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TABLE OF CONTENTS

[INTRODUCTION] [MATERIALS AND...] [RESULTS] [DISCUSSION] [CONCLUSIONS] [REFERENCES] [TABLES] [FIGURES]


The Angle Orthodontist: Vol. 75, No. 5, pp. 723–729.

The Influence of Force Magnitude on Intrusion of the Maxillary Segment

E. van Steenbergen;a C. J. Burstone;b B. Prahl-Andersen;c I. H. A. Aartmand

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


INTRODUCTION Return to TOC

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 studies1,5,6 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 deformation1,2,10 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.


MATERIALS AND METHODS Return to TOC

Sample

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.

Records

A lateral cephalometric radiograph, one set of impressions with a wax bite in centric occlusion, and intraoral photographs were obtained from each patient at the start of intrusion and when intrusion of the maxillary four incisors was completed or stopped in case of clearly visible side effects. The lateral cephalograms were taken with the aid of a cephalostat by the principal investigator. The patient's head position in the cephalostat was documented, so that pre- and postintrusion cephalograms were taken with the patient's head in the same position. To distinguish the patient's right and left side, a ligature wire was tied around the right canine bracket in such a way that it was clearly visible on the lateral cephalogram. Impressions were poured in plaster and trimmed in centric occlusion.

Measurements

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.

Treatment protocol

Patients were recruited after explanation of the treatment plan by the orthodontist. After bands and brackets were placed, alignment was performed in segments, with the anterior segment extending from the right to left lateral incisor and the buccal segment from canine to first molar. When the wire segments were rigid and passive, one lateral cephalogram, five intraoral photographs (one frontal, two buccal, and two occlusal photographs), and one set of impressions with a wax bite in centric occlusion were obtained. To be certain that the segments were passive, they were left in place for five weeks after insertion, before records were taken and intrusion was started. At the same visit the intrusion arch was placed with a force level of 40 g in group 1 and 80 g in group 2 as measured in the midline (20 g per side).

Visits were scheduled every five weeks. During each visit the intrusive force was measured, recorded, and, when necessary, adjusted to the proper level. When the incisors were intruded to the proper level, the intrusion arch was removed and a lateral cephalogram, impressions, and wax bite were obtained again. The same actions were undertaken when side effects were clearly present. Loose bands and brackets were recorded and replaced in a manner that maintained the passivity of the buccal segments.


RESULTS Return to TOC

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 ).


DISCUSSION Return to TOC

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.


CONCLUSIONS Return to TOC


REFERENCES Return to TOC

1. Burstone CJ. Deep overbite correction by intrusion. Am J Orthod. 1977; 72:1–22. [PubMed Citation]

2. Burstone CJ. Biomechanics of deep overbite correction. Semin Orthod. 2001; 7:26–33.

3. Weiland FJ, Bantleon HP, Droschl H. Evaluation of continuous arch and segmented arch leveling techniques in adult patients—a clinical study. Am J Orthod Dentofacial Orthop. 1996; 110:647–652. [PubMed Citation]

4. Melsen B, Agenbæk N, Markenstam G. Intrusion of incisors in adult patients with marginal bone loss. Am J Orthod Dentofacial Orthop. 1989; 96:232–241. [PubMed Citation]

5. Vanden Bulcke M, Sachdeva R, Burstone CJ. The center of resistance of anterior teeth during intrusion using the laser reflection technique and holographic interferometry. Am J Orthod. 1986; 90:211–219.

6. Dermaut LR, Vanden Bulcke MM. Evaluation of intrusive mechanics of the type “segmented arch” on a macerated human skull using the laser reflection technique and holographic interferometry. Am J Orthod Dentofacial Orthop. 1986; 89:251–263.

7. Dellinger EL. A histological and cephalometric investigation of premolar intrusion in the Macaca speciosa monkey. Am J Orthod. 1967; 53:325–355. [PubMed Citation]

8. Steigman S, Michaeli Y. Experimental intrusion of rat incisors with continuous loads of varying magnitude. Am J Orthod. 1981; 80:429–436. [PubMed Citation]

9. Burstone CJ, Koenig HA. Creative wire bending—the force system from step and V bends. Am J Orthod Dentofacial Orthop. 1988; 93:59–67. [PubMed Citation]

10. Burstone CJ. The rationale of the segmented arch. Am J Orthod. 1962; 48:805–821. [PubMed Citation]

11. Smith RJ, Burstone CJ. Mechanics of tooth movement. Am J Orthod. 1984; 85:294–307. [PubMed Citation]

12. van Steenbergen E, Nanda R. Biomechanics of orthodontic correction of dental asymmetries. Am J Orthod Dentofacial Orthop. 1995; 107:618–624. [PubMed Citation]

13. Romeo DA, Burstone CJ. Tip-back mechanics. Am J Orthod. 1977; 72:414–421. [PubMed Citation]

14. Woods MG. The mechanics of lower incisor intrusion: experiments in nongrowing baboons. Am J Orthod Dentofacial Orthop. 1988; 93:186–195. [PubMed Citation]

15. Firouz M, Zernik J, Nanda R. Dental and orthopedic effects of high-pull headgear in treatment of Class II, division 1 malocclusion. Am J Orthod Dentofacial Orthop. 1992; 102:197–205. [PubMed Citation]

16. Dermaut LR, Kleutghen JPJ, De Clerck HJJ. Experimental determination of the center of resistance of the upper first molar in a macerated, dry human skull submitted to horizontal headgear traction. Am J Orthod Dentofacial Orthop. 1986; 90:29–36. [PubMed Citation]

17. Armstrong MM. Controlling the magnitude, direction and duration of extraoral force. Am J Orthod. 1971; 59:217–243. [PubMed Citation]

18. Burstone CJ, Manhartsberger C. Precision lingual arches— passive applications. J Clin Orthod. 1981; 22:444–451.

19. Burstone CJ, Koenig HA. Force systems an ideal arch. Am J Orthod. 1974; 65:270–289. [PubMed Citation]

20. Pocock SJ. Clinical Trials, a Practical Approach. Chichester: Wiley & Sons; 1984:73–76.

21. Baumrind S, Frantz R. The reliability of headfilm measurements: 1. Landmark identification. Am J Orthod. 1971; 60:111–127. [PubMed Citation]

22. Baumrind S. The reliability of headfilm measurements: 2. Linear and angular measurements. Am J Orthod. 1971; 60:506–517.

23. Baumrind S, Miller D, Molthen R. The reliability of headfilm measurements: 3. Tracing superimpositions. Am J Orthod. 1976; 70:617–644. [PubMed Citation]

24. Faber R, Burstone CJ, Solonche DJ. System of computerized treatment planning. Am J Orthod. 1978; 73:36–46. [PubMed Citation]



TABLES Return to TOC

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




FIGURES Return to TOC


Click on thumbnail for full-sized image.

FIGURE 1. Force system from an intrusion arch



Click on thumbnail for full-sized image.

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



Click on thumbnail for full-sized image.

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



Click on thumbnail for full-sized image.

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


aPrivate Practice, 7311 KK Apeldoorn, The Netherlands

bProfessor Emeritus, Department of Orthodontics, University of Connecticut Health Center, Farmington, Conn

cProfessor and Chairperson, Orthodontic Department, Academic Centre for Dentistry, Amsterdam, The Netherlands

dAssistant Professor, Methodologist, Department of Orthodontics and Social Dentistry, Section Social Dentistry and Dental Health Education, Academic Centre for Dentistry, Amsterdam, The Netherlands

Corresponding author: E. van Steenbergen, DDS, MDS, Hofdwarsstraat 1 c, 7311 KK Apeldoorn, The Netherlands (E-mail:
ecbt@vansteenbergen.nl)




© Copyright by E. H. Angle Education and Research Foundation, Inc. 2005