The most important part of the implant is the bearing surface. This is the smooth, polished part of the implant that articulates at the level of the joint, allowing the low-friction motion which is required to walk normally. Over time, surgeons have tried many different combinations of bearing surfaces, each of which has certain advantages and drawbacks. The most predictable results are found using bearing surfaces that have stood the test of time, and have demonstrated decades-long survivorship in longitudinal registry studies.
The hip replacement prosthetic is a ball-and-socket articulation. The ball is made of a highly polished medical-grade ceramic, which is a very smooth and very durable material. The ceramic ball is impacted onto the trunnion of a titanium-alloy stem which has been inserted into the canal of the femur. This titanium stem is coated with a rough layer of a material called hydroxyapatite, which encourages the patient’s own bone to robustly grow onto the new stem.
On the socket side, the bearing surface is a highly cross-linked polyethylene liner, a very durable medical-grade plastic with superior wear properties. This liner is impacted into a titanium hemispherical shell, which has been inserted into the patient’s own hip socket. Like the stem, this new socket has a roughened outer surface that allows new bone to grow in, unitizing the implant to the patient’s body and providing long-lasting, durable fixation. Thus, while the implants themselves are made of titanium alloy, the final bearing surface is ceramic-on-polyethylene.