# A key finding from long-term stability studies following orthodontic treatment is that late anterior mandibular crowding is often independent of the pre-treatment malocclusion. This relapse is primarily attributed to:
A. A continued, anteriorly-directed component of natural craniofacial growth
B. The patient's failure to wear a maxillary removable retainer
C. Improper arch form used during the alignment phase
D. A rebound effect from temporary root resorption during treatment
The correct answer is A. A continued, anteriorly-directed component of natural craniofacial growth
Long-term stability studies, including serial cephalometric analyses by Björk and Skieller, demonstrate that late mandibular anterior crowding (developing or worsening 5–10+ years post-treatment) arises from physiologic late mandibular growth—a forward (anteriorly directed) rotation and elongation of the chin relative to the stable incisor apices, which displaces the lower incisors lingually against lip and tongue pressures, reducing arch perimeter by 1–2 mm on average. This process is largely independent of initial malocclusion severity or treatment modality (e.g., extraction vs. non-extraction), occurring in 60–80% of cases regardless of pre-treatment alignment, as confirmed in cohorts like the University of Washington Post-Retention Study (Little et al.). Patient compliance with maxillary retainers (B) influences upper arch stability but not lower growth; improper arch form (C) affects short-term relapse; and root resorption (D) shows no causal link to late crowding. Indefinite lower retention remains essential to mitigate this growth-driven tendency.
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