Biologically mediated resorption of brushite cement in vitro
Introduction
In vivo studies investigating the biological reaction to and degradation of brushite cements have reported complete or extensive resorption [1], in addition to fragmentation [2] or long-term stability of the cement [3]. Crystallographic and spectroscopic analyses of retrieved brushite cement implants have shown that a marked reduction in the rate of resorption occurs following the formation of hydroxyapatite in the cement, the presence of which is thought to be caused by hydrolysis of the brushite [4]. A previous in vitro ageing study by our group has shown that the rate of mass loss from brushite cement and the formation of hydroxyapatite are affected strongly by ageing medium refreshment rate, the volume of liquid to which the cement is exposed and the presence of protein in the ageing media (which inhibited hydrolysis) [5]. As potential resorption is the major advantage that brushite-based calcium phosphate cements have over hydroxyapatite cements, attempts have been made to prevent brushite hydrolysis in the cement. One approach was the addition of magnesium ions to the cement mix [6]. Magnesium is a potent inhibitor of hydroxyapatite crystallisation since it adsorbs to the surface of newly forming hydroxyapatite crystal nuclei and blocks active growth sites [7]. In an in vitro study, a magnesium salt (8.5 wt% MgHPO4·3H2O) was shown to prevent hydroxyapatite formation when a brushite cement was aged in Hank's solution [8]. Following the implantation of a similar cement formulation containing 5 wt% MgHPO4·3H2O in the distal and proximal humerus of Swiss Alpine sheep, however, hydroxyapatite formation was noted after 4 months [9].
Another apatite crystallisation inhibitor [10], the pyrophosphate (P2O74−) ion has previously been added to brushite cements at very low concentrations as a means of retarding the setting rate of brushite cements. Our group has recently reported the formation of new cements consisting of brushite and calcium pyrophosphate by the addition of a pyrophosphoric acid solution, (rather than orthophosphoric (PO43−) acid solution) to β-tricalcium phosphate [11]. In the present study the influence of powder to liquid ratio on the setting times and compressive strength of a β-TCP–pyrophosphoric acid cement was determined. The cement formulation which exhibited the best handling properties was selected and aged in vitro for periods of up to 90 days. The influence of ageing the cement in PBS and serum on compressive strength, porosity and the phase composition of the cement were determined.
Section snippets
Methods and materials
The β-tricalcium phosphate (β-TCP)–pyrophosphoric acid solution cement was produced from the combination of β-TCP (Plasma-Biotal, Derbyshire, UK) with 1 mL of a liquid component consisting of 540 mg pyrophosphoric acid (Rhodia, West-Midlands, UK) and 720 mg double distilled water. To determine the range of powder to liquid ratios to which the cement could be mixed, the mass of β-TCP added to the liquid phase was varied between 1.25 and 2.75 g at 250 mg increments (powder to liquid ratios of 1.25–2.75
Results
Initial experimentation revealed that workable cement pastes could be mixed at powder to liquid ratios in the range of 1.25–2.75 g/mL. Increasing the powder to liquid mixing ratios of the cement paste to >1.25 g/mL reduced both the initial and final setting times exhibited by the cement formed with pyrophosphoric acid solution (Fig. 1). When mixed to a powder to liquid ratio of 1.25 g/mL the initial setting time of the cement formed with pyrophosphoric acid solution was 21±1 min and the final
Discussion
Cement strength may be influenced by a number of factors including relative porosity, critical flaw size, crystal morphology, degree of conversion and homogeneity of the cement matrix [15], [16]. Of these factors, varying powder to liquid ratio is the most likely to have had the largest affect on degree of conversion and relative porosity. Since the relationship between porosity and strength is inverse logarithmic, a small reduction in relative porosity might be expected to result in a large
Conclusion
We have demonstrated that by using pyrophosphoric acid in a cement paste, it is possible to form a matrix consisting of crystalline and amorphous components. The pyrophosphate ions may have been associated with the amorphous components or have been absorbed to the surface of the brushite crystals. The presence of the amorphous phase and/or the pyrophosphate ions prevented the formation of hydroxyapatite in the cement matrix during in vitro ageing, as has previously been shown to occur in purely
Acknowledgments
The authors acknowledge the financial support of the EPSRC (LMG) and the provision of a CASE studentship by Smith and Nephew Group Research Centre, York, UK. MJT is grateful to the Royal Society for the award of a University Research Fellowship.
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