American Board of Orthodontics (ABO) Practice Exam 2026 - Free Orthodontics Practice Questions and Study Guide

The first appearance of rounding at the lateral margins in SMI (Stage of Mineralization for the Incisor) stages corresponds to the capping stage. During the capping phase of dental development, the shape of the developing tooth structure begins to manifest as the enamel organ differentiates and starts to acquire rounded contours.

In the context of tooth development, each stage signifies different morphogenetic changes. The capping stage is critical because it marks the transition from a flat dental structure to one that begins to take on a more defined and rounded configuration. This rounding is then essential for the proper formation of the tooth's anatomy, impacting its aesthetic and functional aspects.

The other stages described in the question occur later in the overall development process. The fusion stage happens when the dental tissues begin to merge, which is observable after the capping phase. The growth phase encompasses further development and maturation of the tooth, and the development stage refers to the entire process from initial differentiation to final tooth eruption. Therefore, the rounding at the lateral margins is specifically identified during the capping stage, making it the correct answer.