Hip Replacement Surgery: Is the Cement Strong Enough?

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LONDON -- People who have hip replacement surgery need a strong bone cement to hold the new joint in place. They also need antibiotic protection against postoperative bone infection. Research reported this week suggests that the standard way of giving the antibiotic in such cases -- mixed in with the cement -- might actually weaken the mechanical strength of the cement.

At the British Pharmaceutical Conference, researchers from Queen's University Belfast reported laboratory work in which they analysed antibiotic release from bone cement polymers. The usual antibiotic in such cases is gentamicin.

The expectation was that any gentamicin on the surface of the cement would be released, diffusing out in the presence of joint fluid. But the researchers were surprised to see that some gentamicin was also being released from inside the cement. It appeared that the antibiotic was causing cracks in the bone cement, providing more surfaces through which the drug could diffuse.

Researcher Ryan Morrow reported, "Microscopic images revealed a significant quantity of pores, voids and cracks in the surface of the cement, which were found to increase in approximate proportion to the amount of gentamicin loading. The formation of pores and cracks allows a low release of gentamicin from within their structure long after the initial burst of gentamicin from the surface of the cement."

As to possible consequences of the cracks, he added: "They could aid further cracking and subsequent mechanical failure of the bone cement if placed under stress."

The Belfast team is now working on new cement formulations to overcome this problem.

Their theory is that the cracking occurs because the antibiotic is present as small particles dispersed in the cement polymer. The new approach is to instead incorporate the antibiotic as part of the polymer structure.

Morrow reported the development of one such polymer, in which an antibiotic is attached by a chemical linkage that breaks (to release the active antibiotic) when it comes in contact with joint fluid. Being part of the polymer structure, the antibiotic should not affect the mechanical properties of the cement.

Source: The Royal Pharmaceutical Society of Great Britain

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