Metals in Artificial Joints

In my Inorganic Biomaterials class this semester we learn about different materials that have applications in medical devices and how they are used. I’ve learned about metals, ceramics, and composites and how they are processed to improve how they work in the body. We have learned a lot about metals because they are one of the most widely used materials in implants and we’ve recently been focusing on artificial hips. These artificial joints are usually made of a combination of materials with the stem constructed out of titanium or cobalt-chromium. For hip joints the round head is usually cobalt-chromium or ceramic, and the cup that it fits into is made of a polymer, although some are metal or ceramic.

Artificial Hip

The stem is made of metal in all artificial hips to make sure that it is strong enough to withstand the forces in the joint. The problem is that metals are too strong. The metal stem is stronger than the bone that it is implanted into, so it will tend to take on most of the forces that are applied to the joint. The problem is that bone is a living material and when it doesn’t feel the compression forces it normally would it begins to weaken. This can cause loosening as the implant separates from the bone. So, contrary to what you would think, we  actually want metal that is weaker so that it better matches the natural properties of bone.

Bone and MetalAnother problem is that metal and bone have very different structures. The smooth surface of metal is nothing like the porous structure inside our bones. Osteoblasts, the cells that make bone don’t attach well to the smooth surface of metal, so bone won’t grow around the implant to hold it in place. To change this people have developed different ways of added metal coatings to implants that look more like the structure of bone. The osteoblasts like this surface better than the normal smooth surface, so the bone is more likely to grow around the implant, holding it in place and making it more difficult to dislodge. The picture on the right shows bone (left) growing on Trabecular Metal (right) which has been engineered to mimic the natural structure of bone.

Basically, scientists are working to try and make the metal in artificial joints more like bone so that the body responds better to it.

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