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host’s angiogenesis potential and strives to stimulate it through optimizing the scaf­fold material, application of growth factors, or cell seeding in the scaffold. However, the ingrowth of blood vessels into the scaffold is often slower than 5μm/h [77], causing a prolonged healing period associated with major hypoxic conditions, espe­cially in the central portion of the scaffold [8]. The vascular outgrowth, directed from the implanted scaffold to natural bone, has been proposed as an alternative to enhance vascularity. Other strategies of establishing vascularization of scaffold include incorporating the scaffold with preformed microvasculature that can be directly perfused by blood as soon as it establishes interconnection (inosculation) to the vessels of host [7880] or with microvascular fragments that promptly develop into microvasculature network upon implantation [8183].
Buser etal. demonstrated that creating small perforations in the cortical plate permits enhanced migration of cells with angiogenic and osteogenic potential [84]. Decortication provides communication between marrow space and the scaffold. This phenomenon could augment vascularization, promote growth factors, and sub­sequently enhance bone formation [85, 86].
Some conditions can enormously affect vascularity of bone; for instance, prior treatment with radiation as an adjunct therapy for malignant head and neck lesions could heavily enfeeble vascularity, decrease cellularity and osteogenic cell count, hin­der bone remodeling, alter cytokine capacity, and create a hypoxic environment [87
89]. Mineral component of bone is relatively resistant to radiation; on the contrary,
bone marrow containing BMSCs and endothelial cells of vasculature are strongly radiosensitive. Delivery of cell-signaling factors such as VEGF, TGF-β1, and BMP or gene therapy has been proposed to improve bone regeneration in radiated sites, yet regeneration of irradiated critical-sized defects could not be accomplished [9092].
P. Hazrati and A. Khojasteh
6 Treatment Planning
6.1 Scaffold Manufacturing Considerations
6.1.1 Material
Material selection is of signicant importance in designing scaffolds made by addi­tive manufacturing (AM) methods. The size and load-bearing requirements of the defective area inuence the biomaterial selection. To operate effectively invivo, suitable biomaterials must exhibit process compatibility with the particular AM technology used, as well as proper biochemical and physical features [93]. Many varied materials have been used in fabricating biodegradable scaffolds. For exam­ple, because of their improved corrosion resistance and biomimicry, biodegradable metal alloys, including magnesium (Mg) or zinc (Zn), have recently gained popular­ity [9496]. On the other hand, these materials provide particular problems, such as high melting temperatures, ammability, and the formation of metallic vapors, all of which jeopardize process stability [97].
Due to their strong biocompatibility, bioresorption, and processability, various polymers are viable materials for AM production of patient-specic implants [98].
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While polymers, either natural or synthetic, are great materials for accommodating AM processing parameters, single material groups are restricted in their ability to meet both AM processing and clinical usefulness criteria in biomedical applica­tions. Polylactides, for example, have high tensile strength and low degradation rate; therefore, they may last longer invivo than desired. While polyglycolic acid (PGA) and poly(lactic-co-glycolic acid) (PLGA) have better mechanical qualities, they degrade fast when employed as a bioresorbable scaffold, and their tensile strength is decreased by 50% within 2weeks [99]. Furthermore, polymers’ pure use is limited due to improper mechanical properties and inappropriate degradation [100, 101]. Ceramics are often mixed with polymers to overcome this stumbling block to create composite materials with superior mechanical and physiological properties [102, 103]. Also, incorporation of this mineral phase provides osteoin­ductivity for the scaffold [104]. Ceramics’ brittleness and poor mechanical resis­tance barricade pure application of them in load-bearing sites [105]. Naturally, bone is a composite tissue composed of a mineral component dominated by hydroxyapa­tite (HA) nanocrystals and an organic phase made up of extracellular matrix (ECM) proteins, with collagen type 1 accounting for around 90% of the total [106]. Multi­material constructs own a better capacity to imitate hybrid tissue architectures as scaffold material because of the complicated arrangement and composition neces­sary for function [107, 108]. Accordingly, when designing a scaffold for bone regeneration, a mixture of mineral-like and ECM-like components must be consid­ered as scaffold material to achieve the desired outcomes.
Polymers and bioceramics are frequently utilized as ECM-like and mineral com­ponents, respectively. The combination of polymers with bioceramics could help achieve the desired ideal characteristics. However, fabricating scaffolds with a high mineral concentration similar to the actual bone may not be the best way to promote bone growth [1]. For instance, increasing the weight ratio of HA to high percentages in composite scaffolds not only complicates the production procedure but also may decrease osteogenic differentiation, decrease compressive strength, and prolifera­tion of cells [109111]. Increasing the proportion of β-TCP and bioglass has similar effects on scaffolds’ biological and functional characteristics [39, 112114]. The proportion of both ceramic and polymer elements in the composite material has a substantial impact on the composite scaffold’s invivo and invitro behavior [115]. Figure4 demonstrates materials utilized in scaffold fabrication.
Polymers
Polycaprolactone (PCL)
PCL is a biogenic polyphosphate (bio-polyP). It is the most widely utilized polymer biomaterial in AM because of its high mechanical qualities, biocompatibility, sig­nicant printing accuracy, relatively low expenses, FDA approval, osteoconductiv­ity, exibility, lower pH changes, simplicity of processability, low melting point, ease of mixing with other materials, and established efcacy in bone regeneration [105, 116122]. On the other hand, PCL cannot be solely employed for hard tissue regeneration due to its ductility and low elastic modulus [100, 101]. Therefore,
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P. Hazrati and A. Khojasteh
Fig. 4 Materials utilized in the fabrication of polymer-ceramic scaffolds. Al alginate, BD bioden- tine, BCP biphasic calcium phosphate, BO bio-oss, BG bioglass, CaP calcium phosphate, CaC calcium carbonate, CH chitosan, CS calcium silicate, Col collagen, DBB deproteinized bovine bone, DCPD dicalcium phosphate dihydrate, GL gelatin, Gr graphene, HA hydroxyapatite, MgO magnesium oxide, MTA mineral trioxide aggregate, PCL polycaprolactone, PDLLA poly(D,L­lactide acid), PEG poly(ethylene glycol), PEU poly(ester urea), PGF phosphate glass ber, PLLA poly(l-lactic acid), PLA poly lactic acid, PLGA poly(lactic-co-glycolic) acid, PLG polylactide-co­glycolide, POC poly(1,8-octanediol-co-citrate), PVA poly(vinyl alcohol), PHB poly(3­hydroxybutyrate- co-3-hydroxyhexanoate), SF silk broin, TCP tricalcium phosphate. (Reprinted with permission from [1])
many mineral components, like HA, tricalcium phosphate (TCP), calcium silicate, and bioglass, are added to PCL in order to compensate mentioned shortcomings [1]. The combination of PCL and beta-tricalcium phosphate (β-TCP) is one of the most frequently utilized mixtures of PCL in bone tissue engineering [123].
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Poly Lactic Acid (PLA)
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PLA might be considered a choice for hard tissue regeneration due to its superior mechanical characteristics compared to other synthetic polymers. However, its applicability is limited due to its lengthy biodegradation period and the diffusion of acidic metabolites during this process [1, 124]. Poly(D,L-lactide acid) (PDLLA) and poly(l-lactic acid) (PLLA) are other forms of this polymer with enhanced bio­compatibility and mechanical stability [109, 125, 126]. HA, TCP, and bioglass are the most frequent ceramics combined with PLA and its derivatives [1, 125].
Poly(Lactic-co-Glycolic Acid) (PLGA)
PLGA has a faster rate of degradation and lower osteoinductivity [127]. PLGA may degrade to nontoxic acidic compounds, such as lactate, which might trigger an inammatory reaction [128]. Hence, mineral components, especially HA and TCP, are added to PLGA to cover its aws [1].
Alginate
Alginate is often used to repair cartilage due to its ability to induce chondrocyte proliferation and compatibility with cartilaginous tissue [129, 130]. Although because of its very low stiffness and having no interaction with cells, it is not appro­priate for application in load-bearing areas [22, 131].
Chitosan
Chitosan, a natural polycationic linear polysaccharide generated by deacetylation of chitin, has several benets, including biocompatibility, biodegradability, antifungal, antibacterial, and anticancer activity, but it also has certain disadvantages, such as insolubility in physiological pH [132135]. Carboxymethyl chitosan (CMCS) has improved water solubility, hence increasing the processability of the tissue­engineered scaffolds. Grafting CMCS with waterborne polyurethane (WPU) enhances mechanical properties, such as compression strength, and biological qual­ities, such as osteoblast adhesion and proliferation [136].
Bioceramics
Calcium Phosphate Compounds
Calcium phosphate compounds (mostly β-TCP) are structurally comparable to the calcium phosphate substance that makes up 60–70% of normal bone, indicating intrinsic biomimetic properties. It is biodegradable and has been shown to have osteoconductive qualities [137]. β-TCP has good biodegradability, chemical inter­action with bone, and wear resistance, which are all essential characteristics for load-bearing craniofacial defect locations [138]. The proportion of β-TCP in com­posite materials has a direct and inverse relationship with the surface roughness and contact angle, respectively.
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Hydroxyapatite (HA)
P. Hazrati and A. Khojasteh
Because of its stoichiometric closeness to the mineral phase of normal bone, HA has been regarded as a scaffold material for bone healing with excellent biocompat­ibility [139]. However, because of its inherent poor interaction with the binder liq­uid during the 3D printing process, it is commonly mixed with other bioceramics or biopolymers [140].
Bioglass
Bioglasses are also employed because of their high osteoconductivity and ability to attach to hard tissue; nevertheless, they degrade slowly and induce cytotoxicity in the surrounding tissue [141]. Their main benet is that they promote osteogenesis and interact well with cells [13, 142], but they are too brittle to be used in load­bearing craniofacial locations [143]. A study has reported that bioglass affects osteogenic differentiation better than HA [121].
6.1.2 Composition Technique
Melt blending, solvent-based blending, and powder blending are among the compo­sition processes utilized to make printing materials [1]. Melt blending may be done in two ways. To form a composite, either the ceramic powder is added to the molten polymer or a combination of ceramic powder and polymer is melted. A solvent is used to mix polymer and ceramic components in solvent-based blending. Extra steps are usually required to eliminate the solvent before or after constructing the structure. For sintering-based molding methods, such as stereolithography (SLA) and digital light processing (DLP), powder blending is often employed. A combina­tion of ceramic powder and polymer powder is created, and the ceramic and poly­mer components are fused using a laser or light. Solvent blending and melt blending are the most common methods of preparing polymer-ceramic composite scaf­folds [1].
6.1.3 Designing
Compilation of CAD/CAM technologies and tissue engineering-based approaches of maxillofacial reconstructive procedures have yielded application of patients­specic implants as a mean of bone tissue engineering. Unique scaffolds that are highly compatible with the defect’s topography and meet each patient’s particular needs are designed in a virtual environment, according to data provided by 3D imaging modalities [1315]. Metallic restorations of adjacent teeth could distort the images and complicate the CAD process [57].
After obtaining 3D radiographs (i.e., CT) and conveying this dataset as standard stereolithographic les to the CAD program, a 3D model of intended bones with the defect is reconstructed (Fig. 5). These 3D models allow surgeons to witness the exact shape and size of the defect rather than visualizing it. Afterward, the CAD program determines the orientation and location of the osteotomy cuts on the virtual model (Fig.6a). The surgeon’s judgment could modify these cuts; eventually, these virtual osteotomy cuts could be replicated as surgical guides to instruct the surgeon during the procedure.
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ab
c d
Fig. 5 Workow of virtually simulating surgical site before surgery. CT image of coronal (a), axial (b), and sagittal (c) plane. (d) 3D reconstructed image of a defective left posterior mandible
ab
Fig. 6 (a) Dened position and angulation of osteotomy cuts on the virtual model; (b) the defec- tive site lled by designed patient-specic scaffold
The empty space created by osteotomy is then lled with a patient-specic scaf­fold, which is designed according to the normal corresponding contralateral ana­tomic site (Fig.6b). The designed scaffold is then transformed into a CAM program where, in addition to morphology, signicant properties such as porosity and inter­connectivity are specied. As mentioned before, pore size greater than 300μm is a prerequisite for proper vascular and bone ingrowth [31].
6.1.4 Fabrication Technique
Solvent casting, gas foaming, phase separation, particulate leaching, and freeze­drying are some of the conventional techniques used to fabricate tissue scaffolds. However, using these traditional fabrication procedures, it is not possible to produce
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patient-specic scaffolds with unique geometry and a highly porous structure with linked pores [144, 145]. When used as an improved tissue scaffold production tech­nique, 3D printing technology can construct complex geometries with specic ben­ets, such as the ability to t into irregular defective areas and replicate tissue complexity via precise placement and improved control over the porosity and pore pattern [146149]. When 3D printed patient-specic implants are employed for functional bone regeneration in bony defects, it takes less time and needs fewer screws for retention than the classic approaches [142]. Even though all 3D printers follow a dened process to produce constructs (layer-by-layer placement of the material through the prespecied spatial motion) [150], accuracy, expenses, post­printing procedures, and preferred primary material vary between different methods [115, 151]. Because of its simplicity of use, the diversity of biocompatible polymers and composites that may be employed and the accuracy with which porous struc­tures can be created, FDM is particularly appealing for bone tissue engineering applications [39]. FDM is the most frequent method of fabricating composite scaf­folds [1]. In most situations, the melt blending process is employed to create the composite used in the FDM procedure. DIW, sintering-based techniques, and LDM are other methods of printing biodegradable scaffolds. However, these 3D printing techniques have some drawbacks. For example, it has been shown that printing HA scaffolds with a composite mix with a 40% ceramic ratio is not possible [110].
P. Hazrati and A. Khojasteh
6.2 Surgical Technique
General anesthesia with nasal intubation is required for the surgical procedure; how­ever, local anesthesia with or without sedation may sufce when reconstructing small areas. Placing the scaffold into the defective area could be accomplished dur­ing surgical removal of ablated tissue. However, a 1-year interval between resection and regeneration procedures is recommended in the surgical protocols. Flap design should provide the accessibility and visibility required for both resection and regen­eration procedures. In vascular surgery, the location of neurovascular bundles should be predesigned in the scaffold’s morphology. If the morphology of the defective area allows, a series of tiny holes, 1–2mm deep could be constructed on the defect walls with round burs under abundant irrigation to enhance the quantity of bleeding and revascularization at the surgical site. Because utilizing the scaffold as a guide for drilling the hole might cause the fragile scaffold to crack and fracture, a polytetra­uoroethylene (PTFE) scaffold replica could be produced to serve as a guide for the proper placement of the hole for the xation screw [15]. The patient- specic implant is then placed at the defective site and secured to the remaining wall with the titanium mini-screw(s) lengthen 6–8mm inserted through the predrilled hole. Plasma rich in growth factor (PRGF) could be prepared before surgery and applied to the surgical site in order to promote tissue healing and facilitate bone regeneration [15, 152]. An addition to conventional coverage of scaffold with a membrane, a pedicled buccal fat pad ap could be used to cover the scaffold. Figure7 shows application of B-TCP scaffolds to treat a defect in the posterior mandible.
a
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b
c
f
de
Fig. 7 (a) Radiographic view of a large defect at the posterior mandible. (b) Implanting the degradable scaffold and covering it with buccal fat pad pedicle. (c) Postsurgical radiographic view of the defect. (d) Radiographic view after 12months showing signicant amounts of bone forma­tion. (e) Radiographic view after 18months. (f) Clinical view of the defect site after 18months showing bone formation and 90% defect lling
7 Prognosis
All the studies which conducted FBR in humans reported in the literature between 2009 and 2017 demonstrated high success rate with nite serious complications [37]. Long-term stability has been reported in cases, where FBR is employed to prepare the bone quantity for implant placement.
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