A spacer for a knee replacement prosthesis. The spacer may have a lower surface with a locking component adapted to interlock with a tibial tray, an upper surface optionally with a tibial post and a pair of smooth-surfaced and slightly concave condyle support platforms disposed on opposite sides of the tibial post, and a substantially hollow body with an internal reservoir that can be filled with antibiotic material. The reservoir may be accessible from a port on the hollow body that leads into the internal reservoir, and which permits refilling of the reservoir. In some embodiments, the reservoir may be refillable from this port without removal of the spacer from the patient; for example, a percutaneous needle may be inserted into the port. The spacer may dispense antibiotic from this reservoir to the surrounding area in order to treat an infection.