Through the last decades, the success rate of dental implants has evolved from modest numbers to current high results [1,2]. Placement of dental implant to edentulous regions offer specific advantages over the tooth supported fixed dental prosthesis such as preservation of the alveolar bone, avoiding adjacent teeth restoration, better aesthetics and function [3,4].

Extraction of the tooth is indicated when a tooth has nonrestorable condition or cannot be maintained in a long-term period in terms of function or aesthetics. The beginning of the post extraction socket healing process is clinically observed by clot formation and end up with epithelized closure over the bone filled socket [5-7]. The healing of the extraction site may preserve the original bone dimensions with uneventful bone formation. Unfortunately, in most cases, following the tooth extraction, alveolar bone of the socket undergoes a loss of bone of 50% in its width within the first year [7-10]. Radiographic studies showed that loss of alveolar ridge height largely happened within the first 90 days post extraction. Expected horizontal and vertical bone loss after six months following the tooth extraction are 29 - 63% and 11 - 22% respectively [11,12]. Furthermore, traumatizing of extraction socket bony walls during extraction or by some pathological conditions can also evoke adverse dimensional changes.

Bone loss after tooth extraction may lead to two main challenging situations: it can create an aesthetic problem around the fixed dental prosthesis resulting in soft tissue recession, or it can make the application of dental implant more challenging requiring guided bone regeneration.

To decrease post extraction alveolar bone resorption and/or restore damaged socket walls, various treatment protocols have been recommended to preserve/augment the ridge [13-16]. All existing socket preservation techniques may limit the severity of ridge resorption but not totally prevent it. According to current studies, immediate implant placement also can minimize resorption of fresh extraction sockets [17,18]. However, severe bone damage before or during tooth extraction may force the clinician to change the implant placement protocol from immediate to delayed protocol [19].

Whereas post extraction residual hard and soft tissue morphology plays a crucial role in aesthetic and functional outcomes of the future implant therapy, worldwide overview to extraction socket classification system allows clinicians to ease communication and data collection, which may lead to development of more predictable treatment modalities or to establish the most appropriate treatment plan after tooth extraction.

The aim of this study was to review current literature concerning extraction socket classification immediately following tooth extraction and the rationales for socket preservation/augmentation procedures and with reference to it suggest novel clinical decision tree for extraction socket preservation/augmentation in aesthetic and non-aesthetic areas to provide the practitioners planning dental implants with more accurate prognosis in all tooth regions.

Protocol and registration

The methods of the analysis and inclusion criteria were specified in advance and documented in a protocol. The review was registered in PROSPERO, an international prospective register of systematic reviews. Registration number: CRD42019136314.

The protocol can be accessed at:

https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=136314.

The reporting of this systematic analysis adhered to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) Statement [20].

Focus question

The following focus question was developed according to the problem, intervention, comparison, and outcome (PICO) study design (Table 1):

What are the indications for socket preservation/augmentation procedures based on separate clinical and radiographic extraction socket parameters or organized into classification system, in aesthetic and non-aesthetic zones immediately after tooth extraction?

Information sources

An electronic search was performed for articles in English language published from January 1 2009 and May 1 2019, by two reviewers (OD and SG) using MEDLINE (PubMed) and EMBASE databases. Reference lists of studies were also hand-searched for relevance.

Search

The search strategy incorporated the examination of MEDLINE (PubMed) and EMBASE electronic databases. The following keywords were used: "socket augmentation" OR "socket preservation" OR "socket augmentation necessity" OR "socket augmentation indication" OR "extraction socket classification".

Selection of studies

The included articles were selected by OD and SG according to inclusion and exclusion criteria. Any disagreement was resolved through discussion. In addition, an experienced senior reviewer consulted the included articles (Figure 1).

Types of publications

The review included only human studies published in English. Letters, editorials, PhD theses, and abstracts, in vitro and animal studies were excluded.

Types of studies

This present review included all human prospective, follow-up studies, clinical trials, cohort studies, case-control studies, systematic reviews/guidelines of consensus conferences in the English language published from January 1 2009 and May 1 2019.

Types of participants/population

Subjects, whose extraction sockets’ dimensional changes were evaluated by the clinical and/or radiological parameters, were included in this present review.

Inclusion and exclusion criteria

The applied inclusion criteria were as follows:

The following types of articles were excluded as follows:

Sequential search strategy

While initial literature search, all articles were selected based on titles and abstracts. Two independent reviewers (OD and SG) selected the included articles according to inclusion and exclusion criteria. An experienced senior reviewer consulted the included articles for the final stage of screening.

Data extraction

The data were independently extracted from included articles according to aims and themes of this review.

Data items

Data were collected and arranged from selected articles in the following fields:

Risk of bias within studies

The methodological quality of each study was performed by using the risk of bias assessment tool outlined in Cochrane Handbook for Systematic Reviews of Interventions [21]. Seven domains were evaluated random sequence generation, allocation concealment, defined inclusion/exclusion, blinding of participants and/or personnel, blinding of outcome assessment, incomplete outcome data and selective reporting. Absence of information regarding blinding was not treated as relative because it could not have any effect for the study.

Statistical analysis

Heterogeneity between studies was found therefore meta-analysis could not be performed.

Study selection

Article review and data extraction were performed according to the PRISMA flow diagram (Figure 1). The initial search displayed 553 results from the MEDLINE (NCBI Ovid and PubMed Central [PMC]) and EMBASE databases. The inclusion and exclusion criteria were applied to 536 articles. After inclusion and exclusion criteria were applied, 17 articles were selected. Finally, 10 full text articles were included in this study.

Study exclusion

The reasons for excluding were as follows: animal studies (n = 3) [11,22,23], being a case studies (n = 1) [24] and not enough information regarding the topic (n = 3) [25-27].

Quality assessment of the included studies

Only six studies of the included papers were clinical studies. Quality and risk assessment was conducted by one author (AS) and is represented in Table 2. Included studies were assessed following the Cochrane collaboration’s tool [21] for assessing risk of bias. Summarizing, no single study was classified as low risk of bias for all the criteria and most studies demonstrated moderate or unclear risk of bias.

Extraction socket classifications

There have been several proposed systems to extraction socket classification. All included articles are summarized in Table 3. Caplanis et al. [28] introduced a new classification in 2004, which comprised general assessments of hard and soft tissue walls of socket. After the detailed socket evaluation studies, it has become noticeable to the authors that the quality and quantity of the buccal hard tissue is key factor in a long-term healing. In 2007, Elian et al. [29] proposed more simplified extraction socket classification and rationale for socket preservation for the aesthetic region. The classification was based on buccal bone and soft tissue level. Another classification and treatment recommendation of extraction defect suggested by Juodzbalys et al. [30] which was based upon both the hard and soft tissues. Authors proposed extraction socket classification encompassed not only quantitative measurements of hard and soft tissue but also included qualitative parameters of it. Another classification system for posterior teeth extraction socket in terms of septum bone morphology proposed in 2013 [19]. This molars extraction sites classification is based upon the available septum bone for stabilization of immediately placed implant. Meanwhile Chu et al. [31] subclassified previously reported extraction socket classification [29] and comprised extraction sites with bone defect. Extraction socket classification performed by El Chaar et al. [32] subclassified extraction socket in accordance with buccal bone plate loss, interproximal bone height, apical topography and soft tissue biotype.

Indications and reasons for extraction socket preservation/augmentation

Detailed literature scanning showed very little existing accurate knowledge about the indications for extraction socket preservation. Indications for soft tissue grafting after tooth extraction was evaluated by Schlee et al. [33] and decision tree was suggested. Connective tissue grafts were recommended after the assessment of gingival biotype of the extraction site. A study performed by Horowitz et al. [34] showed that extraction socket preservation resulted in more reduced bone resorption compare to dimensional alveolar bone changes after immediate implant placement. Recently, Jung et al. [35] described all treatment modalities, their limitations and presented a valuable decision tree for alveolar ridge preservation in aesthetic zone.

Extraction socket preservation is defined as alveolar ridge preservation within the bone envelope remaining after tooth extraction, meanwhile ridge augmentation is defined as increasing the volume of alveolar ridge beyond the bony envelope at the time of tooth extraction [26]. In clinical practice sometimes it is difficult to separate those two procedures and in literature they are described mainly as extraction socket preservation. In present study authors’ opinion those definitions should be revised and discussed, because it is impossible to preserve not existing damaged socket structures. Definition "extraction socket preservation" is recommended to use in cases where extraction socket anatomy is intact. In contrast definition "extraction socket augmentation" defines alveolar ridge restoration when bony walls of the socket are partly or completely lost. Taking into account fact, that different definitions can cause misunderstandings in present study both definitions were used.

The tooth extraction generally results in some morphological and compositional modifications in the overlying soft tissue of the socket [36]. Three-dimensionally soft tissue changes are expectable both vertically and horizontally immediately after extraction [37]. The quality and quantity of peri-implant soft tissue plays an important role in achieving an aesthetically acceptable outcome of implant-supported prosthesis. Another crucial factor in achieving aesthetic peri-implant soft tissue after applying final prosthesis is biological width. It is obvious that insufficient peri-implant soft tissue can cause deficiencies in terms of peri-implant aesthetic appearance [38]. Different surgical approaches have been suggested to overcome these situations following implant therapy and to enhance aesthetic outcomes of peri-implant soft tissues [39,40]. The connective tissue graft performing in conjunction to immediate implantation have been proposed by various authors [40-43]. Some authors in clinical cases with both soft and hard tissue defects proposed extraction socket augmentation with bone graft substitute placed at the bony level and autologous soft-tissue graft or soft-tissue substitute placed at the level of adjacent soft tissue [44-47]. Jung et al. [44] combining demineralized bovine bone mineral with collagen membrane or autogenous soft tissue graft resulted in significantly reduced horizontal and vertical alveolar resorption compare to spontaneous healing.

It has been suggested that the bone walls of the extraction socket may be preserved by immediate implant placement [17,18]. However, according to animal studies the buccal wall of the socket undergo significant resorption which could not be prevented by immediate implantation [7,10]. Horowitz et al. [34] observed more favourable results after socket preservation compare to site collapse after immediate implant placement. The study comparing alveolar ridge dimensional changes after immediately placed implants with or without additional bone regeneration procedures demonstrated horizontal bone resorption 43.3% in the group without grafting material and only 15.8% in the group with simultaneous bone regeneration procedure [48]. Some studies suggest the minimal buccal bone thickness (2 mm) to maintain the facial aspect of the implant [49-51]. When the buccal bone plate is thin or is dehisced, it is more likely to increased resorption and greater alveolar ridge alterations following tooth extraction [52]. In these types of cases immediate implant placement is not possible without decreased primary implant stability or implant body exposure. The correlation between buccal bone thickness and alveolar ridge dimensional changes were observed by Jung et al. [53] and resulted in more pronounced bone resorption in cases with thin buccal bone plate. However, the same study demonstrated that extraction socket preservation prevented from bone remodelling even in sites with less than 1 mm of buccal bone thickness.

Following the atraumatic tooth extraction, detailed evaluation of the socket bony walls is initially made by bone probing technique. Appreciation of the remaining socket walls and their post extraction conditions is crucial for choosing of the implant treatment protocol [28]. Alveolar ridge preservation procedure has been suggested in cases with severe loss of buccal bone plate (> 50%) [48]. Patients with thin soft tissue biotype and buccal bone loss from 25% to 50% a delayed implant placement with socket preservation is recommended [32]. The horizontal bone loss of the socket walls is evaluated: the distance between the tip of the socket labial plate and coronal border of the buccal bone of adjacent regions. Acceptable distance for immediate implant placement is estimated to be 2 mm [28].

For immediate implant placement protocol, contribution of the apical bone is clinically vital to ensure not only favourable implantation but also to attain sufficient primary implant stability. Several authors suggested at least 3 - 5 mm of periapical intact bone for an intimate bone-to-implant contact [19]. However, it is important to understand and evaluate the anatomy of the region through cone-beam computed tomography for avoiding injuries to the crucial adjacent landmarks [54]. The inferior alveolar nerve, the incisive canal, the mental foramen, nasal floor, and maxillary sinus are the most common adjacent vital anatomical structures, which should be examined thoroughly before the implant surgery [55,56].

The septal bone of the posterior tooth socket is the primary available bone area for immediate implant placement. Therefore, during the multi-rooted tooth extraction, special care should be taken not to compress or damage the interdental septum. In the sockets with impaired septum bone, primary stability of the implant is achieved by the walls of the socket. Therefore, in this type of sockets buccal bone plate should be thick and intact thus wider implants could be placed [19].

The evident advantage of the immediate implantation is reduced treatment time. Immediate implantation is a well-accepted treatment protocol because of the obvious advantages such as; reduced treatment time, extraction socket walls maintenance, and attaining better aesthetic results. However, applying immediate implant placement protocol to the socket with periapical pathology is a matter of debate. The possible complication of this technique might be the microbial contamination of the socket during the initial phase of healing [57,58]. There are several clinical studies which support the high survival rate and normal osseointegration when implants are placed immediately after extraction of teeth presenting endodontic lesions [59-61]. Considering the risk of apical peri-implantitis or even implant failure attention should be focused on alveolar debridement and meticulous cleansing of extraction socket before implant placement and using medications including a bactericide antibiotic and anti-inflammatory analgesics. Meanwhile, a study performed by Kim et al. [62] analysed ridge augmentation in periodontally compromised extraction sockets and resulted in 99.3% safety rate after thorough extraction of infectious source.

The pneumatization of the maxillary sinus is a physiological process that occurs during the growth period, causing to the volume increasing [63]. The apices of the maxillary posterior teeth are inserted within the maxillary sinus cavity in some cases, which could jeopardize the implant insertion [64]. Immediate implant placement is not recommended in these situations without sinus lifting surgery. There have been suggested different sinus augmentation procedures that are performed at the time of dental extraction [65-67].

The study by Al-Hezaimi et al. [22] reported that buccal bone gets blood supply from adjacent teeth and classified extraction socket as a single tooth extraction or multiple teeth extraction. This was in agreement with a study performed by Al-Asker et al. [68] who reported that contiguous tooth extraction resulted in more pronounced alveolar bone loss. The correlation between bone loss and immediate placed implants were more extensive in multiple adjacent teeth extraction [23].

For better identification of the socket status, thoroughly evaluation of the soft and hard tissue around the socket should be performed immediately after tooth extraction. Although there are various types of extraction socket classifications none of them could completely evaluate all morphological parameters of alveolar ridge. Furthermore, present study revealed that indications for extraction socket preservation have wider spectrum than socket morphology and are related to surrounding tissue anatomy or dental implantation operation indications and timing. Taking on account results of analysis of extraction socket classifications and rationales for alveolar ridge preservation we suggested a decision tree for extraction socket augmentation/preservation immediately after tooth extraction in aesthetic and non-aesthetic zones (Table 4).

Indications and reasons for extraction socket preservation/augmentation were subclassified to aesthetic, when impossible to reach sufficient aesthetic result; functional, when impossible to gain implant primary stability; and risk related, when there is possibility of significant alveolar bone resorption, apical peri-implantitis development, and maxillary sinus as well as nasal floor perforation or possibility to reduce need for elevation of the sinus and nasal floor. Different parameters are determined for aesthetic and non-aesthetic zones.

However, this article has some limitations. The clinical decision tree for extraction socket preservation/augmentation is mostly based on bone morphology and its relation with vital structures. Indications do not include detailed soft tissue assessment because it was described by the authors of previous extraction socket classification [48]. Moreover, when interpreting the results of the study, including only human studies and not performing statistical analysis should be taken into consideration as limitations. Since novel clinical decision tree for extraction socket preservation/augmentation in aesthetic and non-aesthetic zones is based on comprehensive indications, authors believe that it can be useful tool to the clinician establishing the most appropriate treatment plan after tooth extraction.

A novel clinical decision tree for extraction socket preservation/augmentation immediately after tooth extraction in aesthetic and non-aesthetic zones has been presented. The proposed indications help to determine the need of extraction socket preservation/augmentation from the aesthetic, functional and risk-related viewpoint. However, thorough clinical and radiographic evaluation of extraction socket should be performed. Based on current evidence, extraction socket preservation/augmentation is suggested not only in clinical cases with dental implants being planned but also to preserve alveolar bone for removable prosthesis fixation or to support soft tissue around fixed dental prosthesis. Further studies are needed to validate the reliability of proposed clinical decision tree for extraction socket augmentation.