Crowded teeth in a nine-year-old child should be treated by expanding the dental arch and extracting the teeth for correction according to the degree of crowding. 1. Mild: Mild crowding of teeth can generally be enlarged by using an arch expander, placing the spiral spring of the expander in the appropriate place, opening the expander by rotating the spiral hole, and transmitting the orthodontic force to the teeth and alveolar bone through the sender to expand the width and length of the upper and lower jaw arches to create enough space for the teeth to grow. 2. Moderate and severe: Patients need to extract the teeth to make room for the crowded teeth, and then slowly restore the remaining teeth to their normal position as well as function by wearing braces. The number of extractions should be decided by the dentist according to the degree of crowding. Parents of nine year olds with crowded teeth should take them to the doctor as soon as possible to avoid missing the best time for orthodontic treatment. If the teeth are severely malformed, surgical intervention may be required.