Three-Dimensional Impression of Biomaterials for Alveolar Graft: Scoping Review

Craniofacial bone defects are one of the biggest clinical challenges in regenerative medicine, with secondary autologous bone grafting being the gold-standard technique. The development of new three-dimensional matrices intends to overcome the disadvantages of the gold-standard method. The aim of this paper is to put forth an in-depth review regarding the clinical efficiency of available 3D printed biomaterials for the correction of alveolar bone defects. A survey was carried out using the following databases: PubMed via Medline, Cochrane Library, Scopus, Web of Science, EMBASE, and gray literature. The inclusion criteria applied were the following: in vitro, in vivo, ex vivo, and clinical studies; and studies that assessed bone regeneration resorting to 3D printed biomaterials. The risk of bias of the in vitro and in vivo studies was performed using the guidelines for the reporting of pre-clinical studies on dental materials by Faggion Jr and the SYRCLE risk of bias tool, respectively. In total, 92 publications were included in the final sample. The most reported three-dimensional biomaterials were the PCL matrix, β-TCP matrix, and hydroxyapatite matrix. These biomaterials can be combined with different polymers and bioactive molecules such as rBMP-2. Most of the included studies had a high risk of bias. Despite the advances in the research on new three-dimensionally printed biomaterials in bone regeneration, the existing results are not sufficient to justify the application of these biomaterials in routine clinical practice.


Introduction
Craniofacial defects can originate from an array of etiological factors including congenital malformations, trauma, infection, rejection or implant failure, infection of bone graft, osteomyelitis, or surgical removal of tumors [1][2][3]. The craniofacial bone can also be impacted by systemic conditions such as osteodegenerative illnesses such as osteoporosis and arthritis, other impactful conditions include osteogenesis imperfecta and bone fibrous dysplasia [4]. All these conditions will compromise functional aspects such as phonation, mastication, and swallowing, which in turn affect the patient's quality of life [5,6]. The two most common craniofacial bone defects are cancer of the head and neck and cleft lip and palate (CLP) [5][6][7][8][9]. CLP is a multifactorial pathology with several genetic and epigenetic factors as well as environmental factors such as geographical location, socioeconomical factors, and race [10,11]. In an attempt to minimize anomalies resulting from CLP, multidisciplinary treatment is initiated from birth and carries on into adulthood in order to achieve optimal results [12].
During the mixed dentition stage, individuals with CLP may require a secondary alveolar bone graft. During this period, this approach can result in relevant improvements such as closure of oronasal fistulae, stabilization of the two maxillary segments, and enhanced support of the alar base, which, in turn, will improve nasal and labial symmetry [13,14]. The secondary alveolar bone graft was introduced by Boyne and Sands in 1972 and it is currently regarded as the gold standard with the iliac crest being the most frequently chosen donor location [13]. In order to assert the proper timing to perform this procedure, the upper canine should have two thirds of its root developed which usually occurs between the ages of 9 and 11 [13].
The autologous bone graft can present with a variety of setbacks including limited amount of grafted bone, immune response risks, procedure time, and heavy costs. Additionally, a year after the procedure, bone reabsorption will happen in 40% of cases creating the need for re-intervention [15,16]. The main donor sites of autologous bone in craniomaxillofacial surgery are iliac crest graft and calvarial graft, but intraoral graft is also a possibility [17]. Currently, regenerative medicine has been established as a viable alternative in treatment of bone defects including CLP [18][19][20][21]. This approach can modulate the bone regeneration process and inflammation and enhance the healing process. Various biomaterials have been developed with the intent of overcoming the limitations of conventional bone grafts [22], such as heterologous or homologous bone graft [23,24]. These substituting materials can be used on their own or combined with an autologous bone graft and/or matrices. The most recognized tissue regeneration approach in the literature in the treatment of alveolar bone defects is bone morphogenetic protein 2 [25,26]. This approach provides comparable outcomes concerning bone volume, filling, and height to the gold standard technique with the iliac crest bone graft [26].
The matrices ( Figure 1) are a subtract that allow for cell differentiation and proliferation. Their biocompatibility, biodegradability, osteoconduction, and mechanical properties are characteristics which can influence the success rate of the bone regeneration process [27]. two most common craniofacial bone defects are cancer of the head and neck and cleft lip and palate (CLP) [5][6][7][8][9]. CLP is a multifactorial pathology with several genetic and epigenetic factors as well as environmental factors such as geographical location, socioeconomical factors, and race [10,11]. In an attempt to minimize anomalies resulting from CLP, multidisciplinary treatment is initiated from birth and carries on into adulthood in order to achieve optimal results [12].
During the mixed dentition stage, individuals with CLP may require a secondary alveolar bone graft. During this period, this approach can result in relevant improvements such as closure of oronasal fistulae, stabilization of the two maxillary segments, and enhanced support of the alar base, which, in turn, will improve nasal and labial symmetry [13,14]. The secondary alveolar bone graft was introduced by Boyne and Sands in 1972 and it is currently regarded as the gold standard with the iliac crest being the most frequently chosen donor location [13]. In order to assert the proper timing to perform this procedure, the upper canine should have two thirds of its root developed which usually occurs between the ages of 9 and 11 [13].
The autologous bone graft can present with a variety of setbacks including limited amount of grafted bone, immune response risks, procedure time, and heavy costs. Additionally, a year after the procedure, bone reabsorption will happen in 40% of cases creating the need for re-intervention [15,16]. The main donor sites of autologous bone in craniomaxillofacial surgery are iliac crest graft and calvarial graft, but intraoral graft is also a possibility [17]. Currently, regenerative medicine has been established as a viable alternative in treatment of bone defects including CLP [18][19][20][21]. This approach can modulate the bone regeneration process and inflammation and enhance the healing process. Various biomaterials have been developed with the intent of overcoming the limitations of conventional bone grafts [22], such as heterologous or homologous bone graft [23,24]. These substituting materials can be used on their own or combined with an autologous bone graft and/or matrices. The most recognized tissue regeneration approach in the literature in the treatment of alveolar bone defects is bone morphogenetic protein 2 [25,26]. This approach provides comparable outcomes concerning bone volume, filling, and height to the gold standard technique with the iliac crest bone graft [26].
The matrices ( Figure 1) are a subtract that allow for cell differentiation and proliferation. Their biocompatibility, biodegradability, osteoconduction, and mechanical properties are characteristics which can influence the success rate of the bone regeneration process [27]. These matrices can be three-dimensional (3D) printed enhancing its adaptation to the bone defect. With the use of 3D technologies, these matrices can be created and adapted according to the specific needs of each patient by changing their internal and external structures whilst using different materials [27,28].
The most commonly used matrices in bone defect treatment are bioceramic and are usually made out of hydroxyapatite (HA) or β-tricalcicum-phosphate (β-TCP). These materials are highly biocompatible and with osteoinductive abilities while also promoting rapid bone formation [29]. Despite a general increase of interest regarding 3D printed These matrices can be three-dimensional (3D) printed enhancing its adaptation to the bone defect. With the use of 3D technologies, these matrices can be created and adapted according to the specific needs of each patient by changing their internal and external structures whilst using different materials [27,28].
The most commonly used matrices in bone defect treatment are bioceramic and are usually made out of hydroxyapatite (HA) or β-tricalcicum-phosphate (β-TCP). These materials are highly biocompatible and with osteoinductive abilities while also promoting rapid bone formation [29]. Despite a general increase of interest regarding 3D printed biomaterials in recent years, a comprehensive study regarding the general effectiveness of these biomaterials is lacking. To clarify this, we conducted a scoping review to assess the effectiveness of 3D printed biomaterials in the treatment of alveolar defects, which would be helpful for readership since it synthesizes what we know and the best future clinical approach in a single paper. Moreover, this knowledge will allow sustaining the realization of new future clinical studies. The aim of this paper is to put forth an in-depth review regarding the clinical efficiency of available 3D printed biomaterials for the correction of alveolar bone defects.

Risk of Bias
The bias risk of the in vitro studies was obtained using the Faggion Jr. norms for pre-clinical studies regarding dental materials [30]. For the in vivo studies, the bias risk tool from the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) was used.

Study Selection
The initial search, performed on the previously mentioned databases, gathered 792 studies. After removing duplicates, 604 studies were scrutinized according to title and abstract. Afterwards, all references deemed irrelevant for this systematic review were excluded, resulting in 123 potentially relevant studies. Given that 31 articles did not report bone regeneration rates, only 92 references were included in the final sample. The identification, screening and eligibility process is summarized in the flow chart ( Figure 2).

In Vitro Studies
Fifty-one articles analyzed the properties of biomaterials in vitro. The year of publication ranged from 2015 to 2022, with the exception of one study conducted in 2006 [31]. The most commonly used biomaterial in the control group was PCL matrix, followed by β-TCP and PLLA. Osteogenic activity through alkaline phosphatase was the most widely used method to assess bone regeneration, having been described in 26 articles. Seventytwo studies evaluated bone regeneration through the expression of osteogenesis-related genes. Only one study [32] reported the release rate of growth factors. On the other hand, one study [33] evaluated the porosity of the matrix and found that the presence of nanotubes is associated with more favorable results for osteogenesis when compared to larger pores. Table 1 summarizes the results of the in vitro studies included in this systematic review.

In Vivo Studies
In vivo bone regeneration was evaluated in 75 articles, published between 2015 and 2022, in various animal species, such as New Zealand rabbits, beagle dogs, and rat models. The number of animals used in each study ranged from 3 to 120, with seven articles not reporting the sample size [30,31,[82][83][84][85].
The most commonly used biomaterial in the control group was β-TCP matrix, followed by PCL matrix. Regarding the evaluation method, microcomputerized tomography was the most used followed by histology. Other methods used were real-time polymerase chain reaction [42,86] and immunohistochemistry [87].

Synthesis of Quantitative Evidence
In the various studies evaluated, many different biomaterials are described. The most referenced biomaterial was β-tricalcium phosphate (β -TCP), used in 16 in vitro studies and 27 in vivo studies. The second most referenced biomaterial was polycaprolactone (PCL), mentioned in 16 in vitro studies and 20 in vivo studies. Hydroxyapatite (HA) was the third most used biomaterial, in 7 in vitro and 16 in vivo studies. There are other biomaterials/biomolecules that were used in more than 3 studies, namely: decellularized extracellular matrix (dECM), human recombinant bone protein type 2 (RhBMP-2), collagen, polylactic acid (PLLA), polylactic acid-co-glycolic acid (PLGA), calcium sulfate (SC), and different types of hydrogel (e.g., bone-derived extracellular matrix, β-TCP, cell-laden, nanocomposite, MicroRNA). All other biomaterials are mentioned only in a few studies, generating a multitude of results, which makes them difficult to analyze, and, consequently, to draw conclusions (Table 3). Table 3. Biomaterials described in the included studies (in vitro and in vivo).

In Vitro Studies
In   The most used evaluation method was different in in vitro and in vivo studies. In the first ones, the most frequent methods were the following: determination of osteogenesisrelated gene expression by qRT-PCR (27 studies), and the evaluation of alkaline phosphatase activity, a mineralization precursor protein, by p-nitrophenol assay (9 studies), and by a staining assay with the AKT assay kit (7 studies). In in vivo studies, radiological methods such as micro-CT (57 studies) and histological methods (56 studies) are the most used (Table 4).

In Vitro Studies In Vivo Studies
The most used 3D printing technique mentioned in both types of studies is extrusionbased 3D printing (23 in vitro studies and 27 in vivo studies). However, there are other techniques used simultaneously in in vitro and in vivo studies, namely: fused deposition modeling (6 and 10, respectively), stereolithography (2 and 7, respectively), and laser sintering technique (3 in both). Other techniques are used, but only occasionally in 1 or 2 studies (Table 5).

Risk of Bias
The risk of bias of the in vitro and in vivo studies is summarized in Tables 6 and 7, respectively. Regarding in vitro studies, none described the methodology to implementation sample. All in vivo studies also lacked information regarding sample allocation, allocation randomization process methodology, implementation, and protocol. All but three of the articles disclose information regarding study financing.
Regarding in vivo studies, most of the studies have serious methodological flaws, leaving out pivotal information such as sequence generation, allocation concealment, and blinding. Only six studies specify investigator blindness as a factor during outcome assessment. Lastly, seven other studies report no additional bias sources.

Discussion
The aim of the present systematic review was to report the current state of the art regarding the clinical efficiency of available 3D printed biomaterials for the correction of alveolar bone defects. Although the quantitative analysis of the results could not be executed due to the heterogeneity of the studies, the qualitative analysis allowed for a better understanding and evaluation of the published studies.
The conventional technique requires an autologous graft of cancellous bone and is considered the gold standard [13]. However, with the limited offer of donor bone as well as the bone reabsorption rate due to its adaptability to the defect site, a re-intervention may be necessary [15,16]. In an attempt to diminish these limitations, studies have been carried out in order to explore different approaches that can accelerate bone formation, reduce bone reabsorption and improve soft tissue scarring. 3D printed biomaterials can be specifically made to adapt to the bone defect site; this has led to an increase in studies regarding this topic over the last five years [27,28].
Out of the 75 in vivo studies included, 17 evaluated the efficiency of the PCL matrix [32,35,37,[50][51][52][53][54][55]62,63,65,66,73,77,81,120]. This biomaterial is the most well reviewed biomaterial in literature due to its high biocompatibility, durability and subsequent extensive use [37]. Despite its low degradation rate, the PCL matrix is limited in terms of cellular adhesion and osteogenic differentiation, several authors [32,35,49,50,53,62] have suggested combining it with different polymers [37] and bioactive molecules such as rBMP-2, that promote proliferation and differentiation of mesenchymal stem cells into osteoblasts resulting in bone formation. Nonetheless, a recently published umbrella review regarding the efficiency of current approaches in regeneration of bone defects in non-syndromic patients with cleft palate concluded that rBMP2 seems to provide results similar to the iliac crest bone graft in terms of bone volume and vertical dimension [121]. Another limitation of the PCL matrix is its low hydrophilia [52], which can be amended when the matrix is combined with a hydrophilic material such as β -TCP [35,66,77] or polydopamine [37]. With the addition of graphene, the PCL matrix increases its capacity to induce the secretion of growth factors that boost angiogenesis [56].
The β-TCP matrix was reportedly used in 12 in vitro and 30 in vivo studies. This calcium phosphate bioceramic presents ideal biocompatibility and osteoconductivity [36,64,85]. In addition to those characteristics, the β-TCP matrix also contains components similar to the bone tissue apatite along with a good balance between reabsorption and degradation during bone formation. Despite all these attributes, the osteogenic abilities of this biomaterial showed subpar results when used in large bone defects [35,48,64] and thus falling short when compared to the autologous bone graft [70].
The hydroxyapatite matrix is one of the most referenced bioceramics in in vivo studies. When combined with β-TCP this matrix becomes highly biocompatible and with a great osteointegration rate [88,90,123]. However, more studies are required in order to fully understand the macro-design that can optimize bone regeneration [90]. Since the bone formation process involves the immune system, this can be modulated by biomaterials such as esphingosine-1-phosphate (S1O) which has been linked to the β-TCP matrix. This sphingolipid has been shown to increase the expression of genes related to osteogenesis, such as osteoporin (OPN), transcribing factor 2 related to a runt (RUNX2), and osteocalcin (OCN) [36]. In addition to this, the combination of β-TCP with strontium oxide (SrO), sillica (SiO 2 ), magnesium (MgO), and zinc (ZnO) also proved to be effective in bone regeneration due to alterations in the physical and mechanical properties of the matrix [48].
Regarding PRF, this biomaterial can improve the reconstruction of the alveolar cleft. It is prepared from centrifuged autologous blood formed by a fibrin matrix that contains platelets, white blood cells, growth factors and cytokines. These factors may promote the uniqueness and differentiation pathways of osteoblasts, endothelial cells, chondrocytes, and various sources of fibroblasts, stimulating the regenerative capacity of the periosteum. Furthermore, the fibrous structure of PRF acts as a three-dimensional fibrin scaffold for cell migration [16]. In this way, PRF can be used with a bone substitute, allowing wound sealing, homeostasis, bone union, and graft stability [16]. In contrast, BMP-2 is usually applied in alloplastic bone grafts or scaffolding and is an effective inducer of bone and cartilaginous formation. Its application avoids the limitation of autologous bone grafts, which may be related to the shorter operative and hospitalization time. However, it has some adverse effects, such as nasal stenosis and localized edema at the graft site [26].
Another promising candidate for bone regeneration is the pure Zn L-PBF porous scaffold [74]. It presented relatively adjusted deterioration rates and mechanical strength for bone implants. Furthermore, they also showed well in vitro cytocompatibility with MC3T3-E1 cells and osteogenic capacity for hBMMSCs. The in vivo implantation results showed that pure Zn scaffolds have potential for applications in large bone defects with osteogenic properties [74].
Additionally, the microstructure of the matrices such as porosity, pore size, and structure play a very important role in cell viability and bone growth [115]. In contrast to traditional methods, the development of three-dimensional printing allows for the control of the microstructure. Therefore, a wide variety of materials and techniques are available to optimize the matrix [124]. Shim et al. reported that porosity affects osteogenesis, with matrices with 30% porosity showing better osteogenic capacity than groups with 50% and 70% porosity [115]. Regarding pore size, the literature suggests that the ideal size should be between 400 to 600 µm [63,103,111]. Finally, the pore configuration should also be considered in terms of the dynamic stability of the matrix. Recently, matrices with hierarchical structures have been studied. Zhang et al. demonstrated that tantalum matrices with hierarchical structures exhibited excellent hydrophilicity, biocompatibility, and osteogenic properties [33]. However, in the future, additional in vivo studies are required as to understand what structure the matrix should present in order to find a balance between cell viability and mechanical properties of the biomaterial, optimizing bone regeneration.
This systematic review presents some limitations that may alter the interpretation of the results, namely: (1) some of the included studies present a small sample size with only three animals; (2) the included studies present high risk of bias; (3) lack of evaluation of variables that interfere with bone regeneration, such as the position of the teeth in the bone graft, the width of the defect, the volume of grafted bone and the experience of the clinician; (4) absence of clinical studies; (5) heterogeneity of the studies in terms of matrix typology and follow-up used may difficult outcome assessment. Due to the heterogeneity in the methodology of the included studies, most of the studies selected in this systematic review were classified as having a high risk of bias, which may decrease the certainty of the results. According to the risk of bias analysis, the analyzed parameters with the highest risk of bias were sample allocation, allocation randomization process methodology, implementation, and protocol. These factors must be considered when figuring out the results of this review. The methodology of the several studies evaluated is very different and is not described enough, which makes their effective comparison impossible. Since there are numerous types of biomaterials/biomolecules and various combinations between them, future studies should define the most appropriate methodology, creating guidelines for its implementation and subsequent comparison.
In addition, future studies should be calibrated in order to use similar parameters and protocols, providing stronger evidence, focusing on the most described materials, namely β-tricalcium phosphate, polycaprolactone, hydroxyapatite with decellularized extracellular matrix (dECM), human recombinant bone protein type 2 (RhBMP-2), collagen, polylactic acid (PLLA), poly(lactic acid-co-glycolic acid (PLGA), and calcium sulfate (CS). Moreover, these promising materials should be evaluated and compared to each other in a single study in order to obtain more effective and clinically applicable conclusions. In the future, additional studies should be performed, more specifically blinded randomized studies with increased control of possible bias sources namely, the randomization process, concealment of the investigators of the experimental groups and description of the limitations of the studies. Moreover, the cost-effectiveness of the proposed new regenerative strategies should be evaluated, as it plays a crucial role in clinical decision making in healthcare systems, especially public institutions.
Lastly, future systematic reviews focused on 3D biomaterials should include only the most referenced evaluation and printing techniques. Therefore, for in vitro systematic reviews, the authors should compare PCL, b-TCP, RhBMP-2, and HA biomaterials created by extrusion printing, fused deposition, stereolithography, or laser sintering techniques. The chosen evaluation methodology should be gene expression by qRT-PCR and alkaline phosphatase activity. On the other hand, for in vivo systematic reviews, the authors should analyze the same biomaterials and the same technique printing, but the evaluation methodology should be based on radiology imaging and histology.

Conclusions
The most reported three-dimensional biomaterials were the PCL matrix, β-TCP matrix, and hydroxyapatite matrix. Despite the advances in the research on new threedimensionally printed biomaterials in bone regeneration, the existing results are not sufficient to justify the application of these biomaterials in routine clinical practice.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.