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Gingival response to mandibular incisor intrusion



American Journal of Orthodontics and Dentofacial Orthopedics, 2007-08-01, Volume 132, Issue 2, Pages 143.e9-143.e13, Copyright © 2007 American Association of Orthodontists


Introduction: The aims of the study were to evaluate the rate of accompanying gingival movement and the changes in attached and keratinized gingivae after orthodontic intrusion of mandibular incisors. Methods: The study was carried out with 16 subjects whose mandibular incisors were orthodontically intruded for the correction of overbite. The orthodontic intrusion was performed with the segmented utility arch technique. Periodontal indexes and the widths of attached and keratinized gingivae were recorded before and after treatment. The gingival movement in relation to orthodontic intrusion was determined by means of radioopacity with a specially designed metal device indicating the position of the gingival margin and the mucogingival junction on the cephalograms taken before treatment and after intrusion. Results: There were no statistically significant changes in the width of attached and keratinized gingivae after treatment ( P >.05). The gingival margin and the mucogingival junction moved in the same direction as the teeth by 79% and 62%, respectively. A statistically significant decrease of the clinical crown length was also observed ( P <.05). Conclusions: These results suggest that orthodontic intrusion does not lead to significant changes in the width of attached and keratinized gingivae when adequate plaque control is maintained. The gingiva moves in the same direction with the tooth, yet considerably less. This might indicate the need for follow-up or gingival correction after intrusion therapy.

The reaction of supporting periodontal structures and the accompanying gingival movements during various orthodontic treatment modalities have been investigated previously. The gingivae around orthodontically intruded, extruded, and rotated teeth move in the same direction of orthodontic movement. The concomitant gingival movement with the teeth that were subject to orthodontic movement in the mesiodistal direction was also demonstrated.

Animal studies on the reaction of healthy periodontal tissues to intrusive loads have been conducted. It was shown that the continuous application of relatively low orthodontic loads during intrusion led to minor decreases in the height of healthy alveolar bone. The accompanying movement of the gingivae after vertical tooth movements has been studied only in monkeys. The findings demonstrated that the gingival margin and the mucogingival junction around orthodontically extruded maxillary incisors move in the coronal direction by 77% and 80%, respectively. According to other findings from histologic sections, the sulcus base and the mucogingival junction move in the apical direction along with the tooth by 60% after orthodontic intrusion of maxillary incisors. Additionally, pocket depth increases, clinical crown length decreases by 40% of the orthodontic intrusion, and the junctional epithelium constantly seats on the cementoenamel junction after orthodontic intrusion.

However, no systematic data related to the accompanying gingival movement after orthodontic intrusion of human incisors are available. Therefore, the aims of this study were to investigate the accompanying gingival movement after orthodontic intrusion of periodontally healthy mandibular incisors and to assess the possible effects of orthodontic intrusion on the width of the attached and the keratinized gingivae.

Material and methods

Sixteen adolescent subjects with at least 4 mm overbite (8 boys, 8 girls; mean age, 13.51 ± 2.61 years), whose mandibular incisors were to be orthodontically intruded, were included in the study. All subjects received full-mouth scaling 2 weeks before the start of orthodontic treatment and were instructed to perform oral hygiene procedures.

Plaque index, gingival index, sulcus depth, bleeding on probing, and attachment levels were measured in the mesiobuccal, midbuccal, and distobuccal aspects of all 4 mandibular incisors. The mucogingival junction was defined with Schiller’s iodine (IKI) solution. Clinical crown length and width of keratinized gingiva of the incisors were measured 3 times midbuccally with a sliding caliper calibrated in 0.10-mm increments, and the average score was recorded. The width of the attached gingiva was calculated by subtracting the depth of the gingival sulcus from the width of the keratinized gingiva.

To determine the location of the gingival margin and the mucogingival junction on the cephalogram, a specially designed metal device was placed into the bracket slot of the mandibular right central incisor ( Fig 1 ). The coronal and apical extensions of the device were adjusted to slightly touch the gingival margin and the mucogingival junction, respectively. A cephalogram was taken with the device in place ( Fig 2 ).

Coronal and apical arms of metal device placed in bracket slot indicating gingival margin ( A ) and mucogingival junction ( B ). Mucogingival junction is defined by IKI solution ( C ).
Fig 1
Coronal and apical arms of metal device placed in bracket slot indicating gingival margin ( A ) and mucogingival junction ( B ). Mucogingival junction is defined by IKI solution ( C ).

Magnified view of anterior region on lateral cephalometric radiograph with metal device.
Fig 2
Magnified view of anterior region on lateral cephalometric radiograph with metal device.

The mandibular right and left first molars were used as anchors for the intrusion of the incisors. The molars were banded (0.018 x 0.025 in), and brackets with 0.018-in slots were placed on the incisors. A segmented utility arch (0.016 × 0.016 in, Blue Elgiloy, Dentaurum, Ispringen, Germany) was prepared, placed into the slots, and secured by ligatures. Thus, 45° buccal root torque for the molars and 15° labial root torque for the incisors were maintained. Twenty grams of orthodontic intrusive force was applied on each incisor.

The patients were instructed to perform bacterial plaque-control measures and placed on chlorhexidine (0.12%) medication twice daily. In triweekly recall sessions, professional bacterial plaque-control procedures were performed, and the archwires were checked. If necessary, the archwires were replaced with the new ones. After the intended intrusive movement, determined according to the treatment plan, the archwires were removed in weeks 17 to 20 (mean, 18.4 ± 1 weeks). The mucogingival junction was determined with IKI solution, and a new metal device was placed into the bracket of the mandibular right central incisor in the same way as the first one. Therefore, it was possible to determine the new positions of the gingival margin and the mucogingival junction on the cephalogram taken after treatment.

To determine the relative movement of the gingival margin and the mucogingival junction to intrusive movement, only the mandibular right central incisors (n = 16) were considered. In the cephalometric analyses, the perpendicular distance of the center of resistance to the corpus axis was accepted as the amount of intrusion ( Fig 3 ). The perpendicular distance of the gingival margin and the mucogingival junction, identified by the radioopacities of the extending arms of the metal devices on the cephalograms, to the corpus axis was accepted as the accompanying movement of the gingiva ( Fig 3 ). All measurements were corrected for the roentgenographic magnification (x 1.180).

Illustration of distance between corpus axis and center of resistance ( a ), gingival margin ( b ), and mucogingival junction ( c ) ( Pm , protuberentia menti on Xi-Pm line [ie, corpus axis]).
Fig 3
Illustration of distance between corpus axis and center of resistance ( a ), gingival margin ( b ), and mucogingival junction ( c ) ( Pm , protuberentia menti on Xi-Pm line [ie, corpus axis]).

Paired samples t tests were performed for statistical analysis with software (version 11, SPSS, Chicago, Ill), and the level of significance was set at α = 0.05.

Results

No statistically significant changes were observed in the scores of plaque index, gingival index, sulcus depth, bleeding on probing, and attachment level recorded before and after treatment ( P >.05) ( Table I ). The width of the keratinized gingiva and the attached gingiva reduced slightly ( P >.05); however, there was a significant decrease in clinical crown length ( P <.05). The distance from the incisal edge to the base of the sulcus and the mucogingival junction also decreased significantly ( P <.05) ( Table II ).

Table I
Periodontal indexes recorded before and after treatment
Before treatment After treatment Test
Plaque index 0.67 ± 0.29 0.74 ± 0.25 NS
Gingival index 0.43 ± 0.17 0.57 ± 0.08 NS
Sulcus depth 1.02 ± 0.37 1.22 ± 0.25 NS
Bleeding on probing 0.35 ± 0.11 0.52 ± 0.13 NS
Attachment level 0.76 ± 0.25 0.88 ± 0.19 NS
NS , Not significant, P >.05.

Table II
Changes in gingival widths and clinical crown lengths, and approximation of incisal edge to either sulcus base or mucogingival junction
Before treatment (mm) After treatment (mm) Test
Keratinized gingiva 3.43 ± 1.26 3.14 ± 1.02 NS
Attached gingiva 2.43 ± 1.28 2.07 ± 1.03 NS
Clinical crown length 8.59 ± 1.24 7.96 ± 1.3
Incisal edge—sulcus base 9.59 ± 1.36 9.03 ± 1.27 >
Incisal edge—mucogingival junction 12.03 ± 1.36 11.06 ± 0.98 >
NS , Not significant, P >.05.

P <.05.

The average intrusion was 2.62 ± 0.53 mm, with a slight labial proclination (mean 4.27° ± 1.15°) ( Fig 4 ). The gingival margin and the mucogingival junction moved apically in the same direction along with the tooth by 79% (2.07 ± 0.69 mm) and 62% (1.62 ± 0.78 mm) of the total amount of dental intrusion, respectively ( Fig 5 ). There was no statistically significant difference between the movement of the gingival margin and the mucogingival junction ( P >.05).

Illustration of labial angulation of incisors after intrusion.
Fig 4
Illustration of labial angulation of incisors after intrusion.

Illustration of displacement rates of gingival margin and mucogingival junction compared with amount of actual intrusion.
Fig 5
Illustration of displacement rates of gingival margin and mucogingival junction compared with amount of actual intrusion.

Discussion

It has been reported that describing intrusion as a cumulative reaction of the teeth against vertical forces and tipping movement leads to a conceptual controversy in orthodontics. Various amounts of intrusion can be achieved, depending on treatment time, force level, and differences in appliance design. Therefore, it is difficult to make reliable comparisons between the results of various studies. However, it was stated that actual intrusion could only be accomplished with a segmented arch, the technique that we used in our study.

The mandibular right central incisor was selected as the primary tooth for cephalometric analysis in this study, because it is the closest tooth to the midline and therefore the least subject to roentgenographic magnification in lateral cephalograms. To determine the relative gingival movements to orthodontic intrusion, tattoo marks were placed in the gingivae in a monkey study, but this method could not be used in humans for ethical reasons. Therefore, we used a specially designed metal device to determine the locations of the gingival margin and the mucogingival junction in the cephalogram.

To comprehend the reaction of the periodontium to orthodontic intrusion, an interpretation of the results from animal studies is necessary. Murakami et al demonstrated that although sulcular epithelium ended at the cementoenamel junction, the gingival sulcus deepened approximately 40% as much as the tooth intrusion when oral hygiene was maintained. Unlike the periodontal ligament and alveolar bone, the gingiva is not subject to resorption; it retracts due to stretching of the gingival fibers and does not peel off from the tooth surface, leading to periodontal pocket formation. The alveolar crest fibers are pressurized, causing alveolar crest resorption after 1.8 to 2 mm of orthodontic intrusion of animal incisors. However, even with oral hygiene measures, if the amount of intrusion is more than 5 mm, dentoperiosteal and dentogingival fibers tear off from the cementum, leading to increased sulcus depth without alveolar crest resorption.

In our study, there were no statistically significant increases in plaque index, gingival index, sulcus depth, bleeding on probing, and attachment level scores; this might be attributable to efficient personal and professional bacterial plaque-control procedures. The average intrusion was 2.62 ± 0.53 mm, and this could explain why sulcus depth did not increase significantly. As mentioned earlier, this amount of intrusion leads to compression of dentoperiosteal and dentogingival fibers and subsequent alveolar crest resorption in animals; this cannot be histologically demonstrated in humans. Therefore, it can be suggested that the reduction we noticed in gingival height was due to compression of the gingiva. As for the gingival movement in relation to orthodontic movement, the buccal gingiva moves distally by 49.4% to 82.4% of the total distal tooth movement, and the gingiva moves almost as much as a rotated tooth. The sulcus base and the mucogingival junction move in the direction of intrusion at a rate of 60% of the actual intrusion. Our findings show that the gingival margin and the mucogingival junction move in the same direction along with the intruded teeth at the rates of 79% and 62% of the actual intrusion, respectively. In comparison, the apical displacement rate of the gingival margin was more than that of the mucogingival junction, but this was not statistically significant.

Orthodontic appliances do not lead to significant reduction in the width of gingivae if effective bacterial plaque control is maintained. Likewise, when bacterial plaque control is adequate, combined retrusive-extrusive and retrusive-intrusive orthodontic movements of the maxillary anterior teeth have no negative effect on the width of keratinized and attached gingivae. On the other hand, lack of oral hygiene during intrusion leads to pocket formation and alveolar bone resorption. As stated by Melsen, the effect of intrusion of teeth is highly related to the standard of oral hygiene. In our study, as a result of strict oral hygiene procedures, there was no significant reduction in the widths of attached and keratinized gingivae.

Despite a slight reduction in gingival width, clinical crown lengths decreased significantly; this might have some clinical consequences. Possibly, the significant decrease in clinical crown length is due to the inability of the gingival complex to keep up with the movement of the teeth in the apical direction. Contrary to our findings, Abdel-Kader reported no significant clinical crown reduction in 91% of mandibular incisors 12 months after correction of overbite. Murakami et al reported that clinical crowns shortened due to gingival accumulation after intrusion without visual and histologic signs of inflammation. Although clinical crown reduction was statistically significant in this study, it was only 0.63 mm on average. However, with more intrusive movement, this figure might rise to clinically significant levels, which can make a periodontal surgical evaluation necessary after treatment. Particulary in patients with gummy smiles, when intrusion of incisors in serious amounts is required, a future gingivectomy might be necessary to regain actual crown length and use the full potential of intrusion therapy. Nevertheless, before deciding for a gingivectomy, follow-up is suggested after intrusion until the gingival tissues settle to a constant position.

This study had certain limitations with regard to precise determination of the movements of soft tissues, but it is still of value because it was carried out in human subjects for the first time. Also, the actual position of the gingival margin could have been determined by completely removing the orthodontic appliances for a certain time, but this was impossible because it could have caused relapse, and the patients were still in active orthodontic treatment. The data on the precise locations of the gingival margin and the mucogingival junction after treatment are being followed up.

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