ORIGINAL ARTICLE

Eruption of the permanent study

upper canine: A radiologic

Enrique Ferndndez, MD, DDS, a Luis Alberto Bravo, MD, DDS, MS, PhD, b and Manuel Canteras, MD, PhD c Murcia, Spain

The early detection of eruptive anomalies of the upper canine requires an understanding of its normal eruptive pattern. We studied this pattern in terms of upper canine inclination and its relation to the lateral incisor, on the basis of the panoramic radiographic records of 305 children aged 4 to 12 years. The study sample comprised 554 maxillary canines in the oral pre-emergence phase of eruption. Subject age, sex, inclination of the canine (CI), its relation to the lateral incisor (RCLI), and development of the lateral incisor (DLI) were evaluated. The results show that the canine erupts, increasing its inclination mesially until a maximum is reached, at about 9 years of age, after which the tooth begins to progressively upright itself. The individual variability of the degree of CI at a given age is considerable. In the initial stages, the RCLI is most commonly characterized by overlapping, a situation rarely seen in the final stages. The DLI effectively discriminates both periods, because when DLI is incomplete, more than half of the cases have an overlapping RCLI. In contrast, when the DLI is complete, this overlapping is seen in only 7% to 11% of the cases. This variable therefore increases the capacity to detect a possible eruptive anomaly at an early stage. In patients with complete DLI and overlapping RCLI, particularly when associated with other clinical signs such as the nonpalpation of the cuspid bulge in the alveolar process, the presence of dental agenesis, ankylosis, malformations, or ectopic eruptions, extraction of the primary canine is advised to prevent impaction. (Am J Orthod Dentofacial Orthop 1998;113:414-20.)

I m p a c t i o n most commonly involves the lower third molar, followed by the permanent upper canine. Approximately 1.5% to 2% of the general population experiences impaction of the upper ca- n ine] -4 which implies a number of repercussions. In esthetic terms, and as pointed out by Dewell, 5 "its strategic position at the angle of the arch is signifi- cant in the maintenance of harmony and symmetry of occlusal relationship and in determining the contours of the mouth as a whole." Functionally, the lack of canine guidance has negative consequences on joint dynamics, 6 and in the neighboring teeth, impaction leads to a high frequency of root resorp- tion. 7-9 Treatment of canine impaction, on the other hand, is complex, prolonged, and of uncertain out- come. In addition, it adversely affects the periodon- tal condition of the canine. 1~

The early detection of this eruptive disorder is therefore of fundamental importance if preventive measures are to be adopted. On this matter, Lap-

From the University Dental Clinic, Faculty of Medicine, University of Murcia, Spain. "Visiting Professor of Orthodofitics. In private practice. bTitular Professor of Orthodontics. ~ and Chairman of Biostatistics. Reprint requests to: Enrique Fernfindez, MD, DDS, Ronda de Levante, 5 - 1 ~ B, 30008, Murcia, Spain. Copyright �9 1998 by the American Association of Orthodontists. 0889-5406/98/$5.00 + 0 8/1/82410

pin ~5 quoted Broadbent who, in a previous commu- nication, recommended extraction of the primary canine when the permanent tooth presents signs of altered eruption. A number of other authors 162a favor the same procedure in the presence of radio- logic evidence of altered eruption, as expressed by an anomalous inclination of the canine and/or over- lapping of the canine and root of the permanent lateral incisor. These characteristics vary, however, in the course of normal eruption.

The aim of the current study was to analyze the eruption of the upper canine, in terms of its incli- nation and relation to the lateral incisor on the basis of panoramic radiographic records.

MATERIAL AND METHODS

A transverse retrospective study was made of the panoramic radiographs of 305 children (145 boys and 160 girls) taken from a pediatric dentistry practice. Inclusion criteria were patient age of 4 through 12 years and the absence of trauma, agenesis, or supernumerary teeth in the study zone. The children evaluated had not been treated orthodontically, and all had good quality pan- oramic radiographs. Patients treated in a pediatric den- tistry practice were selected to secure characteristics com- parable with those of the general population and avoid the potential bias implied by introducing patients seeking

414

American Journal of Orthodontics and Dentofacial Orthopedics Ferndndez, Bravo, and Canteras 415 Volume 113, No. 4

Table I. Sample distribution by age and sex

Age (years) Boys Girls Total

4 15 17 32 5 16 14 30 6 17 23 40 7 20 23 43 8 16 18 34 9 15 17 32

10 16 17 33 11 15 16 31 12 15 15 30

orthodontic treatment. The patient age and sex distribu- tion are shown in Table I.

All radiographs were obtained by the same operator and apparatus (Siemens Model Orthoceph 5 orthopanto- mograph). The radiographic measurements were likewise performed by the same individual (E. F. A.).

The following variables were studied:

(1) Patient sex and age (age recorded to one decimal point).

(2) Canine inclination (CI): Following the method described by Bjerklin and Kuro123 for the first maxillary molars, we measured the external angle formed by the major axis of the canine and the straight line through both suborbitary points (Fig. 1), an angle of 90 ~ thus corresponded with a canine perpendicular to the reference plane. If greater, the angle indicated mesial CI.

(3) Relation of the canine to the lateral incisor (RCLI): Two possibilities were considered ac- cording to whether the crown was distal or mesial to the distal border of the lateral incisor (Fig. 1). When mesial, the two teeth were overlapping.

(4) Development of the lateral incisor (DLI): Devel- opment was regarded as complete when the tooth had fully erupted and possessed a fully formed root, even in the presence of an open apex. This situation corresponded with stage no. 9 in the classification of Nolla. 24 All other cases repre- sented incomplete development.

Finally, the number of cases in which the canine had not yet commenced root formation was recorded (112 canines; 63 from boys and 49 from girls). The number of canines that had reached the alveolar bone crest level also was determined (34 canines; 14 from boys and 20 from girls). Because the study centered on the eruptive period before emergence, we excluded all cases (56 canines) in which the canine had already passed the alveolar bone crest level. As a result, the whole study sample consisted of 554 upper canines (271 from boys and 283 from girls).

Statistical analysis of the results was carried out using BMDP statistical software (University of California Press). In addition to descriptive statistics (mean and

Fig. 1. Canine inclination (CO is measured by the ex- ternal angle formed by the major axis of the canine and a straight line through both suborbitary points. The relation of the canine to the lateral incisor (RCLI) is distal on the right side and mesial on the left side of the figure. The development of the lateral incisor (DLI) is complete on the right side and incomplete on the left side.

standard deviation), the Student t test and analysis of variance were performed for comparing means. Correla- tion and regression analyses between quantitative vari- ables also were used. The X 2 test was used for contrasting qualitative variables, applying the Yates correction in the case of small samples. The adjusted standardized deviates established significantly associated values. Stepwise logis- tic regression was in turn used to jointly relate quantitative and qualitative variables.

Method Error

Different studies have confirmed the reliability of angular measurements in panoramic radiographsY 28 Two radiographs were taken randomly for each age group (18 subjects and 36 canines) to analyze the reliability of the values taken by our variable CI. On three different days, three repeated measurements of the inclination of each canine were performed by the same person (E. F. A.). The "intraclass correlation coefficient ''29 was calculated (r = 0.992), showing a high reliability of the method used. Twelve canines that had not yet begun to develop their roots also were analyzed to test the reliabil- ity of the method in those canines with a lesser develop- ment and therefore with an apparent greater ditficulty in drawing the tooth axis. A high correlation coefifcient also was found in this situation (r = 0.980).

RESULTS Inclination of the Canine

For descriptive purposes, the study sample was divided into age groups to de termine the mean inclination for each age (Table II) . The differences observed were highly significant. No significant dif- ferences were no ted between the sexes. Figure 2

416 Fern~ndez, Bravo, and Canteras American Journal of Orthodontics and Dentofacial Orthopedics April 1998

cl r 108

104

100

96

92

88

84

80

. 9 9

4 5 6 7 8 9 10 11 12

AGE (years)

Fig. 2. Mean canine inclination (Cl) at different ages.

Table I I . Can ine incl inat ion at different ages

A .F r (yearn) N Mean (~ SD (~ Max. (~ Min. (~

4 64 91 5 109 78 5 60 91 5 113 82 6 80 93 5 110 81 7 86 97 6 114 85 8 68 98 6 115 87 9 64 99 7 120 85

10 64 95 7 115 81 11 37 91 6 106 76 12 31 91 5 105 80

The ANOVA test shows that the difference among the means is significant (p < 0.0001).

graphically illustrates the evolution of mean inclina- tion for each age group.

To analyze canine inclination before the start of eruption, we selected those cases in whom root forma- tion had not yet begun. We recorded an angle of 91 ~ _+ 6 ~ (mean _+ standard deviation), On the other hand, inclination at emergence was 8 8 ~ + 5 ~ . In neither group were differences observed between boys and girls.

The behavior of CI in the course of eruption was analyzed by applying a regression analysis between CI and age. We examined the cases in whom the root was at least partly formed. As CI does not behave linearly through eruption (Fig. 2), we ap- plied a second-order polynomial regression analysis. A highly significant correlation was obtained (p = 0.0001; r = 0.41), with no differences observed between the sexes (Fig. 3). The highest point on the curve, that is, the greatest inclination, corresponded with an angle of 98 ~ _+ 6 ~ and an age of 9 years.

c l (o)

108

104

100

96

92

88

84

80 5 6 7 8 9 10 11 12

Age (years)

V=-0'7x2+12'6x+41'3 SE=6 R=0'41 P=0'0001

Fig. 3. Second-order polynomial regression between canine inclination (CI) and age.

Relation of the Canine to the Lateral Incisor

Table III shows the cases in whom the canine was observed distal or mesial to the distal border of the permanent lateral incisor for each age group. Adjusted standardized deviates showed that up to age 6 years, overlapping was significantly more frequent. At age 7 years, differences were no longer observed, and from age 8 years onwards, distal positioning became significantly more frequent.

To analyze the factors that might be related to such changes, we carried out a logistic regression analysis with all the variables studied (Table IV). The DLI was the variable that individually exhibited the closest relation to the RCLI.

Table V shows the frequency distribution of RCLI in the total sample for the different cases of DLI. A X 2 analysis showed the frequency difference to be statistically significant, whereas residuals anal- ysis demonstrated that an incomplete DLI was more related to a mesial RCLI (67% versus 33% distal RCLI). In contrast, a complete DLI was related to a distal RCLI (89% versus 11% mesial RCLI).

To corroborate the discriminating capacity of DLI, however, it would be better to use the ages at which both variables exhibit behavioral changes, rather than the total sample where, as may be seen in Table III, the extreme ages present clear differ- ences. Table VI shows that the DLI is in most cases completed between the ages 7 and 9 years.

Table V presents the frequency distribution of these ages. In a global analysis of the 7-year-old to 9-year-old age interval, 36% of the canines appeared to be overlapping the lateral incisor. When the sample was divided as a function of DLI, however,

American Journal of Orthodontics and Dentofacial Orthopedics Ferndndez, Bravo, and Canteras 4 1 7 Volume 113, No. 4

Table III. D i s t r ibu t ion of the re la t ion of the can ine to the la te ra l incisor ( R C L I ) a t different ages

Distal RCLI

Age I (years) N %

27 17 35 57 68 70 86 89 94

ASD N

Mesial RCLI

I % I ASD

4 17 - 5 47 73 5 5 10 - 5 50 83 5 6 28 - 4 52 65 4 7 49 0 37 43 0 8 46 2 19 30 - 2 9 45 2 19 30 - 2

10 55 5 9 14 - 5 11 33 4 4 11 - 4 12 29 4 2 6 - 4

Adjusted standardized deviates (ASD) are significant when equal or greater than 2. This table shows that mesial RCLI is related to age from 4 to 6 years, whereas distal RCLI is related to age from 8 to 12 years.

overlapping was seen in 57% of cases when devel- opment was incomplete. In contrast, overlapping was observed in only 10% of cases when DLI was complete. Residuals analysis also reflects the statis- tical significance of the association between incom- plete DLI and mesial RCLI, and between complete DLI and distal RCLI for the same age group.

Finally, of the children older than 9 years who exhibited a fully developed incisor (n = 184), the RCLI was 93% distal and 7% mesial, whereas in the four patients in whom the incisor development was incomplete, mesial RCLI appeared in 50% (Table V).

Association Between CI and RCLI

Table VII shows the average CI values for the different RCLI values in the total series and in the subsamples corresponding to different developmen- tal stages of the lateral incisor. On considering both the total sample and the incomplete DLI subgroup, we observed no differences in CI between the dif- ferent RCLI values (94 ~ and 93 ~ respectively). When the DLI was complete, however, the CI was found to be significantly greater in the cases of canine overlapping on the incisor (103 ~ versus 95~

DISCUSSION

The results obtained show that the upper canine initially erupts with increasing mesial inclination (first rapidly and then more slowly) until a maximum point is reached, followed by gradual straightening distally (Fig. 2). This particular behavior was shown by the highly significant statistical correlation of the second-order polynomial regression analysis applied (p = 0.0001, Fig. 3). Broadbent 3~ established that "the upper cuspids move downward, forward and laterally away from the root ends of the laterals". Moyers 31 reported that at age 3 years, the canine is positioned high within the maxilla, with the crown

Table IV. Stepwise logis t ic r egress ion of all the var iables , wi th r e l a t ion of the can ine to the la te ra l incisor ( R C L I ) be ing the d e p e n d e n t va r i ab le

Variab~s Relative

Coefficient SE risk

DLI 1.33 0.13 3.79 Constant 0.71 0.13

Development of the lateral incisor (DLI) is the variable that individually relates to RCLI. SE, standard error.

directed mesially. Posteriorly, it displaces toward the occlusal plane, straightening gradually until it ap- pears to contact the distal aspect of the root of the lateral incisor, deviating toward a more vertical position. According to our results, this "lateral movement" and "gradual straightening" of the ca- nine would take place in a second eruptive stage, at approximately 9 years of age, whereas in a first stage, the canine would move toward the lateral incisor, increasing its mesial inclination.

Before commencing to erupt, the dental follicle of the canine is positioned above the follicle of the first premolar, 3~ because it is limited mesially by the piriform aperture (Fig. 4). Once eruption has started, and as a result of maxillary growth, a certain mesial displacement must occur to eventually reach a correct position within the arch. After the mesial displacement, a rectification is required, with straightening and completion of the eruption verti- cally. This straightening coincides with the age at which an important clinical sign appears, that is, palpation of the cuspid bulge in the vestibular aspect of the alveolar process. This suggests that movement takes place not only distally but also in a vestibular direction.

According to our results, on average, the pre- eruptive canine is vertical (91~ Posteriorly, it

418 Ferndndez , Bravo, a n d Canteras American Journal of Orthodontics and Dentofacial Orthopedics April 1998

Table V. Relation of the canine to the lateral incisor (RCLI) for the different development stages of the lateral incisor (DLI) in the total sample and in the subsample 7 to 9 years of age and greater than 9 years of age intervals

RCLI (total sample) * RCLI (7-9 years) "/- RCLI (>9 years)$

Distal Mesial Distal Mesial Distal

DL1 N % ASD N % ASD N % ASD N % ASD N % ASD N

Incomplete 109 33 - 1 3 217 67 13 52 43 - 7 68 57 7 2 50 - 3 2 Complete 203 89 13 25 11 -13 88 90 7 10 10 - 7 171 93 3 13 TOTAL 312 56 0 242 44 0 140 64 0 78 36 0 173 92 0 15

Mesial

I % ASD

50 3 7 - 3 8 0

*x 2 = 168p < 0.0001; ?x 2 = 50 .7p<0 .0001; ~x a (Yates correction) = 4.8, p < 0.0001. ASD, a~usted standardized deviates.

Table VI. Development of the lateral incisor (DLI) as a function of patient age and sex

Age (years)

DL1 (boys) DL1 (girls)

Incomplete Complete Incomplete Complete

N % % N % % N N I 4 30 100 0 0 34 100 0 0 5 32 100 0 0 28 100 0 0 6 32 94 2 6 46 100 0 0 7 38 95 2 5 33 72 13 23 8 18 56 14 44 14 39 22 61 9 10 33 20 67 7 21 27 79

10 2 6 30 94 2 6 30 94 11 0 0 23 100 0 0 14 100 12 0 0 18 100 0 0 13 100

Table VII. C a n i n e incl inat ion (CI ) as a function of the relation of the canine to the la tera l incisor ( R C L I ) and the development of the lateral incisor ( D L I )

Complete DLI* Total sample N Incomplete DLI

RCLI Mean (~ SD (~ Mean (~ SD (~ Mean (~ SD (~

Distal 312 94 7 109 93 6 203 95 7 Mesial 242 94 7 217 93 6 25 103 7

*t = - 4 . 5 , p < 0.0001.

reaches maximum inclination (98~ to ultimately straighten again in a vertical position (88 ~ at the time of emergence. The degree of inclination at each age (Table II), however, varies greatly from one individual to another, as reflected by the high standard deviation of the samples, and by the low correlation coefficient of the regression (r = 0.41). This considerably limits the predictive capacity of the regression curve; that is, the latter does not allow us to precisely predict canine inclination at a given age.

The results obtained also show that in the initial stages, the most frequent RCLI is mesial, that is, presenting an overlapping between both of them. In

contrast, in the final stages and as a result of straightening of the canine, this situation is rarely seen. To determine the moment at which this change takes place, we performed a logistic regres- sion analysis and observed a greater correlation with the DLI than with age, showing us that this variable is more related to dental development than to chronologic age.

If the total sample were divided as a function of the DLI, 33% of the cases with incomplete DLI were found to exhibit distal RCLI, versus 67% with mesial RCLI. In contrast, when DLI was complete, distal RCLI was observed in 89% of cases and mesial in only 11%. On considering the ages at

American Journal of Ol~hodontic~ and Dentofucial Orthopedics" Ferndndez, Bravo, and Canteras 419 Volume 113, No. 4

which DLI is supposedly completed, that is, the 7-year-old to 9-year-old age interval, the differences remained significant. Forty-three percent of cases with incomplete DLI exhibited distal RCLI, with a mesial relation in 57%. In turn, when incisor devel- opment was complete, 90% of the canines presented a distal RCLI, versus only 10% with a mesial relation. Interestingly, if we concentrated on patient age only, overlapping was observed in 36% of cases of subjects in the 7-year-old to 9-year-old age inter- val, whereas on introducing the variable DLI, over- lapping was found in 57% of cases of incomplete development and in a mere 10% of cases with complete DLI. This variable therefore increases the capacity to detect a possible eruptive anomaly at an early stage. Of the cases of subjects aged 10 years or older, only 4 exhibited incomplete DLI. Of these, 50% (n = 2) presented overlapping, whereas among the 184 subjects with complete DLI, overlapping was seen in only 7%.

Lindauer et al. 32 carried out a retrospective study of 41 impacted canines. In previously obtained panoramic radiographs (average age 12 _+ 1 years), they found that 78% exhibited overlapping between the canine and lateral incisor. In contrast, they found overlapping in only 4% of the 71 control canines that did not develop impaction. They con- cluded that such relations detected in panoramic radiographs during the mixed dentition period con- stitute a prognostic sign of canine impaction. Ac- cording to our results, this sign is not applicable in the entire mixed dentition period, but only after a point that coincides with complete development of the lateral incisor. Therefore DLI is a good refer- ence for defining when a radiograph overlapping image constitutes a sign of altered canine eruption. In the same way, Ericson and Kuro133'34 demon- strated the predictive value of the absence of cuspid bulge palpation in the vestibular aspect of the alveolar process after the age of 10 years.

The "geneticist" or "mechanistic" interpretation of the cause of canine impaction has recently gen- erated controversy, 35'36 although these explanations need not be conflicting. The results of the current study show that the CI is not related to RCLI during the period in which overlapping is most frequent, that is, with incomplete DLI. In the patients with complete DLI, however, a relationship was indeed observed between mesial RCLI and increased incli- nation. In this group, overlapping would be a con- sequence of the displaced eruptive trajectory of the canine, whereas in the former, RCLI would be more dependent on the conditions of space within the

Fig. 4. Pre-eruptive situation of the canine and the first premolar.

apical area, being more probably a distal RCLI when space is greater.

A possible hypothesis for the triggering of canine impaction is that genetic predetermination leads to a canine eruptive pattern with a greater palatal inclination. Because the canine is in a vestibular position with respect to the lateral incisor, 37 in those patients in whom the two teeth are overlapping, the incisor would avoid the palatal deviation of the canine, frequently undergoing some kind of dis- placement or resorption of its root at the vestibular aspect. 8 When overlapping does not occur, however, canine deviation becomes possible. This would hap- pen either because of excess space (a condition to which different authors have related palatal impac- tion of the canine 5,3~'39) or because of agenesis or microdontia of the lateral incisor, to which it also has been related. 4~

In the event of such deviation, the stimulus to initiate resorption of the root of the deciduous canine may not occur, and this tooth (along with the lateral incisor that now lies vestibular to the canine) would prevent posterior physiologic straightening as initially described. The result of this would be palatal impaction. Extraction of the deciduous ca- nine, however, would effectively allow straightening to occur, as demonstrated by a number of au- thors. 21,22 Also the agenesis of the lateral incisor would allow the straightening of the adjacent canine in many instances. This would explain the lower frequency of canine impaction observed in cases of agenesis of the lateral incisor versus cases of a peg-shaped lateral incisor. 41

In contrast to what has been said of those cases with complete DEI, this hypothesis implies that in the initial stages with incomplete DLI, RCLI in

420 Ferndndez, Bravo, and Canteras American Journal of Orthodontics and Dentofacial Orthopedics April 1998

panoramic radiographs would not constitute a good reference for evaluating canine eruption. This is because in the event of a distal RCLI, palatal deviation becomes possible, whereas when mesial, the palatal deviation may already have occurred. Therefore in high risk subjects, that is, subjects with relatives affected by impacted canines or subjects present with other associated disorders such as agenesis, ankylosis, microdontia, 42,43 a careful radio- graphic follow-up study is advisable, and one might even resort to intraoral films or more sophisticated radiologic techniques to obtain the exact position of the canine in relation to the lateral incisor.

CONCLUSIONS

During eruption, the upper canine increases its incli- nation mesially, until a maximum angle is reached at approximately 9 years of age. From this inflexion point onwards, the tooth progressively straightens until it emerges above the gingival margin.

Considerable individual variability exists as to the degree of canine inclination at a given point in the course of eruption, therefore the capacity to predict inclination at a given age is limited.

When the lateral incisor is not yet fully developed, panoramic radiographs more commonly show overlapping of the canine and lateral incisor. In contrast, when lateral incisor development is complete, such overlapping is rare; moreover, the few cases in which it is observed involve a greater mesial inclination of the canine.

The overlapping of the canine and lateral incisor in panoramic radiographs when the incisor has completed its development may be a sign of eruptive disorders of the canine, suggesting the adoption of preventive measures to avoid impaction; for example, extraction of the primary canine. This is particularly applicable to patients in whom the cuspid bulge is not palpated in the vestibular aspect of the alveolar process after age 10 years, or when other dental development disorders are present, such as agen- esis, ankylosis, dental malformations, or ectopic eruptions.

We thank Dr. Luz Aguil6 for permitting us access to the records of the patients used in this work. We also thank Dr. Jiiri Kurol for his assistance and suggestions in the reviewing of this manuscript.

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