Cephalometric assessment of the axial inclination of upper and lower incisors in relation to the third-order angle

Abstract

Background and aim: Estimating incisor inclination cephalometrically by reference lines NA and NB puts the orthodontist in the difficult position of relating these axial inclination data to the bracket's third-order prescription which refers to a perpendicular to the occlusal plane. Purpose of the present study was to evaluate the relationship between the cephalometrically-assessed incisor inclination (using the lines NA and NB for reference) and the third-order angle (syn.: torque angle, TA) according to Andrews' description, and moreover to investigate the correlation between incisor inclination data and skeletal-sagittal and skeletal-vertical findings.

Materials and methods: The lateral cephalograms and corresponding dental casts of 67 subjects between 10 and 25 years of age (regardless of skeletal and dental relationships) were considered in the study. All subjects were Caucasian, and none had undergone orthodontic therapy. Upper (U1) and lower (L1) incisor angulations were cephalometrically assessed in reference to the NA and NB lines and compared to third-order angles obtained from dental cast measurements with an incisor inclination-recording appliance. Incisor inclination data from the two measurements were correlated to craniofacial sagittal (angles SNA, SNB, ANB) and vertical (angles NSL-NL, NSL-ML, ML-NL) findings from the radiographs.

Results: The third-order angles in the upper arch measured on the dental casts were a mean of 16.2 degrees (SD = 5.3 degrees) smaller than the axial inclination according to the NA line; the lower incisor third-order data were less than those of the axial inclination according to the NB line by a mean of 27.8 degrees (SD = 4.75 degrees). In this sample, there was a range of 42.7 degrees for the U1TA variable (mean = 5.6 degrees, SD 9.73 degrees) and 47 degrees for U1NA/ degrees variable (mean = 21.71 degrees, SD = 8.67 degrees). The L1TA variable showed a range of 29 degrees (mean = -2.95 degrees, SD = 7.17 degrees), the radiographic L1NB/ degrees range was 23 degrees (mean = 24.91 degrees, SD = 5.8 degrees). We observed a highly significant correlation (r(NA) = 0.84***, r(NB) = 0.76***) between the Andrews' angle and the inclination estimated in reference to the NA and NB lines. No significant correlation between incisor inclination and craniofacial measurements was detected.

Conclusions: Dental cast measurements seem to be more precise and more valid than lateral radiographs. The method we describe enables clinicians to get a good idea precisely and quickly of how much torque potential remains in the brackets and archwires during treatment. The inclination of the incisors can also be calculated using the regression equations provided, making additional lateral cephalograms unnecessary.