C

A midline diastema shows a strong tendency to relapse; therefore long-term fixed retention is needed. Bonded lingual (canine-to-canine) retainers provide continuous, invisible stabilization and have the lowest failure and relapse rates compared with removable Hawley appliances [Zachrisson, 1977, PMID 322500; Butler & Dowling, 2005, PMID 15789987].