Authored by Abere DV*
Annotation
This work investigated the suitability of the utilization of cowry shell-based hydroxyapatite (HA) in orthopaedic and dental applications. HA was synthesized via aqueous precipitation process and sintered at different temperatures. The pH and density of the synthetic HA were determined before subjecting the samples to mechanical characterization. The chemical analysis of the HA was carried out with the aid of Energy Dispersive X-ray Florescence (ED-XRF), Atomic Absorption Spectrophotometer (AAS), Fourier’s Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) while the microstructural analysis was evaluated using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).
The weight of the precipitate produced at pH 9 and 10 are similar to the theoretical HA which is 8.17 g per precipitation batch assuming complete transformation of Calcium (Ca) to HA while the weight recovered at the pH 10 to 12 are greater than the theoretical value and this might be due to the presence of adsorbed water layers on the surface of the powder at the corresponding pH. The density of the synthetic HA is in the range of 2.66–3.75 glcm3 which falls within the theoretical density of HA. The HA has the optimum hardness value of 742 HV at 900oC. The compressive strength obtained ranges from 252.20-452.5 MPa while the optimum compressive strength is 452.5 MPa at 1200oC. The tensile strength obtained is in the range of 55.84 to 86.41 MPa. The optimum value being 86.41 MPa was obtained at 1200oC and this falls within the range of the tensile strength of dense HA. The range of the elasticity of the synthetic HA is 30.83–65.05 G Pa and it was observed that the elasticity of the material increases as the sintering temperature increases. The value obtained is higher than the modulus of bone and that of human tooth but falls within the range of value of a dense HA. The fracture toughness obtained ranges from 0.65 – 2.55 MPam1/2. The optimum value of the fracture toughness which is 2.55 MPam1/2 at 1200oC is within the range of the fracture strength of human compact bone. The ED-XRF and AAS reveal that the main component of the synthetic HA powder are calcium and phosphorus. It can be deduced from the FTIR that the synthetic sample is hydroxyapatite. Observation from the XRD patterns shows that the material is a crystalline single phase with large amount of amorphous phase which is good because amorphous components present an improve biodegradable attributes. Pure HA and other phases in minute concentration were observed in the XRD results. The SEM analysis of the HA material shows that the particle size of the material has a high dispersion. It can be observed that the images of the synthesized hydroxyapatite are porous in nature and this porous nature is a good desirable property of material for bone substitute. The EDS technique reveals that the elemental constituent of the synthesized HA was obtained to be Ca 55.25 wt%, P 26.91 wt% and O 17.84wt% which implies high purity of the calcium phosphate produced through the continuous precipitation technique. The particle sizes obtained through the SEM micrographs are within the range of the sizes that can enhance bone regeneration.
This synthetic hydroxyapatite will be compatible with the human physiological environment since biocompatibility is a direct result of their chemical constituents which include ions that are commonly found in the physiological environment. The synthetic HA will therefore find applications in filling of bone defects in orthopaedic surgery, coating of dental implants and metallic prosthesis.
Keywords:Orthopaedic; Dental; Hydroxyapatite; Precipitation; Crystalline; Bone; Biocompatibility
Introduction
Bones which exist as a natural composite possess type 1 collagen with calcium phosphate in the form of hydroxyapatite in human body [1]. Bone performs several roles which include serving as a calcium reservoir, aiding the soft tissues and providing the cells found in the marrow that differentiate into blood cells and the likes. However, its main function is to serve as mechanical support for soft tissues as well as anchoring for the muscles which generate motion [2]. The case of fractures increases greatly with age and accident. Especially, fractures due to age is partly as a result of extraosseous factors like impaired reflex of the old age, reduced proprioceptive efficiency, reduced cushioning by fat, weakened musculature and by osseous factors such as the structural changes in the shape and size of the bone and by deterioration of the condition of the bone material also [3].
Bone graft materials are quickly becoming a vital tool in reconstructive orthopaedic surgery and demonstrate considerable variability in their appearance. Functions of bone graft maaterials and bone healing provide a structural substrate for these processes and serve as a vehicle for direct antibiotic delivery [4]. The most prominent and major material in the teeth and human bones are hydroxyapatite (HA). Due to this, HA with similar characteristics to natural HA are widely developed to repair and function as bone substitutes.
HA is a crucial element needed for bone regeneration. Various forms of HA have been utilized for a long time. The essence of bone regeneration always revolves around the healthy underlying bone or it may be the surroundings that give sufficient strength. HA is widely known for bone regeneration through conduction or by acting as a scaffold for filling of defects from ancient times, but emerging trends of osteo-inductive characteristics of HA are much promising for new bone regeneration [5]. Emerging technology has made the dreams of clinicians to realize the use of HA in different forms for various regenerative purposes both in vivo and in vitro.
Hydroxyapatite is among the bioceramics which represents the most commercially available regenerative graft material. Hydroxyapatite also belongs to the inorganic components of the bone and is closely associated with the bony apatite structure. It is bounded in the organic matrix, so that it exists with other mineral trace elements in the normal bone [6]. Due to the attribute of the HA, it is attracting more relevant in regenerative science as a good substitute potential material next to autograft. HA has been applied as a substitute for bone due to its chemical nature which is similar to the natural bone. The major constituent of bone is 69 wt% mineral phase, 22 wt% organic matrix and 9 wt% water [7]. Bone is the most prominent calcified tissue in mammals [6] and is a ceramic–organic bionanocomposite with a complex structure. The general formula of HA is Ca10(OH)2(PO4)6 which is similar to an inorganic component of bone matrix. As a result of this similarity, rigorous and extensive research is in progress to utilize HA as a substitute for bone. HA is one of the most stable and less-soluble calcium phosphate bio ceramics with Ca/P ratio of 1.67 [7,8]. The pure HA powder is white but the naturally occurring HA can as well possess green, yellow, or brown colorations, comparable to the discolorations of dental fluorosis. In biological systems, HA occurs as a principal inorganic component of normal (bone, teeth, fish enameloid, and some species of shells) and pathological (dental and urinary calculus and stones) calcifications. The mechanical properties of HA depend on crystal size, porosity, sinterability, phase composition, density and the likes. The bending, compressive, and tensile strength values of HA ceramics respectively fall in the range of 38–250, 120–150, and 38–300 MPa [7,8]. Young’s modulus of dense HA ceramics varies from 35 to 120 GPa, depending on the residual porosity and impurities. Weibull’s modulus of dense HA ceramics lies in the range 5–18, characteristic of brittle materials. The Vicker’s hardness of dense HA ceramics is 3–7 GPa. The mechanical properties of HA bioceramics strongly depend on the microstructure and sintering ability; densely sintered bodies with fine grains are tougher and stronger than porous ones with larger grains [9].
HA bio ceramics have been widely used as artificial bone substitutes due to their excellent biological properties, which among others include bioactivity, biocompatibility, bio affinity, osteoconduction [15], osteointegration [14] and osteoinduction [20]. HA constitutes only calcium and phosphate ions and hence no adverse local or systemic toxicity has been reported in any study. When implanted, newly formed bone binds directly to HA through a carbonated calcium-deficient apatite layer at the bone implant interface [10]. HA surface supports osteoblastic cell adhesion, growth, and differentiation, and new bone is deposited by the creeping substitution from the adjacent living bone. HA scaffolds can also function as delivery vehicles for cytokines with a capacity to bind and concentrate bone morphogenetic proteins (BMPs) in vivo [11]. HA have been investigated for its clinical viability in various bone defects [12]. Many researchers have demonstrated a better initial osseointegration and a high short-term success rate [13-15]. HA-coated implants showed varying results of survival [16-20]. Different forms of HA have been derived from different origins for various uses. Bovine HA [21-30] and synthetic HA [31- 34] are major sources of HA grafts. These have shown varying success rates.
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