Effects of mandibular incisor extraction on anterior occlusion in adults with Class III malocclusion and reduced overbite

ORIGINAL ARTICLE Effects of mandibular incisor extraction on anterior occlusion in adults with Class III malocclusion and reduced overbite Espen Færøvig, DDS, a and Björn U. Zachrisson, DDS, MSD, PhD b Oslo, Norway The aim of this study was to assess the treatment outcome and changes in mandibular incisor position after extraction of one single incisor in 36 adult orthodontic cases with combined Class III and open bite tendencies. The cases consisted of 21 female and 15 male patients with an average age of 27.8 years (standard deviation [SD], 11.1 years) at the start. Fixed 0.018-inch Edgewise appliances were used in both arches in 19 patients and in the mandibular arch in 17 patients. Average treatment time was 18 months (SD, 7.1 months). Pretreatment ANB was 0.5 o (SD, 2.7 ), overjet 1.4 mm (SD, 1.9 mm), and overbite 1.5 mm (SD, 1.1 mm). The lower incisors were 3.6 mm (SD, 2.8 mm) in front of the APg-line, and the Averaged Irregularity Index was 1.1 mm (SD, 0.6 mm). Records representing pretreatment (T1), posttreatment (T2), and average 4.3 years (SD, 2.3 years) retention (T3) included cephalograms, panoramic films, intraoral and extraoral photographs, and plaster models. All cast measurements were made with digital calipers. On the cephalograms, the lower incisor tips moved posteriorly 1.7 mm (SD, 2.0 mm) and occlusally 1.5 mm (SD, 1.8 mm) from pretreatment to posttreatment. Relative to the x-axis, they tipped lingually 5.9 o (SD, 5.6 ). On the cast analysis, overjet increased 1.0 mm (SD, 1.9 mm) and 1.5 mm (SD, 1.5 mm) for the maxillary central and lateral incisors, respectively, whereas overbite increased 0.6 mm for both the central and lateral incisors. The intercanine width was reduced by 3.3 mm (SD, 2.0 mm), while the intermolar width was unchanged. The Average Irregularity Index was reduced from 1.1 to 0.2 mm. Arch-length was reduced 3.6 mm from pretreatment to posttreatment, and another 0.3 mm from posttreatment to 4.3-years retention. Other changes from posttreatment to 4.3-years retention were slight. No loss of the interdental gingival papillae was normally observed. It is concluded that the extraction of one mandibular incisor can lead to satisfactory treatment results in adults with mild Class III malocclusion and reduced overbite, particularly when coupled with a large mandibular intercanine width and minor crowding, and some mandibular tooth size excess. However, the orthodontic treatment frequently became more complicated and time-consuming than expected at the start. (Am J Orthod Dentofacial Orthop 1999;115:113-24) Treatment by extraction of one single mandibular incisor is not popular in the orthodontic profession despite the apparent advantages of extraction in the region of crowding. 1,2 Objections to this extraction option have been based on case reports or subjective clinical opinions after observing less desirable outcomes in treated Class I and Class II malocclusions. 2-7 Unwanted side-effects have been increases of overbite and overjet beyond acceptable limits, space reopening, partly unsatisfactory posterior occlusion, recurrence of crowding in the remaining three incisors, and unesthetic loss of the interdental gingival papillae in the mandibular anterior region. 2-10 The prevailing opinion appears to be that the one incisor extraction alternative should be reserved for the atypical, compromise, or relapse case. 7 From the Department of Orthodontics, Dental Faculty, University of Oslo. a Clinical Instructor b Professor II. Reprint requests to: Dr Björn Zachrisson, Stortingsgaten 10, 0161 Oslo, Norway Copyright 1999 by the American Association of Orthodontists. 0889-5406/99/$8.00 + 0 8/1/92617 On the other hand, Riedel and co-workers 6,11 have suggested that in patients with severely crowded mandibular arches, the removal of one (or two) mandibular incisor(s) is the only logical alternative that may allow for increased stability of the mandibular anterior region without continuous retention. In some instances, the extraction of two mandibular incisors may satisfy the requirements of maintaining arch form without expansion of intercanine width. Extreme crowding or protrusion, particularly when accompanied by loss of gingival tissue or bone overlying the labial surface of incisor roots, may thus be indications for mandibular incisor extraction according to these authors. 6,11 Kokich and Shapiro 5 stated that if lower incisor extraction is performed without careful planning, the resulting occlusal discrepancy often cannot be resolved satisfactorily. They argued that with careful case selection, single incisor extraction may allow the clinicians to use simple treatment mechanics and achieve good results. A careful and realistic diagnostic set-up was 113

114 Færøvig and Zachrisson American Journal of Orthodontics and Dentofacial Orthopedics February 1999 Table I. Cephalometric variables at T1, T2, and T3 in present sample of 36 patients with Class III/open bite tendencies T1 T2 T3 Variable Mean SD Mean SD Mean SD SNA 80.0 4.1 79.5 4.4 79.6 4.0 SNB 79.5 4.3 79.5 4.9 79.4 4.4 ANB 0.5 2.7 0.0 2.9 0.2 2.8 FH/NA 88.1 4.0 88.3 4.0 88.6 4.0 FH/NPg 89.3 4.3 89.7 4.7 90.2 4.0 FH/ML 23.5 5.7 23.8 5.6 22.8 5.1 SN/ML 31.4 7.2 31.5 8.3 31.3 7.0 A NPg 1.1 mm 3.3 1.6 mm 3.9 1.7 mm 3.5 FA/NBa 90.3 5.4 90.5 6.3 90.2 5.1 UAFH/LAFH 0.8 0.1 0.8 0.1 0.8 0.1 UL E-line 5.7 mm 4.3 6.2 mm 3.7-7.0 mm 3.4 LL E-line 2.6 mm 4.3 3.8 mm 4.1-4.8 mm 4.3 L1 APg 3.6 mm 2.8 2.4 mm 2.4 1.7 mm 2.7 L1/APg 27.4 5.6 23.2 4.8 23.0 5.6 L1/ML 93.6 7.6 88.0 7.5 88.0 8.4 U1 APg 5.6 mm 3.1 5.3 mm 2.7 4.4 mm 2.5 U1/APg 23.0 6.9 23.1 6.0 20.6 6.2 U1/FH 114.0 6.8 115.2 7.5 112.9 6.7 U1/SN 102.2 17.4 106.1 7.3 104.2 6.7 U1/L1 130.0 10.2 133.5 8.8 136.4 9.9 FA, Facial axis. UAFH and LAFH, Upper and lower anterior face height, respectively. Table II. Model measurements (in mm) at T1, T2, and T3 in present sample of 36 patients with Class III/open bite tendencies. Overjet and overbite were measured separately for the maxillary central and lateral incisors. The measurements for intercanine and intermolar widths, arch length and average irregularity index represent the mandible. T1 T2 T3 Variable Mean SD Mean SD Mean SD Overjet central incisors 1.4 1.9 2.4 1.0 2.1 0.7 Overjet lateral incisors 0.7 1.5 2.2 0.8 2.0 0.8 Overbite central incisors 1.5 1.1 2.1 0.9 1.9 0.9 Overbite lateral incisors 1.5 2.5 2.1 1.6 1.9 1.5 Intercanine width 24.8 2.5 21.5 1.6 21.4 1.5 Intermolar width 40.3 3.7 40.2 3.6 39.8 3.9 Arch length 54.0 6.2 50.4 6.3 50.1 0.0 Average Irregularity Index 1.1 0.6 0.2 0.1 0.2 0.1 Lower anterior crowding 3.1 2.1 0.0 0.0 0.0 0.0 considered to be an important aid in determining tooth size discrepancies, and whether or not the occlusal result would be acceptable and consistent with the treatment objectives. The setup will also demonstrate the amount of mesiodistal enamel reduction that will be needed from the upper incisors. Several authors 5,9,13,14 have emphasized that maxillary dentitions with narrow lateral incisors (measurable mandibular Bolton excess 12 ) may represent good indications for extraction of one mandibular incisor. Others 3,10,15-17 have remarked that cases with Class III tendency could be another indication for incisor extraction, because some collapse of the lower arch may be acceptable or even desirable in such instances. There is thus an obvious need for a careful differential diagnosis in orthodontic treatment planning before incisor extraction is performed. A treatment outcome with resultant exaggeration of the overbite and overjet, which might be quite undesirable in Class II deep-bite cases with accentuated overjet, could in fact be beneficial associated with the treatment of Class III malocclusions with reduced overbite. The purpose of the present study was to examine and record the treatment outcome in a group of orthodontic patients with mild-to-moder-

American Journal of Orthodontics and Dentofacial Orthopedics Færøvig and Zachrisson 115 Volume 115, Number 2 Table III. Error of the method Systematic error: t test Houston s at 10% level Dahlberg s coefficient of and its Measurement calculation reliability (%) significance Models (mm) Overbite central 0.09 99.2 6.978* incisor Overbite lateral 0.07 99.5 2.128* incisor Overjet central 0.11 99.6 0.287 incisor Lower intercanine 0.14 99.6 0.955 width Lower intermolar 0.11 99.9 4.035* width Lower arch length 0.21 99.7 9.608* Average Irregu- 0.07 99.0 2.601* larity Index Lower anterior 0.08 99.8 1.878* crowding Cephalograms (mm) AN Pg 0.43 94.7 0.921 ULE line 0.36 99.7 1.010 LLE line 0.27 99.6 2.371* L1 APg 0.43 97.8 0.925 U1 APg 0.69 99.8 0.683 L1-L6 (horizontal) 6 0.60 97.3 1.204 L1-FOP (vertical) 6 0.37 92.4 1.709 L1-Pg Perpen- 1.54 97.1 1.180 dicular 20 Cephalograms ( o ) SNA 0.92 96.5 0.261 SNB 0.94 96.6 0.335 ANB 0.36 98.5 1.090 FH/NA 0.72 97.1 0.043 FH/NPg 0.56 98.8 0.567 FH/ML 0.80 98.6 2.553* SN/ML 1.70 94.1 1.251 FA/NBa 0.96 97.1 1.068 UAFH/LAFH 0.02 99.7 1.010 L1/APg 2.14 84.8 1.108 L1/ML 0.56 99.5 0.885 U1/APg 0.80 98.8 0.589 U1/FH 1.11 97.9 0.428 U1/SN 0.97 98.1 0.243 L1/U1 3.74 88.1 1.056 L1/FOP 6 0.76 99.2 0.004 *Significant at the 10% level (P >.1). NS, Not significant. ate Class III malocclusion, reduced overjet and overbite, and relatively minor mandibular incisor crowding, in whom one mandibular incisor extraction had been selected as the treatment of choice in attempts to improve the anterior occlusion. To rule out the variable effects of posttreatment growth changes in overjet and overbite, 18 all participants in the study were adults. Fig 1. Two mandibular superimposition techniques used in this study to assess changes in horizontal, vertical, and angular positions of the lower incisors. A, Method according to Riedel et al. 6 H and V, horizontal and vertical position, respectively, of lower incisor; A, angular position of lower incisor to x-axis; B, Method of McNamara et al. 20. (See text for details.) MATERIAL AND METHODS Sample The sample consisted of 36 adult patients (21 females and 15 males) with a mean age of 27.8 years (standard deviation [SD], 11.1 years) treated with edgewise orthodontic therapy with 0.018 0.025 inch slot attachments. Nineteen of the patients received orthodontic treatment in both the maxilla and the mandible, and 17 were treated only in the mandibular arch. These persons represented a consecutive series of all patients, who fulfilled certain criteria (see following) for one mandibular incisor to be extracted as part of the orthodontic treatment plan, and who were treated between 1988 and 1995 in two offices. The patients had been treated by one clinician (B.Z.) in his private office (n =

116 Færøvig and Zachrisson American Journal of Orthodontics and Dentofacial Orthopedics February 1999 Fig 2. Young adult female patient before (left row) and after (right row) orthodontic treatment including extraction of the right central mandibular incisor. This patient demonstrates both the indication (see text) and the typical changes in anterior (A and B) and posterior occlusion (C to F) in the present sample. G to H represents the mandibular occlusal changes with some reduction of the lower intercanine width. 29) or under his supervision by graduate students at the Department of Orthodontics, University of Oslo (n = 7). To be included in this study, the patients had to fulfill the following criteria: 1. Be a nongrowing adult. 2. Have an occlusion with tendency toward or established mild-to-moderate Class III malocclusion with reduced or no overbite. The anterior occlusion was a mild anterior crossbite of one or several teeth in 22 cases, an edge-to-edge occlu-

American Journal of Orthodontics and Dentofacial Orthopedics Færøvig and Zachrisson 117 Volume 115, Number 2 A B C D Fig 3. Same patient as in Fig 2 demonstrates typical changes in anterior overjet and overbite (C to D) associated with orthodontic treatment including the extraction of one single mandibular incisor in patients with Class III/open bite tendencies. sion on at least one incisor in 5 cases, and a normal overlap in 9 cases. In 28 cases, there was noticeable wear of the lingual and/or incisal aspects of one or several maxillary incisors. 3. Have mild-to-moderate anterior mandibular crowding. The pretreatment characteristics are demonstrated in Tables I (cephalometric variables) and II (model description). Despite considerable individual variations, the typical features were a slightly retrognathic maxilla and a normal mandible, a tendency to lowangle face type, a normal profile, normal inclination of the maxillary incisors, and slightly proclined mandibular incisors (Table I). The average orthodontic treatment time was 18.0 months (SD, 7.1 months). In the majority of patients (n = 22), some mesiodistal enamel reduction had been performed in the mandibular and/or maxillary dental arches in order to optimize the final result and maintain the interdental gingival papillae. The mean posttreatment observation time was 4.3 years (SD, 2.3 years). All patients still had bonded mandibular lingual retainers at time of follow-up. The retention appliance in all cases were fiveunit retainers made of 0.0215 five-stranded spiral wire (Penta-One), as described previously. 19 In the maxillary arch, generally an upper removable plate was used. Clinical records, consisting of dental casts, orthopantomograms, lateral cephalograms, and intraoral and extraoral photographic slides were collected for each patient at three time intervals, representing: pretreatment (T1), posttreatment (T2), and at follow-up a minimum of 2 years after debonding (T3). Cephalometric Analysis All lateral cephalometric radiographs were hand traced by the same examiner (E.F.). A total of 24 measurements were used. On each T1 tracing, x and y coordinates were constructed according to Riedel et al 6 (Fig 1A). The functional occlusal plane (FOP) drawn through the region of the overlapping cusps of the first premolars and first molars served as the x-axis on the T1 tracing. The y-axis was represented by a line through the average mesial contact point of the mandibular first molar perpendicular to the occlusal plane. Because this reference axis would change with mesial movement of the mandibular molars during the orthodontic treatment (Fig 1A), a second y-axis was also used. The y-axis of McNamara et al 20 represented by the perpendicular line

118 Færøvig and Zachrisson American Journal of Orthodontics and Dentofacial Orthopedics February 1999 A B C D E F G H Fig 4. Young adult male patient with Class III and open bite tendencies before (left row) and after (right row) orthodontic treatment including extraction of the right central mandibular incisor and marked mesiodistal stripping of the maxillary teeth.this patient demonstrates typical changes in anterior occlusion (A to D) and occlusal changes (E to H) in the present material. The mandibular intercanine distance was reduced from 26 to 22.5 mm in this case. from pogonion to the FOP (Fig 1B) provided a means to evaluate possible mesial movement of the mandibular molars with treatment. T2 and T3 cephalograms were superimposed structurally according to the method described by Björk 21 on the T1 tracings, using the inner contour of the symphysis, while obtaining a best fit of the mandibular canal and third molars when present (Fig 1A). The FOP from

American Journal of Orthodontics and Dentofacial Orthopedics Færøvig and Zachrisson 119 Volume 115, Number 2 A B C D Fig 5. Even small increases in overjet and overbite associated with orthodontic treatment including the extraction of one mandibular incisor may be of considerable clinical significance in patients with Class III/open bite tendencies, as demonstrated in this young adult female. Particularly note the vertical positions of the maxillary left lateral incisor (arrows in A and D), and marked incisal and lingual wear of the left central incisor pretreatment (A). the first head film was transferred to the following tracings and was used as a reference for the cephalometric measurements, according to the method of Pancherz 22 and Hansen et al. 23 Three variables were measured: Horizontal position of the mandibular central incisor (Fig. 1A and 1B). The linear measurement from the incisal edge of the mandibular central incisor to the y- axes along a line parallel to the x-axis. Vertical position of the mandibular central incisor (Fig.1A). The linear measurement from the incisal edge of the mandibular central incisor to the x-axis along a line parallel to the x-axis. Angulation of the mandibular central incisor (Fig.1A). The inner angle formed by a line from the most apical point of the root to the midpoint of the incisal edge at its intersection with the x-axis. Model analysis A digital caliper (Jocal One-Hand Caliper, CE Johansson Gage Co., White Plains, N.Y.) calibrated to 0.01 mm was used for model measurements. The same examiner (E.F.) made all measurements. The following definitions were used: Overjet. The shortest distance from the most labial point of each maxillary central and lateral incisor, respectively, to points on the labial surface of the mandibular central and lateral incisors along a line parallel to the occlusal plane. Overjet for the central and lateral incisors was calculated as the average of the right and left measurements. Overbite. The distance from the incisal edge of each mandibular central and lateral incisor, respectively, to points on the labial surfaces denoting the projection, parallel to the occlusal plane, of the incisal mesiodistal midpoint of the corresponding maxillary central and lateral incisors. The calipers were oriented perpendicular to the occlusal plane to obtain the most accurate measurements. Overbite for the central and lateral incisors was calculated as the average of the right and left measurements. Mandibular intercanine and intermolar widths and arch length were measured according to Riedel et al. 6 Irregularity. The Average Irregularity Index (AvIrIn), ie, the average irregularity for each contact point, was calculated as recommended by Riedel et al 6 to compare the amount of crowding before and

120 Færøvig and Zachrisson American Journal of Orthodontics and Dentofacial Orthopedics February 1999 A B C D E F G H Fig 6. Interdental gingival recessions occurring after the extraction of one mandibular incisor and associated orthodontic treatment in adult male patient with triangular incisor morphology. Some loss of gingival papillae is evident pretreatment (A). As a result of the triangular incisor shape, the gingival papillae of the remaining three mandibular incisors almost disappeared in the middle of the orthodontic treatment period (arrows in E). Marked mesiodistal stripping of all maxillary and mandibular incisors and continued orthodontic space closure was needed to obtain a reasonably esthetic final result (F and H). Also note labial gingival recession occurring during rotation of mandibular left lateral incisor (D to E).

American Journal of Orthodontics and Dentofacial Orthopedics Færøvig and Zachrisson 121 Volume 115, Number 2 Table IV. Mean changes and standard deviations (SD) of mandibular superimposition measurements in present sample of 36 patients with Class III/open bite tendencies. Numbers represent changes in lower incisor position associated with orthodontic treatment (T2 to T1) and at follow-up (T3 to T2). Mandibular superimposition measurements Mean change Lower incisor position T1-T2 SD T2-T3 SD Riedel model X-coordinate (mm) -1.7* 2.0-0.4 1.6 Y-coordinate (mm) 1.5* 1.8 0.1 1.0 Angle to x-axis ( ) 5.9* 5.6 0.6 2.7 McNamara model X-coordinate (mm) -1.7* 2.0-0.5 1.6 X-coordinate, Incisal tip moves anteriorly (+), posteriorly (-). Y-coordinate, Incisal tip moves occlusally (+), gingivally (-). Angle to x-axis, Retroclined (+), proclined (-). *P >.05). after one mandibular incisor extraction. The average was computed by dividing the IrIn by the number of contacts in the mandibular anterior segment. Before extraction, the number of contact areas would be five; after extraction the number would be four. Because the IrIn may not optimally describe the crowding in circumstances with zig-zag pattern of neighboring teeth, a conventional measurement of mandibular incisor crowding was also used. Toothsize discrepancies were analyzed by the Bolton analysis. 12 This material showed an average pretreatment mandibular anterior tooth size excess of 1.6 mm (SD, 1.1). Error of the Method All measurements were made by the same observer. Method errors were investigated by rerecording 20 randomly selected sets of models and 20 cephalograms at least 3 weeks after the original recording session. Dahlberg s technique 24 was applied to determine the error between duplicate determinations, systematic error was assessed at the 10% level as recommended by Houston, 25 and the coefficient of reliability was also calculated. 26 The data are presented in Table III. Dahlberg errors ranged from 0.07 to 0.21 mm for model measurements and were small for the overbite and overjet assessments. They were larger for the radiographic measurements, and ranged from 0.27 to 1.54 mm and from 0.02 to 3.74 on the cephalograms. The coefficient of reliability varied between 84.8% for lower incisor angulation relative to the APg-line and 99.9% for lower intermolar width. Significant systematic errors for this observer were found in recording several linear and one angular measurement. Statistical analyses were made by calculating the means and standard deviations, and levels of statistical significance were determined with paired t tests. RESULTS Clinical Findings Figs 2-6 show typical cases demonstrating pretreatment and posttreatment intraoral and extraoral conditions. In most cases, the posttreatment occlusal status was satisfactory with bilaterally good interdigitation of the posterior teeth (Fig 2D, F). In a few cases, however, the posterior occlusion at T2 was less satisfactory with a tendency to or edge-to-edge occlusion of the canines or premolars unilaterally. A detailed analysis of the posterior occlusion will be presented separately. Figs 2A-B, 3C- D, 4C-D, and 5A, D demonstrate typical changes of the anterior occlusion from T1 to T2. With the exception of two cases in which the resolution of a moderate degree of crowding resulted in some interdental recession (Fig 6), the gingival papillae filled the space properly between the three mandibular incisors (Figs. 2B, 3A, 4D, 5D). In four cases, there was slight (< 2 mm) labial gingival recession on one (three patients) or two (one patient) of the lower incisors posttreatment (Fig 6). Cephalometric Analysis Mandibular incisor tip. Tables IV, V, and VI demonstrate the positional changes of the mandibular incisal tip from T1 to T2 and T3. On average, the lower incisors retroclined 1.7 mm (SD, 2.0 mm) from T1 to T2, and had moved slightly further posteriorly at T3 (Table IV). Table V describes subgroups with individual variation in response in the horizontal plane. The mandibular incisor tip was 1.5 mm (SD, 1.8 mm) higher vertically at T2 than at T1 and maintained this position at T3 (Table IV). Table VI describes subgroups with individual variation in response in the vertical plane. The lower incisor retroclination was evident also by the 5.9 (SD, 5.6 ) change in the angle of the incisor relative to the x- axis (Table IV). The same numeric horizontal movement of the lower incisor tip relative to both y-coordinates used (Table IV) demonstrated that the mandibular

122 Færøvig and Zachrisson American Journal of Orthodontics and Dentofacial Orthopedics February 1999 Table V. Individual variation in change of lower incisor horizontal position on lateral cephalograms associated with extraction of one mandibular incisor and orthodontic treatment in present sample Lower incisor tip T1-T2 Table VII. Individual variation in change of mandibular intercanine width on models associated with extraction of one mandibular incisor and orthodontic treatment in present sample Intercanine width T1-T2 Proclined No change Retroclined 1 mm Retroclined 2 mm Retroclined > 2 mm 4 patients 3 patients 11 patients 5 patients 13 patients Increased No change Reduced 1-2 mm Reduced 2-4 mm Reduced > 4 mm 1 patient 4 patients 3 patients 17 patients 11 patients Table VI. Individual variation in change of lower incisor vertical position on lateral cephalograms associated with extraction of one mandibular incisor and orthodontic treatment in present sample Lower incisor tip Intruded No change Extruded 1 mm Extruded 2 mm Extruded > 2 mm T1-T2 4 patients 8 patients 10 patients 5 patients 9 patients first molars had not come significantly forward with treatment. Skeletal variables. The skeletal changes from T1 to T2 and T3 are presented in Table I. Such changes generally were minimal in this sample of adults. Soft tissue profile. On average, the soft tissue profile was slightly, but significantly (P <.05) more retrusive at T2 than at T1 by 0.5 mm and 1.2 mm for the upper and lower lip, respectively (Table I). At T3, the upper and lower lip had retracted further by 0.8 mm and 1.0 mm, respectively. Model Analysis Overjet. The overjet was increased by an average of 1.0 mm for the central incisors and 1.5 mm for the lateral incisors from T1 to T2 (Table II), both of which were statistically significant (P <.05). At T3, there was a nonsignificant slight relapse (Table II). Overbite. The maxillary central incisor overbite increased by a mean of 0.6 mm from T1 to T2 (Table II). This difference was not statistically significant. The change from T2 to T3 was negligible. The maxillary lateral incisor overbite increased by a mean of 0.6 mm from T1 to T2, which was statistically significant (P <.05). The change from T2 to T3 was insignificant (Table II). Mandibular intercanine width. The average reduction in intercanine width from T1 to T2 (Table II) was 3.3 mm, which was statistically significant (P <.05). The change from T2 to T3 was insignificant. Table VII describes subgroups with individual variation in reduction of the mandibular intercanine width. In 10 of 11 patients, where the distance was reduced more than 4 mm, the pretreatment 3-3 distance was markedly wide. Mandibular intermolar width. There were only small and insignificant changes in mandibular intermolar width from T1 to T2 and T3 (Table II). Mandibular arch length. On average, the mandibular arch length was reduced from T1 to T2 by 3.6 mm (Table II), which was statistically significant (P <.05). The 1.5 mm further reduction at T3 was not statistically significant. Irregularity. The average irregularity for each contact point was reduced by 0.9 mm from T1 to T2 (Table II), which was statistically significant (P <.05). This was unchanged at T3. The total pretreatment lower anterior crowding of 3.1 mm (SD, 2.1 mm) was eliminated at T2 and remained unchanged at T3, due to the presence of the fixed retainers. DISCUSSION Most previous comments on treatment results after one mandibular incisor extraction have been supported by clinical case reports of one or a few patients. 5,7,8,13-17,27-29 However, some studies have analyzed patient samples. Richardson 3 evaluated 22 successfully completed cases that fell into three categories: (1) one incisor completely outside the line of the arch (3 cases); (2) narrow incisor apical base with distally inclined canines and fanning or imbrication of the incisors (14 cases); and (3) tendency towards Class III malocclusion with imbrication of the lower incisors (5 cases). Dacre 2 reexamined 32 teenagers a few years after orthodontic treatment. They were either Class I malocclusions with late crowding treated in the mandible only (16 cases) or a varied group in whom additional extractions had been made (16 cases). Riedel et al 6 examined 12 Class I, 6 Class II:1, and 6 Class II:2 adult cases in whom one incisor was extracted, as well as 18 patients in whom two mandibular incisors were extracted as part of orthodontic treatment. Canut 10 reported on the long-term anterior alignment several

American Journal of Orthodontics and Dentofacial Orthopedics Færøvig and Zachrisson 123 Volume 115, Number 2 years out of retention in a sample of 26 young adolescents (mean age at T1, 12.5 years) with different malocclusions. Compared with the above studies, the present material is larger and more homogenous with regard to malocclusion characteristics. The improvement of the anterior occlusion in the present study was due to both posterior and occlusal movement of the mandibular incisors (Table IV). Admittedly, the mean increases in overjet and overbite were small (Tables II). But as demonstrated in Fig 5A-D, even minor vertical and sagittal changes of the incisors may still be adequate to improve the anterior occlusion by a clinically important amount. Whereas the overjet improvement may be due to true lingual movement of the three mandibular incisors, the extrusion relative to the FOP may in part be explained in geometric terms as an effect of the retroclination around the apex (Fig. 1A). In contrast to the present results, Riedel et al 6 in their one-incisor extraction sample of 24 patients found that the mandibular incisor tip on average moved 0.56 mm anteriorly and 0.19 mm gingivally, while proclining 3.5 o from T1 to T2. The explanation for the difference between the two studies is evidently the different types of malocclusions examined and the different treatment goals. The patients in the Riedel et al 6 study exhibited marked mandibular crowding pretreatment, and had an average overjet of 5.2 mm and overbite of 4.0 mm. Thus the treatment goal was to resolve the crowding problem and eliminate the overjet and overbite. In the present material with less crowding, and reduced overbite and overjet, the treatment goal was clearly quite different. The reasons for our selection of patients with less mandibular anterior crowding than in previous studies were (1) that the space from the extracted incisor was largely used for the attempts to improve the overjet and overbite, and (2) to reduce the risk for loss of the gingival papillae, which is inherent in adults when marked crowding is present pretreatment. 30 This means that even though marked anterior irregularity can be resolved adequately by one incisor extraction when observed from an occlusal view (Fig 6C, G), a loss of the gingival papillae between the three remaining incisors may still make the final treatment result esthetically unsatisfactory (Fig 6H). It is our clinical experience that cases with marked crowding and triangular incisors should preferably be treated by nonextraction approaches, including extensive mesiodistal stripping of the incisors, in order to maintain the interdental gingival papillae. The pretreatment mandibular anterior tooth size excess in incisors with normal mesial and distal contours favored the extraction decisions in this sample. The fact that the interdental papillae were generally well preserved after the orthodontic treatment was a result of (1) the careful selection of cases, (2) the mesiodistal enamel reduction of mandibular and/or maxillary incisors, carried out in the individual cases as needed, and (3) the emphasis on optimizing the axial inclinations of each lower incisor. In addition, labial gingival recession was observed infrequently in this material. However, the risk of creating some labial gingival recession while derotating crowded mandibular incisors was evident in some few cases (Fig 6D-E), particularly when the soft tissues were thin. 30 Somewhat unexpectedly, the mean treatment time was 18 months and thus not notably shorter than that of routine orthodontic treatments involving four premolar extractions. This means that one lower incisor extraction case may become more complicated than expected at the start, and may not allow the clinician to use simple mechanics to achieve a good result as suggested by Kokich and Shapiro. 5 The main reason for the relatively long treatment periods in this study were unexpected complications that occurred during therapy, most notably lingual tilting of the mandibular canine crowns and unwanted narrowing of the intercanine width. In particular, adequate torque control of both mandibular canines (Figs 2B,4D,5D,6H) would appear to be necessary throughout the treatment period, and in fact, the crown torque and axial inclinations of all mandibular teeth may need constant monitoring. It is emphasized that all patients in this study were retained with fixed retainers on a prolonged basis. The bonded five-unit retainers will keep the teeth together and prevent rotations, but they are unlikely to prevent vertical movement of the entire canine-tocanine segment. The merits of prolonged retention are discussed elsewhere. 31 The semipermanent retention was partly selected in order to prevent space reopening in the anterior region, because even a small diastema would become visible and unesthetic. No harmful side effects or iatrogenic damage was registered in any of the patients, and definite decisions on when to remove the bonded retainers have therefore not yet been made. CONCLUSIONS The results of the present 36-case study indicate that one single mandibular incisor extraction may be a good orthodontic treatment alternative in selected adult cases with tendency toward or established mild-tomoderate Class III malocclusion with reduced overjet and overbite. The anterior occlusion was improved in all cases and the esthetic outcome was generally satis-

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