American Board of Orthodontics (ABO) Practice Exam 2025 - Free Orthodontics Practice Questions and Study Guide

The effect of a lip bumper on the molars in the acrylic group primarily involves the mechanics of the lip bumper itself, which exerts a force on the teeth. In this scenario, when a lip bumper is employed, it typically creates space by applying pressure from the lip to the anterior teeth, allowing for the posterior teeth to respond.

The correct answer describes a situation where there is mesial crown movement coupled with distal root movement of the molars. This means that as the bumper is activated, the crowns of the molars move toward the midline of the dental arch (mesially), while the roots tend to move in the opposite direction (distally). This differential movement can occur because the lip bumper maintains a constant forward position, allowing the crowns to shift into the newly created space in the arch, whereas the roots adjust their position based on the forces applied.

This movement pattern is significant because it aids in the correction of posterior occlusion and alignment, utilizing the natural growth and repositioning influences of the dentition over time. Understanding the mechanics of these movements is crucial for orthodontic treatment planning, particularly when addressing space issues or guiding eruption patterns in growing patients.