Orthodontically treated midline diastema is best retained by:
Hawley's appliance for 6 months
Hawley's appliance for 12 months
Bonded lingual retainer
No need for retainer as it is self-retained
Advantages of lingual-bonded retainers

Lingual bonded retainers are invisible, hence, aesthetically acceptable.
Patient comfort and compliance.
Recommended in patients with compromised periodontal support.
Effectiveness and reliability in the long term and permanent retention and help in retaining the orthodontically treated midline diastema.

Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities 3rd ed O P Kharbanda pdf no 1880