Mobile Bearing Knee Replacement Offers no Benefits vs. Fixed Bearing

In a previous posting 1, the question was raised as to which is better, fixed or mobile bearing . In the end, the answer was not very clear. But in the view of one implant manufacture, the choice is simple: fixed bearing.

In a recent press release, a major U.S. Orthopedic implant company (Stryker Corporation) has decided not to offer their mobile bearing knee replacement in the U.S. market. Although the company has executed a clinical trial in an effort to gain FDA approval for the device, data gathered during the study has demonstrated no clinical advantages for the mobile bearing knee compared to a comparable fixed bearing design.

The company also cited higher costs and higher revision rates for mobile bearings as further reasons why the mobile bearing design will not be pursued in the U.S. market.

The original press release can be found here:
http://www.prnewswire.com/news-releases/us-ide-study-indicates-mobile-bearing-knees-offer-no-clinical-advantages-over-fixed-bearing-knees-86866577.html

More on Stryker can be found here:
http://www.stryker.com/en-us/products/Orthopaedics/KneeReplacement/index.htm

1 Is One Really Better? Fixed vs. Mobile Bearing Knee replacement

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Knee Implant Wear

Polyethylene Wear and Osteolysis

Although there have been many advances in Total Knee Implants, they are still mechanical devices that can wear over time or if implanted improperly.

Modern have been cited to have a greater than 90% survivorship at 15 years (1) however, over the long term, challenges still remain.

The “weak link” so to speak in a Total Knee is typically the tibial insert (or tibial bearing). The insert is made of ultra-high-molecular-weight-polyethylene or UHMWPE. As your knee moves (to the tune of 1 million steps or more/year!) the surface of the insert will wear and create tiny particles of UHMWPE.

The wear rate in modern knees that are implanted well is very small. But if a knee is unbalanced and/or of poor design, accelerated wear can occur. This accelerated wear, can in turn, lead to failure of the insert (i.e. cracking) or a biologic response to the wear particles known as osteolysis. In the presence of the small particles (recognized as foreign bodies) the bone can “resorb” or waste away around the implants leading to loosening of the implants from the bone.

Knee implant design is a balance between providing freedom of movement and congruity. The less congruity between the tibial insert and femoral component, the more movement can occur (sounds good, but…). However, to distribute forces over a wide enough area to minimize wear (forces at the knee can reach up to 5 times body weight!), a close mating of the femoral component and tibial insert is desired. These competing requirements lead to a design challenge that is constantly being optimized as new implants are developed. Also recent treatment to implant materials can also help.

Surgical technique is also important. The better aligned the components are, the more resistant they will be to wear. Modern instrumentation has made knee replacement very reproducible. In addition, new computer assisted techniques may also be improving accuracy. On the other hand, new “less invasive” techniques offer reduced scarring and quicker recovery, but as you might infer, could make it more challenging to obtain a well aligned knee and hence could lead to long term problems if done incorrectly.

As younger and more active patients wish to maintain active lifestyles in the face of debilitating joint pain, the issue of implant wear will continue to be at the forefront and inspire continued development of implant design and surgical technique.

(1)”Long-term followup of nonmodular total knee replacements.” Keating EM, Meding JB, Faris PM, Ritter MA. Clin Orthop. 2002 Nov; (404):34-9.

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