Tooth-Derived Matrix Granules for Enhanced Bone Healing: Chemical Composition, Morphological Aspects, and Clinical Outcomes

Abstract

Bone grafting has increasingly been used in surgical procedures for enhanced bone augmentation. Tooth-derived graft material has received considerable attention due to its chemical composition and autogenous source that can improve bone tissue healing. The main aim of this study was to provide a short and comprehensive review on the chemical composition, morphological aspects, and clinical outcomes of bone grafting using tooth-derived matrix granules. Dentin tissue has a chemical composition similar to that on bone tissues regarding the presence of hydroxyapatite, type I collagen, and different growth factors. Dentin-matrix granules are often processed at well-controlled size ranging from approximately 300 up to 1300 µm, while maintaining porosity and organic content. In addition, a dense collagen fiber network is still present after the milling and chemical treatment of dentin granules. Thus, dentin-matrix granules can improve the bone healing process considering their chemical composition, porous structure, and adequate size. However, further in vivo and in vitro studies should be performed taking into consideration different demineralization procedures, remnant organic content, porosity, and granule size.

1. Introduction

Restoration of alveolar ridge dimensions or bone reconstruction after tooth loss has been a challenge in implant dentistry. In some clinical cases, tooth forced eruption can be used as a tissue healing procedure and implant site development technique [1,2]. For instance, alveolar ridge preservation is a common clinical procedure performed to control alveolar ridge resorption after tooth extraction and to enable later implant placement and prosthetic rehabilitation [3]. Various types of bone substitutes have commonly been used for decades to restore the alveolar bone. Bone substitutes are classified according to their origin such as autogenous, allograft, xenograft, and alloplastic substitutes [4,5,6,7,8]. Autogenous bone is the first choice regarding its morphological aspects, chemical composition, and biologic content including osteogenic cells and growth factors [9,10,11]. Unfortunately, autogenous bone has limitations related to its availability, morbidity, and remodeling process [12,13]. An adequate grafting material must provide a highly bioactivity and balanced resorption in association with proper mechanical properties that enable the preservation and healing of bone defects. Furthermore, the material should be cost-effective and easily handled at chair-side clinical cases [14].

In dentistry, teeth are often extracted due to several issues such as advanced periodontitis, caries, orthodontic treatment, or wisdom teeth (third molar) extraction indication. Nevertheless, alveolar ridge atrophy can occur as a consequence of gradual alveolar bone loss [5,15,16]. Previous studies have shown different strategies for alveolar atrophy reconstruction or ridge preservation by including autologous tooth matrix [17,18,19,20]. Experimental studies have provided evidence that autologous tooth-derived matrix (ATDM) reveals osteogenic stimuli thanks to its chemical composition and biological content [21,22,23]. The chemical composition of dentin is quite similar to that of bone tissues considering the presence of hydroxyapatite (60–80%), type I collagen, growth factors, and other proteins [21,24,25,26]. Dentin contains the following growth factors: insulin-like growth factor (IGF)-II, bone morphogenetic protein (BMP)-2, and transforming growth factor (TGF)-b [27,28,29,30,31].

Therefore, ATDM is considered as a chair-side bone substitute regarding its chemical composition and autologous source. Thus, a harvested tooth can be mechanically and chemically treated providing a particulate material within 15–20 min [32,33,34]. Mechanical debridement is performed by removing the enamel to expose the dentin and by grinding to produce granules [33,35]. Then, a chemical treatment is carried out to disinfect and remove debris, leading to the exposure of the collagen fiber content and release of proteins. Previous studies have reported the use of demineralizing substances for chemical treatment such as sodium hydroxide or hydrochloric acid, although different parameters have been assessed [32,33,34]. Thus, the clinical feasibility of preparing ATDM granules plays a major role in the use of such material as a bone substitute. However, the amount of ATDM granules is limited, and therefore, a mixture with xenogeneic or synthetic bone substitutes has become a promising strategy for bone healing. In addition, the source of graft material can be absent in some cases that do not show indications for tooth extraction. In fact, the type, concentration, and exposure time of the chemical substances determine the morphological aspects and chemical composition of the tooth-derived matrix. Clinical studies have revealed a higher density of bone formation around ATDM granules after implantation into bone defects as compared with control groups that are free of ATDM granules [36,37,38,39]. Such findings validate the use of ATDM granules as an alternative autologous bone substitute [21,22,23]. Nevertheless, correlations among chemical composition, morphological aspects, and bone formation have not been clear in previous studies, and therefore, bone formation can vary depending on the ATDM granules.

Thus, the main aim of the present study is to perform a concise review on the autologous tooth-derived matrix regarding preparation, chemical composition, and clinical evidence. It is hypothesized that tooth-derived matrix granules enhance interaction with proteins and cells leading to faster bone formation. The current review emphasizes the effects of the chemical composition and morphological aspects of autologous tooth-derived matrix on the clinical procedures for bone healing.

2. Method

A bibliographical search of articles was carried out on PubMed using the following search items: “tooth” OR “dentin” AND “granules” AND “chemical composition” OR “microscopy” OR “morphological” AND “bone healing” OR “bone regeneration” OR “bone repair”. The selection criteria involved articles published in the English language, within the last 15 years, that included in vitro, meta-analysis, and prospective cohort studies. Two of the authors (J.C.M.S. and M.E.R.) independently evaluated the titles and abstracts of potentially relevant studies. For this study, only papers that focused on chemical and morphological aspects of tooth-derived matrix granules were considered. The following variables were retrieved from the selected studies: authors’ names; publication year, purpose of the study, chemical composition, chemical treatment, grinding process, microscopy, and in vivo outcomes.

3. Results and Discussion

Previous studies have reported the chemical composition and morphological aspects of ATDM granules and their effects on bone healing. The dentin-derived granules also reveal micro-scale pores that originate from the dentin tubules. The chemical and morphological factors play a key role in the interaction of ATDM granules with proteins and osteogenic cells that enhance the bone healing process. Thus, the results from previous in vitro and in vivo studies validate the hypothesis of the present study. A detailed discussion of previous findings is described below.

3.1. Chemical Composition and Morphological Aspects of Dentin-Derived Matrix

The chemical composition of bone and dentin are quite similar including hydroxyapatite (Hap) and type I collagen showing a high degree of cross-linking with matrix-binding proteins and growth factors such as IGF-II, BMP-2, and TGF [40,41]. Dentin-derived matrix granules are prepared from teeth which have been extracted due to trauma, advanced periodontal bone loss, or other indications such as wisdom teeth or orthodontic treatment. Immediately after extraction, restorative crowns and fillings must be cut off and removed as well as the tooth crown (Figure 1A). In addition, carious lesions, discolored dentin, periodontal ligament (PDL) remnants, and calculus should be removed by using tungsten carbide burs at high speed, according to previous studies [42,43].

For ATDM preparation, enamel and cementum are removed by using tungsten carbide burs exposing the dentin that is ground and chemically treated with disinfection solutions [44]. Tooth roots should be split out in the case of multi-rooted teeth. Then, the remnant tooth structures are dried using oil-free steam and ground using a grinder apparatus, as illustrated in Figure 1B,C. The grinding procedure can be designed to produce granules with size ranging from 300 up to 1200 µm in the collection tray, as seen in Figure 1D [32,33,34]. Thus, the grinding process is carried out for 3–5 s, and then granules are stirred for 20 s over sieves with different mesh size. Granules are separated by using different sized sieves. Particles with size below 300 µm fall into a waist drawer since they do not show morphological features for bone grating. The grinding process can be repeated depending on the amount of tooth remnant.

The granules can be contaminated revealing a negligible risk of exposing patients to diseases, and therefore, an antibacterial chemical rinsing should be performed in solutions such as sodium hydroxide (NaOH), ethanol, and ethylenediaminetetraacetic acid (EDTA) (Figure 1E). The immersion time can vary depending on the protocol and chemical solutions. For instance, a rinsing procedure has been reported in NaOH and ethanol for 5 min [32,33,34]. Sodium hydroxide solution is a strong disinfection agent that has been used in several medical applications due to its defatting properties [33,34]. Sodium hydroxide is also effective in degrading proteins and nucleic acids as well as inactivating most viruses, bacteria, yeasts, fungi, and endotoxins. Ethanol is used in combination with NaOH to promote penetration of the disinfection solution into dentin tubules exposing the clean surface of the mineralized dentin matrix. Ethanol is safer than other disinfectants since it is rapidly eliminated, and the human body can also metabolize ethanol. As a second step, the application of EDTA, known as a chelating agent, has shown capability to remove bacteria by binding Mg²⁺ and Ca²⁺ ions from the outer cell wall of bacteria [34,45]. The antimicrobial effects of EDTA have been assessed for a wide range of microorganisms that include Gram-negative and Gram-positive bacteria, yeasts, ameba, and fungi. Moreover, EDTA is known to dissolve hydroxyapatite (Hap) from dentin l by chelating calcium, thus, exposing the dentin organic collagenous matrix and partially demineralizing the dentin granules [33,35]. After the EDTA treatment, granules are washed twice in sterile phosphate buffered saline (PBS). Then, PBS is decanted leaving wet particulate dentin ready for grafting into freshly extracted sockets or into alveolar bone defects, as shown in Figure 1F. The process from tooth extraction until grafting takes approximately 15–20 min. It should be noted that the efficiency of selecting the ATDM granules of specific size for grafting is higher than 95%. The volume of ATDM granules is more than twice of the original root volume. Alternatively, the wet ATDM granules can be dried over a hot plate (140 °C) for 5 min [2,33,34,46]. In fact, several protocols for the preparation of ATDM granules have been reported in the literature. Therefore, several other terms have been applied for ATDM depending on the processing protocols such as demineralized dentin-matrix (DDM), deproteinized demineralized dentin-matrix (dDDM), tooth-derived dentin-matrix (TDM), mineralized dentin-matrix (MDM), or partially mineralized dentin-matrix (PDM) [37,40,47].

Thus, the chemical treatment promotes a partial demineralization of the ATDM granules with the exposure of the collagen fibers and the release of dentin- derived growth factors and other Hap-binding proteins [28,48,49]. After immersion in demineralizing solutions, large dentin tubules and loosened collagen matrix may serve as channels for releasing proteins that are essential for proliferation and differentiation of osteogenic cells. ATDM granules reveal opened dentin tubules and porous surfaces that increase the adsorption and release of proteins in the surrounding surgical site [2]. An adequate balance in the demineralization process of ADTM granules should be achieved to provide a Hap-based matrix in combination with collagen fibers and growth factors. Furthermore, a partial demineralization procedure can be optimized for standard chemical composition and morphological aspects that enhance bone healing. Dentin tubules reveal diameters ranging from 1 up to 3 μm, and a proportion ranging from approximately 20 k to 50 k tubules/mm² can remain over dentin granules, as noticed in Figure 2. Depending on the chemical treatment, the size and spatial porous structure of ATDM granules can be enlarged, resulting in increased porosity from 3 up to 20% [50].

The porous structure of ATDM granules increases their wettability and retains blood contents from host tissues including water, proteins, and minerals [51,52], that provides an optimal environment for the adhesion and proliferation of osteogenic cells. In fact, Hap still remains in the ATDM granules and the chemical composition and porous structure allow the release of growth factors [28,48,49]. Micro-scale porous surfaces stimulate the adhesion, proliferation, and differentiation of osteogenic cells leading to enhanced bone healing [51,53,54]. However, micro-scale pores at 1–3 μm do not allow cell infiltration and ingrowth or neither the formation of blood vessels. However, ATDM granules produced with size ranging from 300 to 1200 μm provide a large surface area for protein adsorption and cell adhesion for bone formation. The lack of macro-scale pores in the ATDM granules can slow down bone formation since the formation of blood vessels can not occur in the micro-scale pores of the granules. The ratio of granules in a bone defect determines the three-dimensional distribution of granules and enhancement of the biological events over the early period of bone healing [55]. Thus, the distance among the granules can be established by their amount and size distribution ranging from 300 up to 1200 μm. An increase in the amount of granules to fill a bone defect decreases the intergranule distance. An adequate inter-granule spacing is crucial for proper migration and differentiation of osteogenic cells, leading to the formation of blood vessels and new bone [55]. The formation of blood vessels takes place in the macro-scale spaces among particulate biomaterials with distance between 50 and 400 μm. A combination of granules and hydrogels has been studied to improve clinical handling of particulate bone graft materials and to control the distribution of granules in a bone defect [55,56,57]. Hydrogels (i.e., such as collagen- or fibrin-based materials) have been used to preserve a three-dimensional homogeneous distribution of graft material in a bone defect, maintaining adequate inter-granule spacing [55,56].

3.2. In Vivo Evidence of Bone Healing

Experimental in vivo studies have validated the biological performance of ADTM granules for bone healing. Several studies on the transplantation of human dentin blocks into human bone defects have shown noticeable intimate contacts without soft tissue that reveal progressive dentin-bone ankylosis after 6 months, and then replacement by bone tissue without any inflammatory reactions [58,59,60]. Thus, the feasibility of a clinical procedure with ATDM granules associated with their bone response has been receiving attention by scientists and clinicians [2,35,60]. Histological examinations of grafted sites from multiple studies have revealed the formation of a dentin–bone complex, where the tooth bone graft was surrounded by newly forming bone [61]. In addition, the histological results have shown remodeling processes between dentin and bone without signs of inflammatory reactions [2,21,30]. Cone beam computed tomography (CBCT) images have shown evidence of cortical and cancellous bone formation after 6 years follow-up [62,63,64]. A variable period of time required for resorption of dentin graft has been reported, and histologic studies have shown a characteristic depiction of graft particles encased in newly formed bone [8,60]. A lack of regular resorption time affects the predictably of the dentin-based graft materials, and therefore, promotes obstacles in treatment planning, mainly in cases in which implants are required to be placed at the grafted sites.

Previous animal studies have reported bone formation in the tibia of rabbit and mice [23,45,65,66,67]. Many clinical studies have shown that new bone was formed by osteoinduction and/or osteoconduction on guided bone healing, socket preservation, sinus lift, and ridge augmentation [29,59,60,68,69,70]. Another case series study of 13 patients with 61 dentin-grafted post-extraction sockets compared CBCT images before and 4 months after surgery, and showed a gain in mid-palatal bone height (measured to the top of the graft still in the consolidation phase) and a loss of ridge width. A histological analysis revealed new bone formation in close contact to dentin particles with no signs of inflammatory reactions or fibrous encapsulation of the ATDM [14].

A prospective controlled clinical trial study evaluated the behavior of ATDM blocks during 26 weeks long-term follow-up observations based on 30 patients. Patients were divided into two groups who received ATDM blocks or autogenous bone blocks. The findings were consistent with those of previous short-term studies and indicated that ATDM blocks were capable of continuous remodeling under a functional load with appropriate volume maintenance, showing no statistical significance between both groups (p = 0.241) [26]. Regarding sinus lift procedures, a previous retrospective clinical study on the placement of 100 dental implants in 51 patients showed overall clinical success of 96.5%. A histologic examination showed that ATDM was gradually replaced by new bone to supporting the dental implants [71]. Hence, mineralized dentin became firmly integrated to the newly bone, establishing a solid site for anchorage of dental implants [60,70]. An alveolar ridge preservation technique with the use of ATDM graft has been associated with enhancing bone healing and maintaining bone volume for implant placement. Such previous findings support the strategy of using tooth-derived granules for repairing bone defects or preserving bone loss. Indeed, previous studies have revealed a gradual resorption rate of ATDM granules in late implant placement.

In the previous studies, several chair-side procedures have been described with variable use of chemical substances for partial demineralization of the ATDM granules. Variations in the content and exposure time for chemical treatment lead to different degrees of ATDM demineralization. In addition, the size of tubules and granules also varies depending on the physicochemical procedure, and therefore, further studies should be performed to optimize standard processing of ATDM granules. In addition, the availability of collagen fibers and growth factors in the dentin is also important for the migration and differentiation of osteogenic cells. A correlation between the processing and content of proteins should also be assessed in further studies. Regarding the clinical handling of particulate bone substitutes, the three-dimension distribution of granules in a bone defect cannot be controlled by a clinician. The use of hydrogels should be taken into consideration to provide an adequate inter-granule distance that results in enhanced bone healing. In this way, the combination of ATDM granules with hydrogels can become a promising clinical approach, and therefore, in vitro and in vivo studies should be performed to validate their clinical application.

4. Concluding Remarks

Within the limitations of the previous in vitro and in vivo studies, the main outcomes from the studies on autogenous tooth-derived dentin matrix can be drawn. Different guidelines for manufacturing autogenous tooth-derived dentin matrix have been reported in the literature depending on the grinding process and chemical treatment in demineralizing solutions. However, mean size of granules have been reported ranging from approximately 300 up to 1200 μm, which provided adequate surface area for bone formation and resorption rate with the replacement of granules by new bone. In addition, the dentin-derived granules possess an inherent micro-scale porous structure due to the presence of dentin tubules at 1–3 μm in diameter. Regarding chemical treatment with demineralizing solutions, the dentin tubules are enlarged and the mineral content composed of hydroxyapatite is partially dissolved. Even though the granules are treated with demineralizing solutions, hydroxyapatite is still present and its porous structure allows the release of growth factors which stimulate the osteogenic cells in the bone healing process. Tooth-derived graft material has been shown to perform properly as expected for osteoconductive graft materials since an adequate amount of new bone has been histologically examined after a long healing time. Thus, autogenous tooth-derived dentin matrix disclosed structural, chemical, and biological aspects to serve as an alternative autologous bone substitute. In this context, it must be emphasized that autogenous tooth-derived dentin matrix appears to be clinically suitable for different reconstruction or preservation procedures for enhanced bone healing.