Comparison of the effects on the pharyngeal airway space of maxillary protraction appliances according to the methods of anchorage

Patients with skeletal class III malocclusions have a prominent and protrusive lower face and a relatively inconspicuous and retruding upper face, resulting in a concave profile. Class III malocclusions, which are skeletal facial deformities, are characterized by retrusion and deficiency of the maxilla and excessive growth and protrusion of the mandible. Almost two thirds of skeletal class III malocclusions are due to either retrusion of the maxilla or a combination of maxillary retrusion and mandibular protrusion [1–3].

The prevalence of class III malocclusions is only 1 to 3% in the Caucasian population. However, the incidence is as high as 14% in Asian populations [4–6].

Class III malocclusions are one of the most challenging orthodontic problems for practitioners to correct. The treatment modalities for skeletal class III malocclusions are growth modification for young patients and orthognathic surgery for adult patients. Growth modification treatment should start in earlier ages than treatment for other orthodontic problems, usually in the primary dentition or early mixed dentition stages [7, 8], so the entire treatment time can be extensive.

There are many maxillary protraction appliances for skeletal class III malocclusion which have been used since 1960 [9]. Numerous studies have investigated the dentoalveolar and skeletal effects of maxillary protraction appliances [10–12]. These appliances stimulate sutural growth of the maxillary complex by pulling the complex forward. This results in maxillary protrusion, clockwise rotation of the mandible, counterclockwise rotation of the palatal plane, proclination of the upper incisors, and retroclination of the lower incisors. However, the tooth-borne anchorage facemask (TBFM) has some disadvantages. The protraction forces of maxillary protraction appliances are applied to the first maxillary molar, so the effects are more dentoalveolar than skeletal in nature. This results in a relatively high rate of relapse to reverse overjet after the mandible is completely grown and dental side-effects, such as mesial migration and extrusion of the maxillary molars and labioversion of the maxillary incisors. The optimal age at which to begin treating patient with facemasks is somewhat controversial, but it is usually done at the stage of primary dentition or early mixed dentition [7, 8].

To address the disadvantages of the tooth-borne facemask, the skeletal anchored facemask (SAFM) was invented [10–12]. This appliance obtains stable anchorage in the facial bone and applies direct force to sutures in the maxillary complex.

Recently, many studies have revealed the efficacy of the SAFM compared to the TBFM, focusing on cephalometric measurements [13–15]. Also, there have been studies on the maxillary protraction effects of such facemasks on pharyngeal airway dimensions [16–18]. However, there are few articles comparing the effects of the SAFM and the TBFM on pharyngeal airway dimensions.

This study compares the effects of the SAFM and TBFM on the pharyngeal airway.

Consecutive subjects were recruited from a private orthodontic clinic in Chunan, South Korea. Twenty-eight patients (8 boys and 20 girls) were treated with a TBFM and 24 patients (12 boys and 12 girls) were treated with an SAFM. All subjects had skeletal and dental class III malocclusions with an anterior crossbite or incisor edge-to-edge relationship and class III molar relationships, cervical vertebra maturation stage of 3 or 4. No patient had a congenital deformity with involvement of cervical vertebrae 3 and 4. No previous orthodontic treatment was performed on any of the patients. The initial mean age of the TBFM group was 10.3 ± 1.4 years (range, 8.4–12.6 years). The initial mean age of the SAFM group was 11.2 ± 1.1 years (range, 9.1–13.3 years). The mean duration of TBFM treatment was 14.3 months and that of SAFM treatment was 16.9 months.

Cephalometric analysis

Lateral cephalograms were taken before treatment (T1) and after treatment (T2) and traced by a single investigator. Figure 3 shows cephalometric measurements and maxillary advancement measurement (Co-A) is represented as a dotted line. Figures 4 and 5 describe reference points and cephalometric measurements of the pharyngeal airway. Specific measurements used in this study are categorized into linear measurement and areal measurement as follows:

1. 1)
   Linear measurements (Fig. 4)

   1. 1.

      Superior pharyngeal space (SPPS): The width of the pharynx measured between the posterior pharyngeal wall and the dorsum of the soft palate on a line parallel to the FH plane (the line through Po and Or) that runs through the middle of the line from PNS to P

       
   2. 2.

      Middle pharyngeal space (MPS): The width of the pharynx measured between the posterior pharyngeal wall and the dorsum of the tongue on a line parallel to the FH plane that runs through P

       
   3. 3.

      Inferior pharyngeal space (IPS): The width of the pharynx measured between the posterior pharyngeal wall and the dorsum of the tongue on a line parallel to the FH plane that runs through C2i

       
    
2. 2)
   Areal measurements (Fig. 5)

   1. 1.

      Superior pharyngeal area (SPPA): The area of the pharynx with an inferior border on the ANS-PNS extension line and an anterior border on the line perpendicular to the ANS-PNS line that runs through the pterygoid

       
   2. 2.

      Middle pharyngeal area (MPA): The area of the pharynx with a superior border on the ANS-PNS extension line and an inferior border of the extended occlusal plane

       
   3. 3.

      Inferior pharyngeal area (IPA): The area of the pharynx with a superior border on the extended occlusal plane and an inferior border of the most anteroinferior point on the body of the third cervical vertebra (cv3i)—the most anterior point of the hyoid bone (hy) line

       
    

The length of linear measurements and area of areal measurements were calculated with V-Ceph (version 7.0, Osstem, Seoul, Korea).

At time T1, both the TBFM and SAFM groups were normally distributed, so an independent t test was performed to evaluate the difference between the groups.

Facemask therapy induces forward displacement of the maxilla and decreases forward displacement of the mandible. This is accomplished by stimulating cellular activity in circummaxillary sutures and the maxillary tubercle [19, 20]. Although conventional facemask therapy has proven to be an efficient method for treating skeletal class III malocclusion, because of unwanted dentoalveolar effects of conventional tooth-borne face masks, there have been numerous attempts to develop a method of skeletal face mask anchorage. Kokich et al. used an intentionally ankylosed deciduous tooth as an anchor point, Singer et al. and Enacar et al. introduced osseointegrated implants as anchors, Hong et al. placed a hexagonal implant in the palatal bone and used this as an anchor point, Kircelli and Pektas inserted two miniplates in the lateral nasal wall of the maxilla and placed elastics in the hooks of the miniplates, and De Clerck et al. used two miniplates inserted in the infrazygomatic crest and two miniplates inserted between the canine and the first premolar in the mandible as anchors [12, 21–25].

In SAFM protocol, there are some controversies about proper magnitude of maxillary protraction force. Nguyen et al. [18]. applied initial force of 100 g on each side and increased force up to 200 g after 1 month. But Sar et al. [13] delivered protraction force of 400 g on each side and acquired proper maxillary advancement. Some clinicians think that elastic force of 400 g on each side is insufficient and need even more force for adequate maxillary protraction, but there is lack of substantial studies about proper protraction force.

The relationship between facemask therapy and airway volume increase has been widely investigated [16, 17, 26, 27]. However, controversy remains concerning the effectiveness of facemask therapy on airway dimension increases. Hiyama et al. and Kaygisiz et al. reported that pharyngeal airways improved after correcting maxillary retrusion by facemask therapy [16, 17]. On the other hand, Baccetti et al. and Mucedero et al. found that there were no significant increases in pharyngeal airway volume after face mask therapy [26, 27]. In the present study, we observed increases in pharyngeal airway volume after treatment in both groups of patients.

This study used a modification of Kircelli’s miniplate anchorage system because of its relative simplicity and effectiveness. Conventional miniplates used to treat trauma patients were initially employed, but screw loosening and plate detachment from the maxilla happened frequently because of the bone flexibility in young patients. So, the fixation system was changed to a locking system in order to increase stability and make screw loosening less likely. However, our plate and screw fixation method still has some disadvantages. Although we used locking fixation system, there was one patient who showed loosening of miniplate and consequently needed removal of miniplate and re-operation was performed just lateral to former position. According to Sar et al. [13], loosening of miniplate were up to 7% and in that case, the miniplate should be replaced. In addition to screw loosening, these miniplates were not designed for skeletal anchorage, they need to be modified preoperatively. Another disadvantage involves the hook at the last hole. This hook has internal threads for the locking system. Although surgeons smooth the threads before placement, some sharp edges remain, which can cause the elastics to break. These problems originate from the fact that we used miniplates designed for trauma or orthognathic surgery. If this skeletal anchorage method gains popularity, a plate and screw system optimized for this purpose could be created.

Changes in airway measurements were assessed using lateral cephalometric radiographs. Although lateral cephalograms can only provide two-dimensional information, they have been used to efficiently analyze pharyngeal airway spaces. Not only are they simple and easy to use but there is also a significant correlation between airway space measured with a lateral cephalogram and measured with computed tomography according to Riley and Powell [28]. Various studies have used lateral cephalometric films to investigate pharyngeal airway changes. It is widely accepted that there is a correlation between head posture and pharyngeal airway volume [16, 29]. Therefore, all lateral cephalometric radiographs should be taken with the head in a natural position, and the cephalometric films used in this study were taken following standard rules.

One limitation of this study is that it lacks a control (untreated) group. Comparison of linear and areal measurements between a control group and TBFM and SAFM groups would be ideal. However, in order to acquire cephalometric radiographs of control subjects, patients with skeletal class III malocclusion would have to be left untreated, which causes an ethical dilemma. Also, the study involves exposure to radiation, which is another ethical issue. Fortunately, there is precedence for carrying out studies of this nature without an untreated control group.

Airway passages usually expand as individuals grow up. Some authors have conducted research on the correlation between physical growth and pharyngeal airway volume [30–34]. According to Taylor et al., most posterior pharyngeal wall growth occurred in two spurts, from 6 to 9 years of age and from 12 to 15 years of age; growth of airways in individuals 9 to 12 years old is very limited [32]. The subjects in the current study consisted of 20 boys and 32 girls, with a mean age of 10.7 years and a mean treatment time of 15.5 months. Therefore, we can assume that the extent of natural pharyngeal airway growth in these subjects would be insignificant and the airway increases we observed was induced by face mask treatment.

Both the TBFM and SAFM groups showed increased pharyngeal airway measurements after treatment, but the SAFM group showed statistically significantly greater increases than did the TBFM group in four measurements. The SAFM subgroups had greater SPPA and IPA increases than did the TBFM subgroups. These findings suggest that SAFM therapy is more effective than TBFM therapy for increasing airway volume.

It is uncertain why the upper airway space increased by maxillary protraction. Some possible explanations are as follows. First, the protraction force of the face mask may induce forward movement of the maxilla, especially PNS; this could cause anterior displacement of the soft palate and consequently increase upper airway space [16]. Second, the anterior position of the tongue is altered by facemask treatment. This may be induced by increased volume of the oral cavity or by clockwise rotation of the mandible. The altered tongue posture could lead to an anterior shift of the soft palate and increased upper airway space [35].

Recently, pediatric obstructive sleep apnea (OSA) has attracted a great deal of attention. Many treatment modalities are available for pediatric OSA. Some authors proposed that maxillary protraction can be a solution for OSA [36]. Hiyama et al. [16] suggested that maxillary protraction appliances could contribute to improvements in respiratory function in patients with maxillary hypoplasia. Similarly, Hüsamettin et al. [37] implied that maxillary protraction could alleviate respiratory discomfort in patients with maxillary retrusion, and Verse et al. [38] found that intraoral devices were effective in approximately 50–70 percent of patients with obstructive sleep apnea. Conley [39] evaluated the effects of dental treatment in pediatric patients with obstructive sleep apnea. According to that study, maxillary protraction with palatal expansion has the potential to treat pediatric OSA patients. However, there are still an insufficient number of studies on the effects of maxillary protraction on treatment outcomes of OSA, such as measurements of polysomnography. Given the results of the current study, increased upper airway dimensions resulting from use of SAFM could contribute to the treatment of pediatric OSA. Unfortunately, none of our subjects reported a history of OSA, so further studies using specific measurements of OSA are needed in order to evaluate SAFM as a treatment for OSA.

There were significant increases in upper airway space in the TBFM and SAFM groups after maxillary protraction. The SAFM group exhibited greater overall pharyngeal airway increases than did the TBFM group, and SAFM subgroups showed greater increases in airway measurement values than did the TBFM subgroups, especially measurements of SPPA and IPA. Thus, SAFM was more effective than TBFM in producing airway increases. SAFM treatment may improve respiratory function in patients with maxillary hypoplasia or OSA, but this hypothesis requires further investigations.

To our knowledge, there has been no previous report comparing pharyngeal airway changes between TBFM and SAFM treatment. Therefore, we can say this study has produced meaningful results, even though it lacks data on untreated subjects.