What can be the maximum time period for replanting the knocked out tooth?
Importance of the replanting the tooth in less than 5 minutes: Immediate replacement of tooth within 5 minutes is the most important factor for long-term survival of knocked out (avulsed) teeth.
Beyond 15 minutes: Storage/transportation in cold milk or the patient's own saliva will prevent the avulsed tooth from drying. If replantation of the avulsed tooth takes a fairly longer time then it affects the ability to regenerate periodontal ligament cells — the living cells on the root surfaces that connect them to the bone of the tooth socket. The long term outcome is then less favorable.
By and beyond 12 hours: Avulsed tooth replanted by and beyond 12 hours has the least chances of survival. It causes a fusion of the tooth to bone (ankylosis) followed by gradual root resorption (dissolving away) and tooth loss over a few years. The tooth will survive for a period for time but ankylosis and/or resorption are inevitable.
Summary: Any permanent tooth that is avulsed should be replanted as soon as possible. Teeth replanted in pre-adolescents typically show shorter survival times than replantation in young adults because of the impact of the growth process and the enhanced cellular activity.