An in vitro investigation of lingual bonding.

An in vitro investigation was undertaken to evaluate the bonding of orthodontic appliances onto lingual surfaces; 53 maxillary premolars, 37 mandibular premolars, and 37 mandibular incisors were used. Brackets were bonded onto the lingual and labial surfaces and fractured with an Instron machine. Enamel damage associated with debonding also was assessed. Results indicated comparable bond strengths (t test) on lingual (Li) and labial (La) surfaces: maxillary premolars--Li-138.2 kg/cm2, La-127.7 kg/cm2; mandibular premolars--Li-136.2 kg/cm2, La-121.6 kg/cm2; and mandibular incisors--Li-166.3 kg/cm2, La-161.1 kg/cm2. Adaptation of lingual bracket bases resulted in significantly higher lingual bond strengths for maxillary premolars (166.9 kg/cm2) and mandibular premolars (180.4 kg/cm2), but not for mandibular incisors (149.2 kg/cm2). On debonding, the percentages of lingual surfaces exhibiting horizontal "crescent-shaped" fracture lines and enamel fragment fractures were significantly higher (x2 test) than the corresponding percentages for labial surfaces: maxillary premolars--Li-67.9%, La-5.7%; mandibular premolars--Li-62.2%, La-13.5%; and mandibular incisors--Li-43.2%, La-18.9%. Furthermore, an increase in vertical enamel fracture lines (cracks) subsequent to debonding was seen labially and lingually. Bonding procedures for lingual surfaces should be identical to those advocated for labial surfaces. Care during debonding must be exercised to eliminate possible enamel damage.