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    The Great Debate: Mobile vs. Fixed Bearings in TKA

    At ICJR’s Pan Pacific Orthopaedic Congress, Dr. Rajesh Maniar and Dr. Thomas Schmalzried made the case for their preferred bearings for total knee arthroplasty. Below are the abstracts from their presentations.

    Mobile Is Better
    Rajesh N. Maniar, MS Orth, MCh Orth

    Mobile-bearing knee implants, introduced 4 decades ago, continue to be used globally – with very good reason, which is important to understand.

    Fixed-bearing knees may be widely used, but their bane is polyethylene wear issues that can cause osteolysis and failure. The orthopaedic literature provides ample evidence of this wear at all component interfaces, namely:

    • Articular surface wear
    • Undersurface wear
    • Post wear

    The literature now also contains multiple and varied studies analyzing the kinematics between component interfaces and their influence in ultimately wearing out the implant.

    In addition, such studies have clarified the role of mobile bearing technology in limiting wear at all interfaces.

    Fixed-bearing knees have periodically undergone modifications to improve the wear issue, but at their best, the wear rate remains twice that of mobile-bearing knees, and the improvements have probably served only to delay the wear process by few years.

    It is thought that the difference in wear rate between fixed- and mobile-bearing knees will become most evident in the second decade after implantation. A recent meta-analysis compared the low contact stress (LCS) rotating-platform knee against all other knees in the Swedish joint replacement registry, and it showed that survival is far better with the LCS knee. [1]

    Other studies have shown that for a similar design of fixed- versus mobile-bearing knee, the occurrence of synovitis and osteolysis is significantly less with mobile-bearing knee.

    Experience has revealed 2 issues with the LCS knee:

    • Associated risk of spin-out of the rotating bearing
    • Relatively limited range of movement when compared with fixed-bearing designs

    This led to the evolution of the posterior-stabilized rotating-platform (PSRP) design. The PSRP knee has been in use since year 2000, and several studies have shown much greater range of movement and negligible rate of spin-off.

    Our own published study describes our experience with this design at 5 to 8 years after implantation. [2] The same cohort of patients is now prepared for reporting at 10 to 13 years, and we will show 100% survivorship and no osteolysis at this time point.

    Author Information

    Rajesh N. Maniar, MS Orth, MCh Orth, is from Lilavati Hospital and Research Centre, Mumbai, India

    References

    1. Hopley CD, Crossett LS, Chen AF. Long-term clinical outcomes and survivorship after total knee arthroplasty using a rotating platform knee prosthesis: a meta-analysis. J Arthroplasty 2013 Jan;28(1):68-77.e1-3. doi: 10.1016/j.arth.2012.04.026. Epub 2012 Sep 21.
    2. Maniar RN RN, Singhi T, Gangaraju B, Patil A, Maniar PR. Mid term results of LCS knee: The Indian experience. Indian J Orthop 2013 Jan;47(1):57-62. doi: 10.4103/0019-5413.106903.

     

    Fixed Is Better
    Thomas P. Schmalzried, MD

    Mobile-bearing total knee arthroplasty (TKA) designs can offer increased tibio-femoral conformity (low contact stress) with low rotational constraint. Such designs gained popularity when the survival of fixed-bearing knees was challenged by high wear rates and delamination of machined, modular, polyethylene inserts. This was due to sub-surface oxidation following gamma-in-air sterilization and shelf aging.

    With subsequent improvements in polyethylene manufacturing and implant design, the advantage of mobile-bearing TKAs has not been realized. Fixed-bearing knees have equivalent function and better survivorship into the second decade.

    In a randomized, prospective, Food and Drug Administration (FDA) investigational device exemption (IDE) study, there were no differences in mean clinical assessment scores or mean score changes from baseline at any postoperative interval through 2 postoperative years.

    Nineteen of the 252 mobile-bearing and 13 of the 255 fixed-bearing knees had undergone revision of any component.

    Estimated survival at 6 postoperative years was 90.1% (95% confidence interval [CI], 84.1–93.9) for mobile-bearing and 94.2% (95% CI, 90.1–96.6) for fixed-bearing knees. There was one case of insert dislocation of a mobile-bearing knee.

    In a meta-analysis of 1,910 TKAs, there were no significant differences in clinical performance (clinical scores, range of motion, and radiographic evaluation), component alignment, revision rates, or adverse event rates.

    Utilization of mobile-bearing knees has decreased in the US, and now more than 90% are fixed-bearing knees, including all-polyethylene designs.

    Author Information

    Thomas P. Schmalzried, MD, is from the Joint Replacement Center at St. Vincent Medical Center, Los Angeles, California.

    References

    1. Ball ST, Sanchez HB, Mahoney OM, Schmalzried TP. Fixed versus rotationg platform total knee arthroplasty: a prospective, randomized, single-blind study. J Arthroplasty. 2011 Jun;26(4):531-6.
    2. Mahoney OM, Kinsey TL, D’Errico TJ, Shen J. The John Insall Award: no functional advantage of a mobile bearing posterior stabilized TKA. Clin Orthop Relat Res. 2012 Jan;470(1):33-44.
    3. Moskal JT, Capps SG. Rotating-platform TKA no different from fixed-bearing TKA regarding survivorship or performance: a meta-analysis. Clin Orthop Relat Res. 2014 Jul;472(7):2185-93.
    4. Nguyen LC, Lehil MS, Bozic KJ. Trends in Total Knee Arthroplasty Implant Utilization in the United States. J Arthroplasty. 2015 Jul;30(7):739-742.
    5. AOA National Joint Replacement Registry 2014 Annual Report.