Comparative Efficacy of Different Flapless Ridge Preservation Techniques: A Systematic Review and Meta-Analysis

The peri-implant phenotype consists of site-specific hard and soft tissue components, and average threshold values have been identified to determine the minimal amount necessary for optimal outcomes.¹  In addition, achieving ideal 3-dimensional positioning with a favorable ridge anatomy for good primary stability during surgical placement is a critical consideration during implant treatment planning, and the placement of implants may be complicated by preexisting conditions or anatomy.²  Tooth extraction and the time since extraction are significant influencing factors, with an expected horizontal dimensional reduction of 3.79 mm (29%–63%) and vertical reduction of 1.24 mm (11%–22%) at 6 mo.³  Thus, favorable alveolar ridge anatomy and volume are crucial to achieve an ideal restoration of the implant-supported crown. If unachievable, the clinician must decide between a staged or simultaneous approach to restore appropriate ridge dimensions or an alternative treatment plan not involving implants.

Alveolar ridge preservation has been proposed to reduce the impact of post-extraction hard and soft tissue changes on future implant placement and aesthetic outcomes. Therefore, the main goals of any ridge preservation technique should include preservation of hard and soft tissues, especially the interdental papilla.⁴  Ridge preservation may be defined as “any procedure undertaken at the time of or following an extraction that is designed to minimise external resorption of the ridge and maximise bone formation within the socket.”⁵  This may involve the use of hard and/or soft tissue biomaterials and/or membranes.⁶  If a soft tissue autograft is used to cover the augmented socket, it is known as the socket seal procedure.^6,7  Ridge preservation does not prevent the physiological process of bone resorption following extraction, nor may it enhance new bone formation.⁸  However, when compared to spontaneous healing, alveolar bone resorption may be reduced significantly in a horizontal and vertical dimension.^9–11  Further, the need for additional augmentation at the time of implant placement may be decreased when alveolar ridge preservation is performed.^12–14  Although other terms, such as “socket preservation” and “socket grafting,” have been used, the term “ridge preservation” will be used throughout this article.

While the efficacy of ridge preservation in maintaining post-extraction dimensions has been consistently reported in the literature, these studies often pool the results of studies using different surgical techniques into 1 analysis; thus, evidence is lacking on the specific influence of different surgical procedures on clinical outcomes.^11,13–15  Recently, it has been suggested that a flapless procedure may be used for mild to moderately damaged extraction sockets (<50% missing buccal plate) and a flapped approach for severely damaged sockets (>50% buccal plate missing).¹⁶  Indeed, keratinized mucosa width and soft tissue architecture are thought to be better preserved in a flapless approach, as a flapped approach requires coronally advancing it and displacing the mucogingival margin.^17–19  In cases with severe buccal bone damage (>50%), it may be advisable to raise a mucoperiosteal flap to facilitate visualization and grafting of the defect. However, this approach sacrifices the keratinized mucosa due to the need to coronally advance the flap for full or primary wound closure and is often associated with the need for additional soft tissue grafting. Thus, this article aims to systematically review the efficacy of different flapless techniques used in alveolar ridge preservation involving mild to moderately damaged sockets with <50% buccal bone loss. Considerations for the choice of technique are discussed as well.

This systematic review was performed under the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines²⁰ . The protocol has been registered on PROSPERO (CRD42020200614).

The search was guided by the PICO (population, intervention, comparator, outcome) framework²¹ . Only human clinical studies with a minimum of 10 subjects where alveolar ridge preservation was performed (population) were included in the study. To be eligible for selection, studies had to employ a flapless technique of alveolar ridge preservation (intervention) and compare it to spontaneous healing or flapless ridge preservation with a different material for closure (comparator). All studies had to provide quantitative measurements of post-extraction ridge dimensions using either clinical or radiographic changes (outcome). The PICO question was, “How do different flapless techniques for ridge preservation compare with spontaneous healing, and is there any influence of the material used for closure?”

Inclusion criteria consisted of human randomized clinical trials with a minimum of 10 subjects, where ridge preservation procedures were performed at the time of extraction without immediate implant placement. The outcome of interest for included studies was quantitative measurements of post-extraction vertical and horizontal dimensions, using either clinical or radiographic assessment. Exclusion criteria consisted of studies not in English, <10 subjects, case reports or case series, reviews, in vitro or animal studies, flapped surgeries (full mucoperiosteal flaps raised beyond the extraction socket, even studies where sockets were left to heal without primary closure), studies that had an invalid treatment group (no membrane or bone substitute used, non-intact socket, or >50% buccal bone loss), inadequate description of the extraction socket, or studies with no outcomes of interest.

A systematic literature search was performed in PubMed, EMBASE, and Cochrane Library (Table 1). Literature from 1977 to December 2019 was included in the search. Titles and abstracts were screened by 2 authors (E.N. and J.T.) for relevance according to the eligibility criteria. Full texts were assessed for relevant articles when insufficient information was provided in the titles and abstracts. Any disagreement was resolved with discussion. The list of excluded studies with reasons is provided in Supplementary Table 1.

Data collection was done using data collection forms designed to extract study characteristics and outcomes as well as methodological quality assessment. Data were independently extracted by E.N. and J.T., who were not blinded to the authors and institution of the studies undergoing review.

Risk of bias was independently assessed by E.N. and J.T. using a checklist of items from the Cochrane Handbook 6.0.²²  The following domains were assessed: randomization, effect of assignment to intervention, missing outcome data, measurement of outcome, and selection of reported results. Any disagreement between reviewers was resolved by open discussion, and consensus was obtained.

The primary outcome was post-extraction horizontal and vertical ridge hard tissue changes. The secondary outcome was soft tissue changes following extraction.

Extracted data were presented in tables according to the treatment modality and outcome parameters. Meta-analysis was performed to estimate weighted mean difference (WMD) with a 95% confidence interval. Multiple-arm studies were compared to a common control group (spontaneous healing). Each intervention group was included as a separate comparison, and the common control group was split evenly to avoid the “double count” problem.²²  The random effects model was used in the meta-analyses to account for heterogeneity among studies. Subgroup meta-analysis was performed to test whether there is a statistical difference between techniques. The statistical heterogeneity between studies was assessed using Cochran's Q test and I² statistic. The I² values of 25%, 50%, and 75% were considered as low, moderate, and high levels of heterogeneity, respectively. All analyses were conducted using statistical software (R, version 3.63). A P value <.05 was considered statistically significant.

The search yielded 2091 articles published between 1977 and December 2019. Screening of titles and abstracts led to the exclusion of 1992 articles, and a further 91 articles were excluded after assessing full text. Finally, a total of 8 randomized controlled trials, including 262 patients, were selected (Figure 1).

The general characteristics of the 8 included studies are summarized in Table 2. All the randomized clinical trials included had a parallel-arm design, with 5 studies including multiple treatment arms.^23–27  Most were conducted at a single center in a university setting, and 2 were multicenter studies,^24,28  1 of which was in a private practice setting.²⁸  The dropout rate across studies was minimal, and the main reasons were the need to raise a flap due to a dehiscence >50% or due to a preexisting infection. All patients in the included studies were ≥18 years old, smoked fewer than 10 cigarettes per day, and had no active or untreated periodontal disease, with the exception of 2 studies that included patients who smoked <20 cigarettes per day.^23,27  Periodontal status was not mentioned in 2 studies.^24,28 

Reasons for tooth extraction varied, ranging from chronic periodontitis, dental caries, orthodontic reasons, prosthetic reasons, endodontic complications, or trauma. Three studies did not specify reasons for extraction.^24,25,28  Three studies included molars,^24–26  and 5 studies were in non-molar sites.^23,27–30  Sockets were described as having <2 mm³⁰ or <3 mm^24,28 buccal dehiscence, <50% missing buccal or lingual walls,^23,25–27  or intact.²⁹  Buccal bone thickness at baseline was reported in 6 studies.^{23–25,27,29,30}  Better preservation of crestal bone dimensions was noted when the buccal bone thickness at baseline was ≥1.5 mm.^24,25,29,30  Two studies reported buccal bone thickness ≤1.5 mm only.^23,27  Reevaluation was performed between approximately 3 and 6 mo. Five studies assessed changes with cone beam computed tomography (CBCT),^{23,26,28–30}  1 with a stone cast,²⁷  and 2 with clinical measurements at reentry.^24,25 

This review identified 3 techniques: the Bio-Col technique, the subperiosteal tunnel technique, and a technique that involved grafting hard tissue with an autogenous soft tissue graft or collagen matrix (Figure 2). A description of each technique with their advantages and disadvantages is provided in Table 3. One study used the Bio-Col technique with freeze-dried bone allograft and either a collagen matrix or a collagen sponge.³⁰  Three studies used the subperiosteal technique with xenografts and collagen membranes.^24–26  Five studies used a technique that involved grafting hard tissue with either xenograft or allograft and an autogenous soft tissue graft or collagen matrix.^23,27–30  The bone graft most commonly used was xenograft^23–28  followed by allograft.^29,30  Two studies used alloplast and xenograft in different treatment groups of the same study.^23,27  All studies used either a resorbable dressing or membrane or a soft tissue graft or soft tissue graft substitute for socket coverage. Presurgical antibiotic prophylaxis was prescribed in 1 study,²⁸  postsurgically in 5 studies,^{23,26,27,29,30}  and pre- and postsurgically in 2 studies.^24,25 

Data were extracted on the most coronal level horizontal changes and mid-buccal vertical changes reported by studies (Table 4). Horizontal measurements on the CBCT scan ranged from 1, 3, and 5 mm from the crest;^23,26,28 3, 6, and 9 mm from the crest;²⁹  and 4, 7, and 10 mm from the crest.³⁰  Clinical measurements at reentry were at the most coronal level of the socket²⁴  and 2 mm from the crest of the ridge.²⁵  The horizontal measurements used in this review were at the most coronal level, and this coincided with the most accentuated changes reported by the studies. Vertical measurements were more consistent and measured through the long axis of the tooth to the most coronal aspect of the crest. Four studies included a spontaneous healing control group.^23–26  Some studies reported on intergroup differences only,^24–26,29  while others included intra and intergroup changes.^23,28,30  Except for an alloplast graft, all combinations of xenograft or allograft with a resorbable barrier were effective in limiting ridge changes compared to spontaneous healing regardless of technique used. Two studies included a comparison between a bone graft and collagen membrane combination against a procedure that used only a barrier membrane,^25,29  with 1 study finding the addition of graft to be significantly beneficial.²⁵  When comparing the use of a collagen matrix to a soft tissue graft of collagen sponge, no significant vertical or horizontal changes were observed,^23,28,30 

It was possible to perform meta-analyses for 4 studies that included a spontaneous healing control group. Two studies had 2 different arms of treatment modalities, and these were analyzed separately.^23,24  Clinically measured outcome variables consisted of horizontal dimensional changes, vertical dimensional changes (mid-buccal), and vertical dimensional changes (mid-lingual). Subgroup analysis comparing 2 flapless ridge preservation techniques (the hard tissue graft with soft tissue graft or soft tissue substitute technique and the subperiosteal tunnel technique) was performed. A significant reduction in horizontal ridge width changes (WMD = 2.56 mm, 95% CI [2.18, 2.95], P < .05) was observed in favor of flapless ridge preservation procedures (Figure 3A). Inter-study heterogeneity was not significant (I² = 0%, P = .43). Subgroup analysis demonstrated that the choice of technique had no effect on horizontal dimensional changes, and no heterogeneity was noted (P = .37). Quantitative analysis for vertical dimensional changes revealed fewer dimensional changes on the mid-buccal (WMD of 1.47 mm, 95% CI [1.04, 1.90], P < .05) and mid-lingual aspects (WMD = 1.28 mm, 95% CI [0.68, 1.87], P < .05), and both results were in favor of the flapless ridge preservation procedures (Figure 3B and C). Significant inter-study heterogeneity was observed when analyzing vertical changes (I² = 57%, P = .04, and I² = 75%, P < .01, respectively). Subgroup analysis showed no significant vertical changes on the mid-buccal aspect between techniques (P = .13). The test for subgroup analysis for vertical changes on the mid-lingual aspect revealed an additional benefit when the subperiosteal tunnel technique was used (P = .04).

Two studies included an evaluation of soft tissue changes when a collagen matrix, soft tissue graft, or collagen sponge was used (Table 5). Meta-analysis was not possible, as the studies assessed different soft tissue outcomes. Schneider et al²⁷  evaluated horizontal contour, whereas Natto et al³⁰  reported on gingival thickness and keratinized tissue width and also did not have a control group (spontaneous healing). Although the use of a bone substitute with a collagen matrix or soft tissue graft resulted in fewer horizontal contour changes compared to spontaneous healing, this finding did not reach statistical significance.²⁷  However, the use of a collagen matrix resulted in an increase in gingival thickness compared to baseline, suggesting its potential to improve soft tissue thickness.³⁰ 

Risk of bias of included studies is presented in Supplementary Table 2. The overall risk of bias of most included studies was considered low,^{23–25,28–30}  and 2 had some concerns.^26,27  Three studies received industry funding,^23,26,27  dental companies provided the materials in 2 studies,^25,29  and 3 received no external funding.^24,28,30 

Flapless ridge preservation procedures typically involve the use of biomaterials placed at the bony level of the socket at the time of extraction, followed by a barrier to seal it. The placement of a barrier is important, as it prevents displacement of the graft and soft tissue ingrowth until a preliminary bone matrix is established. Qualitative analysis indicates that the flapless approach is effective in limiting horizontal and vertical hard tissue changes after extraction regardless of the technique used. These studies used either a xenograft or an allograft to graft the extraction socket. Conversely, there is limited evidence that the use of an alloplast (ie β-tricalcium phosphate) resulted in more ridge resorption than spontaneous healing,²³  possibly because of its shorter degradation time and higher turnover. This is consistent with other systematic reviews that also noted that use of alloplast grafting material results in poorer outcomes.^33,34 

Meta-analysis was based on 4 studies in the present review. Compared to spontaneous healing, flapless ridge preservation procedures were more effective in reducing horizontal ridge width changes. Subgroup analysis did not reveal any significant differences between the subperiosteal tunnel technique or the hard tissue graft with soft tissue graft or soft tissue substitute technique. Flapless ridge preservation procedures were also significantly more effective in preserving vertical ridge width changes on the buccal and lingual aspects. Subgroup analysis showed a marginally significant difference for preserving lingual vertical changes when the subperiosteal tunnel technique was used. This observation should be interpreted with caution due to the significant heterogeneity present and the inclusion of molars in the studies using the subperiosteal tunnel technique. The lack of significant vertical or horizontal ridge changes observed comparing the 2 techniques suggests that the type of technique used to cover the socket has a minimal effect on hard tissue changes.

Only 2 studies evaluated soft tissue changes. Although the results were inconclusive, they suggest the potential for collagen matrix or a soft tissue graft to augment soft tissues at the time of the procedure. This observation agrees with Fischer et al,³⁵  who reported thicker mucosa at the time of implant placement when socket preservation was carried out with xenograft and a soft tissue punch.

Most studies included for review were at low risk of bias. Two studies^26,27  had some concerns over the effect of assignment to intervention, and this was due to inadequate analyses in the presence of dropouts. Dropouts occurred after randomization because extraction sockets were deemed to be inappropriate for alveolar ridge preservation after extraction was performed. Randomization of patients should have occurred after the extraction was done to ensure that the extraction sockets fit the inclusion criteria.

Inevitably, operators delivering the intervention would have been aware of the assigned intervention. This was not considered a deviation from the intended intervention due to the trial context; thus, the effect of assignment to intervention is low. For multiple-arm studies, no influence on the outcome was expected, as a similar situation presents for all groups tested. Although it was not always clear whether outcome assessors were aware of the intervention received by study participants,^{23,24,26–28}  the assessment of the outcome was thought not to have been influenced by knowledge of the intervention received.

Although other reviews have assessed the efficacy of ridge preservation as a technique, these have typically combined flapless and flapped approaches with the aim of evaluating different combinations of biomaterials.^9,33,36  To the best of the authors' knowledge, this is the first meta-analysis evaluating different surgical techniques for alveolar ridge preservation, with a focus on flapless procedures.

The results of our study indicate that flapless ridge preservation techniques are as effective in limiting horizontal and vertical ridge changes compared to what is reported in the literature with combined approaches. Further, using a flapless approach may result in more conservation of horizontal width. This agrees with the results of a randomized clinical trial by Barone et al¹⁷  that found an increased width of resorption for a flapped approach compared to a flapless approach. However, this should be interpreted with caution, as studies selected for this review specifically included sockets with mild to moderate defects.

This review has several limitations. Although the aim was to review different flapless ridge preservation techniques, only randomized clinical trials were included. Oher flapless procedures have also been described in the literature, such as the ice cream cone technique;³⁷  however, no eligible studies were found for this review.

There was significant heterogeneity related to study design. These include the regenerative material used, timing and method of reevaluation, and parameters reported. Patient factors, such as reason for extraction, buccal plate thickness, tooth position, degree of socket integrity, smoking, and oral hygiene status, may contribute as well. Furthermore, while a flapless ridge approach has been recommended for alveolar ridge preservation in extraction sockets with mild to moderate defects, it was out of the scope of this article to compare the clinical outcomes with early implant placement. The exact threshold where the expected volumetric bone loss from an extraction socket left to heal spontaneously in cases planned for early implant placement such that aesthetic outcomes are compromised, where a hard and soft tissue graft is needed during implant placement, or where the treatment has to be changed to a delayed implant placement approach (and the avoidance of such outcomes if a flapless ridge preservation were to be done instead) has not been determined.

Description of extraction sockets needs to be better defined. Many studies did not describe the integrity of the socket adequately after extraction and mentioned only that teeth were removed “as atraumatically as possible to avoid harming bony walls.” Others used previously published classifications^37,38  but do not adequately describe indications for the choice of ridge preservation procedure.

If the need for alveolar ridge preservation has been determined, selection of the appropriate technique should be based on the clinical presentation and technical difficulty of the procedure. The integrity of the socket is the most important consideration in this decision-making process, and flapless techniques may have added benefits for buccal defects <50%. It has been suggested that the Bio-Col technique may be used for buccal defects <2 mm.³⁰  The subperiosteal tunnel technique facilitates placement of a barrier membrane beyond defects and thus may be more suitable for larger buccal defects or when fenestrations are present at the time of extraction. A soft tissue punch technique may be used if additional soft tissue augmentation is desired. A collagen matrix may also be used as a substitute to avoid a second surgical site and potentially poorer outcomes due to graft necrosis.

The technical difficulty and time taken for the procedure should also be considered. The Bio-Col technique utses a collagen plug and is an economical and time-efficient option. Indeed, histologic analysis has demonstrated that the use of a collagen sponge as a wound barrier for ridge preservation is as effective as a dense polytetrafluoroethylene membrane.³⁹  Compared to a collagen matrix, the collagen sponge is also able to maintain buccal soft tissue thickness (although to a lesser degree).³⁰  In contrast, the need to tunnel and fit a collagen membrane around the grafted site associated with the subperiosteal technique increases the difficulty of the procedure. The optional incorporation of an interim provisional may also be useful for any of the flapless ridge preservation techniques, as the use of these interim prostheses can preserve the soft tissue contour.⁴⁰ 

Recently, a high-density polytetrafluoroethylene (d-PTFE) membrane has been used in regenerative procedures, including ridge preservation.⁴¹  The 0.2-μm pore size is thought to be protective against bacterial contamination, allowing exposure of this membrane to the oral cavity.⁴²  Ridge preservation with this membrane has the added benefit of increased keratinized tissue formation.⁴³  So far, 1 randomized clinical trial has compared post-extraction ridge dimensional changes with an intentionally exposed d-PTFE membrane (without bone substitute) with spontaneous healing.⁴⁴  Ridge preservation with a d-PTFE membrane increased the formation of keratinized tissue; however, ridge width and height dimensions were not significantly different.

Traditionally, the hard tissue with autogenous soft tissue graft technique has used a free gingival graft obtained with soft tissue punch and adapted to the soft tissue wound margins of the socket. However, a significant proportion of graft necrosis may occur, leading some authors to consider this an unpredictable procedure.⁴⁵  Subepithelial connective tissue grafts have also been used to restore soft tissue architecture or deficiency in periodontal plastic and implant surgery and may be used for ridge preservation procedures as well.⁴⁶  Indeed, the organic extracellular matrix may serve as a space maintainer while carrying over transplanted fibroblasts with tissue-inductive potential.⁴⁷  The subepithelial connective tissue graft may be combined with the subperiosteal tunnel technique, and fixing the graft within a prepared tunnel may increase graft survival.⁴⁸  The use of rotated pedicle flaps, such as the vascularized interpositional periosteal connective tissue flap, also provides a way to close grafted extraction sockets in a flapless manner.⁴⁹  Autogenous periosteal pedicle grafts may also function effectively as barrier membranes for regeneration while minimizing gingival recession.^50,51  Advantages include improved blood supply, similar proximity of the donor site, and optimal aesthetics.

Although this study demonstrated that different techniques may result in similar hard tissue changes, there could be differences in the preservation of soft tissue, such as gingival thickness (occlusal and buccal), amount of keratinized tissue, and horizontal contour. Therefore, in addition to hard tissue changes, future studies should also report on volumetric change to soft tissue contours and whether further soft tissue augmentation is still required after the ridge preservation procedure.

In an era of contemporary dentistry where patient morbidity and patient-reported outcome measures are increasingly as important as the efficacy of the technique used, flapless ridge preservation approaches are gaining traction in the dental implant community. The present review focused on the outcomes of different flapless ridge preservation techniques in terms of post-extraction hard and soft tissue changes. Data comparing flapless ridge preservation techniques in humans are limited, and only a few studies have investigated this in comparison with spontaneous healing. Within the limitations of this review, flapless ridge preservation techniques have good clinical outcomes, and the technique used to close the socket does not seem to play a significant role for hard tissue changes. The additional benefit of using a collagen matrix or soft tissue graft in preserving or augmenting soft tissue remains to be determined. Regardless, it seems prudent to choose a technique that is minimally invasive when the situation allows, and there may be advantages in soft tissue management when using a flapless approach. These include preservation of interdental papilla, opportunity for simultaneous augmentation of soft tissues, and not changing the position of the mucogingival junction through flap advancement.

The authors wish to thank Ms Yu Fan Sim from the Faculty of Dentistry, National University of Singapore, for her advice on biostatistics.

The authors declare that they have no competing interests. No funding was received for this work.