Immediate vs Conventional Loading of Mandibular Overdentures: A Comprehensive Systematic Review and Meta-Analysis of Randomized Controlled Trials

Edentulism is a disability that is highly prevalent in the elderly population around the world. Around 10% of the adult population in the United States is completely edentulous and this percentage is expected to rise owing to an increase in life expectancy.^1,2  For decades, the choice of rehabilitation of edentulous patients has been complete dentures but owing to difficulties in achieving retention, stability, and support in a severely resorbed mandible, implant-supported overdentures have become the mainstay of treatment in such patients.^3,4  Heydecke et al⁴  in a clinical trial comparing implant-supported overdentures with conventional dentures have shown that pretreatment patient expectations are largely met with implant-supported overdentures compared to the conventional prosthesis. It is now well accepted that mandibular overdentures supported by 2 anterior implants is a simple and economical treatment option for edentulous patients.³  However, in an attempt to improve stability, researchers have also used 3 or 4 implants in the interforaminal region to support mandibular overdentures.⁵  On the other hand, recent long-term studies have demonstrated that even a single mandibular midline implant may provide adequate retention to the mandibular overdenture and may be more economical to the patient.⁶ 

In the early days of oral implantology, an unloaded healing time of 3 months for the mandible and 6 months for the maxilla was advocated to facilitate osteointegration.⁷  This traditional 2-stage surgical technique was based on the hypothesis that a stress-free environment is imperative in achieving successful osseointegration.⁵  It was believed that early loading would lead to soft tissue encapsulation, thereby preventing osseointegration and decreasing implant survival.⁷  However, the intervening period between placement of implants and delivery of final prosthesis is often uncomfortable for patients since the esthetics and function offered by a provisional prosthesis are usually suboptimal.⁸  Immediate loading of implants would not only shorten treatment time but also increase patient satisfaction by providing early restoration of function.⁸  In this context, there has been considerable research on the outcomes after immediate loading of implants for mandibular overdentures.^9–11  For inclusion of the immediate loading protocol in routine clinical practice it is essential that outcomes such as implant survival and marginal bone levels are not inferior to the conventional loading protocol.

To date, several systematic reviews and meta-analysis studies have attempted to compare treatment outcomes after immediate loading vs conventional loading of mandibular overdentures.^8–10,12,13  Evidence from reviews published 2010 is, however, limited owing to the inclusion of only 3 randomized controlled trials (RCTs) comparing immediate loading with conventional loading protocols.^8,10  Sanda et al,¹³  in a 2017 meta-analysis, pooled 4 trials comparing immediate and conventional loading but the included studies reported differences in the number of implants used per patient. In another study, Helmy et al⁹  have assessed evidence from 4 RCTs comparing the immediate and delayed loading, but the study was restricted to trials using only 2 unsplinted implants. Thus, to the best of our knowledge, no study has conducted a detailed analysis of Level 1 evidence on clinical outcomes after immediate vs delayed loading of mandibular overdentures. Therefore, this study aimed to perform a systematic literature search of all RCTs comparing immediate and conventional loading of mandibular overdentures irrespective of the number of implants and conduct a meta-analysis of implant failure and marginal bone loss (MBL).

The PICOS (Population, Intervention, Comparison, Outcome, and Study design) outline was used for including studies in the review. We included only RCTs carried out on patients receiving mandibular overdentures (Population). The intervention was to be immediate loading of dental implants compared to the conventional loading protocol. Outcomes measured were to be implant failure and/or MBL. There was no restriction placed on the number, diameter, and splinting of implants in the included studies. Immediate loading was defined as placement of overdenture in occlusion with the opposite dentition within 3 days of implant placement with a 1-stage implant placement protocol.⁵  Conventional loading was defined as a 2-stage implant placement protocol wherein the overdenture is attached over the implant after a healing period of 3 to 6 months. The following studies were excluded from the review: (1) Studies with follow-up of fewer than 12 months; (2) studies comparing early loading with conventional loading or immediate loading with early loading; (3) Non-randomized trials, single-arm studies, retrospective studies, case series, and case reports; and (4) Studies not reporting implant failure or MBL.

Two independent reviewers performed an electronic search of PubMed, ScienceDirect, Ovoid, Springer, and Google Scholar databases up to April 1, 2020 using the following search terms: “implant”; “dental implant”; “immediate loading”; “loading”; and “overdenture.” No language restriction was placed for the literature search. The search strategy and results of the PubMed database are presented in Supplementary Table 1. References of included studies were hand searched for identification of any missed out studies. After evaluating the studies at the title and abstract level, full texts of selected articles were scanned for inclusion in the review. Any disagreements were resolved by discussion. Guidelines of the PRISMA statement (Preferred Reporting Items for Systematic Reviews and Meta-analyses)¹⁴  and Cochrane Handbook for Systematic Reviews of Intervention¹⁵  were followed during the conduct of this review.

Data were extracted from the RCTs by 2 reviewers independently. The following details were obtained using a pre-prepared data collection form: authors, publication year, implant system, sample size, demographic details, number of implants per patient, use of implant splinting, type of prosthesis attachment, loading time, follow-up period, and outcome data. The primary outcome was the implant failure. The secondary outcome was MBL. Any other outcomes reported by the included trials were also extracted and presented in a descriptive form.

All included RCTs were assessed for bias using the Cochrane Collaboration risk assessment tool.¹⁶  Studies were rated as low risk, high risk, or unclear risk of bias for each of the following variables: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other biases.

Intention-to-treat data was used for the conduct of this meta-analysis. Studies were grouped based on the number of implants used per patient. Data on implant failure were summarized using risk difference (RD) and 95% confidence intervals (CI) while MBL was pooled using mean difference (MD) and 95% CI. Review Manager (RevMan, version 5.3, Nordic Cochrane Centre) was used for the meta-analysis. We used a random-effects model to calculate the pooled effect size. Heterogeneity was calculated using the I²  statistic. I²  values of 25%–50% represented low, values of 50%–75% medium and >75% represented substantial heterogeneity. A meta-analysis was conducted when the primary and secondary outcomes were reported by at least 3 coherent studies. Publication bias was to be assessed by funnel plots if more than 10 studies were included in a meta-analysis.

The study flow chart is presented in Figure 1. Twenty-four studies were selected for full-text review. Six studies were excluded as 3 trials^17–19  compared early loading vs conventional loading, 1 trial²⁰  compared immediate vs early loading, 1 study²¹  had a follow-up of less than 12 months and 1 study²²  did not divide the sample into immediate and conventional loading groups. A total of 13 trials corresponding to 18 publications were included in the review.^23–40  Only 1 trial³³  compared immediate and delayed loading of single implant-supported overdenture. Seven trials^{23,28,34,36,37,39,40}  used 2 implants, 1 trial²⁷  used 3 implants while 4 trials^29–32  used 4 implants.

Details of studies using 2 implants for the mandibular overdenture are presented in Table 1. None of the included trials splinted the 2 implants. The type of attachments included ball, locator, magnetic, and stud. The sample size varied from 9 to 18 patients per arm, while follow-up ranged from 12 to 36 months. Narrow diameter implants were used in the trial of Schuster et al.²³  All trials loaded the implants on the day placement for the immediate loading group and after 3 months in the conventional loading group. Characteristics of studies using 4 implants are presented in Table 2. Three studies^29,31,32  splinted the mandibular implants. The sample size varied from 8 to 10 patients per arm and the follow-up ranged from 24 to 36 months. For single³³  and 3-implant²⁷  supported mandibular overdentures, only single studies have compared immediate and delayed loading by means of an RCT. Details of these studies are presented in Table 3. Kern et al³³  compared the immediate loading of single mandibular implant-supported overdentures in a large study of 158 patients with a follow-up of 24 months. On the other hand, Stephan et al²⁷  compared the 2 loading protocols in patients with 3 splinted mandibular implants with a total follow-up of 24 months. All of the included 13 studies reported data on implant failure while 11 trials reported MBL as one of the outcomes.

Data of 86 patients in the immediate loading group and 85 patients in the conventional loading group were pooled for a meta-analysis on implant failure with 2-implant supported overdentures. Our analysis indicated no statistically significant difference in implant failure with either loading protocols (RD: -0.02, 95% CI: -0.11, 0.06, I² = 9%, P = .56) (Figure 2). Data from 6 trials^{23,28,34,37,39,40}  using 2 implants, with 76 patients each in the immediate and conventional loading groups, was pooled for the meta-analysis of MBL. Our results indicated a lack of any statistically significant difference between the 2 study groups (MD: 0.07, 95% CI: −0.10, 0.24, I² = 70%, P = .42) (Figure 3).

For studies using four implants, data of the 3 studies^29,31,32  using splinted implants were pooled for the meta-analysis. Our results indicated that immediate loading did not result in a statistically significant increase in the risk of implant failure compared to conventional loading (RD: −0.04, 95% CI: −0.18, 0.11, I² = 0%, P = .61) (Figure 3). Similarly, no statistically significant difference was noted in the MBL with either group (MD: 0.06, 95% CI: −0.09, 0.20, I² = 0%, P = .44) (Figure 4).

Apart from the pre-defined primary and secondary outcomes of this review, several other outcomes were reported by the included studies. Details of such outcomes in trials using 2 implants are presented in Table 4. Peri-implant tissue variables like plaque index, gingival index, bleeding on probing, probing depths, and length of keratinized mucosa were reported by different included trials. None of the studies reported statistically significant differences for any of these parameters between the immediate and conventional loading groups, except for 2 studies. Elsyad et al⁴⁰  reported significantly higher probing depths at labial and distal sites in immediate loading group at 12 and 36 months, while Schuster et al²³  reported significantly higher probing depth in the conventional loading group at 12 months. Two studies^28,40  reported better implant stability, measured by Periotest (Siemens) or Ostell (Integration Diagnostics AB), in the immediate loading group at 12 months of follow-up with one⁴⁰  of them reporting no difference on long-term follow-up of 36 months. No such difference was noted by 3 other trials^23,34,37  using 2 implants. Quality of life (QoL) outcomes were assessed by 2 studies^23,36  and both reported better QoL in the immediate loading group in the early follow-up periods.

Other outcomes reported by studies using 4 implants are presented in Table 5. Peri-implant tissue variables were not found to be significantly different in the immediate and conventional loading groups by 2 studies.^31,32  No difference in implant stability between the 2 groups was noted by 3 studies^30–32  on long-term follow-up.

Details of similar other outcomes reported by studies using 1 or 3 implants are presented in Table 6. No statistically significant difference was noted in any peri-implant tissue variables by the 2 studies.^27,33  Post-operative pain and discomfort were significantly higher and fracture of the denture bases was more frequently noted with immediate loading of single implants in the study of Kern et al.³³ 

The authors' judgment of risk of bias in included studies is presented in Figure 6 and the overall risk of bias per domain of all studies is presented in Figure 7. Appropriate methods of randomization per reported by all except for 5 studies.^{27–29,31,32}  Allocation concealment was adequately reported by 3 studies.^23,36,39  Due to the nature of the intervention, participants and operators could not be blinded. Blinding of outcome assessment was reported by only 2 studies.^39,40  Only 1 trial was pre-registered.³³  The overall risk of bias in the RCTs was moderate.

This systematic review and meta-analysis aimed to provide a detailed overview of high-level evidence comparing immediate loading and conventional loading of mandibular overdentures, irrespective of the number of implants used. The results of our meta-analysis indicate that there is no difference in implant failure or MBL with immediate or delayed loading when 2 unsplinted or 4 splinted implants are used for the overdenture. Also, descriptive analysis indicates that there may be no difference in peri-implant tissue indices and implant stability on long-term follow-up with either loading protocols with 2- or 4-implant-supported overdentures. Evidence for single and 3-implant supported overdentures was restricted to only single RCTs.

The popularity of immediate loading of dental implants for either single, partial, or complete denture prosthesis stems from the fact that immediate loading enables earlier restoration of esthetics and function and drastically reduces the treatment time for the patient as well as the clinician. Immediate loading, however, is not frequently used by clinicians for all cases owing to the uncertainty whether similar outcomes can be achieved compared to conventional loading.⁴¹  The risk of implant failure is an important variable with immediate vs conventional loading of dental implants although previous meta-analytic studies have demonstrated contrasting results. In a 2015 study, Sanz-Sanchez et al⁴²  pooled data from RCTs using all types of prosthesis and reported a higher risk of implant failure with immediate loading compared to conventional loading. Chen et al⁴¹  in a recent meta-analysis of 39 trials using fixed prosthesis have also reported lower survival rates with immediate loading of dental implants. On the other hand, other reviews have found no significant difference in implant survival with either loading protocols.^43,44  Such variation in literature can be partially attributed to the difference in the prosthesis types used in the included studies. In our analysis of 171 patients with 342 implants in studies with 2-implant supported overdentures, we found no statistically significant difference in implant failure. Detailed analysis of the forest plot revealed that a statistically significant difference in implant failure was not reported by any of the RCT, even the one employing narrow diameter implant.²³  Our results are similar to those of Helmy et al,⁹  which, too, did not report any significant difference in implant failure in an analysis of 103 patients with unsplinted 2-implant supported overdentures.

While 2 unsplinted implants are now commonly used to support mandibular overdentures, clinicians have also used single, 3, and 4 implants in the interforaminal region to support the removable prosthesis. Studies comparing 2 and 4 implants for overdentures have indicated no difference in implant survival, MBL, and patient satisfaction with either treatment modalities.^45,46  Also, while earlier in vitro studies reported that splinting of implants may be favorable from a mechanical point of view,⁴⁷  clinical studies have shown no difference between splinted and non-splinted implants.^48,49  Despite evidence on the lack of difference in clinical outcomes, our review pooled data only from coherent RCTs to provide clear evidence and reduce interstudy heterogeneity. We found no difference in the risk of implant failure with splinted 4-implant supported overdentures. The lone RCTs evaluating the loading protocols with unsplinted 4-implant supported and 3-implant supported overdentures also did not find any significant difference in implant failure.^27,33  However, Kern et al³³  in their large RCT on single-implant supported overdenture has reported a significantly higher risk of implant failure with immediate loading compared to conventional loading. This difference is because as compared to 2 or more implants, a single implant cannot restrict the direction of movement of the overdenture and may act as a rotational axis resulting in strong extra-axial loading and subsequent failure with immediate loading.^33,50 

After immediate loading, micromotions occur at the bone-implant interface, which can affect the osteointegration process.⁵¹  While excessive micromotions lead to fibrous encapsulation of the implant, smaller micromotions allow differentiation of tissue into bone.⁵²  Therefore, monitoring of MBL around the implant has been an important indicator for measuring implant success.¹¹  In our secondary analysis, we found no difference in MBL with immediate or delayed loading protocols for both unsplinted 2-implant supported overdentures and splinted 4-implant supported overdentures. Stephan et al²⁷  using 3-implant supported overdentures also reported no significant difference in MBL after 24 months of follow-up. Our results are consistent with previous systematic reviews assessing outcomes of immediate loading for implant-supported fixed prosthesis,⁴¹  single anterior implants,⁴³  and unsplinted 2-implant supported overdentures.⁹  The lack of difference in MBL indicates that irrespective of the loading protocol, marginal bone has similar behavior post-osseointegration.

On descriptive analysis of other outcomes, the included studies reported no difference in peri-implant tissue variables like plaque index, gingival index, bleeding on probing, and so on, irrespective of the number of implants. Studies also compared implant stability using Periotest (Siemens) or by measuring the implant stability quotient by Osstell (Integration Diagnostics AB). With an exception of Kutkut et al,²⁸  no study reported differences in implant stability in the immediate or delayed loading groups at the last follow-up period. The statistically significant difference in the study of Kutkut et al²⁸  may be due to the lack of extended follow-up in their trial. As for patient-related outcomes, only 3 studies^23,33,36  compared QoL outcomes while only 2 studies^23,36  assessed masticatory performance in our review. Studies did not report statistically significant differences in QoL outcomes between the loading protocols, but 2 studies^23,36  reported earlier improvement of QoL with immediate loading. This can be attributed to the earlier provisioning of the prosthesis in the immediate loading group, which significantly affects patient outcomes. However, no difference in masticatory performance was noted between the 2 groups.

Compared to previous systematic reviews and meta-analysis on immediate and delayed loading of mandibular overdentures, our study has important differences. Foremost, previous reviews have either clubbed outcomes of studies irrespective of the number of implants per arch^10,13  or have focused solely on studies with a fixed number of implants.⁹  On the other hand, our review included all RCTs irrespective of the number of implants while performing a meta-analysis only for coherent studies. Secondly, past reviews on mandibular overdentures have assessed only implant survival and MBL.^9,10,13  The current study provides an overview of all outcomes compared to date in RCTs. Lastly, our study is the most up to date evidence on the subject with 3 new studies added from 2017 to 2020.

The results of our review, however, should be interpreted with the following limitations. The overall quality of studies was moderate and the majority of the studies were of a small sample size. Blinding of outcomes assessment was reported only by 2 studies. This may have resulted in observer bias, especially for the secondary outcome. There was methodological heterogeneity in the included studies with differences in implant systems, prosthesis attachments, and follow-up period. This may have influenced the results of our meta-analysis. Also, the survival of implants can be dependent on many other factors like quality of bone, surgical skills, patients' oral hygiene status, etc. These factors were not taken into consideration in our analysis.

To conclude, our meta-analysis indicates that there may be no difference in implant failure and MBL with immediate loading or conventional loading of 2 and 4 implant-supported mandibular overdentures. The literature review indicates that there may be no difference in peri-implant tissue indices, implant stability, and QoL outcomes between the 2 loading protocols. The overall quality of evidence is moderate and studies for 1-, 3-, and 4-implant supported overdentures are limited. Further, adequately powered RCTs are required to strengthen the evidence.

The authors declare that they have no competing interests.