Background and purpose: Immediate implants placement (IIP) are considered a reliable procedure, with survival rates of 94.9% to 98.4%. Nevertheless, in the posterior mandible it poses high risk of damage to anatomical structures. The aim of this study was to determine the risk of anatomical structures injury associated with IIP in the posterior mandible based on apical primary stability, respecting a safe distance from inferior alveolar nerve (IAN) and lingual plate. Moreover, to evaluate the influence of different factors on those risks. Materials and methods: Pre-extraction cone beam computed tomography (CBCT) scans of 100 patients were retrospectively analyzed. Measurements were taken from tooth apices to lingual plate and to mandibular canal. Values of <4mm of the former and <6mm of the later categorized as considerable risk. Values of <2mm at both measurements considered as high risk. Two-sided p values <0.05 were considered statistically significant.  Results: Mean root-to-alveolar canal (RAC) distance was 7.6±2.7mm in the first molar, 6.5±3mm in second premolar and 5.4±3 mm in the second molar ( p <0.005). Mean distance to the outer lingual cortex (DLC) was 3.9±2.1mm in the first molar and 3.2±0.1mm in the second molar. Thus, second molars were at the 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. Conclusions: 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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