Treatment Effects of Forsus™ Fatigue Resistant Device versus Herbst Appliance in treatment of Class II div 1 Malocclusion

Adel Bahaitham 1, Mithran S. Goonewardene 2, John M. Razza 3

1 Former Graduate Student, Dental School, The University of Western Australia 

2 Program Director, Orthodontics, Dental School, The University of Western Australia 

3 Senior Lecturer, Orthodontics, Dental School, The University of Western Australia

 
ABSTRACT

The objective of this study was to compare the skeletal and dento-alveolar changes associated with the Forsus™ Fatigue Resistant Device and the Herbst appliances in conjunction with full fixed appliances (FFA) in the correction of Class II Division 1 malocclusion. 

 Pre-treatment and post-treatment lateral cephalograms of 27 growing patients treated with the Forsus™ Fatigue Resistant Device and FFA (mean age was 13.62 years) and 20 growing patients treated with the Herbst appliance (mean age was 12.52 years) were digitized and analyzed using the method of Pancherz.

During treatment there was no significant change in the position of the condyle for either group. In the maxilla, A point came forward more in the Herbst group (1.66mm) and came forward less in the Forsus group (0.87mm); however, these changes were not statistically-significant. In the mandible, Pognion came forward (3.76mm) in the Herbst group and (4.35mm) in the Forsus group. Molar correction was (3.6mm) in the Herbst group and (2.03mm) in the Forsus group. Overjet correction was (5.65mm) in the Herbst group and (4.55mm) in the Forsus group. The relative contribution to overjet and molar correction was predominantly skeletal for the Forsus group and approximately equally dental and skeletal for the Herbst group. There were no changes in the vertical planes angles in the Herbst group while the palatal plane angle has statically-significant increase in the Forsus group.

Dental effect of appliance therapy irrespective of the choice of appliance appeared to increase with advancing age as the skeletal effects decreased. Both appliances were effective in managing the Class II div 1 malocclusion.

 
INTRODUCTION

Management of the Class II malocclusion is one of the most challenging procedures in orthodontics, with an estimated one-third of all orthodontic patients treated for this condition1-6. It is well known, however, that Class II malocclusion is not a single diagnostic entity7,8 but may result from various combinations of skeletal and dentoalveolar components. The most consistent finding in Class II malocclusion is mandibular skeletal retrusion9. 

Philosophically therefore, therapy should be directed to enhance mandibular growth in these patients.

Controversy exists as to the mode of action of various appliances and their purported effects including the appropriate timing for maximum efficiency. Appliances such as extra-oral traction, various types of functional appliances and their derivatives and compensatory tooth movement have all been recommended for patients with similar dento-skeletal morphology. What appears to be clear is that most techniques appear to be effective in addressing the dental problems but the relative contribution of skeletal and dental problems may vary slightly with the technique.

The term functional appliance refers to a variety of a removable or fixed appliances designed to alter the mandibular position, both sagittally, vertically and sometimes transversely, resulting in orthodontic and orthopedic changes. Advocates of functional appliances have maintained that stimulation of mandibular growth caused by forward positioning of the mandible may be the primary mechanism by which they effect a change. Histological studies on laboratory animals have consistently shown a significant increase in cellular activity when the mandible is hyperpropulsed, in addition to activation of genes which regulate both pre-chondrobalstic cell differentiation and chondroblast cell division. Remodeling of the articular fossa has also been suggested as a contributing mechanism to the redirection of the mandibular growth. It has been suggested that analogous effects may be observed in humans which aids in the correction of Class II malocclusion. Conversely, investigators have suggested that greater mandibular growth does not occur in humans as had been demonstrated in animals, and these observed changes might be similar to those expected with normal growth or conventional edgewise therapy. Moreover, it has been proposed that the Class II correction observed with functional appliances was caused by a ‘headgear’ effect of restraining maxillary growth, with a combination of dental changes such as retroclination of the maxillary incisors and proclination of the mandibular incisors. It appears that the differences in the long-term effects between the various appliances may distill down to the way in which the maxilla is controlled in both the horizontal and the vertical dimensions. 

The lack of success of orthodontic treatment and in particular, functional appliances has in some circumstances been attributed to lack of patient compliance. In addition, failure to achieve optimum results has also sometimes been attributed to the inability to control the amount and direction of mandibular growth. A variety of treatment techniques, including the Herbst appliance have been devised to overcome this issue in an attempt to exclude compliance demands. The Herbst appliance, originally designed by Emil Herbst in, reintroduced and popularized by Pancherz in eliminates the need for patient cooperation and potentially stimulates the overall amount of mandibular growth, directs this growth in the appropriate direction.

The Herbst appliance is usually a fixed “bitejumping” device that features a bilateral telescoping mechanism. Telescoping tube-and-rod assemblies extend from the region of the maxillary first molars to the region of the mandibular premolars and keep the mandible in a constantly protruded position. Many variations is design have been proposed and well documented reports outlining clinical management procedures. 

Numerous analogous protrusive “bite-jumping” appliances have been developed to be used in conjunction with fixed multibanded appliances. The Forsus™ Fatigue Resistant Device, developed by Bill Vogt in 2001, is one such appliance consisting of a 3-piece telescoping coaxial spring, modified from the Forsus Nitinol Flat Spring (FNFS). Comparison of the effects of the FNFS and Jasper Jumper appliances have been reported, yet effects of Forsus™ Fatigue Resistant Device has not been compared with any other fixed appliance. The purpose of this study was to compare the dental and skeletal therapeutic effects obtained by Forsus™ Fatigue Resistant Device with the historical standard of the Herbst appliance in treating Class II malocclusions in growing children.

 

MATERIALS AND METHODS

A retrospective study was undertaken to assess the treatment effects of the Herbst appliance and the Forsus™ Fatigue Resistant Device in combination with fixed appliances. Ethical approval for this study was granted by the Human Research Ethics Committee of The University of Western Australia.

A total sample of 47 patients presenting with Class II division I malocclusion, treated with either Herbst appliance or the Forsus™ Fatigue Resistant Device in combination with fixed appliances selected from the files of the University of Western Australia, Orthodontic Department’ The following inclusion criteria were applied:

• The patient presented for treatment in the permanent dentition.

• The patient was less than 14 years 6 months of age.

• Satisfactory pre-treatment (T1) and posttreatment (T2) lateral cephalograms were available. 

• All the patients were treated with 0.022-inch X 0.028-inch. Full Fixed edgewise appliances 

The Forsus™ Fatigue Resistant Device group consisted of 27 patients with a mean age of 13.17 and 14.39 years for females and males respectively, The mean age of the 20 patients in the Herbst group was 11.79 and 12.65 years for female and males respectively.

Pre-treatment (T1) and post-treatment (T2) lateral cephalograms used in the present study were taken with the teeth in habitual occlusion and the lips in repose. For all measurements, magnification was adjusted for Films were taken on average, 204 days and 171 days before commencement of the treatment in the Forsus™ Fatigue Resistant Device and Herbst groups respectively, followed by post-treatment cephalograms taken on average, 84 days and 110 days after commencement of the treatment in the Forsus™ Fatigue Resistant Device and Herbst groups respectively.

 
Clinical application of the Forsus™ Fatigue Resistant Device and Herbst appliance

The patients involved in the study were treated by a variety of operators all of whom share similar treatment and diagnostic criteria and methods within the University of Western Australia orthodontic treatment framework, with all appliances designed similarly and fabricated by the same orthodontic laboratory.

The Forsus™ Fatigue Resistant Device (3M Unitek, St Paul, Minn), (Figure1), is produced in four different lengths 25, 29, 32, 35mm. After a leveling phase, 0.019 X 0.025-inch stainless steel arch wires were engaged. The size of the appliance was determined by measuring the distance from the distal end of upper molar tube to a point distal to cuspid bracket while in centric occlusion using the Forsus™ guide (REF 807-014) provided by the manufacturer. The appliance was attached to the headgear tubes of the maxillary first molars through “L” ball pin into spring module distal-end pin hole with leaving 1 to 2 mm clearance between the distal end of tube and pin ball and activating the appliance by crimping a split ring bushing on the push rod distal of the stopper by compressing the spring as needed (usually 2 or 3 mm at a time to achieve midline correction and advancement)122,123 or by enlarging the male component. Patients were observed at 6-week intervals, and the appliances activated as needed. After an edge to edge anterior bite and super-Class I molar relationship was obtained, the appliances were removed. Lateral Cephalograms were taken before comprehensive orthodontic treatment and after the comprehensive orthodontic treatment. The average treatment period of the Forsus™ Fatigue Resistant Device was 130.48 days and the overall average orthodontic treatment period was 768.22 days.
 
Forsus™ Fatigue Resistant Device
Fig 1a: Forsus™ Fatigue Resistant Device
Treatment Effects of Forsus02
The Forsus™ guide (REF 807-014) and the four different lengths of the male parts of the appliance-1

                                      Fig 1b: The Forsus™ guide (REF 807-014) and the four different lengths of the male parts of the appliance

The Herbst appliance design used for all patients in this study was a modification of the original Herbst appliance81,124. Stainless steel crowns were employed rather than bands on the maxillary first molars and the mandibular first premolars. C’ clips were used to lock the attached tube and piston on the axles. (Figure 2).

Fig 2a: Upper component of Herbst appliance, female component of pistons, C-clips
Fig 2b: Lower component of Herbst and Herbst/ RME appliances, male component of pistons, C-clips

At the commencement of treatment, the mandible was protruded anteriorly to an edge-to-edge position between the central incisors according to the method described by Pancherz81. In some cases additional tubing was added to the piston during treatment so as to maintain an edge-toedge anterior relationship. The average treatment period for the Herbst appliance was 258.55 days and the overall average orthodontic treatment period was 819.95 days.

Cephalometric analysis

Pre-treatment (T1) and post-treatment (T2) lateral cephalograms used in this study were taken with the teeth in the habitual occlusion at the same radiological centers to minimize the magnification errors.

The pre-treatment (T1) and post-treatment (T2) lateral cephalogram radiographs of each patient were scanned using AGFA Snap Scan 12364 (Germany), then digitized utilizing Viewbox™ (Viewbox 3.01 dHAL, Kissia, Greece) software, (Fig 3a). For each radiograph 62 variables were determined.

The analysis of the anteroposterior linear changes was constructed using a custom analysis developed to measure 14 grid cephalometric variables according to the method of Pancherz81,83,125 (Fig 3b). In addition, 8 standard cephalometric variables: SNA, SNB, ANB, mandibular plane angle (ML/NSL), nasal plane angle (NL/NSL), Vertical jaw relation (ML/NL), mandibular length (CoPg) and overbite were measured relative to a coordinate system consisting of the occlusal line (OL), and a perpendicular to this line through the point sella (OLP). 

Treatment Effects of Forsus06
Fig 3a: Cephalometric reference points used in the study
Treatment Effects of Forsus07
Fig 3b: Cephalometric reference lines related to OLsp used in the study

Legend for Fig 3

OLs:      Occlusal line superior

LOsp:    Occlusal line superior perpendicular 

sn:         subnasale ss: subspinale 

ss’:        subspinale soft tissue

is:          incision superiors 

ii:           incision inferiors 

li:           labrale inferiors 

sm’:      submentale soft tissue 

pg:        pogonion 

pg’:       pogonion soft tissue 

co:        condylion 

ar:         articulare 

ms:       molar superiors 

mi:        molar inferiors

 

The pre-treatment occlusal plane, otherwise known as occlusal plane superior (OLs), was constructed from the incisal tip of the most prominent upper incisor to the disto-buccal cusp tip of the upper first molar. The occlusal line superior perpendicular (OLsp) was constructed perpendicular to this occlusal plane running through sella. The shortest distance of various points to this perpendicular line (LOsp) was determined. The post-treatment radiograph was then digitally superimposed over the pre-treatment radiograph using what is termed “cranial base best fit”. This was achieved by matching points of the cranial base (Sella, Basion, Spheno-ethomidale and Nasion) as best as possible by minimizing the sum of the square of the distances between the various points according to the Viewbox™ software. The pre-treatment occlusal plane (OLs) was then transferred to the post-treatment radiograph in order that an identical reference was used for both pre-treatment and post-treatment radiographs.

The null hypothesis tested was that there is no difference in treatment effects between the Fatigue Resistant Device group and Herbst group.

Statistics

Differences for the 22 cephalometric variables were assessed including the age between the two groups (Fatigue Resistant Device and Herbst) and within the two groups. 

         • The means and standard deviations were calculated for each of the 22 variables. 

        • For the 2-group analysis, t-tests were calculated to assess the statistical significance of the intergroup and intra-group  differences for each of the 22 variables. 

        • Independent t-tests were used when looking at inter-group differences i.e. comparing different subjects at T1 (adjusted for unequal variance when indicated by Levene’s test) and T2 (Multivariate regression models were fitted. Predictors were the age at start of treatment, sex, the type of device and the pre-measurement. The dependent variable was the post-measurement). Paired t-tests were used when comparing intra-group treatment changes i.e. the same subject at different points in time, namely pre-treatment and posttreatment.

           • The relative contribution (as a percentage of total change) of skeletal and dental changes in the maxilla and mandible to overjet and molar relationship correction between the same two groups were also measured.
 
The total change was measured by the change of the position of the incisor tips and mesial contacts of the first molars for overjet and molar correction respectively. The skeletal change was measured by the change of A-point (for the maxilla) and Pg (for the mandible).The dental change was the difference between the total and skeletal changes.
 
 

RESULTS

2-group analysis

It was determined that the two groups were similar in treatment duration and scheduling or pre- and post-treatment cephalograms however the age of the subjects was statistically-significantly different between the two groups (Table 1).

Table 1: Means and Standard Deviations for the Forsus and Herbst groups

When comparing the 2 groups at T1 there were 10 statistically-significant cephalometric measurements of 23 in total two groups (Table 2). The patients who received Herbst appliance treatment were younger than the Forsus patients, the overbite and Class II molar relationship was more severe in the Herbst group and the Class II molar relationship was mainly due to maxillary molar position. The maxillary and mandibular bases and incisor measures as well as the mandibular length relative to the occlusal line were larger in the Herbst group before treatment.

A linear multivariate regression model to assess the Post-treatment (T2) changes was fitted to compare the P-values (Wald tests) and whether the coefficients related to selection of device (device code), Age, Sex & Pre-treatment measurement in the regression model were significantly different from Zero. This strategy was applied to take into account the variation in pre-treatment condition (Table 3).

 
Independent t- Tests (2 groups), Means and Standard Deviations
Table 2: Independent t- Tests (2 groups), Means and Standard Deviations for the Forsus and Herbst groups at T1
Summary of P-values from Multivarient regression models of Post-measurement.
Table 3: Summary of P-values from Multivarient regression models of Post-measurement. All models were adjusted for Device type, Age, Sex & Pre-measurements
Table 4: Comparison of pre-treatment ages & measurements with post-treatment ages & measurements for Forsus & Herbst devices Paired t-test, Means and Standard Deviations were used

The following results were found when variations in pre-treatment condition were accounted for: 

     •All the variables were statistically-significant between the pre-treatment and post-treatment measurements except the overjet and the molar relation in which none of the predictors were statistically-significantly different.

      •According to the device code there were 4 statistically-significant differences of the total 22 variables between the two groups. The Forsus group had a greater effect on A-point and maxillary dental movements i.e. the dental changes of the maxillary incisor and molar were more than the Herbst appliance group while the Herbst group had a greater effect on B point.

      •On evaluating the effect of age there were 2 statistically-significant differences in mandibular dental change i.e. there was greater  mandibular molar and incisal changes with unit increase of age.

      •On evaluating the effect of sex, there were 7 statistically-significant differences which were SNA,SNB, mandibular plane angle to both cranial base and palatal plane, mandibular length (Pg/Co) and the mandibular dental changes (incisal and molar) as follows:

              ♦ The females had a statisticallysignificant greater change in SNA and SNB. 

              ♦ The males had a statistically-significant greater change in mandibular plane angle relative to the cranial base and palatal plane angles. 

              ♦ The females had a statisticallysignificant greater change in mandibular length (Co/Pg). 

              ♦ The females had a statisticallysignificant greater change in mandibular dental changes (incisal and molar positions).

 
Intra-group analysis

As would be expected, statistically-significant treatment changes (T1-T2) were noted for both the Forsus and Herbst appliances groups (Table 4). For both groups there were statistically-significant treatment changes for ANB, overjet, molar relation, mandibular base, mandibular length, mandibular incisor and molar positions, maxillary molar position, mandibular molar horizontal mandibular base and overbite. The Forsus appliance group also demonstrated statistically-significant treatment changes for SNA, nasal plane angle and maxillary molar horizontal maxillary base. The Herbst group also demonstrated statistically-significant treatment change for SNB. The ANB correction was slightly greater for the Herbst group (1.9°) compared to the Forsus group (1.57°), as was the overjet correction (Herbst 5.65mm, Forsus 4.55mm) and molar correction (Herbst 3.6mm, Forsus 2.03mm). No significant treatment changes were noted for condylar position.

Both appliances were successful in Class II correction as measured by improvement in ANB, overjet and molar relation. A-point moves forward slightly (0.87mm and 1.66mm) in the Forsus and Herbst groups respectively, however, these changes were not statistically-significant. Pogonion came forward more in the Forsus group (4.35mm) than the Herbst group (3.76mm).

The Herbst group had more improvement of the Class II malocclusion in terms of the incisal and molar relationship. (Herbst overjet improvement of 5,65mm, molar relation improvement 3.6mm; Forsus overjet improvement of 4.55mm, molar relation improvement 2.03mm).

Since it was found that there were differences between the two groups before treatment, it was considered to remove selected subjects from the analysis in order that the two groups were similar at pre-treatment for all the 22 variables analyzed. This might be achieved by removing from the analysis those subjects in the Herbst group with the most severe Class II characteristics, as represented by the largest overjet or smallest ANB values. However, this was not deemed to be statistically appropriate, since these subjects could not be considered the outliers (as represented in Fig 4). Instead, a Multivariate regression models were fitted to compare the statically-significant difference between the two groups (Table 3) as discussed earlier.

Relative contribution of skeletal and dental changes to overjet and molar relationship correction

The overall relative skeletal contribution to correction of the overjet and molar relationship change was found to be greater with the Forsus appliance group than the Herbst appliance (Figure 5). Conversely, the overall relative dental contribution to correction of overjet and molar relationship change was found to be greater with the Herbst appliance group than the Forsus appliance group.

A measurement of method error was undertaken according to Dahlberg analysis126. The pretreatment radiographs of ten subjects who were randomly selected from the pool of the 47 subjects were redigitized and re-superimposed at least four weeks after initial digitization to assess the accuracy of data collected. The Dahlberg formula was applied: the magnitude of the combined method error (ME) = ±  where d is the difference between two measurements of a pair and n is the number of pairs. From this, the combined method error for linear measures was ±0.6mm and for angular measures were ± 0.73 degrees.

Fig 4: Boxplots of Pre-treatment (T1) in blue color and post-treatment (T2) in green color cephalometric values for Forsus and Herbst groups, by variable.

Boxplots of Pre-treatment (T1) in blue color and post-treatment (T2) in green color cephalometric values for Forsus and Herbst groups, by variable.
ANB
ii-Olasp
mi pg-Olasp
Relative contribution of skeletal and dental changes to overjet and molar relationship correction for Forsus and Herbst groups

Fig 5: Relative contribution of skeletal and dental changes to overjet and molar relationship correction for Forsus and Herbst groups

DISCUSSION

The present study compared the treatment effects achieved in two Class II treatment modalities, specifically was designed to assess the skeletal and dento-alveolar changes of two appliances, both to correct the Class II division 1 malocclusion, and used in the permanent dentition stage of dental development with the Full Fixed Appliance comprehensive orthodontic treatment.

Although patients were assigned to either group according to the therapeutic need and not randomly assigned to the study groups, the data demonstrated that there were many statisticallysignificant different variables between the groups at T1 (Table 2) which indicates that the patient who received the Herbst appliance treatment had a more severe Class II dental malocclusion, possibly influencing the urgency in commencing treatment and subsequent younger age group when compared to the Forsus appliance group.

It is interesting to notice there were no modifications in the inclinations of the nasal and mandibular plane angles in relation to the cranial base in the Herbst group and this finding is similar to McNamara127 and Windmiller128. This may be explained by the consequent increases in mandibular ramus height. On the other hand the Forsus group showed a statistically significantly increase the nasal plane angle due to the greater effect of the Forsus on the maxillary dento-alveolar structures. 

A statistically significant decrease in the SNA angle was found only in the Forsus group, demonstrating a growth restraining on the maxilla. However, the increase in distance between point A and OLsp has less forward movement in the Forsus group compared to the Herbst (both were not statistically-significant). It is a known fact that point A is influenced by the dentition. When the upper incisors are retracted, labial tipping of the roots may shift point A anteriorly. But with both appliances the change of the upper incisor was not significant.

The effects on the maxilla conflicted with previous Herbst appliance studies and a Forsus study  which reported that maxillary growth was inhibited. On the other hand, the Forsus appliance was reported to have no statistically-significant maxillary growth inhibition similar to the present study.

Forward displacement of the mandible was found in both treatment groups similar to previous studies of the Herbst and Forsus. The appliances have a forward and downward force to the mandible but the Herbst has more forward effect when measured at B point, which may be attributed to further forward movement of the mandibular incisors. Pancherz 9 stated that the major advantage of Herbst treatment in correcting a Class II treatment is that “you get the growth when you need it.”

The magnitudes of dento-alveolar changes were similar in both appliances but the mandibular changes were more pronounced in the Herbst group and the maxillary changes were greater in the Forsus group. These dentoalveolar findings are similar with nearly all the previous studies. The amount of the dentoalveolar change increased with each increase in age unit to compensate for limited amount of skeletal changes with each increase in the age unit, consistent with the findings of Pancherz. Females have more statistically significant mandibular dental changes because most of the females received Forsus orthodontic treatment at an advanced age compared to the Herbst group.

The mean age in both treatment groups was considerably higher than in three major prospective randomized clinical trials involving removable functional appliance therapy and therefore direct comparison in terms of treatment effect is difficult. However, broadly speaking, the overall effects were similar, in that correction of the Class II molar relationship and overjet were achieved through a combination of skeletal and dental changes.

The average durations of treatment for the Forsus and Herbst appliances in the study were approximately 4months and 8 months respectively. 

This is considerably shorter than the treatment period for the removable functional appliances used in the major randomized clinical trials though somewhat longer than the 6 months’ treatment used by Pancherz with Herbst appliance.

The Herbst appliance and its variants appear to be used more frequently as part of a onestage treatment protocol for managing Class II malocclusions. It has been suggested that delaying or eliminating early treatment may be more efficient and lead to similar long-term outcomes. Tulloch et al. reported the 2-phase treatment (functional appliance followed by fixed appliances) did not lead to any reduction in the average time a child is in fixed appliances during a second stage of treatment, and it did not decrease the proportion of complex treatments involving extractions or orthognathic surgery. Furthermore Livieratos & Johnston questioned the value of routine two-stage Class II treatment, finding that one-stage and two-stage non-extraction Class II treatment groups underwent skeletal changes that left them ‘essentially indistinguishable at the end of treatment’. Von Bremen and Pancherz reported that the treatment of the Class II Div 1 malocclusion was more efficient with progression dental development, particularly when the Herbst appliance is utilized.

In the present study the overjet correction in the Herbst appliance group was accomplished by 37 per cent skeletal and 63 per cent dental changes and the molar correction was accomplished by 58 per cent skeletal and 42 per cent dental changes while Ruf & Pancherz reported that the overjet correction in the Herbst/Multibracket appliance treatment in Class II Div 1 malocclusion in late adulthood was 13 per cent skeletal and 87 per cent dental changes contributed to overjet correction and molar correction by 22 per cent skeletal and 78 per cent dental changes. 

Numerous strategies and appliances are recommended for management of the Class II malocclusion, all demonstrating some degree of success in achieving a satisfactory outcome. The relative contribution of skeletal change and the timing of treatment to effect maximal change in the skeleton has been the source of much conjecture. Recent publications appear to downplay the importance of timing therapy during periods of maximum growth. What often overlooked is the fact that the older patients would have already had a significant increase in mandibular length prior to therapy which would reduce the expectation during therapy and this may in part explain the differences observed in these studies.

CONCLUSION

Herbst appliance or the Forsus™ Fatigue Resistant Device both achieved the objectives in treatment planning Class II Div 1 malocclusion with relatively little statistically significant differences in the treatment time. In general the Forsus appliance group has more effect on the maxillary dentoalveolar structures while the Herbst appliance group has more effect on the mandibular dentoalveolar structures. In both groups the dental changes increased with each advancing age. The Herbst appliance did not appear to impose as significant changes in the vertical dimension while the Forsus appliance tended to increase in the palatal plane angle.

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