A hip replacement surgery involves replacing the damaged hip joint with an artificial implant. There are several components to a hip replacement that work together to create a stable artificial hip joint.
The femoral stem
The femoral stem is inserted into the hollow center of the femur (thigh bone). It acts like the “ball” part of the ball-and-socket hip joint. The femoral stem is usually made of metal and fits snugly into the femur, either with or without bone cement. The stem may have a rough, porous surface that allows the bone to grow into it over time for a stable fit.
The femoral head
The femoral head is a ball-shaped component that sits atop the femoral stem. It acts as the “ball” that fits into the socket part of the artificial hip. The femoral head is usually made of a smooth, durable plastic or ceramic material that glides easily against the socket component.
The acetabular shell
The acetabular shell replaces the socket portion of the hip joint. It is a hemispherical “cup” that is inserted into the hollow acetabulum space of the pelvic bone. Like the femoral stem, it may have a textured surface that allows bone to grow into it over time. The shell is usually made of durable metal.
The liner
Inside the acetabular shell is a rounded liner. This acts as the “socket” for the femoral head to move within. The liner is made of plastic, ceramic or a highly cross-linked polyethylene material to provide a smooth, low-friction surface against the femoral head.
Bone cement
In some hip replacements, bone cement is used to hold the implant components in place. Polymethylmethacrylate (PMMA) cement acts like an adhesive when it hardens. The surgeon spreads this around the femoral stem and inside the acetabular shell before inserting the components. As it hardens, it creates a rigid structure that locks the implants in place.
Press-fit components
Many modern hip replacements use press-fit components that don’t require cement. The femoral stem and acetabular shell have a porous, grit-blasted surface that allows the bone to grow into them over several weeks. This creates a biological fixation and stability similar to how cement fixation works.
Screws
Screws are sometimes used to provide extra stability, especially when press-fit components are used. Small screws may secure the metal acetabular shell to the pelvic bone. Larger screws can help anchor the femoral stem in place within the hollow femur.
Conclusion
In summary, a hip replacement is held securely in place through a combination of press-fit components, bone ingrowth surfaces, cement adhesion and supplemental screws. These work together to keep the artificial joint stable as you use your hip. With proper surgical technique and using high-quality materials, a hip replacement can last for many years before any loosening occurs.
