You have made a big decision. You decided to have your hip or knee replaced. And you think- what in the world is this thing made of?
That is a great question and one I will answer today. The materials used in hip and knee replacements are similar, yet different. This has to do with the forces acting on the implant, and the type of stress that differs across the hip and knee joint.
Hip replacements are made of 4 different parts: -Acetabular Cup (Shell) -Polyethylene liner (Plastic liner inside the shell) -Head ball -Femoral stem
Acetabular Cup (Titanium) This is composed of titanium. Most are usually 3-D printed. There is a specific porous structure these cups have to let the host cells of your bone grow into this cup. This allows for permanent long lasting fixation.
Liner (plastic) The liner is composed of a medical grade plastic called polyethylene. This liner has been designed to have very low wear rates, like a truck tire designed to last 20 years.
Femoral Head (ceramic) The femoral head ball is a ceramic. A 4th generation material composed of alumina and zirconia. This has been one of the most significant advances in hip replacement surgery over the last 15 years. It has allowed for your hip replacement to last 20-25 years without needing to be revised.
Femoral stem (Titanium) The femoral stem is a titanium allow. The stem has a special coating on the exterior that allows for friction and stability while your body's cells grow into the stem. Some femoral stems also have a special material coating the stem which allow for faster fixation to the body.
A knee replacement has 3 main components. The surgery must resurface the end of the femur (thigh bone), tibia (shin bone), and there must be a bearing surface placed between the two. The materials are slightly different in composition because of the increased surface area of the knee compared to the hip and the shear forces across a knee replacement.
Femoral Component (Cobalt Chromium) The femoral component is composed of a cobalt chromium alloy. Think the shinny chrome bumper on a car. This component has traces of nickel and other ions in it.
Liner (plastic) The liner is composed of a medical grade plastic called polyethylene. The liner must be designed to be tough against breaking but also not wear out with time.
Tibial baseplate (titanium) The Tibial baseplate is composed of a titanium allow.