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

Abstract

Background

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.

Methods

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.

Results

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).

Conclusions

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.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12903-026-08990-4.

Introduction

Implant placement in the posterior mandible is often challenging because of post-extraction alveolar ridge resorption and the close anatomical relationship between the alveolar crest and the mandibular canal. Following tooth loss, both vertical and horizontal ridge reduction may occur, resulting in insufficient bone volume for implant placement, particularly at molar sites [1–3]. In anatomically compromised cases, additional procedures such as ridge augmentation or inferior alveolar nerve lateralization may be required; however, these approaches are technique-sensitive and may increase treatment complexity and the risk of neurosensory complications [4, 5].

Previous CBCT-based studies have shown that posterior mandibular edentulous sites frequently have anatomical limitations that may necessitate additional surgical intervention during implant planning. In a CBCT-based study, Mateo G-D et al. reported that supplementary bone augmentation procedures were required in approximately 63.7% of posterior mandibular edentulous sites [5]. This finding indicates that rehabilitation of the posterior mandible often involves more than straightforward implant placement. Although several augmentation techniques, including guided bone regeneration, block grafting, titanium mesh application, interpositional grafting, and distraction osteogenesis, have been described, no single approach is universally considered optimal. In addition, these procedures may be associated with increased treatment complexity, morbidity, and complication risk, particularly in anatomically restricted posterior mandibular regions [3, 5].

Cone-beam computed tomography (CBCT) has become an important imaging modality for the preoperative assessment of posterior mandibular anatomy because it enables three-dimensional visualization of alveolar bone morphology, accurate measurement of ridge height and width, and improved delineation of the mandibular canal and lingual cortical contour [2, 6–8]. This information is particularly relevant in anatomically demanding posterior mandibular regions, where conventional two-dimensional imaging may not adequately depict internal osseous morphology. In addition to selecting implant dimensions, detailed evaluation of the mandibular canal and lingual concavity is important for preoperative planning. In this context, Yoon et al. emphasized the value of CBCT-based assessment of posterior mandibular lingual concavity because of its potential implications for implant-related cortical perforation [8].

Successful management of posterior mandibular implant procedures depends not only on imaging but also on the clinician’s ability to interpret radiologic anatomy accurately. In anatomically demanding regions, inadequate anatomical assessment or limited surgical experience may increase the likelihood of complications during implant planning and placement [5]. In recognition of these issues, the European Academy of Dental and Maxillofacial Radiology has emphasized that clinicians involved in prescribing, acquiring, or interpreting CBCT examinations should have appropriate training and technical competence [7]. As implant treatment is increasingly performed in anatomically complex regions, comprehensive radiologic evaluation remains an essential prerequisite for appropriate preoperative planning.

Against this background, site-specific morphometric assessment of posterior mandibular edentulous ridges may provide clinically relevant information for preoperative implant planning. Although posterior mandibular lingual concavity has been investigated previously, CBCT-based studies have generally assessed ridge height, ridge width, or lingual concavity separately or by using isolated linear or qualitative approaches [1, 2, 8–10]. The contribution of the present study lies in combining conventional linear measurements (RBH and ridge width at three standardized subcrestal levels) with two complementary concavity-related parameters, LCA and HCO, within a tooth-region-specific virtual planning frameworkThis integrated approach was intended to characterize both the available bone volume and the internal lingual cortical configuration that may influence surgical planning. The primary objective was to compare RBH, BW1, BW3, BW5, LCA, and HCO across posterior mandibular edentulous regions (PM1, PM2, M1, and M2). The secondary objective was to evaluate the distribution of standard, short, and narrow implant feasibility and augmentation requirements using predefined virtual planning criteria. We hypothesized that distal posterior mandibular regions, particularly molar sites, would have less favorable vertical morphometric characteristics and more pronounced concavity-related anatomical constraints than premolar regions.

Materials and methods

Study design, setting, and ethical approval

This single-center, retrospective, observational study used CBCT-based morphometric analysis. Archived CBCT scans acquired for clinical diagnostic purposes at the Nevşehir Hacı Bektaş Veli University Oral and Dental Health Application and Research Center were evaluated. The study was approved by the Non-Interventional Ethics Committee of Nevşehir Hacı Bektaş Veli University (approval date: 12 June 2025; decision no. 2500051530) and was conducted in accordance with the Declaration of Helsinki. The study design, eligibility criteria, site definition, measurement protocol, and statistical approach were specified before data were extracted from the radiographic archive.

Because the study used anonymized, retrospectively collected radiographic data, no prospective recruitment was undertaken. All eligible CBCT scans identified during the archive review were included. Accordingly, the sample should be regarded as a complete retrospective sample of eligible scans rather than a prospectively powered recruitment cohort.

Eligibility criteria and screening procedure

CBCT scans were eligible when the entire mandible was visualized and at least one posterior mandibular edentulous site could be assigned to the PM1, PM2, M1, or M2 region. Tooth-region localization was based primarily on the presence of an adjacent natural tooth. When no adjacent tooth was present, a clearly identifiable mental foramen was required as the principal landmark. When necessary, the mandibular canal, alveolar crest, and remaining posterior arch contour served as supplementary anatomical references to confirm orientation; they were not used as independent primary criteria for site assignment.

Hemi-arches were excluded when the tooth region could not be assigned reliably because reference landmarks were absent within the region of interest. Scans or sites were also excluded if they showed mandibular cysts, tumors, fractures, motion artifacts, substantial metal artifacts, indistinct canal or cortical boundaries, previous bone grafting, major reconstruction after implant failure, or severely tilted or displaced teeth that precluded reliable site localization. When the superior cortical border of the mandibular canal was not clearly visible, canal continuity was reviewed in the axial, coronal, and sagittal planes. If uncertainty remained after multiplanar review, the site was excluded from analysis as a predefined quality-control measure.

Site definition and localization

The analytical unit was the edentulous posterior mandibular site. Each site corresponding to the first premolar, second premolar, first molar, or second molar position was treated as a separate site-level observation. Because several sites could be obtained from the same patient, site-level observations were not considered statistically independent; patient-level clustering was handled in the statistical analysis by using patient ID as a random intercept.

For bounded edentulous spaces, the virtual site position was assigned according to the adjacent teeth and expected tooth sequence. For distal-extension and longer edentulous spans without adjacent posterior teeth, the mental foramen served as the principal anatomical landmark for regional orientation, whereas the mandibular canal and local mandibular arch contour served as supplementary landmarks. Contralateral homologous information, when available, was used only as an additional orientation aid. Sites that could not be assigned to a tooth-specific region with sufficient confidence were excluded. When an adjacent natural tooth was present, the first virtual implant center was placed at least 5 mm from the neighboring root surface. In longer edentulous spans, subsequent virtual centers were positioned at approximately 8 mm intervals from the center of the previous site to maintain a standardized site-based planning model.

CBCT acquisition and image analysis standardization

All scans were acquired using a Planmeca ProMax 3D Mid system (Planmeca Oy, Helsinki, Finland). Images were obtained at 90 kVp and 8 mA with a voxel size of 0.2 mm. The field of view was selected according to the clinical indication, with a maximum field of view of 16 × 20 cm. Images were reconstructed and analyzed using Romexis software, version 6.2 (Planmeca Oy, Helsinki, Finland).

All measurements were performed in the same DICOM-viewing software environment using a standardized multiplanar reconstruction workflow. Cross-sectional slices were aligned perpendicular to the alveolar ridge before measurement. The default Romexis 6.2 display preset was used for the initial image review. When necessary, minor adjustments to brightness, contrast, and sharpness were made solely to improve visualization of anatomical boundaries; these adjustments did not modify the raw DICOM data. No additional image-enhancement or postprocessing filters were applied during measurement. Standardized display conditions were used because viewer-dependent parameters, including slice thickness, sharpness, and contrast adjustment, may influence visualization and segmentation of the inferior alveolar canal on CBCT images [11, 12].

Primary and secondary outcomes

The primary outcomes were residual bone height (RBH), ridge width measured at 1, 3, and 5 mm apical to the alveolar crest (BW1, BW3, and BW5), lingual concavity angle (LCA), and horizontal concavity offset (HCO). The secondary outcomes were the distribution of sites by virtual implant-feasibility category, the distribution and magnitude of vertical and horizontal anatomical insufficiency, and the exploratory post-augmentation distributions of feasible implant diameters, lengths, and diameter-by-length combinations.

Morphometric measurement protocol

RBH was measured on cross-sectional CBCT images as the vertical distance from the most coronal point of the alveolar crest to the superior border of the nearest limiting neurovascular structure. At molar sites, the limiting structure was generally the superior cortical border of the mandibular canal. At premolar sites where the mental foramen was the nearest limiting anatomical boundary, its superior border was used. Ridge width was measured perpendicular to the alveolar ridge on the same cross-sectional image at 1, 3, and 5 mm apical to the alveolar crest (BW1, BW3, and BW5). These levels were selected to represent crestal and subcrestal regions relevant to implant planning and to characterize changes in ridge contour with increasing depth (Fig. 1).

Fig. 1.

Cross-sectional CBCT image illustrating residual bone height (RBH) and ridge-width measurements. RBH was measured from the most coronal point of the alveolar crest to the superior border of the mandibular canal or, when applicable, the mental foramen. Ridge width was measured at 1, 3, and 5 mm apical to the alveolar crest (BW1, BW3, and BW5)

Lingual concavity morphology was evaluated on the cross-sectional image showing the deepest lingual undercut within the mesiodistal extent of each virtual site. A horizontal reference line was drawn 2 mm coronal to the superior border of the mandibular canal, in accordance with previous CBCT-based morphometric approaches and the conservative safety margin commonly used in implant planning adjacent to the inferior alveolar canal [9, 10, 13]. The intersection of this line with the lingual cortical plate served as the inferior reference point for concavity assessment. LCA was defined as the angle formed by two lines representing the superior and inferior contours of the lingual concavity. HCO was defined as the horizontal distance between the deepest point of the lingual concavity and the most prominent point of the lingual cortical plate at the same reference level. All LCA and HCO measurements were obtained on cross-sectional slices oriented perpendicular to the alveolar ridge (Fig. 2).

Fig. 2.

Cross-sectional CBCT image illustrating the landmarks used for lingual concavity measurements. LCA was defined as the angle formed by lines representing the superior and inferior contours of the lingual concavity. HCO was defined as the horizontal distance between the deepest point of the lingual concavity and the most prominent point of the lingual cortical plate at the same reference level

Fig. 3.

Schematic illustration of the virtual implant-positioning protocol. When an adjacent natural tooth was present, the first implant site was positioned at least 5 mm from the neighboring root surface. In longer edentulous spans, subsequent implant sites were positioned at approximately 8-mm intervals from the center of the preceding implant site

Virtual implant feasibility model

Implant feasibility was evaluated using a virtual radiologic planning model based on CBCT-derived bone dimensions rather than actual implant placement. Accordingly, all feasibility categories should be interpreted solely as anatomical planning categories. Implant-size categories were defined operationally on the basis of commonly used dimensions reported in the implant literature: a standard implant (4.0 mm in diameter and 10 mm in length), a short implant (4.0 mm in diameter and 6 mm in length), and a narrow-diameter implant (3.0 mm in diameter) [14, 15].

The final feasibility categories were mutually exclusive and were generated using a predefined decision tree. Because RBH was measured from the alveolar crest to the superior border of the limiting neurovascular structure, a 2-mm safety margin was incorporated into implant-length selection. Sites with RBH ≥ 12 mm were considered vertically suitable for a standard-length implant, sites with 8 mm ≤ RBH < 12 mm were considered vertically suitable for a short implant, and sites with RBH < 8 mm were classified as requiring vertical augmentation. Horizontally, BW1 was used as the reference for crestal width. Sites with BW1 ≥ 5.5 mm were considered horizontally suitable for a standard-diameter implant, sites with 4.5 mm ≤ BW1 < 5.5 mm were considered horizontally suitable for a narrow-diameter implant, and sites with BW1 < 4.5 mm were classified as requiring horizontal augmentation. Sites meeting both augmentation criteria were classified as requiring combined vertical and horizontal augmentation. This decision tree was used solely to standardize radiologic classification and was not intended to replace individualized surgical decision-making.

For the exploratory post-augmentation simulation, feasible implant diameters and lengths were also tabulated under the assumption that the indicated vertical and/or horizontal augmentation had been successfully completed. Diameter categories were 3.0, 3.5, 4.0, 4.5, and ≥ 5.0 mm, and length categories were 6, 8, 10, and ≥ 12 mm. This secondary simulation was included only to describe the potential distribution of implant dimensions under standardized assumptions and should not be interpreted as actual implant selection or a clinical treatment recommendation.

Reliability assessment

All measurements were performed by a single calibrated observer. To assess intraobserver reliability, 10% of sites (n = 25) were selected at random and remeasured at least 2 weeks later. Intraclass correlation coefficients (ICCs) were calculated for RBH, BW1, BW3, BW5, LCA, and HCO. Because all measurements were obtained by one observer, interobserver reproducibility could not be assessed; this limitation is addressed explicitly in the Limitations section.

Statistical analysis

All statistical analyses were performed using IBM SPSS Statistics, version 23 (IBM Corp., Armonk, NY, USA). Continuous variables were summarized as means, medians, and ranges, whereas categorical variables were presented as counts and percentages. Linear mixed-effects models were the primary analytical approach for continuous morphometric outcomes, with patient ID included as a random intercept to account for clustering of multiple sites within the same patient. Tooth region (PM1, PM2, M1, and M2) was included as a fixed effect in regional comparisons of BW1, BW3, BW5, RBH, LCA, and HCO. Bonferroni-adjusted pairwise comparisons were performed when the overall regional effect was significant.

Separate mixed-effects models were used to evaluate the associations between age and sex and BW1, BW3, BW5, and RBH. Corresponding age- and sex-based models were not fitted for LCA or HCO because these analyses were not part of the predefined demographic assessment. Virtual implant-feasibility and augmentation categories were summarized descriptively because they were deterministic outputs of the predefined dimensional thresholds. Regional differences in the estimated magnitude of vertical and horizontal anatomical insufficiency were evaluated using the Kruskal-Wallis test. For key proportions reported in the abstract, 95% confidence intervals were calculated using the Wilson method. Intraobserver reliability was assessed using ICCs. A two-sided p value < 0.05 was considered statistically significant.

No a priori sample-size calculation was performed because this retrospective study included all eligible scans available during the archive review. As a supplementary assessment, a post hoc power analysis was performed using G*Power, version 3.1.9.7 (Heinrich Heine University, Düsseldorf, Germany), based on a fixed-effects one-way analysis of variance model with four regional groups, a total sample size of 252 sites, an effect size of 0.25, and an alpha level of 0.05. The achieved power was 0.926. This post hoc calculation was not used as a substitute for a priori sample-size planning and is presented only as supplementary information regarding the available dataset.

Results

The study included 45 individuals: 26 female and 19 male participants. The mean age of the study population was 58.1 years (range, 35–81 years). The mean age was 57.1 years (range, 35–75 years) among female participants and 59.5 years (range, 44–81 years) among male participants. Overall, 252 posterior mandibular edentulous sites were evaluated. Of these, 45 sites (17.9%) were in the first premolar region (PM1), 67 (26.6%) in the second premolar region (PM2), 74 (29.4%) in the first molar region (M1), and 66 (26.2%) in the second molar region (M2). Sites were nearly evenly distributed between the right (51.2%) and left (48.8%) sides.

Age was not significantly associated with BW1 (p = 0.132), BW3 (p = 0.217), BW5 (p = 0.185), or RBH (p = 0.858). Sex was not significantly associated with BW1 (p = 0.896), BW3 (p = 0.863), BW5 (p = 0.602), or RBH (p = 0.180). Within this dataset, regional anatomical differences were therefore more pronounced than age- or sex-related differences in the primary linear ridge dimensions. Descriptive morphometric measurements by tooth region are presented in Table 1. 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). Mean LCA ranged from 56.05° in M2 to 73.38° in PM1, whereas mean HCO ranged from 1.96 mm in PM1 to 2.35 mm in M1.

Table 1.

Descriptive morphometric measurements by posterior mandibular tooth region, with regional comparisons based on linear mixed-effects analysis

Region	Variable	Mean	Median	Min	Max	Overall p-value
PM1	BW1 mm	5.06	4.60	2.60	14.01	0.013
	BW3 mm	6.89	6.60	3.40	13.06	< 0.001
	BW5 mm	8.14	8.10	4.62	11.20	< 0.001
	RBH	13.56	13.65	4.24	22.27	< 0.001
	LCA (degrees)	73.38	75.00	55.00	84.00	< 0.001
	HCO (mm)	1.96	2.10	0.50	3.10	0.036
PM2	BW1 mm	4.58	4.20	2.22	10.40	0.013
	BW3 mm	6.43	6.47	3.15	10.45	< 0.001
	BW5 mm	8.06	7.93	4.30	11.20	< 0.001
	RBH	12.55	12.82	2.00	19.67	< 0.001
	LCA (degrees)	69.84	69.00	53.00	80.00	< 0.001
	HCO (mm)	2.15	2.10	0.48	3.28	0.036
M1	BW1 mm	5.16	4.22	1.80	14.20	0.013
	BW3 mm	7.35	7.03	2.20	13.81	< 0.001
	BW5 mm	9.05	9.13	2.20	13.46	< 0.001
	RBH	12.17	12.41	2.04	19.20	< 0.001
	LCA (degrees)	64.05	65.00	47.00	77.00	< 0.001
	HCO (mm)	2.35	2.48	0.50	3.70	0.036
M2	BW1 mm	5.72	5.40	2.20	13.80	0.013
	BW3 mm	8.24	8.23	3.00	14.22	< 0.001
	BW5 mm	10.22	10.40	5.20	15.28	< 0.001
	RBH	12.09	12.26	1.60	19.93	< 0.001
	LCA (degrees)	56.05	56.00	45.00	65.00	< 0.001
	HCO (mm)	2.08	2.05	0.42	3.30	0.036

Linear mixed-effects analysis showed a significant overall regional effect for BW1 (F = 3.669, p = 0.013). Bonferroni-adjusted pairwise comparisons identified a significant difference only between PM2 and M2, with greater BW1 in M2 (p = 0.007). At 3 mm apical to the crest, the overall regional effect was also significant (F = 6.682, p < 0.001). BW3 was significantly greater in M2 than in PM1 (p = 0.025) and PM2 (p < 0.001). At 5 mm apical to the crest, the overall regional effect was significant (F = 14.475, p < 0.001). BW5 was significantly greater in M2 than in PM1 (p < 0.001), PM2 (p < 0.001), and M1 (p = 0.010). M1 also had significantly greater BW5 than PM2 (p = 0.029).

A significant overall regional effect was also observed for RBH (F = 13.886, p < 0.001). Bonferroni-adjusted comparisons showed that RBH was significantly greater in PM1 than in PM2 (p = 0.004), M1 (p < 0.001), and M2 (p < 0.001). In addition, RBH was significantly greater in PM2 than in M2 (p = 0.033). No significant differences were observed between PM2 and M1 or between M1 and M2.

Regional differences were also observed in lingual concavity-related parameters. The overall regional effect for LCA was significant (p < 0.001). Bonferroni-adjusted pairwise comparisons showed that LCA was significantly greater in PM1 than in PM2 (p = 0.011), M1 (p < 0.001), and M2 (p < 0.001). LCA was also significantly greater in PM2 than in M1 (p < 0.001) and M2 (p < 0.001), and greater in M1 than in M2 (p < 0.001). The overall regional effect for HCO was also significant (p = 0.036); however, the only significant pairwise difference was between M1 and PM1, with greater HCO in M1 (p = 0.044). Significant Bonferroni-adjusted pairwise comparisons are presented in Supplementary Table S1.

According to the predefined virtual radiologic planning criteria, 120 sites (47.6%) were classified as suitable for standard implants, 8 (3.2%) for short implants, and 17 (6.7%) for narrow-diameter implants. Eight sites (3.2%) were classified as requiring vertical augmentation, 84 (33.3%) as requiring horizontal augmentation, and 15 (6.0%) as requiring combined vertical and horizontal augmentation (Table 2). These categories represent threshold-based radiologic planning outputs and should not be interpreted as observed surgical treatment decisions or clinical outcomes.

Table 2.

Classification of posterior mandibular sites according to anatomical ridge adequacy

Implant / augmentation strategy	Number of sites (n)	Percentage (%)
Standard implant	120	47.6
Short implant	8	3.2
Narrow implant	17	6.7
Vertical augmentation	8	3.2
Horizontal augmentation	84	33.3
Vertical + horizontal augmentation	15	6.0
Total	252	100.0

Among sites with vertical insufficiency, the estimated mean deficiency was 1.86 mm in PM1, 2.25 mm in PM2, 2.98 mm in M1, and 3.56 mm in M2. Among sites with horizontal insufficiency, the estimated mean deficiency ranged from 1.83 to 2.19 mm across regions. Kruskal-Wallis analysis showed no significant regional differences in the magnitude of vertical or horizontal anatomical insufficiency (p = 0.369 and p = 0.235, respectively) (Table 3).

Table 3.

Magnitude of vertical and horizontal anatomical insufficiency in posterior mandibular regions

Region	Vertical insufficiency: sites (n)	Vertical insufficiency: mean (min-max), mm	Horizontal insufficiency: sites (n)	Horizontal insufficiency: mean (min-max), mm
PM1	4	1.86 (0.39–3.76)	16	1.83 (0.50–2.90)
PM2	7	2.25 (0.23-6.00)	34	2.19 (0.48–3.28)
M1	7	2.98 (1.01–5.96)	28	2.19 (0.50–3.70)
M2	5	3.56 (1.40–6.40)	21	2.00 (0.42–3.30)

Under the assumption that the indicated augmentation procedures had been successfully completed, feasible implant diameters and lengths were estimated for all 252 posterior mandibular edentulous sites. Across all regions, implants 3.5 mm in diameter were the most frequently feasible diameter category, whereas implants ≥ 12 mm in length were the most frequently feasible length category. Wider implant diameters (≥ 5.0 mm) were more frequently feasible in molar regions. However, the distribution of feasible implant diameters did not differ significantly among tooth regions (χ² = 12.80, p = 0.38). Similarly, no significant association was observed between implant-length distribution and tooth region (χ² = 6.59, p = 0.68) (Table 4).

Table 4.

Post-augmentation distribution of feasible implant diameters and lengths by region (total n = 252)

Region	3.0 mm	3.5 mm	4.0 mm	4.5 mm	>=5.0 mm	6 mm	8 mm	10 mm	>=12 mm
PM1	2	22	6	5	10	1	9	13	22
PM2	6	38	5	4	14	1	20	15	31
M1	3	32	10	4	25	3	27	15	29
M2	6	30	3	4	23	3	22	11	30
Total	17	122	24	17	72	8	78	54	112

When implant diameter-length combinations were analyzed under the same post-augmentation assumption, the most frequently observed combinations were 3.5 x > = 12 mm (19.0%), 3.5 × 8 mm (17.5%), and > = 5.0 x > = 12 mm (11.5%). The distribution of diameter-length combinations did not differ significantly across tooth regions (chi-square = 39.63, p = 0.80) (Table 5).

Table 5.

Post-augmentation distribution of feasible implant diameter-by-length combinations by region

Implant size (mm)	PM1	PM2	M1	M2	Total n (%)
3.0 × 10	0	2	1	1	4 (1.6)
3.0 x > = 12	2	4	2	5	13 (5.2)
3.5 × 8	5	13	14	12	44 (17.5)
3.5 × 10	8	10	7	5	30 (11.9)
3.5 x > = 12	9	15	11	13	48 (19.0)
4.0 × 6	0	0	1	0	1 (0.4)
4.0 × 8	2	1	1	0	4 (1.6)
4.0 × 10	1	0	4	0	5 (2.0)
4.0 x > = 12	3	4	4	3	14 (5.6)
4.5 × 6	0	1	0	0	1 (0.4)
4.5 × 8	1	1	1	1	4 (1.6)
4.5 × 10	1	1	0	2	4 (1.6)
4.5 x > = 12	3	1	3	1	8 (3.2)
>=5.0 × 6	1	0	2	3	6 (2.4)
>=5.0 × 8	1	5	11	9	26 (10.3)
>=5.0 × 10	3	2	3	3	11 (4.4)
>=5.0 x > = 12	5	7	9	8	29 (11.5)
Total	45	67	74	66	252 (100.0)

Intraobserver reliability was high, with ICCs ranging from 0.92 to 0.98. Because all measurements were performed by a single observer, only intraobserver agreement was assessed.

Discussion

This retrospective CBCT-based study evaluated posterior mandibular edentulous ridge morphology by combining conventional linear measurements with lingual concavity-related parameters in a standardized virtual planning framework. The principal finding was that posterior mandibular anatomy varied according to tooth region: ridge width tended to increase distally, whereas residual bone height decreased toward the posterior mandible. Lingual concavity-related parameters, particularly LCA, also varied by region. These findings partially support the study hypothesis by indicating that molar regions may be less favorable vertically and may have more complex internal lingual cortical morphology, even when ridge width is greater.

Previous studies have reported increasing ridge width from premolar to molar regions together with decreasing residual bone height toward distal sites [1–3]. Similar patterns were described by Bressan et al. and Braut et al. in CBCT-based analyses of edentulous posterior mandibles and were attributed to the anatomical course of the mandibular canal and the morphology of the submandibular fossa [1, 2]. The present findings are broadly consistent with these observations: ridge width was greater at distal sites, whereas RBH was more favorable in premolar regions. These results support the view that distal regions, particularly molar sites, may have less favorable vertical morphometry despite greater horizontal width [1–3, 16, 17].

The combined interpretation of ridge width and height is clinically relevant. If only horizontal bone width is considered, molar regions may appear more favorable than premolar regions. However, the concurrently reduced RBH at posterior sites indicates that greater ridge width does not necessarily correspond to more favorable overall anatomy for implant placement. This finding reinforces the need to assess vertical and horizontal dimensions together, particularly in posterior mandibular implant planning, where the mandibular canal is a critical limiting structure.

A key aspect of the present study was the assessment of internal lingual cortical morphology using LCA and HCO. Although conventional radiologic evaluation often emphasizes linear ridge height and width, these parameters may provide additional information about the internal contour of the lingual aspect of the posterior mandible. Yoon et al. highlighted the clinical relevance of lingual concavity morphology in posterior mandibular implant planning, particularly in relation to the anatomical risk of cortical perforation [8]. In the present study, LCA decreased progressively toward distal regions, with significant differences among PM1, PM2, M1, and M2. When considered together with the lower RBH values in posterior regions, this pattern suggests that molar sites may require more detailed three-dimensional evaluation before implant placement.

HCO showed a significant overall regional effect, but only M1 and PM1 differed significantly in pairwise comparisons. This pattern suggests that HCO may capture subtler changes in the horizontal extent of lingual concavity than LCA. Therefore, LCA and HCO should not be regarded as interchangeable. Rather, they may provide complementary information: LCA describes the angular configuration of the lingual cortical contour, whereas HCO reflects the horizontal depth of the concavity at the selected reference level. Their combined use may help identify situations in which crestal width alone does not fully characterize the internal morphology of the posterior mandible.

The virtual planning model indicated that standard implant placement was radiologically feasible at 47.6% of sites, whereas horizontal augmentation was the most common insufficiency category. In the exploratory post-augmentation simulation, 3.5 mm was the most frequently feasible diameter and ≥ 12 mm was the most frequently feasible length category; however, neither diameter nor length distribution differed significantly among tooth regions. These findings indicate that the simulation primarily describes standardized implant-dimension categories after the assumed correction of anatomical deficiencies rather than demonstrating a strong regional preference for implant size. Accordingly, these outputs should be interpreted cautiously as radiologic planning descriptors rather than treatment recommendations. Clinical decisions should also consider prosthetic requirements, implant-system characteristics, bone quality, systemic factors, surgical access, clinician experience, and patient-specific risk.

The influence of demographic variables on posterior mandibular ridge dimensions remains controversial. Some authors have reported greater ridge dimensions in male patients, whereas others have found no statistically significant association [2, 16, 18]. In the present study, age and sex were not significantly associated with BW1, BW3, BW5, or RBH. Within this sample, local anatomical variation and tooth-region characteristics may therefore have had a greater influence on posterior mandibular morphometry than demographic variables alone.

From a radiologic perspective, CBCT provides three-dimensional visualization of the mandibular canal and surrounding cortical structures that cannot be fully characterized using conventional two-dimensional imaging [6, 7]. The present findings support the utility of standardized CBCT-based measurement protocols for site-specific assessment of posterior mandibular edentulous regions [11]. Because viewer-dependent factors such as slice thickness, contrast, and sharpness may affect visualization of the inferior alveolar canal on CBCT images, consistent display and reconstruction settings are important when interpreting canal-related measurements [12]. In this context, the combined assessment of LCA and HCO may provide information beyond conventional linear measurements and support a more comprehensive preoperative radiologic evaluation [6–8].

Clinical implications

Although lingual concavity assessment has been described previously, the present study integrates LCA and HCO with conventional ridge-height and ridge-width measurements in a tooth-region-specific feasibility model. This integrated assessment may help clinicians identify sites at which apparently adequate crestal width does not necessarily indicate favorable internal lingual morphology. In practical terms, the model may support the preoperative identification of sites that appear horizontally adequate but still require careful evaluation of the lingual cortical contour, particularly in molar regions, where reduced RBH and altered concavity-related parameters may narrow the surgical safety margin.

The proposed approach should therefore be regarded as a radiologic planning aid rather than as a predictor of surgical complications, augmentation success, or implant survival. Its value lies in standardizing the anatomical description of posterior mandibular edentulous sites and encouraging the simultaneous evaluation of vertical height, crestal and subcrestal width, and lingual cortical configuration.

Limitations

The principal limitation of this study is that all findings are based on CBCT-derived morphometry and virtual implant planning rather than actual surgical outcomes. Consequently, the proposed feasibility categories should not be interpreted as direct predictors of intraoperative lingual perforation, augmentation success, implant survival, or postoperative complications. Second, the retrospective, single-center design may limit generalizability and introduce selection bias. Third, only intraobserver reliability was assessed. The absence of interobserver reproducibility data is a notable limitation, particularly for LCA and HCO, which may be more operator-dependent than conventional linear measurements of ridge height and width.

Fourth, although a standardized image-analysis workflow and display protocol were used, exact values for viewer-specific slice thickness, brightness, contrast, and sharpness were not prospectively evaluated as independent study variables; this limitation should be considered when interpreting measurement reproducibility. Fifth, potentially relevant clinical variables, including duration of edentulism, bone quality, systemic conditions, periodontal history, and detailed clinical history of the edentulous sites, were unavailable for analysis. Sixth, because site localization relied primarily on adjacent teeth or the mental foramen, the protocol may be less applicable to completely edentulous mandibles or to cases in which these landmarks cannot be identified reliably. Finally, the virtual implant-feasibility model and post-augmentation implant-size simulation were based on predefined dimensional assumptions introduced for standardization and may not fully reflect all clinical scenarios, implant systems, prosthetically driven implant positions, or actual surgical decision-making. Future studies should validate these morphometric findings prospectively by examining the relationships of LCA and HCO with surgical decision-making, intraoperative findings, complications, augmentation outcomes, and implant survival. Multicenter studies with broader population sampling and formal assessment of interobserver reproducibility would further clarify the clinical applicability and generalizability of these parameters.

Conclusions

Within the limitations of this retrospective CBCT-based study, posterior mandibular edentulous sites showed region-specific variation in ridge dimensions and lingual concavity-related parameters. Molar regions had lower residual bone height despite greater ridge width, indicating that horizontal and vertical dimensions should not be interpreted in isolation. The combined assessment of RBH, BW1, BW3, BW5, LCA, and HCO may provide a more comprehensive radiologic characterization of posterior mandibular anatomy during implant planning. The post-augmentation implant-size simulation provides descriptive information on feasible diameter and length categories under standardized assumptions. However, because the present findings are based on virtual planning rather than clinical outcome data, their clinical applicability should be confirmed in prospective surgical studies.

Abbreviations

BW1

Bone width at 1 mm apical to the alveolar crest

BW3

Bone width at 3 mm apical to the alveolar crest

BW5

Bone width at 5 mm apical to the alveolar crest

CBCT

Cone-beam computed tomography

HCO

Horizontal concavity offset

ICC

Intraclass correlation coefficient

LCA

Lingual concavity angle

M1

First molar

M2

Second molar

PM1

First premolar

PM2

Second premolar

RBH

Residual bone height

Authors’ contributions

Conceptualization: Okkesim A, Esen C; Formal analysis: Esen C, Okkesim A. Investigation: Okkesim A, Esen C; Methodology: Okkesim A; Writing–original draft: Okkesim A.; Writing – review & editing: Esen C, Okkesim A.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This retrospective study was approved by the Non-Interventional Ethics Committee of Nevşehir Hacı Bektaş Veli University (approval date: 12 June 2025; decision no. 2500051530). At the time of clinical admission and CBCT acquisition, all patients had provided broad written consent for the future use of their anonymized clinical and radiographic data in scientific research. Because the study was retrospective and used anonymized data, the requirement for additional study-specific informed consent was waived by the ethics committee.

All procedures involving human participants or human data were conducted in accordance with the ethical principles of the Declaration of Helsinki and its subsequent amendments.

Consent for publication

Not applicable. The manuscript contains no identifiable individual data, images, or videos.

Competing interests

The authors declare no competing interests.