Effect of upper second molar eruption status on the efficiency of upper first molar distalization: a systematic review and meta-analysis

To assess the distalization, tipping, and vertical movement of the maxillary first permanent molar in patients with and without erupted maxillary second molar (MSM). Eligibility criteria: Randomized/non-randomized clinical studies comparing distalization, tipping, and vertical movement of the maxillary first permanent molar in patients with and without erupted MSM. Information sources: Unrestricted literature search of six databases was conducted up to May, 2024. Risk of bias: The quality assessment of the studies was conducted using the Cochrane Risk of Bias Tool (ROBINS-I). Synthesis of results: Random effects meta- analyses using standardized mean differences (SMDs) and their 95% confidence intervals (CIs) were performed, followed by meta-regressions, sensitivity analyses, and assessment of the quality of evidence using GRADE. Included studies: Eleven studies (8 retrospective and 3 prospective non-randomized trials) involving 562 participants (43.97% male) were included. Synthesis of results: Distalization was significantly more effective when the maxillary second molar was unerupted (9 studies; SMD = -0.41; 95% CI: -0.81--0.004; P = .04; I² = 68.6%). The eruption status of MSM has no significant effect on tipping (8 studies; SMD = -0.27; 95% CI: -0.68-0.15; P = .17), and vertical movement (4 studies; SMD = 0.08; 95% CI: -0.32-0.48; P = .57) of upper first molars during distalization. Sensitivity analyses showed no significant differences based on study design, appliance type, or anchorage type, confirming the robustness of the findings. The certainty in the estimates was very low due to high risk of bias, methodological weaknesses, and small sample sizes. Limitations of evidence: The inclusion of non-randomized, mostly retrospective studies, unmatched baseline, Class II severity, and insufficient reporting of treatment duration are key limitations. Interpretation: Very low level of evidence indicates that it may be preferable to perform distalization before the eruption of the upper second molar. PROSPERO (CRD42024591126).