LONG-TERM STABILITY AND BIOLOGICAL SIDE EFFECTS OF ORTHODONTIC TREATMENT: A NARRATIVE REVIEW OF RELAPSE MECHANICS, RETENTION STANDARDS, AND PATIENT-SPECIFIC EXTERNAL APICAL ROOT RESORPTION

Orthodontic treatment reliably corrects malocclusion, yet two long-standing biological uncertainties continue to limit predictability of outcome: post-treatment relapse despite retention, and external apical root resorption (EARR) as an iatrogenic sequela of tooth movement. This narrative review synthesises the published literature on these two domains. For relapse and retention, the periodontal and gingival fibre apparatus, occlusal and neuromuscular equilibrium, and soft-tissue envelope are identified as the principal determinants of post-treatment stability, yet none of these factors, singly or combined, allow clinicians to predict which patients will relapse without lifelong retention. Fixed and removable retainer failure rates remain substantial over long follow-up periods, and stability appears largely independent of the complexity of the original malocclusion. For EARR, the literature confirms a multifactorial aetiology in which mechanical variables such as force magnitude, duration, and direction of movement interact with a substantial, heritable genetic component acting through interleukin-1, RANK/RANKL/osteoprotegerin, purinergic (P2X7R), and vitamin D receptor signalling pathways. Despite this, no validated pre-treatment screening protocol exists beyond conventional periapical or panoramic radiography, and clinicians remain unable to reliably identify high-risk patients before force application begins. Both domains share a common gap: biological variability at the periodontal, alveolar, and genetic level outpaces the predictive capacity of current clinical tools. Standardised long-term retention protocols and validated multifactorial risk-stratification models, potentially incorporating genetic markers alongside biomechanical and radiographic indices, represent priority areas for future research.