ORIGINAL ARTICLE

Toothmovement after orthodontic treatment with4 second premolar extractions

Kun Chen,a Xianglong Han,b Lan Huang,b and Ding Baic

Chengdu, Sichuan, China

Introduction: This retrospective study was designed to investigate the position changes and movementpatterns of incisors and molars after orthodontic treatment with extractions of 4 second premolars in patientswith mild crowding, slight dental protrusion, and Angle Class I relationship.Methods: Pretreatment and post-treatment 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 tothe posttreatment occlusal plane, the mean incisor movements were 3.3 and 2.9 mm lingually in the maxillaand themandible, respectively. The firstmolars weremovedmesially an average of 3.2 and 3.4mm in themax-illa and the mandible, respectively. The incisor inclination was under proper control. The extraction space wasalmost equally taken up by the anterior and posterior segments. Conclusions: These data can be used toestimate the expected position changes and movement patterns of the incisors and molars in patients withmild crowding and slight bialveolar dental protrusion after orthodontic treatment with 4 second premolarextractions. (Am J Orthod Dentofacial Orthop 2010;138:770-7)

The first premolar has been themost common toothremoved in orthodontic clinics, as suggested byprevious articles.1,2 But, in some patients with

mild crowding, acceptable incisor positions and facialprofiles, the second premolar would be an alternative tothe first premolar to be extracted.3-5 Nance6 was one ofthe first to propose this; underlying his and some other ad-vocates’ recommendations was the concept that secondpremolar removal would result in less incisor retractionand consequently less lip retraction compared with firstpremolar removal.3-6 It is widely accepted that theanchorage potential is highly related to the area of rootsurface involved.7,8 Proffit9 figuredout that less incisor re-traction effect would be anticipated while extracting thefurther posteriorly located tooth. And the space of secondpremolar extraction was mainly occupied by mesialmovement of themolars, but some retractionof the incisorcould occur. This incisor retraction effect has beenconfirmed by other researchers.3,4

From the State Key Laboratory of Oral Disease (Sichuan University), West

China Stomatology Hospital of Sichuan University, Chengdu, Sichuan, China.aPostgraduate student; PhD candidate, Discipline of Orthodontics, Faculty of

Dentistry, University of Hong Kong, Hong Kong SAR, China.bPostgraduate student.cProfessor.

The authors report no commercial, proprietary, or financial interest in the

products or companies described in this article.

Reprint requests to: Ding Bai, 14 3rd section, Ren Min Nan Lu Rd, Chengdu,

Sichuan, China; e-mail, baiding88@hotmail.com.

Submitted, November 2008; revised and accepted, January 2009.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2009.01.030

770

Despite the agreement that improvement in orthodon-tic techniques increases the potential to freelymove teeth3 dimensionally and to correlate these movements withexpected facial growth changes,8 it is still believedthat a particular premolar removal will have predictableincisor and molar position changes.5,6 Diagnosis andtreatment planning are based on this prediction;accordingly, the plan is executed to ensure that the teethwill ultimately reside in the predetermined positions.8

When reviewing studies that examined the effects ofsecond premolar extraction treatment on tooth move-ment, we found that most reports were based on clinicalobservations; there is little scientific information tomake an accurate prediction of extraction space distri-bution.8 Also, controversy surrounds the distances of in-cisor andmolar movement. Some authors reported little,if any, incisor change, and the extraction site was almosttaken up by molars.3,4 But others found that the incisorswere retracted remarkably, even as much as 3 mm.10,11

Underlying this inconsistentphenomenon is the complexityof the subjects studied.Therewere considerable differencesin the pretreatment characteristics in a study and betweenstudies, including severity of crowding, arch discrepancy,vertical skeletal pattern, and other orofacial features,which resulted in the wide range of individualvariations in tooth movement. One example was thatsubjects with blocked-out and impacted second premo-lars were included in the study of Schoppe.4 Since sever-ity of crowding, pretreatment incisor position, FMAangle, and individual response can influence the distanceof incisor movement,11-13 it would, therefore, seem to be

Table I. Pretreatment and posttreatment variables

Variable

Before treatment After treatmentSignificance(P value)Mean SD Mean SD

Age at start (y) 16.1 4.3

Crowding (mm) 2.0 1.3

SNA (�) 84.1 4.2 83.6 4.3 0.1204

SNB (�) 80.6 4.5 80.3 4.8 0.4037

ANB (�) 3.5 1.9 3.3 1.5 0.315

FMA (�) 29.4 6.8 29.5 7.2 0.7141

OP/SN (�) 18.1 5.6 18.3 5.6 0.8383

U1/PP (�) 59.1 8.2 68.0 10.0 0.0002*

U6/PP (�) 78.5 10.1 80.4 5.2 0.429

U1/NA (�) 25.3 5.9 18.5 6.6 0.0009*

U1-NA (mm) 5.0 2.3 2.5 1.9 0.0003*

L1/MP (�) 93.4 8.1 88.3 6.5 0.005*

L6/MP (�) 84.3 5.2 80.3 6.0 0.003*

L1/NB (�) 31.6 5.2 26.8 5.0 0.0051*

L1-NB (mm) 7.4 1.6 5.3 1.7 0.0003*

U1/L1 (�) 118.6 9.2 130.4 8.7 0.0003*

Upper lip to

E-line (mm)

2.7 1.4 1.5 2.0 0.031*

Lower lip to

E-line (mm)

4.1 2.1 2.1 2.3 0.015*

American Journal of Orthodontics and Dentofacial Orthopedics Chen et al 771Volume 138, Number 6

impossible to predict the incisor movement in a patientaccording to the average data obtained from a groupof subjects with various craniofacial features, assuggested by previous authors.8,14,15

To observe tooth response, it is appropriate to studypatients with normodivergent facial type, mild or nocrowding, and Angle Class I relationship. In hyperdiver-gent or hypodivergent subjects, the skeletal patternmight affect horizontal tooth movement. In those withsevere crowding, most extraction space would be usedto reconcile arch length discrepancy, and then the spacedistribution pattern would be affected by the amount ofresidual space and the mechanotherapy used in releas-ing crowding. Class I malocclusions were selected toeliminate excessive molar movement involved in thetreatment of Class II and Class III malocclusions.

Therefore, this study was undertaken to determinethe anterior and posterior dental changes in a group ofpatients with mild crowding, slight dental protrusion,and Angle Class I relationship treated with 4 secondpremolar extractions and preadjusted appliances.

Z angle (�) 79.4 3.7 76.6 3.7 0.032*

Student t test; *P\0.05.

MATERIAL AND METHODS

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

All subjectswere selected frompatients referred to theDepartment of Orthodontics, West China StomatologyHospital, Sichuan University, who fulfilled the followingcriteria: (1) skeletal Class I and dental Class I malocclu-sion, (2) mild arch crowding (0-4 mm), (3) slight dentalbimaxillary protrusion, and (4) normodivergent facetype (24�\FH-MP\34�).

This clinical research was approved by the institu-tional ethics review board of Sichuan University. Thepretreatment characteristics of the subjects are shownin Table I. All patients were diagnosed and treated by1 operator (D.B.) with a 0.028-in slot preadjustededgewise appliance (Roth) and sliding mechanics. Thetreatment protocol was as follows: 4 second premolarswere extracted as part of a comprehensive orthodontictreatment plan. All teeth mesial to the second molarwere bonded. A preadjusted edgewise appliance wasused for all patients. After aligning the maxillary andmandibular dental arches with sequentially changedcontinuous nickel-titanium archwires, maxillary andmandibular 0.018 3 0.025-in stainless steel wires witha reverse curve of Speewere placedwith a depth of about3 to 4 mm. The crown labial torque in the maxillary pos-terior and mandibular wires was eliminated, but main-tained in the maxillary anterior part. The crown labial

torque was about 10� in the maxillary central incisorregion. On these wires, intra-arch nickel-titanium coilsprings were used to close the remaining spaces, and in-terarch Class II elastics were used when required to har-monize the molar relationship. After all spaces werecompletely closed, the second molars were included inthe archwire, and the whole arches were aligned again.No posterior anchorage enhancement appliance (eg,temporary implant anchorage or transpalatal arch) wasused. Normal incisor overbite and overjet and posteriorneutral relationships were achieved at the end of treat-ment. One patient’s records are shown to illustrate thepretreatment orofacial features and treatment changes(Figs 1 and 2).

Patients whose second premolars were extracted forother reasons, such as severe caries, periapical lesions,or blocked out or impacted teeth, and who had systemicdiseases that could affect bone metabolism wereexcluded from this study.

Lateral cephalometric radiographs were taken of allpatients before and after treatment. The cephalogramswere obtained on the same radiographic unit (Orthopanto-mograph OP 100D, Instrumentarium, Tuusula, Finland)under standardized conditions.

The amounts of incisor and molar movement wereassessed by superimposing the maxilla on ANS andthe palatal plane (ANS-PNS), and the mandible by thestructural method.16,17 Retraction of the maxillary and

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

772 Chen et al American Journal of Orthodontics and Dentofacial Orthopedics

December 2010

mandibular central incisors was measured on theposttreatment occlusal plane, from the projection pointof the maxillary and mandibular central incisors’ edgeand apex. The degrees of maxillary and mandibularincisor tipping were measured from the angle betweenthe long axis of the incisor and the palatal planeand mandibular plane, respectively. Molar movementwas measured from a perpendicular between theposttreatment occlusal plane and the most mesialpoint on the molar crown and the mesial apex. Thedegrees of maxillary and mandibular molar tippingwere measured from the angle between the long axisof the mesial root and the palatal plane andmandibular plane, respectively (Fig 3). All tracingand measuring work was done by the same operator(K.C.).

The center of rotation of the incisors can be deter-mined by the method of Christiansen and Burstone18

as the intersection of 2 lines that coincides with the toothaxis before and after treatment (Fig 3).

Statistical analysis

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

Ten randomly selected cephalograms were retracedand measured twice 4 weeks apart. Results of the pairedStudent t test showed no significant difference betweenthe 2 sets of measurements at the 95% CI.

RESULTS

The posttreatment cephalometric measurements areshown in Table I, demonstrating that the problems ofdental and soft-tissue protrusion were solved, but there

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

American Journal of Orthodontics and Dentofacial Orthopedics Chen et al 773Volume 138, Number 6

was no remarkable change in the sagittal and verticalskeletal structures.

The average maxillary central incisor movementswere 3.3 mm palatally at the edge and 0.5 mm at theapex. The average crown palatal tipping was 8.9�. Themaxillary first molar was moved mesially by averagesof 3.2 mm at the crown and 2.2 mm at the apex, witha mean value of 3.8� of mesial crown tipping.

The mandibular incisor was moved by averages of2.9 mm at the edge backward and 1.4 mm in the apex,and was tipped lingually an average of 5.1�. The man-dibular first molar had mean mesial movements of3.4 mm at the crown and 4.6 mm at the apex, anda mean distal crown tip of 4.0� (Table II, Figs 4 and 5).

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

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

DISCUSSION

It is remarkable that many studies reported wideranges of individual variations in incisor changes andmolar movements.1,3,4,8,11,12,14,15 Thus, the average datacould not be directly used to estimate likely toothposition changes in a patient. In this study, froma number of patients who were treated with 4 secondpremolar extractions, we selected those with similarmalocclusions and treatment modalities. That enabledcreation of a more homogeneous group, compared withthose in the previous studies, and therefore made theresults applicable to further estimation in individualpatients.

CR

CR ANS

OP

OP

PP

Fig 3. The maxilla was superimposed on the palatalplane at ANS. The mandible was superimposed by thestructural method. Tooth movement was measured onthe posttreatment occlusal plane. The geometric posi-tion of the incisor’s center of rotation was determinedby the intersection of 2 lines that coincidedwith the toothaxis before and after treatment.

Table II. Distances of tooth movement and tipping

Crown Apex Tipping

Tooth Mean (mm) SD CV Mean (mm) SD Mean (�) SD

U1 �3.3 1.7 0.52 �0.5 1.1 8.9 3.1

L1 �2.9 1.4 0.48 �1.4 0.9 �5.1 5.1

U6 3.2 1.1 0.33 2.2 1.1 �3.8 3.3

L6 3.4 1.4 0.41 4.6 2.0 �4.0 3.7

Positive numbers indicate mesial movement and tooth-axis clockwise

rotation; negative signs indicate distal movement and tooth-axis

counterclockwise rotation.

774 Chen et al American Journal of Orthodontics and Dentofacial Orthopedics

December 2010

Schoppe4 reported on 12 subjects, but there was no-table variability in the malocclusions and the appliancesused to treat them. Some patients with blocked-out andimpacted second premolars were included. Some au-thors 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, weselected patients with mild crowding and slight dentalprotrusion, and individual variations were avoided tothe greatest extent. Therefore, although the samplesize was relatively small, the treatment modalitieswere comparatively standardized. The results underthis condition can reflect the true tooth response to 4premolar extractions. Compared with those of theprevious studies, the individual variations in this studywere so small that the data can be used as guidancewhen treating patients. We introduce the coefficient ofvariation (CV), a parameter measuring the dispersionof a probability distribution, to compare individualvariations in this study with those of previous studies.Ong and Woods11 reported a maxillary incisal tipchange of 1.6 mm with a CV of 100% (1.6/1.6). In thestudy of Shearn and Woods,12 the mandibular incisor’sretraction relative to the APog line had a much higherCV of 580% (2.9/0.5). In this study, all CV values oftooth crown movement were less than 1, reflecting com-paratively small individual variations.

Undoubtedly, all patients treated in our study couldhave been treated in other ways—eg, first premolarextraction or nonextraction with temporary implant

anchorage. We did not intend to investigate the limitfor retracting incisors, but to introduce the generalrule of tooth movement after 4 second premolar extrac-tions with the most commonly used mechanics and theprinciple of simplicity in orthodontic treatment. In ourstudy, a reverse curve of Spee wire and crown labial tor-que in the maxillary anterior part were used to controlincisor labial inclination during retraction. Zigzagelastics (Class II elastics and intra-arch elastics) wereapplied to close the remaining extraction spaces andharmonize the molar relationship. The treatment resultsproved that this force system is successful in treatingpatients with slight dental protrusion with 4 second pre-molar extractions, and in controlling incisor positionand inclination appropriately at the same time.

The posttreatment occlusal plane was used as thereference plane to measure the amount of tooth move-ment in this study. In previous studies, some othercraniofacial reference systems, including the PM, SN,and NPog lines, were used. The major deficiency inthese systems is that relating jaws to these cranial refer-ence planes introduces inherent inconsistencies. Theseinconsistencies arise from variations in craniofacialphysiognomy, including the sagittal spatial relationshipof nasion relative to the jaws and the rotation of the jawsrelative to the cranium.19 On the contrary, relating thedentition to the occlusal plane will definitely not affectthe measurement of tooth movement and thus providesa reliable method to assess that. Moreover, the occlusalplane is clinically relevant and user friendly.20 Theresults can also be used for diagnosis and treatmentplanning directly on the models.

The average changes in the anteroposterior posi-tions of the incisors in this study exceeded those re-ported in previous studies.3,4,8,10-12,14 In our subjects,the maxillary incisor was retracted 3.3 mm, and themandibular incisor was retracted 2.9 mm. Kim et al10 re-ported comparable but a bit smaller results. Subjectsfrom the 2 studies had similar dental discrepanciesand pretreatment incisor labial inclinations. But in the

OP

OP

2.2 0.5

3.2 3.2

3.4 2.9

4.6 1.4

Fig 4. Tooth movement measurements relative to theposttreatment occlusal plane.

OP

OP

PP

68.0

84.3

80.3

93.4

88.3

59.1

Fig 5. Tooth tipping measurements relative to the pala-tal plane in the maxilla and the mandibular plane in themandible.

American Journal of Orthodontics and Dentofacial Orthopedics Chen et al 775Volume 138, Number 6

study of Kim et al, space was closed purely by intrajawelastics; thus, maxillary incisor anchorage loss wasdecreased and resulted in less retraction. The smallerretraction of the maxillary incisors in other studiescould be explained by the greater dental discrepancies(4-7 mm) in their samples.3,4,8,11,12,14 Less space afterreleasing crowding led to less incisor retraction.

The maxillary and mandibular incisors lost part oftheir labial inclination during retraction. This findingcorroborates the previous data of Schoppe4 andLogan.14 The fact that the rotation center had a highlyvariable position indicated that the tipping movementwas under proper control to achieve better incisorinclination on an individual basis with a similar forcesystem. Proper incisor inclination is crucial for an es-thetic smile.21 Chinese people have a relatively convexfacial profile, and the incisor labial inclination is greatercompared with that of white people.22 That requiresproper incisor labial inclination to be maintained aftertreatment to accommodate to the facial profile. How-ever, a straighter and more retrognathic profile is thepreference of most people, so incisor inclination shouldbe properly reduced during retraction.23 Meanwhile, theincisors were only prescribed to a minimal distal repo-sition in patients with slight dental crowding and protru-sion. Because of the play between the wire and braces,crown labial torque was applied in the maxillary incisorregion to prevent too much crown palatal inclinationand unwanted distal movement.24 The result provedthat archwires with a reverse curve of Spee and zigzagelastics can precisely and efficiently control incisorretraction after 4 second premolar extractions.

In our study, there were mean molar mesial move-ments of 3.2 and 3.4 mm in the maxilla and the mandi-ble, respectively. These data were consistent with otherauthors’ 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 facialdivergence at the start of treatment that could haveincreased this movement during space closure.

In our sample, the extraction space was approxi-mately equally taken up by the anterior and posteriorsegments. In a study on various premolar extractions,Williams and Hosila7 found that the anterior segmentoccupied 66.5% of the extraction site in patients whohad 4 first premolar extractions, whereas the percentagewas 56.3% in those who had maxillary first premolarand mandibular second premolar extractions. It iswidely accepted that anchorage potential is highly re-lated to the area of root surface involved.8 Accordingto this rule of thumb, the proportion in patients with 4second premolar extractions should be expected to beless; this was proved by our study. Kim et al10 obtaineda smaller percentage of incisor occupation (44.5%)because of greater than average facial divergence.Meanwhile, our results on space distribution were quitedifferent from those of some previously publishedclassic articles about patients with 4 second premolarextractions; they agreed that the extraction sites weretaken up mostly by the molars instead of the anteriorteeth.3,4,8,11,12,14 In our study, although crown labialtorque was applied to the maxillary anterior region,the molars took up only half of the extraction sites.This might suggest that molars cannot move mesiallyas much or as easily as we expected, and more anterioranchorage reinforcement should be considered formore molar forward movement when a secondpremolar is removed.11

The change of the facial skeletal vertical dimensionafter premolar extraction would be essential to an es-thetic outcome.25 In this study, no significant difference

49.6%53.8%

50.4%46.2%

Fig 6. Extraction space distribution in the maxilla andthe mandible.

A

B

C

A

B

C

Fig 7. Position of the center of rotation. B indicates theaverage position. The region between A and C is therange of individual variation.

776 Chen et al American Journal of Orthodontics and Dentofacial Orthopedics

December 2010

was found in the cephalometric vertical parametersexcept for the height of the mandibular molar. The man-dibular molar was extruded while moving mesially dueto the mandibular archwire with the reverse curve ofSpee and Class II elastics. The extruded molarcontributed to the maintenance of the skeletal verticaldimension. The vertical position of the maxillary inci-sors was well maintained for an esthetic smile arc,which benefited from the intrusive force in the maxillarywire with the reverse curve of Spee.

When these results are applied to individual patienttreatment, it should be remembered that this study onlyprovides the tooth movement pattern in a simple butstandard situation. However, this could serve as a foun-dation for diagnosis and treatment in more complicatedmalocclusions, if considering other influencing factors,such as severity of crowding, pretreatment incisorposition, and vertical growth pattern.

CONCLUSIONS

From these results, the conclusions are as follows.

1. Extraction spaces are approximately equally takenup by the anterior and posterior segments in pa-tients with mild crowding, slight dental protrusion,and Angle Class I relationship after treatment with4 second premolar extractions.

2. Preadjusted edgewise appliances, accompanied bywires with a reverse curve of Spee and zigzag elas-tics (intra-arch and Class II elastics), can ensure thesmooth closure of the extraction spaces and propercontrol of the inclination of the teeth.

3. Incisor anchorage should be reinforced to gain suf-ficient forward molar movement, even for secondpremolar extractions.

4. The posttreatment occlusal plane would be an ap-propriate reference plane to measure tooth move-ments in Angle Class I second premolar extractionpatients.

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