Skeletal and dental effects produced by functional regulator-2 in pre-pubertal class II patients: a controlled study

class II is one of the most prevalent dental and skeletal malocclusion in the sagittal plane, and it occurs in up to one-third of the population [1, 2], with the highest prevalence among northern European descent [1]. Although variable combinations of dental and skeletal factors contribute to this malocclusion, the most frequent diagnostic finding in class II malocclusion is mandibular skeletal retrusion [1, 3]. A therapy able to enhance mandibular growth is thus indicated in class II patients [4]. To this goal, a wide range of functional appliances aimed to stimulate mandibular growth by forward posturing of the mandible is available [5]. Many treatment protocols, sample sizes and research approaches have led to disparate outcomes in studies on human subjects [5].

One of the very earlier available functional appliances is the functional regulator (FR)-2 [6]. The FR-2 has been developed to eliminate functional disorders that can interfere with normal skeletal and dental development [6]. The claimed mechanism of action of this appliance is targeting poor postural behaviour of the orofacial musculature and, in particular for class II malocclusion, advancing the mandible with muscular training [6].

In spite of the number of studies on FR-2 treatment performed to date, there are still controversial aspects mainly regarding the skeletal effects provided by this appliance [7]. In particular, a wide range of different results has been reported, from no skeletal effects [8] to restriction of the maxillary growth [9] or enhanced mandibular length [3, 10, 11]. However, major limitations of most of these previous investigations, which may account for the different conclusions carried out, have recently been uncovered [7].

An important aspect in functional treatment for skeletal class II malocclusion is the growth phase, i.e. pre-pubertal or pubertal, at which treatment is delivered. Indeed, it has been demonstrated how individual responsiveness to treatment, especially in terms of mandible elongation, is critically dependent on this aspect [12–14]. However, studies on the FR-2 treatment have included patients on the basis of dentition phase [3] or chronological age [8–10, 15].

Moreover, the request for early treatment in class II patients has led to a common practice procedure according to which functional treatment is performed during a pre-pubertal growth phase. Therefore, considering important clinical implications, the knowledge on whether or not the FR-2 treatment may be effective in reducing class II malocclusion by relevant skeletal changes when delivered at the pre-pubertal growth phase would be useful. The present controlled study was thus carried out to elucidate on the skeletal and dental effects produced by a FR-2 treatment in subjects with skeletal class II malocclusion and treated during the pre-pubertal growth phase.

Through a strict selection of subjects with skeletal class II malocclusion, the present controlled study addressed what the skeletal and dental effects produced by FR-2 treatment delivered at a pre-pubertal growth phase are. The present results show clinically irrelevant skeletal effects and more pronounced dental changes, especially at the incisors, mainly responsible for the reduction of the overjet. Among the incisors, the maxillary ones underwent more relevant positional changes.

One of the most critical aspects in functional treatment of class II patients relates to the possibility of enhancing mandibular growth to a clinically relevant level. Ideally, this growth should be restored to be comparable to that of class I subjects [5, 29]. In spite of the great number of investigations performed over the last decades [5], most of these were significantly hampered by the designs used [30]. Among the main limitations are improper diagnosis of skeletal class II malocclusion [30] and the lack of evaluation of the growth phase during treatment [5, 7, 31]. Unfortunately, studies on the skeletal effects of FR-2 in growing class II patients are within these general limitations and with few exceptions [11, 12], with a previous meta-analysis [7] that could not clearly discriminate between true treatment effects due to FR-2 wearing and those due to other confounding factors, such as the growth phase at which treatment was delivered.

The present study, even though used a retrospective design, followed a rigorous classification of the class II patients and selected a well-matched control group. For both groups, the whole observational term was within the pre-pubertal growth phase. Having the class II patients included herein characterised by a retruded mandible, the main issue to be addressed is whether or not this treatment protocol is able to enhance mandibular growth on the sagittal plane. In this regard, total mandibular length measured by the Co-Gn (or Co-Menton/Pogonion) distance is a primary parameter. However, one of the major confounding problems in cephalometrics is the use of both Co and Articulare (Ar) as the posterior end point in measuring mandibular total length. Indeed, measurements with Ar as an end point, such as Ar-Gn, might give significant values for supplementary mandibular growth, without a corresponding increase in Co-Gn [15]. In spite of this consideration, several of the previous investigations on functional treatment for class II patients used Ar as the posterior mandibular end point when dealing with different class II appliances [5] or specifically regarding the FR-2 [7]. In the present investigation, the total mandibular length, as Co-Gn, underwent very similar mean increases of 4.1 and 4.0 mm in either the treated and control groups, respectively. Similar results with no or minimal effects of FR-2 treatment on mandibular length (as Co-Gn or Co-Pogonion) were seen by previous investigations [15, 32, 33]. However, these studies did not specify which growth phase the patients had during treatment [15, 32, 33] or had an unmatched control group [15]. On the contrary, McNamara et al. [3] reported a supplementary increment of 3.6 mm/2 years in mandibular length (Co-Gn) in patients treated in the late mixed and early permanent dentitions, presumably with several of these patients treated during the pubertal growth phase. Petrovic et al. [12] reported an additional increase in Co-Pogonion ranging from 0.8 to 5.5 mm/year depending on different biological growth categories in subjects treated with the FR-2 at the pubertal growth phase. Freeman et al. [11] reported a total increase in Co-Gn of 3.0 mm in subjects treated by FR-2 as compared to untreated controls over a 10-year-long follow-up. These subjects showed a pre-pubertal and a post-pubertal growth phase at the beginning and end of the treatment, respectively [11].

In the present study, the treatment effect of FR-2 on the skeletal growth of both the maxilla and mandible was insignificant (Table 1). With the exception of the significant reduction in the ANB angle, all of the other sagittal and vertical parameters showed very similar values between the treated and untreated control groups. Particularly, the ES coefficients were generally low and not significantly different than zero (Table 1). The main treatment effects on the sagittal growth of the maxilla and the mandible are shown in Figure 2. The treatment effects on Co-A and Co-Gn distances were 0.5 and 0.1 mm, respectively. Similarly, the treatment effects on the maxillary and mandibular positions on the sagittal plane, referred to as the A to N perp and Pg to N perp distances, respectively, were also insignificant (+0.3 and +0.6 mm, respectively, Figure 2). These changes were also within the range of the method errors. The previous evidence mentioned above and the present results may thus be explained by the differences in the growth phase at which the patients received treatment. The present controlled study is the only one treating all the patients with FR-2 appliance during the pre-pubertal growth phase. However, pre-pubertal growing patients have been investigated in randomised clinical trials [34, 35] using different functional appliances, although in those studies how the diagnosis of skeletal class II malocclusion was appraised was not clearly reported [36], or they relied on an increased overjet [35] or molar class II (any subdivision) [34] that cannot account for a true skeletal disharmony when used alone [30]. In this regard, the identification of skeletal maturity, with particular regard to the onset of pubertal growth phase, would have major clinical implications when dealing with orthodontic treatment in growing patients with skeletal class II malocclusion [12–14].

Nevertheless, the patients treated herein underwent a noteworthy reduction of the overjet and an improvement of the molar relationship of 2.9 and 1.8 mm, respectively (Table 2). These were, however, due to dental effects produced by the FR-2 treatment. Indeed, a remarkable palatal tipping of the maxillary incisors was seen with a treatment effect on the U1/FH angle of 5.3°. On the other hand, a proclination of the lower incisors was also seen, although only the ES coefficient (0.7), and not the treatment effect on the L1/ML angle (2.8°), was significant. By using different parameters, very similar results have been recorded in previous studies treating patients at the pubertal growth phase [11], late mixed dentition [3] or an unspecified growth/dentition phase [33, 37, 38]. Although Fränkel and Fränkel [6] reported only the insignificant or minimal effects of the FR-2 treatment on the tipping and position of the maxillary and mandibular incisors, all of these evidence would support the concept that FR-2 treatment produced relevant dental effects especially at the incisor level. These dental effects would occur irrespective of the growth phase at which treatment is delivered.

Therefore, considering the lack of skeletal effects by FR-2 treatment and that dental compensation to reduce the overjet may be obtained by a shorter period of fixed appliance therapy, this latter option would be of choice for all pre-pubertal class II subjects with prominent maxillary incisors. This fixed orthodontic treatment, e.g. 2 × 4 appliance, would thus prevent incisor trauma [39], while functional treatment for skeletal growth restoration could be delayed until the onset of the pubertal growth spurt [12–14].

The FR-2 treatment does not have relevant skeletal effects on class II patients treated during the pre-pubertal growth phase, while the overjet reduction is mainly due to dental effects.

LP is an associate professor at the Second University of Naples. AF and SP are post-graduate students of the Orthodontic Program at the Second University of Naples. LC is an assistant professor and head of the Orthodontic Program at the University of Trieste. GP is a research fellow at the University of Trieste.