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The mobile-bearing knee: growing use worldwide

Participants consider the characteristics of mobile-bearing designs, the clinical performance of the devices.

Issue: December 2003

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The growing global interest in the use of mobile-bearing knee designs is manifest by an appreciation of their clinical performance, which like their fixed plateau counterparts is influenced by patient and surgical variables as well as design, material and manufacturing choices.

Their increasing use throughout Europe and the Pacific Rim points to their clinical utility despite restriction in the United States. DePuy’s LCS Complete Mobile-Bearing Knee System was the first design approved by the FDA for use in the United States. The restriction in the United States is largely attributed to the regulatory imperative of demonstrating through both laboratory and clinical evaluation the safety and efficacy of particular designs.

Dual surface articulation between a polyethylene insert and metallic femoral and tibial tray components characterizes this unique class of knee design. They offer significant increases in articulation conformity, distributing contact and subsurface stresses, thus decreasing the potential for polyethylene material damage. Through insert mobility, they minimize constraint forces transferred to fixation interfaces.

This round table brings together experienced orthopedic surgeons whose collective background in the use of various mobile-bearing designs — coupled with an appreciation of their clinical utility, performance and shortcomings — provides the reader with insight into this unique class of knee replacement devices.

A. Seth Greenwald, D.Phil. (Oxon)
Moderator

A. Seth Greenwald, D.Phil. (Oxon): Why do you use mobile-bearing knee designs and in which patients are they indicated?

Giles R. Scuderi, MD: One of the biggest issues in implant failure is polyethylene wear and osteolysis. While this may be related to the material properties of polyethylene, it is also influenced by implant design. Mobile-bearing knees provide a solution to this problem with their dual articulation, which reduces surface contact stress with its conforming design, allows greater freedom of motion, and potentially reduces backside wear. Additionally, if we are looking at knee motion in deep flexion, we need to realize that the tibial rotation does occur and can be reproduced with a mobile-bearing articulation. For these reasons, I would select a mobile-bearing design for a patient who is active with good preoperative range of motion.

David W. Murray, MD: We believe the ideal situation in which to use a mobile bearing is unicompartmental knee replacement (UKR) with intact cruciates. Under these circumstances, an unconstrained, fully congruent mobile-bearing device, such as the Oxford Knee (Biomet), can be used. As this device restores knee kinematics to normal and has negligible wear, it gives excellent function and long-term survival, particularly in young, active patients. We therefore use mobile bearings in all knees that are appropriate for UKR, or about one in three osteoarthritic knees that require replacement.

Greenwald: Mobile-bearing designs have been assumed to require increased bone resection and are technique prone particular to soft tissue balancing. Do you feel there is a learning curve associated with their use that requires increased surgical proficiency?

Frederick F. Buechel, MD: Surgical techniques for mobile bearings are the same as for fixed bearings. That is to say, flexion and extension gap balance through medial or lateral soft tissue release requires no change in surgical technique. In fact, mobile-bearing knees are easier to implant than fixed bearings, since attention to tibial component axial rotation is less critical. In fixed-bearing tibial components, even a slight malposition in rotation can cause maltracking and increased polyethylene wear. Mobile-bearing tibial components are much less sensitive to rotational malalignment and can self-adjust to allow for proper femoral tracking.

Murray: A mobile bearing that is fully congruent in all positions has low contact stresses and, therefore, low wear. This full congruence can only be achieved if the femoral component is spherical; it cannot be achieved with a traditional J-curve-shaped femoral condyle. Our retrieval studies of these fully congruent bearings have confirmed that their wear is extremely slow. They have also shown that the wear is independent of bearing thickness and that it is safe to use polyethylene as thin as 3.5 mm. Therefore, this design of mobile bearing requires less bone resection than an equivalent fixed-bearing device.

We agree that soft tissue balancing is more important with a mobile bearing than with a fixed bearing. We have therefore developed sophisticated instrumentation to accurately balance the ligaments. In addition to preventing dislocation, accurate balance restores ligament function to normal and results in a better functional outcome. We have clear evidence that there is a learning curve associated with the use of mobile-bearing devices; however, we believe that with good instructional courses, many early problems can be avoided.

Greenwald: From your fixed plateau experience, how do your personal outcomes compare and are the additional prosthetic costs justified?

Scuderi: Being a disciple of John Insall, I have vast experience with fixed-bearing posterior stabilized knee designs. Now, as an advocate of mobile-bearing total knee arthroplasty, I am impressed that my clinical results are comparable, with excellent functional outcomes and patient satisfaction. I believe this is attributable to meticulous surgical technique, which focuses on soft tissue balancing, correct component alignment and secure cement fixation. If we can improve implant survivorship and reduce the incidence of implant failure, beyond the 10- to 15-year survivorship of 95% that we are seeing with the fixed-bearing designs, as well as improve knee function, the costs will be justifiable.

Douglas A. Dennis, MD: My personal outcomes with mobile-bearing total knee arthroplasty are similar in the short term to my experience with fixed-bearing total knee replacement. I think the additional prosthetic costs are justified based on the increased contact area available with most mobile-bearing designs. This should result in reduced polyethylene wear and increased longevity of the implant. For this reason, I think the increased cost is justified.

Greenwald: What technique-based clinical problems have you encountered that resulted in revision? How might they be addressed in the future?

Buechel: Flexion instability, allowing rotary bearing subluxation or dislocation in otherwise well-aligned knees, has been a problem in less than 1% of primary knee replacements using mobile bearings worldwide. Maintaining a stable flexion gap (less than 1 mm of medial or lateral distraction) will minimize this problem. Occasionally, it may be necessary to use a rotational stop pin in a rotating-platform device to eliminate recurrent bearing subluxation.

Dennis: The surgical technique problems that I encounter in total knee arthroplasty have essentially been the same, whether it is a mobile bearing or a fixed bearing. I am often asked what I do differently in the operating room while performing a mobile-bearing total knee arthroplasty. My answer is that I truly do nothing differently. The principles of fixed and mobile bearings are the same.

In my first 500 mobile bearings, I have had to perform one reoperation. This was due to a late hematogenous infection. I do, however, have one additional patient who has chronic effusions related to mid-flexion instability, who is contemplating revision total knee arthroplasty.

Greenwald: From your clinical follow-up, how do patient kinematics compare to your fixed plateau experience?

Scuderi: Motion and stability are key elements to a successful total knee arthroplasty. Mobile-bearing designs, especially those designed for high flexion with a posterior stabilized spine cam mechanism, have demonstrated remarkable femorotibial kinematics with a high incidence of posterior femoral rollback, minimal condylar lift-off, and excellent range of motion under weight-bearing conditions. While this is related to implant design, it is greatly influenced by the surgical technique, as previously mentioned.

Our fluoroscopic analysis has demonstrated that these kinematic observations are very similar to our fixed-bearing prosthesis with a comparable design. One observation that favors a mobile-bearing prosthesis is axial rotation, which averaged 5.4º, but was as high as 13.3º in the mobile-bearing design. By allowing the tibial polyethylene to rotate, the surface stress, along with the tibial post stress, will be reduced.

Dennis: We have done extensive in vivo kinematic studies on both fixed- and mobile-bearing knee replacements. Overall, fixed- and mobile-bearing total knees behave very similarly kinematically; the only exception is when mobile-bearing knees are tested in gait. These designs typically show very little anterior or posterior translation of contact area, as compared with fixed-bearing designs, which show more sliding anteriorly and posteriorly. We believe this may account for the very low wear rates associated with mobile-bearing designs in the first two decades of use. Since there is little anterior or posterior translation, this should result in reduced shear forces. We believe that this is related to the increased sagittal conformity that is typically present in most mobile-bearing designs.

The incidence of femoral condylar lift-off and the magnitudes of axial rotation have been quite similar with fixed- and mobile-bearing designs. When studying axial rotation, however, with fixed-bearing designs the axial rotation is obviously occurring on the top side of the polyethylene bearing. We have done two studies placing tantalum beads within the mobile polyethylene bearing to see where the axial rotation is occurring. In both of these studies, the bearing followed the femoral component.

Therefore, in mobile-bearing total knee arthroplasty, at least of the rotating platform design, the axial rotation is occurring on the bottom side of the bearing. This may be another favorable feature of a mobile-bearing design. Professor Greenwald has demonstrated that the contact areas on the bottom side of a mobile bearing are quite large, and the stresses are typically quite low (less than or equal to 8 MPa; Greenwald, AAOS Instructional Course Lecture, 2003).

Murray: The kinematics of a mobile-bearing knee replacement depends upon its design detail and, as a result, it may have better or worse kinematics than its fixed-bearing counterpart. We have shown that an unconstrained mobile-bearing UKR restores knee kinematics to normal. The kinematics are normal not only at one-year postoperatively but also at 10, indicating that the cruciate mechanism continues to function and that the mobile bearing continues to move in the long term. There is some evidence to suggest that the cruciate mechanism in knees with fixed-bearing UKR does not function normally in the long term. This may be because the localized wear that occurs in the fixed polyethylene constrains the movement of the femoral condyle and results in failure of the cruciate mechanism.

Greenwald: Mobile-bearing designs have been assumed to generate increased wear volumes due to their dual surface articulations. Do you have any experience in this regard particular to retrievals and/or osteolytic response? Are they more or less prone to underside wear than their fixed plateau counterparts?

Buechel: Mobile-bearings used with polished tibial components wear significantly less than fixed-bearings. This fact has been documented by both simulator and long-term retrieval studies that show minimal abrasive wear on the underside of the bearings. The upper surfaces of mobile-bearings show an increased abrasive wear pattern commensurate with load and activity. Collectively, these bearings demonstrate less wear than their fixed-bearing counterparts.

Murray: Retrieval studies have shown that when the Oxford Knee (Biomet) is functioning normally with no impingement, the combined wear on both upper and lower surfaces of the mobile-bearing is about 0.01 mm per year (1 mm in 100 years). Because of the large area of contact, this results in a wear volume of about 6 mm³ per year. This wear volume is small and we are not aware of any cases in which it has resulted in significant osteolysis. In contrast, the in vivo data for fixed-bearing designs suggest that their linear wear is perhaps 10 times faster and their volumetric wear is also larger. Perhaps the most important difference is that with a fully congruent mobile-bearing the wear is low and predictable, whereas with fixed-bearing designs the wear is variable and may be large.

Dennis: In my practice, I have seen very few problems with osteolysis in mobile-bearing total knee arthroplasty designs. The few cases that I have seen have been associated with gross malalignment and instability. In rotating platform mobile-bearing designs, the polyethylene is primarily exposed to unidirectional wear vector patterns. It has been shown that when polyethylene is exposed to unidirectional wear patterns, the coefficient of friction is reduced and the wear is minimized.

Outside of the United States, however, there are multiple mobile-bearing total knee designs that allow both axial rotation as well as anterior and posterior translation. These are often referred to as “slide-and-glide” designs. I think we need to follow these types of mobile-bearing knees carefully. They obviously will be exposed to multidirectional wear vector patterns on the bottom side of the insert. John Fisher, PhD, has shown that when polyethylene is exposed to multidirectional wear patterns, as is present in the slide-and-glide designs, the wear rates are increased nearly 10-fold (V. Jones, J. Fisher, et al., BioMed. Mater. Eng., 1999).

Greenwald: In mobile-bearing knee design, do you see a role for the use of highly cross-linked polyethylenes as well as alteration in the femoral surface?

Scuderi: We are going through some dramatic changes in implant design, as well as material changes. While designers are trying to develop implants that produce less wear debris, we are also developing articular surfaces that are more wear resistant. However, the final answer is not yet available. A highly cross-linked polyethylene may have a more wear resistant articular surface but, depending on the formulation, it may be more brittle and a crack in the polyethelene may generate into a fracture or component breakage. The interaction of the polyethylene with the trunnion, tibial stops and tibial posts needs to be considered. Further testing with the amount of radiation that determines the degree of cross-linking needs to be performed.

So I will say that at the present time the net-shaped molded and compression-molded polyethylene components are adequate. In contrast, we do know that a highly polished cobalt chrome tibial tray surface is less abrasive and is ideal for a mobile bearing prosthesis. Finally, regarding the femoral surface, it is intuitive that a highly polished surface will produce less wear debris.

Greenwald: Are these devices compatible with the growing interest in computer assisted surgical navigation to improve component placement and alignment?

Buechel: In Germany, mobile-bearing knees are commonly implanted devices using computer-assisted surgical navigation.

Dennis: I do believe mobile-bearing total knee devices are compatible with computer-assisted surgical navigation. I also believe, however, that fixed bearings are equally compatible. Anything that will facilitate improvement in implant alignment and soft tissue balance would be favorable for the long-term outcome of either fixed or mobile-bearing total knee arthroplasty.

Greenwald: The FDA is considering down classifying mobile-bearing knees, which will make them more generally available. Do you feel this is justified in light of your own experience with these designs?

Buechel: Down-classification for proven mobile-bearing knee designs is justified and should be implemented to improve patient care. A serious concern exists for unproven mobile-bearing designs that deviate from the principles of maximizing congruity, minimizing constraint and allowing for reasonably normal kinematic motion. Devices that provide multidirectional motion and include sudden stops or posts, which can inhibit A-P translation or varus-valgus lift-off, can contribute to increased wear and should be carefully studied in the long term before being accepted over proven devices.

Murray: The mobile-bearing Oxford knee has been in use for 25 years and there is a large amount of information published on it. The indications for its use and the surgical technique, involving sophisticated instrumentation, are well established. When used appropriately it has been shown in a number of series that excellent function and outstanding long-term survival can be achieved. Despite this, the FDA has not approved it.

In order that surgeons and patients in the United States can benefit from the instrumentation, a fixed-bearing version of the Oxford knee has been developed. This, we believe, will function at least as well as other fixed-bearing UKR. The FDA approved this fixed-bearing device despite the fact that it has no clinical results.

The only way to address this anachronistic system that approves a new, unproven fixed-bearing device but does not approve an established mobile-bearing equivalent with advantages in terms of wear and kinematics and excellent long-term results is to “downclassify” mobile-bearing knees.

For more information:

• Bacon RK, Smith TS, Greenwald AS. Joint simulator performance of the New Jersey knee meniscal bearings. DePuy Technical Report 217; 1984.
• Bourne RB, Callaghan JJ, Dennis DA, Dorr LD, Greenwald AS, et al. Mobile-bearing knee Replacement. JBJS. 2000:1020–1041.
• Buechel FF, Pappas MJ. New Jersey LCS Knee Replacement System: 10 year evaluation of meniscal bearings. Clin Orthop. 1989;20:147-177.
• Buechel FF Sr, Buechel FF Jr, Pappas, MJ, et al. Twenty-year evaluation of meniscal bearing and rotating platform knee replacements. Clin. Orthop. 2001;388:41-50.
• Buechel FF: Recurrent LCS Rotating Platform dislocation in revision total knee replacement: mechanism, management, and report of two cases. Orthopedics, 2003;26:647- 649.
• Beverland DE, Jordan LR. LCS Rotating Platform dislocation and spin out – etiology, diagnosis and management. LCS Mobile Bearing Knee Arthroplasty – 25 Years of Worldwide Experience. Edited by K.J. Hamelynck & J.B. Stiehl. Springer-Verlag Berlin Heidelberg 2002:235-240.
• Collier JP, Mayor MB, et al. Analysis of the failure of 122 polyethylene inserts from uncemented tibial knee components. Clin Orthop. 1991;273:232-242.
• Pappas MJ, Buechel FF. Wear in prosthetic knee joints. AAOS Scientific Exhibit # 1305, Washington DC. Feb 1992.
• Strauss JM, Briard J-L, Ruther W. Navigation and Soft-Tissue Balancing of LCS TKA. LCS Mobile Bearing Knee Arthroplasty – 25 Years of Worldwide Experience. Edited by K.J. Hamelynck & J.B. Stiehl. Springer-Verlag Berlin Heidelberg 2002:333-342.
• Goodfellow JW, O’Connor JS. Clinical results of the Oxford knee surface arthroplasty of the tibio-femoral joint with a meniscal bearing prosthesis. Clin Orthop. 1986;205:21-42.
• Fisher J, McEwen H, Stone M. Polyethylene wear – problem & cure. Presented at LCS 25-year anniversary celebration meeting; Amsterdam, Netherlands. Oct 16-20 2002.