Dental crowding is the most common malocclusion. 60-70% of patients with malocclusion have varying degrees of dental crowding, what causes this malocclusion? The cause of overcrowding is tooth and bone disproportion, where the tooth volume (total width of the teeth) is relatively large and the bone volume (length in the alveolar arch) is relatively small, and the length of the dental arch is not enough to accommodate all of the teeth in the dental arch. The following are the main reasons for dental bone disproportion: 1. Evolutionary factors The masticatory organs have shown a tendency to degenerate and weaken during the evolution of human beings. The degree of degeneration of each chewing organ is different, muscle degeneration is the fastest, followed by bone, teeth are the slowest, this unbalanced degradation constitutes the background of the racial evolution of human dental crowding. 2, genetic factors The size, number and shape of teeth, the size, position and shape of jawbone are all subject to strong genetic control. The human masticatory organs are genetically dominated by degenerative shapes. 3.Environmental factors The early loss of the second molar leads to the forward movement of the first permanent molar, the length of the dental arch becomes shorter, and crowding occurs. 4, bad habits The obstacle of replacement of milk and permanent teeth plays an important role in the occurrence of dental crowding. Early loss of milk teeth, especially the early loss of the second molar, causes the first permanent molar to move forward, resulting in a reduction in the length of the dental arch, the permanent teeth eruption position is insufficient and crowding occurs. Some other bad habits can also cause crowding. Changes in food structure have an effect on the occurrence of crowding. Long-term consumption of fine and soft food does not allow the chewing function to function properly, resulting in underdevelopment of the alveolar and jawbone.