Application of Hydroxyapatite in Regenerative Dentistry

Abstract

In clinical practice, dentists face alveolar bone loss that needs to be managed by bone grafts. The basic bone grafting materials are autograft, allograft, xenograft, and alloplasts. Autografts are gold standard because it has osteoconduction osteoinduction osteogenic. However, they possess risk for the morbidity of the donor site and limited availability. Allograft have possibility of disease transmission and immunologic reactions. These problems potentiated the use of alloplasts. For bone regeneration, hydroxyapatite is the reference material because of its biocompatibility, bioactivity, osteoconductivity, and osteoinductive property. Natural hydroxyapatite can be synthesized from fishbone, coral, bovine bone, eggshell, and seashells. Hydroxyapatite bone substitute has ideal properties for socket preservation, sinus augmentation, periodontal regeneration and in restorative and preventive dentistry. When used as implant coatings, they support osseointegration and osteogenesis. Hydroxyapatite known for its bone regenerative capacity. Nano-hydroxyapatite, with smaller size and wider surface area, permits more proteins and cells to attach to the surface speed up regeneration. Hydroxyapatite are used as inorganic building blocks for tissue engineering or as nano-fillers with polymers. Furthermore, ion doping and surface modifications have been reported to prepare functionalized hydroxyapatite. This chapter illustrates the role of hydroxyapatite in regenerative dentistry, and advances and advantages of using it as a component of other dental materials, whether experimental or commercially available.

1. Introduction

Hydroxyapatite (HAp)—the key inorganic element of teeth and bones [1, 2]. It is extensively used in dental clinics and for bone repair, because its good biocompatibility and biological activity. Nanohydroxyapatite (nHA) a type of nanomaterial having small crystal grain diameter, wider surface interface, high surface and binding energy. Nano-hydroxyapatite act as a drug carrier to deliver anti-tumor drugs [3]. High hardness and wear-resistance of tooth enamel depends on the braidedinorganic-organic composite structure and multilayer structure [4]. The natural remineralizing ability of hydroxyapatite is still restricted, but numerous studies have shown a high promoting effect on remineralization. There mineralizing performance is further improved by the introduction of acidic amino acids and flouride. To speed up the drug absorption and to treat periodontal diseases such as jaw cyst, HAp paste is effectively used [5]. Nano-hydroxyapatite with porous microspheres have numerous benefits such as high drug load,large specific surface area, better biocompatibility, pH-responsive degradation, used in tumor detection and tumor drug carrier, in bone repair, the combination of HA with natural and synthetic polymers has effectively solved problems such as high brittleness and uncontrollable degradation rate” [5].

2. Shortcomings of available bone regenerative materials

A bone graft material’s main purpose is to aid osseous healing by providing a cellular environment for new bone creation, a structural framework during the healing process. Tissue viability, defect size, graft size, shape, and volume, biomechanical features, graft handling, cost, ethical considerations, biological traits, and associated repercussions all factor into the optimum bone graft selection [6]. Bone graft material has different attributes for regeneration and they vary according to the graft material used (Table 1) [7].

Attribute	Description
Osteogenic	Ability to differentiate and produce bone
Osteoinductive	Provide a biologic stimulation (proteins and growth factors) that causes mesenchymal stem cells and other osteoprogenitor cells to proceed into the osteoblast lineage
Osteoconductive	Allow for cellular attachment, proliferation, and migration by providing a structure and topography that allows for cell attachment, proliferation, and migration
Osteointegrative [8]	Forms tenacious bond formed between the new mineralized tissue & graft material

Table 1.

Attributes of bone grafts [7].

The materials utilized in grafting of bone can be classified into different categories mainly autografts, xenografts, and allografts. Other types include biologically and synthetic based biomaterials, tissue-engineered biomaterials, and combination of these substitutes (Table 2).

Type	Description
Autografts	Because of osteogenic, osteoconductive, and osteoinductive capabilities, along with the lack of foreign body responses, they are the bench mark. Obtained from intra oral and extra oral sites.
Allografts	Biological materials obtained from the same species. Primarily serve as an osteoconductive or structural matrix, but they lack osteoinductive qualities.
Xenografts	Bone substitutes derived from other species like bovine or swine grafts, and transplanted into humans having osteoconductive property
Alloplasts	Synthetic bone graft substitutes having osteoconductive property

Table 2.

Various kinds of bone grafts [9, 10, 11].

The grafts mentioned above has its own set of benefits and drawbacks. Following are the shortcomings of available bone regenerative materials (Table 3).

Bone graft	Short comings
Autograft [9, 10, 11, 12, 13, 14]	Needs second surgery so surgical morbidity and discomfort, possibility of surgical complications, ankylosis and root resorption, limited graft volume
	Cortical bone grafts-higher resorption rate, less vasculature, resulting in less bone remodeling
	Cancellous bone grafts-mechanically weak
Allografts [11, 12, 13, 14]	Infection and allograft fracture due to decreased revascularization, possibility of transmission of disease, sterilization of graft may compromise its osteoinductive potential, high cost of procurement, the host’s immune reaction, inconsistency of graft integration, high failure rate, expensive, due to ethical and legal concerns.
Xenograft [11, 12, 13, 14, 15, 16, 17]	Antigenicity, tissues must be handled carefully to eliminate organic components, unpredictability of regeneration and resorption rates
Alloplasts [11, 12, 13, 14, 15, 16, 17]	Demineralized bone matrix-lack of structural stiffness caused by processing and the difficulty to locate the material radiographically due to its intrinsic lucency. Ceramics are not osteoinductive, compressive strengths are lower than cortical bone

Table 3.

Shortcomings of bone regenerative materials.

After decades of research works on the best available regenerative materials, studies have reached a prime focus on naturally available resources. Out of which naturally available hydroxyapatite has gained much attention compared to readily available synthetic grafts due to its predictable results. Hydroxyapatite is a natural polymer of calcium phosphate derived from bone or natural materials such as coral, algae, fishes and other marine sources and commonly utilized to accelerate bone repair owing to its potential to operate as a structural scaffold. Because chemical composition of hydroxyapatite closely mirrors that of bone’s inorganic component, it can be employed as a superior biocompatible bone grafting material. Natural HA is less expensive, and the key raw ingredient is readily available. Natural HA shows substitutions and traces of certain chemical elements due to which its bioactivity behavior can be enhanced, compared with that of synthetic HA. Also this kind of substitutions promotes the formation of noval bone. Natural HA is considered as a better regenerative material than synthetic Hap due to its bone bonding ability, significant biocompatibility mediated by its porous architecture [11, 12, 13, 14, 15, 16, 17, 18].

Although synthetic HA is widely known for its capacity to link with bone tissue, it is restricted due to its reduced solubility, sluggish rate of bone binding ability, and bacterium adhesion inhibition. This is a significant disadvantage since patient recuperation is hampered, and infection can result in surgical failure. Because HA’s crystal structure is porous, it can easily accommodate ion replacements. Due of its increased crystallinity and Ca/P ratio, synthetic HA has a very low resorption rate. Another important issue with HA is its low mechanical strength, which prevents it from being employed in high-load-bearing applications [11, 12, 13, 14, 15, 16, 17].

3. Clinical application of hydroxyapatite in alveolar bone regeneration

Many grafting materials are used in dentistry which include allograft, autograft, alloplastic and xenograft graft. The advantages and short comings are mentioned above in this chapter.

Since then there is an interest in the use of alloplastic (synthetic) grafting materials.

The first synthetic bone graft that was used in 1892 by Van Meekeren, who treated bone defect as using calcium sulfate. Since then, materials classified under bioceramics are substituted as bone grafts in humans. The conventional bioceramics material that is used for the regeneration of bone is hydroxyapatite (HA) [19, 20, 21].

Hydroxyapatite (HA) is an important class of material belonging to the calcium phosphate ceramic group which is also applied for regenerative bone surgeries. Normal human bone is mainly comprised of 69 weight% mineral apatite, 22 weight % organic component and 9 weight % water and the mineral components of the bone were formularized as calcium hydroxyapatite, which contains irregularly shaped particles of size ranges from 30 to 45 nm length and width and an average of 5 nm thickness [22].

HA with the general formula of (Ca 10(PO4)6(OH) 2) contains calcium phosphate in ratio of 1.67, that is similar to bone. HA is the stable form of calcium phosphate-based materials, which is less soluble. It contains only calcium and phosphate, so it does not cause any tissue inflammation, and is biocompatible and HA is commonly used for orthopedic, dental, and maxillofacial applications, also as a covering material for metal implants or as a bone filler [23, 24].

In daily practice, dentists encounter alveolar bone loss and the reasons include extraction, tumor surgery, or periodontitis. The resultant bone defect hampers not only for prosthetic reconstruction but also has an esthetic effect. Due to this, bone surgeries are usually performed to regenerate the lost bone and restore the alveolar ridge contour. Taking all this into consideration alveolar bone regeneration has a role in all aspects of dentistry [25, 26, 27].

Dennissen et al. which was the primary study in performing HA in over denture and submerging root therapy in vital tooth for preserving the bulk of the alveolar ridge for better retention of prostheses [28].

John W. Frame Hydroxyapatite have excellent biocompatibility, tolerated by the hard and soft tissues of the mouth and jaws, a great potential for the future. Paper reviewed the material in its physical form in both porous and solid structure and its behavior in relation to its biological effect in various sites noted for implant placement, and its technique of surgery. The known controversies and doubtsabiut the material is mainly regarding alveolar ridge augmentation [29].

Nurul Saadah Razali Alveolar ridge loss is a physiologic consequence of extraction. This event causes feasibility of implant placement, prosthetic rehabilitation and esthetic outcome. An attempt to minimize the shrinkage of the alveolar bone, socket preservation was launched to intercede with the natural process by providing a scaffold with antibacterial and regenerative properties. For decades, hydroxyapatites (HA) used as one among biomaterials used for socket preservation treatments and it was considered as biocompatible and osteoconductive [30].

Anne Handrini Dewi Determined and analyzed the potential use of HA as a substitute of bone for alveolar bone regeneration procedures. The application of HA as bone substitute intervene the healing process [31].

Niranjan Ramesh Hydroxyapatite (HA) is a bioceramic biomaterial that copies the mineral composition of bones and teeth. HA, was commonly made via various techniques in previous years, and it is found to have excellent bioactivity, osteoconductivity, and biocompatibility. HA has been used in combination with polymers in the form of bio composite implants to improve the mechanical properties and it also enhances its activity by exploiting the effects of both HA and the polymer involved in making the biocomposite [32].

Graig D. Brown A newly developed hydroxyapatite cement helps to promote regenaration of bone in craniofacial defects and was assessed to determine its potential in treating osseous defects. HAC appears to be sufficiently structurally stable for reconstruction and augmentation of non-stress-bearing portions osseous defects [33].

Yukna RA Used HA graft material shows more clinical benefits in majority of patients with extensive periodontal defects along with open flap debridement [34].

Aparna Singh HA in conjunction with resorbable collagen membrane is used as an acceptable alternative to autogenous block graft and non-resorbable membrane for treating compromised alveolar ridge deficiencies [35].

4. Applications of hydroxyapatite for regeneration of tooth structures

Regenerative dentistry a branch of regenerative medicine that emphasis on oro-dental pathologies inclusive of bone defects such as periodontitis and alveolar bone resorption and tooth destructive disease like dental carries and pulpal necrosis [36]. All these localized skeletal ailments directly influence the quality of life of patients and healthcare resources. In order to address these aliments effectively, targeted therapies towards regeneration of both tooth and bone [36, 37].

The current treatment methods consist of replacing the lost structure with direct and indirect synthetic restorative materials. In more severe cases of dental caries or traumatic incidents, dental pulp can be compromised in reversible or irreversible inflammatory responses or pulp necrosis. Regenerative endodontics includes biological procedures to replace damaged dentin or root structures as well as pulp-dentin complex cells. Pulp capping and dentin regeneration using current biomaterials have notable limitations [36]. Severe inflammatory reactions induced by the synthetic capping materials are the major drawbacks in this methodology that can result in therapy failure. To address these limitations, research in dentistry continues to bring more decisive and reliable methods. In this way, establishing novel regenerative approaches and regeneration of dentine –pulp complex is the mainstay [37, 38].

Dental pulp regeneration requires the embodiment of a scaffold conducive with the regeneration of the pulp-dentin complex. ECM-derived proteins and or other natural resources can be employed as natural scaffolds for tooth regeneration. These biological materials can be easily engineered for production of a variety of polymeric and composite scaffolds [38]. HA, a glycosaminoglycans in the extracellular matrix, can enhance cellular metabolism and mineralization in hydrogel form, as natural platform for treating dentin/pulp complex in presence of human DPSCs (Dental pulp stem cells) and apical papilla stem cells [39, 40].

Because tooth structure is complicated, complete tooth regeneration will be difficult. As the first step in tissue engineering for the tooth, regeneration of dentine or a substitute should be attempted. To reconstruct a tooth in whole or in part, adoption of a scaffold can be used to form a tooth with three-dimensional structure. Thus for the regeneration of dentine or pulp dentine complex, porous hydroxyapatite scaffold may be used. Therefore, an HA scaffold with a hollow center similar to a tooth structure was devised and used [40]. HA scaffold was effective for dentin or dentin pulp regeneration. Thus it can be a perfect choice for tooth regeneration using a hydroxyapatite cylindrical scaffold. Osteogenesis due to stem cells in the HA was also found to be excellent [40].

Hayakawa S. et al. stated that HA platforms are effective for regeneration of dentin-pulp complex. When hDFSCs seeded on Synthetic HA scaffold (ENGIpore©) and incubated for 6 weeks an intense adherence, colonization of polygonal-shaped cells to the HA platform which was similar to dentin [41].

Campodoni et al. used Mg-Hydroxyapatite on gelatin polymers which were embedded in a matrix from chitosan blend and gelatin, to create a biocompatible 3D porous composite structure. This product is similar to dentin in its architecture and chemical composition, adaptability for cells to adhere, and differentiate [42].

Hydroxyapatite can be commonly fabricated from natural sources like fishbone, coral, bovine bone, eggshell, and seashells through the calcination process. The trace ions consisting of Na+, K+, Mg2+, Sr2+, Zn2+, and Al3+, or anions like F-, Cl-, SO4 2-, and CO3 2- make HA non-stoichiometric [42]. These trace of ions, are beneficial to promote rapid bone regeneration [43].

Natural and Synthetic hydroxyapatite based materials have been preferred over allografts and autografts for hard tissue repair. Commonly associated problems with the grafts include donor site morbidity, graft shortage, graft rejection, and disease transmission [44].

HA application in orthopedics can used in restoring bone defects and augmentation of bone. The interlocked porous structure of hydroxyapatite based implant can function as an extracellular matrix, favoring tissue regeneration and cellular development. Furthermore, HA promotes firm anchorage to the surrounding tissue and the implant thus enhances the osseointegration process. The anchorage of bone for longer periods enhances the successful osseointegration, hence completely restoring the functional ability [45].

Another remarkable application of HA in tooth regeneration as HA cylinders. HA cylinders can be used for replacement of tooth. Earlier HA was used dental cements presently its also used in toothpaste [46, 47].

Porous hydroxyapatite is an excellent biomaterial for tooth regeneration. Yoshikawa et al. [40] reported the susceptibility of HA in porous structures as a platform for tooth regeneration [48].

The another application of HA is in drug delivery system. The high-binding affinity and natural porous structure of HA enhances the drug loading ability [40].

The pure form of HA is not used for hard tissue restoration because of its low and brittle load bearing capability. Hence HA is used along with polymer or in composite form for the application of hard tissue regeneration [49, 50].

In this case, the toughness, elasticity of the composite matrix which also include its compressive strength of HA ceramic phase improves the properties of HA and thus enhances the effectiveness of the scaffold when used in tooth regeneration [51].

The development of new trends lead to the fabrication of nano-HA can accelerate dentin remineralization [52]. Nano- HA provides a rich source of calcium thus it acts as shielding material against caries and dental erosion [53]. HA requires calcium hydroxide in an enormous amount and is calculated by the increased Ca/P value. Additionally the application of nanoHA in toothpaste function as filler for the repair the sunken enamel surfaces and also favors a protective covering on the dentinal tubules that are dissolved, contributing a speedy and quick cure from tooth hypersensitivity [47, 54]. Hydoxyapatite comes with various unique kind of properties like it does not induce any inflammation or toxicity, has the capacity to chemically bond with the bone and has the property of stimulating the growth of bones [50].

5. Hydroxyapatite for dental implant surface coating

The dental implants are used as a substitution for tooth replacement and made from stainless steel, cobalt-chrome alloys or titanium. Among these materials, titanium and its alloys gained high importance due to its highest biocompatibility, mechanical properties, excellent corrosion resistance, strength and relatively low weight [55, 56].

The Ti-based implants still remain many restraints to be implanted in human body. The high modulus of elasticity of Ti compared to the bone can persuade stress shielding also the poor tribological behavior of Ti could cause severe adhesive wear and as a consequence, generate debris in the blood stream leading to bone resorption. Implant loosening may occur because of infection in the neighboring tissues and lead to implant failure [57, 58, 59].

All these limitations can be eliminated by altering the composition, morphology and surface structure making the mechanical properties intact [60].

The implant surfaces can be modified, as there is furtherance in technology and the modified implant surfaces will improve rate of osseointegration. Current modifications include plasma spraying the implant surfaces with either hydroxyapatite (HA) or titanium beads [61, 62, 63, 64].

There are numerous reasons for coating implants with HA. The implementation of HA coatings thus alter the implant surface characteristics has been known since 1980s. Better osteoblastic activity and increased collagen levels are discern in cells growing on HA-coated Ti implants, improve bone fixation and thus increases the lifetime of metallic implants, enhancing the ingrowth of mineralized tissue improved the biological fixation, bioactivity, and biocompatibility of dental implants. Thus, it is concluded that the HA coatings on metallic implants would enhance osseointegration and thus decreasing the time from implant insertion to final reconstruction.

6. Ha coating methodology

The application of apatite coatings on dental implants is a favored surface modification. HA and fluorapatite are incompetent to be used as implants, as they are brittle in nature. Therefore, load–bearing implants have been coated with HA and Fluor apatite to enhance earlier osseointegration. Various techniques are:

1. Sol–gel coating.

2. Plasma spraying.

3. Biomimetic deposition.

4. Electrochemical deposition.

5. Electrophoretic deposition.

6. Ion sputtering

7. Ion plating.

8. Ion implantation

9. Ion-beam-associated deposition

10. Super-high-speed (SHS) blasting process

11. Dip coating

6.1 Sol–gel coating

6.1.1 Preparation of hydroxyapatite sol

For preparation of sol-gel HAp coating, a blend of calcium and phosphorus precursors are used for preparation of sol with addition of solvents like ethanol and water.

Table 4 shows the commonly used precursors and solvents for the preparation of hydroxyapatite.

	Precursor	Solvent
Ca precursors	Calcium acetate monohydrate Calcium nitrate tetrahydrate Calcium nitrate tetrahydrate Calcium nitrate tetrahydrate calcium chloride	Water and 1,2-ethanediol Water Ethanol Water and ethanol Water
P precursors	Phosphoric acid Ammonium phosphate dibasic Triethylphosphite Triethylphosphite Trimethyl phosphate Phosphorus pentoxide diammonium hydrogen orthophosphate Trisodium phosphate	Water Water Water and ethanol Ethanol Water Water

Table 4.

Precursors and solvents of HA [65].

6.2 Plasma spraying (PS)

In PS technology it uses a device to melt and deposit a coating material at a high velocity. A direct-current electric arc created by a high current, low voltage electrical discharge between two electrodes produces a plasma flame. The arc super heats a carrier gas stream that contains the molten HA powder. The HA is deposited on the implant by the plasma flame. Adherence of the HA to titanium is mechanical and can be promoted by a roughened substrate surface [66].

Chen et al. determined functionally graded HA/Ti composite coatings had superior mechanical properties over monolithic HA coatings. He concluded that incorporating titanium to HA coating would significantly improve the bond strength of the PS coating. PS heat treatment affects the HA coating phase by increasing crystallinity [67].

6.3 Biomimetic deposition

Bone regeneration is a biological process, and precipitation of ions from solution to form apatite can be considered. An artificial body fluid with ion concentration similar to human blood plasma is used to form biomimetic apatite [68, 69]. This is a classical method to test the bioactivity of material and, apatite formed on the surface of the material can be considered as bioactive [70].

6.4 Electrochemical deposition (ECD)

Electrochemical Deposition-A rapid and excellent method, gives excellent control of the coating material’s thickness, uniformity, crystallinity, and stoichiometry. The process temperature of ECD is less compared to the plasma spray method. This method is typically used to coat hydroxyapatite (HAp, Ca₅(PO₄)₃OH), which is a commonly used biomaterial for bone implants. HAp is a intrinsically occurring mineral form calcium phosphate (CaP) family [71]. Therefore, HAp modification using ECD method on Ti–6Al–4 V surface have favorable results for osteoconduction. An ECD with optimized redox voltage on implant enhance the osseointegration process. This technique could have promising clinical applications to amplify the healing process and success rates of dental implantation.

6.5 Electrophoresis deposition (EPD)

EPD is a process by which, colloidal particles such as HA nano particles are suspended in a liquid medium drift under the power of an electric field which are deposited onto a counter charged electrode. Pressure is concurrently applied to HA nano particles against the electrode. The coating is formed by pressure exerted by the potential difference between the electrodes [72].

EPD are currently being used due to its low cost, easy methodology, capable of producing coatings of variable thicknesses, high deposition rate, formation of highly crystalline deposits with low residual stresses [73]. EPD can produce HA coatings ranging from <1 micron to >500 microns thick [74]. Surface patterns created on the EPD cathode create a patterned HA coating on an implant substrate to change surface topography and enhance osseointegration [75].

6.6 Ion sputtering

Electron beam evaporation and magnetron sputtering techniques are used to deposit hydroxyapatite Nano coatings, to optimize the deposition conditions and so achieve desired properties. The easy replica for ion sputtering is the diode plasma, having a pair of planar electrodes, an anode and a cathode, inside a vacuum system. Another type of sputtering employs radio frequency (RF) diodes that operate at high frequency [76]. Surface characteristics are essential because of its role in enhancing osseointegration.

6.7 Ion plating

Periimplantitis, which is a bacterial induced infection on dental implant materials in human mouth, is one kind of biomaterial centered infection. Periimplantitis is responsible for losing the bony support of the bioimplants, which may cause great damage to the patient. Bacterial adhesion resistant surfaces, multiple arc ion plating methodology, plasma nitriding are used to fabricate Ti nitride coatings on commercial pure titanium used as dental implant materials [77].

6.8 Ion implantation

Into Ti6Al4V dental implants ion implantation using CO remarkably enhanced the osseointegrationin terms of the bone–implant contact, compared to the untreated dental implants. Ion implantation treatments were carried out using a Danfysik high-current implanter Model 1090. Then the samples were ultrasonically degreased and cleaned preceding to any treatment, which were performed using gaseous precursors in a Chord is ion source [78] Chord is ion source is a filament driven ion source with a small plasma chamber designed for production of singly-charged ions.

6.9 Ion-beam-associated deposition of the HA coating

On Ti-based alloy, using ion-beam-assisted deposition an HA layer can be coated. The deposition methods composed of an electron beam vaporizing a pure hydroxyapatite target.

Argon ion beam was focused on the metal substrate for deposition. The deposited layers were amorphous. The bond strength between the layers can be increased with increasing current.

The dissolution rate in a physiological saline solution decreased remarkably. These enhancements were attributed to an increase in the Ca/P ratio of the layer [79].

With ion-beam assistance, the Ca/P ratio of the layer increased apparently due to the high sputtering rate of P compared to that of Ca from the layer being coated.

7. Super-high-speed (SHS) blasting process

Super-high-speed (SHS) blasting process is a novel method to increase the bond strength between coating layer and implant surface so as to stop exfoliation of weakly layered HA from the titanium surface. An evenly layered micron-thick HA layer could maintain micro texture of the implant surface. The coating showed increased bond strength and magnificent wettability properties [80].

7.1 Dip coating

Hydroxyapatitesol was coated onto titanium rods by a dip coating method. An ultrasonic homogenizer was used for the preparation of HA sols by dispersing HA crystals less than 100 nm length in distilled water or physiological salt solution. Homogeneity of the surface of the HA coated titanium rods were determined by scanning electron microscopy (SEM). Tuantuan Li [79] in his study found out, after implantation of uncoated and HA dip coated titanium rods in dog femurs, new bone formation was seen only over the coated material. The bonding strength of HA coated rod was found to be increased after 4 week’s of implantation, as determined by pull-out testing [79]. The dip coated titanium presented remarkable biocompatibility in bone replacement applications [71].

8. Interactions at the HA coatings-tissue interface

The cellular response at the HA coated implant and tissue interface be contingent on the proteins and bio fluid adsorbed onto the surfaces. The implant surface may release ions into the bio fluid, which in turn react with proteins, water and other constituents of the bio fluid, causing surface remodeling [80, 81]. Thus, the surface quality of dental implant and its interactions with constituents of the bio fluid are critical in determining the nature and degree of cellular behavior, especially attachment and proliferation. According to Kasemo et al. [82] the biomaterial/tissue interactions occur within a narrow interfacial zone of less than 1 nm, and these initial interfacial interactions determine the initial bony attachment [82]. Besides the release of ions, surface energy of the biomaterials also affects the initial cellular contact. 108–111 Non identical materials possess distinct surface energies. Surface energy can provide a primary indicator of potential cellular adhesion and implant surface biocompatibility [83, 84, 85, 86]. It has been observed that materials with critical surface tensions of 20 to 30 dynes/cm exhibit minimal cell adhesion, whereas materials with critical surface tensions above this range have greater degree of bio adhesion. 114 With CaP coatings of different crystallite size, there was no significant difference in the critical surface tension [87]. Biomolecules in the biological fluids continuously adsorb on the implant surfaces are vital for controlling cellular responses. Also changes in protein conformation after adsorption on biomaterials surfaces may occur and conformational changes are suggested to elicit differences in cellular response between different biomaterials (Table 5) [88, 89].

Study/year	Author	Study description	Study arms	Results
In Vivo and In Vitro Analyses of Titanium-Hydroxyapatite Functionally Graded Material for Dental Implants. (2021). PMID: 34036104 [90]	Wang X, Wan C, Feng X, Zhao F, Wang H	Fabrication of atitanium-hydroxyapatite (Ti-HA) functionally graded material (FGM) with superior mechanical and biological properties for dental implantation.	Ti group Ti-HA group	The ALP and TGF-β1 levels were slightly increased. The transcript value of ALP and TGF-βRI were high in the Ti-HA groups TGF-βRII showed no obvious increase. The BIC bone-implant contact (BIC) and bone volume over total volume (BV/TV) did not exhibit significant differences between the Ti and Ti-HA FGM groups (P = 0.0504). BV/TV showed the Ti-HA FGM group had better osteogenesis (P = 0.04). Ti-HA
Induction Plasma Sprayed Nano Hydroxyapatite Coatings on Titanium for Orthopedic and Dental Implants (2011). PMID: 21552358 [91]	Mangal Roy, Amit Bandyopadhyay, Susmita Bose	Preparation of a highly crystalline nano hydroxyapatite (HA) coating on commercially pure titanium(Cp-Ti) using coupled radio frequency(RF) plasma spray	HA coatings were prepared on Ti using normal and supersonic plasma nozzles at different plate powers and working distances.	X-ray diffraction and Fourier transformed infrared spectroscopic analysis revealed the normal plasma nozzle lead to enhanced phase decomposition, increased amorphous calcium phosphate (ACP) phase formation, and serious dehydroxylation of HA. Where as coatings utilized using supersonic nozzle hold on to the crystallinity and phase purity of HA; Microstructural properties,adhesive bond strengths,cytotoxicity of HA coatings showed better osteoblast formation and early implant–tissue integration
Aerosol deposition of hydroxyapatite and 4-hexylresorcinol coatings on titanium alloys for dental implants (2011). PMID: 21821331 [92]	Kim SG, Hahn BD, Park DS, Lee YC, Choi EJ, Chae WS, Baek DH, Choi JY	Aerosol deposition is a newertechnique.4-Hexylresorcinol (4-HR) is an antiseptic. The influence of the 4-HR component of HA coatings on titanium surfaces was studied in vitro and in vivo	Group A HA Group B HA+ 4-HR coating	MG63 cells attachement, increased osteocalcin expression and alkaline phosphatase activity, higher reverse torque value higher in HA 4-HR group.; Histologic analysis, ossteogenesis value and bone implant contact value were significantly higher in the HA,4-HR group 8 weeks after surgery.
Hydroxyapatite-based composite for dental implants: an in vivo removal torque experiment.(2002). PMID: 2418015 [93]	Young-Min Kong 1, Dong-Hwan Kim, Hyoun-Ee Kim, Seong-JooHeo, Jai-Young Koak	Screw-shaped dental implants were fabricated from commercially pure Ti (c.p. Ti) The HA-based composites were made by mixing HA with Al(2)O(3)-coated ZrO(2) powders. The mechanical properties increased by a factor of 3. Reversed torque to loosen the implants in vivo was measured to estimate the osteointegration.	Group A c.p. Ti Group B HA-based composite implants	The composite implants 2-times-higher removal torque to the Ti implants (ANOVA, p < 0.05),
The correlation between osseointegration and bonding strength at the bone-implant interface: In-vivo & ex-vivo investigations on HA and HA/Ti coatings (2022). PMID: 36162145 [94]	Ghadami F, Amani Hamedani M, Rouhi G, Saber-Samandari S, Mehdi Dehghan M, Mashhadi-Abbas F Farzad-Mohajeri S,	The study analyzed the effects of HA and HA/Ti coatings on osseointegration,bonding strength at bone-implant interface.	Three groups 1) (CP-Ti) rods Uncoated commercial pure titanium 2) HAcoated CP-Ti rods, and 3) Composite of 50%wt HA + 50%wt Ti coated CP-Ti rods.	Pull-out tests showed that the ultimate strength of HA and HA/Ti coatings were significantly greater than the uncoated samples (P < 0.05). Histological assessment showed significantly improved osseointegration of HA/Ti composite coatings than with HA coatings (P < 0.05)

Table 5.

In vitro and in vivo studies with HA -coated dental implant surfaces.

9. Advances in nanoparticle incoorporated hydroxyapatite

The fundamental base of the enamel unit is HA particle, size of 20 to 40 nm. The proteins are almost degraded when the enamel reaches its evolvement. This leads to the crystallization of apatite; hence the enamel cannot be biologically remodeled [95].

Hydroxyapatite has poor ability to repair fibrous connective tissue surrounding the granules, at the same time, the porosity, surface geometry and surface chemical properties of traditional hydroxyapatite scaffolds restrict the application themselves, especially to alveolar bone repair [96].

The nano-HA formed has been found to own analogous morphology, structure, and crystallinity as a biological apatite. When compared with larger HA, nano HA constitutes good biocompatibility, nontoxicity and also having higher resorption rate [97].

Nano-HA is the ideal treatment option for treating bone defects caused by trauma or surgery. While macroscale hydroxyapatite particles or blocks have long been used to treat bone abnormalities, nanoscale hydroxyapatite particles recently been clinically introduced in an injectable form. When combined with stem cells or growth factors and placed on a scaffold, nano-HA can be used in tissue engineering and bone or cementum regeneration. It can help with oral surgeries like cleft lip and palate repair and periodontal procedures [98]. Composites containing nano-HA are used to fill the alveolar socket and reduce alveolar bone loss immediately after extraction because they can recreate the bone structure and environment while maintaining the bone’s flexibility and resistance [99].

There are various studies demonstrating applications of nanohydroxyapatite in periodontics. Lowe et al. [100] concluded that chitosan nano-HA-fucoidan scaffold has high biocompatibility and mineralization and also that the composite membrane exhibited a applicable micro architecture for cell growth and nutrient supplementation. Lee et al. [101] found out that reduced graphene oxide/HA nanocomposites accelerated bone regeneration and stimulated osteogenesis. Uysal et al. [102] evaluated the efficacy of treating periodontitis using subgingivalnano-hydroxyapatite powder with an air abrasion device combined with scaling and root planning and found that it improved clinical periodontal parameters more than scaling and root planning alone as it enhances clot adhesion to the surfaces of tooth by improving surface wettability.

The widespread application of HA in implants is due to its bioactive characteristics, which aid in the production of new bone, improve tissue integration, and speed up the healing process. As a result, it’s being used to convert metallic implants’ smooth, harsh surfaces into a more biocompatible, porous environment similar to hard tissues. Nano-HA is the most often employed coating material for titanium and stainless-steel implants, offering benefits such as improved bone bonding and new bone genesis, as well as improved bone-to-implant contact [103]. Another benefit of employing nano-HA as a coating for dental implants is its capacity to prevent bacterial growth, including both Gram-positive and -negative bacteria. Furthermore, implants coated with a small layer of nano-HA had a lower inflammatory response, as HA is a modulator for monocytes and macrophages, which are responsible for the early inflammatory response. Several investigations have demonstrated that, because of its chemical and chrystallographic affinity for the inorganic components of bone, this substance is capable of establishing chemical linkages and ensuring a faster integration of the implants with the bone and surrounding tissue [104]. As grafting materials in relation to dental implants, alloplast materials containing nano-HA are employed. They promote bone repair by enhancing angiogenesis and consequently having a high porosity. Because of the quicker bone growth, an earlier implant placement is permitted, with these materials having a 4-month healing period [105].

Nanohydroxyapatite in toothpaste, in particular, has been demonstrated in studies to promote remineralization and the hardness of dental enamel and dentine. This is owing to the nano-hydroxyapatite particles’ incredibly small size, which allows them to easily enter and interact with sub-micrometer and nanometrescale acidic erosion damage on tooth surfaces (white spots). Calcium and phosphate ions are liberated from the nano-hydroxyapatite particles during the contact. The liberated ions enter the enamel rods and crystallize into apatite [106]. As a result, re-mineralization and restoration of the enamel surfaces occur. Despite the fact that nano particles can penetrate dental porosities, they can form a protective layer on the tooth’s surface. Nano-HA is even used in sports drinks to reduce the impact on teeth. Acidic drinks increase tooth surface degradation.

Several studies have also demonstrated that using nano-hydroxyapatite in dental products can reduce bacterial colonization of tooth surfaces and dentine hypersensitivity. Studies have also indicated that calcium phosphate-based materials can be used to repair or remineralize missing, damaged, or eroded tooth enamel. HA-based materials, in particular, are commonly employed to remedy surface issues like as discolouration, voids, and chips. HA has been employed as filler for strengthening GICs and restorative resin composites on both a micro- and nano-scale [107]. Nano-HA is utilized to remineralize dentin and enamel that has been compromised by caries. The hard tissue loses mineral ions due to acid assault from bacterial metabolism in early-stage caries, while the collagen network is unharmed. The endeavor to remineralize this organic scaffold is realized by the use of nanoparticles (nano-HA, bioactive glass), which act as either direct replacements for final minerals or as a carrier for lost ions during caries assault [108].

Bleaching methods generate reactive oxygen species, which penetrate the enamel and reach the dentin, breaking organic molecules and processing lighter and clearer substances. The most common bleaching chemical is hydrogen peroxide, which has a concentration of 30–35% in a gel product. The gel is loaded with remineralizing chemicals such as fluoride calcium and hydroxyapatite in nano form to reduce hypersensitivity following bleaching, which can occur in up to 70% of bleached patients. The bleaching agent can permeate the enamel through microscopic surface flaws and beneath pores, causing sensitivity. Nano-HA paste can correct these minor enamel flaws, preventing the sensory reaction [109].

Nanomaterials can have a harmful impact on human health if they are present in the environment since their entry into the body is enabled by exposure and subsequent absorption through the skin, digestive tract, and lungs. Ingestion of nanoparticles in dental products during or after treatment; inhaling of aerosols created from nanomaterial-based composites during drilling; and direct interaction between nanomaterials and cellular tissues in the oral cavity can all result in exposure and potential harm [110]. Nanomaterials may easily interact with cell constituents such as DNA molecules, proteins, and intracellular components, which is important. It’s challenging to foresee and comprehend these interaction mechanisms, elimination pathways, and immunological responses. This ambiguity stems from the fact that different nanomaterials of the same material behave differently in different biological tissues. Coatings, for example, can alter the size range, surface charge, and surface chemistry of nanomaterials in relation to cellular tissues [111].

Smart nanomaterials that aid in healing, stimulate cellular regeneration, and help in osseointegration of bioactive dental implants are now being researched. These new technologies, however, are not without their own set of problems. For example, designing a low-cost, mass-produced nano-robotic system that can perform their intended duties is a heavy task. There’s also a demand for smart nanomaterials, protocols, and nano-devices that can give disease monitoring, diagnosis, prevention, and treatment techniques that are tailored to particular individuals.

10. Future perspectives

New dental products are still in high demand, both scientifically and commercially. There is currently no one solution that satisfies all of the required qualities and standards for preventative and restorative applications. The most successful technique of delivering beneficial outcomes for patients, however, is regarded to be breakthroughs in nanotechnology-based strategies for generating new products. Several active research fields are now being studied. Colloidal liquids containing millions of active nanometer scale robots might be delivered into the oral cavity to shut down certain nerves, for example, to lessen anxiety and provide greater patient comfort during dental treatments [112].

The practitioner sends the nano-robots to specific tooth positions or soft tissues once they have entered the oral canal. The nano-robots then travel into tissue structures, shutting off the sensitivity of specific nerves. The practitioner then instructs the nano-robots to restore nerve sensitivity and leave the tissues following the dental surgery. Orthodontic nano-robots could also be utilized to remodel periodontal tissues and allow painless tooth straightening, rotation, and repositioning in minutes to hours. Nano-robotic dentifrices, on the other hand, might be utilized to carry and distribute toothpastes or mouthwashes that break down organic matter or oral bacteria into harmless by-products. Nano-robots could also be utilized to administer antibiotics and medications (nano-encapsulation). While nano-sensors/robots could be used to detect and identify dangerous elements to aid in the diagnosis and treatment of diseases, thereby improving patient well-being [113].

Furthermore, recent research has shown that high-strength nanoparticles can be engineered into dental polymers to improve their strength and durability. For example, dental polymer reinforced with graphene gold nanoparticles increased mechanical characteristics, encouraged tissue formation when graphene oxide was implanted to collagen scaffold, and improved physicochemical and surface properties when graphene oxide was implanted to collagen scaffold. Carbon nanotubes and Boron Nitride nanoplatelets have also sparked interest among scientists as a viable biomaterial for dental applications. A recent study found that Boron Nitride nanoplatelets reinforcement improved the strength and fracture toughness of zirconia composites. However, conflicting investigations have demonstrated that carbon nanotubes have both cytotoxic and non-cytotoxic qualities, sparking a discussion about its possible application as a bioceramic material. As a result, bio-kinetics and organ toxicity are crucial in determining the quantitative risk of using these high-strength nanomaterials.

Furthermore, smart nanomaterials that aid in healing, stimulate cellular regeneration, and osseointegration of bioactive dental implants are now being researched. These new technologies, however, are not without their own set of problems. For example, designing low-cost, mass-produced nano-robotic systems that can perform their intended duties. There’s also a demand for smart nanomaterials, protocols, and nano-devices that can give disease monitoring, diagnosis, prevention, and treatment techniques that are tailored to particular individuals.

11. Conclusion

Hydroxyapatite is biocompatible material that provides cell adhesion & proliferation. It is used as a carrier & a loading agent in the controlled release and delivery of drugs. Also used as a coating materials on orthopedic implant, because of chemical resemblance to the mineral component of mammal bone and hard tissues. Advances related to varieties of methods of synthesis of HAp, as well as knowledge and Applications of Hydroxyapatite.

HAp is generally used to treat bone and periodontal defects, alveolar ridge, as dental materials, middle ear implants, tissue engineering systems and bioactive coatings on metallic osseous implants. Recent studies also suggest that HAp particles impede the progression of cancer cells.

Due to its structural fragility, a great demand for the development of efficient, simple and low-cost methods. HAp has limited applications when the bone defect to be repaired is in anatomical regions that are under constant tension. Thus, researchers have been directing studies in obtaining materials having superior properties.

As a replacement, HAp is used with other materials, increasing its applicability and efficiency for treatment of tissues. This shows a promising methodology and high potential for the development of scaffolds.

The development of optimal bone supports, coatings and release systems are a challenge for the engineering of bone tissue. It is a requisite to study the development and applicability in different anatomical sites to improve mechanical and biological aspects of HAp-based implants and to optimize their safety and efficiency. However, the prospects of novel biomaterials such as HAp in biomedical fields definitely depend upon the advancement of our knowledge, not only of the material, but also its interactions with specific bio-molecules, cells, and tissues.