The influence of prosthetic treatments and implant-supported prostheses on posterior mandibular ridge atrophy: a retrospective cohort study
The Fifth German Oral Health Study  has demonstrated that the replacement of lost teeth remains a high priority in modern dentistry. The population of “young olds” (age range: 65 − 74 years) in this nationwide study was found to have 16.9 residual teeth on average, one in eight (12.4%) being completely edentulous, and the population of “younger adults” (age range: 35 − 44 years) to be missing 2.1 teeth on average . Within this high-priority issue of replacing lost teeth, bone preservation is a critical requirement for the long-term stability of stomatognathic function and oral health-related quality of life . By the same logic, being able to predict the way in which jawbone will likely respond to functional loads exerted by a specific prosthesis is essential to provide patients suffering from tooth loss with appropriate solutions. Any treatment options that might promote bone atrophy and, as a result, jeopardize the long-term goals of prosthetic stability and masticatory function, should be avoided. There is need to ensure high-quality prosthetic treatments that will restore the natural tooth anatomy, reestablish what stability of the residual tissues is attainable by optimizing load distribution, and recover the highest standards of comfort, phonetics, esthetics, and function that can possibly be achieved . Any compromises in restoring partially or fully edentulous patients are liable to have negative long-term consequences for residual tissue structures, cause further deterioration, as well as complicate and narrow down the remaining options for subsequent treatment . The present study aims to investigate the influence of different prosthetic treatments and implant-supported prostheses on posterior mandibular ridge atrophy in partially and completely edentulous patients. Specifically, the effects of various types of prosthetic treatments, including fixed and removable prostheses, and different configurations of implant-supported prostheses are compared. Studies have compared different prosthetic setups and configurations, including complete dentures and implant-supported prostheses, in both the maxilla and the mandible and systematic reviews and meta-analyses have shown significant differences in the effects of various prosthetic treatments on surrounding tissues, highlighting the need for a more holistic analysis of these treatment options . However, these studies often lack a comprehensive approach that integrates and systematically compares both the choice of prosthetic treatment and the morphology of the treatment configurations. The posterior mandible has a crucial role within the stomatognathic system and presents unique challenges for prosthetic interventions due to its anatomical and functional characteristics. The selection of the prosthesis type has significant implications for the long-term integrity of the alveolar bone, as masticatory forces are distributed across all supporting tissue structures. Clinical situations of inadequate support and unbalanced force distribution can promote bone atrophy . Especially in cases of Kennedy Class I and II with wide spans of free-end gaps in the posterior region, it is recommended to adhere to stringent design requirements to minimize negative impacts on the bone . Forces directed toward the prosthesis base occur unconsciously while chewing, typically around 50 N, but can rise to over 400 N during conscious clenching in the molar area. Parafunctional activities like grinding and clenching generate even greater compressive forces, placing higher demands on the mechanical properties of dental prostheses . With these considerations in mind, a retrospective cohort study was conducted to investigate the varying degrees of mandibular posterior ridge atrophy associated with long-term use of specific fixed or removable prosthetic devices supported by implants, natural teeth, or mucosal tissues. The primary outcome measure was the least vertical height of the mandible, measured perpendicular to a tangent along the underside of the mandible. The null hypothesis was that the change in this vertical height between a baseline orthopantomogram (OPT) and a follow-up OPT after ≥ 3 years would show no difference between different types of prosthetic treatments. In contrast, the alternative hypothesis posits that different prosthetic treatments, which may serve as alternative therapeutic options for similar indications, lead to significantly varying degrees of vertical alveolar ridge bone resorption. The study aimed to assess how different types of prosthetic treatments and Kennedy classes influence bone resorption, providing insights into their potential correlations. 

Study preparations and data extraction A retrospective cohort study was designed to examine prosthetic treatments after tooth loss involving the posterior mandible. All treatments had been performed at the Department of Dental Medicine and Oral Health Graz, Austria between 01/2001 and 12/2012. Approval of the study design was obtained in advance from the institutional review board (Ethics committee Graz, Austria ref. 27–172 ex 14/15). Patient records and entries were extracted from the medical documentation and information network (MEDOCS; SAP SE, Walldorf, Germany) and the Sidexis (v 02.20, Dentsply Sirona, Charlotte, NC, USA) dental imaging database. The analysis encompassed a range of prosthetic treatments frequently employed in both partially and fully edentulous mandibles, including clasp-retained and telescope-retained removable partial dentures (RPDs), tooth-supported fixed partial dentures (FPDs), implant-supported FPDs, as well as bar-retained, locator-retained, and mucosa-supported complete dentures. Inclusion and exclusion criteria Patients were initially included in the study if they presented all of the following conditions: Aged ≥ 18 years at the time of prosthetic delivery. Received any prosthesis to replace at least one missing lower premolar or molar. Availability of a baseline OPT obtained immediately after prosthetic delivery. Availability of a follow-up OPT obtained ≥ 3 years after the baseline using the same x-ray unit and resolution specifications (Orthophos XG Plus, software v. 02.20; hardware v. AD; Dentsply Sirona). If follow-up OPTs taken after more than 3 years were available, the later radiograph was used, ensuring one baseline and one follow-up OPT per patient while maximizing the time span between them. The patients were excluded from the study if they presented one of the following conditions: Incomplete baseline records or a history of posterior mandibular inflammation. Presence of distortions, deficits in sharpness, or differences in resolution between the baseline and follow-up OPTs. History of diabetes, smoking, osteoporosis, malignant disease, or radiotherapy. Changes in prosthetic treatment, bone augmentation, or further tooth loss in the mandible during the observation period. Radiograph standardization and analysis All OPTs were captured using the same digital panoramic X-ray system with consistent exposure parameters (panoramic view for adults, 64 kV, 12 mA, exposure time of 14.2 s) as documented by the ortho software. Patients were positioned according to the manufacturer’s recommendations, ensuring the Frankfurt-Horizontal plane was parallel to the floor and the vertical line aligned with the face’s midsagittal plane. An experienced oral radiologist preliminarily reviewed each baseline OPT and the corresponding follow-up OPT for each patient, excluding cases with any distortion, improper exposure, or other factors such as faulty head positioning and varying mouth openings between the corresponding radiographs that could affect comparability. Measuring bone atrophy Calibration and validation of measurement procedures were conducted, with an examiner (A.S.) trained in bone height measurement on OPTs and blinded to patient details. This examiner (A.S.) measured mandibular bone height from all OPTs in a randomized sequence. The primary measurement extracted from the validated OPTs was defined as the vertical dimension of the mandibular bone at its minimum vertical height, as illustrated in Fig. . A widely used image editing and processing program for medical and scientific image analysis (ImageJ; National Institutes of Health, Bethesda, MD, USA)  was employed to facilitate the precise identification of anatomical landmarks, including the borders of the ramus, the ascending branch, crestal alveolar bone contour, and midline in edentulous cases . The pixel-size calibration was directly adopted from the X-ray-specific software (Sidexis) and applied in ImageJ. The objective assessment of the mandibular height involved placing a tangent line along the underside of the mandible. Subsequently, a vertical line was constructed perpendicular to this tangent, intersecting at the lowest height of the crestal alveolar bone contour within the edentulous area between adjacent teeth or implants and the ascending branch. If the area was occupied by an implant, the measurement was taken distal to the implant. In cases of edentulism, this measurement was obtained at the lowest vertical height between the ascending branch and the mandibular midline. This procedure was chosen after a comprehensive analysis of the literature and comparable studies regarding measurements on OPTs concerning the mandible . Depending on the type of the prosthesis, the measurement was performed either unilaterally or bilaterally on a millimeter scale applied to the radiographs. Patients who met the inclusion criteria for both sides of the mandible were randomly selected for measurement and analysis on one side only, either left or right. However, for patients with removable partial dentures of Kennedy Classes I, II, and III, both sides were measured and analyzed in the side-based statistical comparison. The difference between the baseline and follow-up measurements was calculated to determine the extent of alveolar ridge atrophy for each patient, which was then utilized for subsequent analysis. To account for magnification during the X-ray process, a calibration factor of 1.3 was applied, as established by previous research conducted at our center (Department of Oral Surgery and Radiography) . Side-based analysis of measurements Patients who received removable partial dentures underwent measurements at both the edentulous side and the contralateral edentulous gap-space for comparison. Specifically, the comparison groups included Kennedy Class I (representing bilateral edentulous areas with randomization to one side), Kennedy Class II with measurements taken at the edentulous free-end side, Kennedy Class II with measurements taken at the contralateral edentulous gap-side, and Kennedy Class III with measurements at the edentulous space and randomization to one side if the edentulous space was bilateral. Statistical analysis All statistical operations were carried out using statistical (SPSS, version 27; IBM, Armonk, NY, USA), and spreadsheet software (Excel, version 2016; Microsoft, Redmond, WA, USA). To assess sample size adequacy, a post-hoc power analysis was conducted for key comparisons to detect clinically relevant differences. To ensure comparable groups, the effect of gender on bone resorption based on the type of prostheses was controlled using t-tests for independent samples. In total, 16 comparisons were made between groups with different prosthetics to determine which type of prosthetic treatment resulted in higher or lower bone resorption than others. For statistical analysis, one-way analysis of variance (ANOVA) was used, significance was determined using Tukey Post hoc Test and p -values were reported. A significance threshold of p < 0.05 was used. Furthermore, groups were compared based on the prosthetic gap situation to identify potential correlations between a free-end situation and increased bone resorption. Therefore, Kennedy classifications were compared within and between the removable prosthetic treatments of clasp-retained RPDs and telescope-retained RPDs. Coincidentally, this also amounted to 16 comparisons. The paired t-test was used to compare the means of two related groups. Furthermore Scattergrams of posterior mandibular ridge atrophy were created for sites under both, clasp-retained RPDs and telescope-retained RPDs. Significant linear increases in bone loss were analyzed using Pearson correlation analysis. 

A retrospective cohort study was designed to examine prosthetic treatments after tooth loss involving the posterior mandible. All treatments had been performed at the Department of Dental Medicine and Oral Health Graz, Austria between 01/2001 and 12/2012. Approval of the study design was obtained in advance from the institutional review board (Ethics committee Graz, Austria ref. 27–172 ex 14/15). Patient records and entries were extracted from the medical documentation and information network (MEDOCS; SAP SE, Walldorf, Germany) and the Sidexis (v 02.20, Dentsply Sirona, Charlotte, NC, USA) dental imaging database. The analysis encompassed a range of prosthetic treatments frequently employed in both partially and fully edentulous mandibles, including clasp-retained and telescope-retained removable partial dentures (RPDs), tooth-supported fixed partial dentures (FPDs), implant-supported FPDs, as well as bar-retained, locator-retained, and mucosa-supported complete dentures. 

Patients were initially included in the study if they presented all of the following conditions: Aged ≥ 18 years at the time of prosthetic delivery. Received any prosthesis to replace at least one missing lower premolar or molar. Availability of a baseline OPT obtained immediately after prosthetic delivery. Availability of a follow-up OPT obtained ≥ 3 years after the baseline using the same x-ray unit and resolution specifications (Orthophos XG Plus, software v. 02.20; hardware v. AD; Dentsply Sirona). If follow-up OPTs taken after more than 3 years were available, the later radiograph was used, ensuring one baseline and one follow-up OPT per patient while maximizing the time span between them. The patients were excluded from the study if they presented one of the following conditions: Incomplete baseline records or a history of posterior mandibular inflammation. Presence of distortions, deficits in sharpness, or differences in resolution between the baseline and follow-up OPTs. History of diabetes, smoking, osteoporosis, malignant disease, or radiotherapy. Changes in prosthetic treatment, bone augmentation, or further tooth loss in the mandible during the observation period. 

All OPTs were captured using the same digital panoramic X-ray system with consistent exposure parameters (panoramic view for adults, 64 kV, 12 mA, exposure time of 14.2 s) as documented by the ortho software. Patients were positioned according to the manufacturer’s recommendations, ensuring the Frankfurt-Horizontal plane was parallel to the floor and the vertical line aligned with the face’s midsagittal plane. An experienced oral radiologist preliminarily reviewed each baseline OPT and the corresponding follow-up OPT for each patient, excluding cases with any distortion, improper exposure, or other factors such as faulty head positioning and varying mouth openings between the corresponding radiographs that could affect comparability. 

Calibration and validation of measurement procedures were conducted, with an examiner (A.S.) trained in bone height measurement on OPTs and blinded to patient details. This examiner (A.S.) measured mandibular bone height from all OPTs in a randomized sequence. The primary measurement extracted from the validated OPTs was defined as the vertical dimension of the mandibular bone at its minimum vertical height, as illustrated in Fig. . A widely used image editing and processing program for medical and scientific image analysis (ImageJ; National Institutes of Health, Bethesda, MD, USA)  was employed to facilitate the precise identification of anatomical landmarks, including the borders of the ramus, the ascending branch, crestal alveolar bone contour, and midline in edentulous cases . The pixel-size calibration was directly adopted from the X-ray-specific software (Sidexis) and applied in ImageJ. The objective assessment of the mandibular height involved placing a tangent line along the underside of the mandible. Subsequently, a vertical line was constructed perpendicular to this tangent, intersecting at the lowest height of the crestal alveolar bone contour within the edentulous area between adjacent teeth or implants and the ascending branch. If the area was occupied by an implant, the measurement was taken distal to the implant. In cases of edentulism, this measurement was obtained at the lowest vertical height between the ascending branch and the mandibular midline. This procedure was chosen after a comprehensive analysis of the literature and comparable studies regarding measurements on OPTs concerning the mandible . Depending on the type of the prosthesis, the measurement was performed either unilaterally or bilaterally on a millimeter scale applied to the radiographs. Patients who met the inclusion criteria for both sides of the mandible were randomly selected for measurement and analysis on one side only, either left or right. However, for patients with removable partial dentures of Kennedy Classes I, II, and III, both sides were measured and analyzed in the side-based statistical comparison. The difference between the baseline and follow-up measurements was calculated to determine the extent of alveolar ridge atrophy for each patient, which was then utilized for subsequent analysis. To account for magnification during the X-ray process, a calibration factor of 1.3 was applied, as established by previous research conducted at our center (Department of Oral Surgery and Radiography) . 

Patients who received removable partial dentures underwent measurements at both the edentulous side and the contralateral edentulous gap-space for comparison. Specifically, the comparison groups included Kennedy Class I (representing bilateral edentulous areas with randomization to one side), Kennedy Class II with measurements taken at the edentulous free-end side, Kennedy Class II with measurements taken at the contralateral edentulous gap-side, and Kennedy Class III with measurements at the edentulous space and randomization to one side if the edentulous space was bilateral. 

All statistical operations were carried out using statistical (SPSS, version 27; IBM, Armonk, NY, USA), and spreadsheet software (Excel, version 2016; Microsoft, Redmond, WA, USA). To assess sample size adequacy, a post-hoc power analysis was conducted for key comparisons to detect clinically relevant differences. To ensure comparable groups, the effect of gender on bone resorption based on the type of prostheses was controlled using t-tests for independent samples. In total, 16 comparisons were made between groups with different prosthetics to determine which type of prosthetic treatment resulted in higher or lower bone resorption than others. For statistical analysis, one-way analysis of variance (ANOVA) was used, significance was determined using Tukey Post hoc Test and p -values were reported. A significance threshold of p < 0.05 was used. Furthermore, groups were compared based on the prosthetic gap situation to identify potential correlations between a free-end situation and increased bone resorption. Therefore, Kennedy classifications were compared within and between the removable prosthetic treatments of clasp-retained RPDs and telescope-retained RPDs. Coincidentally, this also amounted to 16 comparisons. The paired t-test was used to compare the means of two related groups. Furthermore Scattergrams of posterior mandibular ridge atrophy were created for sites under both, clasp-retained RPDs and telescope-retained RPDs. Significant linear increases in bone loss were analyzed using Pearson correlation analysis. 

The search of MEDOCS and Sidexis (see Materials and Methods) returned 608 patients meeting the inclusion criteria. Of these, 151 cases had to be excluded, the vast majority ( n = 118) due to OPT-related problems, including but not limited to potential distortion, improper exposure, faulty head positioning, angulation, and mouth opening, all of which could affect further analysis, as well as inadequate, missing, or inappropriately timed images. Ten more cases were excluded due to osteosynthesis plates, seven because their prostheses could not be properly categorized, six for peri-implantitis, and ten for other non-OPT-related reasons. Thus 457 patients were left for analysis. As defined by the interval between both measurements in each case, these 457 cases were followed up for a mean of 6.43 ± 2.38 (3.02 − 13.1) years, and the patients were 57.6 ± 12.5 (17.7 − 90.9) years old at the time of prosthetic delivery (Table ). To keep numbers comparable, inclusion of implant-supported fixed prostheses was confined to the 120 cases with the longest observation intervals. Table stratifies patients, the primary criterion of differentiation being prostheses typically used for partial versus complete mandibular edentulism. A total of 457 sites, all measured for alveolar ridge height both immediately upon prosthetic delivery and ≥ 3 years thereafter, were available for analysis. Female ( n = 272) and male ( n = 185) sites were not differently distributed across the different types of prostheses (Table ). There was no significant association between gender and prosthetic treatment. The post-hoc power analysis showed that comparisons such as clasp-RPDs vs. implant-supported FPDs in the partial edentulism group (Cohen’s d = 1.77, power = 1.0) and 4I bar-retained RDPs vs. mucosa-supported RDPs in the complete edentulism group (Cohen’s d = 2.55, power = 1.0) had sufficient statistical power. However, comparisons involving smaller sample sizes, such as 2I bar-retained RDPs vs. Locator-RDPs (Cohen’s d = 1.77, power = 0.62), demonstrated limited power due to the low case numbers. Prostheses-related atrophy in partially edentulous mandibles Table (upper part) lists the posterior ridge atrophy measured at prostheses typically used in partially edentulous mandibles. Mean bone loss was 0.15 ± 0.19 (–0.09 to 1.25) mm for tooth-supported versus 0.29 ± 0.24 (0.00 to 1.05) mm for implant-supported fixed partial dentures (FPDs). Both of these fixed restoration types were thus associated with significantly less bone than 2.01 ± 1.34 (0.01 to 7.91) mm for clasp-retained or 1.93 ± 1.24 (0.16 to 5.11) mm for telescope-retained removable partial dentures. All of these four comparisons were statistically significant at p < 0.001 (Table , upper part). The Pearson correlation analysis, shown in Fig. , disclosed a significant linear pattern of posterior ridge resorption over time for both, the clasp-retained RPDs ( r = 0.193; p < 0.001) and telescope-retained RPDs ( r = 0.348; p < 0.001). RPD-related atrophy according to Kennedy Classes Under clasp-retained RPDs, sites that lacked distal support bilaterally (Class I) or unilaterally (Class II free-end-side) showed significantly more posterior atrophy than sites that did involve distal support (Class II gap-side or Class III). All of these four comparisons were statistically significant at p < 0.001. While a similar picture was obtained for sites under telescope-retained RPDs, it should be noted that the case numbers for Class II and III were relatively low. Both RPD types showed less bone loss in cases of distal support than in cases of bilateral lack of distal support (Class III versus Class I). This difference was statistically significant for clasp-retained RPDs ( p < 0.001). Prostheses-related atrophy in fully edentulous mandibles Table (lower part) lists the posterior atrophy measured at prostheses typically used in fully edentulous mandibles. Bar-retained overdentures on four implants yielded significantly less bone atrophy, at 0.13 ± 0.10 (− 0.02 to 0.52) mm, than mucosa-supported complete dentures at 3.26 ± 1.68 (1.13 to 7.72) mm, Locator abutments on two implants at 3.96 ± 1.36 (1.71 to 5.25) mm, and bar-retained overdentures on two implants at 1.76 ± 0.67 (1.17 to 2.90) mm. All of these three comparisons were statistically significant at p < 0.001 (Table , lower part). 

Table (upper part) lists the posterior ridge atrophy measured at prostheses typically used in partially edentulous mandibles. Mean bone loss was 0.15 ± 0.19 (–0.09 to 1.25) mm for tooth-supported versus 0.29 ± 0.24 (0.00 to 1.05) mm for implant-supported fixed partial dentures (FPDs). Both of these fixed restoration types were thus associated with significantly less bone than 2.01 ± 1.34 (0.01 to 7.91) mm for clasp-retained or 1.93 ± 1.24 (0.16 to 5.11) mm for telescope-retained removable partial dentures. All of these four comparisons were statistically significant at p < 0.001 (Table , upper part). The Pearson correlation analysis, shown in Fig. , disclosed a significant linear pattern of posterior ridge resorption over time for both, the clasp-retained RPDs ( r = 0.193; p < 0.001) and telescope-retained RPDs ( r = 0.348; p < 0.001). 

Under clasp-retained RPDs, sites that lacked distal support bilaterally (Class I) or unilaterally (Class II free-end-side) showed significantly more posterior atrophy than sites that did involve distal support (Class II gap-side or Class III). All of these four comparisons were statistically significant at p < 0.001. While a similar picture was obtained for sites under telescope-retained RPDs, it should be noted that the case numbers for Class II and III were relatively low. Both RPD types showed less bone loss in cases of distal support than in cases of bilateral lack of distal support (Class III versus Class I). This difference was statistically significant for clasp-retained RPDs ( p < 0.001). 

Table (lower part) lists the posterior atrophy measured at prostheses typically used in fully edentulous mandibles. Bar-retained overdentures on four implants yielded significantly less bone atrophy, at 0.13 ± 0.10 (− 0.02 to 0.52) mm, than mucosa-supported complete dentures at 3.26 ± 1.68 (1.13 to 7.72) mm, Locator abutments on two implants at 3.96 ± 1.36 (1.71 to 5.25) mm, and bar-retained overdentures on two implants at 1.76 ± 0.67 (1.17 to 2.90) mm. All of these three comparisons were statistically significant at p < 0.001 (Table , lower part). 

This retrospective study demonstrates that the amount of bone atrophy to be expected in the posterior mandible after prosthetic treatment will greatly depend on the type of prosthesis used. Over a mean follow-up of 6.42 ± 2.37 (3.02 − 13.1) years, tooth- and implant-supported fixed prostheses were found to involve the least resorption for their typical indications of partial mandibular edentulism, and the same was true of bar-retained overdentures supported by ≥ 4 implants for their typical indications of complete mandibular edentulism. When using digital OPTs for precise measurements, it is essential to consider the limitations of two-dimensional imaging techniques. Despite the proven utility of OPTs in a wide range of diagnostic and analytical applications, the technique does exhibit fundamental limitations, particularly concerning the achievement of exact spatial measurements. The two-dimensional nature of OPTs can lead to distortions, magnification errors, and overlapping anatomical structures, all of which can potentially compromise the precision of the measurements obtained . This is consistent with findings by Kim et al., who reported an average magnification rate of 1.27–1.28 in digital OPTs, with variations depending on the anatomical site and clarity of structures . To account for this, a calibration factor of 1.3 was applied, based on established protocols developed through prior research conducted at our center using our specific panoramic X-ray machine validated with cadaveric specimens . Similarly, Raza et al. highlighted that while vertical magnification in digital OPTs aligns closely with manufacturer specifications, horizontal magnification can deviate, emphasizing the need for careful interpretation . In addition to magnification-related errors, improper patient positioning can further exacerbate inaccuracies. Studies have shown that deviations in patient alignment, such as tilting or anterior-posterior displacement, result in pronounced distortions, particularly in the anterior and canine regions . These findings underline the importance of strict adherence to positioning protocols to optimize image quality and minimize errors. Within these constraints, OPTs remain a valuable tool in dental and maxillofacial imaging. Historical data and numerous studies have consistently demonstrated that, although absolute values might occasionally be elusive, OPTs are eminently suitable for drawing valid conclusions regarding general trends, relationships, and changes over time . Cone-beam computed tomography (CBCT) could theoretically provide three-dimensional data such as circumferences, diameters, widths, or volumes, and may offer higher accuracy for specific measurements. However, its application poses challenges, including inconsistent landmark definition, variations in scan field size, and segmentation methods that impact reproducibility. Additionally, exact measurement values in millimeters would require extensive, consistent CBCT imaging, which is not routinely indicated for standard prosthetic cases. Hence, while the exactitude of individual measurements may occasionally be questioned, the overall capability of OPTs to provide clinically relevant insights and data remains well-established. To improve reliability, OPTs that didn’t meet comparability standards were strictly excluded from the study. This ensured that only images meeting predefined criteria were analyzed, reducing variability from technical differences. By filtering out non-compliant OPTs, the study aimed to standardize measurements and minimize outliers. For prostheses typically used in partially edentulous mandibles, the amount of bone loss with implant-supported FPDs in the present study is consistent with findings in the literature, showing that implants properly supported by hard tissue in the absence of inflammation can ensure long-term preservation of the surrounding alveolar bone. A prospective cohort study found high bone stability even in implants stabilized by interpositional block grafts after three years . These results are underpinned, based on a smaller population, by findings of 0.51 ± 1.42 mm of surrounding bone in posterior mandibles after up to 45 months . Similar investigations have also been conducted for the maxilla, such as the study by Alrajhi et al. (2020), which evaluated maxillary bone resorption with conventional dentures versus implant-supported fixed prostheses . Their results demonstrated that maxillary fixed prostheses on four implants opposed by distal-extension RPDs significantly reduced vertical maxillary bone loss compared to conventional dentures, although they did not entirely prevent bone loss. Specifically, the study found that the control group with conventional dentures exhibited significantly higher bone loss (0.270 mm more on average) than the implant-supported prostheses group. Additionally, anterior maxillary areas experienced greater bone loss than posterior areas across both groups, and women exhibited slightly higher bone loss compared to men. These findings further support the evidence that properly designed and supported implant prostheses can contribute to the preservation of alveolar bone over the long term, even in challenging clinical scenarios. A systematic et al. (2023) investigated bone loss in the posterior edentulous mandible restored with complete dentures, two-implant-supported overdentures, or four-implant-supported overdentures . The analysis of 14 studies found no significant difference in bone loss between complete dentures and two-implant-supported overdentures (mean difference − 0.25 mm; p = 0.43). However, bone loss was significantly lower with four-implant-supported overdentures compared to two-implant-supported overdentures (mean difference − 0.96 mm; p = 0.04). The authors emphasized the need for well-designed randomized controlled trials to validate these findings and better assess the long-term outcomes of these treatment approaches. In the present study, although the groups for complete dentures and two-implant-supported overdentures included relatively few patients, this systematic review supports our hypothesis and data, suggesting that significantly greater bone loss can be expected in these cases. Some discrepancies exist in the outcomes observed with removable partial dentures. In a retrospective clinical study by Brandt et al. (2024), which analyzed 842 clasp-retained removable partial dentures with metal frameworks, it was evident that relining was required in a significant number of cases . This observation highlights the changes in bone tissue beneath the framework of RPDs over time. Interestingly, relining was more frequently performed for maxillary RPDs compared to mandibular RPDs. This difference may reflect variations in bone resorption patterns between the maxilla and mandible, further supporting the need to carefully consider the framework design and materials to account for long-term changes in bone support. The results for clasp- and telescope-retained RPDs indicate less distinct differences in bone loss comparable to prior studies. In a systematic review and meta-analysis by Oh et al. (2021) comparing the loss of abutment teeth for double crown-retained removable partial dentures and clasp-retained RPDs, no significant differences were observed in terms of abutment loss or tooth mobility between the two designs . The findings emphasize the importance of proper framework design and maintenance protocols in minimizing long-term complications, suggesting that bone loss might not be significantly influenced by the retention mechanism, although more favorable outcomes were observed with a wider distribution and greater number of abutment teeth. The absence of significant differences could also likely be attributable to clinical preferences at our institution, where telescopic crowns were more frequently used in compromised cases (e.g., Kennedy Class I or patients with a history of periodontal disease), while clasps were often selected for situations with better distal support. These treatment-specific preferences may partly explain the less pronounced differences observed in this study. In a series of doctoral theses performed at the University of Bonn (Germany), Vieweg  first used a hexapod method in a pig model to measure forces that would act upon an isolated tooth either prepared as a primary telescope or fitted with a clasp to retain a short RPD. With 10 N applied to the denture base 15 mm off the abutment tooth, the clasped and telescoped teeth were found to be deflected by up to 2.3°. Schneider  then performed computer simulations, using finite-element method, of RPDs in a Kennedy Class I situation, with a first premolar and contralateral canine serving as support. While the forces along these denture bases were slightly lower with telescopic than with clasped abutments, the stress levels varied more greatly with the length of the edentulous spaces and denture bases. Along these unsupported distances, the simulated forces (maximum of 500 N) impressed the mucosa by up to 0.5 mm. Occlusal interferences must also be considered, as pathological contacts could lead to improper loading of the prosthesis, thereby increasing force peaks. In the current study, only patients who received prosthetic treatment at the study site, meeting the clinic’s high prosthetic standards, were included, thus ensuring that the occlusion was correct at both the time of insertion and at the follow-up. Additionally, all patients in the present study underwent regular periodontal maintenance therapy, and occlusion was controlled at various follow-up appointments. All patients came from a standardized and strict treatment and recall protocol used at the study clinic. However, recall intervals varied depending on the individual patient’s risk classification. To preserve the posterior alveolar ridge under RPDs in situations lacking distal support (Kennedy Class I or II), transforming the morphology of the edentulous space by implant insertion is a recommended approach . A study from Kaufmann et al. found that implants placed in the presence of compromised residual dentitions were capable of facilitating treatment planning for RPDs and to result in better options of designing dentures characterized by more homogeneous patterns of force distribution in a follow-up of up to 8 years . Regarding prostheses in fully edentulous mandibles, findings indicate that, to preserve posterior bone, overdentures in the mandible supported by ≥ 4 implants should be preferred to mucosa-supported complete dentures. Utilizing only two implants for support may enhance patient satisfaction initially, but can lead to specific complications over time, such as the need for prosthesis relining, prosthesis fracture, and replacement of retention elements . Although the case number for overdentures on two implants in the present study was inadequate for any far-reaching conclusions to be drawn, the results do indicate higher rates of posterior ridge resorption. Nevertheless, the two-implant concept is well documented and represents a very good option in older patients, in the presence of substantial comorbidities, or in other situations where compromised health, such as limited physical resilience or systemic conditions, makes additional implants or extensive surgical interventions unfeasible . However, it should not be the first choice for younger, non-compromised edentulous patients, where preserving natural bone structure should be one of the highest priorities. A computer-model study of mandibular overdentures retained by ball attachments on either two or four implants has resulted in maximum stresses as high as 0.80 MPa for the two-implant variant compared to only 0.29 MPa for the four-implant variant . According to a recent systematic review and meta-analysis, which investigated the optimal number of implants for mandibular full-arch prostheses with a focus on comparing less than five versus more than four implants, no differences in implant survival, prosthetic survival, or bone loss were demonstrable from studies with follow-up periods of 5–15 years . Yet the authors did point out that two implants might increase the risk of complications by exposing the prostheses and its surrounding tissues to higher levels of biomechanical stress. Overdentures supported by two or four implants have rarely been compared to each other in clinical studies, but reports on up to one-year outcomes did, in a statistically significant fashion, reveal that two implants were less favorable in terms of marginal bone resorption and implant loss . In another study, which utilized three different landmarks to analyze vertical bone loss in edentulous mandibles with bar-retained overdentures on two interforaminal implants, bone loss was virtually absent close to the implant, significant at the site of the chewing center, and even more pronounced at the distal edge of the denture flange . Hence implants seem to protect the nearby bone from resorption. Additional limitations of the present study notably include its retrospective design and a variability in case numbers for the different prostheses. To address the latter issue, statistical analyses were presented where appropriate, supplemented by descriptive comparisons where statistical analysis was not feasible. Regarding the partially small sample sizes, the post-hoc power analysis provided valuable insights into the statistical robustness of the study. While sufficient power was confirmed for most comparisons, particularly those with larger groups, limitations were observed in subgroups with low case numbers. These findings underscore the reliability of the primary results while acknowledging areas of reduced statistical strength. The retrospective study design provides a comprehensive view by utilizing existing records, acknowledging the practical challenges in confirming every specific detail and achieving complete data. In contrast, the inclusive nature of this study, which encompasses a diverse array of treatments, offers a reflection of real-world clinical practice that is often more varied than the controlled environments of many prospective studies. This approach enhances the relevance and applicability of the findings to daily medical settings. Also, a certain degree of consistency in treatment approaches may be inferred from the fact that all treatments were performed in the same department. Further studies are needed to confirm the observed effects of different prosthetic interventions on residual bone resorption. A prospective, randomized controlled study design is recommended to address potential sample imbalances and enhance the accuracy of results. Additionally, findings obtained with a longer follow-up period could provide further insights into the long-term impact of prosthetic procedures. The choice of treatment should always be based on clinical and patient-specific considerations to avoid unintended outcomes. 

The present study significantly enhances the understanding of alveolar ridge bone atrophy and the role of different prosthetic treatments in mitigating its progression in the posterior mandible. Key findings underscore the importance of prosthetic design in preserving bone integrity and promoting long-term stability. Specifically, tooth- and implant-supported fixed prostheses were associated with the least bone resorption, suggesting, if applicable, their preferential use in clinical practice. Additionally, for fully edentulous mandibles, bar-retained overdentures supported by at least four implants demonstrated superior outcomes by minimizing bone atrophy compared to other approaches such as mucosa-supported dentures or overdentures supported by fewer implants. It is noted that increasing the number of implants beyond four does not significantly enhance outcomes in terms of residual bone preservation. The results highlight that fixed prostheses offer substantial benefits in partially edentulous mandibles, showing less bone atrophy compared to RPDs. Moreover, RPDs with distal support perform better than those without, reinforcing the importance of design considerations in prosthetic selection and the consideration of supporting implants in the posterior region. Ultimately, these insights provide valuable guidance for dental practitioners in choosing appropriate prosthetic treatments that not only fulfill functional and aesthetic needs but also preserve bone health, ensuring enhanced stability and patient satisfaction over the long term. 

