In this study, we examined first premolar inclination in a large sample.
First premolar inclination, canine inclination, and mesiodistal location were measured on 797 panoramic radiographs of orthodontically untreated children (ages, 8-11 years; 381 boys, 416 girls). The sample comprised 1496 premolars and 1496 canines. A linear mixed-effects model was used to determine the contribution of age, sex, canine inclination, canine sector location, second molar maturational stage (D-G), and dental arch side on premolar inclination.
First premolar inclination values (medians and interquartile ranges) were 12.76° (8.12°-19.05°) at 8 years, 11.82° (7.87°-16.04°) at 9 years, 10.40° (6.38°-15.46°) at 10 years, and 9.03° (5.42°-12.81°) at 11 years; 13.86° (8.60°-18.78°) at stage D, 10.56° (7.39°-14.77°) at stage E, 10.43° (6.08°-15.09°) at stage F, and 8.00° (4.62°-10.74°) at stage G. The following equation was selected (Akaike information criteria = 424.99): first premolar inclination (°) = –2.211 + 2.240 (8 years) + 1.363 (9 years) + 0.955 (10 years) + 0.387 (canine inclination) + 0.902 (right side) + 2.320 (stage D) + 6.320 (sector 1) + 5.446 (sector 2) + 3.803 (sector 3). There was no difference between percentiles constructed by age and maturational stage.
First premolar inclination decreases during the mixed dentition and is moderately correlated with canine inclination.
Percentiles by age were computed for first premolar inclination (FPI).
Percentiles by maturation of the mandibular second molar were computed for FPI.
There was no difference between the distributions of these percentiles.
Canine inclination and sector location significantly influenced FPI.
FPI decreases progressively from 8 to 11 years of age.
During eruption, the maxillary permanent canine moves down between the lateral incisor and the first premolar with increasing mesial inclination of the crown until the age of 9 years, after which it progressively uprights. Because of the contact with the erupting canine, the inclination of the crown of the lateral incisor increases distally until 10 years of age, and then it spontaneously decreases. The erupting canine is also close to the first premolar, and this anatomic proximity explains why the greater the uprighting movement of the long axis of the canine over time, the greater that of the first premolar, and vice versa. However, no data are yet available on the distribution in terms of first premolar inclination in the mid-to-late mixed dentition of a large cross-sectional sample.
Dentists have the responsibility to recognize abnormalities in the developing dentition. Since an anomalous intraosseous position of the erupting canine increases the chance for developing eruption problems, knowledge of the changes with the passage of time would allow assessment of whether the erupting canine and the neighboring teeth are following a correct path of eruption in terms of inclination compared with the range of variability at a given age, or whether it is necessary to plan more frequent checkups or to adopt preventive measures for the abnormal eruption paths.
Three-dimensional imaging provides an accurate topographic diagnosis, but panoramic radiographs are more convenient for routine deciduous examinations because of the lower radiation dose. Therefore, in this study, we relied on the first premolar inclination as detected by 2-dimensional panoramic radiographs routinely collected in the mixed dentition for other purposes, with the additional aim of obtaining data comparable with previous reports. The purposes of this study were (1) to investigate the association of demographic or dental parameters (age, sex, canine inclination and mesiodistal position, dental maturational stage, and dental arch side) with the first premolar inclination and (2) to examine the first premolar inclination between 8 and 11 years of age in a large cross-sectional sample.
Material and methods
For this observational cross-sectional study, we collected 797 panoramic radiographs of 381 boys and 416 girls that had been taken in routine examination of the developing dentition with the same apparatus (Pro-Max orthopantomograph; Planmeca, Helsinki, Finland) at the Department of Radiology of the University of Bologna in Italy. All radiographs were already available in the medical records at the time of data collection. Inclusion criteria were white ethnicity, age between 8 and 11 years, and not undergoing or not having undergone orthodontic treatment. Exclusion criteria were not completely erupted maxillary permanent incisors; odontomas, cysts, or supernumerary teeth in the study zone; small or peg-shaped lateral incisors; tooth agenesis (except for third molars); history of trauma in the orofacial region; craniofacial syndromes; cleft lip or palate; and poor-quality radiographs (ie, incisors, canines, and first premolars not clearly distinguishable for the measurements). Patients or parents were informed of the study aims and asked to sign an informed consent to participate.
Demographic data and medical and dental histories were obtained from medical record reviews by 1 operator (G.G.). Panoramic radiographs were saved or digitized with a scanner (Expression 1680 Pro; Epson Italia, Milan, Italy) at a resolution of 300 dpi. A calibrated and experienced orthodontist (G.A.B.), blinded to the age and sex of the children, assessed the maturational stage of the mandibular left permanent second molar using the method of Demirjian et al (stages D-G) because the maturation of this tooth is a reliable indicator for the timing of the spontaneous eruption of the maxillary canine.
One experienced operator (S.I.P.) measured the inclinations of the maxillary canines and the first premolars through the angle formed between the long axes of the teeth and the midline (angle α for the canine, angle π for the maxillary first premolar ) using the ImageJ angle measurement tool. Measurements were rounded to the nearest 0.01°. The mesiodistal position of the canine cusp tip was assessed in accordance with the sector analysis proposed in a previous study : sector 0, area distal to a line tangent to the distal heights of contour of the deciduous canine crown and root; sector 1, area bounded by sector 0 and the long axis of the deciduous canine; sector 2, area bounded by sector 1 and a line tangent to the distal heights of contour of the lateral incisor crown and root; sector 3, area bounded by sector 2 and the long axis of the lateral incisor; sector 4, area bounded by sector 3 and a line tangent to the mesial heights of contour of the lateral incisor crown and root; sector 5, area bounded by sector 4 and the long axis of the central incisor; and sector 6, area bounded by sector 5 and the midline. When the first premolar was erupted on one side of the panoramic radiograph, only the opposite side was considered for tooth measurements. For this reason, 46 right premolars and 52 left premolars were excluded along with their neighboring canines; the study sample comprised 1496 canines and 1496 premolars.
Thirty randomly selected panoramic radiographs were measured 3 times 2 weeks apart to evaluate the intraobserver agreement for first premolar and canine inclinations using intraclass correlation coefficients. Kappa statistics were used to determine the intraobserver agreement in assigning the canine sector location and the dental maturational stage, after controlling for the marginal homogeneity ( P = 1 for canine sector location and P = 0.564 for dental maturational stage). A linear mixed-effects model that accounted for paired data (dental arch side) was used to show the contributions of the demographic parameters such as age and sex and the dental parameters such as canine inclination, canine sector location, and maturational stage of the mandibular second molar on first premolar inclination. A first model included as fixed effects demographic and dental parameters and as random effect the intercept, not taking into account that measurements on both sides were related. To evaluate the goodness of fitting of the model, the Akaike information criteria was used: ie, the best model is the one with the lower Akaike information criteria score (424.99). Considering as random effects the relationship between the measurements on both sides, 2 new models were generated by assuming an unstructured repeated covariance type (unequal variances) and compound symmetry (same variance for repeated measures); −2 logarithm likelihood ratio test was used to compare models. Variables with P values ≤0.05 were considered statistically significant predictors and were included in the final model.
Percentiles relative to first premolar and canine inclinations were constructed by age and maturational stage of the mandibular second molars. The Shapiro-Wilk test was used to verify the Gaussian distribution of the first premolar and canine inclinations; the Wilcoxon test for paired data was used to compare the percentiles classified by age with those classified by maturational stage of the mandibular second molar for both first premolar and canine inclinations. The α level was set a priori at 0.05. All analyses were performed with statistical software (version 21.0; IBM, Armonk, NY).
The numbers of canines classified according to sector and side are shown in Table I ; the numbers of teeth measured for each side at each age and maturational stage are reported in Table II . The intraobserver agreement was excellent for all measurements ( Table III ). Table IV presents medians and interquartile ranges relative to first premolar and canine inclinations by age and maturational stage of the mandibular second molar.
|Canine sector location||Right side||Left side||Total|
|Right side||Left side||Total|
|Maturational stage of the mandibular second molar|
|First premolar inclination||ICC, 0.993; 95% CI, 0.987-0.996 (right side)
ICC, 0.993; 95% CI, 0.988-0.997 (left side)
|Canine inclination||ICC, 0.991; 95% CI, 0.984-0.996 (right side)
ICC, 0.989; 95% CI, 0.980-0.994 (left side)
|Canine sector location||Kappa statistics: 1.000 (right side)
Kappa statistics: 1.000 (left side)
|Dental maturational stage||Kappa statistics: 0.949|
|Age (y), number of children||Median||Interquartile range||Maturational stage of the mandibular second molar and number of teeth||Median||Interquartile range|
|First premolar inclination (°)|
|8, n = 191||12.76||8.12-19.05||D
n = 456
|9, n = 231||11.82||7.87-16.04||E
n = 524
|10, n = 203||10.40||6.38-15.46||F
n = 362
|11, n = 172||9.03||5.42-12.81||G
n = 154
|8, n = 191||16.05||10.02-21.39||D
n = 456
|9, n = 231||15.28||10.60-20.37||E
n = 524
|10, n = 203||14.48||8.97-19.87||F
n = 362
|11, n = 172||12.98||7.45-18.29||G
n = 154
As for premolar inclination, 4 mild outliers (beyond the inner fences: ie, 1.5 interquartile ranges) were identified for the right side and 11 for the left side (1 of them beyond the outer fences). By excluding the outliers, the median and interquartile range values became 10.10° and 5.96° to 15.43° rather than 11.48° and 7.18° to 16.98° for the right side, and 10.09° and 5.96° to 15.43° rather than 10.25° and 5.99° to 15.69° for the left side.
Age among the demographic parameters, and canine inclination, canine sector location, maturational stage of the mandibular second molar, and dental arch side among the dental parameters were significantly associated with the first premolar inclination ( Table V ).
|Age (8 y)||2.240||0.545||0.001|
|Age (9 y)||1.363||0.489||0.005|
|Age (10 y)||0.955||0.479||0.046|
|Maturational stage D||2.320||0.699||0.001|
|Canine inclination (°)||0.387||0.021||0.001|