External root resorption (ERR) is a common iatrogenic complication of orthodontic treatment, characterized by the irreversible loss of root structure due to odontoclastic activity. In periodontally compromised patients, who often present with reduced alveolar bone support and chronic inflammation, the risk of ERR is significantly heightened, potentially leading to tooth mobility, loss of attachment, and compromised treatment outcomes. This narrative review synthesizes recent literature from 2020 to 2025 to elucidate the biological risk factors contributing to ERR in this vulnerable population and to explore preventive strategies. Key biological risk factors include excessive mechanical forces during tooth movement, which upregulate inflammatory mediators such as receptor activator of nuclear factor kappa-B ligand (RANKL), genetic predispositions, prior dental trauma, abnormal root morphology, and systemic conditions like endocrine disorders. Periodontal compromise exacerbates these risks through microbial dysbiosis, increased cytokine expression (e.g., IL-1β, IL-6, TNF-α), and altered periodontal ligament (PDL) stress distribution, as demonstrated in finite element analyses showing elevated resorption risks during rotations and translations after 4 mm bone loss. Prevention emphasizes interdisciplinary management, commencing orthodontic therapy only after achieving periodontal stability (e.g., no probing pocket depths ≥5 mm with bleeding on probing), utilizing light continuous forces (5-10 g for intrusions), incorporating treatment pauses for healing (2-6 months upon detecting resorption), and regular radiographic monitoring with cone-beam computed tomography (CBCT) for early detection. Additionally, endodontic interventions prior to orthodontics may reduce ERR in vital teeth by minimizing pulp-mediated inflammatory responses. This review underscores the need for tailored protocols to mitigate ERR, ensuring safe orthodontic outcomes in periodontally compromised patients while highlighting gaps in long-term prospective studies.
