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External apical root resorption (EARR) is acommon iatrogenic problem associated with orthodon-tic treatment. Although EARR has long been recog-

nized as a consequence of mechanically induced toothmovement,1-3 its causes are still poorly understood.EARR occurs during treatment when forces at the apexexceed the resistance and reparative ability of the peri-apical tissues. Other implicated factors include trauma,tooth devitalization, ectopic eruption of adjacent teeth,traumatic occlusion, recurrent heavy forces to the den-tition such as bruxism, and habits such as tonguethrusting and finger nail biting.1,2,4-8 The literature sug-gests that movement of teeth with mature roots, exten-sive root movement, and intrusive mechanics enhancethe risk of EARR.9-15 It is, however, difficult to isolateand evaluate specific tooth movements likely toenhance EARR because combinations of complex

mechanical tooth movements, such as extrusion, intru-sion, translation, tipping, torquing, and rotations, areproduced by a wide array of orthodontic appliances.

The purpose of this study was to quantify apical andincisal movements of the maxillary central incisorthroughout treatment and determine which movementswere clinically relevant in predicting the extent ofEARR. The maxillary central incisor was used because(1) EARR is most common on this tooth, (2) this toothis easily visualized on a lateral cephalogram, and (3)maxillary incisors often undergo more displacementthan other teeth during extraction treatment.

MATERIAL AND METHODS

Sample Selection

Orthodontic treatment records of 110 cases were used inthis mixed longitudinal retrospective analysis. Forty-onerecords were from a solo Tweed standard edgewise practice,29 were obtained from a solo Begg lightwire practice, and 40were obtained from a solo Roth prescription straightwirepractice. Treatment outcome was not considered in sampleselection. To reduce sample variability, cases were used thatfulfilled the following criteria:

1. Subjects were white and were adolescents at the startof treatment.

2. Subjects had a Class I sagittal molar relationship

aIn private practice.bProfessor in the Department of Orthodontics, The Health Science Center, Uni-versity of Tennessee.Reprint requests to: Edward F. Harris, Department of Orthodontics, College ofDentistry, 875 Union Avenue, University of Tennessee, Memphis, Tennessee38163, e-mail: [email protected] 1998 by the American Association of Orthodontists.0889-5406/98/$5.00 + 0 8/1/88547

677

CONTINUING EDUCATION ARTICLE

Directions of orthodontic tooth movements associated withexternal apical root resorption of the maxillary central incisor

Robert J. Parker, DDS, MS,a and Edward F. Harris, PhDb

Conway, Ark, and Memphis, Tenn

External apical root resorption is a multifactorial problem encountered in all disciplines of dentistry, but it is

most commonly seen in cases treated orthodontically. Specific tooth movements that are most likely to

exacerbate external apical root resorption are poorly understood. Purpose of the present investigation was

twofold: (1) to quantify apical and incisal movements of the maxillary central incisor in the sagittal and

vertical planes from cephalograms and (2) to use stepwise multivariate linear regression analyses to see

which tooth movements and skeletodental relationships are most predictive of external apical root

resorption. The sample consisted of 110 adolescents with similar pretreatment malocclusions (Class I

crowded or bimaxillary protrusive) and treatment planned similarly (extraction of four first premolars) by

experienced private practitioners. Each of three practitioners used a different orthodontic appliance; the

sample was divided proportionately into cases treated with Tweed standard edgewise technique, Begg

lightwire technique, and Roth-prescription straightwire technique. Lateral cephalograms were analyzed atthe start, middle, and end of treatment. There was no statistical difference in average external apical root

resorption between sexes or among techniques. Measures of tooth movement were highly predictive,

explaining up to 90% of the variation in root resorption. Apical and incisal vertical movements and increase

in incisor proclination were the strong predictors of external apical root resorption for each regression

model. Incisor intrusion with increase in lingual root torque together were the strongest predictors of external

apical root resorption. In contrast, distal bodily retraction, extrusion, or lingual crown tipping had no

discernible effect. (Am J Orthod Dentofacial Orthop 1998;114:677-83)

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678 Parker and Harris American Journal of Orthodontics and Dentofacial OrthopedicsDecember 1998

(Angle) at the start of treatment as assessed from thedental study casts.

3. Subjects had undergone extraction of the four first pre-molars as part of treatment to alleviate anterior crowd-ing or dentoalveolar protrusion.

4. Each case had a diagnostic quality cephalogram beforeand after treatment. If a case had cephalograms takenduring the course of treatment, one of these as close tomidtreatment as possible also was assessed.

5. Root formation of the maxillary central incisors wascomplete before treatment was initiated.16

6. There was no history of trauma to the permanent max-illary central incisors as ascertained from patient his-tory and radiographic examination. Permanent maxil-lary central incisors also were intact, caries-free, andwithout endodontic treatment.

Sample Description

The sample consisted of 47 males and 63 females.Mean start age was statistically equivalent in the maleand female patients and among the three treatmentmodalities as judged from a two-way analysis of vari-ance (grand mean, 13.4 yrs; standard deviation [SD],1.78). Beginning of active treatment was defined byplacement of separators and the end of active treatmentwas denoted by removal of the fixed appliances. Activetreatment time ranged from 1.2 years to 4.4 years forthe overall sample. Mean time in active treatment wasstatistically equivalent in the male and female patientsand among the three treatment modalities (grand mean,2.7 yrs; SD, 0.41).

Maxillary Superimposition

Cephalometric images of the maxillae were super-imposed for the pretreatment, intreatment, and post-treatment x-rays of each patient to quantify movementsof the most protrusive maxillary central incisor. Fidu-cial points were located within the bony maxillabecause external landmarks drift and displace at differ-ent rates than those within the maxilla.

The anterior and posterior nasal spine were identi-fied on the pretreatment radiograph, and these land-marks were penetrated on the film with a needle, leav-ing small but highly visible reference points. Theintreatment radiograph then was placed over the pre-treatment film. The palatal cortical plate and trabecula-tion patterns within each maxilla were used to registeron the two bony structures.17,18 A definite change inradiographic opaqueness for the palatal cortical plateand common trabeculations was evident with precisesuperimposition of these anatomic structures. The ANSand PNS reference points were transferred to theintreatment radiograph. The posttreatment radiographwas then superimposed over the pretreatment radi-ograph and these two reference points were transferredin the same manner.

Positional Changes

Fig. 1 illustrates the measurements used to calculatethe seven positional changes of the maxillary centralincisor during orthodontic treatment. Two verticaltooth movements were calculated for each film: (1) the

Fig. 1. Schematic tracing of maxilla and central incisor shows linear and angular mea-

surements for apex and incisal edge of maxillary central incisor. A, Sagittal tooth move-

ments were measured to PNS-perpendicular. Landmark abbreviations are horizontal apical

distance (A_HRZ), horizontal incisal distance (I_HRZ), vertical apical distance (A_VRT),

and vertical incisal distance (I_VRT). Angular position () was the angle formed by long axisof the incisor with the palatal plane. B, Schematic tracing shows changes in tooth position.

Abbreviations are horizontal apical change (_A_HRZ), vertical apical change (_A_VRT),

straight-line movement of apex (A_DIS), horizontal incisal edge movement (_I_HRZ), verti-cal incisal edge movement (_I_VRT), and straight-line movement of incisal edge (I_DIS).

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American Journal of Orthodontics and Dentofacial Orthopedics Parker and Harris 679Volume 114, Number 6

distance from the maxillary central incisal edge topalatal plane (I_VRT), and (2) the distance from thecentral incisor apex to palatal plane (A_VRT). Sagittaltooth movements were measured to a plane perpendic-ular to palatal plane intersecting at PNS. Two parasagit-tal (horizontal) tooth distances were measured on each

film: (1) the distance from the incisal edge to PNS-per-pendicular (I_HRZ), and (2) the distance from the rootapex to PNS-perpendicular (A_HRZ). Two straight-line tooth movements also were calculated from pairsof films: (1) the distance the incisal edge traveledbetween pretreatment, intreatment, and posttreatmentassessments (I_DIS) and (2) the distance the root apextraveled between the three treatment assessments(A_DIS). The other incisor tooth measurement wasangular. The posterior-inferior angle formed by theintersection of the long axis of the maxillary centralincisor and the palatal plane was measured on eachfilm.

Ten skeletodental variables also were measured oneach film. The focus here was in determining whetherthe amount and kind of skeletodental changes duringtreatment were significantly associated with theamount of root resorption. These are defined in Table I.

Measuring Root Resorption

Each cephalogram was assessed to define thedegree and severity of EARR for the most procumbent

maxillary central incisor. Total tooth length was mea-sured quantitatively with the Quick Ceph Image pro-gram, intermittently checking measurements directlywith electronic sliding calipers. Image contrast andmagnification were optimized for each measurement.Total tooth length was measured from the incisal edgeto the most apical limit of the root. Tooth length wasdetermined to the nearest 0.1 mm parallel to the longaxis of the tooth.

Because of the capability with Quick Ceph Imageof optimizing magnification, brightness, and contrastof the radiographic image, intraobserver repeatabilitywas high. Gauged with a repeated-measures analysis ofvariance,19 intraobserver error for root length was just1.7%.

Table I. Cephalometric variables assessed at each examination

SNA: posterior-inferior angle at junction of the sella-nasion and nasion-point A planes.SNB: posterior-inferior angle at junction of the sella-nasion and nasion-point B planes.ANB: obtained by subtraction of SNB from SNA.FMA: anterior-inferior angle at junction of Frankfort horizontal and the mandibular plane (Go-Me).IMPA: posterior-superior angle at junction of the mandibular plane and long axis of mandibular central incisor.

U1-PP: inferior-posterior angle at junction of the line through long axis of the maxillary central incisor and the palatal plane.U1-L1: posterior angle at junction of the long axis of maxillary central incisor and long axis of mandibular central incisor.OP-PP: angle at intersection of palatal plane and Downs occlusal plane. Angle is positive when the lines intersect distal to the arches so

the occlusal plane is tipped down in front.Overjet: distance parallel with occlusal plane between the incisal edges of maxillary and mandibular central incisors.Overbite: distance perpendicular to occlusal plane between incisal edges of maxillary and mandibular central incisors.

Table II. Tests for sexual dimorphism in tooth length (mm)

Males Females Sex difference

Maxillary central incisor n x SD n x SD t Test

Pretreatment tooth length 47 27.0 1.8 63 26.1 1.6 2.8*

Intreatment tooth length 38 26.2 1.9 32 25.1 1.2 2.8*Posttreatment tooth length 47 25.7 1.9 63 24.6 1.8 3.0*Pretreatment to posttreatment length change 47 1.3 1.0 63 1.5 1.2 0.9Pretreatment to intreatment length change 38 0.7 0.6 32 0.9 0.7 1.4Intreatment to posttreatment length change 38 0.7 0.6 32 0.9 0.8 0.9

*P < .05.

Table III. Average changes in root length*

Treatment interval n x SD t Test

Pretreatment to Intreatment 70 0.8 0.7 -8.9Intreatment to posttreatment 70 0.8 0.7 9.8Pretreatment to posttreatment 110 1.4 1.1 13.7

*Negative values indicate decreases in tooth length. Sexes and treat-ment methods were pooled. The one-sample ttests determinedwhether the average change differed significantly from zero. Samplesizes varied because there were no midtreatment radiographs for thestraightwire series.P < .0001.

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RESULTS

Apical Root Resorption

Two-sample t tests confirmed an obvious sexualdimorphism for length of the maxillary central incisor(Table II). Males had a longer tooth, an average of 1 mmlonger, at the pretreatment, intreatment, and posttreat-ment examinations. In contrast, changes in tooth length(EARR) were very similar between males and females;both sexes experienced statistically equivalent decreas-es in tooth length during each treatment interval.

There also was no statistical difference in toothlength among the Begg, Tweed, and straightwire tech-niques at any of the three examinations; the averageamounts of root loss were statistically the same acrossall three treatment modalities. These three techniquesproduce the same degree of apical resorption eventhough they achieve correction by different means.

Resorption resulted in highly significant millimetricdecreases in tooth length (P < .001). Of note, averageloss from the pre- to intreatment examination was thesame as that occurring from the intreatment to post-treatment examinations (x

_= .8 mm) in the Tweed

and Begg groups (who had midtreatment x-rays). Aver-age root loss of 1.4 mm occurred across the entire

course of orthodontic treatment for the total sample(Table III).

Predictors of EARRSkeletodental variables (Table I) and measures of

incisor movements (Fig. 1) were assessed with step-wise multiple linear regression analyses.20,21 The pur-pose was to test whether the amount of apical resorp-tion could be predicted from either (1) starting condi-tions or (2) amounts of intreatment change. Initial runshad the entrance and deletion criteria for variable selec-tion set at = .20; this retains predictors for subsequentanalysis that may not meet a more rigorous probabilityvalue because of multicollinearity.22 Predictors notmeeting the screening criterion (P > .20) were omittedfrom further analysis. The stepwise procedure then was

used with the entrance and deletion criteria set at =0.05, which yielded a set of predictors that was testedfor multicollinearity using variance inflation factorsand analyzing the structure of relationships among thevariables.20-22

Substantial multicollinearity needs to be removedfrom a model because it means that two or more of theindependent variables are more strongly correlatedwith one another than any is with the dependent vari-able. Multicollinearity confounds interpretation of theregression solution, and it can dramatically alter thevalues of the partial correlations (which also confusesinterpretation).23,24 Multicollinearity also decreases theaccuracy of the coefficient estimates.24,25 Use of vari-ance inflation factors to measure and localize multi-collinearity in a model has been described in variousstatistical works.20,24,26

The patients skeletodental morphologic characteris-tics (as assessed from 10 variables, Table I) was testedon the outside chance that there were clinical relevantpredictors of EARR discernible before treatment. Thesewould be useful flags for the orthodontist that a patient

Table IV. Regression results predict EARR fromincisor movements during first half of treatment*

Skeletodental Standardized variable regression Uniquenessentered coefficient index Partial R2

Apical_Vertical_21 1.26 0.41 0.15Incisal_Vertical_21 1.02 0.32 0.12Incisal_Angular_21 0.89 0.20 0.20

*ModelR2 was 47% (P < .0001). Models were evaluated without anintercept.The 21 suffixes denote changes from pretreatment to intreatmentexaminations.

Table V. Regression results predicting EARR fromincisor movements during the second half of treatment*

Skeletodental Standardized variable regression Uniquenessentered cofficient index Partial R2

Apical_Vertical_32 1.23 0.30 0.05Incisal_Vertical_32 1.15 0.34 0.21Incisal_Angular_32 0.81 0.14 0.14Overjet_32 0.22 0.03 0.15

*ModelR2 was 55% (P < .0001). Models were evaluated without anintercept.The 32 suffixes denote changes from intreatment to posttreat-ment examinations.

Table VI. Regression results predicting EARR fromincisor movements during treatment*

Skeletodental Standardized variable regression Uniquenessentered Coefficient index Partial R2

Apical_Vertical_31 1.45 0.75 0.19Incisal_Vertical_31 1.45 0.61 0.37Incisal_Angular_31 0.94 0.27 0.35Overjet_31 0.07 0.00 0.00FMA_31 0.08 0.01 0.01

*ModelR2 was 92% (P < .0001). Models were evaluated without anintercept.The 31 suffixes denote changes from pretreatment to posttreat-ment examinations.

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was particularly susceptible (or resilient) to root resorp-tion during treatment with fixed appliances. In fact,none of the starting conditions was associated with theamount of resorption occurring during treatment.

On the other hand, testing the amount of intreat-

ment change disclosed consistently significant associa-tions. Three regression models were constructed todetect relevant predictors of EARR (Tables IV, V, andVI). In each case the outcome variable was the amountof root resorption, and the three sets of predictor vari-ables were (1) skeletodental changes during the firsthalf of treatment, (2) changes during the second half oftreatment, and (3) changes assessed from start to end oftreatment. Uniqueness indexes were calculated todetermine the percentage of the criterion varianceexplained by the individual predictors. In conjunctionwith uniqueness indexes, standardized regression coef-ficients, and partial coefficients of determination were

evaluated to determine the relative importance of eachpredictors influence in the model.20 A models coeffi-cient of determination (R2) is the variability of EARRexplained by that set of independent variables in linearcombination. An R2 above 40% was considered to beboth clinically as well as statistically significant,although this was just a gauge based on the likelihoodof being able to discern a systematic response clinical-ly. Obviously, much weaker associations wouldachieve statistical significance alone.

EARR that occurred throughout treatment wasregressed on skeletodental variables measured frompretreatment to intreatment (Table IV) to detect any

early treatment changes predictive of EARR. Threevariables, incisal vertical change relative to palatalplane (I_VRT), apical vertical change (A_VRT), andchange in angulation of the maxillary central incisorrelative to the palatal plane (I_ANG) were retained bythe stepwise procedure (ie, the 14 other variables didnot meet the entrance criteria.) In linear combinationthey explained 47% of the variability in EARR.

EARR that occurred throughout treatment also wasregressed on variables measured during the second halfof treatment (Table V) to define which late-treatmentchanges were predictive of EARR. Four variablesentered the model (I_VRT, A_VRT, I_ANG, andOVERJET); in linear combination these explained 55%of the EARR variability. The uniqueness indices, how-ever, revealed that overjet was a weak contributor tothis regression model, but this also was the only modelwhere a skeletal variable was retained.

EARR was regressed to identify overall treatmentcriteria predictive of patients liability to EARR (TableVI). In linear combination, five predictor variableswere found to account for 92% of the criterion variance

(I_VRT, I_ANG, A_VRT, FMA, and OVERJET). Theuniqueness indices revealed that FMA and overjet wereweak contributors to this regression model.

DISCUSSION

External apical root resorption remains a commoniatrogenic problem in orthodontics. There are primari-ly two impediments to preventing resorption: (1) rootapices are prone to resorption when the periodontium iscompressed, so teeth cannot be moved through bonewithout producing some odontoclasia; and (2) no exactcriteria have been found that predict which patientswill experience overt resorption and which will exhibitlittle under the same treatment regimens. Individualresponses to seemingly similar forces can be quite vari-able. However, most patients exhibit little resorption,and the esthetic and functional benefits of treatmentoutweigh the minor iatrogenic sequelae.

Regression Analysis

Standardized multiple linear regression coefficientsrevealed repeatedly that vertical movement of the rootapex (A_VRT), vertical movement of the incisal edge(I_VRT), and change in proclination (I_ANG) were inthe optimum subsets of predictor variables for each ofthe three regression models described. Repeatedly, api-cal and incisal intrusive movements and increase inincisor proclination were the most powerful predictorsof EARR. Orthodontically, this means incisor toothintrusion in the vertical plane and lingual root torque inthe sagittal plane cause the most apical resorption.

Ten skeletodental variables were measured on allcases (Table I), but none was strongly associated withEARR. These variables differed greatly among patientsas a result of differences in facial morphologic charac-teristics, but none was retained in any regression equa-tion, which implies that facial morphologic characteris-tics are effectively unrelated to the variation in EARR.

Prior Studies

Only a few studies have evaluated the extent of api-cal root resorption based on orthodontically inducedtooth movements.10,27,28 There are substantive method-ologic differences among these three studies that makeit difficult to compare results (Table VII). Differencesin research design may explain why the inferred rela-tionship between maxillary incisor movements andEARR remains contentious. Phillips27 concluded thatthere was no association between EARR and angular orsagittal apical movements of the maxillary centralincisor. DeShields,10 in contrast, found significant cor-relations between EARR and sagittal apical move-ments of the maxillary central incisor. Mirabella and

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rtun28 stated, movement of the roots in either ananterior or posterior direction, is associated with apicalroot resorption. Although Mirabella and rtun foundno statistically significant association between verticalmovement of the incisor apex and root resorption, theydid caution about interpretation because few patients intheir study experienced as much as 1 mm of extrusion

or intrusion. In the present study, multiple linearregression revealed that vertical apical movement, ver-tical incisal movement, and incisor proclinationchanges were consistently predictive of EARR.

Root Torque

Apical root resorption decreases a roots surfacearea and, thereby, its periodontal support and ability toresist relapse.29,30 EARR averaged 1.4 mm in the pre-sent sample. Jacobson31 suggested that a loss of a mil-limeter at the apex is trivial because the apical end ofthe root has the smallest diameter, but Kalkwarf et al32

showed that there is a nearly linear relationshipbetween root length and percentage of periodontalattachment, so minor loss in root length may be impor-tant. In addition, loss of root length moves the center ofresistance coronally, so the same amount of torque onthe tooth will have a greater effect than if the root wereintact.33 In the present study, increasing the anglebetween the central incisor and the palatal plane wasstrongly correlated with increasing EARR. This angu-lar increase occurred by tipping the crown facially and

torquing the apex lingually, but the change was mainlydue to lingual root torque rather than labial crown tip-ping because all cases were treated with first premolarextractions and mechanics were designed to retract theincisors. A major goal in extraction treatment with anedgewise appliance is to retract the incisors whilemaintaining torque. In the Begg technique, a major

focus in stage 3 is to torque the maxillary incisors andremove the axial tipping that occurred early in treat-ment. The point is that substantial force is required totorque the apex of the maxillary central incisor lingual-ly, and, unfortunately, these torquing forces concen-trate at the apex which is the smallest and most resorp-tion sensitive area of the tooth.8

Intrusion

Intrusive forces are damaging to root surfacesbecause root shape concentrates pressure at the conicalapex.34 Studies that have explicitly examined theeffects of intrusive mechanics on root resorption con-cluded that intrusive forces produce apical resorp-tion.14,35,36 In the present study, intrusive movement ofthe maxillary central incisor had a strong positive asso-ciation with the amount of EARR. On the other hand,few studies of intrusion have contrasted other vectorsof movement on the same teeth.37 In the present study,the strongest predictors of EARR were vertical incisaland apical movements. It should be noted, however,that no regression model showed intrusion alone to be

Table VII. Summary of characteristics of present and prior studies

Mirabella andCriterion Present study Phillips (1955) DeShields (1969) rtun (1995)

Study type Mixed longitudinal Cross sectional Cross sectional Cross sectionalAge range 11-18 years 11-19 years 11-16 years 20-70 years

Pretreatment malocclusion Class I Class I, II, III Class II Class I, II, IIITreatment plan Extraction* Extraction and Nonextraction Unknownnonextraction

Appliance Edgewise and Begg Edgewise Edgewise EdgewiseReference planes Palatal plane Palatal plane Sella-Nasion Palatal plane

Bolton-ANSRadiograph to assess EARR Lateral ceph Lateral ceph Periapical PeriapicalRace White Unknown Unknown UnknownMovements measured Incisal Apical Apical Angular Apical Angular Incisal Apical

Angular AngularStatistical tests Regression# Student t Pearson rStudent t Pearson r Regression# Student t

Chi-square Chi-square

*Included only four first-premolar extractions.Tweed standard edgewise plus straightwire edgewise.Unknown whether standard or preadjusted appliances.

Measured both vertical and sagittal movements.Straight-line movement calculated.

Measured only sagittal movement.#Stepwise multiple linear regression.

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the cause of EARR. Each clinically significant modelshowed that a combination of movements bestexplained the variation in EARR among patients.These movements primarily were intrusion in combi-nation with lingual root torque.

Sagittal Movements

Studies support the contention that the distance aroot is moved through bone influences the amount ofEARR. DeShields10 found a significant, positive corre-lation between EARR and how far the root was trans-posed horizontally. Sharpe et al29 showed that casesrequiring premolar extraction experienced moreresorption than those requiring less retraction of themaxillary incisors. Harris and Butler38 and Kaley andPhillips39 also reported that the horizontal amount themaxillary incisors needed to be retracted was positive-ly correlated with EARR. In contrast, the present study

showed that exclusively horizontal tooth movementswere not significantly associated with EARR whenother vectors were accounted for. In addition, thestraight-line distance the incisal edge or the apex trav-eled was not strongly correlated with EARR.

CONCLUSIONS

The present investigation quantified apical and incisalmovements of the maxillary central incisor and used multi-variate regression analyses to see which movements andskeletodental variables were most predictive of EARR. Majorfindings were:

1. Incisor tooth movements taken in combination are

strong statistical predictors of the amount of rootresorption experienced during treatment. Specificdirections of movement differentially enhance theextent of EARR, and the amount of EARR is a func-tion of the amount of movement.

2. In combination, intrusive movement and lingual roottorque were the strongest predictors of EARR. In con-trast, bodily retraction in a posterior direction, extru-sion, or lingual crown tipping had no discernible influ-ence.

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