Effectiveness of miniscrew-supported maxillary molar distalization according to temporary anchorage device features and appliance design: systematic review and meta-analysis

Maxillary molar distalization is the most frequently performed treatment for correcting Class II malocclusion and achieving Class I molar and canine relationships without extractions.¹  However, finding appropriate anchorage to avoid side effects is fundamental. Anchorage is crucial for the successful treatment of Class II malocclusion,²  as instability of anchor teeth can result in unfavorable occlusal relationships and an unsatisfactory outcome.³ 

Extraoral appliances (e.g., headgear) and intraoral options (e.g., Nance button) are commonly used to reinforce anchorage. However, extraoral traction poses compliance challenges, while intraoral methods often result in anchorage loss.⁴  To address this, intraoral distalization devices have been used and supported by skeletal anchorage. Dental implants have emerged as a stable solution for orthodontic purposes, benefiting from their osteointegration capabilities⁵ ; implants have demonstrated resilience against forces and remain stable following orthodontic loading over time.⁶ 

Although implants offer clear advantages in preserving anchorage, their invasive insertion and removal techniques hinder widespread adoption in daily practice. To address this limitation, temporary anchorage devices (TADs), including mini-implants,⁷  miniscrews,⁸  and onplants,⁹  have emerged as promising alternatives. Miniscrews especially utilize less-invasive methods than conventional implants and hold great potential for providing stable skeletal anchorage, particularly for posterior tooth distalization.¹⁰ 

TAD-supported appliances can be placed buccally in interdental spaces, palatally¹¹  in the retromolar region,¹²  or even in the zygomatic area.¹³  The varied bone characteristics in these regions require smaller implants, particularly in length, while ensuring stability to withstand orthodontic forces. Factors such as ease of insertion, active treatment and removal, reliable wire fixation, and ease of handling are crucial for successful implant application in orthodontics.¹⁴ 

Recent reviews have examined the use of TAD-supported appliances for nonextraction treatment of Class II malocclusion and highlighted their advantages.^15–18  All of the reviews agreed regarding the advantages provided by TADs, but to date, no systematic reviews have evaluated and compared the overall efficacy of molar distalization performed with different TAD-supported appliances categorized by the location of placement, rigidity of the appliance, and number of TADs used. It is interesting and important to evaluate which types of anchorage and appliance can be used more effectively and efficiently for different malocclusions. Clinically, this information may affect the choice of appliance, depending on specific side effects in terms of changes in molar tipping, vertical movement, or inclination of the occlusal plane. The studies included in this review were stratified based on the location of placement and rigidity of the device used. Therefore, it was possible to evaluate various dental effects and propose a new, different analysis from previous systematic reviews. Therefore, this systematic review and meta-analysis aimed to evaluate the treatment effects of TAD-supported maxillary molar distalization in Class II malocclusion, considering the amount of distalization, tipping, and vertical movement of the maxillary first molars (U6).

This review followed the PRISMA standards of quality for reporting systematic reviews and meta-analyses.¹⁹  The protocol was registered in PROSPERO (CRD42022333115).

An electronic search of PubMed and MEDLINE, Google Scholar, and the Cochrane Oral Health trial registry was conducted from January 2011 to April 2023. Studies with the following characteristics were selected: studies on human subjects, studies published in English, sample size mentioned (at least five patients); prospective and retrospective studies, and random clinical trials (RCTs) that included descriptions of the distalization appliance. Table 1 describes the inclusion and exclusion criteria.

The search strategy included the following terms: Maxillary_molar_distalization_AND_Class_II_AND_skeletal_anchorage_AND_miniscrew_OR_mini-implants_AND_(y_10[Filter“]). ((((“maxilla”[MeSH_Terms]_OR”_“maxilla”[All_Fields]_OR_“maxillary”[All_Fields]_OR_“maxillaries”[All_Fields]_OR_“maxillaris”[All_Fields])_AND_(“molar”[MeSH_Terms]_OR_“molar”[All_Fields]_OR_“molars”[All_Fields]_OR_“molars”[All_Fields])_AND_(“distal”[All_Fields]_OR_“distalization”[All_Fields]_OR_“distalize”[All_Fields]_OR_“distalized”[All_Fields]_OR_“distalizer”[All_Fields]_OR_“distalizers”[All_Fields]_OR_“distalizes”[All_Fields]_OR_“distalizing”[AllFields]_OR_“distally”[All_Fields]_OR_“distals”[All_Fields])_AND_(”“class”[All_Fields]_OR_“classe”[All_Fields]_OR_“classed”[All_Fields]_OR_“classes”[All_Fields])_AND_“II”[All_Fields])_AND_((“skeletal”[All_Fields]_OR_“skeletals”[All_Fields])_AND_(“anchorage”[All_Fields]_OR_“anchorages”[All_Fields]))_AND_(“miniscrew”[All_Fields]_OR_“miniscrews”[All_Fields]))_OR_“mini-implants”[All Fields])_AND_((y_10[Filter])_AND_(humans[Filter])).

Manual searches were performed using reference lists in full-text articles deemed appropriate for inclusion in the study and other relevant systematic reviews. Two authors conducted the search for studies to be included independently, and differences were resolved by discussion and consensus with a third, trained researcher. Data were finally extracted according to the PICOS questions and categorized.

The quality of the included studies was assessed independently by two independent researchers. The Joanna Briggs Institute (JBI) checklist was used for randomized trials (7 questions) and nonrandomized cohort studies (11 questions).²⁰ 

The study design, sample size, age, type of appliance, location of placement, number of miniscrews, and material used for measurements were obtained. The means and standard deviations (SDs) of distalization, distal tipping, and vertical movement of the U6 were calculated. All data were extracted into an Excel spreadsheet by one author and reviewed by another author to confirm accuracy. Studies whose mean was more than double the mean of the subgroups were excluded from the meta-analysis after testing them by analysis of variance.

Meta-analysis was performed using Review Manager software (RevMan version 5.4; The Cochrane Collaboration, 2020) with a generic inverse variance approach. The random-effects method was used because the studies included in the analysis had a high degree of heterogeneity and were quite diverse in terms of the entity of the forces applied. Heterogeneity was first assessed from a clinical perspective based on the position of the TADs, their number, and the rigidity of the distalization system. The significance level was set at 0.05. For meta-analysis, the standard error (SE) value of the SD of the individual results was also calculated.

The database search identified 805 studies, and after the removal of duplicates, titles and abstracts were independently evaluated for inclusion. After the study selection, the percentage of agreement between reviewers reached a Cohen’s kappa coefficient value of 0.93. A third author was consulted for resolution of the disagreements to obtain the final list of included studies. Ultimately, 40 studies met the inclusion criteria for qualitative and quantitative analyses (Figure 1).

The studies included in this review were published between January 2011 and April 2023: 4 RCTs, 13 prospective studies, and 23 retrospective studies.

Distalization groups included 1182 patients. Studies were divided into palatal,^21–47  buccal,^{10,28,32,48–52}  and zygomatic^53–59  according to the position of TADs. For studies with palatal TADs, a distinction was made between rigid^{21–23,25,26,28,29,38,47}  and nonrigid^21,30,35  appliances and between 2-TAD-supported^{21–23,25,26,28–30,35,38,47}  and 3-TAD-supported^{24,27,31–34,36,37,39–46}  appliances. The characteristics of the 40 studies included in the qualitative analysis are summarized in Table 2.

The JBI checklist was used. In the analysis of retrospective and prospective cohort studies, 10 studies were classified as “low” risk, 17 studies as “moderate,” 7 studies as “serious,” and 2 studies as “critical.” The risk of bias (ROB) in the included RCTs was “serious” in two studies and “moderate” in the other two. Tables 3 and 4 show the summary of ROB for nonrandomized and RCT studies, respectively.

U6 distalization with palatal anchorage ranged from 1.65 mm³²  to 5.4 mm,⁴⁴  with tipping values ranging from 0.1°^38,47  to 11.24°.³⁰  Most studies reported U6 intrusion (−0.04 mm⁴³  to −2.6 mm⁴⁶ ), while some reported extrusion (0.2 mm²⁸  to 1.6 mm³⁹ ).

Using buccal TADs, U6 distalization ranged from 1.29 mm⁴⁸  to 5.05 mm,²⁸  with tipping ranging from 0.6°⁵¹  to 7.2°.³²  Vertical movement varied from intrusion of −1.4 mm⁴⁸  to extrusion of 1.3 mm.¹⁰ 

U6 distalization with zygomatic anchorage ranged from 2.93 mm⁵⁴  to 5.31 mm.⁵⁵  Distal tipping varied from 1.21°⁵⁴  to 11.29°.⁵⁹  Vertical movement ranged from −3.7 mm⁵⁶  (intrusion) to 0.6 mm⁵⁸  (extrusion).

Devices with 2 TADs had a distalization range of 2.3 mm²⁶  to 5.3 mm,³⁸  while the distalization range of devices with 3 TADs ranged from 1.65 mm³⁶  to 5.4 mm.⁴⁴  Distal tipping for devices with 2 TADs was 0.1°³⁸  to 11.24°³⁰ , and for devices with 3 TADs, it was 0.28°⁴¹  to 5.09°.⁴⁶  Vertical movement values for the 2-TAD-supported subgroup ranged from −1 mm^22,28  (intrusion) to 0.7 mm²³  (extrusion), and the values for the 3-TAD-supported subgroup ranged from −2.6 mm⁴⁶  (intrusion) to 1.6 mm³⁹  (extrusion).

Nonrigid devices had a slightly smaller distalization range (2.93 mm²¹  to 4.2 mm³⁵ ) than rigid devices (2.3 mm²⁶  to 5.3 mm³⁸ ). Distal tipping was greater in nonrigid devices (8.9°³⁵  to 11.24°³⁰ ) than in rigid devices (0.1°³⁸  to 11.02°²⁹ ). Vertical movement values were similar between nonrigid and rigid palatal devices, ranging from −0.74 mm³⁰  to −0.6 mm³⁵  and from −1 mm²²  to 0.7 mm²³ , respectively.

Figure 2 compares devices with palatal, buccal, and zygomatic TADs in terms of distalization, tipping, and vertical movements. High heterogeneity (I² = 95%) and no significant differences in the amount of distalization (P = .64) were found among palatal (3.74 mm; 95% confidence interval [CI], [3.51, 3.98]; P < .0001; I² = 87%), zygomatic (3.68 mm; 95% CI, [3.21, 4.14]; P < .0001; I² = 95%), and buccal (3.23 mm; 95% CI, [2.16, 4.30]; P < .0001; I² = 98%) anchorage locations.

Distal tipping was higher, although not significantly (P = .30), in the zygomatic group than in the others (4.26 mm; 95% CI, [1.74, 6.78]; P < .0001; I² = 96%) despite four studies being excluded because they had mean values strikingly outside the mean of the single subgroup.^10,28,29,59  Intrusion was also higher (−1.16 mm; 95% CI, [−1.84, −0.48]; P < .0001; I² = 97%) but not significantly (P = .28) different for zygomatic TADs than the other two groups.

Figure 3 shows the comparison between 2-TAD-supported and 3-TAD-supported appliances in terms of distalization, tipping, and vertical movement. The comparison showed high heterogeneity between subgroups (I² = 87%) and no significant differences (P = .86) between the 2-TAD (3.78 mm; 95% CI, [3.31, 4.24]; P < .0001; I² = 87%) and 3-TAD (3.73 mm; 95% CI, [3.43, 4.03]; P < .0001; I² = 88%) subgroups.

There was heterogeneity (I² = 82%) but no significant differences (P = .99) between the 2-TAD (2.37°; 95% CI, [1.26, 3.49]; P < .0001; I² = 88%) and 3-TAD (2.38°; 95% CI, [1.72, 3.04]; P < .0001; I² = 83%) subgroups in terms of distal tipping, even though one study was dropped from the analysis due to a value outside the mean of the subgroup.³⁵  There was also heterogeneity within subgroups for vertical movement (I² = 96%); no significant difference in intrusion (P = .09) was found between devices with 2 TADs (−0.29 mm; 95% CI, [−0.68, 0.10]; P < .0001; I² = 97%) and those with 3 TADs (−0.94 mm; 95% CI, [−1.57, −0.31]; P < .0001; I² = 94%).

Figure 4 shows the comparison between rigid and nonrigid appliances in terms of distalization, tipping, and vertical movement. High heterogeneity (I² = 87%) and no significant differences (P = .63) between the nonrigid (3.61 mm; 95% CI, [2.85, 4.38]; P < .0001; I² = 77%) and rigid (3.85 mm; 95% CI, [3.27, 4.44]; P < .0001; I² = 89%) appliances were found regarding distalization amount.

Distal tipping was significantly higher (P < .0001) in nonrigid (9.84°; 95% CI, [8.08, 11.60]; P < .0001; I² = 60%) than in rigid (1.97°; 95% CI, [1.01, 2.92]; P < .0001; I² = 71%) appliances, with high heterogeneity within subgroups (I² = 95%); one study with mean values outside the mean of the group was excluded.²⁹ 

Finally, for vertical movements, there was considerable heterogeneity (I² = 97%) and no significant difference (P = .06) within the nonrigid (−0.69 mm; 95% CI, [−0.95, −0.44]; P < .61; I² = 0%) and rigid (−0.19 mm; 95% CI, [−0.64, 0.26]; P < .0001; I² = 97%) subgroups.

TADs may reduce the need for tooth extraction for orthodontic purposes and orthognathic surgery. Maxillary molar distalization may be able to correct Class II malocclusion using different TAD-supported appliances. This review assessed the effectiveness of molar distalization based on TAD number, TAD position, and device design.

Data analysis revealed nonsignificant differences in the magnitude of distalization for the subgroup using palatal TADs (3.74 mm) compared to buccal (3.23 mm) and zygomatic (3.68 mm) TADs. Therefore, no significant clinical differences were observed among the different device positions. These results were similar to those of a 2021 review that found distalization values of 2.75 mm for buccal TADs, 4.07 mm for palatal TADs, and 4.17 mm for zygomatic anchorage.¹⁷  A 2022 systematic review also reported similar values for the distalization of molars by buccal TADs (2.44 mm) and palatal appliances (modified C-palatal plates [MCPPs]) in adults (4.00 mm) and adolescents (3.54 mm).¹⁸ 

One possible explanation for any differences would be that buccal TAD root proximity may be a factor limiting distalization, in contrast to extra-alveolar anchorage, which could allow greater movement. Also, palatal appliances allow force application closer to the center of resistance, leading to more distalization. No differences were found between 2-TAD-supported and 3-TAD-supported appliances as well as when comparing nonrigid and rigid devices.

Distal tipping is a common effect of molar distalization. There were no significant differences in distal tipping when comparing by position and number of TADs, but the difference was statistically and clinically significant when rigid (1.97°) and nonrigid (9.84°) appliances were compared. These results were consistent with the specific biomechanical properties involved. Palatal devices with pendulum arms result in distal tipping because the force is applied to the clinical crown, far from the center of resistance.⁶⁰ 

Among all rigid palatal devices, the tipping values were generally low, as expected. The least tipping was shown by 3-TAD-supported devices (MCPPs) that used a controlled force vector that passed through the center of resistance of the U6, increasing distal movement while reducing distal tipping simultaneously.⁶¹  Therefore, the direction of the force vector applied to the distalization system can certainly alter the tipping of teeth during distalization.

In this meta-analysis, no significant differences were found for vertical movements among the buccal, palatal, and zygomatic TADs, but zygomatic TADs showed the greatest degree of intrusion (−1.16 mm). These findings contradicted those reported in two previous reviews.^17,18  It was expected that rigid appliances would provide better vertical control of maxillary molars due to their nonelastic nature, ensuring complete control over vertical movements. However, in a few cases, there was slight extrusive movement (ranging from 0.2 mm²⁸  to 0.70 mm²³ ), observed exclusively with rigid palatal appliances. As discussed above, the direction of the resultant force vector plays a role in the effectiveness of the intrusion of teeth connected to the appliance.

Zygomatic anchorage should allow better vertical control and intrusion since the resulting vector force is above the center of resistance of the U6 and maxilla, therefore enabling intrusion of the upper arch simultaneously with a clockwise rotation of the occlusal plane.

Only 4 RCTs were eligible to be reviewed. To enhance our understanding of distalization effects, prospective and retrospective cohort studies were included. Bias and study design differences may influence the effect estimation, so the results should be interpreted cautiously.

The methods used to assess U6 movement varied among the studies reviewed. Most commonly, measurements were made on lateral cephalograms, cone beam computed tomography (CBCT) images, and three-dimensional (3D) dental models using different landmarks. Although each method was validated and accurate, variations among studies may limit generalizations and comparisons.

Additionally, the limited information provided on the severity of the Class II molar relationships within the studies could confound the meta-analysis.

• There were no significant differences in the magnitude of molar distalization, molar distal tipping, or molar intrusion among distalization appliances using palatal, zygomatic, or buccal TAD anchorages.

• The use of 3-TAD-supported appliances compared to 2-TAD-supported appliances for appliance anchorage did not improve the molar distalization magnitude or modify the extent of tipping and intrusion observed.

• Nonrigid palatal appliances resulted in significantly greater distal tipping than rigid appliances, although rigid and nonrigid appliances showed similar magnitudes of molar distalization and molar intrusion.

• Further well-designed, high-quality RCTs or prospective cohort studies are needed to provide clinical evidence of the efficacy of molar distalization with TADs.