American Board of Orthodontics (ABO) Practice Exam

Severe root resorption occurs when structural loss of the root length is significant, indicating a more critical condition that might necessitate intervention. The correct classification, which states that root resorption becomes severe when there is a loss exceeding 4 mm or one-third of the original root length, aligns with clinical observations that root integrity is compromised at this level.

In orthodontic practice, understanding the thresholds of root resorption is crucial as it helps in assessing the risk of damage to tooth roots during treatment, particularly in cases involving the movement of teeth. Recognizing significant root loss enables professionals to adjust treatment plans to mitigate further resorption and preserve tooth health.

The other classifications do not represent the commonly accepted threshold for severe root resorption in the clinical setting, as they either indicate lesser degrees of resorption or do not align with established definitions used in orthodontics. Thus, the classification in the correct answer serves as a critical reference point for orthodontic diagnostics and treatment planning.