3.1 Study selection
The study selection process is illustrated in Figure 1. A total of 583 articles were identified, and upon review of the titles and abstracts, 420 were excluded, leaving 19 articles. After reading the full texts, 8 studies were included in the present review for qualitative and quantitative synthesis. The 8 remaining studies included three RCTs11,19,20 and five CCTs12,24-27. Information about the excluded records is summarized in Additional file 2.
3.2 Study characteristics
The characteristics of the included studies are presented in Table 1, and the relevant data extracted from the included articles are shown in Table 2. Three RCTs11,19,20 and five CCTs12,24-27 were included in the present review. Six studies investigated direct implant anchorage for canine retraction, while two studies used indirect implant anchorage. Four studies11,25-27 compared TADs with dental anchorage in a split-mouth study. Two studies19,20 compared TPAs with TADs in the maxilla in a parallel study; one20 also compared lingual bars with TADs in the mandible. The other one12 compared midpalatal implant-reinforced TPAs with conventional TPAs in the maxilla. One study24 compared midpalatal implants with dental anchorage in the maxilla and TADs with dental anchorage in the mandible. Three studies11,25,26 inserted implants in both the maxilla and the mandible when the patients’ ANB angle was between 2° and 4° but in only the maxilla when the ANB angle was greater than 5° as a part of camouflage treatment.
3.3 Risk of bias assessment
Three RCTs11,19,20 were considered to have a high risk of bias because none of them had an appropriate strategy for blinding participants and personnel. The study by Davis et al, 201811, used only a computer-generated random allocation of sides and not allocation concealment. The randomization method in the study by Sharma et al, 201219, was random numbers generated by computer, but allocation was performed by alternation, which leads to a high risk of bias in allocation concealment. The third study20 did not report any randomization method, which resulted in an unclear risk of bias. Blinding of the outcome assessment was also difficult in the studies because the TADs could be observed in lateral cephalograms. However, Sharma et al, 201219, removed the miniscrew implants and TPAs before obtaining the postcanine retraction cephalometric radiographs, resulting in a low risk. Davis et al, 201811, used guide wires to differentiate the right and left sides on the lateral cephalograms, resulting in a high risk. Gökçe et al, 201220, did not report a process for blinding the assessor, resulting in an unclear risk. The quality assessment results of the RCTs are summarized in Figure 2.
Five CCTs12,24-27 were assessed using the ROBINS-I23 tool. The study by Hedayati et al, 200724, gave inadequate information regarding the patient inclusion criteria, Thiruvenkatachari et al, 200625, and Thiruvenkatachari et al, 200826, only inserted implants in the maxilla when the ANB angle was greater than 5° as a part of camouflage treatment, which led to a moderate risk of bias in selection. In the measurement of outcomes, Thiruvenkatachari et al, 200625, and Thiruvenkatachari et al, 200826, used wires identifiers, and Chaudhary et al, 201427, used CBCT-generated 2D cephalometric with implants clearly seen on it, which led to a serious risk of bias in the measurement of outcomes. Borsos et al, 201212, used an opaque marker in the approximate position of the implant in both groups, which led to a low risk of bias. Finally, Hedayati et al, 200724, did not present information regarding outcome measurements. Therefore, the overall bias across studies was serious bias in three studies25-27, moderate bias in one study24 and low bias in another study12. The risk of bias information for the included CCTs is summarized in Table 3.
3.4 Primary outcome measures
3.4.1 Mesial molar movement (anchorage loss)
Seven studies11,12,19,20,24,25,27 were qualified for meta-analysis, and the total and subgroup analysis results are given in Figure 3(a, b). In the maxilla, the results showed a total mean difference of 1.56 mm (95% CI: 1.14 to 1.98), with statistical significance (P<0.00001). Subgroup analysis showed a mean difference of 1.74 mm (95% CI: 1.32 to 2.17, P<0.00001) in the direct group and a mean difference of 0.93 mm (95% CI: -1.04 to 2.90, P=0.35) in the indirect group. In the mandible, the results showed a total mean difference of 1.62 mm (95% CI: 1.24 to 2.01), with statistical significance (P<0.00001). Subgroup analysis showed a mean difference of 1.45 (95% CI: 1.13 to 1.78, P<0.00001) in the direct group. Only one study24 included a mandibular indirect group; the results showed a mean difference of 2.73 mm (95% CI: 1.98 to 3.48, P<0.00001). In both the maxilla and mandible, the direct and indirect groups showed substantial heterogeneity, with I2> 50%.
3.4.2 Distal canine movement
Four studies11,20,26,27 were qualified for meta-analysis of both maxillary and mandibular data, and the results are given in Figure 3(c, d). In the maxilla, the results showed a total mean difference of 0.43 mm (95% CI: 0.16 to 0.69), with statistical significance (P=0.001); I2=0. In the mandible, the results showed a total mean difference of 0.26 mm (95% CI: 0.02 to 0.49), with statistical significance (P=0.03); I2=0.
3.5 Secondary outcome measures
One study11 included mesial tipping of the maxillary and mandibular molars with direct TADs. The results showed a mean tipping of 0.30° degrees in the TAD group and a mean tipping of 2.45° in the conventional anchorage group in the maxilla (P=0.000); in the mandible, the values were 0.1875° and 2.6875° (P=0.001), respectively.
One study24 included vertical displacement of the maxillary and mandibular molars with indirect TADs. The results showed a mean intrusion of 0.33 mm in the study group and 0.95 mm in the control group in the maxilla; in the mandible, the results showed a mean intrusion of 0 mm in the study group and 1.02 mm in the control group. With consideration of the cephalometric error (-0.55 mm), slight maxillary molar extrusion and mandibular molar intrusion were observed.
One study27 included distal tipping of the canines in the maxilla and mandible with direct TADs. The results presented tipping of 9.51° in the study group and 6.51° in the control group in the maxilla (P=0.106); in the mandible, the results showed tipping of 7.88° in the study group and 4.34° in the control group (P=0.057).