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    Mobile vs. Fixed Bearings in UKA: Is One Better than the Other?

    The authors investigated survivorship and complications of mobile- and fixed-bearing medial UKAs in a large multi-surgeon orthopaedic surgery group, hypothesizing that no significant differences would be found between the 2 component designs.

    Authors

    Robert F. Murphy, MD; Tyler W. Fraser, BS; and William M. Mihalko, MD, PhD

    Disclosures

    The authors have no disclosures relevant to this article.

    Editor’s Note: This article is excerpted from Mobile Versus Fixed Bearing Medial Unicompartmental Knee Arthroplasty: A Series of 375 Patients, published in the March 2015 issue of the journal Reconstructive Review.

    Introduction

    Unicompartmental knee arthroplasty (UKA) is a reliable surgical option for patients with unicompartmental arthritis of the knee. As implant design and surgical technique have improved, so have survivorship and outcomes. [1,2] Although lateral compartment [3,4] and patellofemoral compartment [5] arthroplasties have been investigated, the most common unicompartmental arthroplasty is medial.

    In medial UKA designs, the bearing surfaces are either mobile or fixed. Proponents of mobile bearings argue that these devices provide superior conformity and improved tibiofemoral biomechanics, thus leading to natural joint motion and low wear rates. [6] Advocates of fixed-bearing implants argue for technical ease in implantation, especially with regard to ligamentous balancing. [7,8]

    Several retrospective and prospective studies have been performed comparing mobile- versus fixed-bearing components in medial UKA. [7,9,10,11] However, each component group was limited to approximately 20 to 50 participants. One meta-analysis has compared both designs, with pooled data from each bearing type, with no significant difference found in clinical outcome or complication rate between mobile and fixed bearing designs. [12]

    We investigated survivorship and complications of mobile- and fixed-bearing medial UKAs in a large multi-surgeon orthopaedic surgery group. Our hypothesis was that no significant differences would be found between the 2 component designs, in that both types of bearings would have similar survivorship and rates of complications.

    Results

    From March 2003 to August 2012, the 12 surgeons at our institution performed 407 UKAs (CPT code 27446). We excluded 32 patients (4 lateral compartment UKAs, 3 patellofemoral UKAs, 25 patients with incomplete medical records), which left us with 375 medial UKAs for the analysis. Of these, 308 were mobile bearing and 67 were fixed bearing.

    Average patient age was similar in the 2 groups (62 years for the mobile-bearing group versus 59 years for the fixed-bearing group; P = 0.12). The percentage of women was higher in the fixed-bearing group (70%) than in the mobile-bearing group (58%; P = 0.06).

    Average time to follow up was 46.75 months (45.4 mobile [range 1-68], 48.1 fixed [range 1-75]; P = 0.15). Patients in the 2 groups had similar knee range of motion at last follow-up: 1-122° for mobile-bearing implants versus 1-120° for fixed-bearing implants (P = 0.34).

    Few complications occurred: 20/307 (6.5%) for mobile-bearing UKA and 5/66 (7.6%) for fixed bearing UKA (P = 0.77). Complications included:

    • Mobile-bearing UKA
      • Progression of lateral compartment disease (7)
      • Component loosening (4)
      • Bearing dislocation (3)
      • Tibial plateau fracture (2)
      • Infection (1)
      • Arthrofibrosis (1)
      • Implant subsidence without fracture (1)
      • Inflammatory synovial disease progression (lipoma arborescens, 1)
    • Fixed-bearing UKA
      • Arthrofibrosis (3)
      • Tibial plateau fracture (2)

    Implant survivorship was not statistically different: 94.8% for mobile-bearing UKA and 96.9% for fixed-bearing UKA (P = 0.44).

    Component revision occurred in 16 mobile-bearing implants. Bearing dislocation resulted in simple polyethylene exchange in 2 cases. Fourteen UKAs required conversion to total knee arthroplasty for the following reasons:

    • Progression of lateral compartment disease (4)
    • Component loosening (4)
    • Tibial plateau fracture (2)
    • Infection (1)
    • Repeat bearing dislocation (1)
    • Component subsidence (1)
    • Inflammatory synovial disease progression (lipoma arborescens, 1)

    Four patients with complications were returned to the operating room for manipulation under anesthesia (1) and arthroscopic debridement of lateral meniscal tear and loose body removal (3).

    In the fixed-bearing UKA, 2 cases returned to the operating room for conversion to total knee arthroplasty for tibial plateau fracture, and 3 underwent manipulation under anesthesia for arthrofibrosis.

    Discussion

    With advances in implant design and surgical technique, UKA has evolved as a safe and reliable intervention for patients with unicompartmental knee arthritis. [1,2] Several previous series have examined outcomes and complications associated with these implants, but their cohort numbers have been relatively low in relation to other arthroplasty literature. [7,9,10,11]

    Proponents of mobile-bearing designs argue for a more normal restoration of knee kinematics, which may theoretically translate to better long-term knee range of motion. Li et al found this to be false, as both mobile- and fixed-bearing patients undergoing kinematic analysis had similar ranges of motion. [11]

    In our groups as well, patients regained excellent range of motion, with no statistical significance between groups (1-122° mobile, 1-120° fixed; P = 0.34). This also confirms other reports that found no difference in clinical outcomes, [10] but we did not gather any validated functional scores.

    Component loosening has been proposed to be one of the leading causes of conversion to TKA [13]. In mobile-bearing implants, the motion and shear force transmission from the mobile bearing interface should theoretically lead to low rates of component loosening.

    In one of the largest comparative series, Emerson found a higher rate (16%) of loosening from tibial components in fixed bearing than in those with mobile implants (2%). [9] In our series, the rate of loosening of mobile components was similar (4/307 = 1.3%); however, none of our fixed-bearing implants showed evidence of loosening at 4 years of follow-up.

    Some authors have argued that the use of mobile-bearing implants may lead to earlier lateral compartment disease progression, [9] and we have found this to be the case in our series. Four patients with mobile-bearing devices required conversion to total knee arthroplasty, while none in the fixed-bearing group were revised for progression of lateral compartment disease.

    Tibial plateau fracture is another known complication of UKA that can occur intraoperatively or can be detected in the postoperative period. [1] We detected 4 tibial plateau fractures, with 2 in each group; all were discovered in the postoperative period.

    Several studies have reported survivorship rates for mobile- and fixed-bearing implants. In fixed-bearing implants, survivorship at 10 to 13 year has consistently been reported as 91-96%. [1,13,14,15] Overall survivorship in mobile-bearing implants has been reported at 85-98% at 10 years of follow up. [9,16] Our survival rates of mobile-bearing (94.8%) and fixed-bearing (96.9%) implants are consistent with these literature reports.

    This study has several limitations:

    • The retrospective design leads the study to incomplete data and inherent biases.
    • We did not collect any validated functional outcome measures on these patients, which may have helped to better differentiate patient satisfaction and clinical outcomes.
    • A large proportion of our data is from mobile-bearing implants, which was due to surgeon preference and outside the control of this retrospective review. A prospective, matched data set may have more precisely defined differences between the 2 implant designs.
    • The groups are not matched in that a higher percentage of fixed bearing patients were female.
    • The number of different surgeons and surgeon experience may play a confounding role in the heterogeneity of the data; however, this is representative of a multi-surgeon group and reflects modern practice.

    Conclusion

    We present the largest single series examining complications and survivorship between mobile- and fixed-bearing medial UKAs. No significant differences were found between these 2 implants. Further studies that are prospective in nature and incorporate validated functional scores may be useful in corraborating these findings.

    Author Information

    Robert F. Murphy, MD, is a pediatric orthopaedic fellow at Boston Children’s Hospital, Boston, Massachusetts. Tyler W. Fraser, BS; and William M. Mihalko, MD, PhD, are from the University of Tennessee – Campbell Clinic, Department of Orthopaedic Surgery and Biomedical Engineering, Memphis, Tennessee. Dr. Murphy was a resident at the University of Tennessee – Campbell Clinic when this article was written.

    Source

    Murphy RF, Fraser TW, Mihalko WM. Mobile Versus Fixed Bearing Medial Unicompartmental Knee Arthroplasty: A Series of 375 Patients. Reconstructive Review 2015;5(1):18-21. http://reconstructivereview.org/ojs/index.php/rr/article/view/96 Copyright 2015 All articles published are the shared property of its authors and Reconstructive Review. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives4.0 International License. JISR gives permission for reproduction of articles as long as notification and recognition is provided.

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