W. Bonfield

Characterization of hydroxyapatite and carbonated apatite by photo acoustic FTIR spectroscopy.

An understanding of the interfacial relationship between a bioceramic implant and the adjacent bonet issue is facilitated by precise characterization of the associated structures. The structure of different commercial synthetic hydroxyapatite powders and a novel carbonated apatite have been studied with photo-acoustic (PAS) Fourier transform infrared (FTIR) spectroscopy. The PAS technique is an ideal method for analysing biomaterials, as materials can be analysed without the need to reduce the particle size or to dilute with KBr. Spectra from carbonated apatite appear to be different from those of commercial hydroxyapatite powders, with the main difference lying in the carbonate and phosphate ratio. Commercial hydroxyapatite powders from different sources have also been analysed and compared.

A comparative study on the in vivo behavior of hydroxyapatite and silicon substituted hydroxyapatite granules

Phase pure hydroxyapatite (HA) and a 0.8 wt % silicon substituted hydroxyapatite (SiHA) were prepared by aqueous precipitation methods. Both HA and SiHA were processed into granules 0.5–1.0 mm in diameter and sintered at 1200 °C for 2 h. The sintered granules underwent full structural characterization, prior to implantation into the femoral condyle of New Zealand White rabbits for a period of 23 days. The results show that both the HA and SiHA granules were well accepted by the host tissue, with no presence of any inflammatory cells. New bone formation was observed directly on the surfaces and in the spaces between both HA and SiHA granular implants. The quantitative histomorphometry results indicate that the percentage of bone ingrowth for SiHA (37.5%±5.9) was significantly greater than that for phase pure HA (22.0%±6.5), in addition the percentage of bone/implant coverage was significantly greater for SiHA (59.8%±7.3) compared to HA (47.1%±3.6). These findings indicate that the early in vivo bioactivity of hydroxyapatite was significantly improved with the incorporation of silicate ions into the HA structure, making SiHA an attractive alternative to conventional HA materials for use as bone substitute ceramics.

Comparison of in vivo dissolution processes in hydroxyapatite and silicon-substituted hydroxyapatite bioceramics

The incorporation of silicate into hydroxyapatite (HA) has been shown to significantly increase the rate of bone apposition to HA bioceramic implants. However, uncertainty remains about the mechanism by which silicate increases the in vivo bioactivity of HA. In this study, high-resolution transmission electron microscopy was used to observe dissolution from HA, 0.8 wt% Si-HA and 1.5 wt% Si-HA implants after 6 and 12 weeks in vivo. Our observations confirmed that defects, in particular those involving grain boundaries, were the starting point of dissolution in vivo. Dissolution was observed to follow the order 1.5 wt% Si-HA>0.8 wt% Si-HA>pure HA and it was found to be particularly prevalent at grain boundaries and triple-junctions. These observations may help to explain the mechanism by which silicate ions increase the in vivo bioactivity of pure HA, and highlight the enhanced potential of these ceramics for biomedical applications.

Relation between age, femoral neck cortical stability, and hip fracture risk

BACKGROUND Hip fracture risk rises 100 to 1000-fold over 60 years of ageing. Loss of resistance to bending is not a major feature of normal ageing of the femoral neck. Another cause of fragility is local buckling or elastic instability. Bones adapt to their local experience of mechanical loading. The suggestion that bipedalism allows thinning of the underloaded superolateral femoral neck cortex arises from the failure of walking to transmit much mechanical load to this region. We aimed to measure whether elastic instability increases greatly with age since it might trigger hip fracture in a sideways fall. METHODS We measured with computed tomography the distribution of bone in the mid-femoral neck of 77 proximal femurs from people who died suddenly aged 20-95 years. We then calculated the critical stress, from the geometric properties and density of the cortical zone most highly loaded in a sideways fall, as a threshold for elastic instability. FINDINGS With normal ageing, this thin cortical zone in the upper femoral neck became substantially thinner. Relative to mean values at age 60 years, female cortical thickness declined by 6.4% (SD 1.1) per decade (p<0.0001), and critical stress by 13.2% (4.3) per decade (p=0.004) in the superoposterior octant compressed most in a sideways fall. Similar, but significantly smaller, effects were evident in men (p=0.004). This thinning compromised the capacity of the femur to absorb energy independently of osteoporosis. Patients with hip fracture had further reduced stability. INTERPRETATION As women age, hip fragility increases because underloading of the superolateral cortex leads to atrophic thinning. Because walking does not sufficiently load the upper femoral neck, the fragile zones in healthy bones may need strengthening, for example with more well targeted exercise.

CHARACTERIZATION OF POROUS HYDROXYAPATITE

Hydroxyapatite has been considered for use in the repair of osseous defects for the last 20 years. Recent developments have led to interest in the potential of porous hydroxyapatite as a synthetic bone graft. However, despite considerable activity in this field, regarding assessment of the biological response to such materials, the basic materials characterization is often inadequate. This paper documents the characterization of the chemical composition, mechanical integrity, macro- and microstructure of a porous hydroxyapatite, Endobon ® (E. Merck GmbH), intended for the bone-graft market. Specimens possesed a range of apparent densities from 0.35 to 1.44 g cm-3. Chemical analysis demonstrated that the natural apatite precursor of Endobon® was not converted to pure hydroxyapatite, but retained many of the ionic substituents found in bone mineral, notably carbonate, sodium and magnesium ions. Investigation of the microstructure illustrated that the struts of the material were not fully dense, but had retained some traces of the network of osteocyte lacunae. Macrostructural analysis demonstrated the complex inter-relationship between the structural features of an open pore structure. Both pore size and connectivity were found to be inversely dependent on apparent density. Furthermore, measurement of pore aspect ratio and orientation demonstrated a relationship between apparent density and the degree of macrostructural anisotropy within the specimens, while, it was also noted that pore connectivity was sensitive to anisotropy. Compression testing demonstrated the effect of apparent density and macrostructural anisotropy on the mechanical properties. An increase in apparent density from 0.38 to 1.25 g cm-3 resulted in increases in ultimate compressive stress and compressive modulus of 1 to 11 MPa and 0.2 to 3.1 GPa, respectively. Furthermore, anisotropic high density (> 0.9 g cm-3) specimens were found to possess lower compressive moduli than isotropic specimens with equivalent apparent densities. These results underline the importance of full structural and mechanical characterization of porous ceramic implant materials. ©1999 Kluwer Academic Publishers

Crack growth resistance in cortical bone: Concept of microcrack toughening

The role of microcracking in cortical bone as a toughening mechanism has been investigated in conjunction with the variation in fracture toughness with crack length. Fracture toughness tests were conducted on miniaturised compact tension specimens made from human and bovine cortical bone and the resultant microstructural damage, present in the form of microcracking on the surface, was analysed around the main propagating crack. It was found that the fracture toughness (Kc) and the cumulative number of microcracks increased linearly with crack extension in human and bovine cortical bone, although both Kc and number of microcracks were considerably higher in the latter case. Based on these results, a mechanism, derived from the resistance (R) curve concept developed for microcracking brittle solids, is proposed to explain the fracture of cortical bone, with microcracking distributed between a frontal process zone and a significant process zone wake. Evidence to support this mechanism is given from the existing bone literature, detailed scanning electron microscopical observations and the distribution of microcracks in the process zone wake.

In vitro and in vivo evaluation of polyhydroxybutyrate and of polyhydroxybutyrate reinforced with hydroxyapatite

Polyhydroxybutyrate (PHB) is a polyester made by many microorganisms under conditions of nitrogen deficiency, and is produced commercially in bulk by biotechnology. It has been suggested that PHB-based materials (copolymers and composites) could be suitable for medical applications and may be biodegradable. This paper presents some findings regarding the degradation and biological properties of polyhydroxybutyrate and composites reinforced with particulate hydroxyapatite. It has been established that the strength and stiffness of these materials reduce on in-vitro environment exposure in phosphate-buffered saline at 37 degrees C for periods up to 4 months, and that the degradation rate is a function of composition and processing conditions. It has also been demonstrated that materials based on PHB produce a consistent favourable bone tissue adaptation response with no evidence of an undesirable chronic inflammatory response after implantation periods up to 12 months. Bone is rapidly formed close to the material and subsequently becomes highly organized, with up to 80% of the implant surface lying in direct apposition to new bone. The materials showed no conclusive evidence of extensive structural breakdown in vivo during the implantation period of the study.

Carbonate substitution in precipitated hydroxyapatite: an investigation into the effects of reaction temperature and bicarbonate ion concentration.

Carbonate substitution in the apatite crystal lattice can occur in either the hydroxyl or the phosphate sites, designated as A or B type, respectively, and previous investigations generally have described precipitated carbonate hydroxyapatite as being B type on the basis of infra red and X-ray data. This paper documents the effects of two precipitation variables, namely temperature and bicarbonate ion concentration, on the morphology, phase composition, and calcium, phosphorus, and carbon contents of precipitated carbonate hydroxyapatite. Variations in both temperature and bicarbonate concentration could yield either acicular or spheroidal crystals. X-ray diffraction and infra red spectroscopy indicated the presence of carbonate in the A site for low carbonate contents (< 4 wt%), and at higher carbonate contents (> 4 wt%), the carbonate was located predominantly in the B site. On the basis of these observations and chemical analyses, a new AB carbonate substitution mechanism is proposed that better describes the experimental data than the B-type models used previously.

Hydroxyapatite-polyethylene composites for bone substitution: effects of ceramic particle size and morphology

Synthetic hydroxyapatite particles of two median sizes and different morphologies have been used to manufacture hydroxyapatite reinforced high density polyethylene composites (HAPEX) for medical applications. The effects of hydroxyapatite particle size on properties of the resultant composites were investigated using various techniques. It was found that composites with smaller hydroxyapatite particles had higher torsional modulus, tensile modulus and tensile strength, but lower strain to failure. Examination of fracture surfaces revealed that only a mechanical bond existed between the filler and the matrix. It was shown that dynamic mechanical analysis is useful in studying the viscoelastic behaviour of the composite.

Chemically coupled hydroxyapatite-polyethylene composites: structure and properties.

Silanation of hydroxyapatite and acrylic acid grafting of polyethylene were employed to improve bonding between hydroxyapatite and polyethylene. The structure and properties of chemically coupled hydroxyapatite reinforced high-density polyethylene (HA/HDPE) composites were subsequently investigated using various techniques. It was shown that there existed a silicon-containing interphase between the hydroxyapatite particle and the polyethylene matrix. The silane coupling agent used promoted chemical adhesion between hydroxyapatite particles and the polymer. The use of the silane coupling agent also facilitated the penetration of polymer into cavities in individual ceramic particles, which resulted in enhanced mechanical interlocking at the matrix-reinforcement interface. With a strong bond (both mechanical and chemical) being formed between hydroxyapatite and polyethylene, chemically coupled HA/HDPE composites possessed improved mechanical properties and fracture behaviour.