Skeletal, dentoalveolar, and buccal bone changes using hybrid and tooth-borne expanders for RME and SARME in different growth stages

Rapid maxillary expansion (RME) has been used for more than a century to correct transverse maxillary deficiencies. The midpalatal suture may be opened orthopaedically before the maxillary sutures become unresponsive. The traditional view regarding RME is that the older the individual, the more interdigitated and complex the sutures become and the more difficult expansion is to achieve.1 Due to increased sutural interdigitation, if RME is performed after the pubertal growth spurt, the skeletal effects reduce and expansion takes place mainly at a dentoalveolar level.2

Bone elasticity decreases as a patient’s skeletal age increases.3 Surgically-assisted rapid maxillary expansion (SARME) was introduced to overcome the side effects of dentoalveolar expansion in adults, by surgical separation of the two maxillae in order to reduce the load on the posterior teeth.4 However even following SARME, the side effects of buccal tipping were noted.5

The use of skeletal anchorage for expansion, with bone-borne or tooth–bone-borne (hybrid) expanders, particularly in adolescents and adults, to maximise the skeletal effects while minimising the dentoalveolar side effects and root resorption, has recently become popular.6–9 Various bone-borne and hybrid expanders have been introduced and involve different mini-implant locations in the paramedian region, alveolar ridge, or anterior palatal slopes.6,7,9 The hybrid expander (Hybrid Hyrax; Tadman GmbH, Gunningen, Germany) is a tooth- and bone-anchored device that uses two palatal mini-implants on the anterior slope of the palate on either side of the midpalatal suture.6

A recent systematic review showed that there are few studies which reported the effects of different types of expanders and skeletal age.10 Although several studies describing the effects of hybrid expanders on RME and SARME have been published,5,11–14 studies based on the skeletal growth stage are warranted in order to justify the common use of tooth–bone-borne (hybrid) maxillary expansion during different growth periods. The aim of this clinical, two-centre, cone beam computed tomography (CBCT) study was to investigate the differences between conventional tooth-borne (TB) and tooth–bone-borne (hybrid) RME in the pubertal and postpubertal stages, and also in surgically-assisted RME (SARME) in young adults in regard to dentoskeletal effects and buccal alveolar bone levels. The null hypothesis stated that hybrid expanders would not offer any significant advantages over traditional expansion appliances during early and late adolescence and in young adults.

The study included CBCT images from before and 6 months after expansion in 60 patients (27 males and 33 females) consecutively treated by either a tooth–bone-borne (H, n = 30) or a tooth-borne (TB, n = 30) RME and SARME. All of the CBCT images were obtained from two previous randomised controlled clinical trials that used similar methods, but in different age groups. The demographic characteristics are listed in Table I.

The patients were divided into six groups (10 patients each) relative to their growth stage according to the cervical vertebrae maturation method (CS).15 The patients in the early adolescent (EA, cervical vertebrae maturation stage, CS = 3–4) and late adolescent (LA, CS = 5) groups were treated by H or TB RME at the Department of Orthodontics at the University of Sydney and the adults (A, CS = 6) were treated at Istanbul University with H or TB SARME (Ethics HREC X17-0075 and HREC/17/RPAH/107; and 2012/641-1044). Ethical approval was provided for the re-evaluation of groups at the two centres by the Non-Interventional Clinical Research Ethics Committee of Istanbul Aydin University (The protocol number: 292/11.11.2020).

The applied selection criteria identified a skeletal transverse maxillary deficiency with a unilateral or bilateral posterior crossbite, maxillary transverse deficiency of more than 5 mm as measured from the cusp of the upper first permanent molar to the lower first permanent molar central groove, CS 3–6, patient records with CBCT images obtained at two time points: before RME (T0) and after a 6-month retention period (T1); the absence of any developmental deformity, and no prior orthodontic treatment.

The TB expanders were of the traditional Hyrax design incorporating bands on the first premolars and molars (Fig. 1A). The H expander used for the EA and LA was a slightly modified design, with two 2×9 mm Benefit mini-screws (PSM, Gunningen, Germany) placed at the level of the third rugae on either side of the mid-palatal suture and bands on the first molars and incorporated palatal extensions extending on the palatal side of the upper premolars (Fig. 1B). The H expander used for adult patients followed the traditional design of bands on only the first molars and two 1.7×10 mm mini-screws (Ortho Easy; Forestadent, Germany) (Fig. 1C).16 In the SARME groups, the same surgical approach was applied and involved a Le Fort I procedure with a midline osteotomy and accompanying pterygomaxillary disjunction. The protocol for expansion for all participants was: each appliance was activated twice/day (0.5 mm daily) and monitored weekly until an overcorrection of 30% was achieved and determined by observing that the palatal cusps of the upper first molars were in contact with the buccal cusps of the lower first molars. After active expansion, the mechanism was fixed and left in situ for 6 months. The appliances were then removed and CBCT records taken.

Figure 1.

The primary outcome was to determine whether there were any significant differences in dental and skeletal changes using the different expansion devices during the three different growth stages. The secondary outcome was to determine the expansive effect on buccal alveolar bone thickness.

The DICOM-formatted CBCT images were rendered into volumetric images, and cross-sectional slices were obtained using InVivo (Anatomage Inc., Santa Clara, California, USA). The measurements are summarised in Table II and Figures 2–4.

Figure 2.

Figure 3.

Figure 4.

Sample size calculations were based on the ability to detect a clinically relevant difference in the increase in inter-premolar crown width data from a previous study,5 a power of 95%, and a significance level of 0.05 in a two-sided paired t-test. The calculations resulted in a required sample size of four patients per group (effect size: d=3.25; r=–0.85). Ten patients per group were recruited to ensure sufficient power.

Table III lists the skeletal-dental characteristics of the TB and H groups. The young adult patients who received the hybrid device had an increased inter-premolar crown distance in comparison with the TB-A group (p=0.02). The other maxillary skeletal, dental and nasal parameters and screw activation protocols were similar between the groups (p>0.05).

Maxillary skeletal, dental, nasal, and buccal alveolar bone changes in three different growth stages in the Hybrid and TB groups are presented in Tables IV, respectively. Intergroup comparisons between the expansion devices for the three growth stages are shown in Table VI.

The present study compared the effects of conventional tooth-borne (TB) and tooth–bone-borne (H) RME during the pubertal and postpubertal stages and of SARME in young adults. The skeletal changes were significant in all groups, and although the expansion at the skeletal level was more pronounced in the H group, this only reached statistical significance for the H-EA group anteriorly and the H-A group posteriorly. The differences, however, may not be clinically significant. Lagravère et al. found no skeletal differences between the TB and bone-borne groups at the posterior level.7 In contrast, Mosleh et al. reported a greater amount of expansion in the TB group in comparison with the bone-borne group for skeletal measurements at the maxillary first molar level.9 Celenk-Koca et al. observed a 1.9 mm increase in the bone-borne group in comparison with the TB group at the midpalatal suture at the level of the first molars, which was slightly higher than the amount observed in the present study.8 The addition of two mini-implants may have accounted for this. In the present H-RME patients, the first premolars expanded significantly and tipped buccally more in adolescents than that seen in the H-SARME adult group. This was considered a result of the palatal extension wire incorporated into the Hybrid expanders in the adolescent patients (Fig. 1).

The nasal width in the EA group increased significantly in comparison with adults in both the H and TB groups. As there were no control groups in the present trial, it was difficult to ascertain the impact that growth might have had on maxillary and nasal changes.17 While some reported that changes at the nasal level were comparable with hybrid and TB expanders in adolescents,11 Mosleh et al. found a greater increase in the TB group.9 It was postulated that this was due to the more anterior force application in their TB group, as their bone-borne expanders placed the anterior mini-implants between the first and second premolars. In contrast, the location of the mini-implants in the present study, in the depth of the palate, closer to the fulcrum, may explain the significant differences.

A recent study, conducted using an alternative measurement method, showed that the amount of nasal width widening was significant, but did not differ between the H-SARME and TB-SARME groups at the anterior level.18 The anterior nasal width findings in the present study were similar and supported the findings of Kayalar et al., who reported comparable results for both SARME groups.19 However, in the present study, posterior nasal width widening and posterior internal skeletal maxillary expansion were significantly greater in the H-SARME than in the TB-SARME group. Although the mini-implants were placed in the anterior palate, the vector of expansion may have had a more parallel trajectory as the SARME was performed with pterygoid disjunction. In contrast, Seeberger et al. found that anterior nasal width widening was significantly greater following bone-borne SARME than with TB-SARME.20 These differences between studies may be due to different observation periods, expander anchorage types and varying surgical and analysis techniques.

In the present study, the inclination of the anterior alveolar crest increased significantly more in the late adolescents after TB-RME than in the young adult TB-SARME. As a general trend, there was an ascending pattern of width increase from the hard palate down to the alveolar crest, a finding that is consistent with the pyramid-like expansion opening commonly described.11 Furthermore, these findings are consistent with those of Mosleh et al.9 and Gunyuz Toklu et al.11 who noted increased linear expansion at the coronal level in comparison with the apical level in nonsurgical RME. The first premolars tipped buccally more in the group with TB devices than in the group with hybrid devices in late adolescents and adults. The anterior alveolar crest angle also increased more in the TB group at the three growth stages, possibly caused by segmental inclination changes in the maxillary halves. However, the first molar angle and posterior alveolar crest angle parameters were comparable between the two groups.

The present findings show that in adults, the first premolars migrated palatally by –1.8°, resulting in an improvement in associated buccal alveolar bone thickness (BABT) of 0.25 mm, when the premolars were not included in the H expander design. This palatal tipping movement may be associated with pressure applied by the buccal muscles or resistance from the palatal mucoperiosteum.5,19 The dental effects of H-SARME expanders on the premolars were similar to those of bone-borne expanders due to the anterior location of the mini-implants which supported the findings of Landes et al. who reported inward dental tipping associated with bone-borne expanders.21 Celenk-Koca et al. also observed uprighting of the maxillary posterior teeth as a result of their bone-borne appliance, which did not include any tooth-borne components, and therefore improved the BABT.8

These findings were however different in adolescents. The premolars tipped buccally by 4.4°, causing a reduction in the BABT of –0.33 mm as the premolars were included in the hybrid expander design in the late adolescent group. However, this effect was still less than the reduction in the BABT of the premolars associated with the TB devices. In contrast, early adolescent patients were not significantly affected by the different appliance designs of the hybrid expanders in relation to premolar inclination and BABT. Although the hybrid expanders had premolar extensions in the early adolescent group, the premolar inclination change was comparable. In addition, there was no significant reduction in the BABT at the premolars. As the hybrid expander incorporated mini-implants, the pressure on the anchorage teeth was likely reduced. This also transmitted the expansion forces closer to the centre of resistance of the maxilla, which may have contributed to the increased skeletal component of expansion in the premolar area, supporting the observation of increased anterior internal skeletal maxillary expansion in early adolescents. Garib et al. found similar dental effects for hybrid expanders incorporating premolar extensions and TB expanders in growing patients.12

Gunyuz Toklu et al. compared a four-band Hyrax with a hybrid appliance and noted a decrease in buccal cortical bone thickness at the premolars only in the Hyrax group, when the appliance was anchored to those teeth.11 Pangrazio-Kulbersh et al. assessed bonded and two-banded Hyrax appliances and observed a reduction of >0.5 mm in the BABT at the first premolars and first molars in both appliance groups.22 A 2018 systematic review by Lo Giudice et al. which assessed BABT changes following TB-RME, demonstrated that the BABT was reduced on the anchorage teeth in the short to medium term.23 The BABT changes were comparable in the intragroup comparison of hybrid devices during the three growth stages. However, there was significantly less BABT reduction in adults than in the early and late adolescents in the TB group. The SARME operation should reduce the dental side effects of TB devices in adults.

According to the findings of the present study, both TB and hybrid RME in adolescents and SARME in adults are effective methods for correcting a transverse maxillary deficiency. However, many adolescents and young adult patients are discouraged from choosing SARME as a treatment due to the risks, complications, and cost of the surgical procedure. It is recommended that orthodontists consider a hybrid RME as a treatment option for a narrowed maxilla in adolescents, due to the lower risk of complications and lesser invasiveness in comparison with SARME, after individual evaluation of the developmental stages of fusion in the maxillary sutures. The palatal extension design used in the present study for adolescent hybrid RME did not impede the skeletal effects nor cause significant BABT reduction in growing individuals. Therefore, if treatment objectives require more movement of the premolars, adding this arm could be desirable.

A limitation of the present study was the short-term follow up and small number of patients. A second limitation was that the study did not include a bone-borne rapid maxillary expansion group. Future studies should incorporate more groups and review patients for a longer period.

Both hybrid and tooth-borne devices were effective in correcting transverse maxillary deficiencies and expanding the nasal width using a RME in early and late adolescents, and SARME in adults. Hybrid expanders increased the internal skeletal maxillary width and skeletal nasal width more than TB devices in early adolescents anteriorly and in adults posteriorly. Tooth-borne devices caused a greater reduction in the buccal alveolar bone thickness at the first premolars and first molars during each growth stage.

Both hybrid and tooth-borne devices were effective in correcting transverse maxillary deficiencies and expanding the nasal width using RME in early and late adolescents and SARME in adults.