Subjects

Cases were selected among patients who finished orthodontic treatment at the Department of Orthodontics, University of Geneva, between 1984 and 1996 and who had dental casts taken approximately 2 and 10 years after the end of orthodontic treatment. Exclusion criteria comprised restoration of the maxillary central incisors and maxillary first molars performed or replaced during the follow-up period, poor quality of dental casts, and periodontal problems noticed by the practitioners at or between the 2 documentations.

Forty-two patients presented the necessary documentation. Seven were excluded, 3 because of inadequate quality of dental casts and 4 because of restorations.

According to the age of the patients at the first documentation used in the study (2 years after debonding and the end of orthodontic treatment), 2 groups were defined. The adolescent group comprised 25 subjects (7 boys, 18 girls), with a mean age of 15.9 years (range: 14.1 to 17.8). The adult group comprised 10 subjects (6 men, 4 women), with a mean age of 31.2 years (range: 21.4 to 46.4).

Measurements of gingival margin and displacement

On the first and second dental casts, teeth 15 and 16, teeth 25 and 26, and teeth 11 and 21 were photographed together in a standardized way. The occlusal plane was horizontal, and the contact point between both pair of teeth was centered in the photographs. The distance between the camera and the models was always the same to keep a constant and easily calculated magnification. This magnification has been calculated by photographing a ruler placed on the dental cast holder.

On the digital images, all teeth photographed were traced with drawing software (FreeHand 8.0, Macromedia Inc., San Francisco, Calif), and the line passing through the surface of contact of both teeth and perpendicular to the occlusal surface was also traced. Both drawings from each case were then superimposed, fitting the vertical placement of the drawing on the angles of the teeth. The measurements were done on the image with both drawings superimposed (Viewbox, version 3.1.1.12, dHAL Sofware, Kifissia, Greece) after magnification adjustment. The most mesial and the most distal points of each tooth were defined, and each tooth was divided into 8 equal parts, with lines parallel to the principal line. The intersections of these lines with the two tracings of the gingival contour were then recorded ( Fig 1 ). The gingival displacement on each line was calculated by the computer, and we calculated gingival displacement for each tooth by averaging the 3 gingival values in the center of the tooth.

Method. A, First photograph 2 years after the end of orthodontic treatment with drawings of teeth 11 and 21, the central line passing through the surface of contact of both teeth and perpendicular to the occlusal plane in black. B, Final photograph 8 years after the initial photograph with the same drawings as in a but in red. C, Tracings of initial and final photographs superimposed by fitting the angles of the incisors. The parallel lines to the central axis divide the maxillary central incisors in 8 equal parts. Arrows show the lines used for measurements.

Secondary eruption measurements

All the casts were scanned with the Laserscan 3D (Willytec, GmbH, Gräfelfing, Germany). The models were put on the articulated base with their occlusal plane parallel to the ground, and the image was then transferred to a computer (Siemens Expert, Siemens AG, Munich, Germany).

On the 3D model, we compared the position of the central incisors and the first molar at the first and second documentation. The structure of reference was the palatal vault, distal of the third rugae. All the points of reference were localized on easily reproducible areas of each tooth and were the same on both sets of casts.

Statistics

All the statistics were conducted using SPSS 13.0 for Windows (SPSS Inc., Chicago, Ill). Statistical significance was set to <0.05. The results of the adolescent and the adult group were compared by using an unpaired t test. The same test was used to compare measurements on incisors and molars. Pearson correlation coefficient was calculated to relate gingival displacement to secondary tooth eruption, molar-to incisor secondary eruption, and molar-to-incisor gingival displacement. Tooth and gingival displacement were evaluated using confidence intervals.

Error of the method

The error of the method for the gingival displacement was tested by measuring the photographs on 2 different occasions on 15 studied cases. The error of the measurements was calculated by using the Dahlberg’s formula ( Se2=∑d2/2n
Se 2 = ∑ d 2 / 2 n
) and ranged from 0.05 to 0.1 mm.

In order to compare our method with a method already described in the literature , the gingival displacement measured on the casts was compared with the 1 using intraoral photographs taken on the same day as the dental casts in 20 cases. The frontal intraoral photographs should have been taken symmetrically from a centric position parallel to the occlusal plane with the central incisors centered in the photograph. High correlation (Pearson correlation) was found between the 2 methods ( r = 0.802; P <0.01).

The method used for the measurements of tooth eruption was evaluated by double localization of the same points of reference with a difference of 10 weeks between the 2 measurements and calculated by Dahlberg’s formula. Standard error (SE) was found to be low (17.1 μm). The differences in the coordinates were assessed and compared using paired t test. A high repeatability of the localization of points of reference and a high correlation between the 2 sets of measurements were found that corresponds to a previous published study.