What is a hip replacement made of?
What are the parts of a hip replacement?
Despite close to 1 million hip replacements being performed in the United States every year, very few patients know the parts and materials that compose a hip replacement.
The hip is a ball and socket joint, and the goal of any hip replacement is to recreate the ball and socket. The socket (acetabulum) is part of the pelvic bones and the ball is the end part of the femur (thigh bone).
There are variations to every rule, but the standard hip replacement is made up of four different pieces: the femoral stem which sits inside the femur bone, the femoral head which recreates the ball of the ball and socket and sits on top of the femoral stem, the acetabular cup, which recreates the socket, and the acetabular liner that sits inside the acetabular cup to provide a new smooth joint surface.
What materials are the pieces made of?
Both the femoral stem and the acetabular cup are usually made from titanium and covered in a roughened surface that allows bone to grow into it, so that it eventually becomes incorporated into a patient’s body. Titanium is inert, which means that the body does not reject it. The femoral head has traditionally been made from a metal cobalt-chromium alloy, but more recently surgeons have been using ceramic heads to try to make hip replacements last longer. The liners have are made from a plastic called polyethylene, but metal and ceramic liners have both been tried and are occasionally still used. Older versions of this plastic had been known to wear out after 10-15 years, but the newer versions have been shown to last significantly longer, and in most cases last for the patient’s entire life.
What is a metal-on-metal hip replacement?
When a metal acetabular liner is used in combination with a metal cobalt chrome femoral head, this is called a metal-on-metal hip replacement (MoM). Initially the thought was that these MoM hips would never wear out and could replace the traditional metal liner and plastic ball joint, but it was discovered that these hips often released tiny metal particles that could cause aggressive reactions around the hip joint as well as problems throughout the body if these particles were absorbed into the blood stream. For these reasons metal on metal hips have mostly fallen out of favor.
What is a squeaking hip?
When a ceramic liner was used with a ceramic femoral head (ceramic-on-ceramic), it occasionally produced a very loud squeak, which patients found very annoying and, thus the ceramic liners also fell out of favor. When the two pieces of ceramic rubbed together very rarely one of the pieces of ceramic would shatter which would be a devastating problem. Both metal-on-metal and ceramic-on-ceramic designs tried to solve the problems caused by the older model plastic liners wearing out. Since the newer models of plastic liners have been shown to be so effective and last longer, the vast majority of hip replacements done today are with a ceramic or metal femoral head and a plastic acetabular liner.