Mobile Bearing Knee Replacement

A particular type of you may hear about is known as the “Rotating Platform” or “Mobile Bearing” knee. In this version the tibial insert (see the Implant post) is designed to move in between the Femoral component and the tibial component.

The femoral and tibial components are fixed to the thigh and shin bones respectively. This allows the insert to remain well mated against the femoral component while still allowing the necessary rotational movement of the tibia (shin) relative to the femur (thigh). The possible benefits are:

  • Increased wear resistance since the femur and tibial insert can be made to “match” more closely. Increasing the contact between the components reduces the pressure on the insert making it potentially last longer. In fixed bearing knees, the rotational movement of the femur relative to the tibia has to occur on top of the insert, so it can’t be made to match as closely.
  • The potential for more natural motion since it allows a greater degree of rotation about the knee compared to fixed designs.

Keep in mind though that since the insert can move it can also dislocate. In addition, the high conformity between the insert and femur of this implant can sometimes limit flexion of the knee compared to other designs. Basically, increased rotation at the knee (like during a golf swing) can come at the expense of increased flexion.

Note that traditional fixed bearing knees also allow rotation to occur, but in a different way. Rotation occurs on the plastic bearing surface between the femoral component and the tibial insert.

Several studies have shown that the long-term results of both implants are comparable.

The study below found similar results in patients who received one of each type of replacement (one in each leg of course!)

Chiu KY, Ng TP, Tang WM, Lam P. “Bilateral total knee arthroplasty: One mobile-bearing and one fixed-bearing”J Orthop Surg (Hong Kong). 2001 Jun;9(1):45-50

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Is One Really Better? Fixed vs. Mobile Bearing Knee Replacement

In our recent post “What’s the ‘Best’ Total Knee Replacement?” it was suggested that more often than not, it “may simply be the one which your experienced surgeon has chosen to utilize”. This sentiment was echoed in a recent publication. 1

In the January 2007 Journal of Orthopedic Surgery and Research article, the authors reviewed two major types of total knee replacements: fixed bearing and mobile bearing designs. The theoretical advantages of the mobile bearing design are discussed and the review is broken down into two main sections, a “Biomechanical Review” and a “Clinical Review” where the results of the two types of designs are compared. (See our section on Mobile Bearing Knee Replacement Here, for a brief description of this type of implant).

In the “Biomechanical Review”, laboratory testing of relative wear rates of the plastic components in each type of design is discussed. Also, studies of knee motion comparing mobile and fixed bearings implanted in both cadaver experiments and in actual patients are summarized. Here, the mobile bearing design seems to have less wear and more natural motion.

In the “Clinical Review”, the results of some earlier clinical studies are examined. In these studies, increased wear rate and subsequent osteolysis (see our section on Implant Wear) in the mobile bearing design was observed. At first, this seems to contradict the previously discussed lab studies on wear (where the mobile bearing was superior). However, as the article goes on to explain the increased wear and osteolysis can be attributed to the presence of the additional wear surface on the underside of the mobile bearing as well as to size of the wear particles. Mobile bearings tend to produce smaller particles which actually result in a greater biologic response and osteolysis. Also, the possibility of tibial insert dislocation or “spinout” in mobile bearings is discussed and some observed cases of insert dislocation are shown.

So at this point in the article, the advantage of one type of design over the other is not very clear. The “Clinical Review” continues on with a review of long term clinical results. Unfortunately, that section concludes with the statement: “So far, the theoretical advantages for mobile bearing design to provide long-term durability have not been demonstrated by any outcome study.” Long term performance simply does not help in differentiating the two designs, with both performing similarly. At the end of the day, the study concludes, (as suggested in the previous post) “For the experienced surgeon, one familiar surgical protocol and instrumentation is suggested rather than implant design, either in fixed bearing or mobile bearing.”

1. Huang CH, Liau JJ, Cheng CK: Fixed or Mobile-bearing Total Knee Arthroplasty. J of Ortho Surg and Res 2007, 2:1

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