How does a fixed functional appliance correct a Class II malocclusion?

Fixed functional appliances are used to correct Class II malocclusions and have supplanted their predecessors, that are removable functional appliances, such as bionator, Frankell, Twin Block, to name just a few.

Yet, it has been shown that no removable functional appliances could make the mandible grow (lengthen) even if these appliances move the lower jaw anteriorly to correct the Class II discrepancy between both jaws.

It was also shown that removable functional appliances are efficient. They correct most of Class II malocclusions, but it is due to dental (dentoalveolar) movements rather than skeletal changes (mandibular lengthening).

What about how fixed functional appliances work?

Review of literature

Let’s look at the articles one by one.

Xbow^®

Flores-Mir et coll.¹, in a retrospective cohort study, compared 69 patients treated consecutively with the Xbow^® (crossbow) appliance with a historical control group of 30 individuals. The Xbow^® appliance was used during 4.5 months in average and the cephalometric X-ray at time T2 was taken 6.5 months in average after the removal of the Xbow^® appliance.

Consequently, the measurements taken at time T2 include the relapse of most of the side effects that could have been caused by the Xbow^® appliance.

Results showed no mandibular changes (no advancement or different growth from the control group). However, a decrease in maxillary protrusion (maxilla that moved backward) and an increase in the vertical dimension were noted.

The dental changes observed were a decrease in the overjet caused by anterior tipping (proclination) of the lower incisors of about 5° (4.7 ± 4.1°). No significant changes were observed in the upper incisors. The upper molars moved backward by about 1 mm (0.9 ± 1.2 mm) and the lower molars moved forward by 1 mm (1.1 ± 1.3 mm).

Xbow^® and Forsus™

The Xbow^® appliance was then compared to the Forsus™ appliance. Miller et coll² in a retrospective cohort study, compared 38 patients having received the Xbow^® appliance (XB group) with 36 patients having received Forsus™ Class II correctors (FO group). T1 is prior to appliance placement (don’t know how many weeks) , T2 was 6 to 8 weeks after the end of the orthodontic treatment rather than at the removal of the functional appliances.

They found that both appliances produced the same amount of tipping of the lower incisors (no differences between both groups). The longer the treatment had been, the more the incisors tipped. The Xbow appliance and Full brackets averaged 24,18 ± 5,26 months and the Forsus with Full Brackets group averaged 30,17 ± 7,66. The difference between the 2 treatment protocol was significant (p = 0,37).

However, it is not clear if the XBow phase 1 treatment time is include phase 2 treatment time with fixed edgewise appliance but it seems so.  In the result section, we see that the treatment time with fixed appliance  in the XBow group is 16,68 months. I assume that the difference between 16 months and 24 months can be explained by a phase 1 of 8 months or so of XBow therapy.

Earlier report (Tullooch et al¹³) compared the treatment time of 2 phase treatment versus one phase treament.  They found that the total duration of treatment beyond phase 1, appears that this was shorter for patients who had had early treatment during phase 1. Therfore, it make sense that phase 2 therapy with full fixed brackets that followed phase 1 therapy with Xbow appliance would appears shorter than a full edgewise therapy with Forsus device.

Xbow^®

A recent prospective study from Erbas et Kocadereli³ analyzed the changes of the Xbow^® appliance and their observation time was at the removal of the functional appliance that was used 6 months in average (5.0 to 8.0 months). The radiographic method used was cone beam computed tomography (CBCT). Their result confirms a skeletal effect of maxillary backward movement (decrease in the SNA angle of 0.67 ± 1.35°) and no significant mandibular changes. The dental movements observed were significant backward tipping of the upper incisors (-3.58 ± 3.56°) and extrusion (egression) of 1.72 ± 1.87 mm. The lower incisors presented a significant anterior tipping of 10.87 ± 4.91°. The upper molars did not show any significant backward movement or intrusion. The lower molars presented a significant forward shift of 2.07 ± 1.24 mm and extrusion of 1.52 ± 1.08 mm.

Forsus™ and Class II elastics

Dubois et coll⁴ prospectively compared patients treated with Forsus™ Class II correctors with patients wearing Class II intermaxillary elastics. The observation time at T2 is at the removal of the Forsus™ Class II correctors and the observation duration is 6 months.

The results could not show evidence of any maxillary or mandibular skeletal changes. The use of Forsus™ correctors resulted in significant backward tipping of the upper incisors and forward tipping of the lower incisors, as well as a change in inclination of the occlusal plane.

Efficiency of the Forsus™ Class II correctors

Franchi L, Alvetro L et coll⁵ compared a retrospective cohort of 32 patients treated consecutively with a historical control group of 27 untreated patients.

The observation times are T0 before the orthodontic treatment and T1 after the orthodontic treatment. The average duration of treatment was 2.4 ± 0.4 years. The average duration of the use of Forsus™ Class II correctors was 5.2 ± 1.3 months.

87.5% of patients saw their Class II malocclusion being corrected successfully. The significant skeletal effect observed was a restriction of maxillary anterior growth (SNA is in average 1.6 ± 1.4° lower than the control value).

The effect on the mandible is mainly dentoalveolar:

1- Anterior movement of the incisors (6.1 ± 6.3°)

2- Anterior movement of the molars (2.4 ± 1.6 mm)

3- Extrusion of the lower molars (3.6 ± 1.5 mm)

A follow-up but retrospective cohort study from the same group of researchers (Cacciatore et coll⁶) analyzed 36 patients 2.3 ± 1.1 years in average (T3) after the end of the orthodontic treatment with Forsus™ Class II correctors compared to an historical control group of untreated individuals.

The results show that, during the observation period post treatment, the maxilla returns to a normal growth and position (SNA increases by 1.4°). The overjet and overbite increase significantly (1.3 mm and 1.5 mm respectively). The upper incisors show a significant intrusion of 1.2 mm.

When the changes between the beginning (time T1) and the end of the follow-up (time T3) are compared, there are no significant skeletal changes in the maxilla or mandible.

The significant dental changes are:

1- Reduction of the overjet (-3.8 mm) and overbite (-1.5 mm)

2- Improvement of the molar relationship (3.7 mm)

3- Retrusion (backward movement) of the upper incisors (-1.1 mm)

4- Intrusion of the lower incisors (-1.2 mm)

2 years after the orthodontic treatment, the efficiency rate of the Class II correction is 83.3% compared to an efficiency rate of 87.5% at the end of treatment.

Forsus™ and temporomandibular joint

Aras et coll⁷ compared 15 individuals before their pubertal growth peak with 14 individuals after their pubertal growth peak. These 29 patients received Forsus™ Class II correctors for a period of about 9 months. Cephalometric X-rays and magnetic resonance imageries were taken. Significant therapeutic effects are about the same as those previously discussed, namely backward tipping and extrusion of the upper incisors, protrusion, intrusion and anterior tipping of the lower incisors, a distal movement of the upper molars and a mesial movement of the lower molars.

Interesting fact, magnetic resonance imagery proved that there were no changes in the position of the articular disc, which is a good thing.

Twin Force^® Bite Corrector™

Chhibber et coll⁸ retrospectively studied the therapeutic effect of the Twin Force^® Bite Correctors™. Like Aras et coll⁷, they compared 23 individuals treated before their growth peak with 18 individuals treated after their pubertal growth peak.

Like the other fixed functional appliances, the skeletal change put in evidence, when the Twin Force^® devices were removed, is a maxillary growth restriction (SNA decreases by 1.22°) and anterior tipping of the lower incisors (dentoalveolar movements) of 4,4 ± 5,5°.

At the end of the orthodontic treatment, there were no differences in the maxillary or mandibular skeletal parameters, no matter if patients were treated before or after their pubertal growth peak.

The orthodontic treatment was significantly longer in individuals whose treatment started earlier (3.67 years versus 2.75 years).

In conclusion, the efficiency of a fixed functional appliance is better in an individual after his/her puberty. Starting too early is not better and leads to a longer treatment.

SUS² correctors

Oztoprak et coll¹⁰ compared, in a prospective study, SUS² correctors with Forsus™ Class II correctors. A control group of untreated individuals was used as a reference. The duration of use of both appliances was 5 ± 1 months in average.

The dentoalveolar effects observed are:

1- Retrusion and extrusion of the upper incisors

2- Protrusion and intrusion of the lower incisors

No significant anteroposterior or vertical skeletal changes were observed for both appliances.

However, if dentoalveolar changes in the lower incisors are compared, the use of Forsus™ Class II correctors significantly leads to more anterior tipping of the lower incisors than SUS² correctors (10.8 ± 3.07° versus 5.78 ± 3.91°).

Study model

	Experimental groups		Observation Time Point
	Sample size (>25)	Untreated Controls	T0 Baseline	T1 Prior Cl II device	T2 Post Cl II device	T3 End of Ortho	T4 Follow-up
Flores-Mir AJODO 2009 R	✔︎ 69	✔︎ 30	✔︎		6 m. post Xbow
Miller AO 2013 R	✔︎ 38/36		✔︎			✔︎
Erbas AO 2014 P	✔︎ 25		✔︎		✔︎
Dubois et al P	✔︎ 23 / 7			✔︎	✔︎
Franchi AO 2011 R	✔︎ 32	✔︎ 27	✔︎			✔︎
Cacciatore AO In press R	✔︎ 36	✔︎ 20	✔︎			✔︎	✔︎
Aras AJODO 2011 P	✔︎ 15 / 14			✔︎	✔︎
Chhibber AO 2013 R	✔︎ 23 / 18		✔︎			✔︎
Oztoprak EJD 2012 R	✔︎ 20 / 20	✔︎ 19		✔︎	✔︎

This table compares the study model of the studies previously discussed. The retrospective cohort studies are identified by the letter R and the prospective ones are identified by the letter P in the first column. Only 4 studies had an untreated control group. Only 3 studies have an observation time before the functional correctors were installed and when the correctors were removed. Only one study presents a follow-up 2 years post treatment.

Thus, there is room for more studies and the best study model will be the one with the most check marks on its row and which will be a prospective study of consecutively treated cases. Making such a study takes a minimum of 4 to 6 years and hundreds of hours of work.

Conclusion

Fixed functional appliances efficiently correct Class II malocclusions. They all function similarly. The correction is mainly done through dentoalveolar changes. No appliances make significant long-term skeletal maxillary and mandibular changes.

Bibliography

1- Flores-Mir C, Barnett G, Higgins DW, Heo G, and Major PW. Short-term skeletal and dental effects of the Xbow appliance as measured on lateral cephalograms. AJODO; 2009;136(6):822-832.

2- Miller, Robert A, Long Tieu, and Carlos Flores-Mir. “Incisor Inclination Changes Produced by Two Compliance-free Class II Correction Protocols for the Treatment of Mild to Moderate Class II Malocclusions.” Angle orthodontist 83, no. 3 (2013): doi:10.2319/062712-528.1.

3- Erbas, Banu, and Ilken Kocadereli. “Upper Airway Changes After Xbow Appliance Therapy Evaluated with Cone Beam Computed Tomography.” Angle Orthod 2014; 84, no. 4: doi:10.2319/072213-533.1

4- Dubois A, Rompré P, Rodrigue C, Remise C, Comparaison des effets sur la croissance des maxillaires de l’utilisation du Forsus versus celle des élastiques de classe II, Thèse de maîtrise

5- Franchi L, Lisa Alvetro, Veronica Giuntini, Caterina Masucci, Efisio Defraia, and Tiziano Baccetti. “Effectiveness of Comprehensive Fixed Appliance Treatment Used with the Forsus Fatigue Resistant Device in Class II Patients.” Angle orthodontist  2011; 81, no. 4: doi:10.2319/102710-629.1

6- Cacciatore, Giorgio, Luis Tomas Huanca Ghislanzoni, Lisa Alvetro, Veronica Giuntini, and Lorenzo Franchi. “Treatment and Posttreatment Effects Induced by the Forsus Appliance A Controlled Clinical Study.” Angle orthodontist (2014):doi:10.2319/112613-867.1

7- Aras, Ada, Saracoglu, Gezer, and Aras. “Comparison of Treatments with the Forsus Fatigue Resistant Device in Relation to skeletal maturity: A cephalometric and magnetic resonance imaging study. AJODO 2011; 140: 616-25: doi:10.1016/j.ajodo.2010.12.018.

8- Chhibber, Aditya, Madhur Upadhyay, Flavio Uribe, and Ravindra Nanda. “Mechanism of Class II Correction in Prepubertal and Postpubertal Patients with Twin Force Bite Corrector.” Angle Orthod. 2013; 83, n718-727: doi:10.2319/090412-709.1

9- Bassarelli T., Does an Anterior Bite Plane Change the Treatment Result Obtained with a Jasper Jumper in Class II Division 1 Growing Patients, Angle East 2013 scientific meeting, New York

10- Oztoprak MO, Nalbantgil D, Uyanlar A, and Arun T. A cephalometric comparative study of class II correction with Sabbagh Universal Spring. European journal of dentistry. 2012 Turkey;6(3):302-10

11- Giuseppe Perinetti, Jasmina Primožič, Giovanna Furlani, Lorenzo Franchi, and Luca Contardo (2014) Treatment effects of fixed functional appliances alone or in combination with multibracket appliances: A systematic review and meta-analysis. The Angle Orthodontist In-Press.

12-Tulloch, J F, C Phillips, and W R Proffit. “Benefit of Early Class II Treatment: Progress Report of a Two-phase Randomized Clinical Trial.” AJODO  1998;113: 62-72.