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Tooth movement after orthodontic treatment with 4 second premolar extractions

收藏 分享 2011-6-18 19:22| 发布者: wwz| 查看数: 34467| 评论数: 0|原作者: Kun Chen, Xianglong Han,Lan Huang, and Ding Bai|来自: Am J Orthod Dentofacial Orthop 2010;138:770-7

摘要: Introduction: This retrospective study was designed to investigate the position changes and movement patterns of incisors and molars after orthodontic treatment with extractions of 4 second premolars ...

Introduction: This retrospective study was designed to investigate the position changes and movement patterns of incisors and molars after orthodontic treatment with extractions of 4 second premolars in patients with mild crowding, slight dental protrusion, and Angle Class I relationship. Methods: Pretreatment and posttreatment cephalograms of 26 subjects were chosen from patients treated by an experienced orthodontist.The movements of the incisors and the molars as well as tooth tipping were measured. Results: Relative to the posttreatment occlusal plane, the mean incisor movements were 3.3 and 2.9 mm lingually in the maxilla and the mandible, respectively. The first molars were moved mesially an average of 3.2 and 3.4mmin the maxilla and the mandible, respectively. The incisor inclination was under proper control. The extraction space was almost equally taken up by the anterior and posterior segments. Conclusions: These data can be used to estimate the expected position changes and movement patterns of the incisors and molars in patients with mild crowding and slight bialveolar dental protrusion after orthodontic treatment with 4 second premolar extractions. (Am J Orthod Dentofacial Orthop 2010;138:770-7)

 

The first premolar has been the most common tooth removed in orthodontic clinics, as suggested by previous articles.1,2 But, in some patients with mild crowding, acceptable incisor positions and facial profiles, the second premolar would be an alternative to the first premolar to be extracted.3-5 Nance6 was one of the first to propose this; underlying his and some other advocates’ recommendations was the concept that second premolar removal would result in less incisor retraction and consequently less lip retraction compared with first premolar removal.3-6 It is widely accepted that the anchorage potential is highly related to the area of root surface involved.7,8 Proffit9 figured out that less incisor retraction effect would be anticipated while extracting the further posteriorly located tooth. And the space of second premolar extraction was mainly occupied by mesial movement of themolars, but someretraction of the incisor could occur. This incisor retraction effect has been confirmed by other researchers.3,4

Despite the agreement that improvement in orthodontic techniques increases the potential to freely move teeth 3 dimensionally and to correlate these movements with expected facial growth changes,8 it is still believed that a particular premolar removal will have predictable incisor and molar position changes.5,6 Diagnosis and treatment planning are based on this prediction; accordingly, the plan is executed to ensure that the teeth will ultimately reside in the predetermined positions.8

When reviewing studies that examined the effects of second premolar extraction treatment on tooth movement, we found that most reports were based on clinical observations; there is little scientific information to make an accurate prediction of extraction space distribution. 8 Also, controversy surrounds the distances of incisor and molar movement. Some authors reported little, if any, incisor change, and the extraction site was almost taken up by molars.3,4 But others found that the incisors were retracted remarkably, even as much as 3 mm.10,11 Underlying this inconsistent phenomenon is the complexity of the subjects studied.There were considerable differences in the pretreatment characteristics in a study and between studies, including severity of crowding, arch discrepancy, vertical skeletal pattern, and other orofacial features, which resulted in the wide range of individual variations in tooth movement. One example was that subjects with blocked-out and impacted second premolars were included in the study of Schoppe.4 Since severity of crowding, pretreatment incisor position, FMA angle, and individual response can influence the distance of incisor movement,11-13 it would, therefore, seem to be impossible to predict the incisor movement in a patient according to the average data obtained from a group of subjects with various craniofacial features, as suggested by previous authors.8,14,15

To observe tooth response, it is appropriate to study patients with normodivergent facial type, mild or no crowding, and Angle Class I relationship. In hyperdivergent or hypodivergent subjects, the skeletal pattern might affect horizontal tooth movement. In those with severe crowding, most extraction space would be used to reconcile arch length discrepancy, and then the space distribution pattern would be affected by the amount of residual space and the mechanotherapy used in releasing crowding. Class I malocclusions were selected to eliminate excessive molar movement involved in the treatment of Class II and Class III malocclusions.

Therefore, this study was undertaken to determine the anterior and posterior dental changes in a group of patients with mild crowding, slight dental protrusion, and Angle Class I relationship treated with 4 second premolar extractions and preadjusted appliances.

MATERIAL AND METHODS

This study was retrospective and included 26 subjects: 15 boys and 11 girls, with an average age of 16 years 1 month at the commencement of treatment and an average treatment time of 2 years 1 month.

All subjectswere selected from patients referred to the Department of Orthodontics, West China Stomatology Hospital, Sichuan University, who fulfilled the following criteria: (1) skeletal Class I and dental Class I malocclusion, (2) mild arch crowding (0-4 mm), (3) slight dental bimaxillary protrusion, and (4) normodivergent face type (24°

Fig 1. Pretreatment records of a 22-year old woman with mild crowding, slight bimaxillary protrusion,and an Angle Class I molar relationship.

Fig 2. Posttreatment records of the same patient who had 4 second premolar extractions

This clinical research was approved by the institutional ethics review board of Sichuan University. The pretreatment characteristics of the subjects are shown in Table I. All patients were diagnosed and treated by 1 operator (D.B.) with a 0.028-in slot preadjusted edgewise appliance (Roth) and sliding mechanics. The treatment protocol was as follows: 4 second premolars were extracted as part of a comprehensive orthodontic treatment plan. All teeth mesial to the second molar were bonded. A preadjusted edgewise appliance was used for all patients. After aligning the maxillary and mandibular dental arches with sequentially changed continuous nickel-titanium archwires, maxillary and mandibular 0.018 3 0.025-in stainless steel wires with a reverse curve of Spee were placed with a depth of about 3 to 4 mm. The crown labial torque in the maxillary posterior and mandibular wires was eliminated, but maintained in the maxillary anterior part. The crown labial torque was about 10°in the maxillary central incisor region. On these wires, intra-arch nickel-titanium coil springs were used to close the remaining spaces, and interarch Class II elastics were used when required to harmonize the molar relationship. After all spaces were completely closed, the second molars were included in the archwire, and the whole arches were aligned again. No posterior anchorage enhancement appliance (eg, temporary implant anchorage or transpalatal arch) was used. Normal incisor overbite and overjet and posterior neutral relationships were achieved at the end of treatment. One patient’s records are shown to illustrate the pretreatment orofacial features and treatment changes (Figs 1 and 2).

Patients whose second premolars were extracted for other reasons, such as severe caries, periapical lesions, or blocked out or impacted teeth, and who had systemic diseases that could affect bone metabolism were excluded from this study.

Lateral cephalometric radiographs were taken of all patients before and after treatment. The cephalograms were obtained on the same radiographic unit (Orthopantomograph OP 100D, Instrumentarium, Tuusula, Finland) under standardized conditions.

The amounts of incisor and molar movement were assessed by superimposing the maxilla on ANS and the palatal plane (ANS-PNS), and the mandible by the structural method.16,17 Retraction of the maxillary and mandibular central incisors was measured on the posttreatment occlusal plane, from the projection point of the maxillary and mandibular central incisors’ edge and apex. The degrees of maxillary and mandibular incisor tipping were measured from the angle between the long axis of the incisor and the palatal plane and mandibular plane, respectively. Molar movement was measured from a perpendicular between the posttreatment occlusal plane and the most mesial point on the molar crown and the mesial apex. The degrees of maxillary and mandibular molar tipping were measured from the angle between the long axis of the mesial root and the palatal plane and mandibular plane, respectively (Fig 3). All tracing and measuring work was done by the same operator (K.C.).

The center of rotation of the incisors can be determined by the method of Christiansen and Burstone18 as the intersection of 2 lines that coincides with the tooth axis before and after treatment (Fig 3).

Statistical analysis

Statistical analysis, including calculations of the means and standard errors of the mean for each variable, was conducted with SPSS software (version13.0, SPSS, Chicago, Ill).

Ten randomly selected cephalograms were retraced and measured twice 4 weeks apart. Results of the paired Student t test showed no significant difference between the 2 sets of measurements at the 95% CI.

RESULTS

The posttreatment cephalometric measurements are shown in Table I, demonstrating that the problems of dental and soft-tissue protrusion were solved, but there was no remarkable change in the sagittal and vertical skeletal structures.

The average maxillary central incisor movements were 3.3 mm palatally at the edge and 0.5 mm at the apex. The average crown palatal tipping was 8.9°. The maxillary first molar was moved mesially by averages of 3.2 mm at the crown and 2.2 mm at the apex, with a mean value of 3.8° of mesial crown tipping.

 

The mandibular incisor was moved by averages of 2.9 mm at the edge backward and 1.4 mm in the apex, and was tipped lingually an average of 5.1°. The mandibular first molar had mean mesial movements of 3.4 mm at the crown and 4.6 mm at the apex, and a mean distal crown tip of 4.0° (Table II, Figs 4 and 5).

In the maxilla, incisor retraction took up 50.4% of the residual space, whereas in the mandible 46.2% of the remaining space was occupied by incisor retraction. The space was distributed to the anterior and posterior segments approximately equally (Fig 6).

The maxillary central incisor’s rotation center was located –4.3 to 12.0 mm apical to the apex, and the mandibular central incisor’s rotation center was located 0.9 to 25.3 mm apical to the root apex (Fig 7).

DISCUSSION

It is remarkable that many studies reported wide ranges of individual variations in incisor changes and molar movements.1,3,4,8,11,12,14,15 Thus, the average data could not be directly used to estimate likely tooth position changes in a patient. In this study, from a number of patients who were treated with 4 second premolar extractions, we selected those with similar malocclusions and treatment modalities. That enabled creation of a more homogeneous group, compared with those in the previous studies, and therefore made the results applicable to further estimation in individual patients.

Schoppe4 reported on 12 subjects, but there was notable variability in the malocclusions and the appliances used to treat them. Some patients with blocked-out and impacted second premolars were included. Some authors did not even mention their selection criteria; this made it impossible to determine the patients’ before-treatment features.3,8,11,12 For this study, we selected patients with mild crowding and slight dental protrusion, and individual variations were avoided to the greatest extent. Therefore, although the sample size was relatively small, the treatment modalities were comparatively standardized. The results under this condition can reflect the true tooth response to 4 premolar extractions. Compared with those of the previous studies, the individual variations in this study were so small that the data can be used as guidance when treating patients. We introduce the coefficient of variation (CV), a parameter measuring the dispersion of a probability distribution, to compare individual variations in this study with those of previous studies. Ong and Woods11 reported a maxillary incisal tip change of 1.6 mm with a CV of 100% (1.6/1.6). In the study of Shearn and Woods,12 the mandibular incisor’s retraction relative to the APog line had a much higher CV of 580% (2.9/0.5). In this study, all CV values of tooth crown movement were less than 1, reflecting comparatively small individual variations.

Undoubtedly, all patients treated in our study could have been treated in other ways—eg, first premolar extraction or nonextraction with temporary implant anchorage. We did not intend to investigate the limit for retracting incisors, but to introduce the general rule of tooth movement after 4 second premolar extractions with the most commonly used mechanics and the principle of simplicity in orthodontic treatment. In our study, a reverse curve of Spee wire and crown labial torque in the maxillary anterior part were used to control incisor labial inclination during retraction. Zigzag elastics (Class II elastics and intra-arch elastics) were applied to close the remaining extraction spaces and harmonize the molar relationship. The treatment results proved that this force system is successful in treating patients with slight dental protrusion with 4 second premolar extractions, and in controlling incisor position and inclination appropriately at the same time.

The posttreatment occlusal plane was used as the reference plane to measure the amount of tooth movement in this study. In previous studies, some other craniofacial reference systems, including the PM, SN, and NPog lines, were used. The major deficiency in these systems is that relating jaws to these cranial reference planes introduces inherent inconsistencies. These inconsistencies arise from variations in craniofacial physiognomy, including the sagittal spatial relationship of nasion relative to the jaws and the rotation of the jaws relative to the cranium.19 On the contrary, relating the dentition to the occlusal plane will definitely not affect the measurement of tooth movement and thus provides a reliable method to assess that. Moreover, the occlusal plane is clinically relevant and user friendly.20 The results can also be used for diagnosis and treatment planning directly on the models.

The average changes in the anteroposterior positions of the incisors in this study exceeded those reported in previous studies.3,4,8,10-12,14 In our subjects, the maxillary incisor was retracted 3.3 mm, and the mandibular incisor was retracted 2.9 mm. Kimet al10 reported comparable but a bit smaller results. Subjects from the 2 studies had similar dental discrepancies and pretreatment incisor labial inclinations. But in the study of Kim et al, space was closed purely by intrajaw elastics; thus, maxillary incisor anchorage loss was decreased and resulted in less retraction. The smaller retraction of the maxillary incisors in other studies could be explained by the greater dental discrepancies (4-7 mm) in their samples.3,4,8,11,12,14 Less space after releasing crowding led to less incisor retraction.

The maxillary and mandibular incisors lost part of their labial inclination during retraction. This finding corroborates the previous data of Schoppe4 and Logan.14 The fact that the rotation center had a highly variable position indicated that the tipping movement was under proper control to achieve better incisor inclination on an individual basis with a similar force system. Proper incisor inclination is crucial for an esthetic smile.21 Chinese people have a relatively convex facial profile, and the incisor labial inclination is greater compared with that of white people.22 That requires proper incisor labial inclination to be maintained after treatment to accommodate to the facial profile. However, a straighter and more retrognathic profile is the preference of most people, so incisor inclination should be properly reduced during retraction.23 Meanwhile, the incisors were only prescribed to a minimal distal reposition in patients with slight dental crowding and protrusion. Because of the play between the wire and braces, crown labial torque was applied in the maxillary incisor region to prevent too much crown palatal inclination and unwanted distal movement.24 The result proved that archwires with a reverse curve of Spee and zigzag elastics can precisely and efficiently control incisor retraction after 4 second premolar extractions.

In our study, there were mean molar mesial movements of 3.2 and 3.4 mm in the maxilla and the mandible, respectively. These data were consistent with other authors’ reports3,4,8,11,12,14 except for that of Kim et al.10 Kim et al10 reported greater mesial molar movements, which might stem from the greater than average facial divergence at the start of treatment that could have increased this movement during space closure.

In our sample, the extraction space was approximately equally taken up by the anterior and posterior segments. In a study on various premolar extractions,

Williams and Hosila7 found that the anterior segment occupied 66.5% of the extraction site in patients who had 4 first premolar extractions, whereas the percentage was 56.3% in those who had maxillary first premolar and mandibular second premolar extractions. It is widely accepted that anchorage potential is highly related to the area of root surface involved.8 According to this rule of thumb, the proportion in patients with 4 second premolar extractions should be expected to be less; this was proved by our study. Kim et al10 obtained a smaller percentage of incisor occupation (44.5%) because of greater than average facial divergence. Meanwhile, our results on space distribution were quite different from those of some previously published classic articles about patients with 4 second premolar extractions; they agreed that the extraction sites were taken up mostly by the molars instead of the anterior teeth.3,4,8,11,12,14 In our study, although crown labial torque was applied to the maxillary anterior region, the molars took up only half of the extraction sites. This might suggest that molars cannot move mesially as much or as easily as we expected, and more anterior anchorage reinforcement should be considered for more molar forward movement when a second premolar is removed.11

The change of the facial skeletal vertical dimension after premolar extraction would be essential to an esthetic outcome.25 In this study, no significant difference was found in the cephalometric vertical parameters except for the height of the mandibular molar. The mandibular molar was extruded while moving mesially due to the mandibular archwire with the reverse curve of Spee and Class II elastics. The extruded molar contributed to the maintenance of the skeletal vertical dimension. The vertical position of the maxillary incisors was well maintained for an esthetic smile arc, which benefited from the intrusive force in the maxillary wire with the reverse curve of Spee.

When these results are applied to individual patient treatment, it should be remembered that this study only provides the tooth movement pattern in a simple but standard situation. However, this could serve as a foundation for diagnosis and treatment in more complicated malocclusions, if considering other influencing factors, such as severity of crowding, pretreatment incisor position, and vertical growth pattern.

CONCLUSIONS

From these results, the conclusions are as follows.

1. Extraction spaces are approximately equally taken up by the anterior and posterior segments in patients with mild crowding, slight dental protrusion, and Angle Class I relationship after treatment with 4 second premolar extractions.

2. Preadjusted edgewise appliances, accompanied by wires with a reverse curve of Spee and zigzag elastics (intra-arch and Class II elastics), can ensure the smooth closure of the extraction spaces and proper control of the inclination of the teeth.

3. Incisor anchorage should be reinforced to gain sufficient forward molar movement, even for second premolar extractions.

4. The posttreatment occlusal plane would be an appropriate reference plane to measure tooth movements in Angle Class I second premolar extraction patients.

REFERENCES

1. Keim RG, Gottlieb EL, Nelson AH, Vogels DS 3rd. 2002 JCO study of orthodontic diagnosis and treatment procedures. Part 1. Results and trends. J Clin Orthod 2002;36:553-68.

2. Proffit WR. Forty-year review of extraction frequencies at a university orthodontic clinic. Angle Orthod 1994;64:407-14.

3. Schwab DT. The borderline patient and tooth removal. Am J Orthod 1971;59:126-45.

4. Schoppe RJ. An analysis of second premolar extraction procedures. Angle Orthod 1964;34:292-302.

5. DeCastro N. Second premolar extractions. Am J Orthod 1974;65: 115-37.

6. Nance KN. The removal of second premolar in orthodontic treatment. Am J Orthod 1949;35:685-95.

7. Williams R, Hosila FJ. The effect of different extraction sites upon incisor retraction. Am J Orthod 1976;69:388-410.

8. Steyn CL, du Preez RJ, Harris AM. Differential premolar extractions. Am J Orthod Dentofacial Orthop 1997;112:480-6.

9. Proffit WR, Fields HW Jr, Sarver DM. Chapter 7 Orthodontic treatment planning: from problem list to specific plan. In: Proffit WR, Fields HW Jr, Sarver DM, editors. Contemporary orthodontics. 4th ed. St Louis: Mosby-Elsevier; 2007.

10. Kim TK, Kim JT, Mah J, Yang WS, Baek SH. First or second premolar extraction effects on facial vertical dimension. Angle Orthod 2005;75:177-82.

11. Ong HB, Woods MG. An occlusal and cephalometric analysis of maxillary first and second premolar extraction effects. Angle Orthod 2001;71:90-102.

12. Shearn BN, Woods MG. An occlusal and cephalometric analysis of lower first and second premolar extraction effects. Am J Orthod Dentofacial Orthop 2000;117:351-61.

13. Al-Nimri KS. Changes in mandibular incisor position in Class II Division 1 malocclusion treated with premolar extractions. Am J Orthod Dentofacial Orthop 2003;124:708-13.

14. Logan LR. Second premolar extraction in Class I and Class II.Am J Orthod 1973;63:115-47.

15. Luppanapornlarp S, Johnston LE Jr. The effects of premolarextraction: a long-term comparison of outcomes in ‘‘clear-cut’’ extraction and nonextraction Class II patients. Angle Orthod 1993;63:257-72.

16. Ricketts RM. A four-step method to distinguish orthodontic changes from natural growth. J Clin Orthod 1975;9:208-15, 218-28.

17. Bjork A. Prediction of mandibular growth rotation. Am J Orthod 1969;55:585-99.

18. Christiansen RL, Burstone CJ. Centers of rotation within the periodontal space. Am J Orthod 1969;55:353-69.

19. Jacobson A. Application of the ‘‘Wits’’ appraisal. Am J Orthod 1976;70:179-89.

20. Jacobson A. The ‘‘Wits’’ appraisal of jaw disharmony. 1975. Am J Orthod Dentofacial Orthop 2003;124:470-9.

21. Nanda R. Biomechanics and esthetic strategies in clinical orthodontics. St Louis: Elsevier; 2005.

22. Chen YX. Lateral cephalometry in 100 Harbin children with normal occlusion. Zhonghua Kou Qiang Ke Za Zhi 1985;20: 45-8.

23. Lu Y, Zhang X. Analysis of facial profile preferences among the Chinese population. Zhonghua Kou Qiang Yi Xue Za Zhi 2000; 35:224-6.

24. Gu M, Bai D, Liang R, Li JZ, Yang P. Study of torsional play angle of orthodontic rectangular arch wires. J Clin Stomatol 2004;20: 171-3.

25. Staggers JA. Vertical changes following first premolar extractions. Am J Orthod Dentofacial Orthop 1994;105:19-24.

 

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