Abstract
The extension of THR to younger and more demanding patients implies the need of bearings enhancing the implants survival, raising the interest on hard-on-hard bearings (metal-on-metal and ceramic-on-ceramic). The standard bearing diameters in THR range from 7/8in (22,225 mm) to 32 mm. Larger diameter bearings were used by McKee-Farrar and Ring THR a solution that was abandoned after the success of Charnley Low Friction Arthroplasty that offered the advantage of much lower torques at the implant-bone interface, due also to the state-of-the-art of the cementation in the early 1960s. Increasing the diameter of THR bearings offers several advantages in terms of increased stability of the joint, as a larger displacement is necessary to produce the joint subluxation, and for a given neck diameter the risk of impingement is reduced while the prosthesis range of motion is increased. The reduced wear of metal-on-metal and ceramic-on-ceramic bearings has led recently to revitalise the design concept of the early THR designs, and large diameter joints today are available in sizes ranging from 36 to 52 mm. Large metal-on-metal bearings are used mostly in resurfacing prostheses. This surgery, which requires a specific surgical training, is indicated for patients performing physical activity, as those patients are relatively young, and have a long life expectation. This raises-some concerns, as increased cobalt and chromium concentrations were measured in the blood and urine of patients having Metal on Metal bearings and the effect of such high metal concentration in the long time is still unknown, as it is controversial if the metal ions are originated by wear debris of by the corrosion of the implant. Recently 36 mm ceramic on ceramic bearings were introduced in THR as a solutions to overcome many of the problems of metal-on-metal joints. Ceramic-on-ceramic bearings have a 35-year clinical history in THR. The extremely low wear of these joints is well assessed , as well as the extreme biocompatibil-ity of the material and the absence of local and systemic negative reaction to ceramic debris. Besides the enhanced safety of the design of the 36 mm heads, the development of thin ceramic inserts allowed to limit the overall diameter of the cup. It is expected that the use of the new alumina matrix composites will allow to further enhance the performances of large diameter ceramic THR bearings.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
