Preclinical and Clinical Applications of Biomaterials in the Enhancement of Wound Healing in Oral Surgery: An Overview of the Available Reviews

Oral surgery has undergone dramatic developments in recent years due to the use of biomaterials. The aim of the present review is to provide a general overview of the current biomaterials used in oral surgery and to comprehensively outline their impact on post-operative wound healing. A search in Medline was performed, including hand searching. Combinations of searching terms and several criteria were applied for study identification, selection, and inclusion. The literature was searched for reviews published up to July 2020. Reviews evaluating the clinical and histological effects of biomaterials on post-operative wound healing in oral surgical procedures were included. Review selection was performed by two independent reviewers. Disagreements were resolved by a third reviewer, and 41 reviews were included in the final selection. The selected papers covered a wide range of biomaterials such as stem cells, bone grafts, and growth factors. Bioengineering and biomaterials development represent one of the most promising perspectives for the future of oral surgery. In particular, stem cells and growth factors are polarizing the focus of this ever-evolving field, continuously improving standard surgical techniques, and granting access to new approaches.


Introduction
The birth of oral surgery is difficult to trace back. Considering exodontics as a part of this specialty, its roots may go back to the origins of dentistry itself. However, since the publication of Anselme Louis Bernard Berchillet's "Traité des maladies et des opérations réellement chirurgicales de la bouche," published in 1778, oral surgery has dramatically evolved and expanded together with the other surgical fields of medicine, effectively encompassing a wide range of procedures beyond exodontics, such as oral pathology, orthodontics, complex reconstructive techniques, and so on. These changes are intertwined with the continuous development of evidence-based medicine and new technologies that constantly challenge the very logic behind the surgical approach, from diagnosis to treatment. Improving oral surgery gives access to new and more complex clinical scenarios stemming from the increase in patients'

Criteria for Review Selection and Inclusion
The review selection was performed by two independent reviewers. Disagreements were resolved by a third reviewer. Selection was limited to reviews that included clinical studies describing both clinical and histological effects of biomaterials on post-operative wound healing in oral surgical procedures. Reviews analyzing non-surgical oral procedures (e.g., orthodontics and conservative dentistry therapies) were discarded. A time limitation of a minimum of 6 weeks for the postoperative

Criteria for Review Selection and Inclusion
The review selection was performed by two independent reviewers. Disagreements were resolved by a third reviewer. Selection was limited to reviews that included clinical studies describing both clinical and histological effects of biomaterials on post-operative wound healing in oral surgical procedures. Reviews analyzing non-surgical oral procedures (e.g., orthodontics and conservative dentistry therapies) were discarded. A time limitation of a minimum of 6 weeks for the postoperative evaluation period was applied.

Results
A total of 41 articles were included in the final review. Final papers are summarized in Table 1, while excluded articles are specified in Table 2.   There is strong evidence that recombinant human plateletderived growth factor (rhPDGF) is efficient in the regeneration of intrabony defects when applicated in combination with a bone matrix. In particular, rhPDGF benefits from the delivery with an osteoconductive scaffold matrix. Clinical and histological results confirmed that rhPDGF in combination with a scaffold was also efficient in the treatment of furcation defects.
Pranskunas et al. [36] 2019 11 (9 human and 2 animal studies) stem cells socket preservation procedures The use of bioactive osteogenic molecules or mesenchymal stem cells supports bone regeneration after tooth extraction. Histologically, no particular differences are revealed between test and control groups.   Results are discussed below and divided into specific topics: stems cells, bone grafts, and growth factors.

Background
Stem cells can be defined as undifferentiated cells characterized by a set of unique properties. A stem cell is capable of proliferation, self-renewal, production of differentiated daughter cells, self-maintenance of their population, and regeneration of injured tissue. An additional key aspect behind stem cells behavior is the flexibility in their behavior based on environmental conditions [58,59]. Stem cells belong to two main subtypes: pluripotent (or totipotent), able to differentiate in any kind of human cell, and multipotent, that can develop into multiple cell types within their lineage. They can be successfully isolated from the inner part of the blastocyst, prior to the implantation of the embryo, together with fetal and adult tissue. Adult stem cells are generally multipotent and are found in most human tissues, as they support the active cell turnover for tissues undergoing self-renovation at different degrees, and enable tissue repair by replacing damaged or lost cells. Adult stem cells can be harvested from various tissues, such as the bone marrow and the oral cavity. In fact, it is possible to find multipotent adult stem cells in exfoliated deciduous teeth, dental pulp, and periodontal ligaments that show osteogenic and neurogenic capacities [60]. Mesenchymal stem cells obtained from the bone marrow are able to differentiate into various cell types and can respond to the medium where they are inserted to differentiate themselves in the appropriate tissues, as needed [6]. Donor area of choice is usually the iliac crest bone marrow; however, the harvesting procedure from this site may be trivial and painful, so new solutions are being looked for to bypass this kind of inconvenience. Interestingly, mandible periosteum and maxillary tuberosity have been proved as a reliable source of mesenchymal stem cells with osteogenic potential, easy to access under local anesthesia and with low to no post-operative discomfort [61].

Overview of Reviews
In total, six reviews were found on the topic, one considering only animal studies, three only human study, and two accounting for both animal and human studies. The review from Amghar-Maach et al. [8], focused on animal studies, assesses the efficacy of dental pulp stem cells (DPSC) in the regeneration of periodontal defects, but remarks how the biomaterial architecture is relevant to the regeneration outcome. In fact, grafting stem cells in the form of cell sheets leads to better results when compared to the injection of dissociated cells. Additionally, pairing stem cells with growth factor such as hepatocyte growth factor (HGF) may favor DPSC differentiation. Correia et al. [16] and Mangano et al. [30] discuss the impact of mesenchymal stem cells in maxillary sinus augmentation, even if paired up with other biomaterials. Mesenchymal stem cells (MSCs) show a positive impact on wound healing and bone regeneration considering vital bone and vital bone percentage, leading to better outcomes in terms of osteogenesis and bone volume gain. Socket preservation procedures, analyzed by Pranskunas et al. [36], show an increase in the clinical and radiographical aspects of wound healing in both animal and human studies; however, no significant difference from a histological point of view was found. Considering implant-related bone regeneration procedures [45] (Varshney et al.), both adipose-derived and bone marrow-derived stem cells have been proved to improve the expected results. However, while this efficacy is particularly relevant in animal models, the treatment of large defects in humans does not always relate to a predictable outcome in terms of regeneration. Therefore, even if very promising, stem cell use in the improvement of oral wound healing is not highly predictable. A better understanding of cellular interactions in the healing phases could help overcome this flaw. The combined use of stem cells and growth factors may improve the efficacy of the regenerative approach in a significant way.

Background
Bone grafts are natural or synthetic biomaterials used in the regeneration of defective bone volumes. They can be classified according to their source, microscopic architecture, form, and blood supply [62].
Considering their origin, bone grafts can be defined as: -Autografts, obtained from the same individual that receives the graft; -Isografts, from an individual from the same species sharing the same antigenic profile (twins); -Allografts, harvested from an individual from the same species but with a different antigenic profile; -Xenografts. obtained from species other than human; -Alloplastic materials, synthetic bone graft substitutes [63,64].
Moreover, bone grafts can exhibit different properties that provide the rationale for their use in regenerative procedures: -Osteogenesis: the graft contains living osteoblasts that contribute to new bone formation; -Osteoinduction: the graft is able to stimulate the differentiation of osteoprogenitor cells into osteoblasts; -Osteoconduction: the graft acts as a scaffold to sustain the development of capillaries and precursor bone cells [65].
Osteogenesis requires the presence of mesenchymal cells able to differentiate into mature osteoblasts (such in autografts). Osteoinduction usually relies on the presence of growth factors, molecules able to mediate cells recruitment, proliferation, and differentiation, and represents one of the most challenging tasks for the development of bone graft substitutes [66].
Regardless of their osteogenetic and osteoinductive properties, every bone graft has to grant a three-dimensional mechanical structure that hosts and supports cells and extra-cellular matrix [62].
The key feature to a scaffold is the porosity of its structure, since pores increase contact surface of the bone graft, favoring its degradation, and allow cell migration and proliferation [30]. Pore diameter, together with pore morphology and interconnectivity, [67] seem to affect cell behavior, favoring neoangiogenesis with a diameter greater than 300 µm, and osteoblasts migration, adhesion, and proliferation with a diameter of 200-400 µm. Current literature suggests that a porosity of more than 50% by volume and pore sizes of 200-800 µm are the most adequate feature for the development of bone tissue [62,68,69]. These grafting properties are defined as osteocondustive, as described above.
The ideal bone graft should exhibit osteogenic, osteoinductive, and osteoconductive properties while lacking antigenic, teratogenic, or carcinogenic reactions, favor neoangiogenesis, be resorbable, possess a hydrophilic nature, and have low morbidity and cost. Resorbable membranes are devices commonly paired up with bone grafting materials; among them, resorbable collagen membranes (RCMs) are the most commonly found in clinical practices [70]. RCMs are manufactured from allogeneic or xenogeneic sources to manage oral wounds such as extraction sockets, sinus-lift, and ridge augmentation procedures, and periodontal and endodontic surgeries [71][72][73][74]. They are one of the essential tools in guided bone regeneration (GBR) techniques, enhancing wound healing through promotion platelet aggregation, clot stabilization, and fibroblast attraction [75,76]. Time of resorption varies from 2 to 32 weeks and they are biocompatible, easy to manipulate, and with low immunogenicity [77]. They are available as membranes, plugs, or pads for ease of use [70].

Bone Grafts Categories
Autografts Autografts are often regarded as the 'gold standard' among bone grafts due to their osteogenic and osteoinductive properties. Additionally, they are almost safe from the risk of immune reaction/rejection, being harvested from the same subject that receives the graft itself. However, one of their major drawbacks is represented by the necessity of a surgical intervention to collect the graft, and this may affect a patient's systemic health, increase morbidity, and expose the subject to the risk of chronic postoperative pain and hypersensitivity of the donor area [78]. In oral surgery, autografts are proven to not be able to counteract the volume contraction of the hard tissues of the edentulous sites [79]. Autograft can be harvested by various intra-and extra-oral donor sites. Intraoral donor sites include edentulous ridges, extraction sockets, mandibular ramus, symphysis, and maxillary tuberosity, while extraoral donor sites are tibia, iliac crest, and calvarium [80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95].

Allografts
Allografts are collected from individuals, either dead or alive, of the same species but with a different genotype, processed in order to prevent the host's immune response and transmission of infectious diseases [89]. They are available as cortical, cancellous, or cortico-cancellous grafts, in various shapes and sizes. Allografts processing has evolved over the years, from the use of fresh frozen bone, simply frozen at −80 • C and no longer used due to the risk of disease transmission and immune response, to the demineralized freeze-dried bone allograft (DFDBA), processed to preserve the organic part that contains bone morphogenetic proteins, growth factors responsible for the graft osteoinductive properties [96].

Xenografts
Xenografts are obtained from donors of a species other than the host's one, and mostly act as scaffolds showing osteoconductive features and slow resorption time. They can be used both alone and paired up with growth factors or other grafts to enhance their properties. Their lack of osteogenetic and osteoinductive properties is balanced by their availability and relatively low cost. Originating from non-human species, the risk for disease transmission and immunogenicity has to be accounted when using xenografts [97].
Among the many available xenografts, the main categories refer to bovine substitutes, equine substitutes, porcine substitutes, algae substitutes, and coral substitutes.

Alloplastic Materials
Alloplastic materials are biomimetic synthetic bone substitutes, characterized exclusively by osteoconductive features, with no osteoinductive or osteogenic properties. Therefore, they act as a scaffold to support cell migration, proliferation, growth and bone tissue formation [98]. Considering the many chemical and physical properties, they are considered the most heterogeneous group of materials that includes, among the many synthetic bone substitutes, calcium phosphate, calcium carbonate, calcium sulfate, bioactive glasses, and polymers.

Overview of Reviews
A total of twenty-five reviews were found regarding bone grafts and different surgical procedures, such as alveolar socket/ridge preservation, periodontal regeneration, atrophic jaws augmentation, sinus augmentation procedures, and alveolar ridge splitting/espansion technique (ARST).
Preservation procedures found controversial results in literature, with not always clear evidence regarding new bone formation when comparing natural healing to grafts [10,19,31,48].
Magnesium-enriched hydroxyapatite (mHA), calcium sulphate, and porcine grafts granted a better outcome when compared to natural healing, while DFDBA proved to be the most efficient allograft among all [10].
Short term bone wound healing, from 3 to 4 months from dental extraction, shows similar clinical, radiographical, and histological characteristics regardless of the use of a bone graft.
Chan et al. [14] found conflicting results concerning the percentage variation of the vital bone with the use of xenografts, ranging from -22% (decrease) to 9.8% (increase), while connective tissue formation likely decreases with the use of bone substitute. Only limited evidences support an increase in vital bone formation following the use of alloplasts. Significant amounts of hydroxyapatite and xenograft particles (15 to 36%) were found at the healing site at an average of 5.6 months after grafting, as a proof of their stability and resistance to resorption. According to Horvath et al. [25], only a limited cluster of studies report a statistically significant increase in trabecular bone formation when using bone grafts in the alveolar ridge preservation; the use of bone substitute does not prevent ridge resorption but rather delays it, due to the permanence of graft particles inside the healing sockets. From the histological point of view, conflicting evidences are found regarding the benefits of ridge preservation, with no active promotion of bone formation sustained by bone grafts, and rather peculiar histological pictures of what resembles a foreign-body reaction from the host to the bone substitute particles [19,34].
While showing an impact on the reduction in the vertical bone dimension following tooth extraction, socket grafting showed no clear evidence of bone dimensional preservation, bone formation, or keratinised tissue dimensions [29].
On the other hand, Willenbacher et al. [46] found an increase in the preserved bone quota, approximately 1.31 to 1.54 mm bucco-oral bone width and 0.91 to 1.12 mm bone height, in alveolar sockets preserved with grafting materials.
The use of alternative graft solutions, such as tooth-bone graft, demonstrated no added benefits over conventional graft materials [22].
Allogeneic bone blocks represent a good alternative to autologous bone blocks, however, histological analysis highlights differences in their behavior during the healing phases. At 6 months. no connective tissue was found and the presence of inflammatory cells was meaningfully lower when recurring to autologous bone, while in the allogeneic blocks large segments of necrotic bone with empty osteocytes lacunae and little osteoclastic activity were found, along with blood vessels invading the Haversian canals of the graft [33].
The use of bone blocks enables vertical-deficient sites to be rehabilitated with implants in animal models [40].
Advanced atrophic bone augmentation techniques, such as ARST, may benefit from the use of bone grafting to preserve buccal bone height and width [11].
Autologous bone shows the best results in sinus augmentation procedures despite its high resorption rate (40%) in animal studies. This downside can be overcome by mixing it to other bone grafts, such as porous hydroxyapatite or bioglasses [12,15,26].
Periodontal regeneration of intrabony defect using bone grafts proved to be superior in terms of regenerative outcomes when compared to simple flap surgery with no use of biomaterials in both animal and human studies, with autologous bone showing the most favorable results [27,34,47].
The use of grafts combined with membranes proved the best result in terms of periodontal regeneration [37], especially in supra-alveolar and two wall intrabony defects models in animals. Three wall intrabony defects do not benefit consistently of the use of grafts or membrane systems [41].
Regeneration of gingival defects is, however, according to Danesh-Meyer et al. [17], compromised by the wound stabilizing effect of the membrane itself, which does not provide adequate space to promote periodontal regenerational while simultaneously impeding apical migration of the gingival epithelium.
Interestingly, alloplastic grafts likely support periodontal repair rather than regeneration [38] and appear to show limited amounts of periodontal regeneration when compared to the other biomaterials [42].
Overall, bone grafts and membranes represent an essential tool in the hard and soft tissue regeneration. In recent years, the efficacy of bone grafts in the socket/ridge preservation techniques has been debated and more and more controversial evidences are emerging, while technology evolves and opens up new scenarios, such as in the case of Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) manufactured bone scaffolds. The key to an optimal wound healing relies, as always in the choice of the correct device, in a deep comprehension of their biological and mechanical properties.

Background
Growth factors (GFs) are molecules able to regulate DNA synthesis, chemotaxis, matrix synthesis, and promote cellular growth, proliferation, and cellular differentiation, usually of proteic or steroid nature.
Among the wide category of growth factors, some have polarized the research attention over the recent years, such as: Platelet-derived growth factor (PDGF): known as one of the initiators of wound healing, with multiple functions ranging from chemotaxis and mitogenesis to promotion of angiogenesis, acts on both soft and hard tissues; -Transforming growth factor beta (TGF-β) and insulin-like growth factor (IGF): regulate collagen and fibronectin synthesis through osteoblasts or fibroblasts stimulation; -Amelogenins: extracellular matrix proteins secreted by ameloblasts that regulate hydroxyapatite crystal growth and orientation and are able to promote periodontal tissues regeneration; in the clinical practice, they are commonly found in enamel matrix derivatives (EMD) compounds, a mix of enamel matrix proteins (EMP), of which amelogenins represent circa 90% of the total protein quota [99,100]; -Statins: recently discovered to possess anti-inflammatory, antimicrobial and pro-osteogenic properties.
An important biological vehicle responsible for delivering GFs to wounded sites is represented by platelets that, in addition to their procoagulant effect, release many biomolecules like PDGF, TGF-β, VEGF, etc.
Therefore, despite the availability of recombinant GFs, the use of autologous platelet concentrates has found many applications in oral surgery.
Platelet concentrates belong to four main categories: 1. Pure Platelet Rich Plasma (P-PRP) or leukocyte-poor PRP that does not contain leukocytes; 2.
PRP is a first-generation platelet concentrate containing platelets in super-natural concentration and minimal amount of natural fibrinogen. Platelets' α granules are responsible for the release of growth factors within 3-5 days of platelet activation, which sustain their stimulation of proliferative phase for 10 days after release. However, calcium chloride and bovine thrombin are added to reach gel consistency and these components may interfere with wound healing [101,102].
Preparation rich in growth factors (PRGF-Endoret) technology was invented as an answer to some of the limitations of PRP preparations. The clot activator, calcium chloride, leads to the formation of native thrombin. This mimicked physiological clotting process enables a more sustained release of growth factors. Moreover, this procedure reduces the risk of immunological reactions and disease transmission associated with the use of exogenous bovine thrombin [103].
PRF represents a new generation platelet concentrate, an evolution of PRP. Similar to the blood clot, it is a tetramolecular fibrin matrix that contains all the molecular and cellular elements, such as platelets, leukocytes, cytokines, and circulating stem cells, that promote healing simultaneously being more stable and homogenous. Furthermore, 20 PRF does not require addition of bovine thrombine or other substances, thus it does not share the coagulant-related drawbacks of PRP [104,105].

Overview of Reviews
A total of thirteen reviews were considered regarding the use of growth, both autologous or recombinant.
In sinus augmentation, PRP does not significantly affect the histological density and quality of the regenerated bone; however, early wound healing was observed [9].
According to Stähli et al. [43], while having no evidence supporting the clinical benefit of PRP in healthy patients, PRP might have a positive effect on wound healing and bone regeneration in compromised patients.
PRF, however, showed superior outcomes in bone regeneration procedures, as per ridge dimension, bone regeneration, osseointegration process, and soft tissue healing [44].
According to Darby and Morris [18], periodontal regeneration performed through the use of PDGF led to greater CAL gain of around 1mm, a greater percentage bone fill of around 40%, and an increased rate of bone growth, compared to an osteoconductive control (β-TCP), with no particular adverse effects. This consideration is backed up by the systematic review of Giannobile and Somerman [23], assessing that PDGF promotes periodontal regeneration at the histological level.
The efficacy of PDGF may be further improved, associating it with an osteoconductive scaffold matrix [35].
Similarly, EMD were found to consistently promote CAL gain and probing depth reduction when compared to flap surgery alone, and its effect is improved using it in combination with graft materials [23,28].
The efficacy of EMD is further highlighted in the treatment of gingival recession [16], improving soft tissue height and thickness; using EMD together with coronally advanced flaps in root coverage seemingly leads to periodontal regeneration with formation of root cementum, periodontal ligament, and alveolar bone [32]. Platelet concentrates showed positive effects on the healing outcome of both soft and hard tissue in the post-extraction alveolar socket, with a significant increase in the keratinized mucosa quota and in the new bone formation percentage, although this result is controversial [20,21].
Local application of statins shows an apparent osteogenic and angiogenic effect in periodontal defects models in animal studies; topical simvastatin enhances wound healing and improves patient outcomes, stimulating bone formation, promoting soft tissue healing, as well as reducing post-operative pain and inflammation [24,39].

Conclusions
The development of biomaterials represents one of the most promising perspectives for the future of oral surgery. In particular, stem cells and growth factors are polarizing the focus of this ever-evolving field, continuously improving standard surgical techniques, and granting access to new approaches. Bone grafts and membranes usually play a pivotal role in GBR procedures. Despite their long history as essential tools in regenerative procedures, controversial evidences are emerging regarding the socket/ridge preservation techniques, that represent the basic approach in the modern oral surgery to the post-extraction socket and edentulous ridge. Technology evolves and opens up new scenarios, such as in the case of CAD/CAM manufactured bone scaffolds.
The regenerative properties of the biomaterials used in oral surgery may be improved thanks to growth factors. Their combined use, in fact, likely enhances the healing processes and favors early wound healing after oral surgical procedures.