While modern artificial hips are made of a number of high-tech materials, metal is still often the material of choice for younger, more active patients. This is due mainly to the fact that it’s so robust. Unfortunately, however, difficulties can arise in the metal ball-and-socket interface – where the artificial head of the femur meets the artificial socket of the pelvis – if things aren't perfectly aligned. In particular, the metal surfaces can wear against one another, decreasing the longevity of the implant and potentially causing health problems in the patient. Now, researchers from Germany’s Fraunhofer Institute for Manufacturing Engineering and Automation are developing a new type of heavy-duty artificial hip, that contains no metal at all.
Both parts of the system take the form of thin shells, that are attached over top of the existing bone surfaces.
The socket implant (which is attached to the pelvis) is made of carbon fiber-reinforced PEEK, a biocompatible, high-strength, wear-resistant polymer composite. The femoral head is made from ceramic, and incorporates a coating of the natural bone component hydroxylapatite, where it joins the patient’s original femur. This attracts stem cells from the adjacent bone marrow, helping the bone to grow into the implant.
Along with the hydroxylapatite on the femoral head, the implants both feature a scaffold-like material containing tiny pins, on the surfaces where they join the bone. This allows them to be pressed and tapped securely into place, using a specialized disposable tool that is attached to standard surgical instruments. They can also be detached and repositioned as needed, while being implanted.
No bone cement is required – bone cement can sometimes crack and fail, causing the implant to come loose. Additionally, because it’s just a shell, the femoral head doesn’t require the traditional long attachment stem that extends down into the inside of the femur. This means that less of the patient’s own bone material needs to be removed.
Because both of the implants are reasonably flexible, more like bone and less like steel, they have been shown to transfer less stress into the surrounding bone than steel systems. Their performance has been tested using a robotic rig that simulates activities such as walking and climbing stairs, while the bone attachment scaffold has been demonstrated on cadavers.
Fraunhofer is developing the hip implants as part of an international team of organizations, that are taking part in the ENDURE (Enhanced Durability Resurfacing Endoprosthesis) project.