by Dr. Jason Hochfelder, Phelps Hospital
Close to 1 million hip replacements are performed in the United States every year, but very few patients know how the replacement parts are constructed or what materials are used. Can you provide some details about the implants?
The hip is a ball-and-socket joint. The socket (acetabulum) is part of the pelvic bones, and the ball is the end part of the femur (thigh bone). The goal of any hip replacement is to recreate the ball and socket.
There are some variations, 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 and sits on top of the femoral stem; the acetabular cup, which recreates the socket; and the acetabular liner, which sits inside the acetabular cup to provide a new, smooth joint surface.
Q: What materials are used to produce the pieces?
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 the 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 are usually 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 – in most cases for the patient’s
Q: What is a metal-on-metal hip replacement?
When a metal acetabular liner is used in combination with a metal cobalt chrome femoral head, it is called a metal-on-metal hip replacement (MoM). Initially the thought was that MoM hips would never wear out and could replace the traditional metal ball and plastic liner. However, it was discovered that the friction of metal against metal often released tiny particles that could cause aggressive reactions around the hip joint as well as problems throughout the body if the particles were absorbed into the blood stream. For these reasons metal-on-metal hips have mostly fallen out of favor.
Q: 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. Understandably, patients found this very annoying. In addition, the rubbing together of the two pieces of ceramic very rarely caused one of the pieces of ceramic to shatter, creating a devastating problem. For these reasons, the ceramic liners fell out of favor. 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 today are done with a ceramic or metal femoral head and a plastic acetabular liner.