Trends in Total Knee Arthroplasty

By Douglas A. Dennis, MD

Abstract

The success of total knee arthroplasty (TKA) over the past two decades of use has resulted in the implantation of TKA into younger patients who have increased functional requirements and demand increased implant longevity. Improved functional performance requires enhanced range of motion, increased motor performance, and creation of kinematic patterns that more closely resemble the normal knee. Increased longevity necessitates more durable implant fixation, improvements in bearing materials, and lower polyethylene stresses. Considerations to reduce polyethylene wear include increased cross-linking, improved femoral component surface finish, better modular tibial locking mechanisms, and the use of mobile-bearing TKA designs that allow increased implant conformity and reduced contact stresses without increasing loads transmitted to the fixation interface.

Reports of device failure in the early years of total knee arthroplasty (TKA) were frequently secondary to aseptic loosening and were often associated with malalignment, instability, or use of implants with excessive prosthetic constraint. With improvements in surgical instrumentation and operative techniques (ie, improved alignment and ligamentous balancing), and due to the use of lower conformity prosthetic devices, loosening rates have been minimized. Low conformity TKA designs, however, resulted in a reduction of polyethylene contact area, and premature polyethylene wear and periprosthetic osteolysis became a prominent cause of TKA failure.¹

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There has been excellent 10- to 15-year outcomes of primary TKA. Future TKA designs, therefore, should improve functional performance and reduce articular bearing surface wear, and maintain the excellent long-term fixation typically obtained in properly aligned and balanced TKAs currently in use.

Improving Functional Performance

Figure 1: Diagram demonstrating the modification of the J curve of the femoral component of some “high-flexion” TKA designs.

Enhancing range of motion (ROM) and muscle function are crucial to improving functional performance of TKA. Replication of the normal knee kinematic pattern is critical to maximize knee flexion and physical function. In vivo fluoroscopic studies of normal knee kinematics demonstrate that posterior femoral roll-back of the lateral femoral condyle, averaging 14.1 mm, routinely occurs during deep flexion. Deep flexion is also associated with substantial axial rotation, averaging 16.8°.² Additional similar studies of patients implanted with TKA show significant reductions in posterior femoral roll-back and axial rotation, as well as the presence of paradoxical anterior femoral translation during deep flexion.^3,4 These kinematic differences from the normal knee likely account, at least in part, for the reduction in knee flexion and reduced motor function typically observed following TKA when compared with the normal knee. Future designs of TKA should focus on providing reproducible roll-back and increased axial rotation.

Recent design efforts to improve knee flexion have been incorporated into high-flexion TKA designs.^5-7 These high-flexion devices allow for posterior cruciate ligament substitution, which enhances posterior femoral roll-back, and bearing mobility, which permits increased amounts of axial rotation without creating excessive rotational polyethylene stresses. Many of these newer designs have also reduced the radius of curvature (“J” curve) of the posterior femoral condyles, both to increase the posterior femoral translation distance and reduce polyethylene stresses in deep flexion (Figure 1).

Reducing Polyethelene Wear

Numerous design improvements have been introduced to reduce polyethylene wear. Some focus on improving the bearing materials, including increasing the cross-linking of polyethylene and improving the surface finish of the femoral component. Multiple knee simulator analyses demonstrate marked reductions in adhesive and abrasive wear when comparing highly crosslinked tibial insert specimens with those manufactured with standard polyethylene.^8-10 Longer in vivo clinical evaluations are needed to assess the value of increased cross-linking of polyethylene in TKA. Computer-directed, precision-grind finishing of femoral components is shown to increase polyethylene contact area by 29%.¹¹ A knee simulator analysis of femoral components manufactured with oxidized zirconium show substantial reductions in polyethylene wear when compared with components manufactured from cobalt-chromium alloys.¹²

Figure 2: Contact area and stress analysis demonstrating high polyethylene contact areas (mm2) and low peak stresses (MPa) of three mobile bearing TKA designs.

Accelerated wear from micromotion and backside wear of modular tibial components is associated with premature failure and periprosthetic osteolysis.¹³ Improvements in modular locking mechanisms and the polishing of modular cobalt-chromium tibial trays were introduced to reduce this mechanism of polyethylene wear.¹⁴ Additionally, use of all-polyethylene or other designs of monoblock tibial components are shown to minimize polyethylene wear.¹⁵

Use of implant designs with increased implant conformity reduces contact stresses and the potential for polyethylene wear but can increase wear and fixation stresses in fixed-bearing TKA if conformity is excessive. Mobile-bearing TKA allows increased implant conformity and contact area without dramatically increasing stresses transmitted to the polyethylene material or fixation interface.

Figure 3: Histogram of a high kinematic knee simulator analysis demonstrating polyethylene wear per million cycles of a fixed-bearing versus rotating-platform TKA with identical femoral component geometry.

By increasing sagittal plane conformity in mobile-bearing TKA, in vivo fluoroscopic analyses demonstrate improved control of anteroposterior translation with reduced paradoxical anterior femoral translation, particularly when tested during gait.⁴ The increased coronal plane conformity typically present in mobile-bearing TKA increases the contact area and lessens the increased contact stresses, which are present if femoral condylar lift-off occurs.^16,17 The increased conformity and subsequent reduction in contact stresses in mobile-bearing designs are shown to substantially lower polyethylene stresses in numerous evaluations.^17,18-21 Greenwald demonstrated contact areas of mobile-bearing TKA during gait range from approximately 400 mm² to 800 mm², which keeps contact stresses at 14 MPa or less (Figure 2).²² The contact areas are substantially greater than is typically observed in most fixed-bearing TKA designs (200 mm² to 250 mm²). Lastly, the advantage of increased contact area is reflected in knee simulator wear studies of fixed-versus mobile-bearing TKA. McEwen et al¹⁹ noted over a four-fold reduction in wear in knee simulator testing of a rotating-platform TKA versus a fixed-bearing design with identical femoral component geometry (Figure 3).

To avoid high polyethylene stresses typically observed with highly conforming, fixed-bearing TKA, rotational bearing mobility must be present and has been documented to occur under in vivo conditions.^23,24 The majority of axial rotation observed in these in vivo studies occurred at the polyethylene bearing-tibial tray interface, with the polyethylene bearing typically following the rotation of the femoral component.

Rotation of the rotating-platform polyethylene insert with the femoral component reduces stresses transmitted to the fixation interface and creates the potential for self-alignment of the polyethylene bearing with the femoral component. Rotational post impingement in posterior cruciate-substituting TKA systems can be reduced in rotating-platform designs due to post rotation with the femoral component intercondylar box rather than by attempts to rotate against it. The self-aligning behavior of the polyethylene bearing with the femoral component is shown to maintain large, centrally located surface contact areas at the femorotibial articulation during flexion and extension movements and during axial rotation of the knee. These advantages are typically more difficult to achieve with fixed-bearing TKA designs.²¹ An additional advantage of the self-aligning feature of rotating-platform TKA systems is the potential facilitation of central patellar tracking.²⁹ In a fixed-bearing TKA, if substantial internal rotation of the tibial component relative to the femoral component is present, the tibial tubercle is lateralized, enhancing the risk of patellar subluxation. A rotating-platform design, because of bearing rotation, permits greater self-correction of component rotational malalignment and allows better centralization of the extensor mechanism.

An in vivo fluoroscopic evaluation of over 1000 TKAs involving 33 different fixed and mobile-bearing TKA designs demonstrated that most patients with TKAs experience less than 10° of axial rotation with normal postoperative activities.³ However, in this large multi-center analysis, a number of subjects experienced axial rotational magnitudes greater than 20°, which is beyond the rotational boundaries of most fixed-bearing TKA designs. Greater axial rotation is an additional advantage for rotating-platform TKA designs, which can accommodate a wider range of axial rotation without creation of excessive polyethylene stresses.

Other Advances

Other future improvements to potentially enhance TKA include the introduction of newer, cementless ingrowth materials such as trabecular metal, use of less invasive operative techniques, and the use of computer-assisted navigation. Computer-assisted navigation is shown to reduce component alignment errors and has the potential to increase the precision of soft-tissue balancing.^26,27

References

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Author

Dr Dennis is from the Department of Biomedical Engineering, University of Tennessee, Knoxville, Tenn, Oak Ridge National Laboratory/University of Tennessee Center for Musculoskeletal Research, Knoxville, Tenn, and Rocky Mountain Musculoskeletal Research Laboratory, Denver, Colo.