Definition/Introduction

There is a need to reconstruct damaged hard tissue for several reasons that include traumatic or non-traumatic events, congenital abnormalities, or disease. Damaged tissues stemming from these events can become a major issue in orthopedic, dental, and maxillofacial surgery. A study on numerous biomaterials revealed that calcium phosphates had been used in hard tissue reconstruction for more than 6 decades. Hydroxyapatite (HA) was the primary material used in orthopedics and dentistry.

Hydroxyapatite (HA) is an inorganic mineral that has a typical apatite lattice structure as (A10(BO4)6C2) where A, B, and C are defined by Ca, PO4, and OH. Pure HA contains 39.68% by weight calcium and 18% by weight phosphorus resulting in a Ca/P mole ratio of 1.67. In fact, there are commercial HA products with the Ca/P ratio bigger or smaller than 1.67. The variety in the Ca/P ratio indicates the phase shift between tricalcium phosphate (TCP) and calcium oxide (CaO). HA with Ca/P ratio bigger than 1.67 comprises more CaO than TCP and vice versa[1][2].

HA crystals present in the human body both inside bone and teeth. In terms of human bone, the HA crystals as a bioactive ceramic cover 65 to 70% by weight of the bone. Furthermore, the architecture of the bone comprises of type-I collagen as an organic component and the HA as an inorganic component. These 2 components form a composite structure at the nanoscale, in which nano-HA is interspersed in the collagen network. This composite forms mineralized collagen and is the precursor of biological mineralized tissue from tendons and skin to hard mineralized tissues such as bone and teeth. Moreover, in the bone, the HA crystals present in the shape of plates or needles, are about 40 to 60 nm long, 20 nm wide, and 1.5 to 5 nm thick[3]. The arrangement of different HA crystalline sizes and shapes provides support for this tissue's structural stability, hardness, and function[4][5].

About concerning the dental role of HA crystal, it covers 70 to 80% by weight of dentin and enamel. Within the human body, the enamel is the hardest substance consisting of relatively large HA crystals (25 nm thick, 40 to 120 nm wide, 160 to 1000 nm long)[6]. Different from bone, enamel does not contain collagen. Amelogenins and enamelins replace the function of collagen by providing a framework for mineralization. Besides, HA is the main material of enamel that screens the diffuse reflectivity of light by covering the pores on the enamel surface, thus makes the appearances of enamel is semitranslucent[4][7].

Overall, the pressure-point in hard tissue reparation is on HA due to its chemical proportion that occupies the majority of hard tissue composition, and also its mechanical properties that support tissue integrity[8]. Currently, HA is widely used as implant material due to its excellent osteoconductive property that supports osseointegration and osteogenesis processes. The biological response to HA implants is influenced by its raw-materials and synthesis process that make product properties vary.

Issues of Concern

Hydroxyapatite has been long used in hard tissue engineering due to its chemical similarity to the mineral of hard tissue. The era of hydroxyapatite (HA) in regenerative science dates back to the 1950s when bioceramics were used to fill the bone defects. However, after more than 6 decades of scientific innovation through research and development, HA has restructured the traditional philosophy of using ceramics in medical sciences through the wide range of its applications in dentistry and drug delivery.

HA application in orthopedics can vary from bone defects repair and bone augmentation to coatings for human body metallic implants. The HA-based implant can provide an interlocked porous structure[9][10]. This structure can act as the extracellular matrix, promoting the natural process of cellular development and tissue regeneration[11]. Furthermore, HA can enhance the osseointegration process by promoting rigid anchorage between the implant and the surrounding tissue without the growth of fibrous tissue. The successful osseointegration retains the bone anchorage for a long period, hence completely restoring functional ability[12][13].

Another significant application of HA can be seen in dentistry since 1979. HA cylinders have been used for tooth replacement. This application was followed by the utilization of HA blocks and coating to enhance the bone fixation in a restorative dental procedure in the early 1980s. Now, HA is found not only in dental cement and fillings but also in toothpaste. HA in toothpaste acts as a polisher to decrease the deposition of accretions on teeth[14][7].

The application of HA also can be found in drug delivery. The naturally porous structure with a high-binding affinity of HA provides a niche for drug loading, thus makes HA a good fit as a drug carrier[15]. The low solubility of nano-HA in physiological conditions contributes to its longer degradation rate[4]. This condition can be useful as a carrier for local drug delivery either by surgical placement or injection. This controlled drug delivery using HA can maintain drug concentration in the blood, and hence, reduce the toxicity to other organs[4][15].

The applications of HA in hard tissue restoration and drug delivery do not use HA in pure form. The mechanical properties of pure HA are relatively low and brittle for load-bearing applications. Therefore, HA is usually incorporated in composite or polymer to increase its application[16][11]. In this case, improvement properties of HA are a result of the compressive strength of the HA ceramic phase as well as the toughness and elasticity of the polymer or composite matrix. Generally, HA is resistant to resorption in vivo that occurs at a rate of 1 to 2% per year. Thus, this condition provides long structural support in the defect area[8][12].

There are several methods to produce HA either from synthetic material or natural sources. Synthetic HA uses raw materials in the form of calcium carbonate, calcium hydroxide, calcium nitrate, diammonium hydrogen phosphate, and ammonium hydroxide. The fabrication process of HA is known as a wet method and solid-state reaction, followed by calcination or sintering process. Both of these methods use chemical reaction by varying the content of calcium oxide (CaO) and tricalcium phosphate (TCP) to reach HA stoichiometric conditions.

The wet method produces non-stoichiometric HA powder, with impurities such as ions of hydrogen phosphate, carbonate, chloride, and sodium. These impurities cause the formation of calcium-deficient HA[1]. Previous works have identified the impurities as an uncontrollable variable that may promote significant changes in the crystallographic arrangement and chemical properties, which later affect the dissolution process of HA. On the other hand, the solid-state reaction produces a stoichiometric and well-defined crystalline shape of HA product, yet solid-state reaction requires high temperatures and long heat treatment procedures. The raw materials of the solid method should have Ca/P = 1.67 and are ball-milled to ensure the product uniform in size[2]. The solid method solely depends on the solid diffusion of ions into the raw materials, thereby needs a high temperature around 1250ºC to start the reaction[2]. Moreover, prolonged heat treatment transforms the singular crystalline particle to more blocky crystals. The increase of crystalline size causes a decrease in porosity, which is associated with the aging process[6].

Hydroxyapatite from the natural source is commonly fabricated from fishbone, coral, bovine bone, eggshell, and seashells through the calcination process. HA produced from natural sources is non-stoichiometric due to the presence of trace ions found in the natural sources[17][13]. These trace of ions, which consist of cations, such as Na+, K+, Mg2+, Sr2+, Zn2+, and Al3+, or anions like F-, Cl-, SO4 2-, and CO3 2-, are beneficial to promote rapid bone regeneration[18].

The mechanical properties of HA depend on several variables, such as phase composition, crystal size, and the synthesis process. Pure HA has bending, compressive, and tensile strength is in the range of 38 to 250 MPa, 120 to 150 MPa, and 38 to 300 MPa, respectively. Young’s modulus varies from 35 to 120 GPa, depending on the impurities[19]. Whereas, Weibull’s modulus with value in the range 5 to 18 exhibits that HA is a brittle material. To increase the mechanical properties of HA, for instance, tough HA is obtained when the composition containing tricalcium phosphate (TCP) that makes HA has high flexural strength, meanwhile the flexural strength decreases to the minimum value if HA contains calcium oxide (CaO)[19][10]. Furthermore, the temperature of sintering also contributes to the change of HA mechanical properties. The rise of sintering temperature causes an increase in density, compressive strength, grain size, and torsional strength[1]. 

Phase composition and preparation method affect the chemical stability of HA. For example, the exchange between magnesium, carbonate, or strontium with the apatite promotes an increase in solubility[17]. In contrast, the exchange with fluoride causes a decrease in the solubility. Sintered HA has higher chemical stability compared to non-sintered HA, which causing sintered HA less soluble in vivo[1].

Clinical Significance

Synthetic and natural hydroxyapatite have been long preferred as the material used in hard tissue repair over autografts and allografts. This is due to problems that the grafts are naturally associated with several issues including graft shortage, donor site morbidity, disease transmission, and graft rejection.

In bone tissue engineering, the bioactivity of HA, which is marked by osteoconductive and osteoinductive processes, has proved to support osseointegration. The osteoconductive property of HA provides a template to guide the new bone formation on its surface down to the pores of the implant body[20]. The HA osteoconductivity allows the osteoblast to attach, proliferate, grow, and express the phenotype in a direct contact manner, thus forming a strong tissue-implant interface. This osteoconductive property depends on the specific geometry and pore size of HA[17][21]. On the other hand, the osteoinductive property of HA encourages tissue ingrowth that allows the neoformation of bone even in the non-bone-forming area. Equally important, the coating of an implant using HA enhances initial mechanical stability post-implantation, resulting in a decrease of aseptic loosening. In this situation, HA facilitates the chemical bonding of the implant with surrounding tissue by absorbing protein into the implant surface[10]. The presence of protein on the surface is favorable for an early healing event at the tissue-implant interface. The high stability of the implant makes immediate loading more predictable. The chemical resemblance of HA to the bone minerals ensures its ability to bond directly to bone tissue without an intervening fibrous layer[17][11]. Overall, osteoinduction, osteoconduction, and osseointegration properties of HA are complementary, not the same, phenomena. All of these properties of HA serve as a fact that the application of HA as the cellular matrix is of great interest.

The advance in the materials fabrication process leads to the development of nano-HA particles which can induce fast dentin remineralization[22]. Nano-HA diffuses into the demineralized collagen matrix of dentin, changing the environment into a suitable scaffold for the remineralization process and acts as the mineral precursor. Nano-HA provides a good source of free calcium and is an important element to promote protection against dental erosion and caries[14]. This application of HA generally requires a high amount of calcium hydroxide marked by an increased Ca/P value. Furthermore, the presence of nano-HA in toothpaste can act as a filler to repair the holes and the sunken surface of enamel[14]. In this reparation process, nano-HA gets through the surface of the enamel to replace the phosphate and calcium ions that have dissolved, thereby remineralizing damaged enamel and reconstruct its structural integrity[5][16]. Moreover, nano-HA in toothpaste also provides a protective coating over dissolved dentinal tubules, offering fast and potential remedy from tooth hypersensitivity.

Atomic bonds in HA are quite strong contributes to the fact that HA does not swell or change in size under the range of PH and temperature[18].  This low swelling ratio of HA forbids the outburst of drugs, a common problem in drug delivery. Bone cement usually has HA both as fixating materials and drug carriers[15]. It is because of its capacity for controlled-drug release via diffusion from the cement rather than via dissolution of the apatite material, as the cement has less in vitro solubility than typical block apatites[23]. Preferably, HA is used to deliver the skeletal drug system in diseased-bone rather than the oral therapeutic system as the acid in the gastric environment can degrade its structure.

There are several problems regarding HA application in medicine. For instance, the use of HA as an implant has inherent defects/fine porosity that could act as a crack initiator. Following this event, the crack propagation can cause catastrophic deterioration during application. Furthermore, the application of bulk HA sometimes can cause the modulus mismatch between bone and the implant which later causing the disproportionate load sharing[10]. On the other hand, regardless of their source, HA always contains traces of elements, such as fluoride ions (F-) and hydroxyl ions (OH-) which causes an increase in crystallite size and a decrease in solubility that can increase the apatite strength[19]. Whereas elements such as phosphide ions (PO3 3-), chloride ions (Cl-) have been known to decrease the HA mechanical properties by causing a reduction in crystallite size and an increase in solubility[12][10].

Another issue that occurs by using HA in medical applications is how to fine-tune the degradation rate. Poor mechanical properties of the HA-based implant can induce not only fast degradation but also implant failure and chronic inflammatory reaction[24][11][24]. For example, rapid degradation leads to the rapid release of the calcium content of HA to the environment, later raising the concentration of calcium locally[9]. Naturally, high calcium concentration is important for bone regeneration. Nonetheless, when the degradation is too fast, it may cause structural collapse of the implant and induce too much graft resorption[19][16]. Controlling HA degradation is important for the implant to induce tissue regeneration promptly. Regarding this condition, the controlled release of HA particles can be carried out by manipulating particle size. Small-sized particles have a wider surface than larger-sized particles of the same weight. Thus, the smaller size particle is easier to detach from the implant body[18].