Estimation of the optimum loading of an antibiotic powder in an acrylic bone cement: gentamicin sulfate in SmartSet HV.

BACKGROUND

In some countries, commercially available antibiotic powder-loaded acrylic bone cement is routinely used in joint replacement, while, in others, "off-label" formulations are used in selected procedures (where the antibiotic powder is blended manually with the powder of a plain cement in the operating room/theater by either the surgeon or approved personnel). In the latter situation, an arbitrary rather than a rational approach is used for deciding on the amount of the antibiotic that is blended with the cement powder (herein referred to as "the antibiotic powder loading").

RESULTS

The first objective of this study was to present two methods for estimating the optimum loading of gentamicin sulfate powder that may be blended manually with the powder of a commercially available acrylic bone cement, ABC (Wopt). The second objective was to define the challenges associated with each of these methods. The loading (W) was optimized with respect to two key properties of the cured cement that were obtained simultaneously, namely (1) fatigue life in phosphate-buffered saline solution (PBS) at 37 degrees C, and (2) the rate of elution of the gentamicin from the cement (E) in that medium. Three sets of specimens were used, containing 2.25, 4.25 and 11.50 wt/wt% of the gentamicin that was blended manually with the cement powder. The fatigue tests involved determining the number of cycles at which a specimen fractured (Nf) when subjected to fully-reversed tension-compression sinusoidal load, +/- 15 MPa at 2 Hz. E was determined from the concentration of gentamicin in the PBS solution when the specimen fractured, using fluorescence polarization immunoassay for the measurement. Consistent with a priori expectations, it was found that, with increase in W, Nf decreased while E increased. Two approaches for obtaining an estimate of Wopt are described, a mathematical method and an empirical one, that lead to Wopt values of 6.50 wt/wt% and 4.78 wt/wt%, respectively. Thus, this antibiotic loading ranges from about the same to about 92% greater than that in SmartSet GHV Gentamicin, which is a commercially available bone cement used clinically and which has the same composition as SmartSet HV except for the presence of gentamicin sulfate in its powder (blended in by the manufacturer).

CONCLUSIONS

We present and critically compare two rational methods of determining the optimum loading of an antibiotic powder in an acrylic bone cement, which should serve as a guide when using "off-label" antibiotic powder-loaded acrylic bone cements in cemented joint replacements.