Mobile-Bearing Knee Replacement

Humza S. Shaikh, MD

Malcolm E. Dombrowski, MD

Richard A. Wawrose, MD

Lawrence S. Crossett, MD

HISTORY AND DEVELOPMENT OF MOBILE BEARINGS

The need for mobile bearings in knee replacement developed after analysis of failed fixed-bearing knee replacements. Significant component loosening was seen in early fixed-hinge devices and congruent fixed-bearing nonhinged devices in less than 2 years of active use.¹,² Incongruent knee replacements that were developed to avoid loosening problems were plagued with wear-related problems in less than 5 years.³,⁴ Modular fixed-bearing polyethylene inserts were developed to allow intraoperative adjustment of component stability, but their locking mechanisms created a new source of wear debris that compounded the existing wear problems.⁵

It was established that congruity without mobility, and mobility without congruity, were flawed design concepts that caused premature loosening or excessive wear. Congruity with mobility has become the ideal stress and movement concept to minimize loosening and wear problems. That is to say, mobile bearings without locking mechanisms represent the most effective way to avoid the identified problems seen with fixed-bearing knee designs over the past three decades.

The kinematic tibiofemoral motion requirements dictate the use of spherical upper tibial bearing surfaces and a flat undersurface to accommodate the variety of movements in the most congruent way. The Oxford meniscal knee uses matching spherical surfaces for the femoral component and the upper meniscal-bearing surface and a flat surface to match a flat tibial component.⁶

This preferred geometry appears to work well as a medial unicompartmental replacement but has had dislocation problems in other applications.⁶,⁷ These problems most likely are caused by a larger than normal single radius of curvature of the femoral component, which under the pull of the posterior cruciate ligament (PCL) in flexion moves the bearing too far posteriorly.

A design solution to the Oxford problem in the presence of cruciate ligaments is seen in the low contact stress (LCS) femoral component, which uses the same spherical surface of revolution in the medial-lateral plane but decreases the radius of curvature from extension to flexion, thus maintaining full-area contact on the upper meniscal-bearing surface from 0° to 45°, where walking loads are encountered, and maintaining at least spherical line contact at deeper flexion angles.⁸ This surface geometry allows a more central femoral component position in flexion by reducing the PCL tension, which tends to pull the femur posteriorly when overstretched. Another design solution to prevent meniscal-bearing dislocation is the use of radial tracks on the LCS tibial components. These tracks allow axial rotation and controlled anteroposterior translation, which impedes direct dislocation by means of the cruciate bone bridge posteriorly and the patellar tendon anteriorly. When combined with stable flexion and extension gaps at surgery, the LCS meniscal bearings could be safely used when both cruciate ligaments are intact or if only the PCL is intact.

In the event of a nonfunctional or absent PCL, central stability with the ability to axially rotate is essential. Long-term survivorship studies have demonstrated that a centrally stabilized total condylar knee replacement is predicted to last for 15 years in more than 90% of cases when used in elderly patients with low loading demands.⁹,¹⁰ These important studies prove that cruciate function is not essential for successful long-term fixation and function in low-demand situations.

Because wear increases as the loads and demands increase, it seems most appropriate to use the proven fixation and central stabilizing concepts of the total condylar device and provide a more wear-resistant and dislocation-resistant bearing surface to achieve better long-term survivorship and reduce wear-related failures. These concepts led to the development of a rotating-platform total knee device that uses the same spherical surface geometry as the meniscal bearings (Fig. 36E-1).

The patellofemoral design process, like the tibiofemoral design process, seeks to provide proper motion and maintain contact stresses below the ideal 5 megapascals (MPa) during walking, stair climbing, and deep-knee bending.¹¹ Button or nonrotating anatomic-type patellar replacements suffer from either point or line contact stresses or from overconstraint. High contact stress will cause early wear failure, whereas overconstraint will cause early loosening failure.¹² For these reasons, a rotating bearing patellar replacement was developed to maintain spherical area contact on the medial and lateral facets while congruently matching the surface of revolution of the deep sulcus femoral groove. Rotating bearing patellar replacement of the LCS design greatly improves on the contact stress seen in other design configurations.¹¹

FIGURE 36E-1 LCS rotating-platform bearing design: spherical surface geometry matches meniscal-bearing designs + rotating bearing patella maintains spherical contact on the medial and lateral facets while congruently matching the surface of revolution of the deep sulcus femoral groove.

EVOLUTION OF THE NEW JERSEY LOW CONTACT STRESS MOBILE-BEARING KNEE

The first mobile-bearing application was seen in shoulder replacement in which two eccentrically placed spherical elements improved the range of motion over simple ball-and-socket systems. These “floating-socket” bearings were developed in 1974 and used clinically from 1975 to 1979, when less constrained shoulder implants were developed.¹³ Later, knee and ankle bearings were developed using similar concepts.⁸

FIGURE 36E-2 History of mobile-bearing designs, from right to left: unicondylar, bicondylar, LCS RP, revision LCS RP.

The first complete systems approach to total knee replacement (TKR) using meniscal bearings, introduced in 1977, was the LCS Complete Knee System. At the time, the LCS system managed unicompartamental, bicompartamental, and tricompartamental disease by providing three options, the bicruciate-retaining, posterior-cruciate retaining, and the cruciate sacrificing rotating-platform design. Additionally, the first metal-backed, rotating bearing patellar replacement was developed in 1977 to provide mobility with congruity in patellofemoral articulation. This LCS Total Knee System, which initially received U.S. Food and Drug Administration Investigational Device Exemption (FDA-IDE) for clinical trials of cemented knees in 1980, and for uncemented knees in 1983, first came to market in 1985. Buechel and Pappas published the earliest clinical results of cemented LCS in 1986, reporting that 88.3% of the first 123 cemented knees had good to excellent outcomes at 2 to 7-year follow-up, with no reported mechanical failures or bearing dislocations.¹⁴ The LCS remains the only knee system in the United States to have undergone formal FDA-IDE clinical trials in both cemented and uncemented applications before being released for general clinical use (Fig. 36E-2).

Early Results

While the majority of early LCS knee implants were posterior cruciate-retaining meniscal-bearing designs, early clinical results challenged their efficacy leading to a paradigm shift. In 2001, Buechel reported minimum 10-year follow-up results for 373 LCS prostheses. Survivorship for the posterior cruciate-retaining uncemented meniscal-bearing group was 83%, for the cemented cruciate sacrificing rotating-platform group was 97.7%, and for the uncemented cruciate sacrificing rotating-platform group was 98.3%. Thus, affirming what most had come to find
over the years, that the rotating-platform prosthesis was not only an easier implant to utilize, but yielded superior survivorship than meniscal-bearing prosthesis.

BIOMECHANICAL CONSIDERATIONS

Component Surface Geometry

Surface congruence is essential to improve wear life in ultra-high-molecular-weight polyethylene bearings, especially in major repetitive load-bearing activities, such as walking, which generates loads of 2.5 times body weight, and stair climbing, which can generate loads of eight times body weight. Aside from direct compressive loading, however, the tibiofemoral bearings must be able to accommodate flexion of 155°, varus-valgus movements of 10°, axial rotation of 30°, and anteroposterior translation of 15 mm when the cruciate ligaments are retained. The patellofemoral articulation is also loaded mainly in compression and needs to accommodate similar flexion to 155°, axial rotation of 6°, and the ability to tilt laterally and medially in the femoral groove without dislocating or rubbing on an edge.

FIGURE 36E-3 A: Industry recommendations are to keep contact stress between femoral implant and polyethylene, ideally, less than 5 MPa, and no greater than 10 MPa. B: Maximizing the contact surface area between the femoral component and the polyethylene insert by maximizing conformity accomplishes this goal by reducing contact stress.

The LCS knee was designed to accommodate for these parameters. Medical-grade polyethylene manufacturers recommend that contact stress ideally remain less than 5 MPa, and no greater than 10 MPa (Fig. 36E-3A). Maximizing the contact surface area between the femoral component and the polyethylene insert by maximizing conformity accomplishes this goal (Fig. 36E-3B). The unique geometry of the LCS knee maximizes conformity by matching the curvature of radii between femoral component and tibial insert, and femoral component with patellar insert. This conformity occurs between femur and tibia from 0° to 30° during gait, and between femur and patella from 0° to 110°. In the weight-bearing segment of the femoral component, segment 2, the anteroposterior radius of curvature equals the medial-lateral
radius (Fig. 36E-4). Therefore, during gait the contact area between the femoral and tibial components is a large sphere, 877 mm².

FIGURE 36E-4 Curvature of radii between femoral component and tibial insert, and femoral component with patellar insert is matched in LCS knee. This conformity occurs between femur and tibia from 0° to 30° during gait, and between femur and patella from 0° to 110°. In the weight-bearing segment of the femoral component, segment 2, the anteroposterior radius of curvature equals the medial-lateral radius.

The importance of maximizing surface area to maximize conformity was elucidated by Kuster and Stachowiak, who examined the interplay between conformity and load.¹⁵ A conformity ratio of 0.99 connotes fully conforming femoral and tibial components, while a ratio of 0 represents a femoral condyle and a flat tibial insert. Using finite element modeling, the authors found increasing conformity ratios has a greater effect on polyethylene stress than reducing loads. In their analysis, as expected, polyethylene stress was higher for a 1000 N load (standing) on a flat tibial insert than a 6000 N load (running) on a conforming insert. However, an increase in the conformity ratio from 0.95 to 0.99 had a greater effect on surface and shear stresses than a load increase of 3000 N. Thus, they posited that a high conformity ratio reduced tibial polyethylene insert delamination and accelerated wear more significantly than load restrictions due to activity modification in nonconforming total knee arthroplasty (TKA) designs. The LCS Total Knee System has a conformity ratio of 0.99 in gait.

Wear Properties

Recent advances have significantly improved the quality of polyethylene inserts. Gamma irradiation in a vacuum pouch has replaced gamma sterilization in air, and calcium stearate has been eliminated from polyethylene resins, both reducing the potential for polyethylene oxidation which is known to reduce polyethylene insert mechanical properties. In spite of this, retrieval of component studies continue to demonstrate reductions in volumetric wear under high loads for LCS total knees in comparison to fixed-bearing prosthesis.⁵,¹⁶,¹⁷,¹⁸ Similar retrieval analyses of meniscal bearings, rotating-platform bearings, and rotating patellar bearings demonstrated significantly less wear than with fixed bearings. Although mobile bearings allow reduced contact stress, they can be overloaded to failure by excessive weight, excessive activity, malalignment, or a combination of these factors. However, by their nature, spherically surfaced mobile bearings accommodate malalignment without overload more easily than fixed bearings. Whether the overall knee alignment is in neutral (5° valgus) or not, the spherical bearing surface always sees congruent contact with the femoral component as compared with a flat-on-flat bearing surface that becomes overloaded during normal condylar liftoff. By design, the rotating platform couples rotational forces between the femoral component and tibial polyethylene insert, and between the tibial insert and the tibial tray surface. In the LCS knee, rotation primarily occurs between the distal polyethylene surface and tibial tray, producing a unidirectional wear pattern. Similarly, rotation at the femoral component and polyethylene interface is largely during flexion-extension, again producing unidirectional wear. Thus, the LCS rotating-platform design reduces complex knee kinematics into predominately unidirectional motions (Fig. 36E-5). As various studies have shown, unidirectional motion reduces polyethylene wear rates, particularly with the newer, highly cross-linked polyethylene inserts¹⁹,²⁰ (Fig. 36E-6).

FIGURE 36E-5 The rotating platform decouples the femoral-bearing contact area and tibial-bearing mobility to reduce bone-prosthesis interface stressed as well as polyethylene stress.

In their biomechanical study, Wang et al analyzed the source of microscopic wear particles generated from modern polyethylene articular surfaces.¹⁹ They found that the polyethylene chain reorganizes due to surface strain secondary to femoral component motion. When stressed, the molecular chain responds favorably when loaded on-axis, but unfavorably when loaded off-axis. Multidirectional joint simulators, mimicking fixed-bearing femoral component-tibial tray interaction, create complex, off-axis loads. They found that this multidirectional motion causes failure along three modes, in accordance with increasing degree of off-axis motion: tensile rupture, shear rupture, and transverse splitting. These mechanisms all produce fiber-like wear particles, with increased wear associated with large degrees of off-axis motion. Another biomechanical study by Marrs et al examined polyethylene wear rates, comparing unidirectional and multidirectional motion.²⁰ They similarly found that unidirectional motion reduces ultra-high-molecular-weight polyethylene wear (Fig. 36E-7). Numerous studies directly comparing the wear rates between fixed and mobile-bearing designs have corroborated these biomechanical findings. McEwen et al investigated polyethylene wear using various total knee prosthesis and stressing them in knee simulators, finding that rotating-platform mobile-bearing implants had reduced wear rates in comparison to fixed-bearing components.²¹ Examination of wear patterns suggested improved wear characteristics in this prosthesis was due to redistribution of knee motion to two articulating interfaces with more linear motion at each interface.

FIGURE 36E-6 Wear patterns for fixed versus mobile bearing, early moderately cross-linked versus newer highly cross-linked. Results show significant improvement in wear rate for fixed-bearing prosthesis when using highly cross-linked polyethylene inserts, but no significant change in wear rates between the two inserts between fixed- and mobile-bearing knees.

FIGURE 36E-7 Multidirectional wear patterns introduce friction vectors in various planes, increasing wear rates in the polyethylene insert. As motion is made more unidirectional, friction vectors align, minimizing the polyethylene wear rate.

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