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Postretention mandibular incisor stability after premolar serial extractions
Postretention Mandibular Incisor Stability After Premolar Serial Extractions Donald G. Woodside, P. Emile Rossouw, and David Shearer The purpose of this study was to evaluate the mandibular incisor alignment in serial extraction cases, using the longitudinal dental cast records of the Burlington Growth Center as a control sample. Various parameters were investigated and the statistical differences determined between the treated and untreated groups. The results were also compared with data from serial extraction groups that subsequently had orthodontic treatment. Untreated subjects and subjects treated only with serial extractions showed similar longitudinal changes. However, the extraction group that also received orthodontic treatment appeared to show more lower incisor crowding long-term. No predictors for stability of clinical significance could be determined. Mechanotherapy influences the craniofacial and dentoalveolar dimensions, which appear to cause more long-term lower incisor crowding. (Semin Orthod 1999;5:181-190.) Copyright© 1999by W.B. Saunders Company
he controversy regarding the extraction of p e r m a n e n t teeth to create space in the dental arch has b e e n a heated topic for the better part of the century. 1-3 N u m e r o u s factors have b e e n suggested in an a t t e m p t to explain postretention crowding. However, no variables have yet b e e n f o u n d as predictors of such crowding? It is c o m m o n l y believed that the extraction of the four first premolars ensures stability of incisor alignment. 5 It was, therefore, disheartening when Little et al 4 dispelled this belief by showing that 70% of all patients with premolars extracted as part of the orthodontic t r e a t m e n t had a degree o f p o s t r e t e n t i o n crowding that was considered unacceptable. Such postretention incisor crowding can be somewhat distressing to the u n i n f o r m e d patient. It is, therefore, i m p o r t a n t to continue to look
T
From the Discipline of Orthodontics, Faculty of Dentistry, University of Toronto, Toronto, Canada. This study was made possible by the use of material from the Burlington Research Centre, Faculty of Dentistry, University of Toronto, which was supported byfunds provided by National Health Grant (Canada) No. 6057-299. Address co~veapondence to P. Emile Rossouw, BSc, BChD, BChD (Hons), MChD, PhD, Faculty of Dentistry, Unive*:~ity of Toronto, 124 Edward St, Toronto, Ontario, M5G 1 G6 Canada. Copyright © 1999 by W.B. Saunders Company 1073-8746/99/0503-0007510. 00/00
for factors or combinations of factors that might provide insight into the cause of late developmental crowding a n d thereby possibly facilitate the establishment of some f o r m of biologic control. As an alternative, lifelong retention or a second orthodontic treatment later in life may b e c o m e necessary.
Premolar Extraction and Postretention Stability In a study of 80 orthodontic patients (22 nonextraction a n d 58 extraction cases) in which either the first or second premolars were extracted, a net arch length decrease was observed during the postretention phase. This decrease in arch length continued during a m i n i m u m 10-year postretention period, and it was observed regardless of whether the patient had b e e n treated with or without extractions. 6 Swanson et al v also investigated the stability of rotated teeth in individuals who had b e e n treated either by nonextraction therapy or by extraction of first bicuspids in one or b o t h arches, followed by comprehensive orthodontic treatment. All patients had b e e n out of retention for a minim u m of 10 years. Canines were observed to be the most rotated teeth in either arch, both before t r e a t m e n t and at the postretention examination. These results suggest that certain teeth
Seminars in Orthodontics, Vol 5, No 3 (September), 1999." pp 181-190
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Woodside, Rossouw, and Shearer
are m o r e p r o n e to rotational relapse than others. No associations could be: f o u n d between rotational relapse of the canines a n d p r e m o l a r extraction. This study showed that factors, such as sex, age at the end of treatment, Angle classification, arch length changes, and growth of the jaws, were not associated with rotational relapse. However, the a m o u n t of rotational relapse experienced during the postretention phase was directly p r o p o r t i o n a l to the a m o u n t of rotational correction achieved during treatment. In 1976, Riedel 8 m a d e the suggestion that the removal of m a n d i b u l a r first or second premolars is " n o t always, and is perhaps not usually related to relieving m a n d i b u l a r anterior incisor crowding in the long-term." It was his contention that a shift toward a nonextraction a p p r o a c h to orthodontic therapy largely resulted f r o m the knowledge that m a n d i b u l a r first p r e m o l a r extraction does not necessarily result in the long-term stability of the m a n d i b u l a r anterior incisor segment. Little et al 4 evaluated mandibular incisor alignm e n t in 65 patients treated by conventional edgewise orthodontic m e c h a n o t h e r a p y subsequent to the extraction of all 4 first premolars a m i n i m u m of 10 years postretention. Alignment was assessed by Little's irregularity index. 9 Before treatment, m o r e than 70% of the patients displayed m o d e r a t e or severe irregularity (23% moderate, and 54% severe). After treatment, 70% of the patients had m o d e r a t e or severe irregularity but in different proportions (56% moderate, and 16% severe). All but one of the patients who showed minimal irregularity before treatment, b e c a m e worse during the postretention phase. Those patients who had severe irregularity initially still showed a net improvement, despite the fact that the postretention irregularity was unacceptable. In general, they also f o u n d that m a n d i b u l a r intercanine width t e n d e d to decrease during the postretention phase. These results also showed that the long-standing concept that m a i n t e n a n c e of intercanine width during t r e a t m e n t would confer stability of incisor alignment was untrue ,4 Thus, arch width changes, which are established during treatment, are a p o o r predictor of long-term stability. Arch length decreased during the postretention stage in all but two patients with only a weak correlation between the a m o u n t of postretention arch length reduction and the severity ofpostretention irregu-
larity. It was concluded that no single parameter, such as sex, age, Angle classifications, length of retention, or values derived f r o m the m o d e l analysis, served as a reliable predictor of future stability. Combinations of the a f o r e m e n t i o n e d p a r a m e t e r s also proved to be of little value in predicting a long-term result. Certain factors may have affected the results of Little et al. 4 T h e p r e t r e a t m e n t age range of the patients/vas large (7 years and 10 m o n t h s to 18 years and 2 months). T h e patients were separated on the basis of their Angle classification, whereas the differences in skeletal morphology between patients were not taken into consideration. A flaw that was pointed out by Little involved the use of m a n d i b u l a r removable retainers in some patients. T h e degree of patient compliance, and extent of retainer wear was known to be variable. Little et al 4 therefore stated, "interpretation of statistical results must be somewhat g u a r d e d . " A continuation of the Little et al 4 study aimed to clarify whether any clinically significant predictors and correlations existed a m o n g cephalometric and dental-cast data when considering longt e r m stability. The study comprised a 10-year postretention cephalometric appraisal of first p r e m o l a r extractions in 54 patients treated by traditional edgewise orthodontics. 5 G o o d mandibular anterior alignment, with an Irregularity I n d e x 9 of less than 3.0 m m , was displayed. As with the case in the 19814 study, the investigators n o t e d that " m a r k e d variation" was an apt description of the postretention sample. Again, they concluded that no specific cephalometric param e t e r or combination of p r e t r e a t m e n t and postt r e a t m e n t cephalometric values were able to predict m a n d i b u l a r incisor irregularity. Associations between cephalometric and dental-cast data were minimal and of limited clinical value. Although unexpected, an association between pret r e a t m e n t and posttreatment cephalometric records was f o u n d for those patients with anatomical characteristics usually representative of Class II, Division 2 patients, which suggests that the n e u r o m u s c u l a r characteristics of this group may be a source of overbite relapse. Little et aP ° e x t e n d e d their previous m o d e l to 10 and 20 years postretention. T h e sample consisted of 31 patients, treated with 4 first p r e m o l a r extractions and conventional edgewise orthodontics. The Little Irregularity I n d e x 9 showed that
Postretention Mandibular Incisor Stability
30% of the 10-year postretention patients (1981 considered clinically acceptable, and at 20 years postretention (1988 study),1° only 10% of the sample cases had clinically acceptable m a n d i b u l a r anterior alignment. It is i m p o r t a n t to note that this was only a m e a n 1 m m increase in irregularity between 10 and 20 years postretention. It was concluded f r o m the above, that mandibular incisor irregularity continues to increase well after active growth has ceased a n d is variable. These long-term changes have also b e e n r e p o r t e d in a sample of untreated n o r m a l individuals, u It was concluded that the only way to ensure acceptable alignment of the dentition was through lifelong retention. The p r e m o l a r extraction study by Little et al 4 was also c o m p a r e d with the incisor irregularity in untreated normals 12 and postretention nonextraction cases. 13 It is remarkable that the a m o u n t of postretention irregularity was m o r e than twice as great in p r e m o l a r extraction cases ( + 2.9 m m ) c o m p a r e d with treated nonextraction cases ( + 1.2 m m ) , and a b o u t four times as great as untreated normals (+0.7 m m ) . T h e extraction t r e a t m e n t seemed to do s o m e t h i n g which actually increased posttreatment irregularity when comp a r e d with untreated normals. Lower arch expansion occurred in the treated patients, a p a r a m e t e r which decreased over time a n d could have contributed to the long-term changes described. T h e long-term occlusal changes of 96 patients treated with edgewise m e c h a n o t h e r a p y f r o m 20 to 25 years previously, showed that, at the postretention follow-up, 72% of the patients h a d at least one p a r a m e t e r that was outside of the ideal range. TM Patients with Class III initial malocclusions were excluded f r o m the sample, 38% h a d extractions, whereas 62% were treated with nonextraction. Thirty-five percent of the pretreatm e n t m a n d i b u l a r incisor crowding was outside the ideal range, whereas, at the time of postretention follow-up, this value was 15%. This presents a m o r e positive picture ofpostretention mandibular incisor alignment. 14 Franklin a5 s u p p o r t e d this conclusion. However, the 3.0 m m of incisor crowding selected by Sadowsky and Sakols 14 may not be r e g a r d e d as n o r m a l by most clinicians. The separation of patients into groups based on their Angle classification without considering differences in skeletal m o r p h o l o g y also creates differences that could influence the o u t c o m e of study) 4 were
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the t r e a t m e n t results. Finally, fixed lingual retainers were used in most patients for varying lengths of time. The m e t h o d and duration of retaining the m a n d i b u l a r incisors in the r e m a i n i n g patients was not stated. U h d e et aP 6 selected a g r o u p (79 patients: 27 extraction, 45 nonextraction) f r o m the records used in the study by Sadowsky and Sakols. 14 No significant difference was f o u n d postretention in the m a n d i b u l a r incisor crowding between extraction and nonextraction treatments in b o t h the Class I and Class II groups. Multiple regression analysis i n d i c a t e d that o t h e r p o s t r e t e n t i o n changes evaluated (maxillary and m a n d i b u l a r intercanine width, maxillary a n d m a n d i b u l a r i n t e r m o l a r width, overjet, overbite, left and right m o l a r relationships, and maxillary incisor crowding) were related to 41% of the variability, and m a n d i b u l a r intercanine width accounted for 12.5% (more than any o t h e r p a r a m e t e r ) .
Postretention Stability in Serial Extraction Cases With Subsequent Treatment A F r e n c h m a n , Robert B u n o n (1743), is credited with the first attempt to guide the eruption of p e r m a n e n t teeth by the selective removal of deciduous a n d p e r m a n e n t teeth. 17 T h e goal 200 years ago was, as now, to p e r m i t the p e r m a n e n t dentition to e r u p t into an acceptable occlusion in deficient arches. T h e r e was little published on the subject after the middle of the last century until Hotz ("active supervision of dental eruption" or "guidance of dental eruption by means of extractions") TM and Kjellgren ("serial extraction") 10 published their respective articles. It has since b e e n assumed that if teeth e r u p t into positions of alignment, stability of such alignm e n t will be c o n f e r r e d in comparison to teeth m o v e d into alignment after eruption. In the early 1940s, it was recognized that dental arch expansion was not the p a n a c e a it was once t h o u g h t to be for the elimination of crowdingY ° Thus, serial extraction was t h o u g h t to bring the teeth and n e u r o m u s c u l a r and skeletal systems into balance. Dewel 2~ stated that teeth in m a r k e d discrepancy cases are not first required to assume positions of e x t r e m e irregularity and then subjected to extensive orthodontic movem e n t with extraction to establish acceptable occlusal relations. They are, instead, " p e r m i t t e d
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to take these positions in the first place." Tweed z2 also stressed the i m p o r t a n c e of working with "growth and its potentials" and the acquisition of "a better understanding of when and how to intervene in the guidance of growth processes so that nature may better a p p r o x i m a t e her growth plan," thereby eliminating the n e e d for "harsh mechanics" to be i m p l e m e n t e d at the time of adolescence. In Class II and Class III malocclusions, serial extractions should be a p p r o a c h e d with caution because the skeletal abnormality must be addressed and corrected as the p r i m a r y focus of the treatment. 21 It has b e e n stated that increased posttreatment stability of m a n d i b u l a r incisors can be achieved with serial extraction followed by comprehensive treatment, s,z°,23-25 Dale 25 also stated that i f a tooth were allowed to complete its formation in a position where it would remain once t r e a t m e n t was completed, it would be m o r e stable. Graber, 2° however, warned that serial extraction, originally advocated to reduce the severity of crowding in deficient arches, is " a n o r t h o d o n tic decision and requires the knowledge, knowhow, and clinical experience of the specialist who ultimately must complete the therapy in almost all cases." H e a d d e d that a poorly guided serial extraction p r o g r a m may be worse than p e r f o r m i n g any t r e a t m e n t at all. Little et a117 evaluated the case records of 30 patients who had u n d e r g o n e serial extraction of deciduous teeth and four first premolars, after which active standard edgewise m e c h a n o t h e r a p y was initiated when all p e r m a n e n t teeth erupted. These cases were followed by a period of retention of usually 2 years or more. All cases were assessed after a m i n i m u m postretention period of 10 years or more. Five variables were measured f r o m pretreatment, posttreatment, and postretention dental casts. These included the Little I r r e g u l a r i t y Index, 9 m a n d i b u l a r arch length, m a n d i b u l a r intercanine width, overbite, and overjet. T h e results of this study showed that, during the observation stage, m a n d i b u l a r incisor crowding usually improved. However, at the postretention stage, this alignment was no better than 30% successful. Surprisingly, this was very similar to that seen in the late first p r e m o l a r extraction group (Little et a14; satisfactory mandibular anterior a l i g n m e n t was less than 30% with almost 20% showing m a r k e d crowding). No
d o u b t the results of this s t u d y 17 w e r e disconcerting to many, because the future stability predicted by Graber, 2° Dale, 25 and Tweed, 26 was not confirmed. T h e consequences of m a n d i b u l a r second prem o l a r extraction were studied in 46 patients (congenitally missing m a n d i b u l a r second premolars or who had had t h e m extracted).27 Comprehensive edgewise o r t h o d o n t i c t r e a t m e n t followed a period of approximately 1 year of physiological drift. O r t h o d o n t i c t r e a t m e n t was initiated shortly after the removal of teeth and b o t h groups were analyzed a m i n i m u m of 10 years postretention. T h e r e was no statistically significant correlation in any dental cast or cephalometric m e a s u r e m e n t between the two groups. The p r e t r e a t m e n t irregularity indices of the groups were similar, and both groups displayed an increase in lower incisor crowding postretention, however, the difference between the two groups was not significant. No associations between incisor alignment and any other variable could be found. Although g e n d e r differences did not show any association with incisor aligmnent at any time interval for either group, p o o l e d males were f o u n d to have significantly m o r e crowding than females at the postretention stage. This contrasts with Sinclair and Little's 28 study in which they f o u n d that, in untreated normals, w o m e n had developed m o r e crowding than men. The authors were careful not to draw any conclusions f r o m these results but stated that they considered that growth may be a factor in the long-term stability of the dentition.
Serial Extraction and Follow-Up Stability T h e r e is limited literature available on the stability of serial extraction cases without subsequent orthodontic treatment. Little et a117 referred to the results of an u n p u b l i s h e d thesis in which 50 serial extraction cases which had not b e e n followed by subsequent orthodontic treatment were studied at 10 years follow-up. T h e results of this study showed greater long-term stability comp a r e d with patients with malocclusions, who were treated with first p r e m o l a r extraction in the p e r m a n e n t dentition and followed by comprehensive orthodontic treatment. Although interesting, Little et al n o t e d that the sample may have b e e n biased and tlae results should be viewed with a certain degree o f caution.
Postretention Mandibular Incisor Stability
Persson et a129 studied a group of 42 serial extraction patients who had their first premolars removed at a mean age of 10½ years as the sole treatment of the malocclusion. At an average age of 30 years posttreatment, a redevelopment of crowding had occurred, t h o u g h this was less than it had been originally. The serial extraction group was compared with a g r o u p of 29 untreated cases who had a " n o r m a l occlusion" at age 13 years. The degree of crowding in the normal group over the same time period was similar to that of the serial extraction group. No significant differences in axial inclination of the mandibular incisors as measured on cephalograms between the two groups were shown at follow-up. Correlations or indicators of future crowding were not sought. A linear assessment of crowding was used in this study instead of Little's Irregularity Index, thereby making direct comparisons with studies conducted by Riedel, Little, and others, difficult. Although the serial extraction group was c o m p a r e d with a control group, the two groups were not matched. Moreover, differences in skeletal m o r p h o l o g y were not taken into consideration in selecting the sample.
Purpose of the Present Study It is important to identify the specific factors which are related to mandibular incisor instability. Some clinicians believe that stability is related to treatment and mandibular incisor teeth should be moved to a p r e d e t e r m i n e d ideal. Others hold that mandibular incisors should not be moved from their original positions. There are very few patients in whom serial extractions were perf o r m e d and mandibular incisors were permitted to erupt in the arch without the influence of subsequent orthodontic treatment. The purpose of the present study was to evaluate mandibular incisor alignment in nonorthodontically treated serial extraction patients, using long-term dental cast records of patients from the Burlington Growth Centre. The aim was to compare patients treated with serial premolar extractions (followed by a period of physiological drift without subsequent orthodontic treatment other than minimal space maintenance therapy) with a m a t c h e d untreated control group (nonextraction) who received no such supervision. The results of this study were
185
c o m p a r e d with those obtained by studies conducted at the University of Washington.
Materials and Methods The experimental sample consisted of 22 patients (12 males and 10 females) from the records of the Burlington Growth Center who had u n d e r g o n e serial extractions of four premolars. Nineteen patients had four first premolars extracted, and three patients had their maxillary first and mandibular second premolars removed. Serial extraction treatment was used to correct a tooth size arch length discrepancy. 3° None of the patients in the experimental group received major orthodontic treatment involving tooth m o v e m e n t or extractions other than the serial extractions. However, a n u m b e r of the patients had received m i n o r orthodontic treatment, in addition to the serial extractions. Treatment methods that were suspected of influencing mandibular incisor alignment were accounted for in the statistical analysis. The patients were m a t c h e d with subjects from the untreated control group to form pairs. 31 Matching criteria included variables such as skeletal age, gender, face height ratio, ANB angle and incisor irregularity (Table 1). All controls were from the Burlington Growth Center and had not received any orthodontic treatment or extractions. Incisor irregularity could not be properly assessed before the initiation of serial extraction treatment (T1), therefore, no measurements were made at this time. Posttreatment (Tz) measurements were made at the time of eruption of the mandibular canines. Long-term (T3) measuremerits were made on the final records. Digital callipers (Mitutoyo Corporation, Tokyo, Japan) were used to measure the following at T2 and T3: irregularity index, mandibular incisor crowding, mandibular intercanine width, mandibular intermolar width, mandibular arch length, overbite, and overjet.
Analysis of Data Analysis of Treatment, Time, and Gender Effects To reduce interexaminer bias, all parameters were measured by one individual. The significance level was set at P < 0.05. A repeated
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Woo&ide, Rossouw, and Shearer
Table 1. Example of Data for Matching the Treatment (Serial Extractions) and Control (Untreated, Nonextraction) Groups: Skeletal Age and Irregularity Index Skeletal Age in Years
Irregularity Index (mm) T2
T3
T~
T3
Gender
No.
Treat
Contr
Treat
Contr
Treat
Contr
Treat
Contr
Female
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 11 12
2.30 3.69 6.18 4.24 1.56 2.01 2.64 4.94 4.34 0.37 2.27 0.45 3.76 2.45 0.59 1.58 2.80 4.21 3.94 4.16 5.20 1.28
2.10 2.15 6.21 4.05 1.84 3.75 2.46 6.47 3.39 1.86 3.69 0.00 4.81 1.71 0.62 1.42 3.01 5.33 1.89 4.90 3.29 2.22
3.36 7.58 3.95 2.72 1.70 2.26 6.96 5.22 8.91 2.01 1.94 3.09 2.86 3.62 7.25 3.99 5.80 5.46 7.41 3.60 10.27 0.44
2.32 4.00 6.97 7.12 1.34 5.04 4.24 7.28 4.89 3.05 3.34 0.47 7.52 1.32 1.07 1.47 4.59 6.49 3.32 4.38 5.33 2.03
10.0 11.0 12.0 11.0 11.0 10.0 10.0 14.0 10.0 10.0 13.0 11.0 11.0 11.0 11.0 13.0 11.0 12.0 10.0 12.0 11.0 12.0
11.0 12.0 11.0 10.0 11.0 10.0 13.0 13.0 10.0 12.0 13.0 12.0 13.0 12.0 13.0 11.0 12.0 12.0 12.0 12.0 12.0 10.0
16.0 17.0 17.0 19.0 19.0 17.0 16.0 20.0 20.5 18.0 18.0 18.0 18.0 16.0 17.0 20.0 16.0 20.0 18.0 20.0 18.0 18.0
17.0 18.0 16.0 19.0 19.0 18.0 19.0 18.0 20.0 20.0 18.0 19.0 19.0 17.0 19.0 18.0 17.0 19.0 20.0 20.0 19.0 16.0
Male
m e a s u r e s analysis o f v a r i a n c e was u s e d to perf o r m a c o m b i n e d analysis o f t r e a t m e n t , time, a n d g e n d e r effects.
Error Analysis M e a s u r e m e n t e r r o r was e v a l u a t e d by r a n d o m s e l e c t i o n o f 10 p a t i e n t s a n d r e m e a s u r e m e n t o n a s e p a r a t e o c c a s i o n . Systematic e r r o r was m e a s u r e d with a S t u d e n t t test. E r r o r was r e p r e s e n t e d e i t h e r as a m i l l i m e t e r m e a s u r e m e n t o r as a c o e f f i c i e n t o f variation.
Analysis of the Effects o f Nonideal Treatments or Measurements At T2, several factors a s s o c i a t e d with t h e p a t i e n t s w e r e i d e n t i f i e d , w h i c h m i g h t have significantly i n f l u e n c e d t h e results. T h e s e i n c l u d e d differe n c e s in skeletal m o r p h o l o g y o r t r e a t m e n t as follows: an a b n o r m a l o v e r b i t e in c o m b i n a t i o n with an a b n o r m a l f a c e / h e i g h t ratio, an a b n o r m a l A N B angle, m o n o b l o c t r e a t m e n t , a n d t h e e x t r a c t i o n o f m a n d i b u l a r s e c o n d p r e m o l a r s ins t ead o f t h e m a n d i b u l a r first p r e m o l a r s . T h e d a t a w e r e a n a l y z e d with a n d w i t h o u t the inclusion o f e a c h o f t h e s e g r o u p s o f individuals. I f t h e i r i n c l u s i o n d id n o t r e s u l t in a si g n i f i can t
d i f f e r e n c e , they w e r e i n c l u d e d in all f u r t h e r analyses.
Results Error Analysis T h e e r r o r associated with this study was small. It was b e l i e v e d t h a t t h e m a r g i n o f e r r o r d i d n o t i n f l u e n c e t h e clinical m e a n i n g o f t h e results.
Treatment, Time, and Gender Effects T h e d i f f e r e n c e s b e t w e e n c e r t a i n variables studied, a n d g e n d e r a n d time, ar e s h o w n in Tab l e 2. N o t ev er y d e p e n d e n t v a r i a b l e c h a n g e d significantly b e t w e e n T2 a n d T3. I n c i s o r i r r e g u l a r i t y a n d c r o w d i n g i n c r e a s e d (in m a g n i t u d e ) , w h e r e a s intermolar width and arch length decreased. I n c i s o r i r r e g u l a r i t y a n d c r o w d i n g c h a n g e s are obviously r e l a t e d to time. N o d i f f e r e n c e b e t w e e n t r e a t m e n t g r o u p s was i n d i c a t e d . I n t e r m o l a r width also s h o w e d s i g n i f i c a n t d i f f e r e n c e s in association with t h e c o m b i n e d effects o f t r e a t m e n t a n d gender. The treated female group's intermolar w i d t h d e c r e a s e d significantly with time. A r c h
Postretention Mandibular D~cisor Stability
18 7
Table 2. PValues From Repeated Measures Analysis of Variance for Treatment, Time, and Gender Effects Variable
Gender
TMT
Time
Time and TMT
TMT and Gender
Arch length IMW ICW Overbite Overjet Irregularity Crowding
0.0236 0.0230 0.5920 0.8657 0.8125 0.8731 0.3334
0.0001" 0.1268 0.1700 0.0202 0.8840 0.5240 0.7122
0.0001" 0.0499 0.0272 0.0154 0.0379 0.0048* 0.0006*
0.0771 0.0785 0.7639 0.9479 0.5374 0.838l 0.7529
0.2702 0.0002* 0.1598 0.4651 0.6916 0.1705 0.5588
*Pvalues < 0.01. TMT, treatment; IMW,intermolar width; ICW,intercanine width.
length decreased significantly with the effects of both treatment and time (Table 2).
Correlations Between Postretention Measures of Incisor Stability (T3) and Other Posttreatment (T2) and Postretention (T3) Measures Pearson correlation coefficients were used to determine if there were simple pairwise correlations between postretention measures of mandibular incisor stability and other posttreatment and postretention measures. No clinically significant variables were identified to predict postretention incisor irregularity.
Discussion To determine the effect of serial premolar extraction treatment on long-term mandibular incisor stability, it was necessary to examine a group of patients treated with serial premolar extractions, but without subsequent orthodontic mechanotherapy. A sample of such patients is housed in the Burlington Growth Center. Although a minim u m postretention period of 5 years is short, it allows comparisons to be made with untreated normal controls, and with similar groups assessed in the literature. There was a significant decrease in arch length with time in both the treated and control groups (P = 0.0001); however, the treated group underwent a significantly greater decrease than the control group. This was the only statistically significant difference between the treated and control groups that could be attributed to treatment differences. It may be explained by physiological drift, which would have been greater in the treated group, because of the removal of four premolars. I n t e r m o l a r width decreased in the treated
female group, and this is reflected in the very significant association between intermolar width, and the c o m b i n e d effects of treatment and gender (P = 0.0002). In all other groups (treated males, control males, and control females), intermolar width did not change significantly. However, the change in intermolar width indicated a significant difference (P = 0.02) between males (treated and controls) and females (treated and controls). In their study of untreated normals, Sinclair and Little ~2 also showed that intermolar width did not change in males, whereas a decrease occurred in females between 13 and 20 years. In the present study, there were no statistically significant associations between the control and treated groups and intercanine width. Nor was there a statistically significant association between intercanine width and gender. The intercanine width, however, decreased in all groups with time (P = 0.03). The results of the present study show that with respect to time, intercanine width responds in the same way, regardless of whether the patient was treated with serial extractions or was an untreated control. These results are very similar to those of Sinclair and Little, 12 whose study showed intercanine width to be very stable in n o r m a l untreated males (there was no statistically significant difference between T 2 and T3), with only m i n o r decreases in females from 13 to 20 years ( P < 0.05). This is in contrast to the marked decrease in intercanine width that has typically been shown to occur in premolar extraction patients with orthodontic mechanotherapy. 4 Little et al 4 f o u n d that most patients treated with the extraction of four premolars u n d e r w e n t a postretention decrease in intercanine width of more than 2 m m ( P < 0.0001). In more than 60% of these patients, intercanine width had been expanded by more than 1 m m during
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treatment. It is possible that the increase in intercanine width caused by orthodontic treatm e n t m o v e d the canines into an unstable position, which led to postretention change. A similar result was f o u n d by Glenn et al, 1~who studied the long-term stability of nonextraetion orthodontic treatment patients. Sixty-eight p e r c e n t of the patients h a d an increase in intercanine width with treatment. A decrease of at least 1 m m ( P < 0.001) in intercanine width was shown to occur at postretention. McReynolds and Little 27 studied long-term stability in patients treated with either late second p r e m o l a r extractions or serial extraction of second premolars, followed by a period of orthodontic mechanotherapy. T h e posttreatment increase in intercanine width was significant in both groups (P < 0.05). The postretention decrease was at least 1.5 man (P < 0.05). From McReynolds and Little's study, 27 it can be seen that the intercanine width r e s p o n d e d in the same way, regardless of whether second p r e m o lars were extracted early (as in the serial extraction patients) or late. W h e n the results of the study by McReynolds and Little 27 are c o m p a r e d with the present study, it seems that the postretention change in intercanine width is not dependent on serial p r e m o l a r extractions, but on the type of orthodontic m e c h a n o t h e r a p y provided. It is possible that the increase in intercanine width that occurs with orthodontic m e c h a n o therapy p o s t t r e a t m e n t 4,13,27 led to a significant postretention decrease in intercanine width. Overbite and overjet showed a tendency to decrease with time, and though this change was not significant, it was observed in b o t h the serial extraction treated and n o r m a l untreated control groups. These results are similar to those of Sinclair and Little, 12 who f o u n d a m o d e s t decrease in overbite and overjet f r o m 13 to 20 years in untreated normals. T h e increase in incisor irregularity with time in the present study was statistically significant ( P = 0.005), but there was no difference between the treated (serial extraction) and control (untreated) groups. Little et al 4 f o u n d long-term changes in incisor irregularity in late p r e m o l a r extraction patients with orthodontic t r e a t m e n t were three to four times greater than the change observed hy Sinclair and Little a2 in untreated normals. This is in contrast to the present study, where the serial extraction g r o u p r e s p o n d e d in the same m a n n e r as the control group. Little et
aP 7 and McReynolds and Little 27 f o u n d that postretention incisor irregularity in serial extraction cases with orthodontic treatment, did not differ f r o m late p r e m o l a r extraction cases followed by orthodontic treatment. T h e difference between the present study and the studies by Little et aP 7 and McReynolds and Little 97was that the serial extraction cases in the present study did n o t receive any o r t h o d o n t i c m e c h a n o therapy. T h e orthodontic m e c h a n o t h e r a p y used in the studies by Little et al, 4,17,27 may have placed the teeth in an unstable position with respect to the neuromusculature, leading to postretention relapse. It appears that without mechanical intervention, the long-term irregularity of the mandibular incisors may be less severe. Irrespective of these observations, lower incisor alignment changes continue well into adulthood, u T h e increases in m a n d i b u l a r incisor crowding with time in the present study are very similar to those of Persson et al, 29 who showed that the long-term crowding of the entire arch in a group of serial p r e m o l a r extraction patients did not differ significantly f r o m untreated normals. Attempts at finding clinically useful associations between measures of incisor stability and other dental p a r a m e t e r s proved unsuccessful. T h e results of this study showed that mandibular incisor stability in serial p r e m o l a r extraction patients, without subsequent orthodontic mechanotherapy, is similar to that in untreated n o r m a l (nonextraction) cases. Although the reason for this similarity in incisor stability is unknown, it is t h o u g h t to be related to the neuromusculature. Graber 2° stressed the i m p o r t a n c e of the dynamic balance between the three tissue systems, teeth, neuromusculature, and bones, which provide the physiological basis of a stable tooth position. T h e results of this study suggest that serial p r e m o l a r extraction t r e a t m e n t is not a panacea. In almost all serial extraction patients, a period o f orthodontic m e c h a n o t h e r a p y is required to achieve a satisfactory occlusion. T h e reason for the increased lack of stability in serial p r e m o l a r extraction cases with subsequent orthodontic m e c h a n o t h e r a p y 17,27 is not known, but is related to some aspect of treatment. T h e results of the present study a p p e a r to show that the effect of the serial extraction treatment, by itself, cannot account for the discrepancy. It is possible that the orthodontic m e c h a n o t h e r a p y used in these studies 17,27 and the resultant increase in intercanine
Postretention Mandibular Incisor Stability
width, left the teeth in an inherently unstable p o s i t i o n w i t h r e s p e c t to t h e s u r r o u n d i n g
neuro-
musculature, leading to postretention relapse. To treat patients so that long-term stability can be improved, it may be important to gain a better understanding of the surrounding neuromusculature and its effects on the other tissue systems. Clinically, the importance of developmental incisor irregularity cannot be underestimated. Patients and their parents must be informed, at the onset of treatment, of the expected long-term changes in tooth position. Patients may find this p h e n o m e n a easier to accept if it is explained that this "wrinkling of the teeth" occurs naturally with age and is analogous to wrinkling of the skin. Although Little 32 stated that deterioration after age 30 years is less significant, it was shown that incisor irregularity changes rapidly to approximately 40 years of age when the rate of change tends to decrease, n Retention may be a lifelong process for which the patient must assume responsibility. Alternatively, a second orthodontic treatment at a later age may be an option.
Conclusions 1. There were no clinically significant differences in the variables studied between serial premolar extraction patients, but without active orthodontic treatment, and untreated normal cases at postretention. 2. There were no clinically or statistically significant differences in the amount of mandibular incisor irregularity or crowding between serial extraction cases and untreated normal cases at postretention. 3. There were no clinically significant differences in the variables studied between males and females. 4. There were no clinically significant correlations between any variables studied, at posttreatment or postretention, and between mandibular incisor irregularity or crowding. N o predictors of clinical value were found. 5. Mandibular incisor irregularity and crowding at postretention were independent of serial premolar extraction treatment, and as noted previously occurred to the same degree in a group of untreated normal patients. 6. Orthodontic mechanotherapy appears to affect the incisor irregularity.
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Acknowledgment The authors thank DI: G. Altuna, Dr. P. Sectakof, Dr. K. Tidey, Mrs. G. Jorgensen and Mrs. J. Hofmeister for their assistance during the prqject.
References 1. Case CS. The question of extraction in orthodontics. Am J Orthod 1964;50:658-691. 2. Cole HJ. Certain results of extraction in the treatment of malocclusion. Angle Orthod 1948;18:103-113. 3. Tweed CH. Interview with Dr D.S. Brandt.J Pract Orthod 1968;2:11-19. 4. Little RM, Wallen T, Riedel RA. Stability and relapse of mandibular anterior alignment-First premolar extraction cases treated by traditional edgewise orthodontics. AmJ Orthod 1981;80:349-365. 5. Sheilds TE, Little RN, Chapko MI~ Stability and relapse of mandibular anterior alignment: A cephalometric appraisal of first premolar extraction cases treated by traditional edgewise orthodontics. AmJ Orthod 1985;87: 27-38. 6. Shapiro E Mandibular dental arch form and dimension. AmJ Orthod 1974;66:58-70. 7. Swanson WD, Riedel RA, D'Anna JA. Postretention study: Incidence and stability of rotated teeth in humans. Angle Orthod 1975;45:198-203. 8. Riedel RA. Dr. Richard A. Riedel on retention and relapse. Interview.J Clinic Orthod 1976;10:454-472. 9. Little RM. The Irregularity Index: A quantitative score of mandibular anterior alignment. Am J Orthod 1975;68: 554-563. 10. Little RM, Riedel RA, ArtunJ. An evaluation of changes in mandibular anterior alignment from 10 to 20 years postretention. Am J Orthod Dentofac Orthop 1988;93: 423-428. 11. Eslambolchi S. A serial study of mandibular incisor alignment from age 20 to 70 years. Diploma thesis, University of Toronto, Toronto, Canada, 1994. 12. Sinclair PM, Little RM. Maturation of untreated normal occlusions. AmJ Orthod 1983;83:114-156. 13. Glenn G, Sinclair PM, Alexander RG. Nonextraction orthodontic therapy: Posttreatment dental and skeletal stability. AmJ Orthod Dentofac Orthop 1987;92:321-328. 14. Sadowsky C, Sakols El. Long-term assessment of orthodontic relapse. AmJ Orthod 1982;82:456-463. 15. Franklin S. A longitudinal study of dental and skeletal parameters associated with stability of orthodontic treatmerit. Diploma thesis, University of Toronto, 1995. 16. Uhde MD, Sadowsky C, BeGole EA. Long-term stability of dental relationships after orthodontic treatment. Angle Orthod 1982;53:240-252. 17. Little RM, Riedel RA, Engst ED. Serial extraction of first premolars-Postretention evaluation of stability and relapse. Angle Orthod 1990;60:255-262. 18. Hotz RP. Active supervision of the eruption of the teeth by extraction. Trans Eur Orthod Soc 1947;48:34-47. 19. Kjellgren B. Serial extraction as a corrective procedure in dental orthopedic therapy. Trans Eur Orthod Soc 1947-48;134-160.
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20. Graber TM. Serial Extraction. AmJ Orthod 1971;60:541575. 21. Dewel BE A critical analysis of serial extraction in orthodontic treatment. AmJ Orthod 1959;45:424-455. 22. Tweed CH. Treatment planning and therapy in the mixed- dentition. AmJ Orthod 1963;49:881-906. 23. Norman E Serial extraction. Angle Orthod 1965;35:149157. 24. Mayne WR. Serial extraction, in Graber TM, Swain BF (eds): Current Orthodontic Concepts and Techniques. St Louis, MO, Mosby, 1975:259-364. 25. Dale JG. Guidance of occlusion: Serial extraction, in Graber TM, Swain BF (eds): Orthodontics: Current Principles and Techniques. St Louis, MO, Mosby, 1985: 260-369. 26. Tweed CH. Indications for the extraction of teeth in orthodontic procedure. AmJ Orthod Oral Surg 1944;30: 405-428.
27. McReynolds DC, Little RM. Mandibular second premolar extraction-Postretention evaluation of stability and relapse. Angle Orthod 1991;61:133-144. 28. Sinclair PM, Little RM. Dentofacial maturation of untreated normals. AmJ Orthod 1985;88:146-156. 29. Persson M, Persson E, Skagies S. Long-term spontaneous changes following removal of all first premolars in Class I cases with crowding. EurJ Orthod 1989;11:271-282. 30. Burlington Progress Report No. 1, Burlington Growth Center, University of Toronto, 1956. 31. Shearer D. Evaluation of postretention mandibular incisor stability in premolar extraction cases treated by serial extraction without subsequent fixed orthodontic mechanotherapy. Diploma thesis, University of Toronto, Toronto, Canada, 1994. 32. Little RM. Stability and relapse of dental arch alignment. BrJ Orthod 1990;17:235-241.
he controversy regarding the extraction of p e r m a n e n t teeth to create space in the dental arch has b e e n a heated topic for the better part of the century. 1-3 N u m e r o u s factors have b e e n suggested in an a t t e m p t to explain postretention crowding. However, no variables have yet b e e n f o u n d as predictors of such crowding? It is c o m m o n l y believed that the extraction of the four first premolars ensures stability of incisor alignment. 5 It was, therefore, disheartening when Little et al 4 dispelled this belief by showing that 70% of all patients with premolars extracted as part of the orthodontic t r e a t m e n t had a degree o f p o s t r e t e n t i o n crowding that was considered unacceptable. Such postretention incisor crowding can be somewhat distressing to the u n i n f o r m e d patient. It is, therefore, i m p o r t a n t to continue to look
T
From the Discipline of Orthodontics, Faculty of Dentistry, University of Toronto, Toronto, Canada. This study was made possible by the use of material from the Burlington Research Centre, Faculty of Dentistry, University of Toronto, which was supported byfunds provided by National Health Grant (Canada) No. 6057-299. Address co~veapondence to P. Emile Rossouw, BSc, BChD, BChD (Hons), MChD, PhD, Faculty of Dentistry, Unive*:~ity of Toronto, 124 Edward St, Toronto, Ontario, M5G 1 G6 Canada. Copyright © 1999 by W.B. Saunders Company 1073-8746/99/0503-0007510. 00/00
for factors or combinations of factors that might provide insight into the cause of late developmental crowding a n d thereby possibly facilitate the establishment of some f o r m of biologic control. As an alternative, lifelong retention or a second orthodontic treatment later in life may b e c o m e necessary.
Premolar Extraction and Postretention Stability In a study of 80 orthodontic patients (22 nonextraction a n d 58 extraction cases) in which either the first or second premolars were extracted, a net arch length decrease was observed during the postretention phase. This decrease in arch length continued during a m i n i m u m 10-year postretention period, and it was observed regardless of whether the patient had b e e n treated with or without extractions. 6 Swanson et al v also investigated the stability of rotated teeth in individuals who had b e e n treated either by nonextraction therapy or by extraction of first bicuspids in one or b o t h arches, followed by comprehensive orthodontic treatment. All patients had b e e n out of retention for a minim u m of 10 years. Canines were observed to be the most rotated teeth in either arch, both before t r e a t m e n t and at the postretention examination. These results suggest that certain teeth
Seminars in Orthodontics, Vol 5, No 3 (September), 1999." pp 181-190
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are m o r e p r o n e to rotational relapse than others. No associations could be: f o u n d between rotational relapse of the canines a n d p r e m o l a r extraction. This study showed that factors, such as sex, age at the end of treatment, Angle classification, arch length changes, and growth of the jaws, were not associated with rotational relapse. However, the a m o u n t of rotational relapse experienced during the postretention phase was directly p r o p o r t i o n a l to the a m o u n t of rotational correction achieved during treatment. In 1976, Riedel 8 m a d e the suggestion that the removal of m a n d i b u l a r first or second premolars is " n o t always, and is perhaps not usually related to relieving m a n d i b u l a r anterior incisor crowding in the long-term." It was his contention that a shift toward a nonextraction a p p r o a c h to orthodontic therapy largely resulted f r o m the knowledge that m a n d i b u l a r first p r e m o l a r extraction does not necessarily result in the long-term stability of the m a n d i b u l a r anterior incisor segment. Little et al 4 evaluated mandibular incisor alignm e n t in 65 patients treated by conventional edgewise orthodontic m e c h a n o t h e r a p y subsequent to the extraction of all 4 first premolars a m i n i m u m of 10 years postretention. Alignment was assessed by Little's irregularity index. 9 Before treatment, m o r e than 70% of the patients displayed m o d e r a t e or severe irregularity (23% moderate, and 54% severe). After treatment, 70% of the patients had m o d e r a t e or severe irregularity but in different proportions (56% moderate, and 16% severe). All but one of the patients who showed minimal irregularity before treatment, b e c a m e worse during the postretention phase. Those patients who had severe irregularity initially still showed a net improvement, despite the fact that the postretention irregularity was unacceptable. In general, they also f o u n d that m a n d i b u l a r intercanine width t e n d e d to decrease during the postretention phase. These results also showed that the long-standing concept that m a i n t e n a n c e of intercanine width during t r e a t m e n t would confer stability of incisor alignment was untrue ,4 Thus, arch width changes, which are established during treatment, are a p o o r predictor of long-term stability. Arch length decreased during the postretention stage in all but two patients with only a weak correlation between the a m o u n t of postretention arch length reduction and the severity ofpostretention irregu-
larity. It was concluded that no single parameter, such as sex, age, Angle classifications, length of retention, or values derived f r o m the m o d e l analysis, served as a reliable predictor of future stability. Combinations of the a f o r e m e n t i o n e d p a r a m e t e r s also proved to be of little value in predicting a long-term result. Certain factors may have affected the results of Little et al. 4 T h e p r e t r e a t m e n t age range of the patients/vas large (7 years and 10 m o n t h s to 18 years and 2 months). T h e patients were separated on the basis of their Angle classification, whereas the differences in skeletal morphology between patients were not taken into consideration. A flaw that was pointed out by Little involved the use of m a n d i b u l a r removable retainers in some patients. T h e degree of patient compliance, and extent of retainer wear was known to be variable. Little et al 4 therefore stated, "interpretation of statistical results must be somewhat g u a r d e d . " A continuation of the Little et al 4 study aimed to clarify whether any clinically significant predictors and correlations existed a m o n g cephalometric and dental-cast data when considering longt e r m stability. The study comprised a 10-year postretention cephalometric appraisal of first p r e m o l a r extractions in 54 patients treated by traditional edgewise orthodontics. 5 G o o d mandibular anterior alignment, with an Irregularity I n d e x 9 of less than 3.0 m m , was displayed. As with the case in the 19814 study, the investigators n o t e d that " m a r k e d variation" was an apt description of the postretention sample. Again, they concluded that no specific cephalometric param e t e r or combination of p r e t r e a t m e n t and postt r e a t m e n t cephalometric values were able to predict m a n d i b u l a r incisor irregularity. Associations between cephalometric and dental-cast data were minimal and of limited clinical value. Although unexpected, an association between pret r e a t m e n t and posttreatment cephalometric records was f o u n d for those patients with anatomical characteristics usually representative of Class II, Division 2 patients, which suggests that the n e u r o m u s c u l a r characteristics of this group may be a source of overbite relapse. Little et aP ° e x t e n d e d their previous m o d e l to 10 and 20 years postretention. T h e sample consisted of 31 patients, treated with 4 first p r e m o l a r extractions and conventional edgewise orthodontics. The Little Irregularity I n d e x 9 showed that
Postretention Mandibular Incisor Stability
30% of the 10-year postretention patients (1981 considered clinically acceptable, and at 20 years postretention (1988 study),1° only 10% of the sample cases had clinically acceptable m a n d i b u l a r anterior alignment. It is i m p o r t a n t to note that this was only a m e a n 1 m m increase in irregularity between 10 and 20 years postretention. It was concluded f r o m the above, that mandibular incisor irregularity continues to increase well after active growth has ceased a n d is variable. These long-term changes have also b e e n r e p o r t e d in a sample of untreated n o r m a l individuals, u It was concluded that the only way to ensure acceptable alignment of the dentition was through lifelong retention. The p r e m o l a r extraction study by Little et al 4 was also c o m p a r e d with the incisor irregularity in untreated normals 12 and postretention nonextraction cases. 13 It is remarkable that the a m o u n t of postretention irregularity was m o r e than twice as great in p r e m o l a r extraction cases ( + 2.9 m m ) c o m p a r e d with treated nonextraction cases ( + 1.2 m m ) , and a b o u t four times as great as untreated normals (+0.7 m m ) . T h e extraction t r e a t m e n t seemed to do s o m e t h i n g which actually increased posttreatment irregularity when comp a r e d with untreated normals. Lower arch expansion occurred in the treated patients, a p a r a m e t e r which decreased over time a n d could have contributed to the long-term changes described. T h e long-term occlusal changes of 96 patients treated with edgewise m e c h a n o t h e r a p y f r o m 20 to 25 years previously, showed that, at the postretention follow-up, 72% of the patients h a d at least one p a r a m e t e r that was outside of the ideal range. TM Patients with Class III initial malocclusions were excluded f r o m the sample, 38% h a d extractions, whereas 62% were treated with nonextraction. Thirty-five percent of the pretreatm e n t m a n d i b u l a r incisor crowding was outside the ideal range, whereas, at the time of postretention follow-up, this value was 15%. This presents a m o r e positive picture ofpostretention mandibular incisor alignment. 14 Franklin a5 s u p p o r t e d this conclusion. However, the 3.0 m m of incisor crowding selected by Sadowsky and Sakols 14 may not be r e g a r d e d as n o r m a l by most clinicians. The separation of patients into groups based on their Angle classification without considering differences in skeletal m o r p h o l o g y also creates differences that could influence the o u t c o m e of study) 4 were
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the t r e a t m e n t results. Finally, fixed lingual retainers were used in most patients for varying lengths of time. The m e t h o d and duration of retaining the m a n d i b u l a r incisors in the r e m a i n i n g patients was not stated. U h d e et aP 6 selected a g r o u p (79 patients: 27 extraction, 45 nonextraction) f r o m the records used in the study by Sadowsky and Sakols. 14 No significant difference was f o u n d postretention in the m a n d i b u l a r incisor crowding between extraction and nonextraction treatments in b o t h the Class I and Class II groups. Multiple regression analysis i n d i c a t e d that o t h e r p o s t r e t e n t i o n changes evaluated (maxillary and m a n d i b u l a r intercanine width, maxillary a n d m a n d i b u l a r i n t e r m o l a r width, overjet, overbite, left and right m o l a r relationships, and maxillary incisor crowding) were related to 41% of the variability, and m a n d i b u l a r intercanine width accounted for 12.5% (more than any o t h e r p a r a m e t e r ) .
Postretention Stability in Serial Extraction Cases With Subsequent Treatment A F r e n c h m a n , Robert B u n o n (1743), is credited with the first attempt to guide the eruption of p e r m a n e n t teeth by the selective removal of deciduous a n d p e r m a n e n t teeth. 17 T h e goal 200 years ago was, as now, to p e r m i t the p e r m a n e n t dentition to e r u p t into an acceptable occlusion in deficient arches. T h e r e was little published on the subject after the middle of the last century until Hotz ("active supervision of dental eruption" or "guidance of dental eruption by means of extractions") TM and Kjellgren ("serial extraction") 10 published their respective articles. It has since b e e n assumed that if teeth e r u p t into positions of alignment, stability of such alignm e n t will be c o n f e r r e d in comparison to teeth m o v e d into alignment after eruption. In the early 1940s, it was recognized that dental arch expansion was not the p a n a c e a it was once t h o u g h t to be for the elimination of crowdingY ° Thus, serial extraction was t h o u g h t to bring the teeth and n e u r o m u s c u l a r and skeletal systems into balance. Dewel 2~ stated that teeth in m a r k e d discrepancy cases are not first required to assume positions of e x t r e m e irregularity and then subjected to extensive orthodontic movem e n t with extraction to establish acceptable occlusal relations. They are, instead, " p e r m i t t e d
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to take these positions in the first place." Tweed z2 also stressed the i m p o r t a n c e of working with "growth and its potentials" and the acquisition of "a better understanding of when and how to intervene in the guidance of growth processes so that nature may better a p p r o x i m a t e her growth plan," thereby eliminating the n e e d for "harsh mechanics" to be i m p l e m e n t e d at the time of adolescence. In Class II and Class III malocclusions, serial extractions should be a p p r o a c h e d with caution because the skeletal abnormality must be addressed and corrected as the p r i m a r y focus of the treatment. 21 It has b e e n stated that increased posttreatment stability of m a n d i b u l a r incisors can be achieved with serial extraction followed by comprehensive treatment, s,z°,23-25 Dale 25 also stated that i f a tooth were allowed to complete its formation in a position where it would remain once t r e a t m e n t was completed, it would be m o r e stable. Graber, 2° however, warned that serial extraction, originally advocated to reduce the severity of crowding in deficient arches, is " a n o r t h o d o n tic decision and requires the knowledge, knowhow, and clinical experience of the specialist who ultimately must complete the therapy in almost all cases." H e a d d e d that a poorly guided serial extraction p r o g r a m may be worse than p e r f o r m i n g any t r e a t m e n t at all. Little et a117 evaluated the case records of 30 patients who had u n d e r g o n e serial extraction of deciduous teeth and four first premolars, after which active standard edgewise m e c h a n o t h e r a p y was initiated when all p e r m a n e n t teeth erupted. These cases were followed by a period of retention of usually 2 years or more. All cases were assessed after a m i n i m u m postretention period of 10 years or more. Five variables were measured f r o m pretreatment, posttreatment, and postretention dental casts. These included the Little I r r e g u l a r i t y Index, 9 m a n d i b u l a r arch length, m a n d i b u l a r intercanine width, overbite, and overjet. T h e results of this study showed that, during the observation stage, m a n d i b u l a r incisor crowding usually improved. However, at the postretention stage, this alignment was no better than 30% successful. Surprisingly, this was very similar to that seen in the late first p r e m o l a r extraction group (Little et a14; satisfactory mandibular anterior a l i g n m e n t was less than 30% with almost 20% showing m a r k e d crowding). No
d o u b t the results of this s t u d y 17 w e r e disconcerting to many, because the future stability predicted by Graber, 2° Dale, 25 and Tweed, 26 was not confirmed. T h e consequences of m a n d i b u l a r second prem o l a r extraction were studied in 46 patients (congenitally missing m a n d i b u l a r second premolars or who had had t h e m extracted).27 Comprehensive edgewise o r t h o d o n t i c t r e a t m e n t followed a period of approximately 1 year of physiological drift. O r t h o d o n t i c t r e a t m e n t was initiated shortly after the removal of teeth and b o t h groups were analyzed a m i n i m u m of 10 years postretention. T h e r e was no statistically significant correlation in any dental cast or cephalometric m e a s u r e m e n t between the two groups. The p r e t r e a t m e n t irregularity indices of the groups were similar, and both groups displayed an increase in lower incisor crowding postretention, however, the difference between the two groups was not significant. No associations between incisor alignment and any other variable could be found. Although g e n d e r differences did not show any association with incisor aligmnent at any time interval for either group, p o o l e d males were f o u n d to have significantly m o r e crowding than females at the postretention stage. This contrasts with Sinclair and Little's 28 study in which they f o u n d that, in untreated normals, w o m e n had developed m o r e crowding than men. The authors were careful not to draw any conclusions f r o m these results but stated that they considered that growth may be a factor in the long-term stability of the dentition.
Serial Extraction and Follow-Up Stability T h e r e is limited literature available on the stability of serial extraction cases without subsequent orthodontic treatment. Little et a117 referred to the results of an u n p u b l i s h e d thesis in which 50 serial extraction cases which had not b e e n followed by subsequent orthodontic treatment were studied at 10 years follow-up. T h e results of this study showed greater long-term stability comp a r e d with patients with malocclusions, who were treated with first p r e m o l a r extraction in the p e r m a n e n t dentition and followed by comprehensive orthodontic treatment. Although interesting, Little et al n o t e d that the sample may have b e e n biased and tlae results should be viewed with a certain degree o f caution.
Postretention Mandibular Incisor Stability
Persson et a129 studied a group of 42 serial extraction patients who had their first premolars removed at a mean age of 10½ years as the sole treatment of the malocclusion. At an average age of 30 years posttreatment, a redevelopment of crowding had occurred, t h o u g h this was less than it had been originally. The serial extraction group was compared with a g r o u p of 29 untreated cases who had a " n o r m a l occlusion" at age 13 years. The degree of crowding in the normal group over the same time period was similar to that of the serial extraction group. No significant differences in axial inclination of the mandibular incisors as measured on cephalograms between the two groups were shown at follow-up. Correlations or indicators of future crowding were not sought. A linear assessment of crowding was used in this study instead of Little's Irregularity Index, thereby making direct comparisons with studies conducted by Riedel, Little, and others, difficult. Although the serial extraction group was c o m p a r e d with a control group, the two groups were not matched. Moreover, differences in skeletal m o r p h o l o g y were not taken into consideration in selecting the sample.
Purpose of the Present Study It is important to identify the specific factors which are related to mandibular incisor instability. Some clinicians believe that stability is related to treatment and mandibular incisor teeth should be moved to a p r e d e t e r m i n e d ideal. Others hold that mandibular incisors should not be moved from their original positions. There are very few patients in whom serial extractions were perf o r m e d and mandibular incisors were permitted to erupt in the arch without the influence of subsequent orthodontic treatment. The purpose of the present study was to evaluate mandibular incisor alignment in nonorthodontically treated serial extraction patients, using long-term dental cast records of patients from the Burlington Growth Centre. The aim was to compare patients treated with serial premolar extractions (followed by a period of physiological drift without subsequent orthodontic treatment other than minimal space maintenance therapy) with a m a t c h e d untreated control group (nonextraction) who received no such supervision. The results of this study were
185
c o m p a r e d with those obtained by studies conducted at the University of Washington.
Materials and Methods The experimental sample consisted of 22 patients (12 males and 10 females) from the records of the Burlington Growth Center who had u n d e r g o n e serial extractions of four premolars. Nineteen patients had four first premolars extracted, and three patients had their maxillary first and mandibular second premolars removed. Serial extraction treatment was used to correct a tooth size arch length discrepancy. 3° None of the patients in the experimental group received major orthodontic treatment involving tooth m o v e m e n t or extractions other than the serial extractions. However, a n u m b e r of the patients had received m i n o r orthodontic treatment, in addition to the serial extractions. Treatment methods that were suspected of influencing mandibular incisor alignment were accounted for in the statistical analysis. The patients were m a t c h e d with subjects from the untreated control group to form pairs. 31 Matching criteria included variables such as skeletal age, gender, face height ratio, ANB angle and incisor irregularity (Table 1). All controls were from the Burlington Growth Center and had not received any orthodontic treatment or extractions. Incisor irregularity could not be properly assessed before the initiation of serial extraction treatment (T1), therefore, no measurements were made at this time. Posttreatment (Tz) measurements were made at the time of eruption of the mandibular canines. Long-term (T3) measuremerits were made on the final records. Digital callipers (Mitutoyo Corporation, Tokyo, Japan) were used to measure the following at T2 and T3: irregularity index, mandibular incisor crowding, mandibular intercanine width, mandibular intermolar width, mandibular arch length, overbite, and overjet.
Analysis of Data Analysis of Treatment, Time, and Gender Effects To reduce interexaminer bias, all parameters were measured by one individual. The significance level was set at P < 0.05. A repeated
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Woo&ide, Rossouw, and Shearer
Table 1. Example of Data for Matching the Treatment (Serial Extractions) and Control (Untreated, Nonextraction) Groups: Skeletal Age and Irregularity Index Skeletal Age in Years
Irregularity Index (mm) T2
T3
T~
T3
Gender
No.
Treat
Contr
Treat
Contr
Treat
Contr
Treat
Contr
Female
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 11 12
2.30 3.69 6.18 4.24 1.56 2.01 2.64 4.94 4.34 0.37 2.27 0.45 3.76 2.45 0.59 1.58 2.80 4.21 3.94 4.16 5.20 1.28
2.10 2.15 6.21 4.05 1.84 3.75 2.46 6.47 3.39 1.86 3.69 0.00 4.81 1.71 0.62 1.42 3.01 5.33 1.89 4.90 3.29 2.22
3.36 7.58 3.95 2.72 1.70 2.26 6.96 5.22 8.91 2.01 1.94 3.09 2.86 3.62 7.25 3.99 5.80 5.46 7.41 3.60 10.27 0.44
2.32 4.00 6.97 7.12 1.34 5.04 4.24 7.28 4.89 3.05 3.34 0.47 7.52 1.32 1.07 1.47 4.59 6.49 3.32 4.38 5.33 2.03
10.0 11.0 12.0 11.0 11.0 10.0 10.0 14.0 10.0 10.0 13.0 11.0 11.0 11.0 11.0 13.0 11.0 12.0 10.0 12.0 11.0 12.0
11.0 12.0 11.0 10.0 11.0 10.0 13.0 13.0 10.0 12.0 13.0 12.0 13.0 12.0 13.0 11.0 12.0 12.0 12.0 12.0 12.0 10.0
16.0 17.0 17.0 19.0 19.0 17.0 16.0 20.0 20.5 18.0 18.0 18.0 18.0 16.0 17.0 20.0 16.0 20.0 18.0 20.0 18.0 18.0
17.0 18.0 16.0 19.0 19.0 18.0 19.0 18.0 20.0 20.0 18.0 19.0 19.0 17.0 19.0 18.0 17.0 19.0 20.0 20.0 19.0 16.0
Male
m e a s u r e s analysis o f v a r i a n c e was u s e d to perf o r m a c o m b i n e d analysis o f t r e a t m e n t , time, a n d g e n d e r effects.
Error Analysis M e a s u r e m e n t e r r o r was e v a l u a t e d by r a n d o m s e l e c t i o n o f 10 p a t i e n t s a n d r e m e a s u r e m e n t o n a s e p a r a t e o c c a s i o n . Systematic e r r o r was m e a s u r e d with a S t u d e n t t test. E r r o r was r e p r e s e n t e d e i t h e r as a m i l l i m e t e r m e a s u r e m e n t o r as a c o e f f i c i e n t o f variation.
Analysis of the Effects o f Nonideal Treatments or Measurements At T2, several factors a s s o c i a t e d with t h e p a t i e n t s w e r e i d e n t i f i e d , w h i c h m i g h t have significantly i n f l u e n c e d t h e results. T h e s e i n c l u d e d differe n c e s in skeletal m o r p h o l o g y o r t r e a t m e n t as follows: an a b n o r m a l o v e r b i t e in c o m b i n a t i o n with an a b n o r m a l f a c e / h e i g h t ratio, an a b n o r m a l A N B angle, m o n o b l o c t r e a t m e n t , a n d t h e e x t r a c t i o n o f m a n d i b u l a r s e c o n d p r e m o l a r s ins t ead o f t h e m a n d i b u l a r first p r e m o l a r s . T h e d a t a w e r e a n a l y z e d with a n d w i t h o u t the inclusion o f e a c h o f t h e s e g r o u p s o f individuals. I f t h e i r i n c l u s i o n d id n o t r e s u l t in a si g n i f i can t
d i f f e r e n c e , they w e r e i n c l u d e d in all f u r t h e r analyses.
Results Error Analysis T h e e r r o r associated with this study was small. It was b e l i e v e d t h a t t h e m a r g i n o f e r r o r d i d n o t i n f l u e n c e t h e clinical m e a n i n g o f t h e results.
Treatment, Time, and Gender Effects T h e d i f f e r e n c e s b e t w e e n c e r t a i n variables studied, a n d g e n d e r a n d time, ar e s h o w n in Tab l e 2. N o t ev er y d e p e n d e n t v a r i a b l e c h a n g e d significantly b e t w e e n T2 a n d T3. I n c i s o r i r r e g u l a r i t y a n d c r o w d i n g i n c r e a s e d (in m a g n i t u d e ) , w h e r e a s intermolar width and arch length decreased. I n c i s o r i r r e g u l a r i t y a n d c r o w d i n g c h a n g e s are obviously r e l a t e d to time. N o d i f f e r e n c e b e t w e e n t r e a t m e n t g r o u p s was i n d i c a t e d . I n t e r m o l a r width also s h o w e d s i g n i f i c a n t d i f f e r e n c e s in association with t h e c o m b i n e d effects o f t r e a t m e n t a n d gender. The treated female group's intermolar w i d t h d e c r e a s e d significantly with time. A r c h
Postretention Mandibular D~cisor Stability
18 7
Table 2. PValues From Repeated Measures Analysis of Variance for Treatment, Time, and Gender Effects Variable
Gender
TMT
Time
Time and TMT
TMT and Gender
Arch length IMW ICW Overbite Overjet Irregularity Crowding
0.0236 0.0230 0.5920 0.8657 0.8125 0.8731 0.3334
0.0001" 0.1268 0.1700 0.0202 0.8840 0.5240 0.7122
0.0001" 0.0499 0.0272 0.0154 0.0379 0.0048* 0.0006*
0.0771 0.0785 0.7639 0.9479 0.5374 0.838l 0.7529
0.2702 0.0002* 0.1598 0.4651 0.6916 0.1705 0.5588
*Pvalues < 0.01. TMT, treatment; IMW,intermolar width; ICW,intercanine width.
length decreased significantly with the effects of both treatment and time (Table 2).
Correlations Between Postretention Measures of Incisor Stability (T3) and Other Posttreatment (T2) and Postretention (T3) Measures Pearson correlation coefficients were used to determine if there were simple pairwise correlations between postretention measures of mandibular incisor stability and other posttreatment and postretention measures. No clinically significant variables were identified to predict postretention incisor irregularity.
Discussion To determine the effect of serial premolar extraction treatment on long-term mandibular incisor stability, it was necessary to examine a group of patients treated with serial premolar extractions, but without subsequent orthodontic mechanotherapy. A sample of such patients is housed in the Burlington Growth Center. Although a minim u m postretention period of 5 years is short, it allows comparisons to be made with untreated normal controls, and with similar groups assessed in the literature. There was a significant decrease in arch length with time in both the treated and control groups (P = 0.0001); however, the treated group underwent a significantly greater decrease than the control group. This was the only statistically significant difference between the treated and control groups that could be attributed to treatment differences. It may be explained by physiological drift, which would have been greater in the treated group, because of the removal of four premolars. I n t e r m o l a r width decreased in the treated
female group, and this is reflected in the very significant association between intermolar width, and the c o m b i n e d effects of treatment and gender (P = 0.0002). In all other groups (treated males, control males, and control females), intermolar width did not change significantly. However, the change in intermolar width indicated a significant difference (P = 0.02) between males (treated and controls) and females (treated and controls). In their study of untreated normals, Sinclair and Little ~2 also showed that intermolar width did not change in males, whereas a decrease occurred in females between 13 and 20 years. In the present study, there were no statistically significant associations between the control and treated groups and intercanine width. Nor was there a statistically significant association between intercanine width and gender. The intercanine width, however, decreased in all groups with time (P = 0.03). The results of the present study show that with respect to time, intercanine width responds in the same way, regardless of whether the patient was treated with serial extractions or was an untreated control. These results are very similar to those of Sinclair and Little, 12 whose study showed intercanine width to be very stable in n o r m a l untreated males (there was no statistically significant difference between T 2 and T3), with only m i n o r decreases in females from 13 to 20 years ( P < 0.05). This is in contrast to the marked decrease in intercanine width that has typically been shown to occur in premolar extraction patients with orthodontic mechanotherapy. 4 Little et al 4 f o u n d that most patients treated with the extraction of four premolars u n d e r w e n t a postretention decrease in intercanine width of more than 2 m m ( P < 0.0001). In more than 60% of these patients, intercanine width had been expanded by more than 1 m m during
188
Woodside, Rossouw, and Shearer
treatment. It is possible that the increase in intercanine width caused by orthodontic treatm e n t m o v e d the canines into an unstable position, which led to postretention change. A similar result was f o u n d by Glenn et al, 1~who studied the long-term stability of nonextraetion orthodontic treatment patients. Sixty-eight p e r c e n t of the patients h a d an increase in intercanine width with treatment. A decrease of at least 1 m m ( P < 0.001) in intercanine width was shown to occur at postretention. McReynolds and Little 27 studied long-term stability in patients treated with either late second p r e m o l a r extractions or serial extraction of second premolars, followed by a period of orthodontic mechanotherapy. T h e posttreatment increase in intercanine width was significant in both groups (P < 0.05). The postretention decrease was at least 1.5 man (P < 0.05). From McReynolds and Little's study, 27 it can be seen that the intercanine width r e s p o n d e d in the same way, regardless of whether second p r e m o lars were extracted early (as in the serial extraction patients) or late. W h e n the results of the study by McReynolds and Little 27 are c o m p a r e d with the present study, it seems that the postretention change in intercanine width is not dependent on serial p r e m o l a r extractions, but on the type of orthodontic m e c h a n o t h e r a p y provided. It is possible that the increase in intercanine width that occurs with orthodontic m e c h a n o therapy p o s t t r e a t m e n t 4,13,27 led to a significant postretention decrease in intercanine width. Overbite and overjet showed a tendency to decrease with time, and though this change was not significant, it was observed in b o t h the serial extraction treated and n o r m a l untreated control groups. These results are similar to those of Sinclair and Little, 12 who f o u n d a m o d e s t decrease in overbite and overjet f r o m 13 to 20 years in untreated normals. T h e increase in incisor irregularity with time in the present study was statistically significant ( P = 0.005), but there was no difference between the treated (serial extraction) and control (untreated) groups. Little et al 4 f o u n d long-term changes in incisor irregularity in late p r e m o l a r extraction patients with orthodontic t r e a t m e n t were three to four times greater than the change observed hy Sinclair and Little a2 in untreated normals. This is in contrast to the present study, where the serial extraction g r o u p r e s p o n d e d in the same m a n n e r as the control group. Little et
aP 7 and McReynolds and Little 27 f o u n d that postretention incisor irregularity in serial extraction cases with orthodontic treatment, did not differ f r o m late p r e m o l a r extraction cases followed by orthodontic treatment. T h e difference between the present study and the studies by Little et aP 7 and McReynolds and Little 97was that the serial extraction cases in the present study did n o t receive any o r t h o d o n t i c m e c h a n o therapy. T h e orthodontic m e c h a n o t h e r a p y used in the studies by Little et al, 4,17,27 may have placed the teeth in an unstable position with respect to the neuromusculature, leading to postretention relapse. It appears that without mechanical intervention, the long-term irregularity of the mandibular incisors may be less severe. Irrespective of these observations, lower incisor alignment changes continue well into adulthood, u T h e increases in m a n d i b u l a r incisor crowding with time in the present study are very similar to those of Persson et al, 29 who showed that the long-term crowding of the entire arch in a group of serial p r e m o l a r extraction patients did not differ significantly f r o m untreated normals. Attempts at finding clinically useful associations between measures of incisor stability and other dental p a r a m e t e r s proved unsuccessful. T h e results of this study showed that mandibular incisor stability in serial p r e m o l a r extraction patients, without subsequent orthodontic mechanotherapy, is similar to that in untreated n o r m a l (nonextraction) cases. Although the reason for this similarity in incisor stability is unknown, it is t h o u g h t to be related to the neuromusculature. Graber 2° stressed the i m p o r t a n c e of the dynamic balance between the three tissue systems, teeth, neuromusculature, and bones, which provide the physiological basis of a stable tooth position. T h e results of this study suggest that serial p r e m o l a r extraction t r e a t m e n t is not a panacea. In almost all serial extraction patients, a period o f orthodontic m e c h a n o t h e r a p y is required to achieve a satisfactory occlusion. T h e reason for the increased lack of stability in serial p r e m o l a r extraction cases with subsequent orthodontic m e c h a n o t h e r a p y 17,27 is not known, but is related to some aspect of treatment. T h e results of the present study a p p e a r to show that the effect of the serial extraction treatment, by itself, cannot account for the discrepancy. It is possible that the orthodontic m e c h a n o t h e r a p y used in these studies 17,27 and the resultant increase in intercanine
Postretention Mandibular Incisor Stability
width, left the teeth in an inherently unstable p o s i t i o n w i t h r e s p e c t to t h e s u r r o u n d i n g
neuro-
musculature, leading to postretention relapse. To treat patients so that long-term stability can be improved, it may be important to gain a better understanding of the surrounding neuromusculature and its effects on the other tissue systems. Clinically, the importance of developmental incisor irregularity cannot be underestimated. Patients and their parents must be informed, at the onset of treatment, of the expected long-term changes in tooth position. Patients may find this p h e n o m e n a easier to accept if it is explained that this "wrinkling of the teeth" occurs naturally with age and is analogous to wrinkling of the skin. Although Little 32 stated that deterioration after age 30 years is less significant, it was shown that incisor irregularity changes rapidly to approximately 40 years of age when the rate of change tends to decrease, n Retention may be a lifelong process for which the patient must assume responsibility. Alternatively, a second orthodontic treatment at a later age may be an option.
Conclusions 1. There were no clinically significant differences in the variables studied between serial premolar extraction patients, but without active orthodontic treatment, and untreated normal cases at postretention. 2. There were no clinically or statistically significant differences in the amount of mandibular incisor irregularity or crowding between serial extraction cases and untreated normal cases at postretention. 3. There were no clinically significant differences in the variables studied between males and females. 4. There were no clinically significant correlations between any variables studied, at posttreatment or postretention, and between mandibular incisor irregularity or crowding. N o predictors of clinical value were found. 5. Mandibular incisor irregularity and crowding at postretention were independent of serial premolar extraction treatment, and as noted previously occurred to the same degree in a group of untreated normal patients. 6. Orthodontic mechanotherapy appears to affect the incisor irregularity.
189
Acknowledgment The authors thank DI: G. Altuna, Dr. P. Sectakof, Dr. K. Tidey, Mrs. G. Jorgensen and Mrs. J. Hofmeister for their assistance during the prqject.
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27. McReynolds DC, Little RM. Mandibular second premolar extraction-Postretention evaluation of stability and relapse. Angle Orthod 1991;61:133-144. 28. Sinclair PM, Little RM. Dentofacial maturation of untreated normals. AmJ Orthod 1985;88:146-156. 29. Persson M, Persson E, Skagies S. Long-term spontaneous changes following removal of all first premolars in Class I cases with crowding. EurJ Orthod 1989;11:271-282. 30. Burlington Progress Report No. 1, Burlington Growth Center, University of Toronto, 1956. 31. Shearer D. Evaluation of postretention mandibular incisor stability in premolar extraction cases treated by serial extraction without subsequent fixed orthodontic mechanotherapy. Diploma thesis, University of Toronto, Toronto, Canada, 1994. 32. Little RM. Stability and relapse of dental arch alignment. BrJ Orthod 1990;17:235-241.