Abstract
Objective
The aim of this study was to retrospectively evaluate the complications associated with implant-supported overdentures (IODs) by comparing splinted and unsplinted attachment systems with varying numbers of implants in the maxilla and mandible.
Results
A statistically significant difference in complication rates was found between splinted and unsplinted systems (p < 0.05), with unsplinted systems demonstrating higher rates. The number of implants did not significantly affect the incidence of complications (p > 0.05). During the first two years, deformation of the retentive part was the most common complication in all the groups. From the third year to the fifth year, four-implant systems more frequently exhibited retention loss and attachment wear, whereas two- and three-implant systems continued to show deformation of the retentive part.
Keywords: Dental implants, Overdenture, Complications
Introduction
Edentulism is generally defined as the partial or complete loss of teeth due to dental caries or periodontal diseases [1]. This irreversible condition is widely accepted as an epidemiological indicator of oral health across different countries and ages [2, 3]. Complete edentulism refers to the loss of all teeth in both the maxilla and the mandible. This condition negatively affects not only chewing and speech functions but also esthetic appearance and psychosocial well-being [4]. Recognized by the World Health Organization (WHO) as a form of disability, complete edentulism significantly reduces quality of life, and its prevalence increases with age [5]. Therefore, various treatment approaches are needed to minimize the impact of edentulism on individuals.
In cases of complete edentulism, conventional dentures (CDs), implant-supported overdentures (IODs), and implant-supported fixed prostheses are considered the main treatment options. The choice of treatment depends on the patient’s systemic health status, available alveolar bone volume and quality, esthetic expectations, functional needs, and financial circumstances. Although CDs are still widely used, IODs offer superior retention and stability, leading to increased patient satisfaction and improved functional outcomes [6, 7].
Owing to their well-established clinical success compared with CDs, IODs have become a widely accepted treatment option for edentulous patients since the publication of the McGill Consensus in 2002 [8]. In accordance with this consensus, a minimum of 2 implants is recommended for the mandible and 4 implants for the maxilla in IOD therapy. With the increasing clinical use of these prostheses and the expansion of related knowledge, the types of attachment systems and application methods have also diversified [9].
Attachments are classified as splinted (bar-type) or unsplinted (solitary) on the basis of their connection to the implants [10]. Bar types such as the Hader bar, Dolder bar, and milled bar help distribute occlusal forces more evenly and provide clinicians with greater flexibility in implant placement. However, they involve complex fabrication procedures and require approximately 17 mm of interocclusal space [11]. In contrast, unsplinted systems such as ball attachments, magnetic retainers, and Locator attachments are easier to clean and more suitable for limited interarch space. However, they require parallel implant placement and generally offer less prosthesis stability compared to bar systems [12].
Mechanical complications are also commonly reported in implant-supported prostheses. In addition to biological stability, clinicians must frequently manage events such as abutment-screw loosening, loss of retention, and wear or deformation of attachment components. These complications may require repeated maintenance visits and can negatively influence long-term patient satisfaction [13].
Over the past two decades, implant research has focused primarily on biological outcomes related to bone stability around implants. However, treatment success is now evaluated not only by the long-term survival of the implant but also by the esthetic and functional adequacy of the prosthesis [13–15]. Recent studies have increasingly focused on prosthesis-related factors that influence patient satisfaction. This shift highlights the fact that mechanical complications commonly observed in implant-supported prostheses may negatively impact overall treatment success [16]. Despite this growing body of evidence, the influence of attachment design and the number of supporting implants on prosthesis-related complications in IODs remains insufficiently clarified, representing an important gap in the existing literature.
The aim of this study was to retrospectively evaluate different treatment options for IODs, specifically by examining the use of splinted and unsplinted attachment systems and the number of supporting implants. This entailed identifying the types of complications encountered and providing insights for clinical applications. The null hypothesis of the study is that there is no statistically significant difference in treatment success or prosthetic complications between splinted and unsplinted attachment systems or on the basis of the number of supporting implants in IOD applications.
Materials and methods
Study design and participants
This retrospective study was approved by the Non-Interventional Clinical Research Ethics Committee (Project No: D-KA22/13). Patients who applied to the Department of Prosthodontics at Başkent University Faculty of Dentistry between January 2014 and January 2022 and whose treatment data were recorded in the NUCLEUS MBS patient management system and the PACS imaging system were included. The inclusion criteria were the use of overdentures supported by at least two implants in the mandible or four implants in the maxilla and the completion of regular follow-ups for at least five years. Cases involving dual bars in the posterior regions, as well as patients who missed regular check-ups or lacked five-year follow-up data, were excluded.
The study population consisted of 360 patients aged 26–99 years. Initially, 1078 patients were screened. After applying the inclusion and exclusion criteria, 360 patients were eligible.
The patients were divided into two main groups on the basis of implant number (2, 3, or 4 implants) and attachment design (splinted or unsplinted). Subgroups were created to evaluate the combined effect of these variables. The number of implants and splint preference were retrospectively verified from patient records and panoramic radiographs. All prostheses were categorized accordingly, allowing for a comparative analysis of complication rates across both implant number and attachment design groups.
Clinical and radiographic data collection
Complication data from the five-year follow-up period were retrospectively evaluated from patient records. According to these records, control sessions were conducted regularly during the first two years and subsequently during the 3rd and 5th years, with all examinations performed by the same physician. The records also confirmed that panoramic radiographs were taken preoperatively, postoperatively, after prosthetic loading, and at each follow-up visit. The number of implants and the splint preference were verified using both anamnesis records and radiographic images.
On the basis of the retrospective review of records, intraoral and extraoral examinations were performed during control sessions to evaluate occlusal harmony, prosthesis adaptation, soft tissue health, and patient complaints. The documented complications included loss of retention and the need for relining, denture base fracture, detachment of housing parts, deformation of retentive elements, attachment fracture or wear, screw loosening or fracture, and implant fracture or loss. Initial posttreatment complaints such as mucosal irritation were not included in the analysis.
Statistical analysis
Statistical analyses were performed using SPSS Statistics version 25.0 (IBM Corp., Armonk, NY, USA). A priori sample size estimation was performed using G*Power 3.1. Based on a medium effect size (w = 0.30), α = 0.05 and a power of 0.80, the minimum required sample size was calculated as 346 participants; the final sample (n = 360) provided adequate power for the planned comparisons. Categorical variables were summarized as frequencies and percentages. The Pearson chi-square test was used to examine differences in complication rates according to implant number and attachment type. Multiple response tests were used to evaluate the distribution of complication types by year. A p value of < 0.05 was considered to indicate statistical significance.
Results
Based on a retrospective analysis of patient records over a five-year period, a total of 360 patients (222 women, 138 men; aged 26–99 years) with 819 implants were evaluated. Most overdentures were supported by two implants (81.7%), whereas fewer were supported by three or four implants. Unsplinted attachments were used in 76.4% of the cases, and splinted attachments were used in 23.6%. The baseline characteristics of the study population are summarized in Table 1.
Table 1.
Baseline characteristics of the study population (n = 360)
| Variable | Value |
|---|---|
| Age range (years) | 26–99 |
| Sex, n (%) | Female: 222 (61.6%) |
| Male: 138 (38.3%) | |
| Total number of implants | 819 |
| Implant number per overdenture, n (%) | 2 implants: 294 (81.7%) |
| 3 implants: 33 (9.2%) | |
| 4 implants: 33 (9.2%) | |
| Attachment type, n (%) | Unsplinted: 275 (76.4%) |
| Splinted: 85 (23.6%) |
Values are presented as number (percentage). Age is expressed as range. Percentages for implant number and attachment type are calculated per prosthesis
Overall, 229 patients (63.6%) experienced no complications, while 131 patients (36.4%) experienced at least one complication. The number of implants was not significantly associated with the complication rate (p = 0.865; Table 2). In contrast, attachment design significantly influenced outcomes, with unsplinted overdentures showing higher complication rates than splinted systems (p = 0.016; Table 3).
Table 2.
Complications according to the number of supporting implants
| Implant number | Complication present, n(%) | Complication absent, n(%) | p value |
|---|---|---|---|
| 2 implants | 109 (83.2) | 185 (80.8) | |
| 3 implants | 11 (8.4) | 22 (9.6) | 0.865 |
| 4 implants | 11 (8.4) | 22 (9.6) |
Chi-square test, significance level set at p < 0.05
Values are presented as the number of cases (percentage)
Table 3.
Complications according to attachment type
| Attachment type | With complication n(%) | Without complication n(%) | p value |
|---|---|---|---|
| Unsplinted | 109 (83.2) | 166 (72.5) | |
| Splinted | 22 (16.8) | 63 (27.5) | 0.016* |
Chi-square test, significance at p < 0.05
Across all groups and time intervals, retentive element deformation was the most frequent complication, followed by loss of retention, whereas screw fracture was rare (Table 4). This trend was consistent regardless of the number of implants (Table 5). When evaluated by attachment design, unsplinted overdentures were primarily affected by retentive element deformation, whereas splinted overdentures showed a predominance of retention loss (Table 6).
Table 4.
Complications by follow-up period (first 2 vs. 3–5 years) n: complication number
| Complications | First 2 years n(%) | 3rd–5th years n(%) |
|---|---|---|
| Loss of retention | 36 (27.1) | 11 (25.6) |
| Fracture of denture base | 10 (7.5) | 6 (14.0) |
| Detachment of housing | 14(10.5) | 4 (9.3) |
|
Deformation of retentive Part |
52 (39.1) | 13 (30.2) |
| Fracture of attachment | 5 (3.8) | 1 (2.3) |
| Wear of attachment | 8 (6.0) | 5 (11.6) |
| Loss of implant | 2 (1.5) | 2 (4.7) |
| Screw loosening | 5 (3.8) | 1 (2.3) |
| Screw fracture | 1 (0.8) | 0 (0.0) |
| Total | 133 (100) | 43 (100) |
Table 5.
Complications according to implant number and follow-up period
| Complication | First 2 years | 3rd–5th years | ||||
|---|---|---|---|---|---|---|
| 2 implants n(%) | 3 implants n(%) | 4 implants n(%) | 2 implants n(%) | 3 implants n(%) | 4 implants n(%) | |
| Loss of retention | 29 (32.2) | 3 (30.0) | 4 (36.4) | 10 (29.4) | 0 (0.0) | 1 (50.0) |
| Fracture of denture base | 8 (8.9) | 1 (10.0) | 1 (9.4) | 6 (17.6) | 0 (0.0) | 0 (0.0) |
| Detachment of housing | 12 (13.3) | 2 (20.0) | 0 (0.0) | 4 (11.8) | 0 (0.0) | 0 (0.0) |
| Deformation of retentive part | 40 (44.4) | 7 (70.0) | 5 (45.5) | 11 (32.4) | 2 (100.0) | 0(0.0) |
| Fracture of attachment | 5 (5.6) | 0 (0.0) | 0 (0.0) | 1 (2.9) | 0 (0.0) | 0 (0.0) |
| Wear of attachment | 7 (7.8) | 1 (10.0) | 0 (0.0) | 4 (11.8) | 0 (0.0) | 1 (50.0) |
| Loss of implant | 1 (1.1) | 0 (0.0) | 1 (9.1) | 2 (5.9) | 0 (0.0) | 0 (0.0) |
| Screw loosening | 4 (4.4) | 0 (0.0) | 1 (9.1) | 1 (2.9) | 0 (0.0) | 0 (0.0) |
| Screw fracture | 0 (0.0) | 0 (0.0) | 1 (9.1) | 0 (0.0) | 0 (0.0) | 0 (0.0) |
| Total prostheses with complications | 90 | 10 | 11 | 34 | 2 | 2 |
n: number of complications
Table 6.
Complications according to attachment type and follow-up period
| Complication | First 2 years | 3rd–5rd years | ||
|---|---|---|---|---|
| Unsplinted n(%) | Splinted n(%) | Unsplinted n(%) | Splinted n(%) | |
| Loss of retention | 26 (29.2) | 10 (47.6) | 8 (23.5) | 3 (75) |
| Fracture of denture base | 9 (10.1) | 1 (4.8) | 6 (17.6) | 0 (0.0) |
| Detachment of housing | 13(14.6) | 1 (4.8) | 4 (11.8) | 0 (0.0) |
| Deformation of retentive part | 42 (47.2) | 9 (42.9) | 12 (35.3) | 1 (25) |
| Fracture of attachment | 5 (5.6) | 0 (0.0) | 1 (2.9) | 0 (0.0) |
| Wear of attachment | 6 (6.7) | 2 (9.5) | 5 (14.7) | 0 (0.0) |
| Loss of implant | 2 (2.2) | 0 (0.0) | 2 (5.9) | 0 (0.0) |
| Screw loosening | 2 (2.2) | 3 (14.3) | 1 (2.9) | 0 (0.0) |
| Screw fracture | 0 (0.0) | 1 (4.8) | 0 (0.0) | 0 (0.0) |
| Total prostheses with complications | 89 | 21 | 34 | 4 |
n: number of complications
Discussion
In this retrospective study, prosthetic complications in IODs were evaluated in relation to implant number and attachment design. The main findings were as follows:
the number of implants did not significantly influence overall complication rates.
attachment design had a statistically significant effect, with unsplinted systems demonstrating higher complication rates.
the most common complication across all groups and time periods was retentive element deformation, followed by loss of retention, whereas screw fracture was the least frequent complication.
Several previous studies reporting that increasing the number of implants does not necessarily reduce mechanical complications and that two implants remain a predictable and cost-effective option for mandibular overdentures [17–19]. Although Srivastava et al. [20] demonstrated in a biomechanical analysis that adding a midline implant could reduce rotational movements and improve load distribution, the higher rate of retentive element deformation in our 3-implant group suggests that such biomechanical advantages may not always translate directly into clinical outcomes. This discrepancy may be related to patient-specific factors such as occlusal forces, insertion–removal frequency, or differences in component wear patterns that are not captured in simulation models.
Leão et al. [21], Ülkü et al. [14], and Di Francesco et al. [12], who reported that unsplinted systems are more prone to component wear, whereas splinted systems, although mechanically more stable, may be associated with long-term retention loss. Our observations are in agreement with these reports, particularly regarding the predominance of retentive element deformation in unsplinted systems and the greater tendency for retention loss in bar-retained overdentures.
A plausible explanation is that solitary attachments concentrate functional loads on individual matrices, accelerating deformation, whereas bar systems distribute forces across implants but may gradually develop misfit and wear along the framework, eventually compromising retention. In addition, the more complex laboratory procedures and hygiene challenges associated with bar designs should be weighed during clinical decision-making.
Consistent with the findings of Ülkü et al. [14], Leão et al. [21], and Dhillon et al. [16], retentive element deformation was most common during the early follow-up period, whereas loss of retention became more prominent over time. The progressive nature of retention loss likely reflects cumulative wear of matrices and housings. Meanwhile, the low incidence of screw fracture may indicate that regular maintenance visits allowed early management of loosening before structural failure occurred. From a clinical perspective, these observations emphasize the maintenance-dependent character of overdentures and the importance of long-term follow-up.
Taken together, these findings highlight that attachment type may play a more critical role than implant number in determining the complication profile of IODs. Clinically, this highlights the importance of attachment selection, patient education, and scheduled maintenance visits to reduce long-term complications and ensure prosthesis success.
This study has several limitations. Its retrospective and single-center design limits the generalizability of the findings. The relatively small sample size in the 3-implant group reduced the statistical power for this subgroup. Biological complications were not included, and the follow-up period was restricted to five years. Additionally, patient-related factors such as smoking, systemic conditions, and parafunctional habits could not be fully standardized. Another limitation is that complications were not analyzed according to demographic variables such as sex and age. Although demographic information was recorded, subgroup analyses could not be performed due to insufficient stratified data. Future studies with larger and more balanced populations are needed to clarify the potential influence of these variables on prosthetic outcomes.
Conclusion
Within the limitations of this retrospective study, attachment design was found to play a more critical role than implant number in determining the prosthetic complications of implant-supported overdentures. Two-implant overdentures remain a predictable and cost-effective treatment option. These findings highlight the importance of attachment selection, patient education, and regular maintenance for long-term treatment success. Future prospective, multicenter studies with longer follow-up periods are needed to validate these results.
Acknowledgements
Not applicable.
Author contributions
Ezgi Erdenol: Data collection, methodology design, statistical analysis, interpretation of results supervision, manuscript writing.Ecem Sancar: Manuscript writing.Ozge Celik: Conceptualization, supervision, Manuscript writing.Selim Erkut: Project administration, supervision, and final approval of the manuscript.
Funding
No funding was received for this study.
Data availability
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
Ethical approval for this retrospective study was obtained from the Non-Interventional Clinical Research Ethics Committee of Başkent University, Faculty of Dentistry, Ankara, Türkiye (Project No: D-KA22/13). The study was conducted in accordance with the principles of the Declaration of Helsinki. As this study involved only retrospective data analysis and did not include any direct patient intervention, the requirement for informed consent was waived by the ethics committee.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.