Posterior mandibular edentulous ridge morphometry on CBCT: anatomical constraints relevant to surgical planning

Accurate preoperative assessment of posterior mandibular edentulous sites is essential for implant planning because ridge resorption, proximity to the mandibular canal, and lingual cortical morphology may limit implant placement. This retrospective cone-beam computed tomography (CBCT)-based study aimed to compare residual bone height (RBH), ridge width, lingual concavity angle (LCA), and horizontal concavity offset (HCO) among posterior mandibular tooth regions and to evaluate implant feasibility and augmentation requirements using a standardized virtual planning model.

This single-center, retrospective, CBCT-based observational study included 45 individuals and 252 posterior mandibular edentulous sites. Sites were assigned to the first premolar (PM1), second premolar (PM2), first molar (M1), or second molar (M2) region primarily according to the presence of adjacent teeth. When adjacent teeth were absent, the mental foramen was used as the principal landmark, with the mandibular canal and local ridge morphology serving as supplementary anatomical references. Contralateral information, when available, was used only as an ancillary orientation aid and not as the primary determinant of site localization. The site was the unit of analysis, and patient-level clustering was accounted for by including patient ID as a random intercept in linear mixed-effects models. RBH, BW1, BW3, BW5, LCA, and HCO were measured using a standardized multiplanar workflow. Implant feasibility categories were generated using predefined vertical and horizontal thresholds and were interpreted as virtual radiologic planning categories rather than clinical outcome measures. An exploratory post-augmentation simulation was also performed to summarize feasible implant diameter and length distributions after assuming that the required augmentation had been achieved.

The study included 45 patients (26 female and 19 male; mean age, 58.1 years) and 252 posterior mandibular edentulous sites. Mean BW1 ranged from 4.58 mm in PM2 to 5.72 mm in M2, mean BW3 ranged from 6.43 mm in PM2 to 8.24 mm in M2, and mean BW5 ranged from 8.06 mm in PM2 to 10.22 mm in M2. Mean RBH was highest in PM1 (13.56 mm) and lowest in M2 (12.09 mm). A significant regional effect was observed for BW1 (p = 0.013), BW3 (p < 0.001), BW5 (p < 0.001), RBH (p < 0.001), LCA (p < 0.001), and HCO (p = 0.036). Standard implant placement was virtually feasible at 47.6% of sites (95% CI, 41.5%-53.8%), whereas horizontal augmentation was the most common insufficiency category (33.3%; 95% CI, 27.8%-39.4%). In the post-augmentation simulation, implants 3.5 mm in diameter and implants at least 12 mm in length were the most frequently feasible options; neither diameter nor length distribution differed significantly among regions. Age and sex were not significantly associated with BW1, BW3, BW5, or RBH (all p > 0.05).

Posterior mandibular edentulous sites showed region-specific variation in ridge dimensions and lingual concavity-related parameters. Distal regions, particularly molar sites, had lower residual bone height despite greater ridge widths. The combined assessment of RBH, ridge width, LCA, and HCO may facilitate a more comprehensive radiologic evaluation during implant planning. However, because these findings are based on CBCT-derived morphometry and virtual planning, they should be validated in prospective studies that include clinical outcomes.