The role of macrophages and giant cells in loosening of joint replacement.

Macrophages and foreign-body giant cells from the bone-cement interface of loosened joint replacement were assayed for direct bone resorption in an in vitro experimental model. Human osteoclasts from giant cell tumors and experimental animal osteoclasts were not capable of resorbing the bone under experimental conditions, but osteoclasts are. It is suggested that macrophages and giant cells in loosened implants may initiate bone resorption, removing the extra-cellular barrier that normally protects mineral crystals from osteoclastic recognition.