A method of treating or preventing infection at a surgical site comprising a bony defect and an implanted metal device is disclosed. Biodegradable microspheres are placed at the site and are capable of near-linear controlled release of an antibiotic agent for a predetermined period of time. The microspheres are configured to be large enough to avoid being phagocytosed and removed from the body, and small enough in diameter to not physically inhibit bone growth at said bony defect site. The microspheres are formed of polylactic-co-glycolic acid (PLGA), with or without polyethylene glycol (PEG), and sufficient antibiotic agent to produce bactericidal levels in body tissues. The microspheres exhibit near-linear delivery of the antibiotic agent for at least 4 weeks at levels exceeding the minimum inhibitory concentration (MIC) for organisms commonly found to be the cause of infections, and facilitate bone ingrowth or regrowth at the site.