Treatment effects of myofunctional appliances in different jaw rotations in Class II division 1 malocclusion

INTRODUCTION

Class II malocclusions can manifest in various skeletal and dental configurations.^[1-5] Most Class II patients have a deficiency in the anteroposterior position of the mandible.^[6] Several treatment options are available for managing Class II problems, and functional appliances have been used for many years in the treatment of Class II Division 1 malocclusions. Several varieties of functional appliances are currently in use that aim to improve skeletal imbalances.^[7-9] The term functional appliance refers to a variety of orthodontic appliances designed to induce a change in activity of the various muscle groups that influence the function and position of the mandible in order to transmit forces to the dentition and the basal bone. Alteration of maxillary growth, possible improvement in mandibular growth and position, and change in dental and muscular relationships are the expected effects of these functional appliances.^[10]

It has been claimed that the forward growth of the maxilla can be inhibited,^[3-16] redirected,^[17] or unaffected^[11-14] by functional appliances. The effect of functional appliances on mandibular growth is controversial. Some authors suggested that mandibular growth can be increased with functional appliance treatment,^[15-17] but others believe the appliances have no real effect on mandibular length.^[18,19] However, most researchers agree that the appliances produce retroclination of the maxillary incisors^[10,14,16] and proclination of the mandibular incisors.^[18] There is no consensus on how the molar correction occurs.

Two of the more popular functional appliances used today are the activator^[18] and Clark’s Twinblock.^[20] Few studies have compared the effects of these appliances.^[21,22] Both are tooth-borne, but the twin-block is designed for full-time wear to take advantage of all functional forces applied to the dentition, including the forces of mastication.

According to Schudy,^[23] the rotation of the mandible is the result of disharmony between vertical growth and antero-posteior or horizontal growth of the jaws. Accordingly, he describes two types of growth rotations of the mandible. If the condylar growth is greater than the vertical growth in the molar area, the mandible rotates counter-clockwise resulting in more horizontal change of the chin and less increase in anterior face height. Conversely, if vertical growth at the molar area is greater than at the condyles, the mandible rotates clockwise resulting in increased anterior face height and less horizontal change of the chin. Clockwise rotators are supposed to exhibit an unfavorable reaction to functional therapy due to a posterior mandibular rotation.^[19]

The purpose of this study was to determine the skeletal, dentoalveolar and soft tissue changes in children with different jaw base rotations treated with either the twin block appliance or the activator for the treatment of Class II Division 1 malocclusion.

MATERIALS AND METHODS

The sample consisted of the patients who were already treated in the Department. Individual standardized lateral cephalograms of 32 patients (18 boys and 14 girls) taken from the X-Trophy Cephalostat manufactured by Kodak were selected. All the cephalometric radiographs were with the teeth in occlusion.

The inclusion criteria were as follows:

• Age group was between 11 and 14 years

• The subjects were treated with a mandibular protrusive appliance (twin block or activator) for the treatment of Class II Division 1 malocclusion.(8-10 h for the activator patient, 14-16 h for the twin block patients, per day)

• The mean duration of the protrusive appliance treatment was 1 year

• Inter-occlusal acrylic trimming was performed in a low angle patients to allow unhindered vertical development of the mandibular buccal segments

• No adjunctive orthodontic treatment either prior to or during the period of functional treatment.

The subjects were divided into two groups:

• A high angle group (n = 16) with frankfort mandibular plane angle (FMA) >27° and

• A low angle group (n = 16) with FMA < 20°.

Cephalograms taken at T1 (pretreatment) and T2 (after 1 year of wearing myofunctional appliance) for all subjects in both the groups were collected. These cephalograms were traced manually and analyzed on the basis of various landmarks chosen to determine the skeletal, dentoalveolar and soft tissue changes in both the groups. The identification of these landmarks was based on the definition by Riolo et al.^[24] A vertical reference plane (VRP) through the sella at 97° to the sella–nasion (SN) plane was constructed [Figure 1]. Forty-two angular, linear, and proportional measurements were recorded. The measurements were verified by two authors, and the mean was considered. The data collected were subjected to statistical analysis.

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Figure 1

A vertical reference plane through the sella at 97° to the sella–nasion plane

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RESULTS

The results are presented in tabular form. The mean ages and length of treatment for both groups were similar.

DISCUSSION

The discrepancy between the jaws in the development of the stomato-gnathic system plays an important role in the development of Angles Class II Division 1 malocclusions. After 3.8 years of study on a lot of 23 Class II untreated patients, Baccetti et al.^[25] have reported that under the influence of normal growth, the length of the mandible is changing with 0-1.2 mm. A previous study, 14.5 months long, on Brazilian Class II untreated subjects of Gomes and Lima,^[26] has reported an annual growth rate of 2.16 mm for the horizontal ramus, 3.16 mm for the vertical ramus and 4.31 mm for the total mandibular length.

McNamara (1981)^[27] studied the components of Class II malocclusion in children and 8-10 years of age and revealed that the majority have a component of mandibular deficiency. Changing the function of the mandible by inducing the patient to function with the lower jaw placed forward could stimulate mandibular growth thereby correcting a Class II problem. It has been established that functional appliances play a significant role in the correction of sagittal and vertical malrelationship (Schmuth 1983).^[6] The change in maxillo-mandibular relationship after forward alteration of the mandibular position results from an interrelated series of changes in several regions within the craniofacial complex (Elgoyhen et al. 1972).^[28] Mamandras and Allen (1990)^[29] suggested that the persons who have small mandibles may benefit from functional appliance therapy than patients with normal-sized mandibles. This study was conducted to determine the skeletal, dentoalveolar and soft tissue changes in children with different jaw rotations treated with either the twin block appliance or the activator for the treatment of Class II Division 1 malocclusion.

All 32 patients selected for this study had undergone functional appliance therapy for correction of a Class II Division I incisor relationship. The subjects were divided into two groups as high angle group (n = 16) with FMA >27° and a low angle group (n = 16) with FMA <20°. The statistically significant changes were seen with the SNB angle and the linear distance from the VRP-Pog in both the high and low angle groups suggesting favorable response to myofunctional appliances by posturing the mandible forward. There was an increase in the corpus length (Go-Gn) in both the groups but not significantly.

The SNA angle, the horizontal distance from VRP-A and VRP-ANS, remained the same for both the groups, suggesting that the functional appliances had a restraining effect on the maxilla. When the mandible was postured forward by the functional appliances, a reciprocal force acted distally on the maxilla, redirecting growth.^[30]

Amongst the dentoalveolar changes, both the groups showed statistically significant reduction in the overjet by retroclination of the upper incisors and proclination of the lower incisors. Mandibular molars significantly moved ahead in relation to the VRP and also showed significant vertical eruption in both the groups. The forward movement of mandibular molars could be attributed to the repositioning of the mandible. The vertical eruption of the molars in the low angle group was because of interocclusal acrylic trimming which was carried out to allow unhindered vertical development of the mandibular buccal segments. As a protocol, all the high angle patients were treated with activator only to restrict further vertical growth. In the high angle group inter-oclussal trimming was not done of the activators, but there was vertical eruption of molars that may have been due to the part-time wear of the activator in the high angle patients. In the upward and forward rotators, that is, in low angle group, these changes are expected, and the literature also supports these findings. But significant findings were also found in the high angle group, which were unexpected. As our study was limited only to a 1 year duration, long-term follow-up of this group is required to further evaluate these findings.

In the low angle group, we found that there is a statistically significant increase in the gonial angle and the LAFH which could be attributed to the upward and mesial movement of the mandibular molars indicating that the horizontal growers showed a favorable response to mandibular advancement by enhancing eruption of posteriors.

In the high angle group, though there was a significant increase in the S-N-Pog and VRP-B, which indicates forward movement of the mandible. This forward movement did not cause a significant increase in the LAFH; that shows that in the high angle group there was a good control on the downward and backward rotation of mandible. In this group, we also found that the lower incisors are significantly proclined because of the forward positioning of the mandible.

Functional appliances have been shown to stimulate bone apposition at the posterior pole of the condyle,^[31-34] this therapeutic effects coincides with the inherited condylar growth direction in vertical growers^[35] and this could explain the unexpected favorable skeletal changes in high angle group.

CONCLUSION

In this study, it was found that both the high angle and low angle groups responded equally well to the myofunctional therapy, in fact, in the high angle group, we found significant changes in more number of variables including skeletal and dentoalveolar parameters.

LIMITATIONS

• Duration of the study was short. This study evaluates the effects of myofunctional appliances after 1 year only, which may not be permanent. So a further study with an increased sample size and followed for a longer time period will greatly improve the authenticity of our results

• The amount of sagittal and vertical advancement during bite registration for myofunctional appliances was not taken into consideration.