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. 2021 May 21;11(3):447–450. doi: 10.1016/j.jobcr.2021.05.008

Sutural deformation during bone-anchored maxillary protraction

Taylor Rae Vracar a,∗, Wanda Claro a, Michael Eli Vracar II b, Randall Stetson Jenkins a, Lane Bland a, Ayman al Dayeh a
PMCID: PMC8167158  PMID: 34094844

Abstract

Introduction

Bone-anchored maxillary protraction (BAMP) is an emerging treatment option for orthopedic correction of maxillary deficiency in young patients. Compared to reverse pull headgear (RPHG), it is believed that forces generated during BAMP result in greater circum-maxillary sutural separation, mandibular retrusion, and improved maxillary protraction. Mechanical loading of the circum-maxillary sutures during BAMP is still poorly understood.

Methods

20 ex-vivo pig heads were used. Miniplates and molar tubes were installed like clinical procedures. A series of five 200 ​g-force (gf) elastics were applied on the right and left side until 1000gf were reached. Strain gauges were installed across the zygomatico-maxillary (ZMS), zygomatico-temporal (ZTS), and nasofrontal suture (NFS). Differential variable reluctance transducers (DVRTs) were installed across the ZTS. Deformation of the sutures during BAMP and RPHG was measured and compared.

Results

Higher average sutural deformation of the ZTS and ZMS was seen in BAMP than RPHG: 36.6 ​± ​20.6με vs 18.0 ​± ​12.4με and 54.7 ​± ​28.5με vs 12.5 ​± ​14.8με, respectively. Similarly, higher NFS deformation was seen in BAMP (18.4 ​± ​12.9με vs. −0.8 ​± ​12.0με). DVRT data showed higher ZTS separation in BAMP than RPHG (6.3 ​± ​5.2 ​μm vs. 1.7 ​± ​2.1 ​μm). These differences were all statistically significant using the Wilcoxon-signed rank test.

Conclusion

Both RPHG and BAMP forces separate the ZTS and ZMS. BAMP resulted in higher levels of sutural separation at the ZTS and ZMS by 2- and 5-fold, respectively.

Keywords: Maxillary protraction, Maxillary deficiency, Bone-anchored maxillary protraction, Reverse-pull headgear, Class III

1. Introduction

Class III malocclusion is characterized by various arrangements of maxillary deficiency and mandibular excess.1 It is ideal to correct the skeletal discrepancy with orthopedic intervention at the appropriate developmental stage. For years, the most used maxillary protraction device was the reverse pull headgear (RPHG).2 RPHG works by applying a traction force to the maxilla using teeth as point of force application.3 Maxillary protraction will result in separation and bone formation along the circum-maxillary sutures,4,5 resulting in anterior movement of the maxilla.6,7 Success of maxillary protraction is dependent on patency of circum-maxillary sutures. Currently, it is recommended to start RPHG in the early mixed dentition as studies reported more skeletal effects in this age group, suggesting more patent sutures.7 While RPHG has proven to be effective, its disadvantages include high rates of relapse, a narrow time interval for intervention, and unwanted dental movements.6 It is believed that less than 50% of the RPHG correction is due to skeletal change.8

Advances in skeletal anchorage had revolutionized our understanding of sutural patency and its clinical implications. Treatments like maxillary expansion that were restricted to young age groups are now successfully used in older patients using bone-borne expanders.9 This suggests that the concept of sutural patency is relative. Since traditional orthopedic devices apply load using teeth as points of force application, the success is likely dependent on the difference between tensile strength of the target suture and mechanical properties of the anchor teeth/alveolar bone complex. Increased sutural interdigitation will result in force concentration on the anchor teeth that might cause undesirable dental side effects and treatment failure. Thus, increased sutural complexity was often considered a limitation to orthopedic treatment.10 Bypassing teeth and applying load directly to bone surrounding the sutures will, theoretically, result in better force delivery and separation of more complex sutures. Several studies have reported on applying RPHG load to temporary skeletal anchorage devices in the lateral nasal wall and malar bone.11,12 They reported that applying a protraction force directly to the maxilla resulted in a more favorable clinical outcome compared to RPHG treatment.

The extraoral traction utilized with RPHG may contribute to the reported limited patient compliance.13 To overcome this, intraoral bone-anchored maxillary protraction (BAMP) has been suggested.14 It works by fixing surgical miniplates into the infrazygomatic crest anterior to the ZMS and into the body of the mandible apical to the canine root and running intermaxillary elastics between them. Forces generated during BAMP are thought to produce better maxillary protraction compared to RPHG.14, 15, 16 In addition to its effects on the maxilla, BAMP resulted in improved protraction of the zygoma and malar bone prominence compared to RPHG.14 However, BAMP is associated with significantly higher treatment costs and 2 additional minor surgeries, so improving predictability and optimizing treatment outcomes is essential.

Comparing sutural deformation during RPHG and BAMP may aid in explaining some of the differences in clinical outcomes among these modalities and optimize their clinical protocols. In a recent study, Al Dayeh et al. (2019) reported that BAMP results in greater ZMS deformation compared to RPHG.17 However, the effects of BAMP on the ZTS were not investigated. Assessing ZTS deformation during BAMP and RPHG will help further our understanding of skull loading during protraction and the effects of such loading on skull morphology. In a clinical study, Hino et al. (2013) reported that BAMP resulted in more favorable protraction of the zygoma compared to RPHG. We hypothesize that this difference is due to increased separation of the ZTS during BAMP.

The purpose of this study was to measure and compare sutural deformation during BAMP versus RPHG at the ZTS and ZMS. The null hypothesis is that there is no difference in sutural deformation and separation between the 2 treatment modalities.

2. Methods

This study was performed using 20 ex-vivo pig heads obtained from a local abattoir. They were in mixed dentition and about 6 months in age, corresponding to growing, skeletally immature human subjects as these are the usual candidates for BAMP. The heads were stored frozen at - 20 ​°C and then left to thaw at room temperature for 12 ​h before each experiment. This animal model was selected because the overall arrangement of the circum-maxillary sutures relative to the occlusal plane is comparable to humans suggesting comparable pattern of sutural loading.

To simulate BAMP, miniplates were fixed to the zygomatic process of the maxilla ~6–10 ​mm anterior to the ZMS and body of the mandible in the canine/first premolar area using 1.4 ​× ​11 mm screws (KLS Martin, FL). The location of the miniplate arms were adjusted so the protraction force was 20°±2° to the occlusal plane. To simulate RPHG, a 0.022 Standard Ormco first molar bracket was bonded to the first permanent molar bilaterally. The molar was prepared with Transbond™ Plus Self-Etching Primer and Assure® Plus Bonding Resin. The bracket was placed using Transbond™ Adhesive. A 1/16” drill bit was used to create a pilot hole in the mandible for a mini-screw to be placed to set up a force system at a similar angle to the occlusal plane as BAMP (Fig. 1).

Fig. 1.

Fig. 1

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A: (Above) DVRT is comprised of a rod and a core. The attachment screws are placed in bones across the suture. When the suture is deformed, the rod will slide in and out of the core. The sliding movement will result in change of the core differential reluctance, enabling the calculation of the core movement at a high resolution of ±1 ​μm. (Below) Single-element strain gauge. Suture deformation causes flexing of the foil.This change is read by pre-calibrated software and the amount of deformation is calculated. B: Schematic image displaying location of strain gauges and DVRTs installed. Strain gauges were installed spanning the ZMS, ZTS, and NFS. DVRT was installed spanning the ZTS.

Bone on each side of the sutures was cauterized then dehydrated using Ethyl Alcohol. Teflon tape was placed on the surface of each suture to prevent accidental adhesion. Strain gauges were bonded across each suture using M-Bond 200 Adhesive (Vishay Precision Group, NC). A 1/16” drill bit was used to create pilot holes to install DVRTs in the bone across the ZTS (Fig. 1). Protraction force was applied between miniplates using 5 series of 200gf elastics (Ormco). Each elastic pair was applied bilaterally starting on the right side. One minute separated each force increment. Strain gauge and DVRT data were collected using a wireless node (V-link, Microstrain, VT) and broadcasted to a nearby computer operating NodeCommander software (LORD Microstrain Inc.) to allow real-time measuring and recording of sutural deformation. The elastics were removed and sutures were allowed to return to their original position before beginning RPHG therapy using the same force level and order of elastics (Fig. 1). Data was analyzed in AcqKnowledge software (Biopac Systems, CA) and then summarized using Excel (Microsoft, Redmond, Wash). Statistical analysis was performed using Wilcoxon signed rank test (IBM SPSS statistics for Windows, NY) due to the limited sample size and variability.

3. Results

During BAMP, sutural deformation at the ZTS ranged from 34 ​± ​22 to 40 ​± ​30με per each 200 gf, while RPHG displayed a range of 17 ​± ​12 to 21 ​± ​14με (Fig. 2). No significant difference was found in sutural deformation across all force orders in both treatment modalities (p ​> ​0.8). Thus, values were averaged. The average ZTS deformation during BAMP was measured to be 36.6 ​± ​20.6 με while RPHG resulted in an average deformation of 18.0 ​± ​12.4 με. This difference was statistically significant.

Fig. 2.

Fig. 2

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Deformation observed at the ZTS (above) and ZMS (below) per each 200 ​g of protraction force from 200gf to 1000gf, along with the average deformation observed at each location.

During BAMP, for each force increment, sutural deformation at the ZMS ranged from 49 ​± ​29 to 57 ​± ​32 με, while RPHG displayed a range of 10 ​± ​8 to 15 ​± ​21με (Fig. 2). No significant difference was found in sutural deformation across all force orders in both treatment modalities (p ​> ​0.8). Thus, values were averaged. The average deformation of the ZMS was significantly higher in the BAMP (54.7 ​± ​28.5με) compared to the RPHG (12.5 ​± ​14.8με) group.

During BAMP, for each 200 gf increment, sutural deformation at the NFS ranged from 14 ​± ​18 to 23 ​± ​14με, while RPHG displayed a range of −3±15 to 0 ​± ​12με (Fig. 2). No significant difference was found in sutural deformation across all force orders in both treatment modalities (p ​> ​0.4). Thus, values were averaged. At the NFS, the average sutural deformation during BAMP (18.4 ​± ​12.9με) was significantly higher than that recorded during RPHG (−0.8 ​± ​12.0 με).

Due to the lower resolution of DVRTs, ZTS separation is reported per 1000gf. BAMP resulted in significantly higher separation of the suture compared to RPHG (6.3 ​± ​5.2 ​μm vs 1.7 ​± ​2.1 ​μm, respectively).

There was no correlation between strain values at ZMS and ZTS for BAMP. However, moderate positive correlation (r ​= ​0.6) was found during RPHG treatment between ZTS and ZMS.

4. Discussion

This study reports on deformation of three circum-maxillary sutures during BAMP and RPHG. To our knowledge, this is the first study to assess ZTS deformation during BAMP.

In a pioneering work, Smalley et al.11 applied maxillary protraction force (600 ​gm) to osseointegrated implants placed in the maxilla and/or zygoma of monkeys. Their results suggested that bone-borne protraction resulted in better protraction and disarticulation of the ZMS and ZTS.11 In our study, we applied a series of five- 200gf elastics to determine whether a specific force magnitude results in disarticulation of the circum-maxillary sutures. Surprisingly, the ZMS and ZTS deformation was directly proportional to force magnitude, with limited difference in sutural deformation at each force level. This suggests that both protraction devices function within the suture's elastic range, and no disarticulation of either sutures happened. While comparing the 2 studies is complicated by the difference in the experimental species and study design, it is important to notice that Smalley's study was performed in vivo and animals were observed for an extended period. Thus, the observed disarticulation could be a result of sutural remodeling. Our study was conducted ex vivo and the impact of protraction on sutural histology was not measured.

Deformation of the ZMS and NFS reported in this study are comparable to those reported previously,17 especially for the RPHG. While our measurements at the ZMS during BAMP were slightly higher (~54 vs. 34 με), such differences are common in strain gauge studies and could be due to the high resolution of the gauges and experimental variability. These comparable findings further validate current results. A major advantage of this study over previous work17 is in assessing the deformation of the ZTS in addition to the ZMS and NFS. Since the ZTS join the zygoma (malar bone) to the temporal bone, measuring its deformation will help understand whether, and to what extent, protraction moves the zygoma forward. The impact of the zygoma position on facial esthetics has long been known.18 Several surgical procedures have been suggested to improve facial esthetics by increasing the prominence of the zygoma.19 In class III patients, maxillary deficiency is often accompanied with weak projection of the zygoma20 resulting in “flattening” of the face and compromised esthetics. Contrary to its effects on the maxilla,21 few studies assessed the impact of protraction on the zygoma.16 Understanding the displacement of the zygoma during maxillary protraction treatments will greatly enhance our knowledge of the effects of such treatment on facial esthetics. Hino (2013) reported increased anterior movement of the zygoma in BAMP compared to RPHG.16 In this study, two approaches were utilized to measure ZTS deformation: strain gauges and DVRT. While both sensors displayed more changes during BAMP, this was more pronounced in the linear deformation measured by the DVRT (6.3 vs. 1.7 ​μm). This can be attributed to the differences in the mechanism of the action of each of the sensors. Since the strain gauge is glued across the suture, lateral deformation can affect the recorded results. As the DVRTs are specialized linear displacement transducers, they provide a better measure of linear sutural separation. Our results show that, when using the same force level, BAMP results in more ZTS separation compared to RPHG, suggesting a better zygoma protraction. However, clinically, the recommended force magnitude for RPHG (400–800 ​g) is much higher than that of BAMP (100–250 ​g).14, 15, 16 Although the ZTS separation was calculated per 1000gf, the linear relation between the force level and sutural deformation allow estimating the amount of separation for each force interval. Thus, under clinical setting where the recommended RPHG force magnitude is 2–4 times that of BAMP, our results suggest that ZTS separation is comparable between the two devices. These findings does not support the zygoma projection seen in BAMP patients.16

Our results on linear ZTS separation combined with those reported previously, using a different cohort of pigs, of the ZMS (Al Dayeh, 2019) suggest that the overall protraction of the midface in BAMP was almost double that of RPHG (12.2 vs. 5.7 ​μm) per 1000gf.17 Adjusting the numbers based on the force magnitude used clinically suggests that protraction force of 250 ​g in BAMP will result in ~3.1 ​μm of protraction and 600gf with RPHG will result in ~3.2 ​μm. These findings do not support the favorable outcome of BAMP observed clinically. Recently, the clinical superiority of BAMP over RPHG has been questioned.22 Our results suggest that while BAMP results in better loading of circum-maxillary sutures, this effect is greatly neutralized by the higher force magnitude used in RPHG. Thus, the difference in clinical outcome between the 2 devices14,16 is not due to better sutural loading, but probably due to compliance, and BAMP effects on the mandible23 both possibilities warrant further investigation.

Although both devices resulted in tension at the ZMS, their effects on the NFS were consistently different. BAMP resulted in tension at the NFS compared to minimal compressive deformation during RPHG. Combined, these results suggest that BAMP causes more bodily anterior movement of the midface compared to upward and backward rotation in RPHG around the NFS. This supports clinical findings of downward and backward rotation of the mandible after RPHG3,6,8 compared to more anterior bodily movement of the maxilla during BAMP.14 This finding might be only applicable to the protraction angle used in the study. Changes in force direction have long been known to alter the path of protraction.24

Due to the complex nature of the ZMS, we hypothesized that increased sutural complexity will result in decreased deformation of the ZMS and more load transmission to the ZTS. This will be manifested as negative correlation between the deformations of the two sutures. No such correlation was found. In fact, RPHG showed a moderate positive correlation. While our results may be affected by the limited sample size and age range, our findings suggest that ZTS and ZMS deformation during protraction is independent. Therefore, future studies are needed to assess the mechanical properties of the ZMS and ZTS and their impact on sutural deformation. Linking the mechanical properties to the radiographic presentation of the sutures can be a great clinical tool in predicting the outcome of protraction.

Findings of this study need to be interpreted with care as several limitations can affect the conclusion. The study was conducted ex vivo. While this allows for a better controlled protocol, the biologic impact of protraction was not assessed and warrants an in vivo investigation. Nonetheless, sutural separation has long been known to induce bone formation at the sutures.5While we selected animals of comparable dental age, the level of skeletal maturity of the ZMS and ZTS were not assessed. The impact of sutural maturity on response of the suture to protraction force has been recently investigated.25 This was controlled by each animal receiving both treatments. Additionally, this study only assessed the sutural loading during BAMP and RPHG. Other clinical advantages of BAMP, such as minimizing dental side effects, mandibular restriction and improved aesthetics and compliance were not investigated.7,14, 15, 16

Within the limitation of this study, our results show higher ZMS and ZTS separation during BAMP. However, when adjusting for the higher force level used clinically in RPHG, both devices result in comparable sutural separation. This suggest that both devices are equally efficient in protracting the midface and that the reported improved treatment outcome during BAMP is not due to improved sutural loading. Additionally, BAMP resulted in bodily protraction of the midface compared to upward and backward rotation in the RPHG. Thus, it might be more suitable if control of vertical growth is a concern.

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