American Board of Orthodontics (ABO) Practice Exam

The center of resistance (C.o.R) for the acrylic group is located at the apex due to the nature and distribution of the forces acting on the teeth and their supporting structures. Understanding the biomechanics of tooth movement reveals that the C.o.R serves as a pivot point around which the tooth will tend to move when subjected to orthodontic forces.

In the case of the acrylic group, the forces applied to the dentition converge effectively at the apical area of the roots. This anatomical location is critical as it minimizes unwanted tipping of the teeth during movement, leading to more controlled and predictable movements. The apex acts as a stable point that counters any horizontal components of force, thus helping in achieving desired tooth positioning.

Focusing on the other options, the crown would not serve effectively as a center of resistance since forces applied here would lead to tipping or uncontrolled movement of the teeth. The molar area, while significant for anchorage, does not represent the center of resistance. Lastly, a point below the roots would be unsuitable for this purpose, as it does not correspond to the actual stabilization needed for effective orthodontic tooth movement.