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Influence of surface porosity and pH on bacterial adherence to hydroxyapatite and biphasic calcium phosphate bioceramics

¹ Department of Clinical Microbiology, Fundación Jiménez Díaz-UTE, Avda de Reyes Católicos 2, E-28040 Madrid, Spain

² Department of Orthopaedic Surgery and Traumatology, Fundación Jiménez Díaz-UTE, Avda de Reyes Católicos 2, E-28040 Madrid, Spain

³ Department of Otolaryngology, Helsinki University Central Hospital, PO Box 220, FI-00029 HUS, Finland

⁴ Department of Medicine, Helsinki University Central Hospital, PO Box 700, FI-00029 HUS, Finland

⁵ Instituto de Ciencias de los Materiales, Universidad de Valencia, PO Box 22085, E-46071 Valencia, Spain

⁶ Department of Inorganic and Bioinorganic Chemistry, Faculty of Pharmacy, Complutense University of Madrid, Plaza Ramón y Cajal s/n, E-28040 Madrid, Spain

Correspondence
Teemu J. Kinnari
teemu.j.kinnari{at}helsinki.fi

Received April 22, 2008
Accepted September 19, 2008

Hydroxyapatite (HA) and biphasic calcium phosphate (BCP) ceramic materials are widely employed as bone substitutes due to their porous and osteoconductive structure. Their porosity and the lowering of surrounding pH as a result of surgical trauma may, however, predispose these materials to bacterial infections. For this reason, the influence of porosity and pH on the adherence of common Gram-positive bacteria to the surfaces of these materials requires investigation. Mercury intrusion porosimetry measurements revealed that the pore size distribution of both bioceramics had, on a logarithmic scale, a sinusoidal frequency distribution ranging from 50 to 300 nm, with a mean pore diameter of 200 nm. Moreover, total porosity was 20 % for HA and 50 % for BCP. Adherence of Staphylococcus aureus and Staphylococcus epidermidis was studied at a physiological pH of 7.4 and at a pH simulating bone infection of 6.8. Moreover, the effect of pH on the potential of HA, BCP and of both staphylococci was evaluated. Results showed that when pH decreased from 7.4 to 6.8, the adherence of both staphylococci to HA and BCP surfaces decreased significantly, although at the same time the negative -potential values of the ceramic surfaces and both bacteria diminished. At both pH values, the number of S. aureus adhered to the HA surface appeared to be lower than that for BCP. A decrease in pH to 6.8 reduced the adherence of both bacterial species (mean 57 %). This study provides evidence that HA and BCP ceramics do not have pores sufficiently large to allow the internalization of staphylococci. Their anti-adherent properties seemed to improve when pH value decreased, suggesting that HA and BCP bioceramics are not compromised upon orthopaedic use.

Abbreviations: BCP, biphasic calcium phosphate; CDA, calcium-deficient apatite; HA, hydroxyapatite; β-TCP, β-tricalcium phosphate.