A 32-year-old male patient presented with upper central diastema resulting from upper bilateral smallsized lateral incisors. The patient also had a deep bite of dental origin. A relatively simple mechanotherapy with pre-adjusted edgewise appliances was applied. A favorable and stable result of the ideal overbite and overjet after the correction of central diastema. Besides the above primary complaints, the prolonged retained roots of upper left first molar caused difficulty for proper space closure since the maxillary sinus wall was downward extended. The lower sinus floor may impede the orthodontic movement, but was very likely to be overlooked. The upper left molars were finally mesialized to occlude with the lower molars in compromised axial inclination.