Special Focus Section
Fixed Bearings versus Rotating Platforms in Total Knee Arthroplasty Peter B. White, BA1
Amar S. Ranawat, MD1
Chitranjan S. Ranawat, MD1
1 Department of Othopedics, Hospital for Special Surgery, New York,
New York
Address for correspondence Peter B. White, BA, Department of Orthopaedics, Hospital for Special Surgery, 535 E 70th st, 6th floor, New York, NY 10021 (e-mail: [email protected]).
J Knee Surg 2015;28:358–362.
Abstract
Keywords
► ► ► ►
fixed bearing rotating platform total knee TKA
Rotating platforms were introduced as an alternative to fixed bearings in hopes of providing more natural kinematics and improved clinical outcomes. In-vitro wear studies showed significantly less wear with rotating platforms as opposed to fixedbearing designs. Kinematic follow-up studies showed significantly improved axial rotation with rotating-platform knees. However, these benefits have failed to translate into improved clinical outcomes or survivorship. This article reviews our institution’s experience, as well as the latest clinical research on wear, kinematics, clinical outcomes, and survivorship of fixed-bearing and rotating-platform knees.
The goal of total knee arthroplasty (TKA) is to reduce pain and restore function in patients with advanced arthritis.1–3 As restoration of function and motion is highly dependent on joint kinematics, it is widely believed that optimal kinematics after TKA should best approximate that of a native, nonarthritic knee. While many implant designs are available, most can be classified as either fixed or mobile bearing. Implants are considered fixed bearing when the polyethylene is locked onto the tibial tray (►Fig. 1) and mobile bearing when the polyethylene freely articulates on the tibial tray. The principle behind mobile bearings is to create a rotational articulating surface, thereby uncoupling the translational and rotational forces about the knee. Two types of mobile bearings exist: meniscal bearings that translate in a multitude of directions and rotating platforms that rotate based on a centralized pivot (►Fig. 2). This article outlines our institution’s experience, as well as the latest clinical research on (1) wear, (2) kinematics, (3) clinical outcomes, and (4) survivorship of fixed bearings versus rotating platforms in TKA. Furthermore, we will highlight our institution’s indications for using rotating platform in TKA.
Wear Durability and high survivorship rates are important factors to determine the success of TKA.4 Currently, aseptic loosening remains the primary mechanical cause of revision TKA and is
received February 12, 2015 accepted February 26, 2015 published online May 5, 2015
highly dependent on polyethylene wear.5 One of the proposed benefits of rotating platforms is decreased polyethylene wear as a result of better force distribution. The popularity of rotating platforms stems largely from the results of early in-vitro wear studies. McEwen et al6 performed a laboratory study comparing two different rotatingplatform systems with a fixed-bearing system under both “intermediate” and “high kinetic conditions.” They noted that both rotating-platform systems resulted in significantly less wear compared with a similar fixed-bearing design. In a similar study, Fisher et al7 confirmed less wear with rotating-platform designs. They proposed that the reduced wear was a result of less cross-shear forces during flexion, allowing anteroposterior (AP) translation to more readily align with the ultra-high-molecular-weight molecules. They also noted evidence of backside wear in rotating-platform knees, similar to what is seen with fixed-bearing knees. This proposed benefit led to the popularity of rotating platforms. Two recent in-vitro studies,8,9 however, have reported similar wear rates in fixed-bearing and rotating-platform designs. Grupp et al8 attributed their similar wear rate to their kinematic approach and additional wear seen on the tibia. Several retrieval analyses of failed TKA have noted similar wear destruction on both fixed bearing and rotating platforms.10–13 Others12,13 who have measured wear visually have generally agreed that rotating platforms cause more backside wear, evidenced by burnishing, scratching, and
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0035-1550338. ISSN 1538-8506.
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third-body abrasions. Conversely, Berry et al14 measured wear through thickness measurements and found overall wear rates to be lower in rotating platforms. Overall, there is considerable in-vitro support for the use of rotating platforms in the literature,6,7 which has led to their popularity.
Fig. 1 Fixed-bearing total knee replacement.
Fig. 2 Noncemented, cruciate-retaining, rotating-platform primary knee prosthesis.
Rotating platforms were originally developed to provide a dual-surface articulation that uncouples axial and translational forces of the knee. The belief was that uncoupling these forces could reduce the stress that led to high rates of loosening seen in early fixed-bearing designs. The etiology of loosening in these designs was believed to stem from high rotational torque forces seen at the implant–bone interface. Therefore, it was proposed that uncoupling the rotational and translational forces could provide more normal kinematics as well as improve survivorship. In general, native knee kinematics is a complex mechanism, especially during flexion. Native deep flexion involves 10 to 15 degrees of tibial internal rotation,15 little translation of the medial femoral condyle, and 15 to 25 mm of posterior translation on the lateral side. Both fixed-bearings and rotating-platform designs often deviate from native knee kinematics. Therefore, many believe that restoring native knee kinematics could provide improved outcomes in TKA. Studies have analyzed flexion mechanisms of fixed bearing and rotating platforms using both fluoroscopy and computer modeling.16,17 Femoral rollback mechanisms are similar irrespective of design,16,17 but can depend on the state of the posterior cruciate ligament. With respect to the sagittal plane, both bearing types have different AP translation kinematics than the native knee.16 It has been reported that the medial condyle translates posteriorly more than the native knee, while the lateral condyle translates less. There have also been reports of anterior translation and instability with both fixed bearing and rotating platforms.18,19 Both bearing types have similar sagittal kinematics. Our institution performed a fluoroscopic study comparing the axial rotation of fixed-bearings and rotating-platforms knees in-vivo.20 Not surprisingly, we found that rotating platforms provide more axial rotation (7 degrees) than fixed bearings (4 degrees). Others have confirmed this result with computer modeling (8 vs. 3 degrees).16 While rotating platforms provide better axial rotation, it is still less than the 10 to 15 degree range found in the native knee. Better axial rotation was shown to have diverse kinematic benefits. Many21,22 have reported rotating platforms to reduce stresses at the implant–bone interfaces. Others23 have found that better rotation reduces the risk of cam-post levering and eccentric post wear. Coupled with reduced cross-shear forces due to improved axial alignment, many believe that improved axial rotation provides a forgiveness of rotational malalignment.24 Finally, some report improved patellar tracking and reduced patellofemoral stresses.25 Overall, these proposed benefits improve the margin for surgeon error and could also improve clinical outcomes. The Journal of Knee Surgery
Vol. 28
No. 5/2015
Downloaded by: Karolinska Institutet. Copyrighted material.
Kinematic Principles
Fixed Bearings versus Rotating Platforms in TKA
White et al.
Unlike fixed-bearing knees, reports demonstrate that the axial rotation exhibited by rotating-platform knees may change over time.26,27 Some have reported an increase in axial rotation,26 whereas others have reported a decrease in axial rotation.27 A decrease in axial rotation may ultimately represent a decline in function over time. Some have also reported that axial rotation can be absent or even reverse over time.26,28 Ultimately, this may indicate the erratic nature of axial rotation with rotating-platform designs. While both fixed bearings and rotating platforms fail to exactly reproduce native knee kinematics, it is evident that rotating platforms produce more normal axial rotation than fixed bearings.
Clinical Outcomes Rotating platforms gained popularity largely because of their theoretical advantages over fixed bearings. Many believed that improved axial rotation could lead to reduced pain and improved function. While some reports show less anterior knee pain with rotating platforms,29,30 most studies have not found an improvement in pain or function.3,31–39 Similarly, our institution has performed several studies,1,40–42 all of which have failed to find any advantage of rotating platforms. Although, most metaanalyses31,43,44 confirm that there is no difference in pain, motion, or function; one recent meta-analysis45 did report a lesser incidence of pain with rotating platforms. Studies that report less pain generally have only short-term follow-up.19,30,45 In fact, after reporting less anterior knee pain with rotating platforms at minimum 1-year follow-up (4.3 vs. 18.9%),29 Breugem et al34 reported fixed-bearing knees to have similar anterior knee pain (13 vs. 17%) at 8-year follow-up. If this is the case, this could be further evidence that rotating platforms may decline in function over time.26,27 More long-term randomized control trials are necessary to further evaluate this. Other reports have revealed that rotating platforms may be associated with an increased incidence of crepitation.1,46 Our institution performed a large matched-pair study and found higher incidences of painless and painful crepitation, albeit not significant.1 In a separate study, we found 1.5% of fixed-bearing knees to have mild, painful crepitation, whereas 6% of rotating-platform knees demonstrated anterior knee pain and 12% had painless crepitation.41 Others have reported a similar incidence of painless crepitation, up to 16.7%, with rotating-platform designs.42,46–48 While some have attributed these high rates to trochlear groove and intercondylar shape,26 most studies report crepitation to be around 1% with fixed-bearing designs.41,46 Overall, most studies indicate that there are no clinically relevant advantages of rotating platforms with respect to outcomes. There have been some reports of less pain with rotating platforms, but there have also been reports of more crepitation.
Osteolysis and Survivorship Cemented, fixed-bearing TKAs are well known to have excellent in-vivo survivorship rates, up to 90% at 15-year The Journal of Knee Surgery
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follow-up.2,49,50 Most studies indicate no difference in survivorship or incidence of osteolysis between bearing types.35,43,49,50 Several meta-analyses31,43,44,50 have also agreed that there is no difference between fixed bearings and rotating platforms. Survivorship rates often vary in the literature depending on end points; however, most indicate close to 96% survivorship with both bearing types.42,49,50 Our institution reported an excellent survivorship of 10 years with a rotating-platform design.42 Kaplan–Meier analysis revealed 100% survivorship with mechanical failure as an end point, 97.7% as revision for any reason as an end point, and 95% with any reoperation as an end point. We found no instance of osteolysis. Kim et al50 looked at a large number of younger patients ( 65 years old) and found similar survivorship rates, but also found an incidence of osteolysis of 1.6 and 2.2% for fixed bearings and rotating platforms, respectively. Most randomized control trials lack significant follow-up to make a definitive conclusion regarding survivorship, but it is evident that there is no midterm difference in survivorship or radiographic lucencies. Both bearing types show excellent survivorship up to 96% at 10 to 15 years. Longer studies are ultimately needed to reveal the benefit of one system over another.
Current Trends and Indications More recently, the use of rotating platforms in TKA has decreased considerably. Nguyen et al51 reported that the use of rotating platforms has dropped significantly from 2005 to 2012 (19 to 7%), whereas the use of fixed bearings is on the rise (81 up to 91%). This decrease is likely due to the reports of bearing dislocations,31 similar clinical outcomes,31,45 and similar survivorship.31,43,44,50 In spite of the similarities between fixed bearings and rotating platforms, we tend to use rotating-platform designs in younger, more active patients, in hopes of superior wear rates. Furthermore, we believe rotating platforms can be especially helpful in cemented and noncemented revision TKA where more constraint is needed. We feel that rotating platforms can help decrease strain forces at the bone–implant interface. This can be especially helpful with hinges and higher levels of constraint. Ultimately, we hope rotating platform can increase the survivorship of revision TKA.
Conclusion Overall, both kinematic and in-vitro wear studies have highlighted certain benefits of rotating-platforms over fixed-bearing knees in primary TKA. However, the improved axial rotation has failed to show any benefits with respect to clinical outcomes or survivorship. Most survivorship studies lack the follow-up necessary to show a difference with respect to survivorship. While rotating platforms have yet to show any clinical advantages, we use them regularly during primary TKA in young and active patients, and both cemented and noncemented revision TKA.
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20 Ranawat CS, Komistek RD, Rodriguez JA, Dennis DA, Anderle M. In
1 Ranawat AS, Meftah M, Rodriguez-Elizalde S, et al. The incidence of
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anterior knee pain and crepitation after total knee replacement: a matched pair analysis between rotating platform and fixed bearing posterior stabilized designs. ASTM Int 2012;STP 1531:46–54 Feng B, Weng X, Lin J, Jin J, Wang W, Qiu G. Long-term follow-up of cemented fixed-bearing total knee arthroplasty in a Chinese population: a survival analysis of more than 10 years. J Arthroplasty 2013;28(10):1701–1706 Vasdev A, Kumar S, Chadha G, Mandal SP. Fixed- versus mobilebearing total knee arthroplasty in Indian patients. J Orthop Surg (Hong Kong) 2009;17(2):179–182 Rand JA, Trousdale RT, Ilstrup DM, Harmsen WS. Factors affecting the durability of primary total knee prostheses. J Bone Joint Surg Am 2003;85-A(2):259–265 Sharkey PF, Hozack WJ, Rothman RH, Shastri S, Jacoby SM. Insall Award paper. Why are total knee arthroplasties failing today? Clin Orthop Relat Res 2002;(404):7–13 McEwen HM, Barnett PI, Bell CJ, et al. The influence of design, materials and kinematics on the in vitro wear of total knee replacements. J Biomech 2005;38(2):357–365 Fisher J, McEwen H, Tipper J, et al. Wear-simulation analysis of rotating-platform mobile-bearing knees. Orthopedics 2006;29(9, Suppl):S36–S41 Grupp TM, Kaddick C, Schwiesau J, Maas A, Stulberg SD. Fixed and mobile bearing total knee arthroplasty—influence on wear generation, corresponding wear areas, knee kinematics and particle composition. Clin Biomech (Bristol, Avon) 2009;24(2):210–217 Haider H, Garvin K. Rotating platform versus fixed-bearing total knees: an in vitro study of wear. Clin Orthop Relat Res 2008; 466(11):2677–2685 Kelly NH, Fu RH, Wright TM, Padgett DE. Wear damage in mobilebearing TKA is as severe as that in fixed-bearing TKA. Clin Orthop Relat Res 2011;469(1):123–130 Schwarzkopf R, Scott RD, Carlson EM, Currier JH. Does increased topside conformity in modular total knee arthroplasty lead to increased backside wear? Clin Orthop Relat Res 2015;473(1): 220–225 Lu YC, Huang CH, Chang TK, Ho FY, Cheng CK, Huang CH. Wearpattern analysis in retrieved tibial inserts of mobile-bearing and fixed-bearing total knee prostheses. J Bone Joint Surg Br 2010; 92(4):500–507 Garcia RM, Kraay MJ, Messerschmitt PJ, Goldberg VM, Rimnac CM. Analysis of retrieved ultra-high-molecular-weight polyethylene tibial components from rotating-platform total knee arthroplasty. J Arthroplasty 2009;24(1):131–138 Berry DJ, Currier JH, Mayor MB, Collier JP. Knee wear measured in retrievals: a polished tray reduces insert wear. Clin Orthop Relat Res 2012;470(7):1860–1868 Most E, Li G, Schule S, et al. The kinematics of fixed- and mobilebearing total knee arthroplasty. Clin Orthop Relat Res 2003;(416): 197–207 Delport HP, Banks SA, De Schepper J, Bellemans J. A kinematic comparison of fixed- and mobile-bearing knee replacements. J Bone Joint Surg Br 2006;88(8):1016–1021 Shi X, Shen B, Yang J, Kang P, Zhou Z, Pei F. In vivo kinematics comparison of fixed- and mobile-bearing total knee arthroplasty during deep knee bending motion. Knee Surg Sports Traumatol Arthrosc 2014;22(7):1612–1618 Daniilidis K, Höll S, Gosheger G, et al. Femoro-tibial kinematics after TKA in fixed- and mobile-bearing knees in the sagittal plane. Knee Surg Sports Traumatol Arthrosc 2013;21(10):2392–2397 Tibesku CO, Daniilidis K, Vieth V, Skwara A, Heindel W, FuchsWinkelmann S. Sagittal plane kinematics of fixed- and mobilebearing total knee replacements. Knee Surg Sports Traumatol Arthrosc 2011;19(9):1488–1495
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vivo kinematics for fixed and mobile-bearing posterior stabilized knee prostheses. Clin Orthop Relat Res 2004;(418):184–190 Bottlang M, Erne OK, Lacatusu E, Sommers MB, Kessler O. A mobile-bearing knee prosthesis can reduce strain at the proximal tibia. Clin Orthop Relat Res 2006;447:105–111 Malinzak RA, Small SR, Rogge RD, et al. The effect of rotating platform TKA on strain distribution and torque transmission on the proximal tibia. J Arthroplasty 2014;29(3):541–547 Zingde SM, Leszko F, Sharma A, Mahfouz MR, Komistek RD, Dennis DA. In vivo determination of cam-post engagement in fixed and mobile-bearing TKA. Clin Orthop Relat Res 2014;472(1):254–262 Okamoto N, Nakamura E, Nishioka H, Karasugi T, Okada T, Mizuta H. In vivo kinematic comparison between mobile-bearing and fixed-bearing total knee arthroplasty during step-up activity. J Arthroplasty 2014;29(12):2393–2396 Sawaguchi N, Majima T, Ishigaki T, Mori N, Terashima T, Minami A. Mobile-bearing total knee arthroplasty improves patellar tracking and patellofemoral contact stress: in vivo measurements in the same patients. J Arthroplasty 2010;25(6):920–925 Russo A, Montagna L, Bragonzoni L, Visani A, Marcacci M. Changes in knee motion over the first 3 years with a mobile-bearing prosthesis. Knee 2006;13(4):301–306 Wolterbeek N, Garling EH, Mertens B, Valstar ER, Nelissen RG. Mobile bearing knee kinematics change over time. A fluoroscopic study in rheumatoid arthritis patients. Clin Biomech (Bristol, Avon) 2009;24(5):441–445 Garling EH, Kaptein BL, Nelissen RG, Valstar ER. Limited rotation of the mobile-bearing in a rotating platform total knee prosthesis. J Biomech 2007;40(Suppl 1):S25–S30 Breugem SJ, Sierevelt IN, Schafroth MU, Blankevoort L, Schaap GR, van Dijk CN. Less anterior knee pain with a mobile-bearing prosthesis compared with a fixed-bearing prosthesis. Clin Orthop Relat Res 2008;466(8):1959–1965 Kim YH, Yoon SH, Kim JS. Early outcome of TKA with a medial pivot fixed-bearing prosthesis is worse than with a PFC mobile-bearing prosthesis. Clin Orthop Relat Res 2009;467(2):493–503 Smith H, Jan M, Mahomed NN, Davey JR, Gandhi R. Meta-analysis and systematic review of clinical outcomes comparing mobile bearing and fixed bearing total knee arthroplasty. J Arthroplasty 2011;26(8):1205–1213 Bailey O, Ferguson K, Crawfurd E, et al. No clinical difference between fixed- and mobile-bearing cruciate-retaining total knee arthroplasty: a prospective randomized study (e-pub ahead of print). Knee Surg Sports Traumatol Arthrosc 2014; doi: 10.1007/ s00167-014-2877-9 Shemshaki H, Dehghani M, Eshaghi MA, Esfahani MF. Fixed versus mobile weight-bearing prosthesis in total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2012;20(12):2519–2527 Breugem SJ, van Ooij B, Haverkamp D, Sierevelt IN, van Dijk CN. No difference in anterior knee pain between a fixed and a mobile posterior stabilized total knee arthroplasty after 7.9 years. Knee Surg Sports Traumatol Arthrosc 2014;22(3):509–516 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;470(1):33–44 Lampe F, Sufi-Siavach A, Bohlen KE, Hille E, Dries SP. One year after navigated total knee replacement, no clinically relevant difference found between fixed bearing and mobile bearing knee replacement in a double-blind randomized controlled trial. Open Orthop J 2011;5:201–208. doi:10.2174/1874325001105010201. [Epub 2011 May 27] Hanusch B, Lou TN, Warriner G, Hui A, Gregg P. Functional outcome of PFC Sigma fixed and rotating-platform total knee arthroplasty. A prospective randomised controlled trial. Int Orthop 2010;34(3): 349–354
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References
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Fixed Bearings versus Rotating Platforms in TKA
White et al.
38 Rjonild M, Soballe K, Hansen PM, et al. Mobile- vs fixed-bearing
45 Li YL, Wu Q, Ning GZ, et al. No difference in clinical outcome
total knee replacement: a randomized radiosterometric and bone mineral density study. Acta Orthop 2015;86(2):208–14 Aglietti P, Baldini A, Buzzi R, Lup D, De Luca L. Comparison of mobile-bearing and fixed-bearing total knee arthroplasty: a prospective randomized study. J Arthroplasty 2005;20(2): 145–153 Ranawat AS, Rossi R, Loretti I, et al. Comparison of PFC sigma fixedbearing and rotating-platform total knee arthroplasty in the same patient. J Arthroplasty 2004;19(1):39 Ranawat AS, Ranawat CS, Slamin JE, Dennis DA. Patellar crepitation in the P.F.C. sigma total knee system. Orthopedics 2006;29(9, Suppl):S68–S70 Meftah M, Ranawat AS, Ranawat CS. Ten-year follow-up of a rotating-platform, posterior-stabilized total knee arthroplasty. J Bone Joint Surg Am 2012;94(5):426–432 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;472(7):2185–2193 Cheng M, Chen D, Guo Y, Zhu C, Zhang X. Comparison of fixed- and mobile-bearing total knee arthroplasty with a mean five-year follow-up: a meta-analysis. Exp Ther Med 2013;6(1):45–51
between fixed- and mobile-bearing TKA: a meta-analysis. Knee Surg Sports Traumatol Arthrosc 2014;22(3):565–575 Choi WC, Ryu KJ, Lee S, Seong SC, Lee MC. Painful patellar clunk or crepitation of contemporary knee prostheses. Clin Orthop Relat Res 2013;471(5):1512–1522 Hamilton WG, Sritulanondha S, Engh CA Jr. Prospective randomized comparison of high-flex and standard rotating platform total knee arthroplasty. J Arthroplasty 2011;26(6, Suppl):28–34 Fukunaga K, Kobayashi A, Minoda Y, Iwaki H, Hashimoto Y, Takaoka K. The incidence of the patellar clunk syndrome in a recently designed mobile-bearing posteriorly stabilised total knee replacement. J Bone Joint Surg Br 2009;91(4):463–468 Carothers JT, Kim RH, Dennis DA, Southworth C. Mobile-bearing total knee arthroplasty: a meta-analysis. J Arthroplasty 2011; 26(4):537–542 Kim YH, Choi Y, Kim JS. Osteolysis in well-functioning fixed- and mobile-bearing TKAs in younger patients. Clin Orthop Relat Res 2010;468(11):3084–3093 Nguyen LC, Lehil MS, Bozic KJ. Trends in total knee arthroplasty implant utilization (e-pub ahead of print). J Arthroplasty 2014; doi: 10.1016/j.arth.2014.12.009
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The Journal of Knee Surgery
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No. 5/2015
Fixed Bearings versus Rotating Platforms in Total Knee Arthroplasty Peter B. White, BA1
Amar S. Ranawat, MD1
Chitranjan S. Ranawat, MD1
1 Department of Othopedics, Hospital for Special Surgery, New York,
New York
Address for correspondence Peter B. White, BA, Department of Orthopaedics, Hospital for Special Surgery, 535 E 70th st, 6th floor, New York, NY 10021 (e-mail: [email protected]).
J Knee Surg 2015;28:358–362.
Abstract
Keywords
► ► ► ►
fixed bearing rotating platform total knee TKA
Rotating platforms were introduced as an alternative to fixed bearings in hopes of providing more natural kinematics and improved clinical outcomes. In-vitro wear studies showed significantly less wear with rotating platforms as opposed to fixedbearing designs. Kinematic follow-up studies showed significantly improved axial rotation with rotating-platform knees. However, these benefits have failed to translate into improved clinical outcomes or survivorship. This article reviews our institution’s experience, as well as the latest clinical research on wear, kinematics, clinical outcomes, and survivorship of fixed-bearing and rotating-platform knees.
The goal of total knee arthroplasty (TKA) is to reduce pain and restore function in patients with advanced arthritis.1–3 As restoration of function and motion is highly dependent on joint kinematics, it is widely believed that optimal kinematics after TKA should best approximate that of a native, nonarthritic knee. While many implant designs are available, most can be classified as either fixed or mobile bearing. Implants are considered fixed bearing when the polyethylene is locked onto the tibial tray (►Fig. 1) and mobile bearing when the polyethylene freely articulates on the tibial tray. The principle behind mobile bearings is to create a rotational articulating surface, thereby uncoupling the translational and rotational forces about the knee. Two types of mobile bearings exist: meniscal bearings that translate in a multitude of directions and rotating platforms that rotate based on a centralized pivot (►Fig. 2). This article outlines our institution’s experience, as well as the latest clinical research on (1) wear, (2) kinematics, (3) clinical outcomes, and (4) survivorship of fixed bearings versus rotating platforms in TKA. Furthermore, we will highlight our institution’s indications for using rotating platform in TKA.
Wear Durability and high survivorship rates are important factors to determine the success of TKA.4 Currently, aseptic loosening remains the primary mechanical cause of revision TKA and is
received February 12, 2015 accepted February 26, 2015 published online May 5, 2015
highly dependent on polyethylene wear.5 One of the proposed benefits of rotating platforms is decreased polyethylene wear as a result of better force distribution. The popularity of rotating platforms stems largely from the results of early in-vitro wear studies. McEwen et al6 performed a laboratory study comparing two different rotatingplatform systems with a fixed-bearing system under both “intermediate” and “high kinetic conditions.” They noted that both rotating-platform systems resulted in significantly less wear compared with a similar fixed-bearing design. In a similar study, Fisher et al7 confirmed less wear with rotating-platform designs. They proposed that the reduced wear was a result of less cross-shear forces during flexion, allowing anteroposterior (AP) translation to more readily align with the ultra-high-molecular-weight molecules. They also noted evidence of backside wear in rotating-platform knees, similar to what is seen with fixed-bearing knees. This proposed benefit led to the popularity of rotating platforms. Two recent in-vitro studies,8,9 however, have reported similar wear rates in fixed-bearing and rotating-platform designs. Grupp et al8 attributed their similar wear rate to their kinematic approach and additional wear seen on the tibia. Several retrieval analyses of failed TKA have noted similar wear destruction on both fixed bearing and rotating platforms.10–13 Others12,13 who have measured wear visually have generally agreed that rotating platforms cause more backside wear, evidenced by burnishing, scratching, and
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0035-1550338. ISSN 1538-8506.
Downloaded by: Karolinska Institutet. Copyrighted material.
358
Fixed Bearings versus Rotating Platforms in TKA
White et al.
359
third-body abrasions. Conversely, Berry et al14 measured wear through thickness measurements and found overall wear rates to be lower in rotating platforms. Overall, there is considerable in-vitro support for the use of rotating platforms in the literature,6,7 which has led to their popularity.
Fig. 1 Fixed-bearing total knee replacement.
Fig. 2 Noncemented, cruciate-retaining, rotating-platform primary knee prosthesis.
Rotating platforms were originally developed to provide a dual-surface articulation that uncouples axial and translational forces of the knee. The belief was that uncoupling these forces could reduce the stress that led to high rates of loosening seen in early fixed-bearing designs. The etiology of loosening in these designs was believed to stem from high rotational torque forces seen at the implant–bone interface. Therefore, it was proposed that uncoupling the rotational and translational forces could provide more normal kinematics as well as improve survivorship. In general, native knee kinematics is a complex mechanism, especially during flexion. Native deep flexion involves 10 to 15 degrees of tibial internal rotation,15 little translation of the medial femoral condyle, and 15 to 25 mm of posterior translation on the lateral side. Both fixed-bearings and rotating-platform designs often deviate from native knee kinematics. Therefore, many believe that restoring native knee kinematics could provide improved outcomes in TKA. Studies have analyzed flexion mechanisms of fixed bearing and rotating platforms using both fluoroscopy and computer modeling.16,17 Femoral rollback mechanisms are similar irrespective of design,16,17 but can depend on the state of the posterior cruciate ligament. With respect to the sagittal plane, both bearing types have different AP translation kinematics than the native knee.16 It has been reported that the medial condyle translates posteriorly more than the native knee, while the lateral condyle translates less. There have also been reports of anterior translation and instability with both fixed bearing and rotating platforms.18,19 Both bearing types have similar sagittal kinematics. Our institution performed a fluoroscopic study comparing the axial rotation of fixed-bearings and rotating-platforms knees in-vivo.20 Not surprisingly, we found that rotating platforms provide more axial rotation (7 degrees) than fixed bearings (4 degrees). Others have confirmed this result with computer modeling (8 vs. 3 degrees).16 While rotating platforms provide better axial rotation, it is still less than the 10 to 15 degree range found in the native knee. Better axial rotation was shown to have diverse kinematic benefits. Many21,22 have reported rotating platforms to reduce stresses at the implant–bone interfaces. Others23 have found that better rotation reduces the risk of cam-post levering and eccentric post wear. Coupled with reduced cross-shear forces due to improved axial alignment, many believe that improved axial rotation provides a forgiveness of rotational malalignment.24 Finally, some report improved patellar tracking and reduced patellofemoral stresses.25 Overall, these proposed benefits improve the margin for surgeon error and could also improve clinical outcomes. The Journal of Knee Surgery
Vol. 28
No. 5/2015
Downloaded by: Karolinska Institutet. Copyrighted material.
Kinematic Principles
Fixed Bearings versus Rotating Platforms in TKA
White et al.
Unlike fixed-bearing knees, reports demonstrate that the axial rotation exhibited by rotating-platform knees may change over time.26,27 Some have reported an increase in axial rotation,26 whereas others have reported a decrease in axial rotation.27 A decrease in axial rotation may ultimately represent a decline in function over time. Some have also reported that axial rotation can be absent or even reverse over time.26,28 Ultimately, this may indicate the erratic nature of axial rotation with rotating-platform designs. While both fixed bearings and rotating platforms fail to exactly reproduce native knee kinematics, it is evident that rotating platforms produce more normal axial rotation than fixed bearings.
Clinical Outcomes Rotating platforms gained popularity largely because of their theoretical advantages over fixed bearings. Many believed that improved axial rotation could lead to reduced pain and improved function. While some reports show less anterior knee pain with rotating platforms,29,30 most studies have not found an improvement in pain or function.3,31–39 Similarly, our institution has performed several studies,1,40–42 all of which have failed to find any advantage of rotating platforms. Although, most metaanalyses31,43,44 confirm that there is no difference in pain, motion, or function; one recent meta-analysis45 did report a lesser incidence of pain with rotating platforms. Studies that report less pain generally have only short-term follow-up.19,30,45 In fact, after reporting less anterior knee pain with rotating platforms at minimum 1-year follow-up (4.3 vs. 18.9%),29 Breugem et al34 reported fixed-bearing knees to have similar anterior knee pain (13 vs. 17%) at 8-year follow-up. If this is the case, this could be further evidence that rotating platforms may decline in function over time.26,27 More long-term randomized control trials are necessary to further evaluate this. Other reports have revealed that rotating platforms may be associated with an increased incidence of crepitation.1,46 Our institution performed a large matched-pair study and found higher incidences of painless and painful crepitation, albeit not significant.1 In a separate study, we found 1.5% of fixed-bearing knees to have mild, painful crepitation, whereas 6% of rotating-platform knees demonstrated anterior knee pain and 12% had painless crepitation.41 Others have reported a similar incidence of painless crepitation, up to 16.7%, with rotating-platform designs.42,46–48 While some have attributed these high rates to trochlear groove and intercondylar shape,26 most studies report crepitation to be around 1% with fixed-bearing designs.41,46 Overall, most studies indicate that there are no clinically relevant advantages of rotating platforms with respect to outcomes. There have been some reports of less pain with rotating platforms, but there have also been reports of more crepitation.
Osteolysis and Survivorship Cemented, fixed-bearing TKAs are well known to have excellent in-vivo survivorship rates, up to 90% at 15-year The Journal of Knee Surgery
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follow-up.2,49,50 Most studies indicate no difference in survivorship or incidence of osteolysis between bearing types.35,43,49,50 Several meta-analyses31,43,44,50 have also agreed that there is no difference between fixed bearings and rotating platforms. Survivorship rates often vary in the literature depending on end points; however, most indicate close to 96% survivorship with both bearing types.42,49,50 Our institution reported an excellent survivorship of 10 years with a rotating-platform design.42 Kaplan–Meier analysis revealed 100% survivorship with mechanical failure as an end point, 97.7% as revision for any reason as an end point, and 95% with any reoperation as an end point. We found no instance of osteolysis. Kim et al50 looked at a large number of younger patients ( 65 years old) and found similar survivorship rates, but also found an incidence of osteolysis of 1.6 and 2.2% for fixed bearings and rotating platforms, respectively. Most randomized control trials lack significant follow-up to make a definitive conclusion regarding survivorship, but it is evident that there is no midterm difference in survivorship or radiographic lucencies. Both bearing types show excellent survivorship up to 96% at 10 to 15 years. Longer studies are ultimately needed to reveal the benefit of one system over another.
Current Trends and Indications More recently, the use of rotating platforms in TKA has decreased considerably. Nguyen et al51 reported that the use of rotating platforms has dropped significantly from 2005 to 2012 (19 to 7%), whereas the use of fixed bearings is on the rise (81 up to 91%). This decrease is likely due to the reports of bearing dislocations,31 similar clinical outcomes,31,45 and similar survivorship.31,43,44,50 In spite of the similarities between fixed bearings and rotating platforms, we tend to use rotating-platform designs in younger, more active patients, in hopes of superior wear rates. Furthermore, we believe rotating platforms can be especially helpful in cemented and noncemented revision TKA where more constraint is needed. We feel that rotating platforms can help decrease strain forces at the bone–implant interface. This can be especially helpful with hinges and higher levels of constraint. Ultimately, we hope rotating platform can increase the survivorship of revision TKA.
Conclusion Overall, both kinematic and in-vitro wear studies have highlighted certain benefits of rotating-platforms over fixed-bearing knees in primary TKA. However, the improved axial rotation has failed to show any benefits with respect to clinical outcomes or survivorship. Most survivorship studies lack the follow-up necessary to show a difference with respect to survivorship. While rotating platforms have yet to show any clinical advantages, we use them regularly during primary TKA in young and active patients, and both cemented and noncemented revision TKA.
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