An examination of proportional root lengths of the mandibular canine and premolars near the time of eruption

Introduction

To make treatments more efficient, orthodontists require a more precise means of estimating tooth eruption. The purpose of this article was to extend the information derived from dental staging techniques by incorporating direct measurements of root lengths for 3 mandibular teeth: the canine and the 2 premolars.

Methods

The full sample consists of 227 panoramic films from 77 female patients and 229 films from 74 male patients treated at a practice in Texas. A subsample of paired preeruption and posteruption films was examined in greater detail (46 subjects for the canine, 42 for the first premolar, and 46 for the second premolar).

Results

Canines will most likely have root length ([root length/total tooth length] × 100) of approximately 70% and premolars will have approximately 65% near the time of alveolar eruption. In addition, the walls of the root canal will probably approach a parallel configuration at the time of eruption.

Conclusions

The percentage of root length, along with root canal parallelism, should allow improved prediction of eruption timing.

Orthodontists often must extend their treatment times waiting for teeth to erupt. Many reports of clinical (gingival) eruption timing have been published, most of which examined children cross-sectionally. Fewer studies have explored tooth development as a maturational process. Among those that did, Moorrees et al and Demirjian are frequently cited. The system of Moorrees et al uses the morphology and extent of crown development and the fractional root lengths to gauge development; that of Demirjian provides descriptive information to allow assessment of stages, similar to the method of Tanner et al for hand-wrist skeletal maturation. Just as Tanner’s system enables assignment of a skeletal age, Demirjian’s allows one to assign a dental age to a patient based on the overall maturity of the mandibular dentition. However, it can be difficult to select the best Demirjian stage to assign, and children can remain in 1 stage for more than 2 years in some cases. Most importantly, the degree to which overall dental maturity corresponds to the probability of imminent eruption of a particular tooth remains largely unknown. A combination of morphologic description and metric analysis might provide the best approach for predicting eruption in clinical settings.

A selective overview of previous studies evaluating the relationship between root length and eruption follows. For a more extensive literature review, see the article by Smith and Buschang. Importantly, all approaches require knowledge about the final, adult root lengths (which obviates their use in clinical settings) and considerable expertise. With respect to expertise, Demirjian criticized the use of fractional staging, noting that even experienced raters can have difficulty deciding between close fractions and criticized that there is considerable individual variation in absolute root lengths.

Using the technique of Moorrees et al, Grøn reported that the mandibular premolars erupted with approximately three quarters of their root complete, with the canine erupting just beyond this fractional root length. Consistent with this finding, Moorrees et al reported that the canine tended to have proportionally longer roots than did premolars at clinical emergence.

Haavikko, in a large cross-sectional study of children in Helsinki, Finland, developed a staging technique similar to that of Moorrees et al. She distinguished between alveolar and clinical eruption, both assessed from radiographs. Roots three quarters complete closely preceded alveolar eruption of the canine; the same fractional root length lagged behind alveolar eruption for the premolars, with a delay of as much as 1.5 years ( Table I ).

Table I
Median and mean ages (y) of eruption derived from previous studies
Haavikko Anderson et al Demirjian and Levesque
Root ¾ Alveolar eruption Stage 11 (root ¾) Stage G Emergence
Girls
Canine 8.7 8.8 8.1 9.6 9.6
First premolar 10.4 9.1 8.9 10.1 10.3
Second premolar 10.7 9.2 9.6 11.1 11.2
Boys
Canine 9.6 9.8 9.4 10.6 10.5
First premolar 10.4 9.6 10.2 10.8 10.7
Second premolar 11.1 10.3 10.6 11.6 11.6

Anderson et al, using the method of Moorrees et al to stage radiographs from the Burlington longitudinal study, reported mean ages closer to the median ages of the root at three quarters found by Haavikko for boys than for girls; Demirjian and Levesque found that the onset of stage G, when the 2 sides, or walls, of the root canal assume a parallel configuration, nearly coincided with clinical emergence in their longitudinal series of French-Canadian children ( Table I ).

Marks and Schroeder stressed that root formation is a consequence, not a cause, of eruption. Nonetheless, there is clearly a functional association. A tooth cannot erupt into functional occlusion without sufficient supporting root length. Root lengthening is a progressive process only approximately delineated by fractional root lengths or shape descriptions. Perhaps a quantified root length combined with both a morphologic evaluation of an individual tooth and consideration of the state of overall dental maturation can provide a better assessment of maturity. The purpose of this article was to explore the relationship between proportional root length (ie, root length relative to total tooth length) and tooth eruption for the mandibular canine and premolars. The goal was to provide orthodontists with a clinical tool to optimize the timing of treatment.

Material and methods

The full sample consisted of 77 girls (aged 6.25-15.36 years; 227 panoramic films) and 74 boys (aged 6.32-16.10 years; 229 films) who received orthodontic treatment at a practice in north-central Texas. Most of the patients were white, and the data did not allow separate analyses by ancestral group. Patients with recorded mandibular deciduous or permanent (except third molar) extractions were excluded, as were those with supernumerary or missing teeth and those having had mandibular oral surgery. Dental maturity was assessed by using the staging technique of Demirjian for 7 mandibular teeth (incisors, canine, premolars, and first and second molars). Alveolar eruption was recorded as having occurred once the level of the bony margin had been exceeded by any cusp. Total tooth length was measured as the distance from cusp tip to root apex for canines and from the higher cusp to the root apex for premolars. Root length was measured from the root bifurcation at the apex formed by the walls of the pulp chamber (ie, from the most superior aspect of the walls of the pulp chamber) to the root apex ( Fig 1 ). The percentage of root length was calculated as root length divided by total tooth length, multiplied by 100.

Fig 1
Demirjian stages E, F, and G for canines and premolars. Measurements of root length and total tooth length are shown for a premolar in stage F.

A subsample examined in greater detail consisted of paired preeruption and posteruption films for each subject, for each tooth considered separately. Subjects whose closest preeruption and posteruption films for each tooth considered individually did not exceed a time interval of 2 years were chosen for inclusion. For the canine, 46 subjects (23 boys, 23 girls) met this requirement; for the first premolar, 42 subjects (21 boys, 21 girls) had such paired films, and for the second premolar, 46 subjects (29 girls, 17 boys) did. Statistical analyses were performed by using SPSS software (SPSS, Chicago, Ill).

Results

An intraobserver reliability test conducted with 50 films yielded 89% agreement in staging and 98% agreement in judging whether alveolar eruption had occurred. The means and standard deviations of the differences in measurement and percentages of root length are given in Table II .

Table II
Intraobserver mean differences (and standard deviations) for measurements and percentages
Mean (SD)
Canine root length (mm) 0.50 (0.96)
Canine total length (mm) 0.27 (0.60)
First premolar root length (mm) 0.32 (0.84)
First premolar total length (mm) 0.22 (0.51)
Second premolar root length (mm) 0.23 (0.53)
Second premolar total length (mm) 0.34 (0.44)
Second molar width (mm) 0.06 (0.25)
Canine root length (%) 1.19 (3.20)
First premolar root length (%) 0.77 (2.93)
Second premolar root length (%) −0.08 (2.67)

Correlations for the full sample (456 films from 77 girls and 74 boys) among ages, root lengths, and percentages of root lengths are presented in Table III . All correlations were significant at the 0.01 level. As expected, there was a strong correlation (r = 0.91) between chronologic age and dental age. Dental age correlated marginally better with root lengths and percentage of root lengths than did chronologic age. The first premolar root length correlated with the root lengths of both physically adjacent teeth at about the same level (r = 0.94 with the second premolar; r = 0.91 with the canine), whereas the second premolar root length correlated slightly better with the first premolar root length than with canine root length (r = 0.86 for the latter). As expected, each root length correlated well with its own percentage of root length (r ≈ 0.9), but, although canine root length correlated nearly as well with percentage of root lengths for the premolars as with its own percentage of root length, for the premolars, the correlations were better with the percentage of root length of the other premolar than they were with the percentage of canine root length. Correlations among percentages of root lengths were moderately high (r ≈ 0.8).

Table III
Full sample (n = 456) correlations
CA CRtL P1RtL P2RtL %CRtL %P1RtL %P2RtL
DA 0.91 0.84 0.90 0.92 0.73 0.84 0.85
CA 1 0.81 0.89 0.89 0.67 0.81 0.80
CRtL 1 0.91 0.86 0.87 0.84 0.82
P1RtL 1 0.94 0.76 0.92 0.86
P2RtL 1 0.72 0.85 0.92
%CRtL 1 0.77 0.76
%P1RtL 1 0.85
CA , Chronologic age (y); DA , dental age (y); CRtL , canine root length (mm); P1RtL , first premolar root length (mm); P2RtL , second premolar root length (mm); %CRtL , percentage of canine tooth length comprised of root; %P1RtL , percentage of first premolar tooth length comprised of root; %P2RtL , percentage of second premolar tooth length comprised of root.

All correlations were significant at the 0.01 level.

Correlations for the smaller sample, individually by tooth class, are given in Table IV . For each tooth considered separately, these sampled subjects had 2 films no more than 2 years apart between which alveolar eruption occurred. Before eruption, dental age correlated better with percentage of root length than did chronologic age. After eruption, this was not the case for the first premolar; for the canine, posteruption age correlations with percent root length were nonsignificant. Correlations of root length with percentage of root length were strong before eruption (r = 0.80-0.86, P <0.001) but declined after eruption, especially for the canine (r = 0.60, P <0.001), indicating greater variation in the length of canine roots for a given tooth length than for premolars. For the canine before eruption, dental age correlated with percentage of root length at the same level as it did with maturational age, defined as the difference between dental age and chronologic age. In other words, children with higher dental ages tended to have greater percentages of root lengths to the same degree that they were early maturers. For the premolars, however, correlations between dental age and percentage of root length were greater than those between dental age and maturational age, so being an early maturer was less strongly associated with dental age per se than was having a greater root length for tooth length. Posteruption maturational age correlations with dental age were low and nonsignificant; dental age correlations with percentage of root length remained moderate for the premolars, but this correlation declined to nonsignificance for the canine. Only for the canine in the preeruption period was maturational age significantly correlated with root length (r = 0.30, P = 0.045) and with percentage of root length (r = 0.32, P = 0.033). Being an early maturer showed some positive relationship with having longer roots, and longer roots for a given tooth length, for the canine but not for the premolars.

Table IV
Small sample correlations
Preeruption Posteruption
CA RtL %RtL MatAge CA RtL %RtL MatAge
Canine (n = 46)
DA 0.78 0.79 0.48 0.49 0.67 0.62 0.27 0.28
CA 1 0.67 0.31 −0.17 1 0.54 0.18 −0.52
RtL 1 0.80 0.30 1 0.60 0.02
%RtL 1 0.32 1 0.07
First premolar (n = 42)
DA 0.80 0.82 0.63 0.33 0.69 0.71 0.50 0.26
CA 1 0.81 0.56 −0.31 1 0.76 0.51 −0.52
RtL 1 0.85 0.02 1 0.78 −0.18
%RtL 1 0.12 1 −0.10
Second premolar (n = 46)
DA 0.76 0.75 0.63 0.29 0.63 0.64 0.49 0.09
CA 1 0.67 0.49 −0.39 1 0.59 0.36 −0.72
RtL 1 0.86 0.08 1 0.77 −0.19
%RtL 1 0.17 1 −0.02
CA , Chronologic age (y); DA , dental age (y); RtL , root length (mm); %RtL , percentage of tooth length comprised of root; MatAge , maturational age (ie, DA – CA). Eruption was alveolar.

Significant at the 0.01 level

significant at the 0.05 level.

Means and standard deviations for the smaller samples are given in Table V . For the canine, boys and girls were significantly different for both chronologic and dental ages before and after eruption, but no root lengths or percentage of root lengths differed significantly by sex. For the first premolar, root lengths were significantly longer in the boys, but the percentages of root length differences remained insignificant. For the second premolar, no sex differences were significant, probably because fewer than 20 boys were represented. The average midpoints of the percentage of root lengths suggested that the canine undergoes alveolar eruption when its root length is approximately 70% of the total tooth length, whereas the premolars erupt with slightly shorter percent root lengths, closer to 65%.

Table V
Small sample means and standard deviations
CAa0 CAa1 DAa0 DAa1 RtLa0 RtLa1 %RtLa0 %RtLa1 Mid%RtL
Canine
Girls (n = 23)
Mean 8.58 10.21 9.20 11.16 14.31 18.45 67.80 72.87 70.33
SD 0.87 0.95 1.17 0.95 2.08 1.74 5.25 3.99 4.34
Boys (n = 23)
Mean 9.74 11.32 10.25 11.98 15.64 19.54 67.13 72.87 70.00
SD 1.28 1.19 1.41 1.03 2.98 2.34 6.02 3.24 3.87
First premolar
Girls (n = 21)
Mean 8.65 10.30 9.35 11.22 9.75 14.54 60.35 68.28 64.31
SD 1.06 1.10 1.22 1.16 1.59 1.82 4.22 2.95 2.97
Boys (n = 21)
Mean 9.70 11.26 10.28 11.98 11.12 16.08 61.95 69.32 65.63
SD 1.38 1.36 1.30 1.07 1.74 2.61 4.61 4.23 3.90
Second premolar
Girls (n = 29)
Mean 9.97 11.37 10.68 12.34 10.91 15.53 61.99 70.21 66.10
SD 1.17 1.17 1.26 1.02 2.17 2.51 5.41 5.29 4.79
Boys (n = 17)
Mean 10.12 11.74 10.92 12.51 10.88 16.88 61.12 71.26 66.19
SD 1.42 1.33 1.12 0.49 2.46 2.16 4.90 2.83 3.43
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Apr 13, 2017 | Posted by in Orthodontics | Comments Off on An examination of proportional root lengths of the mandibular canine and premolars near the time of eruption
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