Replacements for the temporomandibular joint were developed in the early 1960s. Problems with various prostheses, notably the Kent VK1, led to detailed analysis of their risks and complications. In 1999 one type of prosthesis (the Christensen) was converted from an acrylic condyle on cobalt-chromium fossa to metal-on-metal cobalt-chrome condyle and fossa. This has been popular for the last 15 years in the UK, but since it was converted to the metal-on-metal variant there have been several cases of foreign body giant-cell reactions to the prosthesis. Of the 9 cases seen (out of 106 placed), 4 were found retrospectively to be sensitive to one of the metal components of the prosthesis; the others have not been tested to date. Other potential causes of this reaction are point contact, micromovement, or a lymphocyte-mediated response to the prosthesis.