Differential premolar extractions

Differential premolar extractions C. L. Steyn, BChD, MChD, a R. J. du Preez, BSc(l=ng), MS, Dr.(Eng), b and A. M. P. Harris, Hons.BSc, BChD, Dip.Ter.Educ., MChD, FFD(SA)Orth c

Tygerberg, South Africa The primary object of this research project was to establish the amount of incisor retraction to be expected during full fixed mechanotherapy and to generate regression models that could provide more accurate prediction of each of the three most popular combinations of symmetrical premolar extractions, where extraoral anchorage would not be used. Pretreatment and posttreatment records of 73 cases that had their four first premolars extracted (group 44), 74 cases with four second premolar extractions (group 55), and 59 cases with upper first and lower second premolar extractions (group 45) were selected. All these cases were treated by one orthodontist, who used the same edgewise technique throughout. The results indicate that, on average, maxillary retraction in relation to the facial plane (N Po) differed only slightly between group 55 (mean 4.2 + 2.4 mm) and group 44 (mean 4.7 + 2.3 mm), with relatively more retraction for group 45 (mean 6.6 _+ 2.5 mm; p < 0.05). In contrast, the mandibular incisors were retracted slightly more in group 44 than in the other two groups (/9 < 0.05). The regression models that were developed could be useful as an additional tool to assist the practitioner in the selection of which teeth to extract for a particular case. (Am J Orthod Dentofac Orthop 1997;112:480-6.)

T h e posttreatment anteroposterior position of the incisors determines the drape of the lips and is therefore very often at the heart of orthodontic treatment planning. The anticipated positioning of anterior teeth has been made more accurate since the computer has been used during research, enhancing our knowledge and understanding of growth and the mechanics of tooth movement. At the same time, it has also resulted in the visual treatment objective (VTO) becoming more sophisticated and precise. In this regard, we are indebted to researchers like Tweed, 1 Steiner, 2 Begg, 3 Holda w a y , 4 and Ricketts. 5 Tweed 1 introduced his Diagnostic Triangle, which is used as an instrument in deciding on the new positions and angulations of the anterior teeth. In the past, many clinicians have used the Steiner 2 chevrons to aid them in positioning the incisors. Begg 3 and Ricketts 5 relate the incisor position to the point A-Pogonion line and Holdaway 4 emphasizes the position of the upper incisors. For most clinicians, the treatment plan revolves around the evenaParttime lecturer, Department of Orthodontics, Faculty of Dentistry, University of Stellenbosch; orthodontist in private practice. bprofessor and Head of Center for Computational Mechanics, University of Pretoria. CProfessor and Head of Department of Orthodontics, Faculty of Dentistry, University of Stellenbosch. Reprint requests to: Prof. A.M.P. Harris, Department of Orthodontics, Faculty of Dentistry, University of Stellenbosch, Private Bag X1, P.O. Tygerberg 7505, Republic of South Africa. E-mail: [email protected] Copyright © 1997 by the American Association of Orthodontists. 0889-5406/97/$5.00 + 0 8/1/75350

480

tual position of the anterior teeth in the face and the establishment of an excellent occlusion. It is widely accepted that there is a strong relation between root surface area and anchorage and that the choice of teeth extracted therefore has a direct effect on the amount of anterior segment retraction. This has led to the concept of differential extractions. However, there is very little information available to make possible an accurate prediction of the anterior segment retraction that can be expected to occur during the closure of premolar extraction spaces. Various authors have reported on the effect of premolar extractions on the positions of incisors. By using the NA line as reference, Luppanapornlarp and Johnston 6 compared the long-term outcome of four first premolar extraction versus nonextraction treatments. They showed that the extraction cases had a 2.8 mm retraction, with an increase in the interincisal angle of 3.8 °. Using the APo line as reference, Bishara et al. 7 compared the results obtained between a nonextraction group and a four first premolar extraction group. Where extractions were performed, the retraction of the upper incisors was 4.6 __ 2.4 mm for male patients and 4.6 _+ 1.9 mm for female patients. After upper and lower first premolars were extracted, Schwab 8 found that the upper incisors were retracted 2.6 mm, with Sella being used as reference. After upper and lower second premolars were extracted, the upper incisors were retracted by 1.6 ram. In his study comparing the results of Begg versus edgewise-treated cases,

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Carter 9 found that upper incisal edge retraction in relation to the NPo line was 8.2 +- 2.31 mm for the edgewise treated cases, as compared with 8.27 +_ 2.1 mm for the Begg treated cases. Concurrently, the interincisal angle was increased with 22.27 ° +_ 8.91 ° for edgewise and 19.58 ° +- 8.14 ° for Begg treatments. This research supports our belief that there is very little difference in the anchorage available from the edgewise technique, as compared to the Begg technique. The contour of the palate was used as reference by Cusimano et al. 1° They found that the upper incisors retracted 1.9 +- 2.4 mm when four first premolars were extracted. Nel, 11 like Bishara et al., 7 used the APo line as reference. He found that when upper first premolars and lower second premolars were extracted, the upper incisors were retracted by 2.9 mm. Williams and Hosila, ~2 also using the palatal anatomy as reference, researched the effect of different combinations of extraction and nonextraction treatment on incisor retraction. By using the Begg technique, they reported that four first premolar extractions resulted in a combined upper and lower incisor retraction of 10.3 +- 4.5 ram. Extraction of upper first and lower second premolars resulted in a retraction of 9.3 +- 3.7 mm. They also mentioned the possibility that an altered axial inclination could affect the anchorage. The primary object of this research was to establish the amount of incisor retraction to be expected from the three most popular combinations of premolar extractions, namely, four first premolar extractions, four second premolar extractions, and two upper first premolars, along with two lower second premolars extractions.

METHODS AND MATERIALS Two hundred and six actively growing white girls and boys (114 girls and 92 boys) from a private practice were selected according to the following criteria: 1. All the cases were treated by one operator, who used a similar edgewise technique throughout. 2. Extraoral anchorage was not used in any of these cases, and second molars were never included during space closure. 3. A Class I buccal occlusion, along with a normal overbite and normal overjet, was obtained with intraoral Class II elastic traction, when required. 4. The original mandibular intercanine and mandibular intermolar widths were maintained as much as possible. 5. Before and after treatment cephalograms were taken with the same cephalostat (Fiad Rotograph). 6. All cephalograms were directly digitized by one person. 7. Only cases that had a total space shortage, anterior of the first molars, of no more than 12 mm, were included. Such a restriction meant that there was at

Steyn, du Preez, and Harris 481

2

Fig. 1. Some cephalometric parameters used in study. (1) Distance of maxillary central incisor incisal edge perpendicularly projected on SN, to Nasion. (2) Distance of mandibular central incisor incisal edge perpendicularly projected on SN, to Nasion. (3) Facial axis angle. (4) Upper incisor to NA angle. (5) Interincisal angle. (6) Lower incisor to mandibular plan angle (IMPA). (7) Angle between upper central incisor and NPo line.

least a total of 16 mm of space to be eliminated during treatment. This was decided on because it was argued that when a space shortage equal to the combined width of four premolars (_+28 ram) was reached, no intramaxillary anchorage was available because there would be no spaces to close. To include such cases in the study would obscure the outcome. A Houston Instrument Hipad Digitizer (Houston Instruments) feeding into an IBM compatible computer, with software especially developed for this project by the engineer/author, was used for recording all the data. The data were divided into three groups, according to the type of extractions performed. Group 55 contained all the cases that had extractions of all four second premolars, group 45 had all the cases with upper first premolars and lower second premolars extractions, and group 44 had extractions of four first premolars. The following parameters were used in this study (Fig. 1): 1. A space analysis measured directly on study models with standard vernier calipers. 2. Skeletal convexity at point A. 3. The distance of the incisal edge of the maxillary

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4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

central incisors, perpendicularly projected on SN, to Nasion. The distance of the incisal edge of the mandibular central incisors, perpendicularly projected on SN, to Nasion. The perpendicular distance of the incisal edge of the upper central incisors to the NPo line. The perpendicular distance of the incisal edge of the lower central incisors to the NPo line. The angle between the upper central incisors and the NPo line. The angle between the lower central incisors and the NPo line. The incisor overjet measured along the occlusal plane. The SNA angle. The BaNA angle. The NBa-PNSANS angle. The facial axis/BaN angle. The interincisal angle. The IMPA. The upper central incisors to NA angle. The distance of the incisal edge of the lower incisors to the APo line. The facial axis length. The BaN length. The length of the perpendicular from N to ANSPNS. The facial divergence expressed as a percentage of SGo:NMe.

Because the esthetic excellence of the end result is of great importance, it was decided to relate incisor position to skeletal points close to the soft tissue profile. First, Nasion (Na) was chosen as the anteroposterior reference point and Sella-Nasion (SNa) was chosen as the line of reference, because it has been shown by Ricketts 5 and also by Holdaway 4 as being second best only to Basion-Nasion (BaNa) in its stability. In addition, the NPo line was chosen as reference line because of its intimate relation to the soft tissue profile and, in our experience, NPo is clinically more relevant and user friendly. The cephalometric points of S, Na, and Po have been shown to be easily selected and accurately positioned. 13 Ricketts 5 has also shown that there is very little change in the SNA angle with growth, so that the anteroposterior position of the upper incisor in its relation to Nasion will not be affected much during growth without treatment. The position of pogonion, on the other hand, is affected by growth as well as direction of growth. Because of the differences in root areas involved, we were expecting quite a big difference in incisor retraction between the four second premolar extraction group (group 55) and the four first premolar group (group 44), with the upper first and lower second premolar extraction group (group 45) somewhere inbetween the other two, much like what Williams et al. 12 reported. The choice of which extractions to perform was based on the following: Extraction of four second

American Journal of Orthodontics and Dentofacial Orthopedics November 1997

premolars (group 55) was decided on when limited retraction of the incisors was required. This decision was largely based on the same arguments as brought forward by Strang 14 and de Castro. 15 The extraction of four first premolars (group 44) was resorted to when a greater amount of incisor retraction was required, e.g., bimaxillary protrusion cases. The decision to extract upper first premolars and lower second premolars (group 45) was used when a large overjet existed or a relatively large space shortage was present in the upper jaw, the lower incisor position and angulation being close to the ideal, and there was very little or no space shortage in the lower arch. These were the kind of cases where one could consider the extraction of upper first premolars only or perhaps headgear to distalize the upper dentition. It was argued that by using this strategy, one could eliminate the need for headgear by using the anchorage available from the lower first molars, with Class II elastics, to eliminate the overjet and at the same time achieving a Class I molar relationship, by closing the upper extraction spaces from anterior and the lower extraction spaces from posterior directions. The basic treatment technique used for these cases was the following. Bands were cemented to the first molars only. Molar tubes had no built-in torque and had 6° offset. Bonded Lewis brackets (Ormco Co.) were used on the remaining teeth: Premolar bracket slots had zero torque and zero angulation, canines zero torque and 8 ° distal root angulation, lower incisors -1 ° buccal root torque and slight distal root angulation, upper lateral incisors 8° palatal root torque and 8° distal root angulation, and upper central incisors 12° palatal root torque and 4 ° distal root angulation. First, the upper and lower anterior six teeth and the teeth of the buccal segments were aligned and derotated, with preformed nickel titanium arch wires (Nitinol, Unitek/3M Corp.). When necessary, open coil spring was used locally to open enough space for a particular tooth. No tiebacks were used at this stage, to allow the arch wire to slide freely in the buccal segments, so that anchorage was neither actively prepared nor wasted. After alignment, 0.016 × 0.022 heat treated Yellow Elgiloy arches (Rocky Mountain Orthodontics) were placed with required torque bent in and with double helix closing loops always directly distal to the canines and, when required, Class II elastic traction was used. With space closure complete, 0.017 × 0.025 finishing arches were placed with, when required, occlusally soldered Class II elastic hooks mesial to the upper canines and incorporating the necessary torque and curve of Spee adjustments. Interproximal contacts were held tight by cinching the arch wires distal to the molar tubes. At no time was any form of extraoral anchorage used in any of these cases and buecal anchorage was never enhanced by including second molars into the mechanics during space closure or during intermaxillary Class II traction. Much more Class II elastic traction was required to obtain a Class I occlusion in group 45 than in the other two groups. Statgraphics software (Statgraphics Statistical Graph-

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Table I. Means and standard deviations (SD) of parameters pretreatment.

Mean Number of cases

I

Group 44

Group 45

Group 55

SD

Mean

I

SD

Mean

[

SD

73

59

74

Parameters." Distance measurements (mm) 5.6 10.5 9.7 4.0 2.0 3.1 5.8 109.0 103.3 105.5 51.7 1.0 1.7 -2.7

5.5 4.8 2.9 2.1 1.9 2.3 2.4 6.4 6.4 4.0 3.6 2.8 1.9 3.9

4.6 11.3 12.1 4.3 1.3 4.7 8.0 107.1) 100.0 105.8 51.7 -0.1 -1.5 -1.6

4.8 4.3 2.1 2.3 1.8 2.0 2.8 6.1 5.2 5.0 3.1 2.5 1.9 3.7

5.9 10.4 11.4 6.0 3.4 4.1 5.4 107.7 103.0 104.2 51.1 -1.8 -2.9 -4.6

4.9 4.7 2.7 2.2 1.9 2.0 2.3 6.0 6.4 4.6 3.0 3.0 2.3 4.1

Parameters." Angle measurements (degree) Upper I:NPo 27.5 Lower I:NPo 28.8 Upper l:Na 24.4 Lower l:GoMe 97.2 Interincisal 123.7 SNA 82.2 BaNA 62.1 BaN:ANSPNS 26.2 Facial axis 87.0 Facial divergence (%) 68.6

6.8 4.2 7.2 5.7 8.5 3.6 2.9 2.9 3.1 4.0

30.7 30.1 26.0 100.3 119.3 82.7 63.2 26.3 87.3 69.3

7.0 5.4 8.1 6.3 7.5 3.3 3.0 3.4 3.7 4.3

28.0 30.5 23.9 97.3 121.5 81.9 62.2 25.5 85.5 66.9

6.2 4.8 6.7 6.5 7.7 3.4 3.1 2.8 3.6 4.2

Upper l:Nasion Lower l:Nasion Upper I:NPo Lower I:NPo Lower I:APo Convexity Overjet Mand length Facial axis BaN length Na-ANS/PNS Arch disc Mx Arch disc Md Arch disc Mx + Md

Group 55: Extraction of four second premolars. Group 45: Extraction of upper first premolars and lower second premolars. Group 44: Extraction of four first premolars. Upper 1: Upper incisor. Lower 1: Lower incisor. Arch disc Mx: Archlength discrepancy in maxilla. Arch disc Md: Archlength discrepancy in mandible.

ics System, Version 5, Statistical Graphics Corporation) was used for the statistical analyses. RESULTS

The means and standard deviations of the pretreatment parameters are presented in Table I. Table II was created by subtraction of the pretreatment parameters from the posttreatment parameters and therefore shows the means and standard deviations of the changes effected by treatment. Note that upper incisor retraction differed only slightly between the groups 55 and 44, namely, 4.0 mm and 4.2 mm, respectively, when measured from Nasion and 4.2 mm and 4.7 mm, respectively, when measured from NPo. Slightly more retraction was recorded for group 45, namely, 6.3 mm from Nasion and 6.6 mm from NPo. The lower incisors were retracted slightly less in group 55 (1.6 mm from Na and 1.3 mm from NPo) than in the other two groups (group 44 = 2.1 mm from Na and NPo); group 45 = 1.8 mm from Na and 1.4 mm from NPo).

The position of the lower incisors to the APo line was not affected very much in groups 55 and 45 (0.1 mm reduction), with a reduction of 0.9 mm for group 44. This figure for group 45 differs only slightly from the figure of 0.52 mm reported by Burger et al. a6 and proves the usefulness of this extraction combination for cases selected according to the criteria outlined previously, especially if the use of headgear is to be avoided. Facial divergence was virtually unaffected in all three of the groups, even though much more Class II elastic traction, along with stronger curve of Spee, was usually used in group 45. In all three groups, there was an increase of 0.8% in facial divergence. The pretreatment length of the facial axis was shortest (100 mm), and the overjet (8.0 ram), as well as convexity (4.7 mm), the greatest for group 45. This suggests greater Class II tendencies for this group than for the other groups. Because of the fact that these figures represent

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T a b l e IL M e a n s a n d s t a n d a r d d e v i a t i o n s o f p a r a m e t e r s p o s t t r e a t m e n t m i n u s p r e t r e a t m e n t v a l u e s

Group55 Mean Number of cases

I

Group 45 SD

Mean

74

I

Group 44 SD

Mean

59

I

SD

73

Parameters." Distance measurements (ram) Upper l:Nasion 4.0 Lower l:Nasion 1.6 Upper I:NPo -4.2 Lower I:NPo -1.3 Lower I:APo -0.1 Convexity -1.9 Overjet -3.1 Mand length 5.9 Facial axis 6.2 BaN length 2.8 Na-ANS/PNS 2.5

3.6 2.4 2.4 1.8 1.8 1.4 2.6 3.1 3.6 2.3 1.7

6.3 1.8 -6.6 -1.4 -0.1 -2.4 -5.4 6.5 6.6 3.9 2.8

3.4 2.5 2.5 1.6 1.6 1.6 2.7 3.2 3.2 3.0 1.8

4.2 2.1 -4.7 -2.1 -0.9 -2.2 -2.6 6.5 6.3 3.4 2.7

3.6 2.5 2.3 1.8 1.9 1.6 2.6 3.5 3.6 2.5 1.9

Parameters." Angle measurements (degree) Upper I:NPo 0.3 Lower I:NPo 2.5 Upper l:Na 2.3 Lower I:GoMe 2.2 Interincisal -2.1 SNA - 1.5 BaNA - 1.4 BaN:ANSPNS 0.6 Facial axis 0.04 Facial divergence (%) 0.8

8.8 6.0 9.2 6.0 10.6 1.6 1.4 1.7 1.9 1.7

-5.5 1.9 -3.0 1.7 3.6 - 1.9 - 2.0 0.6 -0.1 0.8

10.0 6.1 10.7 6.0 12.1 1.8 1.5 1.6 1.8 1.8

0.3 0.2 2.6 -4.4 -0.5 - 1.6 - 1.7 0.8 0.2 0.8

9.8 6.4 10.6 6.3 12.2 1.6 1.5 1.6 1.5 1.8

If after-treatment value is greater than the before-treatment value, the sign is positive. If after-treatment value is smaller than the before-treatment value, the sign is negative. Group 55: Extraction of four second premolars. Group 45: Extraction of upper first premolars and lower second premolars. Group 44: Extraction of four first premolars.

m e a n values a n d t h e r e f o r e a r e indicative o f t e n d e n cies only, it is n e c e s s a r y to have m o r e a c c u r a t e m e a n s o f p r e d i c t i n g t h e o u t c o m e o f an individual e x t r a c t i o n p r o g r a m . T o this end, r e g r e s s i o n m o d e l s w o u l d p r o v i d e a m u c h m o r e a c c u r a t e p r e d i c t i o n for t h e individual case. O n l y t h e m o d e l s for p r e d i c t i o n o f t h e u p p e r incisor r e t r a c t i o n in r e l a t i o n to the N P o line a r e p r e s e n t e d here, b e c a u s e they p r o d u c e d substantially b e t t e r R - S Q ( A D J ) figures t h a n t h o s e r e l a t e d to Nasion. This finding was u n e x p e c t e d b e c a u s e it was a s s u m e d that, as m e n t i o n e d previously, g r o w t h w o u l d have a m o r e v a r i e d effect on the changes in p o s i t i o n o f N P o t h a n o n Nasion. Model for group 55: y = - 2 . 0 0 6 3 7 - × 1 * 0.134185 - × 2 * 0.319749 + × 3 • 0 . 1 1 6 9 3 4 - × 1 0 * 0.556251 + × 1 1 * 0.720941 T h e R - S Q ( A D J ) = 0.8472 Model for group 44: y = - 3 . 2 3 4 6 3 6 - × 7 * 0.592537 + × 8 * 0.424336 + ×9 * 0.128155- ×10 * 0.259062- ×12 * 0 . 2 8 0 2 9 7 - × 13 * 0.356977

T h e R - S O ( A D J ) = 0.8335 Model for group 45: y = 1.705068 - × 1 * 0 . 2 0 6 3 1 4 - × 2 * 0.421965 - × 4 • 0.104654 + × 5 * 0 . 1 3 0 5 9 2 - × 6 * 0.133146 T h e R - S Q ( A D J ) = 0.8367 w h e r e y equals the a m o u n t o f r e t r a c t i o n in m i l l i m e t e r s o f the u p p e r incisor incisal e d g e in r e l a t i o n to N P o . A negative a n s w e r indicates a c h a n g e posteriorly. (* indicates a m u l t i p l i c a t i o n sign.) × 1 = T h e c o m b i n e d u p p e r a n d lower space analysis. × 2 = T h e b e f o r e t r e a t m e n t overjet. × 3 = T h e b e f o r e t r e a t m e n t convexity. × 4 = T h e b e f o r e t r e a t m e n t angle b e t w e e n upp e r incisor a n d the N A - l i n e . × 5 = T h e b e f o r e t r e a t m e n t facial divergence. × 6 = T h e b e f o r e t r e a t m e n t facial axis angle. ×7 = The before treatment distance of upper incisors incisal e d g e to t h e NPo-line. x 8 = T h e b e f o r e t r e a t m e n t distance of lower incisors incisal e d g e to t h e N P o - l i n e .

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Plot of NPoB (post-treatment) - NPoB (pre-treatment): Group 44

l

-1 .=

. . . . . . .

L . . . . . . .

a . . . . . . .

=.~..e-

";'5'

J

ee

-3

O

•

e,

"O

P 0~ £1

,

. . . . . . .

-5

'

,. . . . . . . .

, ....

•

O OI

-7 M

•

fJ

%

•

0 j

-9

-11 -11

-9

-7

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1

Predicted

Fig. 2. Graph of observed:predicted values of upper I:NPo (posttreatment) --upper I:NPo (pretreatment) for group 44.

×9 = The before treatment angle between BaN and ANS/PNS. × 10 = The expected change during treatment of the angle between the upper incisors and the NA-line. × 11 = The expected change during treatment of the angle between the upper incisors and the NPo-line. × 12 = The expected change during treatment of the angle between the lower incisors and the GoMe-line. × 13 = The expected change during treatment of the interincisal angle. A negative (-) sign should be given to a decreased value, compared with the original value. The graph presented is an observed: predicted plot (Fig. 2), which is typical for the three groups. A complete set of graphs for all the groups and the computer program are available on request. The change in the angle of the upper incisors to

the NA-line was a parameter included in the group 55 regression formula. This made it possible to determine the effect that an increase or decrease in torque to the upper incisors would have on the amount of retraction of the upper incisors. For every 10° that the angle between the upper incisors and the NA-line was decreased, the upper incisors' incisal edge would be retracted an additional 1.84 mm. We deliberately included parameters that would indicate facial growth type strongly. Among these are the facial divergence and the angle between the facial axis and BaN. Table I shows that there were no major differences in skeletal pattern among the three groups (p > 0.55). Interesting is the fact that no single skeletal parameter was common to all three of the regression models. DISCUSSION

To interpret correctly the various authors' results, it is necessary to take into account each one's

486

Steyn, du Preez, and Harris

references from where the changes are measured. We have seen that the NA-line, the APo-line, the NPo-line, sella, and the palatal anatomy were used by the authors quoted previously. These references are all affected by growth and/or treatment in a different way and this makes the interpretation of the results reported by different authors somewhat confusing, which leads to the erroneous impression that the results are inconsistent. Our results, as depicted in Table II, differ slightly from those of other researchers, because we used Nasion and NPo as references. Unexpectedly, we found that there is, on average, less than a millimeter greater retraction to be expected from group 44 than from group 55. Also unexpectedly, group 45 gave approximately 2 mm more retraction than group 44. We suspect, however, that the answer lies in the far longer use of Class II elastic traction because of the greater overjet reduction necessary in group 45. Before our research results became available, we believed that there is a strong correlation between the anteroposterior response of the incisors and the actual change in interincisal angle, and expected to find that the anterior resistance (anchorage) would be strongly affected by the amount of torque applied to the incisors. This was shown to be less than we anticipated. Overbite correction, when required, w a s achieved by incisor intrusion, as well as some premolar extrusion, erected by an accentuated curve of Spee in the arch wires. Ricketts ~7believes that, if the lower anterior segment is already in an extruded position, the premolars should not be extruded. This was not evident in this research project, as demonstrated by the increase in facial divergence of only 0.8% (Table II). These averages suggest that, on average and for the same patient, the choice of which premolars are extracted would eventually have virtually an unnoticeable effect on the soft tissue appearance of the patient. Clinical experience on the other hand has led practitioners to believe, as we did, that there would definitely be noticeable differences with different choices of premolar extractions. It is an unfortunate fact that it is impossible to test two different treatment approaches on the same patient at the same time. We believe that the answer is to be found in the fact that each person has a very unique combination of factors that will determine his or her response. This is why the regression models are useful in

American Journal of Orthodontics and Dentofacial Orthopedics November 1997

getting a little closer to the requirements of the patient. Testing these formulas against 18 randomly selected cases from this study showed that, in most of them, there would indeed have been no more than a millimeter or two difference in the end result, had any one of the three extraction choices been made. CONCLUSION

This study shows conclusively that, on average, whether four first premolars or four second premolars are extracted, the soft tissue appearance of the patient after orthodontic treatment will be virtually the same, providing everything else remains constant. If upper first and lower second premolars are extracted in cases answering to the criteria outlined in our text, slightly more incisor retraction can be expected to occur. However, a much more accurate prediction of the amount of incisor retraction for a person can be made with the use of the regression models shown. We express our gratitude to Ms. Annemarie Olivier for preparing this manuscript, to Dr. Amos Lambrechts, Mr. Arend Louw for technical assistance with the figures and graphs, and to Professor J.S. Maritz for guidance with the statistical analyses.

REFERENCES

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