Accessory Mandibular Foramina: An Anatomical Study in Dry Mandibles and Meta-Analysis

Background/Objectives: Accessory mandibular foramina (AMaFs) are small osseous openings of the mandible that are clinically relevant anatomical variations. This study aimed to characterize the morphology and spatial distribution of AMaFs in dry mandibles and to integrate the existing anatomical evidence through a systematic review and meta-analysis, with the goal of clarifying their potential clinical relevance. Methods: A series of dry mandibles from human adults of unknown age and sex from our laboratory collection was examined to document AMaFs using direct osteological inspection. Stainless steel wire threads and digimatic caliper measurements were utilized by two separate raters. Cluster analysis was employed for the classification of foramina into distinct spatial groups. Furthermore, in accordance with the PRISMA guidelines, an unrestricted literature search was conducted across PubMed, Scopus, SciELO, and Google Scholar using appropriate database-specific combinations of the terms "accessory mandibular" and "foramen/foramina" to search for studies on the prevalence and morphology of AMaFs in dry mandibles or cadaveric material. Radiological studies were excluded. The search was completed on 13 July 2025. Study quality was evaluated using the appropriate AQUA tool. Data synthesis was carried out using STATA 19. No external funding was received. Results: A total of 96 dry mandibles (50 dentate and 46 edentulous) were analyzed. AMaFs were detected in 8/96 mandibles (8.3%). In these mandibles, a total of 25 accessory mandibular foramina, all superior to the mandibular foramen, were identified (mean: 3.13 foramina/mandible), with a mean diameter (SD) of 0.56 ± 0.10 mm and a mean distance from the mandibular foramen of 11.34 ± 1.29 mm (mean vertical distance: 10.32 ± 1.35 mm; mean absolute horizontal distance: 3.78 ± 0.49 mm). Of these foramina, 21/25 (84%) had a diameter ≥0.5 mm; the number, diameters, and distances from the mandibular foramen were comparable between left and right hemimandibles. Based on their positioning relative to the mandibular foramen, the AMaFs were classified into two distinct groups (clusters). In the meta-analysis, a total of 36 studies were included. In most of the mandibles (65.1%; 95% CI: 57.7-72.2%; I²: 94.9%), no AMaFs were detected. The unilateral presence of one or more AMaFs was observed in 20.9% of the mandibles (95% CI: 16.3-25.9%; I²: 91.3%), while bilateral occurrence was identified in 10.6% (95% CI: 6.9-15.0%; I²: 93.0%). Additionally, 2.4% of the mandibles (95% CI: 1.0-4.2%; I²: 86.3%) exhibited multiple AMaFs (≥2) on at least one side. On average, each hemimandible contained 0.253 AMaFs (95% CI: 0.198-0.312; I²: 96.9%). The overall mean diameter of AMaFs was estimated to be 0.65 ± 0.33 mm. The substantial heterogeneity observed was not explained by geographic origin, sample size, publication period, or publication bias. Conclusions: AMaFs were detected in approximately one-third of the mandibles in the studies included in the meta-analysis. AMaFs are typically located superior to the mandibular foramen and may represent additional anatomical pathways associated with inferior alveolar nerve branching. Awareness of these features could help clinicians to anticipate anatomical variability during mandibular surgery and when applying local anesthesia. In addition, it should be acknowledged that inferior alveolar nerve block failure is multifactorial and not solely determined by the presence of AMaFs.