 2-7 During the teenage years between 13 and 18, an increase in lower arch crowding (2.0 m m or m o r e on average) has b e e n r e p o r t e d by Siatowski, s Sakuda et al, 9 Moorrees et al, 1° a n d Sampson et al. u O t h e r investigators have r e p o r t e d an increase in crowding over longer observation periods, extending f r o m the early teens to between 21 and 31 years of age. 12-19 No intermediate examinations were m a d e in these studies, therefore, it is not known whether the crowding developed in the years immediately after e r u p t i o n of the second p e r m a n e n t molars
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From the Department of Orthodontics, School of Dentistry, Belfast. Address correspondenceto MargaretE. Richardson, 33 CherryValley Park, Belfast BT5 6PN, N Ireland Copyright© 1999 by W.B. Saunders Company 1073-8746/99/0503-0004510. 00/0
or whether the crowding continued to increase u p to the later age. I n f o r m a t i o n on changes in lower arch a l i g n m e n t after the age of 18 years is sparse. Bishara et al so f o u n d small increases in crowding between 0.5 and 0.9 m m in 30 patients between 25 and 46 years. Duterloo 21 n o t e d small dimensional arch changes in 26 patients between the m e a n ages of 17 to 28 years. Crowding was n o t measured, but the illustrations showed slight crowding increases in three patients. His sample included some patients y o u n g e r than 17 years, the age at the first observation r a n g e d f r o m 14 to 18 years. Carter and McNamara 19 f o u n d a significant increase of 0.48 m m in lower incisor irregularity in 10 patients between 32 and 45 years of age. To u n d e r s t a n d the relationship between the n o r m a l process of maturation and the changes after o r t h o d o n t i c t r e a t m e n t and retention, it is i m p o r t a n t to chart the progress of the developm e n t of crowding in the u n t r e a t e d lower arch 22 to ascertain whether it is a continuous, gradual, slowly diminishing change, or whether it occurs rapidly at certain stages interspersed with periods of relative stability. Such information may have a bearing on its cause.
S e v e n to Fifteen Years Models of 48 children with a full c o m p l e m e n t of p e r m a n e n t teeth and no early loss of deciduous teeth were selected f r o m the Belfast growth study 22 a n d m e a s u r e d at four stages, z3
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1. T1. P e r m a n e n t lower incisors at least partially erupted when the average age was 7.75 years. 2. T2. Lower p e r m a n e n t canines at least partially erupted when the average age was 10.85 years. 3. T3. Lower second molars at least partially erupted when the average age was 12.5 years. 4. T4. Three years after full eruption o f the lower second molars, the average age was 15.5 years. Lower incisor space condition (crowding) at each stage was calculated as the difference between arch perimeter (between the mesial contact points of contralateral deciduous or permanent canines) and the sum of mesiodistal incisor widths. In a well-aligned arch with all the teeth in contact, arch perimeter was equal to the sum of the tooth widths (Fig 1A). Where spaces occurred, these were a d d e d to the total tooth width to find the arch perimeter (Fig 1B). Where teeth were rotated or displaced, the space between adjacent teeth was measured and substituted for the size of the tooth in question to calculate arch perimeter. The change in lower incisor space condition between each of the four stages was calculated and the mean changes in this and in the measurements made in the following investigations were tested for significance with the paired samples t test. From T1 to T2, there was a significant average decrease in crowding of 0.91 ram. Crowding continued to decrease between T2 and T3, but the change was small and not significant (0.15 m m on average). After eruption of the second p e r m a n e n t molars, there was a significant increase in crowding of 0.41 ram.
Thirteen to Eighteen Years Lower arch space condition (crowding) was calculated from measurements made on models of 5 ! patients from the Belfast third molar study 24'25 at T1 (13 years) and T2 (18 years) in a similar m a n n e r to the previous study, 23 but this time, the arch perimeter was measured between the mesial contact points of the first p e r m a n e n t molars. The change in crowding was calculated as before. O n 60 ° left and right cephalometric radiographs of the same patients, the change in anteroposterior position of the lower first molar (mesial drift) was measured by projecting the anterior contact point of the first molar onto a
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C Figure 1. Measurement of space condition. Arch perimeter minus tooth size. Arch perimeter = sum of mesiodistal tooth widths = zero space condition (A). Arch perimeter = sum of mesiodistal tooth widths + spaces = positive space condition (B). Space between adjacent teeth is substituted for the size of a rotated or displaced tooth in the calculation of arch perimeter = negative space condition (crowding) (C). (Reprinted with permission from Lundy HJ, Richardson ME: Developmental changes in alignment of the lower labial segment. Brit J Orthod 1995;22:339-345.23 Reprinted with permission of Oxford University Press.) maxillary horizontal on a tracing of the T1 film. The T1 tracing was superimposed on the T2 film, registering on internal structures in the mandible in a m a n n e r similar to that described by Bj6rk 26 (Fig 2). M1 patients had untreated lower arches with a full c o m p l e m e n t o f teeth, including third molars u n e r u p t e d at T1. Crowding increased significantly by 2.3 m m on average, ranging from 0 to 6.0 mm. The first molars moved forward significantly by an average of approximately 2.0 lnm on each side, ranging from 0 to 7.0 ram.
Eighteen to Twenty-One Years The Belfast adult growth study 27-29 i n c l u d e d 60 students with intact lower arches anterior to and including second molars. None had been treated
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Figure 2. Measurement of mesial drift on 60 ° cephalograms. Change in position of the mesial contact point of the first permanent molar measured on a maxillary horizontal with a tracing of the first film superimposed on the second registering on the inner outline of the mandibular symphysis and the inferior dental canal. Solid line, first film; broken line, second film.
orthodontically. F r o m m e a s u r e m e n t s m a d e o n models taken at 18 a n d 21 years, the c h a n g e in lower arch c r o w d i n g was calculated as before. C h a n g e in position o f the first m o l a r was measured o n 60 ° c e p h a l o m e t r i c r a d i o g r a p h s using the m e t h o d described previously. O n average, there was n o significant c h a n g e in crowding. O n l y 8% o f the students s h o w e d small increases in crowding. With o n e exception, all were less than the e r r o r o f m e a s u r e m e n t . T h e r e was a significant f o r w a r d m o v e m e n t o f the first m o l a r o n the left side 0.07 m m . Ninety-four p e r c e n t o f students h a d n o c h a n g e . T h e r e m a i n i n g 6% showed small a m o u n t s o f mesial drift r a n g i n g f r o m 0.5 to 2.0 m m .
Twenty-One to Twenty-Eight Years Forty-six o f these students were r e c o r d e d again at age 28 years. 2s Between ages 21 a n d 28 years, there was a significant average increase in lower arch c r o w d i n g o f 0.2 ram, r a n g i n g f r o m 0.2 to 1.4
Second Molar Extraction A small pilot study o n 10 patients 3° followed by a larger study o n 30 patients 31 investigated the effect o f extraction o f the s e c o n d p e r m a n e n t molars o n lower arch crowding. T h e m e a n age at extraction was 13.9 years (range, 11-17 years). Models a n d 60 ° c e p h a l o m e t r i c r a d i o g r a p h s were taken i m m e d i a t e l y before extraction (T1) a n d r e p e a t e d after an interval o f 5 years (T2). In b o t h studies, a c o r r e s p o n d i n g n u m b e r o f n o n e x t r a c d o n subjects r e c o r d e d at 13 years (T1) a n d 18 years (T2) were used as controls. C h a n g e s in lower a r c h c r o w d i n g a n d first m o l a r position were m e a s u r e d as before. In the s e c o n d m o l a r extraction g r o u p there was a slight decrease in lower arch c r o w d i n g o f 0.75 ram, significantly different f r o m the 2.0 m m increasein crowding in the n o n e x t r a c t i o n group. T h e lower first molars m o v e d slightly distally in the extraction g r o u p (0.4 m m left, 0.6 m m right), significantly differe n t f r o m mesial m o v e m e n t in the n o n e x t r a c t i o n g r o u p (2.0 m m left, 1.7 m m right).
mill.
Discussion Eighteen to Fifty Years A n o t h e r small g r o u p o f 16 adults were r e c o r d e d at age 18 years a n d again at age 50 years. 29 Two o f these subjects h a d n o c h a n g e in lower arch crowding. All the others h a d an increase in crowding, r a n g i n g f r o m 0.2 to 2.5 ram.
T h e decrease in lower arch c r o w d i n g in the years after e r u p t i o n o f the lower p e r m a n e n t incisors 23 was consistent with the findings o f others. 2-7 T h e m e c h a n i s m s responsible f o r this decrease seem to be mainly dentoalveolar, b e c a u s e there is little o r n o skeletal growth in the a n t e r i o r p a r t o f the lower jaw at this time. ~,7,32~34A d a m s a n d Richard-
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son 35 f o u n d that the lower incisors proclined relative to the mandibular plane by an average of 13 ° between 5 and 11 years. This is probably the most important means of creating space for incisor alignment. An increase in arch width across the canines caused by alveolar growth just before and during the eruption of the permanent i n c i s o r s 2,4,36 may also help to create space. The significant increase in crowding between 12 and 15 years 23 was consistent with, t h o u g h smaller than, increases in crowding reported over longer observation periods after the age of 12 to 13 years, a2-a9In a larger sample drawn from the same source and including the present sample, Richardson ~7f o u n d a slightly larger increase in crowding of 0.7 m m in the same 3-year period. Between 13 and 18 years, 24 the considerably larger average increase in crowding of 2.3 m m was comparable with that f o u n d by Sampson et al n in the same age range. Sakuda et al, 9 using a scoring system to measure crowding, recorded average increases of 4.9 m m in boys and 6.8 m m in girls between the same ages. Figure 3 shows
models of an untreated boy who had a typical average increase in crowding between 13 and 18 years. From 18 to 21 years, 27 the lower arch appeared to be relatively stable in contrast to the preceeding teenage years. Only one person had an increase in crowding, which was clinically obvious at 1.8 mm. A few people had small increases scarcely visible to the naked eye. The majority had no measurable change. In the following years from 21 t o 28, 28 the increase in crowding of 0.2 mm, t h o u g h statistically significant, was of little importance clinically. Figure 4 shows models of a girl who had the largest increase of 0.7 m m in crowding during this period. The cumulative change from 18 to 28 years, 1.3 m m is only just perceptible. For the small group studied between 18 and 50 years, 29 mean values have little clinical importance. Almost half of them had increases in crowding between 1.5 and 2.5 mm, obvious to the naked eye. These studies 24,27-2u imply that the greatest
Figure 3. Occlusal view of models of an untreated boy at age 13 years (A), and age 18 years (B) showing an average increase in lower arch crowding commonly found at this stage of development. (Reprinted with permission from Lundy HJ, Richardson ME: Developmental changes in aligmnent of the lower labial segment. BritJ Orthod 1995;22:339-345.s7 Reprinted with permission of Oxford University Press.)
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Figure 4. Occlusal models of an untreated girl who had the greatest increase in lower arch crowding, 0.7 mm, between 21 and 28 years. Age 18 years (A), age 21 years (B), age 28 years (C). The cumulative increase in crowding from 18 to 28 years, 1.3 mm, is only just perceptible. (Reprinted with permission from Richardson ME, GormleyJS: Lower arch crowding in the third decade. Eur J Orthod 1998;20:597-607d 8 Reprinted with permission of Oxford University Press.)
a m o u n t of crowding occurs during the teenage years between 13 and 18. The period between the late teens and early twenties seems to be a relatively stable period in terms of lower arch alignment, with variable small amounts of crowding occurring in the third and fourth decades. This suggests that the causes of late crowding may vary at different stages of development. The cause of increased crowding in the intact lower arch is not fully understood. It is obviously multifactorial, and for this reason, it is difficult to show a cause and effect relationship. Mesial migration of h u m a n teeth has been recognized since the late 18th century, when itwas described by J o h n H u n t e r 3s and was shown by the forward m o v e m e n t of the first molars f o u n d in the patients studied between 13 and 18 years of
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age. 24 T h e r e is evidence to s u p p o r t the view that it is largely responsible for the increase in crowding during the teenage years. Mesial migration may be caused by physiological mesial drift, by the anterior c o m p o n e n t of the force of occlusion on mesially inclined teeth, the mesial vectors of muscular contraction, or the contraction of the transseptal fibres of the periodontal ligament. Third m o l a r agenesis and extraction studies 39-42 suggest that mesial migration is greater in the presence of a developing third molar. This suggestion is strongly reinforced by second m o l a r extraction studies. 3°,~1 Removal of the second m o l a r effectively isolates the third m o l a r from the rest of the arch. T h e reduction in crowding and the distal m o v e m e n t of first molars in patients whose second molars have b e e n extracted c o m p a r e d with the increase in crowding and mesial m o v e m e n t of first molars in nonextraction subjects ~°,31 provides convincing evidence of the effects of developing third molars on the anterior part of the arch. O t h e r studies on patients treated by extraction o f second molars 43-46 r e p o r t e d similar results. It is a mistaken impression that it is only i m p a c t e d third molars that cause the problem. A third m o l a r that erupts is likely to exert m o r e pressure on the dental arch than one that remains impacted, and some i m p a c t e d third molars may exert m o r e pressure than others. T h e lower third m o l a r starts to develop in the ramus of the mandible in a mesioangular position relative to the mandibular plane and gradually b e c o m e s m o r e upright until it is in a position to erupt. Impactions can develop in several different ways. 47 1. T h e milder mesioangular impactions start to upright in the n o r m a l way, but the degree of uprighting is insufficient to p e r m i t eruption. 2. T h e m o r e severe mesioangular impactions seem to remain in their original angular position, but on closer inspection, they upright a little and then tip mesially again. 3. Horizontal impactions may start in an apparently favorable position and tip mesially to the horizontal. 4. Some third molars remain vertically impacted after they have uprighted but are unable to e r u p t because of lack o f space. 5. O t h e r third molars can actually tip distally once they have r e a c h e d the vertical position and b e c o m e distoangularly impacted.
T h e third molars that tip mesially are unlikely to exert m u c h pressure on the dental arch. The milder mesioangular and the vertical third molars may cause pressure while they are trying to erupt, but once they b e c o m e impacted and especially once the apices close, they probably cease to do so as do the distoangular impactions once they start to tip distally. Those third molars that force their way into the arch and e r u p t are the ones most likely to cause crowding. T h e cause of late crowding may not be the same in every patient, and in any one individual, there may be m o r e than one contributing factor. Some factors may have m o r e influence than others or may act at different stages of development. The a m o u n t and direction of late mandibular growth, skeletal structure and complex growth patterns, soft tissue maturation, occlusal factors, tooth morphology, periodontal forces, and degenerative tissue changes have b e e n implicated to varying extent and in different combinations. 4~54 An ever-increasing volume of literature reports a variable and unpredictable deterioration in lower arch alignment f r o m 1 to 20 years after retention after various orthodontic treatment regimens in b o t h extraction and nonextraction. 18'55-79It is often claimed that this is caused by the n o r m a l physiological process of maturation f o u n d in untreated s u b j e c t s . 2°,s° Nevertheless, it is not unreasonable to expect that teeth that have b e e n m o v e d orthodontically might be m o r e susceptible to the pressures that cause arch length reduction and crowding. T h e Belfast studies on long-term changes in u n t r e a t e d lower a r c h e s 27-29 suggest that increased crowding in later life may be caused by degenerative changes associated with aging or periodontal disease. In the small group e x a m i n e d at ages 18 and 50 years, 29 though periodontal status was not quantified, all subjects showed some sign of periodontal change ranging f r o m gingival recession with consequent elongation of clinical crowns, to established periodontal disease, treated and untreated. Such changes would result in weakening of the supporting structures of the teeth, rendering t h e m less resistant to pressures that they previously withstood. O r t h o d o n t i c tooth movem e n t may accelerate this process by causing root resorption and reduced alveolar b o n e levels, which may not recover their p r e t r e a t m e n t status.81-89 Difficulties arise in c o m p a r i n g changes in
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lower arch alignment in untreated arches with those that have been treated orthodontically. Methods of measuring changes in alignment may differ. Many o f the postretention studies 6°6.%67,68,69-79 used Little's irregularity index, 9° or used different methods for measuring arch perimeter. 65,7°'73,74,77 S o m e investigators measured total lower arch crowding, 65,77 others investigated anterior lower arch crowding. 7°,7-~ Franklin et a176 measured both. The main problem is the wide range of treatment age in most o f the postretention studies, 6°,63,67,68,7°,73,79 with early and late treatments included in the same sample, and the variable lengths o f postretention time. Patients treated early will have c o m p l e t e d retention well before the age o f 18 years and will be influenced by the same forces responsible for increased crowding in untreated subjects, at their maxim u m in the teenage years. In subjects completing retention after the age o f 18 years, these forces should be largely e x p e n d e d and a degree of stability similar to that f o u n d in untreated arches in the third decade 27,28 might be expected. It could then be argued that retention after 18 years is unnecessary. That this is patently not so indicates that factors associated with o r t h o d o n t i c tooth m o v e m e n t contribute to postretention instability by accelerating the aging process. Further postretention studies o f subjects treated at the same age, reviewed at regular intervals and c o m p a r e d with untreated controls over the same age range would contribute to a better understanding o f the problem.
Acknowledgment The author thanks her colleagues and others who participated in these studies and to Sheena Sloane for preparation of the figures,
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67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
of mandibular anterior alignment: A cephalometric appraisal of first-premolar-extraction cases treated by traditional edgewise orthodontics. AmJ Orthod 1985;87: 27-38. Glenn G, Sinclair PM, Alexander RG. Nonextraction orthodontic therapy: Posttreatment dental and skeletal stability: AmJ Orthod Dentofac Orthop 1987;92:321-328. Little RM, Riedel RA. Postretention evaluation of stability and relapse-Mandibular arches with generalised spacing. AmJ Orthod Dentofac Orthop 1989;95:3%41. Ades AG, Joondeph DR, Little RM, Chapko M1L A long-term study of the relationship of third molars to changes in the mandibular dental arch. Am J Orthod Dentofac Orthop 1990;97:323-335. Kuitert RB, Prahl-Andersen B. Long-term post-treatment observations: Development, stability, relapse. Nederlandse Vereniging Voor Orthodontische Studie, Studyweek 1990; 140-174. McReynolds DC, Little RM. Mandibular Second premolar extraction-Postretention evaluation of stability and relapse. Angle Orthod 1991;61:133-144. Paquette DE, Beattie JR, Johnston LE. A long-term comparison of nonextraction and premolar extraction edgewise therapy in "borderline" class II patients. A m J Orthod Dentofac Orthop 1992;102:1-14. Freeman BV. A comparison of postretention mandibular incisor irregularity in treated Cass II division 1 malocclusions versus untreated Class I "normals." Am J Orthod Dentofac Orthop 1994;105:318-319. Sadowsky C, Schneider BJ, BeGole EA, Tahir E. Long term stability after orthodontic treatment: Non-extraction with prolonged retention. Am J Orthod Dentofac Orthop 1994;106:243-249. Dugoni SA, Lee SJ, Varela J, Dugoni AA. Early mixed dentition treatment: Postretention evaluation of stability and relapse. Angle Orthod 1995;65:311-320. Franklin GS, Rossouw PE, Woodside DG. A longitudinal study of dental and skeletal parameters associated with stability of orthodontic treatment. Am J Orthod Dentofac Orthop 1995;108:452-453. Kahl-Nieke B, Fischbach H, Schwarze CW. Post-retention crowding and incisor irregularity: A long-term follow-up evaluation of stability and relapse. B r J Orthod 1995;22: 249-257.
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78. Moussa R, O'Reilly MT, Close JM. Long-term stability of rapid palatal expander treatment and edgewise mechanotherapy. Am J Orthod Dentofac Orthop 1995;108:478488. 79. Artun J, Garol JD, Little RM. Long-term stability of mandibular incisors following successful treatment of Class II, Division 1, malocclusions. Angle Orthod 1996;66: 229-238. 80. Little RM. Stability and relapse of dental arch alignment. BrJ Orthod 1990;17:235-241. 81. Sjolien T, Zachrisson BU. Periodontal bone support and tooth length in orthodontically treated and untreated persons. AmJ Orthod 1973;64:28-37. 82. Zachrissou BU, Alnaes L. Periodontal condition in orthodontically treated and untreated individuals. II. Alveolar bone loss: Radiographic findings. Angle Orthod 1974;44:48-55. 83. Hollender L, R6nnerman A, Thilander B. Root resorption, marginal bone support and clinical crown length in ortbodontically treated patients. EurJ Orthod 1980;2:19% 205. 84. Hamp SE, Lundstr6m F, Nyman S. Periodontal conditions in adolescents subjected to multiband orthodontic treatment with controlled oral hygiene. Eur J Orthod 1982;4:77-86. 85. Kennedy DB, Joondeph DR, Osterberg SK, Little RM. The effect of extraction and orthodontic treatment on dentoalveolar support. Am J Orthod Dentofac Orthop 1983;84:183-190. 86. Polson AM, Reed BE. Long-term effect of orthodontic treatment on crestal alveolar bone levels. J Periodont 1984;55:28-34. 87. Sharpe W, Reed B, SubtelnyJD, Polson A. Orthodontic relapse, apical root resorption, and crestal alveolar bone levels. AmJ Orthod Dentof~ac Orthop 1987;91:252-258. 88. Ogaard B. Marginal bone support and tooth lengths in 19-year-olds following orthodontic treatment. E u r J Oi~ thod 1988;10:180-186. 89. Bondemark L. lnterdental bone changes after orthodontic treatment. A 5-year longitudinal study. Am J Orthod Dentofac Orthop 1998;114:25-31. 90. Little RM. The irregularity index: A quantitative score of mandibular anterior alignment. Am J Orthod 1975;68: 554-563.
he stability of orthodontic t r e a t m e n t must be j u d g e d against the b a c k g r o u n d of naturally occurring changes in the untreated dentition. It is generally recognized that the h u m a n dentition is in a dynamic state, continually changing t h r o u g h o u t life. Particularly noticeable in this respect is the change in alignment of the lower arch which may be crowded in the early mixed dentition, less so in the years between 8 and 12, and b e c o m e m o r e crowded after eruption of the second p e r m a n e n t molars, a process which is said to continue well into the third decade and beyond.1 T h e decrease in lower incisor crowding in the years immediately after their eruption has b e e n well d o c u m e n t e d . 2-7 During the teenage years between 13 and 18, an increase in lower arch crowding (2.0 m m or m o r e on average) has b e e n r e p o r t e d by Siatowski, s Sakuda et al, 9 Moorrees et al, 1° a n d Sampson et al. u O t h e r investigators have r e p o r t e d an increase in crowding over longer observation periods, extending f r o m the early teens to between 21 and 31 years of age. 12-19 No intermediate examinations were m a d e in these studies, therefore, it is not known whether the crowding developed in the years immediately after e r u p t i o n of the second p e r m a n e n t molars
T
From the Department of Orthodontics, School of Dentistry, Belfast. Address correspondenceto MargaretE. Richardson, 33 CherryValley Park, Belfast BT5 6PN, N Ireland Copyright© 1999 by W.B. Saunders Company 1073-8746/99/0503-0004510. 00/0
or whether the crowding continued to increase u p to the later age. I n f o r m a t i o n on changes in lower arch a l i g n m e n t after the age of 18 years is sparse. Bishara et al so f o u n d small increases in crowding between 0.5 and 0.9 m m in 30 patients between 25 and 46 years. Duterloo 21 n o t e d small dimensional arch changes in 26 patients between the m e a n ages of 17 to 28 years. Crowding was n o t measured, but the illustrations showed slight crowding increases in three patients. His sample included some patients y o u n g e r than 17 years, the age at the first observation r a n g e d f r o m 14 to 18 years. Carter and McNamara 19 f o u n d a significant increase of 0.48 m m in lower incisor irregularity in 10 patients between 32 and 45 years of age. To u n d e r s t a n d the relationship between the n o r m a l process of maturation and the changes after o r t h o d o n t i c t r e a t m e n t and retention, it is i m p o r t a n t to chart the progress of the developm e n t of crowding in the u n t r e a t e d lower arch 22 to ascertain whether it is a continuous, gradual, slowly diminishing change, or whether it occurs rapidly at certain stages interspersed with periods of relative stability. Such information may have a bearing on its cause.
S e v e n to Fifteen Years Models of 48 children with a full c o m p l e m e n t of p e r m a n e n t teeth and no early loss of deciduous teeth were selected f r o m the Belfast growth study 22 a n d m e a s u r e d at four stages, z3
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1. T1. P e r m a n e n t lower incisors at least partially erupted when the average age was 7.75 years. 2. T2. Lower p e r m a n e n t canines at least partially erupted when the average age was 10.85 years. 3. T3. Lower second molars at least partially erupted when the average age was 12.5 years. 4. T4. Three years after full eruption o f the lower second molars, the average age was 15.5 years. Lower incisor space condition (crowding) at each stage was calculated as the difference between arch perimeter (between the mesial contact points of contralateral deciduous or permanent canines) and the sum of mesiodistal incisor widths. In a well-aligned arch with all the teeth in contact, arch perimeter was equal to the sum of the tooth widths (Fig 1A). Where spaces occurred, these were a d d e d to the total tooth width to find the arch perimeter (Fig 1B). Where teeth were rotated or displaced, the space between adjacent teeth was measured and substituted for the size of the tooth in question to calculate arch perimeter. The change in lower incisor space condition between each of the four stages was calculated and the mean changes in this and in the measurements made in the following investigations were tested for significance with the paired samples t test. From T1 to T2, there was a significant average decrease in crowding of 0.91 ram. Crowding continued to decrease between T2 and T3, but the change was small and not significant (0.15 m m on average). After eruption of the second p e r m a n e n t molars, there was a significant increase in crowding of 0.41 ram.
Thirteen to Eighteen Years Lower arch space condition (crowding) was calculated from measurements made on models of 5 ! patients from the Belfast third molar study 24'25 at T1 (13 years) and T2 (18 years) in a similar m a n n e r to the previous study, 23 but this time, the arch perimeter was measured between the mesial contact points of the first p e r m a n e n t molars. The change in crowding was calculated as before. O n 60 ° left and right cephalometric radiographs of the same patients, the change in anteroposterior position of the lower first molar (mesial drift) was measured by projecting the anterior contact point of the first molar onto a
A
%aa¢
C Figure 1. Measurement of space condition. Arch perimeter minus tooth size. Arch perimeter = sum of mesiodistal tooth widths = zero space condition (A). Arch perimeter = sum of mesiodistal tooth widths + spaces = positive space condition (B). Space between adjacent teeth is substituted for the size of a rotated or displaced tooth in the calculation of arch perimeter = negative space condition (crowding) (C). (Reprinted with permission from Lundy HJ, Richardson ME: Developmental changes in alignment of the lower labial segment. Brit J Orthod 1995;22:339-345.23 Reprinted with permission of Oxford University Press.) maxillary horizontal on a tracing of the T1 film. The T1 tracing was superimposed on the T2 film, registering on internal structures in the mandible in a m a n n e r similar to that described by Bj6rk 26 (Fig 2). M1 patients had untreated lower arches with a full c o m p l e m e n t o f teeth, including third molars u n e r u p t e d at T1. Crowding increased significantly by 2.3 m m on average, ranging from 0 to 6.0 mm. The first molars moved forward significantly by an average of approximately 2.0 lnm on each side, ranging from 0 to 7.0 ram.
Eighteen to Twenty-One Years The Belfast adult growth study 27-29 i n c l u d e d 60 students with intact lower arches anterior to and including second molars. None had been treated
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PNS.
.AN~ I
/
'
I
Figure 2. Measurement of mesial drift on 60 ° cephalograms. Change in position of the mesial contact point of the first permanent molar measured on a maxillary horizontal with a tracing of the first film superimposed on the second registering on the inner outline of the mandibular symphysis and the inferior dental canal. Solid line, first film; broken line, second film.
orthodontically. F r o m m e a s u r e m e n t s m a d e o n models taken at 18 a n d 21 years, the c h a n g e in lower arch c r o w d i n g was calculated as before. C h a n g e in position o f the first m o l a r was measured o n 60 ° c e p h a l o m e t r i c r a d i o g r a p h s using the m e t h o d described previously. O n average, there was n o significant c h a n g e in crowding. O n l y 8% o f the students s h o w e d small increases in crowding. With o n e exception, all were less than the e r r o r o f m e a s u r e m e n t . T h e r e was a significant f o r w a r d m o v e m e n t o f the first m o l a r o n the left side 0.07 m m . Ninety-four p e r c e n t o f students h a d n o c h a n g e . T h e r e m a i n i n g 6% showed small a m o u n t s o f mesial drift r a n g i n g f r o m 0.5 to 2.0 m m .
Twenty-One to Twenty-Eight Years Forty-six o f these students were r e c o r d e d again at age 28 years. 2s Between ages 21 a n d 28 years, there was a significant average increase in lower arch c r o w d i n g o f 0.2 ram, r a n g i n g f r o m 0.2 to 1.4
Second Molar Extraction A small pilot study o n 10 patients 3° followed by a larger study o n 30 patients 31 investigated the effect o f extraction o f the s e c o n d p e r m a n e n t molars o n lower arch crowding. T h e m e a n age at extraction was 13.9 years (range, 11-17 years). Models a n d 60 ° c e p h a l o m e t r i c r a d i o g r a p h s were taken i m m e d i a t e l y before extraction (T1) a n d r e p e a t e d after an interval o f 5 years (T2). In b o t h studies, a c o r r e s p o n d i n g n u m b e r o f n o n e x t r a c d o n subjects r e c o r d e d at 13 years (T1) a n d 18 years (T2) were used as controls. C h a n g e s in lower a r c h c r o w d i n g a n d first m o l a r position were m e a s u r e d as before. In the s e c o n d m o l a r extraction g r o u p there was a slight decrease in lower arch c r o w d i n g o f 0.75 ram, significantly different f r o m the 2.0 m m increasein crowding in the n o n e x t r a c t i o n group. T h e lower first molars m o v e d slightly distally in the extraction g r o u p (0.4 m m left, 0.6 m m right), significantly differe n t f r o m mesial m o v e m e n t in the n o n e x t r a c t i o n g r o u p (2.0 m m left, 1.7 m m right).
mill.
Discussion Eighteen to Fifty Years A n o t h e r small g r o u p o f 16 adults were r e c o r d e d at age 18 years a n d again at age 50 years. 29 Two o f these subjects h a d n o c h a n g e in lower arch crowding. All the others h a d an increase in crowding, r a n g i n g f r o m 0.2 to 2.5 ram.
T h e decrease in lower arch c r o w d i n g in the years after e r u p t i o n o f the lower p e r m a n e n t incisors 23 was consistent with the findings o f others. 2-7 T h e m e c h a n i s m s responsible f o r this decrease seem to be mainly dentoalveolar, b e c a u s e there is little o r n o skeletal growth in the a n t e r i o r p a r t o f the lower jaw at this time. ~,7,32~34A d a m s a n d Richard-
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son 35 f o u n d that the lower incisors proclined relative to the mandibular plane by an average of 13 ° between 5 and 11 years. This is probably the most important means of creating space for incisor alignment. An increase in arch width across the canines caused by alveolar growth just before and during the eruption of the permanent i n c i s o r s 2,4,36 may also help to create space. The significant increase in crowding between 12 and 15 years 23 was consistent with, t h o u g h smaller than, increases in crowding reported over longer observation periods after the age of 12 to 13 years, a2-a9In a larger sample drawn from the same source and including the present sample, Richardson ~7f o u n d a slightly larger increase in crowding of 0.7 m m in the same 3-year period. Between 13 and 18 years, 24 the considerably larger average increase in crowding of 2.3 m m was comparable with that f o u n d by Sampson et al n in the same age range. Sakuda et al, 9 using a scoring system to measure crowding, recorded average increases of 4.9 m m in boys and 6.8 m m in girls between the same ages. Figure 3 shows
models of an untreated boy who had a typical average increase in crowding between 13 and 18 years. From 18 to 21 years, 27 the lower arch appeared to be relatively stable in contrast to the preceeding teenage years. Only one person had an increase in crowding, which was clinically obvious at 1.8 mm. A few people had small increases scarcely visible to the naked eye. The majority had no measurable change. In the following years from 21 t o 28, 28 the increase in crowding of 0.2 mm, t h o u g h statistically significant, was of little importance clinically. Figure 4 shows models of a girl who had the largest increase of 0.7 m m in crowding during this period. The cumulative change from 18 to 28 years, 1.3 m m is only just perceptible. For the small group studied between 18 and 50 years, 29 mean values have little clinical importance. Almost half of them had increases in crowding between 1.5 and 2.5 mm, obvious to the naked eye. These studies 24,27-2u imply that the greatest
Figure 3. Occlusal view of models of an untreated boy at age 13 years (A), and age 18 years (B) showing an average increase in lower arch crowding commonly found at this stage of development. (Reprinted with permission from Lundy HJ, Richardson ME: Developmental changes in aligmnent of the lower labial segment. BritJ Orthod 1995;22:339-345.s7 Reprinted with permission of Oxford University Press.)
LowerArch Crowding
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Figure 4. Occlusal models of an untreated girl who had the greatest increase in lower arch crowding, 0.7 mm, between 21 and 28 years. Age 18 years (A), age 21 years (B), age 28 years (C). The cumulative increase in crowding from 18 to 28 years, 1.3 mm, is only just perceptible. (Reprinted with permission from Richardson ME, GormleyJS: Lower arch crowding in the third decade. Eur J Orthod 1998;20:597-607d 8 Reprinted with permission of Oxford University Press.)
a m o u n t of crowding occurs during the teenage years between 13 and 18. The period between the late teens and early twenties seems to be a relatively stable period in terms of lower arch alignment, with variable small amounts of crowding occurring in the third and fourth decades. This suggests that the causes of late crowding may vary at different stages of development. The cause of increased crowding in the intact lower arch is not fully understood. It is obviously multifactorial, and for this reason, it is difficult to show a cause and effect relationship. Mesial migration of h u m a n teeth has been recognized since the late 18th century, when itwas described by J o h n H u n t e r 3s and was shown by the forward m o v e m e n t of the first molars f o u n d in the patients studied between 13 and 18 years of
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age. 24 T h e r e is evidence to s u p p o r t the view that it is largely responsible for the increase in crowding during the teenage years. Mesial migration may be caused by physiological mesial drift, by the anterior c o m p o n e n t of the force of occlusion on mesially inclined teeth, the mesial vectors of muscular contraction, or the contraction of the transseptal fibres of the periodontal ligament. Third m o l a r agenesis and extraction studies 39-42 suggest that mesial migration is greater in the presence of a developing third molar. This suggestion is strongly reinforced by second m o l a r extraction studies. 3°,~1 Removal of the second m o l a r effectively isolates the third m o l a r from the rest of the arch. T h e reduction in crowding and the distal m o v e m e n t of first molars in patients whose second molars have b e e n extracted c o m p a r e d with the increase in crowding and mesial m o v e m e n t of first molars in nonextraction subjects ~°,31 provides convincing evidence of the effects of developing third molars on the anterior part of the arch. O t h e r studies on patients treated by extraction o f second molars 43-46 r e p o r t e d similar results. It is a mistaken impression that it is only i m p a c t e d third molars that cause the problem. A third m o l a r that erupts is likely to exert m o r e pressure on the dental arch than one that remains impacted, and some i m p a c t e d third molars may exert m o r e pressure than others. T h e lower third m o l a r starts to develop in the ramus of the mandible in a mesioangular position relative to the mandibular plane and gradually b e c o m e s m o r e upright until it is in a position to erupt. Impactions can develop in several different ways. 47 1. T h e milder mesioangular impactions start to upright in the n o r m a l way, but the degree of uprighting is insufficient to p e r m i t eruption. 2. T h e m o r e severe mesioangular impactions seem to remain in their original angular position, but on closer inspection, they upright a little and then tip mesially again. 3. Horizontal impactions may start in an apparently favorable position and tip mesially to the horizontal. 4. Some third molars remain vertically impacted after they have uprighted but are unable to e r u p t because of lack o f space. 5. O t h e r third molars can actually tip distally once they have r e a c h e d the vertical position and b e c o m e distoangularly impacted.
T h e third molars that tip mesially are unlikely to exert m u c h pressure on the dental arch. The milder mesioangular and the vertical third molars may cause pressure while they are trying to erupt, but once they b e c o m e impacted and especially once the apices close, they probably cease to do so as do the distoangular impactions once they start to tip distally. Those third molars that force their way into the arch and e r u p t are the ones most likely to cause crowding. T h e cause of late crowding may not be the same in every patient, and in any one individual, there may be m o r e than one contributing factor. Some factors may have m o r e influence than others or may act at different stages of development. The a m o u n t and direction of late mandibular growth, skeletal structure and complex growth patterns, soft tissue maturation, occlusal factors, tooth morphology, periodontal forces, and degenerative tissue changes have b e e n implicated to varying extent and in different combinations. 4~54 An ever-increasing volume of literature reports a variable and unpredictable deterioration in lower arch alignment f r o m 1 to 20 years after retention after various orthodontic treatment regimens in b o t h extraction and nonextraction. 18'55-79It is often claimed that this is caused by the n o r m a l physiological process of maturation f o u n d in untreated s u b j e c t s . 2°,s° Nevertheless, it is not unreasonable to expect that teeth that have b e e n m o v e d orthodontically might be m o r e susceptible to the pressures that cause arch length reduction and crowding. T h e Belfast studies on long-term changes in u n t r e a t e d lower a r c h e s 27-29 suggest that increased crowding in later life may be caused by degenerative changes associated with aging or periodontal disease. In the small group e x a m i n e d at ages 18 and 50 years, 29 though periodontal status was not quantified, all subjects showed some sign of periodontal change ranging f r o m gingival recession with consequent elongation of clinical crowns, to established periodontal disease, treated and untreated. Such changes would result in weakening of the supporting structures of the teeth, rendering t h e m less resistant to pressures that they previously withstood. O r t h o d o n t i c tooth movem e n t may accelerate this process by causing root resorption and reduced alveolar b o n e levels, which may not recover their p r e t r e a t m e n t status.81-89 Difficulties arise in c o m p a r i n g changes in
Lower Arch Crowding
lower arch alignment in untreated arches with those that have been treated orthodontically. Methods of measuring changes in alignment may differ. Many o f the postretention studies 6°6.%67,68,69-79 used Little's irregularity index, 9° or used different methods for measuring arch perimeter. 65,7°'73,74,77 S o m e investigators measured total lower arch crowding, 65,77 others investigated anterior lower arch crowding. 7°,7-~ Franklin et a176 measured both. The main problem is the wide range of treatment age in most o f the postretention studies, 6°,63,67,68,7°,73,79 with early and late treatments included in the same sample, and the variable lengths o f postretention time. Patients treated early will have c o m p l e t e d retention well before the age o f 18 years and will be influenced by the same forces responsible for increased crowding in untreated subjects, at their maxim u m in the teenage years. In subjects completing retention after the age o f 18 years, these forces should be largely e x p e n d e d and a degree of stability similar to that f o u n d in untreated arches in the third decade 27,28 might be expected. It could then be argued that retention after 18 years is unnecessary. That this is patently not so indicates that factors associated with o r t h o d o n t i c tooth m o v e m e n t contribute to postretention instability by accelerating the aging process. Further postretention studies o f subjects treated at the same age, reviewed at regular intervals and c o m p a r e d with untreated controls over the same age range would contribute to a better understanding o f the problem.
Acknowledgment The author thanks her colleagues and others who participated in these studies and to Sheena Sloane for preparation of the figures,
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[~'~
5. Lee KP. Behaviour of erupting crowded lower incisors. J Clin Orthod 1980;14:24-33. 6. Hagberg C. The alignment of permanent mandibular incisors in children. A longitudinal prospective study. EurJ Orthod 1994; 16:121-129. 7. Van der Linden FPGM. Transition of the human dentition. Monograph 13, Craniofacial Growth Series. Ann Arbor, MI, 1984. Center for Human Growth and Development, U Mich. 8. Siatkowski RE. Incisor uprighting: Mechanism for late secondary crowding in the anterior segments of the dental arches. AmJ Orthod 1974;66:398-410. 9. Sakuda M, Kuroda Y, Wada K, Matsumoto M. Changes in crowding of the teeth during adolescence and their relation to growth the facial skeleton. Eur Orthod Soc Trans 1976;93-104. 10. Moorrees CFA, Lebret LML, Kent RL. Changes in the natural dentition after second molar emergence (13-18 years). Int Ass Dent Res 1979:58 (special issue A):276 (abstr 737). 11. Sampson WJ, Richards LC, Leighton BC. Third molar eruption patterns and mandibular dental arch crowding. Aust OrthodJ 1983;8:10-20. 12. Brown VP, Daugaard-Jensen I. Changes in the dentition from the early teens to the early twenties. Acta Odont Scand 1951;9:177-192. 13. Lundstr6m A. Changes in crowding and spacing of the teeth with age. Dent Practit 1969;19:218-224. 14. Humerfelt A, Slagsvold O. Changes in occlusion and craniofacial pattern between 11 and 25 years of age. Eur Orthod Soc Trans 1972;113-122. 15. Sinclair PM, Little RM. Maturation of untreated normal occlusions. Am J Orthod 1983;83:114-123. 16. Meng HE Gebauer U, Ingervall B. Die Entwicklung des Engstandes der unteren Inzisiven im Zusammenhang mit Veranderungen der Zahnbogen und des Gesichtsschadels bei Individuen mit guter Okklusion yon der Puberat bis zum Erwachsenenaeter. Schwiez Mschr Zahnmed 1985;95:762-777. 17. Bishara SE,JakobsenJR, TrederJE, Stasi MJ. Changes in the maxillary and mandibular tooth size-Arch length relationship from early adolescence to early adulthood. AmJ Orthod Dentofac Orthop 1989;95:46-59. 18. Persson M, Persson EC, Skagius S. Long-term spontaneous changes following removal of all first premolars in Class 1 cases with crowding. EurJ Orthod 1989;11:271282. 19. Carter GA, McNamara JA. Longitudinal dental arch changes in adults. AmJ Orthod Dentofac Orthop 1998; 114:88-99. 20. Bishara SE, Treder JE, Jakobsen JR. Facial and dental changes in adulthood. Am J Orthod DentoFac Orthop 1994;106:175-186. 21. Dnterloo HS. Development of the dentition under the influence of functional factors. Hunter WS, Carlson DS (eds): Essays in honor of Robert E. Moyers, Craniofacial Growth Series 24, Center for Human Growth and Development, U Mich Ann Arbor, MI, 1991;103-122. 22. Adams CP. A comparison of 15-year-old children with excellent occlusion and with crowding of the teeth, Angle class 1 malocclusion, in respect of face size and shape and tooth size. Swed DentJ 1982;15:11-26. (suppl)
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