Effect of porosity reduction by compaction on compressive strength and microstructure of calcium phosphate cement

Abstract

Hydroxyapatite (HA) calcium phosphate cements (CPCs) are attractive materials for orthopedic applications because they can be molded into shape during implantation. However their low strength and brittle nature limits their potential applications to principally non-load-bearing applications. Little if any use has been made of the HA cement systems as manufacturing routes for preset HA bone grafts, which although not moldable pastes, are resorbable, unlike HA sintered ceramic. It is known that the strength of cements can be increased beyond that attainable from slurry systems by compaction, and this study investigates whether compaction significantly alters the specific surface area and pore-size distribution of CPC prepared according to the method of Brown and Chow. Compaction pressures of between 18 and 106 MPa were used to decrease the porosity from 50 to 31%, which resulted in an increase in the wet compressive strength from 4 to 37 MPa. The Weibull modulus was found to increase as porosity decreased; in addition the amount of porosity larger than the reactant particle size increased as porosity decreased. It is proposed that this was caused by a combination of voids created by the aqueous solvent used in fabrication and shrinkage that occurs on reaction. The specific surface area was unchanged by compaction.

An evaluation of methods to determine the porosity of calcium phosphate cements

“…For basic CPCs it is common to use the tabulated density of the material [i.e., 3.16 g/cm 3 for hydroxyapatite (HA)] as the skeletal density in Eq. (1).…”

“…The CPCs that have been tested with this method have so far mostly been basic CPCs. 3,11,12 Solvent resaturation is a method that is commonly used for other types of porous materials. 5,7 In this method, dry samples are immersed in a solvent, normally water or alcohol and the weight change is monitored until a constant weight is achieved.…”

“…The skeletal density was calculated using the phase compositions resulting from the XRD measurements. Densities that were used in the calculations were calcium pyrophosphate 3.09 g/cm 3 , TCP 3.14 g/ cm 3 , brushite 2.33 g/cm 3 , and monetite 2.93 g/cm 3 . The porosity was calculated using the calculated skeletal density and the apparent density according to Eq.…”

“…However, their main limitation in comparison to the more traditionally used acrylic cements are their mechanical properties, leading to numerous studies focusing on improving the mechanical resistance of CPCs. Since the mechanical properties are strongly linked to the porosity (and the pore size distribution) of ceramic materials, 3,4 it is of high interest to have reliable and fast methods to determine the porosity of such materials.…”

“…For CPCs, mainly three methods are utilized: helium pycnometry, [8][9][10] mercury intrusion porosimetry (MIP), 3,11,12 and using the tabulated density value of the material 11,[13][14][15] to calculate the porosity. The main drawbacks of these methods are the destructiveness, the long analysis times and the need for special equipment, especially for MIP and helium pycnometry, which may entail that fewer samples are analyzed, compromising statistical power.…”