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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 knee replacement. 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

Press-Fit Total Knee Replacement: Working to Prevent Loosening and Bone Loss

In total knee replacement, the worn ends of the bones that make up the joint are replaced with metal and plastic implants. These implants are secured to the bone either with “bone cement” which the surgeon applies to the implant and bony surfaces or they are simply “press-fit” into place. In the press-fit scenario, the surface of the implant which contacts the bone is typically coated with a porous structure. This structure is designed to mimic the porous or sponge-like structure of the prepared bone in order to promote bony in-growth into the implant coating.

In addition to utilizing a bone-like porous structure for press-fit implant coatings, it is desirable that the “flexibility” or “elasticity” of the implant material also approach that of bone. This is because bone, which is a dynamic living tissue, actually responds and adjusts to the stresses placed upon it. If an implant is too stiff relative to the bone, then over time, the bone will become less and less dense, literally disappearing under the implant due to what the body perceives as lack of use (the old “use it or loose it” adage). This is known as “stress-shielding.” The resulting bone loss may eventually result in loosening of the implant.

It is possible that such bone loss and resulting implant failure can be avoided by using materials and coatings which better approximate the properties of real bone. This is because as the implant material properties approach that of normal bone, loads are better transferred through the implant to the bone, thus allowing the bone to perceive more normal use and encourage the continued maintenance of bone under the implant.

In a recent article in the Journal of Bone and Joint Surgery (American) Minoda et. al1. looked at how bone in the tibia (shin bone) responded to a particular type of press-fit implant. The implant utilized a tantalum porous coating. The property of the bone that was measured to gage how it responded to the implant was bone mineral density (BMD). In the study, a group of implanted knees (28) receiving the press-fit implant was followed and compared over a period of two years, two a group of knees (28) with a cemented implant. BMD scans taken were taken at various intervals.

The researchers found that BMD decreased in the tibia in both groups. However, in the press-fit (porous tantalum) group, the decrease was only 6.7% +/- 22.9% vs. 36.8% +/- 24.2% in the cemented group. So it appears that this particular porous coated implant may have been able to allow more natural loading of the tibia, thereby reducing bone loss compared to the cemented implant. Only time will tell if these apparent benefits continue long-term.

References:
1. Yukihide M, Kobayashi A, Iwaki H, Ikebuchi M,I nori F, Takaoka K. Comparison of Bone Mineral Density Between Porous Tantalum and Cemented Tibial Total Knee Arthroplasty Components. Journal of Bone and Joint Surgery (American). 2010;92:700-706

Knee Replacement Implants

The Parts of a Total Knee Replacement

Knee Implant Wear

Knee Implant Manufacturers

Knee Replacement FAQ

Q. What, exactly is “replaced” in knee replacement?

A. The worn ends of the femur ( thigh bone), tibia (shin) and patella (knee cap) are removed with a saw and replaced with metal and plastic. More on Knee Replacement Surgery

Q. What, is a Knee Replacement Implant made of?

A. Implants are typically made of a combination of Cobalt Chrome and/or Titanium metals and UHMWPE (ultra- high-molecular-weight polyethylene) plastic. More on Knee Implants.

Q. Are there different kinds of implants?

A. Yes! There are many types of implants that fit into broad categories related to their interaction with the remaining natural soft tissue. For example a CR or Cruciate Retaining knee implant is designed to work with the PCL (posterior cruciate ligament) intact. Other types of implants are designed to work without it. In addition, there are many manufacturers of implants which create several brands of knees in each of the broadcategories. Surgeons have many, many choices. More on Types of Knee Implants. Links to Implant Manufacturers

Q. How long will my Knee Replacement last?

A. There are many studies on this topic and the actual time an implant will survive before needing revision will vary on a case by case basis. Several studies demonstrate greater than 90% survivorship at 15 years (1)

Q. How are the implants held in place?

A. Total knee replacement implants are secured to the bone either with a special polymeric compound (“bone cement”) or are “press-fit” into place. Press fit implants have rough or porous under surfaces into which bone will grow. More on Knee Implants.

Q. What is a Uni (or Partial or Half-knee) Replacement?

A. A “Uni” or unicompartmental knee replacement replaces only the medial (inner) or lateral (outer) half of the knee joint (the tibio-femoral joint). The knee cap or patella is not replaced. More on Unicompartmental Knee Replacement

Q. What is MIS or Minimally Invasive Knee Replacement?

A. MIS or Minimally Invasive Knee Replacement is like standard knee replacement except that the incision is smaller, and care is taken not to resect the quadriceps muscle (also known as “quad sparing”). Also, the patella is not “everted” or “flipped” over This is all intended to result in less trauma and speed recovery. More on Minimally Invasive Knee Replacement.

Q. What about recovery, pain etc?

A. Every case is unique and this discussion is best left for a medical professional. You can read about the experience of other recipients at a very interesting discussion forum at Robin’s Total Knee Replacement.

References 1) Dixon MC, Brown RR, Parsch D, Scott RD. Modular fixed-bearing  total knee arthroplasty with retention of the posterior cruciate  ligament. A study of patients followed for a minimum of fifteen years. J Bone Joint Surg Am. 2005 Mar;87(3):598-603

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Normal Knee Anatomy

• The knee joint is the mating and movement of three bones; the femur or thigh bone, the tibia or shin bone and the patella or knee-cap. The end of the femur rides on the top surface of the tibia and the patella moves within a groove on the femur.
• The bones are joined together by ligaments and tendons. These soft-tissue structures guide the movement of the bones and provide the stability needed for normal knee motion. Unlike the ball and socket hip joint, the bones are not interconnected; it is the soft-tissue around the bones which holds the joint in place. This is why one of the key factors in a successful total knee replacement is surgical management and later rehabilitation of the remaining soft-tissue (i.e. muscles, ligaments and tendons).
• In a normal knee, the tendons attach the muscles to the bones and the ligaments attach the bones to one another. During the knee replacement operation, one or more of the ligaments is removed. Often the normal function of one or more of the ligaments is severely compromised due to the deterioration in the knee joint. One of the main jobs of the knee implants is to recreate the normal function of these removed ligaments.

Between the femur and tibia is a soft-tissue pad known as the meniscus. This pad acts as a “shock absorber” helping to evenly distribute the loads or forces between the bones of the knee. The meniscus also provides additional stability during motion. The link below provides a more in-depth review of basic knee anatomy: http://www.kneepaininfo.com/kneeanatomy.html

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