Knee and hip joint replacements are some of the most common procedures performed and they are also the focus of some of the most dynamic changes in materials, design and surgical technique. McLeod Orthopedic Specialist Michael Sutton, DO, describes how the materials and the approach to installing the replacements has changed since he began practicing:
Here is a summary of Dr. Sutton’s description:
Some of the advances have been improvements in the design, the changing of metals, and how they are fixed in the bone itself.
When I first started my practice, the hip joint replacement was a metal prosthesis (which is what we call the implant), and it was cemented into the bone. The cup part, or the hollow part where the implant fitted into, was plastic. Since that time we’ve gone from that implant to a metal implant with metal beads on it. So, instead of using cement to fasten it in the bone, it’s put into the bone without cement and the bone actually grows into the prosthesis. The plastic cup that we used to cement into the socket now has a metal back. The plastic sits inside the metal that has beads on it, allowing the bone to grow into it. In hip surgery today, many hip replacements are cemented less, which is a great advancement as typically the cement would fail first, limiting the useful life of the replacement to about 15 years. The cement immediately fixed the implant in place, but the cementless version takes about 8 weeks or so for the bone to grow in and fix the metal in place.
Knee implants were also cemented with metal and into the top part of the knee. On the bottom part of the knee, plastic was cemented into the bone itself. Now, it sits on top of a metal tray, which makes it last longer. The tray and the femur – which goes on the thighbone – can either be cemented or also have this bone-in growth potential. As we get older, bone quality declines due to aging and disease so this is generally used for younger patients. The non-cemented prosthesis does not do as well if the bone is not healthy.
The metal component, which used to be stainless steel, is now chromium cobalt or even titanium. This is another great advance in the prosthesis because some metals react less with the skin, meaning fewer complications. These are some of the changes I’ve seen just in my lifetime and the designs are constantly changing.