[Use of hydroxyapatite for determination of antibiotic concentrations in bone]

A new method to determine antibiotic bone concentrations is presented. The bone fragments, obtained during hip joint replacement operations in 10 patients were investigated for antibiotic activity after i.v. application of 2 g of oxacillin and 4 g of mezlocillin. We were able to show that both penicillins do not penetrate into the inorganic bone part (hydroxylapatite). Therefore the concentrations measured are related to the organic bone part and are expressed in units per volume rather than in units per weight. With this method the differences between spongious and cancellous bone observed so far are diminished to values without any statistical difference. After 2 g of oxacillin and 4 g of mezlocillin the concentrations are 13.1 mg/l and 34.0 mg/l, respectively. The serum concentrations measured are compared to the bone concentrations. The terminal serum half-life is 63 min for oxacillin and 80 min for mezlocillin when the drugs are given simultaneously.