Temporary tooth remaining after eruption of permanent tooth, canines and incisors most commonly affected.
If not removed, affect direction of eruption of permanent tooth causing malocclusions and gingivitis.
Mandibular canines: permanent tooth erupts medial to temporary. Retention of temporary mandibular canines causes permanent tooth to tip laterally and impinge on hard palate/gingiva/other teeth causing pain on closure of mouth.
Maxillary canines: permanent tooth erupts rostral to temporary. Retention of temporary maxillary canines permanent tooth erupts into space for mandibular canine when mouth closed. May result in:
Impaction of mandibular or maxillary canine.
Maxillary lateral incisor and/or canine pushed laterally by mandibular canine (often with impaction of mandibular canine).
Mandibular canine pushed medial relative to maxillary canine impinging on hard palate.
Incisors: permanent tooth erupts caudal to temporary - retention may affect scissor bite resulting in local soft tissue trauma.
Extraction of temporary tooth . Take care not to disturb permanent tooth and not to fracture temporary tooth root.
Do not put elevator between temporary and permanent teeth but loosen temporary tooth on other three sides: do not use dental forceps: use sharp elevator, luxator, size 11/15 scalpel blade to cut periodontal ligament, be very patient.
If tooth root fractures; less damaging to remain in situ, monitor with radiograph after 2 months to assess situation (absorption).
Subsequent management
Monitoring
Check direction of eruption of permanent tooth; if deviating from normal encourage better direction of eruption by digital pressure < 2 hours daily.
malocclusion. 
Take care not to disturb permanent tooth and not to fracture temporary tooth root.