PGO Scientific journal

An extraction leads to a resorption of the alveolar ridge. The magnitude of these changes is important in decision making and comprehensive treatment planning. In order to minimise these changes as much as possible, different techniques known as alveolar preservation techniques (PRA) have been described, in which bone grafts, membranes (Guided Bone Regeneration), growth factors, as well as immediate implant placement are used. 25 articles were included in this literature review, 15 were systematic reviews and 10 were clinical trials. All clinical studies had an evaluation period of at least 3 months.

It is concluded that the preservation of the alveolar ridge with the use of different bone substitutes represents an effective method to decrease the physiological resorption process after tooth extraction. The use of barrier membranes (ROG) is of great benefit in the preservation of bone height. Plasma rich in growth factors stimulates hard and soft tissue repair and regeneration, resulting in higher quality bone. Immediate implant placement can be considered as an alveolar preservation technique. Although PRA is a widely studied technique, more research is needed, as well as more standardised protocols.

Key words: socket, preservation, biological, ridge

Tooth extraction is indicated when a tooth cannot be restored or maintained in an acceptable condition for long-term health, function and/or aesthetics. Tooth loss has a direct impact on the quality of life by affecting the ability to chew, speak and, in some cases, socialise.¹

Exodontia leads to resorption of the remaining alveolar ridge as a result of bone remodelling. This resorption process begins immediately after tooth extraction, with a 40-60% decrease in the horizontal and vertical dimensions of the alveolar ridge during the first year.

Radiographic studies showed that the loss in height of the alveolar ridge occurred largely during the first 90 days after extraction ². In the same line, Tan et al.,³ in their 2012 systematic review indicated, for post-extraction sockets in which no alveolar preservation technique was used, a resorption of 3.8 mm of horizontal bone and 1.2 mm of vertical bone loss within the first 6 months after extraction. In other words, this corresponds to 29-63% horizontal bone loss and 11-22% vertical bone loss.

Vignoletti et al.⁴ also reported that the resorption of the alveolar ridge was most evident in the horizontal component and that the concomitant vertical loss was most evident at the vestibular level. The practice of bone preservation after tooth extraction in an attempt to maintain ridge height and width was first described as “bone maintenance”. ^5,6,7

Different terms were then used to describe the same procedure, such as ‘cavity preservation’, ‘cavity augmentation’, ‘cavity grafting’, ‘ridge preservation’, ‘alveolar bone grafting’ and ‘alveolar augmentation’, which is defined in the Glossary of Prosthodontic Terms as “any surgical procedure used to alter the contour of the residual alveolar ridge”. (Academy of Prosthodontics 2005).

To avoid ambiguity, the term “alveolar ridge preservation” (ARP) will be used throughout this review. PRA is defined as the procedure of halting or minimising the resorption of the alveolar ridge after tooth extraction for future prosthodontic treatment, including the placement of dental implants.

Objective: To analyse the different regenerative and preservation strategies of a post-extraction socket existing in the current literature.

The protocol carried out to develop this literature review comprises the following steps: formulating questions following the PICO (Patient Population, Intervention, Comparison and Outcome) format, defining inclusion and exclusion criteria, searching for articles, selecting them, assessing the quality of the studies, extracting the data and producing a table of results, analysing the data and interpreting and discussing the evidence.

-Pico format
The PICO format was used to formulate a search question; what treatments, techniques and biomaterials are used in alveolar preservation techniques, and which is the most effective or has the best long-term results? 

-Patient/Population:
The population of interest in this literature review was limited to humans with recently extracted teeth, the subject of the study, who had undergone alveolar preservation techniques defined as “any therapeutic method performed immediately after tooth extraction to preserve the dimensions of the alveolus in order to maximise bone availability for dental implant placement”.

-Intervention:
Alveolar preservation

-Comparison:
To elucidate this aspect, different therapeutic interventions were compared with spontaneous healing of the alveolus post-extraction.

-Outcome measures
The two variables chosen were the dimensional changes occurring in the alveolar walls after tooth extraction and alveolar preservation measured in both height and width (in mm or %). Soft tissue changes (in mm or %) were also considered.

Selection criteria
When defining the inclusion and exclusion criteria, we sought to find the largest number of systematic reviews and clinical trials on the topic addressed, including randomised controlled clinical trials, case-control studies, prospective longitudinal studies and cohort studies, where alveolar preservation therapies were performed, with or without serial exodontia; studies reporting changes in bone levels and evaluating alveolar bone dimensions in height and thickness, conducted in humans and published in English or Spanish with a publication date from 2011 onwards. Articles in other languages and older than 10 years were excluded, as well as those that included case reports, letters and narratives, abstracts, historical reviews or in vitro and animal studies.

Search strategy
Between March and June 2021, an electronic search was carried out in the MedLine (PubMed) and Cochrane Plus databases for articles on the subject, in accordance with the Oxford evidence-based medicine (EBM) system, in an attempt to obtain a significant degree of scientific evidence with an optimum degree of clinical recommendation. To find these studies, we searched using possible combinations of the following keywords: “socket, preservation, biological, ridge”.

The end result was: “(socket[All Fields] AND (“preservation, biological”[MeSH Terms] OR (“preservation”[All Fields] AND “biological”[All Fields]) OR “biological preservation”[All Fields] OR “preservation”[All Fields])) OR (ridge[All Fields] AND (“preservation, biological”[MeSH Terms] OR (“preservation”[All Fields] AND “biological”[All Fields]) OR “biological preservation”[All Fields] OR “preservation”[All Fields]))”

In addition, to complement the electronic search process, an additional manual search was carried out by retrieving potential articles from the bibliographies of the retrieved studies.

Evaluation and selection
Initially, the articles were screened, first by title and then by abstract. In a second stage the full-text articles were read in detail. Papers that met all the selection criteria were processed for data extraction.

The initial search showed 1882 results from the MEDLINE database (NCBI Ovid and PubMed Central [PMC]) and 384 from the Cochrane Plus database. Of these, those that were duplicated were discarded and a selection was made by title and, subsequently, by abstract, selecting a total of 44 articles, which were analysed in full text. Finally, after applying the selection criteria, 25 articles that strictly met the inclusion and exclusion criteria were included, as shown in the PRISMA flow chart in figure 1.

Of the 44 articles that were read in full text, 19 were excluded as they did not meet the stated inclusion criteria. The reasons for the exclusion of these articles were as follows:

o It did not provide enough information on the subject (n=5);
o In animals (n=1);
o Study with long-term non-evaluated sample (n=3);
o No clinical analysis (histology only) (n = 1);
o Treatment of severely compromised alveoli (n = 1);
o Non-randomised clinical trial (n=2);
o In vitro study (n=1);
o Lack of sufficient sample size (n = 5).

Study design and evaluation period
Of the selected studies, 15 were systematic reviews and 10 were clinical trials. All clinical studies had an evaluation period of at least three months. Once the articles had been reviewed and selected, the next step was to analyse and interpret the results, which are reflected in the following tables 1 and 2.

Table 1. Description of articles reviewed with systematic review design

Table 1. Description of articles reviewed with systematic review design

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In an attempt to prevent and resolve alveolar resorption, different alveolar preservation procedures (PRA) have appeared in the literature.  The aim of some of them has been to fill the alveoli with materials such as bone grafts including autograft, allograft and xenograft. Some alloplastic materials that aim to maintain the bone volume of the alveoli are also used.

Bone grafts

Most studies investigating the dimensional changes of hard and soft tissues after tooth extraction state that the ideal material used as a bone graft should not only exhibit osteoconductive properties, but should also promote osteoinduction and osteogenesis. Only autologous bone has these three properties and is therefore considered the gold standard for bone augmentation procedures today 27. However, the additional operating site, the prolonged surgery time, donor-side morbidity, the limited availability of autologous bone and postoperative discomfort lead to the use of alternative bone substitutes for bone regeneration ²⁶. 

The present investigation was able to analyse a large number of studies by grouping the results according to the source of bone substitutes used. While this method of managing the available data and analysing a large heterogeneous sample has inherent limitations, the results revealed similar trends in the included studies. As such, two main conclusions can be drawn: (1) as previously reported, PRA has the ability to slow the resorption process after tooth extraction and (2) there appear to be only minimal differences between bone substitutes. These results are in agreement with previous research reporting similar clinical outcomes associated with PRA using different bone substitutes24,25,26. Despite this, there is one systematic review and meta-analysis that has reported superior outcomes attributed to xenogeneic or allogeneic bone substitutes in combination with a collagen sponge or membrane37. Jambjekar et al35 in 2015 and Joudzbalys et al28 in 2019, concluded in their systematic reviews that alveolar preservation performed with xenogeneic and allogeneic grafts had slightly lower resultant bone resorption than other bone substitutes and were considered the most optimal grafts.

Stumbras et al²⁶ In his systematic review, he stated that allografts, and especially demineralised freeze-dried bone allograft (DFDBA), possessed osteoinductive and osteoconductive properties and therefore not only served as a scaffold for the regeneration of new bone, but also stimulated the differentiation of mesenchymal cells into osteoblasts, making it the best bone graft for alveolar preservation. On the other hand, the authors Jambjekar et al³⁵ and Avila-Ortiz et al³⁷ agreed in their systematic reviews that not only allografts, but also xenografts were biomaterials with good osteoconductive properties and that they generated similar results to allografts, concluding that both bone grafts were valid and optimal for alveolar preservation.

The results of the systematic review conducted by Majzoub et al²⁷ corroborate previous studies in which an average horizontal resorption rate of 3.4 (SD 1.07) mm was demonstrated for unaided alveolar healing, compared to an average of 1.43 (SD 0.89) mm, 1.52 (SD 1.29) mm and 1.84 (SD 1.08) mm with the use of xenogeneic, allogenic and alloplastic graft materials, respectively. In addition, the results of this review also confirm, although based on a limited sample of studies, that the cavity proximal sites exhibited less reduction in vertical dimensionality compared to the mid buccal and mid lingual sites.

On the other hand, it should be taken into account that the dimensional changes following tooth extraction are dictated and influenced by a variety of systemic and local factors to be studied, such as; the extent of traumatic injury during extraction, the morphology of the socket, the presence of infection, smoking, the type and position of the tooth, the presence of periodontal disease, the phenotype of the hard and soft tissues, the compliance of the patient and, most importantly, the number and thickness of the remaining intact socket walls³². However, this literature review did not analyse the effect of such variables due to insufficient data and/or significant heterogeneity among the included studies.

Previous systematic reviews and meta-analyses have demonstrated a superior outcome in ridge preservation associated with a basal buccal bone thickness greater than 1 mm37. In contrast, a recent RCT concluded that PRA only influences the degree of ridge resorption at sites with ≤ 1 mm buccal wall thickness 46. Histologically, Mac Beth et al³³.observed in his systematic review an increase in bone tissue in a group of patients treated with alveolar preservation techniques compared to spontaneously healing alveoli. This increase in bone tissue, in histological terms, was also favoured in the results obtained by Jambjekar et al³⁵. In this systematic review, more favourable results were obtained when the healing period was longer than 12 weeks before placing dental implants.

Guided Bone Regeneration (GBR)

Several authors agree that the use of barriers for Guided Bone Regeneration (GBR) seems to be the most effective technique, thus limiting the dimensional changes of the alveolar ridge after tooth extraction.^23,29,33,34,. This was one of the variables studied in the meta-analysis by Avila-Ortiz et al³⁷. in which the use of barrier membranes had a great benefit in preserving the bone height of the alveolar ridge. 

Dong-Joo in 2019⁴¹ investigates the effect of PRA with a dense polytetrafluoroethylene (d-PTFE) membrane and freeze-dried irradiated allogenic bone for bone-deficient sockets and compares it with spontaneous healing of the socket (control group), concludes that RFA with d-PTFE membrane and allogeneic bone substitute reduced horizontal bone resorption in bone deficient sockets, and the need for bone augmentation at the time of implant placement.
Scheyer et al. in 2016⁴³ has developed a clinical trial comparing the efficacy of two ridge preservation treatments, one using demineralised allograft plus collagen membrane and the other using bovine bone mineral xenograft plus collagen membrane, with bone dimensions recorded at baseline and at 6 months, The conclusion was that the xenograft plus collagen membrane provided better soft tissue healing and better ridge preservation, with greater horizontal bone regeneration, however the vertical lingual and buccal bone changes were not significant between the two treatment modalities. Deeper extraction sockets with higher and more intact bone walls responded more favourably to ridge preservation therapy.

In the systematic review conducted by Lindhe et al⁴⁶. clinically analysed bone quality after alveolar preservation techniques using bone grafts and found that the proportion of biomaterial remaining after 6 months was between 13% and 26% when a collagen membrane-covered xenograft was used. On the other hand, they noted that in some cases fibrous encapsulation of particles of the graft itself occurred. In this aspect they agree with Mardas et al34 who also reported this problem. In addition, in this recent review from Lindhe et al⁴⁶. the authors emphasised that the tissue remodelling process in alveoli treated with Bio-Oss Collagen® (Geistlich) took longer to occur. Calasasns-Maia⁴⁷ compared the efficacy of two bovine xenografts (BiOss and Osseus), finding no significant differences between them.

According to Barone et al⁴⁵. in 2015, where they compared the changes produced in post-extraction alveoli treated with alveolar preservation versus untreated alveoli, they concluded that following the protocol they had chosen, bone resorption was reduced by almost half. This protocol consisted of filling the alveolus with cortico-cancellous collagenised porcine bone and covering it with a pericardial membrane of heterologous origin.

Only two of the studies assessed whether performing alveolar preservation by opening a mucoperiosteal flap was more effective than performing it without opening the flap, Barone in 2016⁴⁴ found no significant differences either clinically or histologically, while Majdi et al in 2019⁴⁰ concluded that leaving the flap open had no effect on dimensional changes in bone height or width, however, there was a wider band of keratinised tissue and less pain with the closed flap technique compared to the open flap.

Brend et al³⁸ in 2020 compares ridge preservation using a xenogeneic bone substitute covered with a collagen matrix, ridge preservation using a xenogeneic bone substitute covered with a free palatal graft or spontaneous healing (control) of the alveolus. They concluded that ridge preservation using a xenogeneic bone substitute covered with a collagen matrix or a palatal graft results in less bone resorption compared to spontaneous healing. In addition, no significant differences were found in graft assessment, postoperative complications and soft tissue contour.

Growth factors

Studies are now describing the potential of plasma rich in growth factors in tissue regeneration after oral surgery. This potential translates into stimulation of hard and soft tissue repair, regeneration and reduction of inflammation and its associated pain and discomfort. Further studies are needed as there is much variability in methods and results in the literature.^30,31,36.

According to Moraschini et al³⁶, conclude that growth factors accelerate the soft tissue healing process, but cannot affirm their effect on bone regeneration, particularly in the late phases of alveolar healing. Although platelet-rich concentrates demonstrated a high rate of vertical resorption³⁰, histological findings show that it stimulated the formation of new bone of higher quality. Micro-CT analysis of platelet-rich concentrates compared to natural clot revealed significantly denser bone in the platelet-rich concentrate group (820 mmHA / ccm vs. 780 mmHA / ccm)³⁶. Platelet concentrates appear to not only aid in the formation of better quality bone, but also improve the qualities of other biomaterials when combined, resulting in smaller dimensional changes. These findings are consistent with results published in other reviews in which autologous platelet concentrates were shown to be beneficial for both the quantity and quality of newly formed bone³¹. 

Clark et al⁴², in 2018 studies advanced platelet-rich fibrin (A-PRF), an autogenous blood product with applications in dentoalveolar surgery. However, there is minimal information on its optimal clinical application or efficacy, so Clark D. developed a randomised controlled clinical trial in which he demonstrated that A-PRF is a suitable biomaterial for ridge preservation. The use of A-PRF produced significantly more vital bone compared to allograft, while preserving ridge dimensions similar to allograft and better than blood clot alone. Modest improvements in ridge dimensional changes were demonstrated when using A-PRF + allograft without a significant decrease in vital bone formation. These findings demonstrate the regenerative potential of A-PRF in post-extraction sites and suggest broader applications of A-PRF outside of ridge preservation. Future studies should extrapolate the osteogenic potential of A-PRF in more extensive ridge augmentation procedures and investigate further regenerative capabilities in periodontal regeneration.

These systematic reviews^30,31,36 revealed that there is currently insufficient clinical evidence to establish which method of post-extraction socket preservation would be optimal for reducing bone resorption and improving bone quality of regenerated bone. In addition to that, there was a lack of indications on when to perform post-extraction socket preservation. In cases of thick periodontal biotype in non-aesthetic zone without demand for vertical bone dimension, spontaneous healing or platelet-rich concentrates are suggested. Furthermore, it is impossible to determine which method for alveolar ridge preservation is better because preservation techniques were analysed in different locations of the jaws and studies compared different evaluation/investigation methods. Considering the correlation between bone quality and quantity, future studies on the combination of platelet-rich concentrates and bone graft materials are needed.

A clinical trial comparing delayed socket implant placement with alveolar preservation using a bovine xenograft together with a collagen membrane and early socket implant implantation post-extraction by Lim, Hyun-Chang³⁹ in 2020 concluded that both protocols produced comparable results in soft tissue levels, periodontal parameters and level of patient discomfort. Therefore, immediate implant placement can be considered as an alveolar preservation technique.

Based on the evidence available in the literature, alveolar bone resorption after tooth extraction is an inevitable process in its entirety. Given the great variability of the studies included in this review, and due to the diversity of patients, materials and interventions, it is not possible to make any statements regarding the proposed procedures for alveolar preservation. One of the major methodological problems is the lack of parameters for measuring the alveolar ridge following extraction, as these are not reproducible, verifiable or quantifiable clinically or through diagnostic imaging. This lack of criteria makes it impossible to compare the morphological state of the alveolar process after an extraction with any procedure for preserving or even reconstructing alveolar processes. It is therefore not possible to make any clinical recommendations in this respect. Nevertheless, the results found in this review favoured the intervention group over the control group and this suggests that alveolar preservation therapies do decrease the bone resorption process. Therefore, there are no absolute rules to help us make one decision or another. Each situation is unique and must be carefully assessed.

Two main limitations were found in the present investigation, which were; the inclusion of multiple grafting techniques and barrier membranes in the analysis with a lack of standardisation in the bone regeneration methods used; and the lack of evaluation of local and systemic factors known to play an important role in the remodelling process. Finally, by focusing on Clinical Trials and Systematic Reviews, with the exclusion of other types of studies, significant information may have been omitted, which could have conditioned the results obtained.

Future lines of research: Although cavity preservation is a well-studied technique, further clinical studies are needed to investigate the dehiscence and fenestration present after tooth extraction and their influence on the dimensional changes in PRA. Further and more homogeneous studies involving a follow-up period of more than 5 years are needed to evaluate the success and survival of implants placed after performing PRA techniques. It would also be interesting to study the biological behaviour of the different biomaterials and membranes on the market. Finally, it would be interesting to establish protocols based on scientific evidence.

1. The preservation of the alveolar ridge with the use of different bone substitutes represents an effective method to decrease the physiological resorption process after tooth extraction. In addition, minimal differences in the resorption rate between allogeneic, xenogeneic and alloplastic graft materials were observed.
2. The use of barrier membranes (ROG) has a great benefit in preserving the bone height of the alveolar ridge.
3. Plasma rich in growth factors stimulates hard and soft tissue repair and regeneration, resulting in higher quality bone, and reduces inflammation and its associated pain and discomfort.
4. Immediate implant placement can be considered as an alveolar preservation technique.
5. Although PRA is a widely studied technique, more research is needed, as well as more standardised protocols.

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