RME has been suggested by McNamara  as a method to relieve mandibular retrusion in skeletal Class II patients. Twin-Block has been commonly used for phase I treatment in skeletal Class II patients; however, previous studies have only compared the effects of the two therapies on mandibular growth after phase I treatment. No study has compared the final effects of the two therapies on mandibular growth after phase II fixed appliance treatment. Therefore, this study enrolled patients who had completed 2-phase treatment and compared the final results, which were clinically more important.
The physiological age and CVM stage were well matched between the two groups before treatment. No differences in the cephalometric measurement between the two groups at T1 were found. Since the two groups were both treated with four 1st premolar extraction and mini-screws maximum maxillary anchorage for Phase II treatment, the long-term effects of different Phase I orthopedic therapies were responsible for the differences between the final results of the two groups.
In our research, skeletal changes in the mandible in the vertical dimension were analyzed by angular measurements, including FMA, and liner measurements, such as PFH, LAFH, PFH/LAFH and Co-Go. Although the results indicated an overall increase in the vertical cephalometric measurements after treatment in both groups, a significant difference in the vertical relationship between the two groups was noted. For example, the mean skeletal changes in the mandible in the vertical dimension were much smaller in the RME group than those in the TB group, as reflected by the changes in FMA (Fig. 3). The RME group showed only a slight increase of approximately 0.35° in FMA, suggesting that the vertical dimension was well controlled and that RME therapy might not cause clockwise rotation of the mandible. However, the change in FMA reached 2.65° in the TB group, clearly indicating that Twin-Block therapy might cause clockwise rotation of the mandible.
The difference in the FMA change between the two groups could be confirmed by the increase in LAFH, which was 5.12 mm in the RME group and 10.19 mm in the TB group (Fig. 4). In addition, the increase in PFH was similar between the two groups. As a result, the difference in the vertical changes between the two groups revealed an increased tendency of mandibular clockwise rotation following TB therapy. This observation had also been noted by Mills and McCulloch . Moreover, et al.  reported a slight increase in the mandibular plane angle and a slight backward, downward rotation of the mandible after RME treatment, which were consistent with our results.
To evaluate skeletal changes of the mandible in the sagittal dimension, relative angular measurements such as SNB, NSAr, SNPog, and NSGn, and liner measurements such as Pog-N|FH, Ar-Gn, Co-Gn, Go-Gn, SE, and SL, were analyzed. ANB significantly decreased and SNB significantly increased in both groups after treatment, with no significant difference in the changes between the two groups. The mandibular length significantly increased in both groups, as measured by Co-Gn, Go-Gn and Ar-Gn; however, TB group did not show more mandibular growth than the RME group.
Significant differences in the changes in SE and NSAr were found between the two groups. SE and NSAr are related to the sagittal position of the mandible and reflect the position of the condyle . The results showed that the increase in SE in the RME group was small, namely 1.10 mm. However, the increase in SE in the TB group was 2.23 mm, which was much larger than that in the RME group. Our results indicated that the condyle was inclined to significantly remodel backward after Twin-Block therapy. This finding is consistent with the finding of Yildirim, Karacay, Erkan , who reported a backward and upward remodeling of the condyle after Twin-Block treatment. Moreover, the decrease in NSAr was much larger in the RME group, suggesting that the point Ar moved forward after RME therapy. This phenomenon might be explained by the obvious forward posture of the condyle after RME treatment . 3D studies would provide more essential information.
The reason why RME therapy promotes mandibular growth in skeletal Class II patients has been controversial. The most accepted reason is that the increased space created by RME therapy relieves the block between the maxilla mandible, and enabled the mandible to naturally grow in three dimensions . The second possible reason is the functional shift, created by occlusion disruption . The boned acrylic and rapid displacement of the maxillary lateral segments might disrupt the occlusion and cause the patient to posture the mandible forward to a more comfortable position. Subsequently, with the remodeling and growth of the condyle, the initial postural change of the mandible becomes permanent [19, 20]. The third hypothesis suggested that the well-controlled vertical dimension in the RME group allows greater forward growth of the mandible, with less downward and clockwise rotation. Our results might support the second and the third reasons.
The facial profile improvements reflected by the Z Angle did not differ between the two groups. Both therapies proved to be useful in the correction of skeletal Class II malocclusion; however, RME therapy resulted in a similar sagittal mandibular growth with less clockwise rotation of the mandible in a shorter Phase I treatment period. This finding suggests that RME should be considered not only in patients with posterior crossbite, but also in hyperdivergent patients where an active growth response of the mandible is expected.
There are some limitations of this research. First, a control group is lacking. Mandibular growth could also be observed in a patient with no treatment. Therefore, a pure treatment effect on the mandibular growth following RME or Twin-Block therapy is lacking. Second, the lack of a CBCT examination resulted in debates about the relative contributions of real mandibular growth, mandibular functional shift and rotation of the mandible. Further studies using three-dimension evaluation might be helpful to provide more information.