Patient characteristics and treatment variables associated with apical root resorption during orthodontic treatment Leif Linge, DDS, Dr. Odont., and Brita Ohm Linge, DDS Skien, Norway Multivariate analysis of patient characteristics and clinical variables was carried out with the maximum single maxillary incisor apical root resorption for each patient as the dependent variable. Root lengths were measured in standardized intraoral radiographs from 485 consecutively treated patients, 11.5 to 25 years of age. The correlation matrix revealed a complex pattern of positive and negative associations between the six pretreatment and seven treatment variables. Variables found to contribute significantly to apical root resorption were overjet, history of trauma to maxillary incisors before initiation of treatment, time of treatment with rectangular arch wires, time of treatment with Class II elastics, lip/tongue dysfunction, and/or history of finger-sucking habits persisting beyond the age of 7 years, and impacted maxillary canines to be corrected orthodontically. Clinical application of an open activator was significantly correlated with overjet but negatively correlated with apical root resorption, with the use of rectangular arch wires and/or Class II elastics, and with total banding time. (AM J ORTHOD DENTOFACORTHOP 1991;99:35-43.)

P r e v i o u s investigations indicate that apical root resorption occurring in conjunction with conventional orthodontic treatment is a multifactorial problem, associated with patient characteristics as well as with treatment factors. H7 On the other hand, current literature does not seem to contain multivariate analysis of the variables involved. A reason for this lack of research may be lack of representative patient samples with adequate clinical records of sufficient detail. The present study was designed to evaluate the relative contribution of pretreatment patient characteristics and treatment variables to apical root resorption of maxillary incisors in consecutively treated orthodontic patients. MATERIAL AND METHODS Comprehensive orthodontic records, including anamnestic information and standardized intraoral radiographs of maxillary incisors before treatment and at completion of retention were available from 485 orthodontic patients--288 girls and 197 b o y s - - I 1.5 to 25 years of age. The sample was selected from a series of 719 consecutively treated orthodontic patients who had participated in an earlier, bivariate study. ~5 The age limitation was made to reduce the problem of residual root growth, which can conceal the amount of root resorption during treatment, and to homogenize treatment circumstances. The average observation pe8/1/17408

riod, from start of treatment to the end of retention, was 3.3 years. Anamnestic information was collected by means of a questionnaire and an interview during each patient's first visit. This included history of any trauma to maxillary front teeth that caused discomfort to the patient and finger-sucking habits that had persisted beyond the age of 7 years. On the same occasion, soft tissue dysfunctions noted in posture, speech, and during swallowing were registered. Overjet and overbite were measured to the nearest 0.5 mm, and conventional orthodontic records were made, including intraoral radiographic cephalograms of maxillary incisors taken with a parallel, longcone technique and 2 × 3 cm films. Corresponding clinical records were taken at completion of retention.'5 All patients were treated in one private orthodontic practice by two orthodontists working in close cooperation. Treatment was carried out by means of an 0.018inch edgewise technique exclusively (60.2%), with removable appliances exclusively (9.1%), or with combinations of fixed and removable appliances (28.5%). The rest of the patients (2.3%) were treated with fixed appliances, but without banding of maxillary incisor teeth. Edgewise therapy included the use of extraoral anchorage with face-bows and intermaxillary elastics (generally 3/16 inch, producing approximately 150 gm of force), usually worn 4 to 6 hours a day and all night. 35

36

am. J. Orthod. Dentofac."Orthop.

Linge and Linge

January 1991

1. X-ray

Z"1

f

C1

2. X-ray

r 2

c 2

1 CI C2

Fig. 1. Correction factor for enlargement difference, f = - - .

CI Apical root resorption: r, - r2 • C'-~

Root torque was carried out by means of 0.016 × 0.022-inch archwires. Removable appliances consisted of open activators with an edge-to-edge working bite (worn about 4 hours a day and all night) or maxillary bite plates with clasps. Root and crown lengths were measured to the nearest 0. I mm directly from the radiographs. Crown length registrations served the purpose of correcting enlargement differences between exposures, assuming crown lengths to be unchanged over the observation period (Fig. 1). The variables were evaluated as shown in Table I. Statistical methods

Stepwise regression was performed with program 2R in the BMDP statistics package, which also produced the correlation matrix. The program was run on a VAX-8650 computer. Findings

Root length measurements in pretreatment and postretention radiographs, corrected for enlargement differences (Fig. 1), revealed the values of apical root resorption in individual maxillary incisor teeth shown in Fig. 2. Mean values for individual teeth and the corresponding banding times are given in Table II. Maximum single apical root resorptions per patient are depicted in Fig. 3. On average, the maximum single-root shortening was 1.54 mm _ 1.11 with a corresponding banding

time of 11.5 months ± 7.6. Eighty patients (16.5%) had root shortenings >2.5 mm for one or more maxillary incisors. Correlation coefficients (r) between all the variables are given in Table III. Correlation coefficients significant at 1% and 5% levels are printed in boldface and underlined, respectively. It is seen that age and sex are not significantly correlated with maximum single-root resorption, and neither are overbite or clinical use of vertical elastics, open activators, or plates with clasps. Some of the variables that were significantly correlated with maximum single-root resorption were also correlated with each other. For example, the strongest variable--banding time--is also strongly correlated with the presence of impacted maxillary canines, clinical use of rectangular arch wires, Class II elastics, and vertical elastics, but not with overjet, history of trauma, and LTD (lip/tongue dysfunction, including history of finger-sucking habits). On the other hand, overjet, which is also well correlated with maximum single-root resorption, is positively correlated with trauma, LTD, clinical use of rectangular arch wires, Class II elastics, and open activators. Overbite was negatively correlated (r = - 0 . 0 0 3 7 ) with maximum single-root resorption and only insignificantly correlated with clinical use of rectangular arch wires and Class II elastics. Stepwise regression analysis resolved the complicated interrelationships and selected predictor variables that, within limits, were mutually independent. A first run, with all variables included, gave the results summarized in Table IV. In this run, banding time and overjet ruled out many of the treatment modality variables because of mutual correlations. In the second run, banding time was excluded from regression. The results are summarized in Table V. Although it yielded more clinically relevant information, the second run lost some prediction p0wer compared to the first run (explained variance, R 2, dropped from 14% to 10%). The relative contribution of each independent variable to the total R 2 can be seen in the column "Change in RL" Trauma seemed to be the most powerful predictive variable in the second run--closely followed by clinical use of rectangular arch wires-whereas LTD, impacted maxillary canines, and Class II elastics were of a lower range. It may be noted that clinical use of open activators, plates with clasps, and vertical elastics had a very low partial correlation with maximum single-root resorption at all steps. A graphic summary of significant findings

Volume99 Number 1

Variables associated with root resorption

37

Table I. Treament variables

Variable

I

Unit

Pretreatment patient characteristics 1. Age 2. Sex 3. Overjet 4. Overbite 5. History of trauma to maxillary front teeth 6. Lip/tongue dysfunction diagnosed clinically at the patient's first visit, including the history of finger-sucking habits persisting beyond the age of 7 years (LTD) 7. The presence of impacted maxillary canines to be corrected orthodontically

(Units) Years 0/1 mm 171111

Yes/no Yes/no

Yes/no

Treatment variables 8. Rectangular arch wires (0.016-inch × 0.022-inch) used in treatment 9. Class II elastics (usually 3116-inch) used in treatment 10. Vertical elastics (usually 3/16-inch) used in treatment 11. Open activators (4 hours a day and all night) 12. Maxillary bite plates with clasps (I0 hours and all night) 13. Time with bands or attachments on maxillary incisors

Months Months Months Yes/no Yes/no Months

Postretention findings 14. Apical root length changes for each maxillary incisor 15. Apical root length changes expressed by the single most severely resorbed maxillary incisor within a patient; this was

mm mm

chosen as the dependent variable throughout the study.

with regard to assumed cause-and-effect relationships is given in Figr 4. The regression formula of the second run was as follows: Maximum single-root resorption = 1.23 (constant) + (condition/treatment variable)* *0.05 x months treated with rectangular arch wires 0.03 × months treated with Class II elastics 0.57 if history of trauma 0.49 if LTD registered 0.69 if impacted maxillary canines Mean treatment time with rectangular arch wires (in 140 patients) was 6 months, and mean treatment time with Class II elastics (also in 140 patients) was 6.8 months. If this is typical, the above formula may be written in short as follows: Maximum single-root resorption = 1.23 (constant) + (condition/treatment variable)* *0.30 if treated with rectangular arch wires 0.20 if treated with Class II elastics 0.57 if history of trauma 0.49 if LTD registered 0.69 if impacted maxillary canine

Table II. Mean apical root resportion and banding times for each of the maxillary incisors (12, 11, 21, 22)

Tooth 12 11 21 22

I

N 428 422 392 414

Mean apical root resorption (nun) 1.0 0.8 0.7 1.0

± ±

1.0 0.9 0.9 1.0

Mean time with bands (months) 10.5 12.2 12.2 10.8

--± ± --

7.0 7.6 7.5 7.0

This formula gives an impression of influences from risk variables in terms of apical root resorption in millimeters. Note the relatively high constant of 1.23, which indicates that, even with all independent variables being ineffective, the regression still forecasts a maximum single-root resorption of 1.23 mm. Standard error of estimate was 1.05 mm, indicative of considerable uncertainty of prediction. DISCUSSION

The age of the subjects, 11.5 to 25 years at treatment initiation, seemed to solve most of the problem of re-

38

Am. J. Orthod.Dentofac. Orthop. January 1991

Linge and Linge

600 n

507 500 I358

400 300

235

168

200

92

100 4 3 6 8

"-282011

3 3

3

5

1

0 -2

-1

0

1

2

4

6

rnm

Fig. 2. Apical root resorption of maxillary incisors N = 1656 (negative value: root elongation).

Table III. Correlation coefficients (r) b e t w e e n all variables (n = 485)

Variable Age Sex Overjet Overbite Trauma Lip/tongue dysfunction (LTD) Impacted maxillary canines Rectangular arch wires Class II elastics Vertical elastics Open activators Plates with clasps Maximum banding time Maximum root resorption

Age

I

Sex

Overjet

Lip I tongue disfunction

Overbite

Trauma

1.00(30 0.1178 0.1717 0.1004

1.0000 0.0474 - 0.2730

1.0000 - 0.04 11

1.0(300

Impacted maxillary canines

1.0000 0.0075 0.0870 0.0701 0.0395 0.0050

1.00130 0.0788 0.0801 0.1278 - 0.0439

0.2245

-0.0871

-0.1481

0.0462

-0.0592

-0.0232

1.0000

0.238

0.0754

0.2088

0.0208

-0.0408

0.0652

-0.0021

0.0453 0.0418 -0.1644 -0.020i 0.0655

-0.0110 -0.0790 0.0780 0.0990 -0.0876

0.1219 -0.0442 0.2326 -0.1475 -0.0097

0.0129 -0.3249 0.1810 -0.1918 -0.0255

-0.0172 -0.0525 0.0025 -0.0326 -0.0173

0.0083 0.1691 0.0279 0.0601 0.0087

-0.0617 -0.0528 -0.0247 -0.0283 0.2040

0.0862

0.0703

0.1429

-0.0037

0.1652

0.1221

0.1035

-

r >" 0.117 is significant at 1% level and printed bold. r > 0.090 is significant at 5% level and printed underlined.

sidual root growth. O n l y one person had a root elongation > 0 . 5 m m . That does not m e a n that all o b s e r v e d root shortenings during treatment are expressions o f root resorption alone. Incidents o f g r o w t h f o l l o w e d by resorption still are possible. Raising the l o w e r age limit to 12 or 13 years w o u l d reduce the problemeven m o r e ,

but p r e l i m i n a r y w o r k with the data s h o w e d that this strategy w o u l d c h a n g e regressions v e r y little. Surprisingly e n o u g h , the limitation o f the material reduced the age factor from being highly correlated with root resorption in the total sample o f 719 cases t5 to insignificant in the present sample. For patients y o u n g e r

Volume 99

Variables a s s o c i a t e d with root resorption

Number 1

140R 120L

39

115

100 83 80

BO

58

60

4?

40

1

30

IIIIII

20 i

_..

10

2 3

I

0 -2 Fig. 3.

-i

0

1

2

3

4

5

6

mm

Maximum single-root resorption/patient N = 485 (negative value: root elongation).

Ma-rirnum

Rectangular archwire

Class 11 elastics

Vertical

Open

elevators

activators

Maximum banding Plates w/clasps

time

single-root resorption

l.O000 0.1004

1.0000 0.0752

1.0000

- 0.0577

- 0,0517

- 0.0210

1.00(30

0.0909

0.0978

0.0221

- 0.0280

0.2172

0.2834

O. 1825

- O. 1664

1.130(O -- 0 . 2 6 7 0

1.0000

0.1674

0.1235

0.0289

-0.0022

-0.0772

0.2930

-0.0185

than 11.5 years at treatment initiation, this correlation again was highly significant. This finding suggests that correlation or regression may give misleading conclusions when variables are not linked linearly or when unknown interactions take place. Our interpretation of the age factor is that it carried along indirect information about root length development. After 11.5 years, most of the upper incisor roots

1.0000

are fully developed. Is The age factor between 11.5 and 25 years then seemed to reflect a more general but insignificant aging process. The factor of age and its significance with regard to apical root resorption caused by various influences of orthodontic treatment warrant further investigation. Well-controlled clinical and experimental studies of root length changes incident to overjet correction with fixed appliances and with

40

Linge and Linge

Am. J. Orthod. Dentofac. Orthop. Janua O" 1991

02,[ o.V,TOR OVERJET I"7 i REC BEFORZ TX J ' ~

.

1

I TIME ~ITH

O. 101 LIP/TONGUE

/ / I0.~//

DYSFUNCTION

I

XI,'OM SINC'EI /-- t_______i ROOT RESORPTIONj

e,i,ES o.I"2

bivariate correlations

part correlations

Fig. 4. A graphic summary of some of the findings. Arrows indicate what is assumed to be the direction of cause and effect. Number accompanying the arrows on the left side are significant bivariate correlation coefficients read from Table I1. Numbers on the right side are significant part correlations based on a simple regression analysis.

Table IV. Stepwise regession analysis with maximum single root resorption as dependent variable (First run, with all variables included)

Step

I

1 2 3

Variable inctuded

I

Muhiple R-'*

Banding time History of trauma Lip/ tongue dysfunction Overjet

4

I

Change in R"

0.0858 0.1148 0.1309

0.0858 0.0290 0.0160

0.1419

0.0110

*R2 is the multiple correlation coefficient squared and an expression of "predicted" or "explained" variance.

Table V. Stepwise regession analysis with maximum single root resorption as dependent variable (Second run, excluding banding time)

Step 1 2 3 4 5

I

Variable i,lcluded Rectangular arch wire used History of trauma Lip/tongue dysfunction Impacted maxillary canine Class II elastics used

activators in young patients may be of specific interest. Mean apical root resorption for the single most severely resorbed maxillary incisor per patient was 1.5 mm, and the mean for individual maxillary incisors was close to 0.9 mm. Apical root resorption thus does not seem to be a major problem in orthodontic therapy. On the other hand, 16.5% of the patients had root short-

I

Muhiple R" 0.0280 0.0577 0.0716 0.0854 0.0990

I

Change in R" 0.0280 0.0296 0.0140 0.0138 0.0137

ening of > 2 . 5 in one or more maxillary incisors, which may be regarded as.significant from a clinical point of view. More knowledge about the patient characteristics and clinical features that contribute to apical root resorption would be desirable for finding ways of reducing this unwanted side effect from orthodontic treatment. Preliminary data analysis showed that the mean

Volume 99

Variables associated with root resorption

41

Number !

resorption for all four maxillary incisor teeth was a slightly more sensitive variable than the maximum single-root resorption per patient. Nevertheless, because of its greater clinical relevance, the latter was chosen as the dependent variable throughout the study. In general, regression was a poor predictor in terms of explained variance and error of estimate. This finding means that real causative factors were missing. Biologic variations, such as the potential of functional adaptation, individual tissue characteristics and cell reactions, patient cooperation, etc., probably play a major role. In addition, there are also biomechanical variables that are not specifically registered, such as the clinical use of round and square wires for some patients without the use of active torque and/or Class II elastics and the levels of force applied. This might be the reason why unspecified banding time was a good predictor from a s t a t i s t i c a l point of view. We suggest that future studies be designed as controlled experiments, encompassing more details on biomechanics, dentofacial functions, and actual tooth movements. For the aim of the present study, to single out pretreatment and treatment risk factors among the variables at hand and range them in relation to each other, the regression model seemed well suited. A normality plot of reciduals showed that the data fit well into the chosen simple linear model. Fig. 4 presents a summary of significant interrelationships between the most important variables. Overjet seemed to have an effect on resorption in the first regression (Table IV) and may be regarded as a risk factor. The graphic representation (Fig. 4) shows in more detail the conditions under which overjet probably would and would not result in apical root resorption. It is well known from other studies, as also shown here, that overjet promotes trauma to incisor teeth--another risk factor (Table IV). Overjet often requires treatment with fixed appliances, active torque with rectangular arch wires, and the use of Class II elastics, both risk factors for root resorption (Table V). Some cases of overjet are well treated with activators, which were n o t found to promote root resorption in our study. In patients treated both with activators and with fixed appliances, the total banding time was short, and the use of rectangular arch wires and Class II elastics was reduced, as evidenced by negative correlations in Table III. Lip/tongue dysfunction, including the history of prolonged finger-sucking habits, may be both a result for and a cause of increased overjet. The path of influence from lip/tongue dysfunction on apical root resorption may thus be indirect, through maintenance or increase of overjet, and direct, through action on therapeutic tooth movements.

The reasons for increased apical root resorption associated with clinical use of rectangular arch wires and Class II elastics probably are quite complex. Biomechanical force application to the apical periodontium is, by nature, indirect. Forces are applied to the dental crown, and local manifestations depend on a series of integrated tissue responses and functional adaptations in the system of dental support, as well as in related dentofacial structures. The extent and nature of such responses are the results of biomechanical, physiologic, and genetic influences. Clinical problems, therefore, are multivariate and should be approached methodologically as such whenever possible. From a biologic point of view it was intriguing to find that trauma to maxillary front teeth that had taken place more than 1 year before the initiation of treatment was the factor most strongly associated with increased root resorption during treatment. The anamnestic information included any acute trauma to the maxillary front segment noted by the patient or the parents to the extent that the patient experienced discomfort. The factor of trauma thus was not limited to individual incisors, to crown fractures, severe mobility or dislocation, or pulp devitalization. Individual observations showed that apical root resorption may occur independently of pulp obliteration and/or endodontic treatment, and it did not necessarily start early in treatment. The present study also suggests that persistent trauma, such as lip/tongue dysfunction and possibly inadequate adaptation to morphologic changes during orthodontic treatment, in its end effect, may come close to apical root resorption caused by acute trauma. There are indications that root resorption associated with trauma in general is dependent on tissue changes very near the root surface. 16 In the case of apical root resorption, that would mean very near the root tip. By extrapolation from other studies, such changes may be linked to cementum, t6 epithelial rests of Malassez, t8-2° pulp and periodontal vascularization, H'~3'16 cementoblast and periodontal fiber reorientation, 2! and diffusion of mineralized tissue. 22 Clinical implications of pulp tissue devitalization and inflammation have been revealed in studies related to endodontic treatment of traumatized teeth. 23'24 The role of epithelial cells has gained new interest in connection with periodontal reattachment studies.2~ Controlled studies of long-term effects and interactions between various qualities and extents of trauma are needed at both the clinical and the experimental levels. The factor of lip/tongue dysfunction was combined with the history of finger-sucking habits persisting beyond the age of 7 years (LTD). This was done with the hypothesis in mind that jiggling forces from soft tissues

42

Linge and Linge

over a period o f time m a y contribute to the occurrence o f apical root r e s o r p t i o n : Obviously, the clinical registration o f this variable was not precise. On the other hand, the registration was made independently o f the intention to study apical root resorption, and in the manner routinely available to the clinician. LTD turned out to be significantly associated with apical root resorption in the regression analysis. F r o m a clinical point o f view, this observation should increase our attention to details o f soft tissue function before and during orthodontic treatment. Diagnostic methods for soft tissue dysfunction should be improved. Orthodontic correction o f impacted maxillary canines in our study was identified as a risk factor for apical root resorption o f incisor teeth. This resorption was not confined to the adjacent lateral incisor. The possibility of root resorption's being caused by the ectopic eruption path o f the canine should not be overlooked, but it does not fully explain the pattern o f root resorption found in these patients. The function as anchorage for extrusive forces directed to the impacted canine implies intrusive forces to the incisors and thus compressive forces to the periodontal ligaments. Previous studies have shown compression to be closely related to resorptive changes at the corresponding root surfaces. H13 Intrusion o f maxillary incisors with a slightly modified Burstone technique in an experimental study I° caused considerably more apical root resorption than generally observed in samples o f orthodontic patients. 15.26.27 From the correlation matrix (Fig. 2), it m a y be seen that the presence o f impacted maxillary canines is significantly correlated with banding time. Since apical root resorption tends to remain active as long as treatment influences are maintained, 15 special attention should be given to these patients. Careful design of force systems and well-dispersed anchorage forces, combined with respectful consideration o f any other risk factors present, m a y help keep the risk o f apical root resorption at an acceptable level. Regression analysis, to some extent, may be helpful in predicting the overall risk for apical root resorption in a given patient. The rate o f success with alternative models o f risk prediction is the topic o f an ongoing study. The evidence that specific treatment variables m a y be responsible for increased apical root resorption suggests a need for further studies, also, with regard to timing, sequence, and methods o f treatment. In clinical work, even with knowledge o f any risk factors identified in the present study, it remains necessary to carry out radiographic control routinely and in all patients. Anatomic, physiologic, and probably genetic variations must be expected to cause corresponding

Am. J. Orthod. Dentofac. Orthop. January 1991

variations in tissue response, which at times may result in iatrogenic damage. For the benefit o f the patients, and as basis for evaluating alternative treatment approaches, these factors should be kept under control during treatment and eventually the process o f apical root resorption registered at early stages by routine, standardized, and adequate methods. We are grateful to Leiv Sandvik, cand. real., and to Dr. Marek Rrsler for valuable advice and for carrying out the statistical analysis.

REFERENCES

1. Ketcham AH. A progress report of an investigation of apical root resorption of vital teeth. Ircr J OR'mOO 1929;15:310-28. 2. Stuteville OH. Injuries caused by orthodontic forces and the ultimate result of these injuries. AM J ORTIIODORAL8URG 1938;24:103-16. 3. Rudolph CE. An evaluation of root resorption occurring during orthodontic treatment. J Dent Res 1940;19:367-71. 4. Rosenberg HN. An evaluation of the incidence and amount of apical root resorption and dilaceration occurring in orthodontically treated teeth having incompletely formed roots at the beginning of Begg treatment. AM J OR'moP 1972;61:524-5. 5. Morse PH. Resorption of upper incisors following orthodontic treatment. Dent Pract Dent Rec 1971;22:21-35. 6. Wainwright WM. Faciolingual tooth movement: its influence on the root and cortical plate. AM J On'moP 1973;64:278-302. 7. Hoeve AT, Mulie RM. The effect of anteroposterior incisor repositioning on the palatal cortex as studied in lamino,graphy. J Clin Orthod 1976;10:804-22. 8. Goldson L, Henrikson C. Root resorption during Begg treatment: a longitudinal roentgenographic study. AM J OR'moP 1975; 68:55-66. 9. Malmgren O, Goldson L, Hill C, Orwing A, Petrini L, Lundberg M. Root resorption after orthodontic treatment of traumatized teeth. AM J ORTnOD 1982;82:487-91. I0. Dermaut LR, De Munck A. Apical root resorption of upper incisors caused by intrusive tooth movement: a radi~raphic study. AM J ORTHODDEN'rOFACOR'rHOP1986;90:321-6. II. Reitan K. Initial tissue behaviour during apical root resorption. Angle Orthod 1974;44:68-82. 12. Kvam E. Adverse effects of orthodontic treatment. In: Thilander B, R0rming O, eds. Introduction to orthodontics. Stockholm: Tandl~karfrrlaget, 1985:225-52. 13. Rygh P. Orthodontic forces and tissue reactions. In: Thilander B, R0nning O, eds. Introduction to orthodontics. Stockholm: Tandl~ikarffdaget, 1985:205-24. 14. Newman WG. Possible etiologic factors in external root resorption. AM J OR'I'HOD1975;67:522-39. 15. Linge BO, Linge L. Apical root resorption in upper anterior teeth. Eur J Orthod 1983;5:173-83. 16. Andreasen JO. Summary of root resorption. In: The biol~ical mechanisms of tooth eruption and root resorption. Birmingham, Alabama: EBSCO Media 1988;399-401. 17. Remington DN, Joondeph DR, ]u'tun J, Riedel RA. Long-term evaluation of root resorption occurring during orthodontic treatment. AM J ORTHODDENTOFACORTHOP1989;96:43-6. 18. Spouge JD. A new look at the rests of Malassez: a review of their embryologic origin, anatomy, and possible role in periodontal health and disease. J Periodontol 1980;51:437-44.

Volume 99

Variables associated with root resorption

Number 1

19. Lindskog S, Bloml0f L, Hammarstrrm L. Evidence for a role of odont~enie epithelium in maintaining the periodontal space. J Clin Periodontol 1988;15:371-3. 20. Reitan K. Behaviour of Malassez' epithelial rests during orthodontic tooth movement. Acta Odontol Seand 1961;19:443-68. 21. Linge L. Tissue reactions in facial sutures subsequent to external mechanical influences. In: McNamara JA Jr, ed. Monograph No. 6, Craniofaeial Growth Series, Ann Arbor: Center of Human Growth and Development, University of Michigan, 1976:25175. 22. Grz G, Rakosi T, Rahn BA. Die Bedeutung der parodontalen Zirkulationsst6mng fiir Umbau und m6gliche parodontale Sch~idigung im Laufe einer kieferortop~idischen Behandlung. Fortschr Kieferorthop 1987;48:34-40. 23. Cvek M. Endodontie treatment of traumatized teeth. In: Andreasen JO, ed. Traumatic injuries of the teeth. 2nd ed. Copenhagen: Munksgaard, 1981:321-84.

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24. Hammarstrrm L, Bloml0f L, Feiglin B, Andersson L, Lindskog S. Replantation of teeth and antibiotic treatment. Endol Dent Traumatol 1986;2:51-7. 25. Egelberg J. Regeneration and repair of periodontal tissue. J Periodont Res 1987;22:233-42. 26. Sj01ien T, Zachrisson BU. Periodontal bone support and tooth length in orthodontically treated and untreated persons. AM J ORa-HOD 1973;64:28-37. 27. Ogaard B. Marginal bone support and tooth lengths in 19-yearolds following orthodontic treatment. Eur J Orthod 1988;180-6. Reprint requests to: Dr. Leif Linge Frognerveien 5, 3700 Skien Norway

AAO MEETING CALENDAR

1991--Seattle, Wash., May 11 to 15, Seattle Convention Center 1992--St. Louis, Mo., May 10 to 13, St. Louis Convention Center 1993--Toronto, Canada, May 16 to 19, Metropolitan Toronto Convention Center 1994--Orlando, Fla., May 1 to 4, Orange County Convention and Civic Center 1995--San Francisco, Calif., May 7 to 10, Moscone Convention Center 1996--Denver, Colo., May 12-15, Colorado Convention Center