Patients always ask “what type of joint replacement will I get?”, or they often go as far as specifying which type of implant they want. As surgeons, we’re always happy to answer these questions because we love our trade and love talking shop!
But here’s the interesting point: ask a room full of hip and knee surgeons, “What is the most important factor when considering a joint replacement?” and they will overwhelmingly respond “the surgeon.”
Maybe that’s us being a little narcissistic, but it’s also us acknowledging that the skills matter a lot more than the equipment–given all implants meet the necessary safety/performance requirements.
That being said let’s talk about implants.
Your natural hip is composed of a ball (femoral head) and a socket (acetabulum). The implants are similar. Unfortunately, we don’t have the same sublime materials to work with as nature (bone and cartilage) so we make do with what we have–metals, plastics, and ceramics, which give us very high performance hip components.
The ball can be made of an alloy of chrome and cobalt. This is not the same as stainless steel (people often ask). In my city, plenty of individuals work in manufacturing of tools and aerospace devices, so I have to be ready to answer these questions in detail.
Otherwise, the ball (head) can be made of ceramic materials. These have become very popular of late and some manufacturers have some clever ceramics. These are very hard and slippery. They are also very smooth. This is important because we often use them against plastic in the socket.
Let’s talk about the socket.
The sockets also can be made of multiple materials each having its individual advantages. Let’s consider the most common materials. These would be titanium and polyethylene.
We attached a picture to help you organize the arrangement of titanium and plastic, but suffice it to say that the titanium allows the plastic a way to attach to your bone. The titanium never touches the ball, if it did the hip would wear very quickly.
The lining of the most common socket is polyethylene plastic, which is also used in household garbage bags. But don’t ask us to stuff one of those into your body!
The polyethylene manufactured for hips has been studied and modified over and over to provide a plastic that is hard, smooth, and durable in the human body.
Have we solved every problem of our plastic hip part? No. We don’t yet know how long it will last. That’s a good thing because our polyethylene wears out so slowly that we have yet to see many replacements for that reason.
In fact, the pieces that wear out early are usually installed incorrectly or broken by some extraordinary means (trauma).
Some discussion must be devoted to metal on metal implants. I have experience with devices that have worked very well as well as some that have not. One was even recalled. Suffice it to say that hip replacements that use metal on metal are still used in limited applications (young males). Chosen correctly these can provide exceptional service and performance. Please ask if you are interested.
It’s also important to cover the femoral stem. The ball needs to be attached to the femur bone just like the polyethylene was attached to the pelvis. In the case of the ball, a stem of metal is fitted to the canal of the femur. The ball is then attached to the stem. This occurs via a taper (many people know about these also).
The stems have some type of surface roughening. These allow the bones to attach to them and fix them to the body. These surfaces are variable. Some look like piles of tiny ball bearings, others like woven wool, and others look like bone under a microscope. They all work if used properly.
So now you know a little more about the implant options available to you.
If you want to have a question answered by one of Rockford Orthopedic’s physicians, feel free to ask in the comments or visit our Patient Question Contact Form.