The prosthetic implant used in hip replacement procedures is composed of three parts. These include the acetabular cup that acts as the socket; the femoral head or “ball” component that sits inside this socket; and the articular interface that lies between these components cushioning them to allow ease of movement as the structures interact with each other.
The acetabular cup
The acetabular cup is the structure that is placed inside the acetabulum or hip socket. The cup is cemmented to the bone surface after cartilage and bone are removed from the acetabulum.
The cup may be singular or modular. One-piece cups are made of either polyethylene or metal, with the polyethylene form held in place via cement and the metal form via a metal coating on the outside of the cup.
Modular cups are made up of a metal shell and a polyethylene liner. The shell has a porous coating on the outside and a locking mechanism on the inside which is designed to hold the liner. The porous coating may be made up of sintered beads or a foam metal design that mimics the actual trabeculae of cancellous bone (spongy appearing bone). As bone grows into the porous coating, the structure becomes more fixed in place. Screws are used to further secure the cup to the bone. Polyethylene liners placed inside the shell are held in place via a rim locking mechanism, while ceramic and metal ones are attached with a Morse taper.
The femoral head
This is the component that fits into the hip socket. The femur is shaped to receive the femoral stem with a prosthetic femoral head attached, which may be via acrylic bone cement or a special surface coating that stimulates the bone to bond to the implant.
The stems may be made of titanium, cobalt chromium or stainless steel and may be monolithic or modular. The femoral head may be metal or ceramic. Ceramic heads often have a lower coefficient of friction than metal heads and may therefore wear down the liner in the socket more slowly. On the other hand, ceramic implants are more brittle and prone to breakage. These can also be monolithic or modular. For modular prosthetics, the heads come in different sizes and can be attached using Morse taper.
The articular interface
The articular interface lies between the femoral head and the acetabular cup. The properties of the interface can be selected after weighing up the patient’s demand for optimal implant function and the associated risks. The size of the interface is determined by measuring either the outside diameter of the femoral head or the inside diameter of the socket.
Reviewed by Sally Robertson, BSc