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ABSTRACT

The purpose of this study was to determine whether the magnitude of intrusive force to the maxillary incisors influences the rate of incisor intrusion or the axial inclination, extrusion, and narrowing of the buccal segments. Twenty patients between the ages of nine and 14 years who needed at least two mm of maxillary incisor intrusion were assigned to one of two equal groups. In group 1 patients, the teeth in the maxillary anterior segment were intruded using 40 g, whereas in group 2 patients, 80 g was used. Records were taken from each patient at the beginning and end of intrusion. There was no statistically significant difference between the 40- and 80-g groups in the rate of incisor intrusion, or the amount of axial inclination change, extrusion, and narrowing of the buccal segments.

KEY WORDS: Intrusion, Force level, Biomechanics.

Accepted: August 2004. Submitted: June 2004

Correction of a deep overbite is often one of the major steps in orthodontic treatment. Depending on the diagnosis and treatment objectives, deep overbites can be treated orthodontically by intrusion of maxillary or mandibular incisors (or both), extrusion of buccal segments, or a combination of these.^1,2 This study focused on correction of deep overbite by intrusion of maxillary central and lateral incisors and evaluated various options to decrease side effects and to increase efficiency with minimal dependence on patient cooperation. To date, very few clinical studies have focused on intrusion.^3,4

Most investigations were performed to compare different methods of deep overbite correction.^3,4 Other reports on intrusion are based on in vitro or laboratory studies^1,5,6 and animal studies.^7,8 Because intrusion is often the preferred way of deep overbite correction, a randomized clinical trial focusing on all aspects related to intrusion is needed as a scientific basis for clinical work and to increase treatment efficiency.

Axial inclination change of the buccal segment is caused by the moment M = F × D, in which F is the intrusive force and D is the distance from the point of force application to the center of resistance (Figure 1 ). The line along which distance D is measured is perpendicular to the line of action of the intrusive force.^1,2,9–13 Several methods have been suggested to decrease this side effect including increasing the number of teeth included in the buccal segment,^1,2,9–11 high-pull headgear wear, and decreasing the amount of intrusive force.^1,2,10

Extrusion of the buccal segment is caused by force F, which is equal in magnitude but opposite in direction to the intrusive force (Figure 1 ).^1,2,9–11 Occlusal forces in part counteract extrusion.^1,2,9,13,14 To decrease the possibility of extrusion, the clinician has the options of keeping the intrusive force on the anterior segment as low as possible, increasing the size of the buccal segment, or counteracting the extrusive force on the buccal segment by, for example, a high-pull headgear.^{1,2,9,15–17}

In the frontal view, the extrusive force is delivered buccal to the center of resistance of the maxillary molar or buccal segment and therefore it creates a moment that can decrease the maxillary arch width.^1,2,9 Besides keeping the forces as low as possible and using a high-pull headgear to counteract the vertical force, the clinician can use a passive transpalatal arch to maintain the intermolar distance.¹⁸

The force should be delivered at a constant and optimal level,^7,8,15,18 and this requires a spring with a low load-deflection ratio. A large fluctuation in force level causes side effects when the forces are too high or no movement at all when the forces are too low, thereby decreasing efficiency. A low load-deflection rate also makes the amount of activation by the clinician less critical and decreases the need for frequent reactivations.

The force level required for intrusion has been reported as low as five g per tooth in patients with decreased periodontal attachment.⁴ Commonly, 10–20 g of force is advocated for maxillary incisor intrusion.^1–3 This recommendation is based on clinical experience. A comparison of different force levels would be meaningful in creating a more efficient approach toward intrusion. It is important to investigate what amount of force intrudes incisors as fast as possible with the least amount of side effects.

To be able to calculate the magnitude of the moments and forces delivered, the force system should be determinate.^9–11,19 The intrusive force has to be delivered through a point contact to the anterior segment. This can be achieved by tying the intrusion arch in a piggyback fashion onto the anterior segment.^{1–3,6,9–13,19} The segments should be as rigid as possible to minimize side effects from wire deformation^1,2,10 and to evenly distribute the moment and forces over the buccal segment as a unit.^1,2,9–11

This study evaluated the effects of intrusive force. The purpose of this study was to determine whether the magnitude of the intrusive force influences the rate of intrusion or the amount of axial inclination change, extrusion, and narrowing of the buccal segments. In this study, the null hypothesis is that the magnitude of the intrusive force has no effect on axial inclination change, extrusion, and narrowing of the buccal segment or on the rate of intrusion.

Orthodontic patients needing at least two mm of maxillary central and lateral incisor intrusion were recruited for this study from all patients referred to the principal investigator's practice. Treatment was performed by one orthodontist only. Patients included in the sample were between nine and 14 years of age and had at least maxillary first molars, first and second premolars, canines, and all maxillary incisors present and fully erupted. Patients with periodontal disease and patients with extremely flared or upright incisors (such as in Class II, division 2 patients) were excluded. Patients with crowding to the extent that they needed extractions to perform alignment were also excluded. No other form of orthodontic treatment was performed in these patients during the time of maxillary incisor intrusion. All patients willing to participate in this study were included in the sample if they met the above-mentioned requirements. During a four-year period, 40 patients were recruited, and these patients were divided into four groups by simple randomization.²⁰ In the present study, two groups of 10 patients each were used. In group 1 patients, the teeth in the maxillary anterior segment were intruded using 40 g, whereas in group 2 patients, 80 g was used.

Lateral cephalograms were traced on a computer screen and on acetate paper.^21,22 From each set of lateral cephalograms a maxillary superimposition (structural) was made.²³ The tracings were digitized and analyses performed by computer.²⁴ The following measurements were performed:

1. Vertical movement of the center of resistance of the maxillary central incisor (indicating the amount of intrusion). Vertical was defined as perpendicular to the palatal plane (Figure 2 , measurement 1). The center of resistance (Cr) of the maxillary incisor was selected as a measurement point instead of the center of resistance of the anterior segment because of its easier to locate and more reproducible location. Because of the rigidity of the anterior segment and the small sagittal distance from the Cr of the maxillary central incisor to the Cr of the anterior segment in this sample, the possibility of error created by using this measurement is small.

2. Change in axial inclination of the buccal segment, which was determined by measuring the angle between the buccal segment and the palatal plane (Figure 2 , measurement 2).

3. Change in axial inclination of the anterior segment, which was determined by measuring the angle between the maxillary central incisor and the palatal plane (Figure 2 , measurement 3).

4. Vertical movement of the buccal segment, which was determined by the distance between the center of resistance of the maxillary first molar and the palatal plane (Figure 3 , measurement 4). The center of resistance of the maxillary first molar was selected as the measurement point instead of the center of resistance of the buccal segment because of its easier, more reliable, and reproducible location. The location of the center of resistance of the maxillary first molar was the trifurcation.^{1,2,9–13,15,16}

5. Change in intermolar width, measured on the models.

6. Change in distance between the incisal edge and the distal side of the maxillary first molar, measured parallel to the palatal plane (Figure 3 , measurement 5).

7. Change in distance between the point of intrusive force application and the center of resistance of the maxillary central incisor, measured parallel to the palatal plane (Figure 4 , measurement 6).

8. Rate of intrusion, expressed in millimeters per week.

To make the measurement error as small as possible, the digital image was enlarged to the extent that the crosshair symbol used for landmark identification was much smaller than the enlarged landmark itself. The next step was to make tracings on acetate paper, make digital images of these tracings and of the cephalograms directly, and trace both.

To make the superimpositions more reliable it was decided to make structural superimpositions on maxillary skeletal structures, which were made using the tracings on acetate. This has the clear advantage over the computer superimposition because the complete outlines of the skeletal structures are used and not just a few digitized points. This method was checked independently by reanalyzing the start and finish cephalograms of 10 patients. The mean differences between both measurements varied from 0.01° for the angular measurement between the central incisor and the palatal plane and 0.01 mm between the distance from the auxiliary tube to the point of intrusive force application to 0.24 mm for the distance between the incisal edge and the maxillary first molar. None of the differences were statistically significant.

To test the null hypothesis, an analysis of variance for repeated measures (measurements at start and finish) was used with group as the independent variable (General Linear Models procedure in SPSS 10). Tables 1 and 2 display the differences and the statistical significance of these differences between both groups in intrusion, arch width, axial inclination change of the buccal segment, and extrusion of the buccal segment.

Table 3 shows the changes in vertical incisor position, arch width, axial inclination of the buccal segment, and vertical movement of the buccal segment of both groups combined between start and finish of incisor intrusion.

The mean intrusion of the anterior segment in both groups was more than two mm (Table 1 ). There was no statistically significant difference in the amount of intrusion between the groups (Table 2 ). The vertical incisor movement of both groups combined was statistically significant (Table 3 ).

The mean intermolar width decreased slightly (0.27 mm, Table 1 ) in the 40-g group and remained about the same (0.04 mm increase, Table 1 ) in the 80-g group. The difference between both groups was not statistically significant (Table 2 ). The change in intermolar width of both groups combined was also not statistically significant (Table 3 ).

In both groups the axial inclination change of the buccal segment increased a small amount: 0.63° in the 40-g group and 1.49° in the 80-g group (Table 1 ). The difference was not statistically significant (Table 2 ). The change in axial inclination of the buccal segments in both groups combined, however, was statistically significant (Table 3 ).

Both groups experienced a small amount of buccal segment extrusion: 0.13 mm in the 40-g group and 0.06 in the 80-g group (Table 1 ). This difference was not statistically significant (Table 2 ). Also, the extrusion of both groups combined was not statistically significant (Table 3 ).

The mean rate of intrusion was 0.15 (SD 0.05) mm per week, with a range from 0.08 to 0.26 mm per week in the 40-g group. In the 80-g group the mean rate was 0.16 (SD 0.05) mm per week, with a range from 0.07 to 0.23 mm per week. The difference between the groups was not statistically significant (Table 2 ).

In both groups a statistically significant amount of incisor intrusion of more than two mm was performed, with no statistically significant difference between the groups. Because the amount of intrusion was predetermined by the patient's treatment plan, a difference in amount of intrusion between the groups was not expected.

A higher intrusive force results in a higher extrusive force buccal to the center of resistance, which increases the likelihood of maxillary arch constriction.^1,2,9 There was, however, no statistically significant difference between the 40- and 80-g groups in intermolar width. This indicates that, in this intrusive force range, the occlusal forces in combination with the size of the buccal segments were sufficient to hold the maxillary intermolar width.

In the 80-g group the mean angulation of the buccal segment increased less than 1° more than that of the 40-g group. The difference between the groups was not statistically significant. The change in axial inclination change in both groups combined was statistically significant, even though the absolute change was small. These findings suggest that up to 80 g of intrusive force does not result in a clinically significant amount of axial inclination change of the buccal segments when the segments consist of canines, first- and second premolars, and first molars.

A higher intrusive force on the incisors results in a higher extrusive force on the buccal segments and thereby increases the likelihood of extrusion of the buccal segment.^1,2,9–11 In this study the amount of extrusion in the 40- and 80-g groups was not statistically significant. The difference in extrusion between the two groups was also not statistically significant. These findings indicate that intrusive forces of up to 80 g do not result in significant extrusion of the buccal segments when canines, first- and second premolars, and first molars are present and included in the buccal segments.

To make treatment as efficient as possible, a high rate of intrusion combined with the smallest possible amount of side effects is preferred. In this study the effect of intrusive force on the rate of intrusion was tested. Between the 40- and 80-g groups no statistically significant differences were observed. The range in rate was quite large but similar when comparing both groups. This indicates that increasing the force from 40 to 80 g does not increase the rate of intrusion.

Force magnitude can be related to anchorage loss.^1,2,10–13 This is of particular interest when intrusion is combined with space closure. This was, however, not the purpose of this investigation. To determine the influence of force level on anchorage loss, more different force levels, with higher and lower forces than the ones used in this study, will have to be studied.

• Maxillary incisors could be intruded with forces of 10 to 20 g per tooth.

• There was no statistically significant difference in extrusion of the buccal segments between the 40- and 80-g groups.

• There was no statistically significant difference between the 40- and 80-g groups in the axial inclination of the buccal segments. In both groups combined the amount of axial inclination change in the buccal segments was statistically significant but too small to be of clinical importance.

• The change in intermolar width was not statistically significantly different between the 40- and 80-g groups. In both groups combined the intermolar width did not change either.

• An intrusive force of 80 g did not increase the rate of intrusion as compared with 40 g.

TABLE 1.Start and Finish Measurements and the Differences Between Them of the 40- and 80-Gram Groups. The Standard Deviations are in Parentheses

TABLE 2.Statistical Significance of the Differences Between the 40- and 80-Gram Groups for the Different Measurements, Showing the F Values with the Degrees of Freedom in Parentheses and the P values

TABLE 3.Changes of the 40- and 80-Gram Groups Combined Between Start and Finish

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FIGURE 1. Force system from an intrusion arch

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FIGURE 2. Measurement 1 indicates the distance between the center of resistance of the maxillary central incisor and the palatal plane, measurement 2 is the angle between the central incisor and the palatal plane, measurement 3 is the angle between the buccal segment and the palatal plane

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FIGURE 3. Measurement 4 is the distance from the center of resistance of the first molar to the palatal plane, measurement 5 the distance from the incisal edge to the distal side of the first molar parallel to the palatal plane

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FIGURE 4. Measurement 6 is the distance from the point of force application to the center of resistance of the central incisor. To clarify the figure the point of force application was moved further anteriorly than its actual location in the study