The force required for intrusion of incisor teeth is 15-25 gm because these teeth have relatively thin periodontal ligaments and delicate root structures. Applying excessive force can cause root resorption and damage to the supporting alveolar bone. Controlled, light forces allow for effective tooth movement while minimizing trauma to the periodontal ligament and pulp. Intrusion requires less force compared to other types of tooth movements due to the direct axial pressure needed to move the tooth into the alveolus. Hence, maintaining force within this range ensures safe and predictable orthodontic intrusion.

Reference: Proffit, W. R., Contemporary Orthodontics, 6th Edition.