Because implant denture becomes an excellent modality for edentulous or partially edentulous patients, the purpose of reducing treatment time and maximizing patient comfort has urged the dentists to explore different surgical techniques in placing implants, and the issue about which is the best technique remains controversial. Typically, a flap is elevated to get better visualization of implanting site and to reduce the risk of bone perforations and fenestrations in placing implant.1 In addition, flapping can also help to protect some important anatomical landmarks (eg, foramina, lingual undercuts, maxillary sinuses) from being injured accidentally. However, during the initial healing stage, flap procedures may decrease supraperiosteal blood supply and theoretically lead to bone loss along with patient discomfort, including pain, bleeding, and edema.2
Inserting an implant with a flapless procedure was first reported by Campelo in 2002.2 In such a procedure, dental implant was inserted through the mucosal tissues without elevating a flap. Flapless procedures have several merits, including less surgical trauma, fewer postsurgical complications, higher patient satisfaction, shorter operative time, better postsurgical healing, and so on.3 In addition, relevant literature had revealed that flapless surgery may help extend the length of the junctional epithelium to coronal, which was an effective way to prevent periimplantitis.3 Nevertheless, because flapless procedure was typically a “blind” surgery, care must be taken to avoid bone fenestrations or perforations; hence, Campelo indicated that flapless procedure was restricted to selected sites where a bone width over 7 mm was available.2
Though bunches of relevant clinical trials were conducted to compare the clinical outcomes of different procedures for implant placement (flapped or flapless), no definitive conclusion was able to be drew from these studies due to the lack of statistical power. Therefore, a meta-analysis of published clinical trials was conducted to test the null hypothesis of no difference between failure rate and marginal bone loss of dental implants placed by flapless and flapped procedures.
Materials and Methods
This meta-analysis followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis protocols 2015 statement guidelines.4
A search for literature within 10 years was undertaken through December 2016 in PubMed, Web of Knowledge, and Cochrane Library, using the following key words: (implant OR implants) AND (flapless OR flapped OR flap) AND (oral OR dental).
To be included, studies should meet the following eligibility criteria: human clinical studies, either randomized or not, with the data on comparison of implant failure rate and marginal bone loss between flapless and flapped procedure, with a minimum of 10 implants in either group. Exclusion criteria included review articles, case reports, animal studies, and in vitro studies.
The identified publications were reviewed for their relevance to the research topic by 2 independent authors (Z.J.B. and Z.D.). The reference lists of the identified studies along with the relevant reviews and meta-analysis were also manually searched to identify additional studies.
A standardized protocol and reporting form were used to extract the relevant data from the included publications: the first author, the year of publication, study design, number of patients, number of placed and failed implants, marginal bone loss, patient's age, healing strategies, follow-up period, and loading time.
The primary outcome of this meta-analysis was the failure rate of the implants, with marginal bone loss as the secondary outcome. The failure rates were treated as the dichotomous measures, whereas the marginal bone loss was regarded as a continuous variable. Moreover, different loading time was used to stratify the subgroup in the analysis of the failure rate of the implants. The statistical units for “failure rate” and “marginal bone loss” were the implants rather than patients. The estimates of relative effect were expressed in risk ratio (RR) in relation to dichotomous outcomes and in mean difference (MD) in millimeters for continuous outcomes, both with a 95% confidence interval (CI). Heterogeneity among studies was assessed with the I2 statistics, with 25% corresponding to low heterogeneity, 50% to moderate, and 75% to high.5 High heterogeneity led to random-effects models, whereas fixed-effects models suited the opposite.6 Results from flapless and flapped procedures were graphically represented along with their CIs in a “forest plot.” A funnel plot shall be drawn to detect the publication bias corresponding to sample size.6
The data were analyzed using the statistical software Review Manager (Version 5.2.8; the Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark). It was considered statistically significant when 2-sided P values were less than 0.05.
After the search strategy, a total of 2717 records were identified, and the selection process is summarized in Figure 1. One thousand eighty records were excluded for duplicated titles and abstracts, with 1637 records retained. After screening the titles and abstracts, 1594 records were further excluded based on the prespecified inclusion and exclusion criteria. Consequently, 43 records were suitable for full-text screening. After full-text screening, 14 articles that did not meet the requirement were further excluded, of which 5 did not have control group, 1 was an updated study, 3 were animal experiments, 5 were reviews or meta-analysis. In addition, 2 articles were considered eligible after screening the reference lists of selected studies. Eventually, a total of 31 publications were included in the meta-analysis.
Description of the Studies
These included studies consisted of 9 retrospective studies and 22 prospective studies. The descriptive details of these studies are listed in Table 1. A total of 1681 patients with 4138 implants were included in the meta-analysis. The age of the patients ranged from 16 to 89 years. Of the 31 included studies, 28 shared at least 12-month follow-up time and 3 only had a follow-up time up to 3 months.7–9 Of the included studies, 7 studies involved implant placement for single-tooth missing,10–16 and the rest studies placed implants in edentulous or partially edentulous jaws. Fourteen studies included smokers of varying degrees,7,15,17–27 2 studies included several controlled diabetic patients,14,24 whereas 1 study focused on postmenopause patients.28 Meanwhile, with regard to healing strategies, some used a submerged protocol,1,10,14 whereas some used a nonsubmerged protocol.9,16,22,28,29 In addition, information about loading time was also provided in these studies. In 8 studies, implants were loaded within 2 weeks after the placement, both in flapless and flapped groups.13,15,17,22,24,25,30,31 Thirteen studies applied a delayed loading protocol,1,10,12,14,16,21,24,26–28,32–34 whereas 6 studies18,19,23,24,30,31 involved both immediate/early loading and delayed loading.
Among the 31 studies, 1970 implants were placed through a flapless procedure, with a failure of 63 implants; 2168 implants were placed through a flapped procedure, with a failure of 39 implants. No implant failure was found in 11 studies.7–9,11,12,16,18,21,23,27,33 None of these 31 studies showed a statistical difference regarding implant failure between the flapless and flapped procedures. Twenty-one studies provided information about marginal bone loss,1,7–13,16,21–23,25–32,34 among which only 5 showed a statistical difference.8,10,13,27,32
Quality assessment was performed on each trial, including the following aspects: random sequence generation, allocation concealment, blinding of outcome assessments, and incomplete outcome data. If all criteria were met, the study was considered to be at low risk of bias. If 2 or more criteria were not met, the study was classified as having a high risk of bias. The scores of each trial are summarized in Table 2. Among the 31 studies, 22 were considered of high risk of bias,1,7,8,10,13,15,17,19–21,23–28,30–35 3 were judged to be at moderate risk of bias,11,14,18 and 6 were at low risk of bias.9,12,16,22,29,36
In this study, failure rate of dental implants which suited the fixed-effects model was statistically affected by different insertion procedures (flapless vs flapped) of the implants and ended up with a RR of 1.70 (95% CI: 1.13–2.55; P = 0.01; heterogeneity: I2 = 0.0%; Pheterogeneity = 0.97; Fig. 2), indicating that placing an implant through a flapless procedure increased the risk of implant failure compared with using a flapped protocol. No evidence of publication bias was found in funnel plot (Fig. 3). Subgroup analysis indicated that in the situation of delayed loading, flapless or flapped procedure showed no statistical effect on the failure rate of dental implants (RR = 1.18; 95% CI: 0.59–2.38; P = 0.64; heterogeneity: I2 = 0.0%; Pheterogeneity = 0.87; Fig. 4). Whereas in the situation of immediate/early loading, implantation with flapless procedure increased the failure rate of dental implants, with a RR of 2.24 (95% CI: 1.05–4.78; P = 0.04; heterogeneity: I2 = 0.0%; Pheterogeneity = 0.91; Fig. 5).
The second outcome marginal bone loss, as a continuous outcome, suited the random-effects model because the heterogeneity was found. There was a statistical difference on the marginal bone loss, between flapless and flapped procedures, with a MD of −0.10 mm (flapless vs flapped; 95% CI: −0.18 to −0.02; P = 0.02; heterogeneity: I2 = 82%; Pheterogeneity = 0.00; Fig. 6), indicating that flapless procedure produced less marginal bone loss in comparison with flapped procedure.
Nowadays, it remains controversial over the difference on the failure rate between dental implants inserted by flapless and flapped procedures. Some held that flapless procedure can be applicable due to the development of computer-guided technology,19 whereas others concerned the uncertainty and the potential risk of this “blind” procedure. Though bunches of trials were performed on this issue, no statistical difference was found owing to small sample size or difference on the trial design. Therefore, the present meta-analysis aimed to pool the data of these individual studies together and to gain a higher statistical power for the measure of interest, thus getting a more specific conclusion.
To our knowledge, the latest meta-analysis concerning the outcome of the flapless and flapped implant placement was published in 2014 by Chrcanovic et al.37 They found a statistical difference between the failure rates of dental implants inserted by flapless and flapped procedure, with a RR of 1.75. In the current meta-analysis, it was identified that the risk of implant failure in flapless implantation procedure was 1.70 times higher than that in flapped implantation procedure, which was a little bit lower than the finding by Chrcanovic et al.37 One of the possible reasons for the higher implant failure rate in flapless implant surgery may be the interference of osseointegration, due to the potential contamination of the implant surface by the epithelial or connective cells coming from mucosa into the hole during the drilling process.19 Another possible reason is the poor vision during flapless procedure, as the soft tissue flap is not elevated. For this reason, the preoperative assessment of the quantity and morphology of the bone by computer tomographic scan is necessary for the flapless implant surgery. In recent years, the computer-guided implantation system and surgical template are more and more widely used to control the direction and depth of the implant and prevent the bone from dehiscence, making flapless implant surgery more predictable.19,20,23 The computer-guided implantation system19,38 is able to provide real-time imaging to adjust the position and angulation of the dental drill, thus protecting the critical anatomical structures and enables flapless implant surgery fully monitored.38 However, a meta-analysis conducted by Van Assche et al39 had revealed that there was a considerable positional and angular deviation in surgical template–guided implant placement, with a mean deviation of 0.99 and 1.24 mm for entry point and apex, respectively, along with a mean angular deviation of 3.8 degrees, which may also be responsible for the higher implant failure rate of flapless procedure. Therefore, with the limitation of the present techniques, there are prerequisites in selecting patients for flapless implantation. Campelo and Camara2 suggested a requirement of at least 7 mm bone width when placing implants by flapless procedure because the surgeons were unable to see the direction of the drill. Improper drill angulation often led to dehiscence or perforation of the cortical plates, especially at the anterior maxilla and the lingual side of the mandibular molar area.2 Moreover, it was reported that flapless procedure was considered optimal only in patients with adequate keratinized mucosa.16 Some studies demonstrated that proper width of keratinized mucosa, namely over 2 mm, was a necessary factor for maintaining periimplant health.40,41 It had been recognized that dental implants with narrow keratinized mucosa were prone to suffer from plaque accumulation, mucosal inflammation along with bleeding on probing (BOP), and experienced more bone loss.41 However, further clinical trials are required to compare the preservation of keratinized mucosa between flapless and flapped implant placement.
Although having the aforementioned limitations, flapless procedure does possess several exclusive merits. First of all, as Boardman et al42 suggested, flapless procedure yielded higher Pink Esthetic Score than flapped procedure, especially during immediate implant placement. Second, flapless procedure was beneficial to the health of periimplant soft tissue. Several studies3,9,16,43 found that flapless implantation produced better values concerning parameters such as probing depth (PD), gingival index, and BOP, compared with flapped implantation. One study performed on dogs showed that flapped group had higher length of the junctional epithelium and deeper zone of connective tissue integration than those of flapless group, which implied that the former had more apically positioned junctional epithelium and might eventually result in an increased PD around the implant.3 In general, high PD comes up with great risk of suffering periimplantitis; hence, flapless procedure with lower PD is beneficial to the health of periimplant tissue. Finally, flapless procedure comes up with better vascularized periimplant mucosa, which will directly result in sufficient blood supply and consequently lead to stronger resistance to inflammation and bacterial invasion.43,44
Different from the meta-analysis of Chrcanovic et al37 in 2014, the immediate/early loading and delayed loading are pooled separately to conduct a subgroup analysis in this article. Traditionally, dental implants were placed and left to a submerged healing period of 3 to 4 months. However, the demands on the shorter and more comfortable healing periods brought in the concept of immediate/early implant loading.24 Several literatures had revealed that immediate or early implant loading can be as effective as conventional delayed loading with a submerged healing period.18,22 Given this, subgroup analysis was carried out in the present meta-analysis to see if there was a difference on the failure rate between implants inserted by flapped or flapless procedures, in the situation of immediate/early loading or delayed loading. All the implants in the selected literatures of the immediate/early loading subgroup were loaded through a provisional or definitive prosthesis immediately or within 2 weeks after implantation. In the subgroup analysis concerning immediate/early loading, there was a statistical difference between the failure rates of dental implants inserted by flapless and flapped procedures, revealing that the flaplessly inserted and immediate/early loaded implants suffered higher failure rates, which was in accordance with several literatures' conclusion.17,25,31 However, our result contradicted with the opinion of several previous reports, which recommended flapless procedure to be used in immediate/early loaded implants,15,42,45 due to better esthetics and less treatment time. We analyzed the studies included in the present meta-analysis and found that 6 studies included in the subgroup analysis for immediate/early loading were performed on edentulous or partially edentulous patients, whereas only 2 studies13,15 were performed on single-implant patients. When these 2 studies were excluded, the result was still statistically significant. Additionally, all those studies that came up with results against ours were performed on single-implant patients as well. Therefore, we presumed that flaplessly inserted and immediate/early loaded implants may not suit edentulous or partially edentulous patients. Whereas, in the situation of delayed loading, there was no statistical difference between the failure rates of dental implants inserted by flapless and flapped procedures, indicating that both procedures were equally reliable when the implants were delayed loaded.
The meta-analysis published by Chrcanovic et al37 failed to illustrate the statistical difference in marginal bone loss between flapless and flapped procedures. In the present meta-analysis, 8 recently published studies13–16,25,27,28,32 were added. Among the 21 included studies evaluating bone changes, 15 concluded that flapless procedure came up with less bone loss, whereas the rest 6 studies took the opposite point of view.10,16,21,25,26,29 The result of the present meta-analysis showed that flapless procedure might reduce the bone loss to some extent, with a MD of −0.10 mm, compared with the flapped procedure. It is believed that good blood supply of periimplant tissue is an essential prerequisite to preserve the bone tissues around the implant and get a successful implant therapy outcome.2 In contrast to the flapped implant surgery that might jeopardize the branches of the supraperiosteal vessels during flapping procedure, the flapless implant surgery keeps the periosteum intact and retains the blood supply from periosteum.2 An experimental study performed on domestic pigs found that flapless implant placement gained better vascularization of periimplant mucosa in comparison with flapped surgery, which was beneficial to the regeneration of periimplant tissue.46 Therefore, flapless placement was better in preserving bone tissues around the implant.46 Besides, covering by the periosteum, the implant placed by flapless procedure may have a positive healing environment during the early bone remodeling process. This opinion was in agreement with the findings of Villa and Rangert,30 who demonstrated that flapless procedure took 6 months to stabilize the bone remodeling, whereas flapped procedure took 12 months.
The results of the present meta-analysis should be interpreted with caution, due to several limitations. To start with, among the 31 included studies, only 7 contained single-tooth implant placement, which may be a confounding factor for this meta-analysis. When these 7 studies were further excluded, the remaining studies were pooled and came up with a RR of 1.72, which was statistically significant. What is more, other confounding factors that may influence the failure rate and marginal bone loss of dental implants were not analyzed, for instance, types of implants, follow-up periods, healing strategy, patients with smoking habits or diabetes, and so on. Finally, 9 retrospective studies were included to acquire sufficient data, and the nature of retrospective study was inherently flawed.
This meta-analysis revealed that flapless procedure may increase the failure risk of the dental implants in comparison with flapped procedure, especially in the situation of immediate/early loading. Nevertheless, flapless procedure showed a superiority in preserving bone tissues over flapped procedure. However, the results must be interpreted with caution, as some uncontrolled confounding factors were presented in the included studies.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
Roles/Contributions by Authors
J. Zhuang: Look up the information; reviewing the included articles; statistical analysis; manuscript preparation. D. Zhao: Look up the information; reviewing the included articles; statistical analysis; manuscript preparation. Y. Wu: Look up the information; reviewing the included articles; statistical analysis. C. Xu: Study design; revise the manuscript.
Drs. J. Zhuang and D. Zhao contributed equally to this work. This work was supported by National Natural Science Foundation of China (Grant numbers 31470903, 31270991, 30900282); Shanghai Pujiang Program (Grant number 13PJD021); Science and Technology Commission of Shanghai (Grant numbers 10QA1404200, 08411961500, 07ZR14070); Shanghai Summit & Plateau Disciplines; Shanghai Leading Academic Discipline Project (Grant numbers S30206-sms02, T0202).
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