The Effect of Chin-cup Therapy in Class III Malocclusion: A Systematic Review

INTRODUCTION

Skeletal Class III malocclusion is clinically presented as a result of maxillary retrusion, mandibular protrusion or a combination of the two [1-3]. It is often associated with complex dento-alveolar problems, which include an anterior edge-to-edge relation or anterior and/or posterior crossbite [4]. Patients with Class III malocclusion exhibit potential esthetic problems presenting a concave profile, and a vertical function pattern [3, 4], which limits their function to vertical movements.

The prevalence of Class III malocclusion presents a wide variety among and within populations [5, 6], as declared by a high 23% rate in Asian populations contrasted to a smaller 5% rate in Caucasians [1, 7]. Treatment of Class III malocclusion has been a challenge for orthodontists [1]. Among various treatment modalities, the chin-cup is considered a traditional appliance for the early orthopedic management of prognathic growing patients [2, 8-11]. Its clinical effectiveness has been investigated over the years with many studies confirming a general improvement of Class III malocclusion through backward and downward mandibular rotation, retardation of mandibular growth, remodeling of the mandible and the temporomandibular joint (TMJ), retroclination of mandibular incisors and closing of the gonial angle [1, 2, 7, 10, 11].

Despite the plethora of the available evidence, clinical results achieved with the chin-cup constitute a controversial issue among researchers. Owing to the absence of a standard protocol concerning the chin-cup’s appropriate use and the contradicting results of the reported outcomes and outcomes measures, a clear interpretation of data is not possible [1, 2, 7,12]. Although a great number of studies focus on the skeletal and dento-alveolar changes [2, 7, 10-12], the effects of chin-cup therapy on facial soft tissues remain uninvestigated. Furthermore, the vast majority of studies report outcomes of the chin-cup use obtained over a short-term period [2, 7, 10, 12], while there is a lack of studies examining the long-term effectiveness of chin-cup therapy.

For the aforementioned reasons the purpose of the present systematic review is the investigation of the short- and long-term effects on both the hard and soft tissues induced by chin-cup therapy in the Class III malocclusion of growing patients, based on the existing scientific evidence.

MATERIALS AND METHODOLOGY

-Secondary: Short and long-term effectiveness of chin-cup therapy, if provided, in order to assess the stability of the results.

All reported time-points after the completion of the treatment, judged by the authors of the trials were included. The short-term effectiveness is related to the outcomes obtained upon the end of the active treatment with chin-cup therapy, while the long-term effectiveness is related to the ones obtained at a post-treatment observation after the end of the active treatment.

Search Strategy

A literature search was carried out by applying the Medline database (PubMed www.ncbi.nih.gov) and the Cochrane Central Register of Controlled Trials covering the period from January 1954 to October 2015 and using the following Medical Subject Heading (MeSH) terms; “Malocclusion, Angle Class III”, “chin-cup therapy”, “chincap therapy”, (Table 1). A schematic representation of the search strategy is shown in Fig. (1).

Selection Criteria

Articles selected for this study fulfilled the criteria for inclusion, (Table 2). The criteria included randomized clinical trials (RCTs), prospective and retrospective clinical trials (CCTs) with untreated control groups. The retrieved studies had to use cephalometrics for analyzing the effects of chin-cup therapy contrasted to untreated Class III control groups. Table 2 also presents in detail the exclusion criteria.

Data Extraction

Two independent reviewers (SM, EF) made the assessment of the articles individually in predefined data extraction forms. No blinding to the authors during data extraction was made and any inter-examiner conflicts were resolved by discussion with a third reviewer (IT). The same reviewers performed the risk of bias assessment of the articles, with one author (AT) acting as the coordinator.

Quality Analysis

For the qualitative evaluation of the retrieved studies the risk of bias was assessed by two reviewers (SM, EF) independently. The assessment was based on the following tool: A Cochrane Risk of Bias Assessment Tool for Non-Randomized Studies on Interventions (ACROBAT-NRSI) [13]. This tool addresses seven domains of bias; bias due to confounding, bias in selection of participants into the study, bias in measurement of interventions, bias due to departures from intended intervention, bias due to missing data, bias in measurement of outcomes and bias in selection of the reported result.

Important confounders with regard to chin-cup therapy were considered those that could have an impact on the reported results. Thus, the following confounders were taken into account both for patients and controls: ethnicity (as Asian populations have a higher prevalence of Class III malocclusion) [1, 7,14-16], age in relation to the skeletal maturity stage, pre-treatment skeletal Class of malocclusion (when it was not skeletal Class III both for patients and controls), soft-tissue profile individual variation in thickness and in tension, and pre-treatment overjet. Moreover, co-interventions were considered the use of additional appliances, such as an occlusal bite plate, a quad-helix appliance, a lingual arch, etc., and the utilization of force magnitude of the chin-cup traction.

Three different outcomes were investigated; skeletal, dento-alveolar and soft-tissue effects of chin-cup therapy in Class III malocclusion both in the short- and in the long-term. For every different outcome of each study an initial risk of bias for every domain was assessed, as indicated by the ACROBAT-NRSI [13]. Because same issues applied to all outcomes, a grouped assessment was made. Finally, an overall risk of bias judgement for each study was achieved.

RESULTS

Our search strategy resulted in 3285 articles. After selection, according to the inclusion/exclusion criteria (Table 2), 46 studies were gathered and read in full-text. Finally, 14 studies were retrieved for the final analysis. The remaining 32 articles were excluded, mainly, because there were no untreated controls or the controls were not skeletal Class III patients, the treatment was combined with extractions or surgery and their objective was out of the scope of interest of the present study. Table 3 summarizes the data of the 14 included studies.

Clinical heterogeneity among studies (different outcome assessment, variable age of patients and different follow-up duration), and the high risk of bias in general precluded the quantitative synthesis of results in a meta-analysis.

Dento-Alveolar Effects

The main dento-alveolar changes produced by the chin-cup were the achievement of a significant overjet [17, 20-22, 24, 29] and retroclination of the lower incisors [17, 19, 21, 22, 24]. More precisely, Ritucci and Nanda [8] declared that transition in overjet occurred with a marked degree of flaring of the maxillary incisors, followed by a variable amount of uprighting, based on lateral cephalograms. Overjet correction was, also, reported byAlacrόn et al. [25], mainly achieved by mandibular incisor retroclination. Moreover, Barrett et al. [21] noted the uprighting of the lower incisors, indicated by the decreased IMPA, as the most significant dental change between the chin-cup and the control groups. Significant proclination of the upper incisors [8, 17, 19, 20, 24] was also pointed out. However, the aforementioned results, especially those regarding the proclined upper incisors, should be carefully interpreted in order to clarify whether they constitute net effects of the chin-cup alone or the additional appliances that were used and were co-interventions.

Changes regarding overbite varied, depending on the appliance that was used. More specifically, Arman et al. [17] noted a significant decrease in overbite in all the treated groups (chin-cups only, chin-cups with removable bite plate, reverse headgear with rapid maxillary expansion devices).

Regarding the molar relationship after the active treatment, Ritucci and Nanda [8] declared that chin-cups accelerate the mesial movement of maxillary molars, without any effect on their eruption rate, while Wendell et al. [27] manifested that the initial Class III occlusion was corrected to Class I relationship in all of the patients.

Soft-Tissue Effects

The effects of chin-cup therapy on the soft tissues were reported in five studies [17, 19, 21, 24, 28]. Significant forward movement of the upper lip was declared in four studies [17, 19, 24, 28] with a concomitant forward movement of the soft-tissue point A [17], while the movement of the lower lip presented differing results. Arman et al. [17], Alacrόn et al. [28] and Barrett et al. [21] stated a decreased distance of the lower lip to E plane (LL-E Ricketts line [17, 21, 28]) and lower lip’s retraction (LL-VR [17]) with a concomitant backward movement of the soft-tissue point B [17] and the soft chin (Pg_(s) [17, 28]). However, Abu Alhaija and Richardson [24] showed significant forward movement of the lower lip [24]. A general soft-tissue facial profile improvement was attributed to the chin-cup by Alacrόn et al. [28], who demonstrated similar correlations between the changes in the hard and in the soft tissues, especially the one between a significant reduction of the facial convexity angle and a significant pogonion retrusion in the chin-cup group.

Stability

Two studies [23, 24] reported information concerning the stability of treatment outcomes, using cephalometric x-rays at a post-treatment observation. Abu Alhaija and Richardson et al. [24], following a one-year post-treatment cephalometric observation, reported a significant increase in mandibular length, which was in accordance with Sakamoto et al. [23], whose study found a forward displacement of the mandible in one-year post-treatment observation and total relapse in the original mandibular growth pattern after two years. Both studies [23, 24] showed a significant increase in the anterior face height.

As for the dental effects, the significantly increased overjet achieved by chin-cups was maintained one year after the end of the treatment [24].

Although stability in the soft-tissue profile was evident at the post-treatment observation, the upper, the lower lip and the chin continued to grow forward following the skeletal pattern [24].

Quality Analysis

The overall judgement for the risk of bias was found serious for all the retrieved studies, (Table 4). All had a serious risk of bias concerning the selection of participants into the study (selection bias). Based on the ACROBAT-NRSI [13], all the studies were found to have some important problems in the corresponding domains, indicating cautious interpretation of the reported results.

DISCUSSION

In this systematic review, our primary goal was to search the existing literature for randomized and control clinical trials regarding the short- and long-term effects of chin-cup therapy on hard and soft tissues of growing patients. These had to include untreated patients as controls.

Although this was not the first time that this issue has been addressed in the literature, researchers in previous systematic reviews did not investigate the long-term effects of chin-cup therapy [2, 10, 12], the soft tissue changes [2, 10, 12] and the adolescence as a study growth period [10].

Our search strategy resulted in only CCTs, thirteen of retrospective [8, 17-28] and one of prospective design [29], with no RCT found. One possible reason is that RCTs are not common in orthodontics, since various parameters are required. These include patient/observer blinding to treatment and ethical matters regarding the control group whose decision of participation is negatively affected by receiving no treatment.

The final studies were cohort studies with weaknesses due to the serious risk of bias, as it is described in detail in Table 4. All the studies were found to have selection bias, as the selection of both participants and controls was related to the received intervention and likely to the outcomes.

Furthermore, the studies were judged to have a serious risk of bias concerning the outcomes’ measurements when the knowledge of the received intervention by the assessors was likely to influence the outcomes in a way that it could cause statistically significant differences. Thus, three studies [8, 23, 27] received that characterization, as the way that the outcome measure was conducted, was considered to have the potential to significantly affect the outcomes. The risk of bias was judged low, when blinding of outcome assessors was reported [25, 28]. These studies were considered comparable to a well-performed randomized trial with regard to this domain, according to the ACROBAT-NRSI [13]. Consequently, studies pertaining to neither categories, were judged to have a moderate risk of bias [17-22, 24, 26, 29]. Based on the ACROBAT-NRSI [13] in these studies the outcome measure was only minimally influenced by the awareness of the received intervention and any error in measuring the outcome was only minimally related to intervention status. The methods of outcome assessment were comparable across intervention groups both for the studies with a moderate and a low risk of bias.

Another weakness of the observational studies, both prospective and retrospective, is the presence of confounders. In the present systematic review, we considered confounders, all those factors that were possibly related to the chin-cup therapy and could cause significant changes in the results. Ethnicity was needed to be taken into account, as Class III malocclusion is more frequently seen in Asian populations [1, 7, 14-16] and consequently these patients may be more often treated with chin-cups. Moreover, patients of Asian ancestry may present different baseline characteristics, as well as a different growth pattern than other populations, thus significantly affecting the results. The age of the participants in relation to their skeletal maturity stage was also accounted for. This was mainly due the fact that the prepubertal patients may present different results from patients that are in the peak of their growth or later. Skeletal Class of malocclusion was considered a confounder when there was doubt on whether the treated and/or the control group had skeletal Class III malocclusion or when some controls had skeletal Class I. Soft-tissue individual variation in thickness and in tension was co-estimated, since it could affect the reported results regarding the soft-tissue changes, as it was highlighted by Arman et al. [17] and Alacrόn et al. [28]. Finally, pre-treatment overjet was also considered a confounder.

In addition, co-interventions were addressed. More specifically, the use of additional appliances, such as a lingual arch to flare the maxillary incisors [23, 26] or a quad-helix appliance [21] were considered critically important co-interventions that could significantly alter the outcomes. To illustrate this, in four studies [17, 19, 20, 24] the declared proclination of the upper incisors [17, 19, 20, 24] followed by forward movement of the soft-tissue point A [17] and the upper lip [17, 19, 24] was probably the result of an additional occlusal bite plate [17, 19], an upper removable appliance [24] and of the combination of maxillary protraction and chin-cup traction in an occipitomental anchorage appliance [20]. The significantly increased overjet [17, 19-21, 24] that was noted, was expected to be a result of the aforementioned additional appliances. However, it was also reported in studies where patients, treated solely with chin-cups, were contrasted to untreated controls [8, 28, 29]. One possible reason is the occlusal interferences in the transition of the occlusion from a one with underjet to one with overjet [8], that flare the upper incisors. It could also be the result of the significant retroclination of lower incisors caused by the chin-cup [8, 22]. At last, utilization of force magnitude was considered a co-intervention as well, since significant reduction in ramus height was noted when lighter force in chin-cup traction was used [22].

Patients under chin-cup therapy showed an improved facial profile, merely induced by the backward and downward rotation of the mandible [17-20, 26, 28]. This was documented by a decrease in the SNB [17-22] and closure of the gonial angle [20, 23, 25, 26]. It was also correlated with an increase in the anterior facial height [17, 18, 22, 24]. In contrast, Wendell et al. [27] recorded significant decreases in the anterior face height during chin-cup therapy in comparison with untreated controls. This was attributed to the 43% decrease in the downward displacement of pogonion during treatment, which was not stable at the post-treatment observation, when it was increased by 60% [27]. The backward and downward rotation of the mandible was correlated with an increase in the ANB angle as well [17, 19, 21-23]. However, there is ambiguity in whether only the mandible or both the mandible and the maxilla are responsible for this.

Moreover, there is controversy among researchers regarding the retardation of the mandibular growth during chin-cup therapy. A significant reduction of the mandibular length (ramal, body and total length) was reported in five studies [22, 23, 26-28] indicating an improvement in the skeletal profile of the treated patients. Most interesting were the findings of Wendell et al. [27], whose study presented a reduction in absolute mandibular length, which continued after the end of the active treatment. In contrast, the studies of Gökalp and Kurt [29] and Abu Alhaija and Richardson [24] showed significantly increased mandibular body [24, 29] and total mandibular length [24]. Gökalp and Kurt [29] attributed these alterations in the forward bending of the condyle, as a result of bone deposition between the condylar head and neck during chin-cup therapy.

The aforementioned controversy led to further investigating attempts by researchers in order to elucidate the role of chin-cup therapy in the retardation of mandibular growth. Similar attempts were also made to assess the potential influence of chin-cup therapy in the appearance of Temporomandibular Joint Disorders (TMD). It has been speculated that internal derangement of the TMJ is likely to occur due to the direct application of the backward chin-cup’s force on the mandibular condyle [7]. This was recently evaluated in a systematic review [7] by Zurfluh et al. who, interestingly, concluded that despite the craniofacial adaptations induced by chin-cups in patients with Class III malocclusion, chin-cup therapy does not constitute a risk factor for the development of TMD, as the existence of insufficient or low-quality evidence in the literature do not allow clear statements regarding the influence of chin-cup treatment on the TMJ. Nevertheless, they related TMD with age and a stressful lifestyle that seem to differentiate the effects imposed on TMJ.

As for the soft-tissue effects, although confounding was evident, the documented results indicate a general soft-tissue profile improvement when the chin-cup is used in skeletal Class III patients. However, in the lack of studies that evaluate the long-term stability of the aforementioned changes, no definite conclusions can be reached.

In the basis of these manifestations, it is evident that the effects of chin-cup therapy both in the short-and especially in the long-term need further investigation and better substantiation with more high-quality evidence to draw reliable conclusions.

CONCLUSION

In summary, the present systematic review shows that the chin-cup therapy can be considered for the short-term treatment of growing patients with Class III malocclusion. More specifically, the following are evident:

• The skeletal profile is improved, as it is confirmed by significant changes in measured variables, which indicate a downward and backward rotation of the mandible.
• Favorable dento-alveolar changes, such as a significant increase in overjet are also observed. However, data need to be carefully interpreted in the presence of co-interventions, such as additional appliances that could have an impact on the outcomes.
• The soft tissues show a general improvement in the facial profile, following the accompanying skeletal and dento-alveolar changes, but with uncertain long-term stability.

Nevertheless, existing limitations that do not permit a clear judgement need to be taken into account. The unclear role of chin-cup therapy in the retardation of mandibular growth, the need for further investigation of the long-term effectiveness and the general lack of high quality evidence suggest cautious interpretation of the reported findings and highlight the need for future research with more high-quality evidence-based clinical trials, in order to draw reliable conclusions.

REFERENCES