Methods: The sample included 17 patients between 8.4 and 11.3 years of age at the start of treatment. All patients were treated using prefabricated LBs, with thick acrylic shields from canine to canine in the lower arch, and rapid maxillary expansion on the upper arch. Lateral cephalograms were used to evaluate each subject at pretreatment-T1 (9.6 years of age); post-lip bumper therapy-T2 (13.3 years of age); and post-fixed appliance therapy-T3 (14.9 years of age). Using Dolphin® Imaging Software (Dolphin Imaging, Chatsworth, CA), cranial base, maxilla, and mandibular superimpositions were performed based on naturally stable structures to evaluate the first molars changes.
Results: During the LB phase (T1-T2), there was mesialization of the lower molars (0.62 mm) relative to the upper molar. The upper molars migrated mesially significantly (p <.05) more than the lower molars (difference ≈ 1.1 mm), but the anterior displacement of the mandibular molars were significant greater (difference ≈ 1.7 mm) than maxillary molar displacement. No significant jaw differences were observed during fixed appliance therapy (T2-T3). The overall changes between pretreatment and post-fixed appliance therapy (T1-T3) showed greater forward displacement of the lower molar (difference ≈ 2.6 mm) and greater mesial migration of the upper molar (difference ≈ 1.4 mm), resulting in greater overall movement of the lower molars (difference ≈ 1.2 mm) .
Conclusions: Lip bumper therapy does not increase the risk of producing Class II molar relationship. Anterior growth displacement of the mandible compensates the lesser mesial migration of the lower molars compared to the upper ones.