How do Artificial Knee Implants Work?

Stephen White, General Manager and Vice President – Knee Division, Zimmer Holdings, Inc:

“Knee replacement designs have evolved a lot in the last twenty to thirty years.  We've seen an improvement in materials.  We've seen an improvement in the quality of product that we're manufacturing, but we're not done yet.  I think what we'll see, going forward in the next ten years, is really a focus on personalizing those implant designs and personalizing the way we deliver the implant into each patient.” 

Voice of:  Stephen White

Early knee replacements used a hinge, which enabled basic movement but did not closely simulate the way an actual knee moves.  Remarkably improved over several decades, today's sophisticated prostheses provide joint mobility that allows recipients to enjoy activities such as walking and swimming.  We test our product over millions of cycles to ensure the implant isn't going to wear out.  From an engineering perspective, we like to consider that a million cycles of testing will equal one year's worth of use. 

Stephen White:

“Other test that we will do will be biocompatibility tests.  We'll make sure that the particles and the prosthesis aren't going to have an adverse reaction in the body and then we will also do extensive testing on individual components and make sure those components are robust and biocompatible. 

Primary total knee replacement can be broken out into, really two categories, a fixed bearing design, in which the tibial insert is fixed to the tibial baseplate or a mobile bearing design, in which that tibial surface is free to float and rotate on that tibial baseplate." 

Voice of:  Stephen White

A surgeon attaches a tibial component with polyethylene to the metal beneath it.  A femoral component rolls along the cushioned surface.  Mobile bearing devices also use the three primary components, but they allow the polyethylene insert to rotate slightly within the metal tibial tray.

Stephen White:

“When we talk about the differences between different implant designs, one of the considerations is also the mechanism that we use to fix the implant to the bone surface.  Here we have an example of a cemented implant.  The surgeon will cement this surface to the bone using what we call bone cement.  Bone cement is an acrylic cement that is designed specifically to be used in the body and it's very biocompatible.  The surgeon will apply the cement to the undersurface of the base and along the stem component.  If your surgeon chooses to use a cementless or a cement-free device, what you should expect is a design that will sit flush to the bone surface.  Over time bone will incorporate into these pores and that will provide the long-term fixation of this implant in your knee replacement.” 

Voice of:  Stephen White

Once implanted in the body, either of these types of systems, will provide greater flexion and range on how far you can bend or flex your knee.  Today's artificial knees closely mimic the natural motion of a knee and are allowing many people to lead healthy, active lives without pain.  Eighty percent of knee replacements last twenty years, letting recipients swim, walk, bicycle and play tennis as they age. 

Stephen White:

“You will find that there are a lot of choices out there for total knee replacements and I think the thing that you have to convince yourself is:  first, that you're confident with your surgeon and their experience with the prosthesis that they choose and secondly, you want to make sure that you've communicated to the surgeon what types of activities you want to get back to doing.  Going through that process and being open and communicating with your surgeon will only provide you with a better outcome after the procedure.”