Immediate implant placement (IIP) is considered a reliable procedure, with survival rates of 94.9%–98.4%. Nevertheless, in the posterior mandible, it poses a high risk of damage to anatomic structures. The aim of this study was to determine the risk of anatomic structures injury associated with IIP in the posterior mandible based on apical primary stability, respecting a safe distance from the inferior alveolar nerve and lingual plate, and to evaluate the influence of different factors on those risks. Pre-extraction cone beam computed tomography scans of 100 patients were retrospectively analyzed. Measurements were taken from tooth apices to lingual plate and to mandibular canal. Values of <4 mm of the former and <6 mm of the latter were categorized as considerable risk. Values of <2 mm at both measurements were considered high risk. Two-sided P < .05 was considered statistically significant. Mean root-to-alveolar canal distance was 7.6 ± 2.7 mm in the first molar, 6.5 ± 3mm in the second premolar, and 5.4 ± 3 mm in the second molar (P < .005). The mean distance to the outer lingual cortex was 3.9 ± 2.1 mm in the first molar and 3.2 ± 0.1 mm in the second molar. Thus, second molars were at higher risk of inferior alveolar nerve injury and lingual plate perforation during IIP. Background factors associated with higher IIP risk were female sex and age < 40 years. In the mandible, the anatomic risk posed by IIP is greatest for second molars and lowest for first molars. Several background factors affect the distances between root apices and the mandibular canal.

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