Material and methods

Lateral headfilms and corresponding dental casts of 75 white subjects (26 male and 49 female; 16-26 years of age) were arbitrarily and sequentially taken from the files of a private orthodontic practice in Göttingen, Germany, according to the following exclusion criteria: deciduous teeth, missing teeth, filled incisors, crowned teeth, and morphologic tooth anomalies. Four study groups were formed according to the general inclusion criterion of incisor inclination that was considered acceptable or adequate by visual and cephalographic screening (ie, proper interincisal support, no midline deviation, good esthetics, and adequate function), with the exception of minor rotations or minor mandibular crowding of 2 mm or less that would not significantly affect axial incisor inclination, by 2 orthodontists at the University Medical Center of Göttingen, Germany, but otherwise different inclusion characteristics: group A comprised subjects who had not previously undergone orthodontic treatment and who had “ideal” occlusion characteristics, as indicated by an Angle Class I posterior and canine occlusion; group B subjects had the same morphologic dental characteristics after 0.022-in straight-wire treatment (torque-tip incisor prescription, +7°/+5° maxillary incisors, and −1°/+2° mandibular central incisors; Ormco, Orange, Calif) with a standardized archwire sequence, starting with 0.014-in nickel-titanium, followed by a 0.014-in Australian wire, 0.016-in and 0.018-in Australian wires, and a 0.016 × 0.022-in stainless steel wire, and finished with a 0.017 × 0.025-in stainless steel wire; group C comprised subjects who had undergone the same straight-wire treatment routine as did group B, but were finished with a posterior occlusion that deviated slightly from what is considered the “ideal” Angle Class I occlusion, apparent as a distal cusp-to-cusp occlusion of quarter to half cusps, and group D had the same occlusal characteristics, but no orthodontic treatment. This study had a retrospective design, and the orthodontic treatments in groups B and C had already been completed at the time of the assessments. Some subjects in group A later received orthodontic treatment because of minor rotations or minor crowding of less than 2 mm, whereas the subjects in group D later received comprehensive orthodontic care. There was no double assignment of subjects—eg, to group A before treatment and then to group B after treatment.

In the case of disagreement between the assessors concerning a subject’s eligibility, that subject was excluded from the trial sample.

Cephalometric and third-order cast measurements were made. Incisor axis inclination and sagittal craniofacial structures were analyzed by using lateral cephalograms and the landmarks and angles indicated in Figure 1 . Upper (U1) and lower (L1) central incisor inclinations were assessed with reference to the cephalographic lines NA and NL (U1NA, U1NL), and NB and ML (L1NB, L1ML), respectively. Sagittal craniofacial relationships were classified by using the ANB angle.

Cephalometric landmarks and angles used for capturing craniofacial relationships and axial incisor inclinations: sella ( S ), nasion ( N ), A-point ( A ), B-point ( B ), tip and root apex of upper and lower central incisor ( U1a, U1t, L1a, L1t ), anterior nasal spine ( ANS ), posterior nasal spine ( PNS ), menton ( Me ), and the most inferior point on the outline of the mandible at the gonial angle ( G ).

Third-order angles were measured directly from the dental plaster casts of each subject matching corresponding lateral radiograph by using a gauge for measuring third-order angles ( Fig 2 ). Upper and lower right central incisors (U1, L1) were chosen and prepared for assessment by marking the middle of the labial long axis of the incisor’s clinical crown. Third-order values (U1TA, L1TA) were defined as positive if the gingival portion of the facial tangent was lingual to the incisal portion and vice versa.

The gauge used for third-order assessments.

Both cephalographic tracings and third-order cast measurements were made manually by 1 examiner (M.K.), twice for each headfilm or dental cast, at an interval of 1 week. Repeated measures were subjected to error analysis. The median (minimum, maximum) difference between these 2 measurements for distinct features were U1NA, 0.1° (−6.3°, 5.5°); L1NB, 0.1° (−6.1°, 2.3°); U1NL, 0.1° (−5.0°, 7.4°); L1ML, −0.1° (−7.6°, 4.7°); U1TA, 0.0° (−2.5°, 2.0°); L1TA, −0.5° (−1.5°, 2.0°); and ANB 0.7° (−1.4°, 2.6°).