The present manuscript aims to critically detail the physiologic process of socket healing, in the absence or presence of grafting materials or platelet concentrates, addressing the associated molecular and cellular events that culminate in the restoration of the lost tissue architecture and functionality.
An electronic search in the National Library of Medicine database MEDLINE through its online site PubMed and Web of Science from inception until May 2019 was conducted to identify articles concerning physiologic process of socket healing, in the absence or presence of grafting materials or platelet concentrates. The search was restricted to English language articles without time restriction. Additionally, a hand search was carried out in oral surgery, periodontology and dental implants related journals.
In total, 122 literature sources were obtained and reviewed. The detailed biological events, at the molecular and cellular level, that occur in the alveolus after tooth extraction and socket healing process modulated by grafting materials or autologous platelet concentrates were presented as two entities.
Tooth extraction initiates a convoluted set of orderly biological events in the alveolus, aiming wound closure and socket healing. The healing process comprises a wide range of events, regulated by the interplay of cytokines, chemokines and growth factors that determine cellular recruitment, proliferation and differentiation in the healing milieu, in a space- and time-dependent choreographic interplay. Additionally, the healing process may further be modulated by the implantation of grafting materials or autologous platelet concentrates within the tooth socket, aiming to enhance the regenerative outcome.
The surgical extraction of teeth is one of the most common dental procedures and the subsequent process of socket healing became a fundamental topic of research and discussion, both within the frame of biomedical and clinical-based dental sciences. The continuous demand for improved aesthetics and functionality have led to the need of preserving and maintaining an adequate tissue volume, of both soft and hard tissues, in order to achieve a successful and long-lasting implant-based oral rehabilitation. Notwithstanding, healing is an intricate process that runs from a timely organized interplay of distinct biological systems, involving a plethora of molecules and cells throughout distinct phases [
The presence or absence of teeth influences alveolar socket remodelling. After tooth extraction, a series of orderly biological events occurs in the alveolus, which results in socket healing [
In the recent literature, the process of socket healing was studied in humans, as well as in distinct experimental animal models, as mice, rats, dogs, and monkeys (
Coagulation and haemostasis, which immediately follows the teeth extraction;
Inflammation, that is initiated shortly thereafter;
Proliferation, initiated in the subsequent days and incorporating the majority of the healing process;
Modelling and remodelling, aiming to restore the lost architecture and functionality, and lasting for several months [
Studies that evaluated the process of socket healing with different type of model and different time of healing
Study |
Year of |
Model |
Time of |
---|---|---|---|
Vieira et al. [3] | 2015 | Mice | 0 - 21 days |
Smith [4] | 1974 | Rat | 1 - 20 days |
Cardaropoli et al. [5] | 2003 | Dog | 1 - 80 days |
Scala et al. [6] | 2014 | Monkey | 4 - 180 days |
Amler et al. [10] | 1969 | Human | 2 - 32 days |
Trombelli et al. [14] | 2008 | Human | 2 - 24 weeks |
Lin et al. [33] | 2011 | Rat | 3 - 14 days |
Hsieh et al. [43] | 1994 | Rat | 5 - 14 days |
Devlin and Sloan [44] | 2002 | Human | 2 weeks |
Lindhe et al. [51] | 2012 | Human | Varying periods of time |
Araújo and Lindhe [52] | 2005 | Dog | 1 - 8 weeks |
Biological events occurring in the socket after tooth extraction described into a sequence of four time-dependent phases.
The purpose of the present study was to detail the biological events of the socket healing process after tooth extraction, and further provide a comprehensive overview of the molecular and cellular aspects that occur through the healing, in the absence or the presence of graft materials and autologous platelet concentrates, aiming improved tissue healing.
The protocol for this study was registered prospectively in the PROSPERO, an international prospective register of systematic reviews.
The protocol can be accessed at:
Registration number: CRD42019132277.
The reporting of this study followed, the guidelines of PRISMA statement [
Focused question 1: What are the detailed biological events, at the molecular and cellular level, that occur in the alveolus after tooth extraction?
Focused question 2: How is the socket healing process modulated by grafting materials or autologous platelet concentrates?
The search strategy was based on electronic database examination. A search was implemented on the National Library of Medicine database MEDLINE through its online site PubMed and Web of Science, and was conducted from their inception until May 2019, to identify articles concerning the physiologic process of socket healing.
The following terms and their combinations were used for the search: "tooth extraction AND socket healing OR alveolar healing AND bone repair OR bone regeneration OR bone healing". Details of the electronic search strategy are presented in
Flowchart of article selection procedure.
The articles, at any stage (abstract or full-text assessment) were independently reviewed by 4 of the authors (P.S.G, L.P, P.D. and L.M) to confirm each study’s eligibility. Any discrepancies between reviewers were resolved by discussion and consensus, and by consulting an additional experienced senior reviewer if needed (M.H.F).
The review included all human and animal studies published in English. Letters, editorials, PhD theses, and abstracts were excluded.
Present review included all retrospective and prospective follow-up studies, case-control studies, case report series, cohort studies, experiments, comparative analyses and observational studies without time cut.
The subjects of the included studies were humans or animals submitted to tooth extraction procedures, with or without any further intervention.
The inclusion criteria were as follows:
No systemic disorders associated;
Clinical, histological, morphological or cellular/molecular outcome parameters and/or imagiologic alveolar bone dimensions outcome parameters.
The exclusion criteria were:
Studies that involved the application of any additional therapy, other than graft materials and autologous platelet concentrates, that could have affected socket healing outcomes;
Studies involving the same population and reporting the same outcome variables, as other included studies.
Following the initial literature search, all article titles were screened to eliminate irrelevant publications. Next, within the abstract reading stage, inclusion and exclusion criteria were applied to the information given in abstracts; if any information was missing, the full-text reading was performed. At the final stage, all the included articles were carefully screened and only relevant articles were included for further analysis.
Two independent reviewers extracted the relevant data using an Excel spreadsheet (Microsoft, Redmond, WA). The extracted data were study design, study setting, follow-up duration, wound healing process phases, classification of the cytokines, chemokines and growth factors and regenerative biomaterials types. Additional data were extracted and descriptive from relevant aspects involved in the research.
No meta-analyses could be performed due to the heterogeneity between the studies (different study designs, control groups, and observation periods).
Immediately after tooth extraction, the socket is filled with blood resulting from the haemorrhagic process, followed by the formation of a stable blood clot embedded in a network of fibrin [
Mechanistically, teeth extraction leads to microvascular damage and blood extravasation, processes that are rapidly controlled by the reflex vasoconstriction, responsible by the retrenchment of vascular smooth muscle cells - able to control bleeding in arterioles up to 0.5 cm in diameter [
Platelets release clotting factors upon activation, which occurs following the contact with extracellular matrix molecules. Blood clot and associated platelets, besides haemostatic functions, further play a fundamental role for proper tissue healing due to the presence of many cytokines (as those from chemokine and interleukin families) and growth factors (e.g., tumour necrosis factor alfa family, transforming growth factor beta family, fibroblast growth factor family, epidermal growth factor family, and individual factors such as platelet-derived growth factor, vascular endothelial growth factor, granulocyte macrophage colony stimulating factor and connective tissue growth factor, among others), that are able to modulate subsequent cellular processes - cell migration, proliferation and differentiation - fundamental to promote angiogenesis and bone regeneration [
Platelets also contain and release vasoactive amines and arachidonic acid-derived metabolism products that play a fundamental role in the initiation and modulation of the subsequent inflammatory phase. In this context, adhesion molecules - as vitronectin and distinct integrins - were found to be positively regulated during the initial events, assisting on the subsequent cell recruitment, adhesion and activation [
A transitory and moderate inflammatory process is essential for adequate bone healing/regeneration, as embraced by the concept of constructive inflammation, with the activation of both humoral and cellular inflammatory response [
Upon blood clot establishment, the recruitment and migration of inflammatory cells are verified, throughout the first days following tooth extraction [
Macrophages are following recruited, from circulating monocytes that experience phenotypic maturation, being responsible for the continuation of phagocytosis and further providing the release of effective growth factors - transforming growth factor-alfa, transforming growth factor-beta, fibroblast growth factor, and epidermal growth factor - that activate subsequently recruited fibroblasts and osteoblasts [
The proliferative phase is initiated by fibroplasia. At this time, there is an intense fibroblast migration and proliferation, as well as an increase in collagen synthesis and other extracellular matrix proteins. The newly formed abundant extracellular matrix further supports cell migration, allowing for enhanced cell adhesion and anchorage thorough filopodia and pseudopodia extensions that attach fibronectin and collagen proteins of the matrix [
Histological data reveals that, during the first week of healing, extraction socket is suffused with loosely organized cell-rich granulation tissue with an intense infiltration of inflammatory cells, vascular sprouts and fibroblasts - replacing the initial blood clot that undergoes coagulative necrosis, in a centrifugal process [
Early activation of TGF-β1 and FGF-2 seem to modulate activation and proliferation of fibroblastic populations, greatly determining the synthesis and maturation of the extracellular matrix and organization of the granulation tissue [
In fact, modelling and blood circulation re-establishment is essential and elapses from the fine balance of pro- and anti-angiogenic factors that populate the microenvironment, modulating endothelial cells’ functionality [
Progressively, the granulation tissue is replaced by a provisional matrix formed by immature connective tissue, rich in collagen fibres and recruited cells. Experimental animal studies have revealed that residual periodontal ligament fibres - identified perpendicularly to the socket wall and inserted in the bundle bone - are embedded within the newly formed matrix in the direction to the socket centre. This matrix progressively replaces the remnants of periodontal ligament, as well as those of the blood clot and granulation tissue [
At this phase of healing, the formation of woven bone begins through the penetration of undifferentiated mesenchymal cells that differentiate into the osteogenic lineage. Fingerlike projections of mineralized tissue are broadly identified, which progressively extend from the walls of the socket to the centre of the wound, in a connective tissue matrix laid with collagen fibres, without a structured organization [
Throughout the bone formation phase and osteogenesis activation, several GFs, such as platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), transforming growth factor β (TGF- β), bone morphogenetic proteins (BMPs), vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF), have been evidenced in the early and intermediate stages, with some differences in peak expressions [
BMPs and TGF-β are classically known to regulate a variety of cellular processes, such as cell-cycle progression, differentiation, motility and adhesion, as well as tissue specific functions such as neuronal growth, bone morphogenesis and wound healing [
Interestingly, BMPs may also stimulate the synthesis and secretion of VEGF, PDGF, FGF, and IGF which regulate angiogenesis [
In addition to the progressive formation of woven bone, this phase is also characterized by the absence of the periodontal ligament in experimental animal models, such as the rat [
Throughout this period, histomorphometric data from the dog revealed that provisional matrix reduced from 75 (SD 14.4)% to 22 (SD 7.4)% within the most coronal region of the socket, from 44 (SD 5.7)% to 10 (SD 2.2)% at mid region, and from 28 (SD 10.4)% to 5 (SD 2.4)% at the most apical portion. Contrariwise, mineralized bone tissue increased significantly within this period, from 15 (SD 11.1)% to 78 (SD 7.1)% at the marginal region, from 56 (SD 7.3)% to 90 (SD 2.1)% at mid-defect and from 72 (SD 11.2)% to 95 (SD 2.7)% at the apical region, sustaining the high metabolic activity of this phase with a high proliferation rate [
The final phase of the socket healing process embraces changes in the structure of the bone tissue, which can occur with modification of the architecture and shape (modelling), or without the modification of these parameters (remodelling). Organizationally, the substitution of woven bone with lamellar bone tissue/bone marrow is regarded as remodelling, whereas the resulting dimensional alterations of the alveolar ridge, at the socket wall, comprise bone tissue modelling [
Of additional relevance, it is possible to verify that the mesio-distal distance of the extraction socket becomes progressively reduced, due to the mature bone remodelling in the coronal region. This corticalization process - corresponding to the formation of the hard tissue bridge covering the socket entrance, separating the marginal mucosa from the extraction socket [
90 days after tooth extraction, it becomes very difficult to identify the socket limits in the different experimental models and human clinical studies. Within monkeys’ experimental socket healing model, mature bone was identified within all the assessed regions (i.e., coronal, middle and apical), with emphasis of the formation of mature cortical bone within the coronal region, leading to the socket corticalization, with the middle and apical locations found to be filled by newly formed trabecular bone and bone marrow [
In humans, 12 - 24 weeks after tooth extraction, lamellar bone and bone marrow were broadly identified, populated with vessels, adipocytes, mesenchymal stem cells and inflammatory cells [
In summary, the modelling and remodelling processes promote qualitative and quantitative alterations in the socket, which results in a reduction of the bone crest dimension. These dimensional changes occur most significantly during the first 8 weeks after extraction, due to the high osteoclastic activity present in this period [
Ridge preservation, or socket preservation involves placement of graft material within the socket, which can be further combined with a membrane, or rotated flap. The rationale for socket preservation is sustained for the fact that, once positioned in the fresh socket, graft materials act as solid scaffolds which assist on coagulum stabilization during the early phases of healing, by impeding the interference of destabilizing factors on the clot maturation process [
Ideally, the graft material should sustain the entire healing process of an extraction socket, being progressively resorbed and replaced by vital, mature bone [
Evidence from the experimental data from the dog model reveal that, after tooth extraction and socket grafting procedure, a fibrin network entraps the graft particles. Inflammatory cells and osteoclasts migrate to the surface of the particles, originating a slow and minimal removal of material from the outer surface of the graft particles [
From the histological point of view, the implantation of distinct grafts promoted new bone formation, possibly by osteoconduction at the apical and the middle part of the socket, while the coronal and central part of the socket have been found to be mainly occupied by graft particles surrounded by dense connective tissue, even some months following the alveolar ridge preservation surgery [
The composition of vital bone formation recorded in the literature was highly variable, with an amount ranging from 19.3% [
Overall, and despite the verified hindrances on the initial phases of the healing process, socket grafting seems to be effective on allowing for the formation of mature bone, further promoting ridge preservation by limiting the physiologic reduction, as compared to ungrafted healing, particularly on the preservation of the mid-buccal and mid-lingual height [
In addition to the use of autogenous, allogeneic, xenogeneic, and alloplastic biomaterials for socket preservation, autologous platelet concentrates (APCs) are widely reported in the literature to enhance extraction socket healing [
The beneficial effect of APCs in extraction socket healing to promote osteoid formation and extraction socket epithelization is mainly attributed to the release of cytokines and growth factors, immersed in the fibrin mesh, platelets, and leukocytes [
According to the fibrin structure, APCs can be present as liquid solutions or in an activated gel form [
The idea of using platelet supplementation to enhance extraction wound healing is primarily based on the ability of platelets to trigger healing response upon release of various growth factors. Platelets contain alpha granules, which degranulate upon platelet activation following the release of growth factors and stimulate cell migration and enhance cellular-level events to expedite wound healing. Experimental and clinical studies have found that platelet growth factors, such as the FGF and TGFβ-1, stimulate bone formation during osseous healing [
The leukocytes within the leukocyte-containing APCs mainly consist of lymphocytes, followed by neutrophils, monocytes, eosinophils, and basophils [
Neutrophils are known to participate in the antimicrobial host defence, both as the first line of the cell-based innate immune defence, as well as within the initiation of the adaptive immunity [
Monocytes, within L-PRP and L-PRF delivered to the alveolar socket, differentiate into tissue macrophages. Macrophages are key mediators of the wound healing process, playing a pivotal role in the transition between wound inflammatory and repair phase, with particular emphasis on osteogenesis [
Apart from the fundamental contribute of leukocytes to the immune-inflammatory activation, major cell populations also release various bioactive factors, when at the extraction wound site. Leukocytes present anti-nociceptive effects through the release of different chemokines, anti-inflammatory interleukins (IL-4, IL-10, and IL-13), and opioid peptides (b-endorphin, metenkephalin, and dynorphin-A), and can, therefore, promote a clinically relevant inhibition of postoperative pain. During the inflammatory phase of wound healing, these cytokines counteract the effects of the pro-inflammatory mediators generated naturally in the early stages of inflammation [
Socket preservation procedure demands slow resorption and adequate space maintaining biomaterials to stabilize the coagulum and counteract post extraction ridge resorption process, as suggested by Araújo et al. in 2008 [
In this manuscript, the alveolar socket healing process following tooth extraction was critically described, from a molecular and cellular point of view, further addressing histological and histomorphometric data from experimental animal models and human clinical studies. It was verified that each tissue component participating in the healing process (i.e. the blood clot, granulation tissue, provisional matrix, woven bone and lamellar bone) revealed a time- and space-dependent interplay, with discrete differences between models and a high inter-individual variability, but converging to mature bone formation and regeneration of the lost tissues. The biological aspects of the healing process following socket grafting were also detailed, sustaining the capability of particulate grafts and autologous platelets concentrates to modulate the molecular and cellular events throughout the healing process.
Overall, the detailed knowledge of the biological events associated with the socket healing process, as well as the temporal sequence of the healing events, is of foundational importance to improve clinical treatment outcomes and assist on the development of innovative biologically-based regenerative approaches for enhanced tissue healing.
Of additional relevance, and aiming to establish more robust and reliable comparisons and conclusions between assayed interventions for enhanced socket healing, future studies shall focus on unbiased control reports and experimental data homogeneity, assisting on effective determination of efficacy, effectiveness and patient satisfaction of the assayed therapeutic options. Also, experimental outcomes should be reformed to include clinical-meaningful and patient-centred outcomes, improving evidence-based decision making. Lastly, and since the current body of evidence derives majorly from single socket healing experiments, this poses potential limitations on the translational application to multiple extraction sites, being imperative to increase the evidence base within this path.
The work was partially supported by the project UID/QUI/5006/2019 with funding from FCT/MCTES through national funds. The authors report no conflicts of interest related to the present review.