Article: Current Principles of Design for Cemented and Cementless Knees

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Current Principles of Design for Cemented and Cementless Knees

David F. Bindelglass, M.D., Jondy L. Cohen, M.D.,
and Lawrence Dorr, M.D.

Twenty years of experience with modern knee arthroplasty have shown certain designs of prosthesis to be extremely successful. Modifications have been made to earlier designs in an attempt to improve fixation and decrease the potential for excessive wear, but new problems sometimes develop. Based on this experience, certain principles become evident which should guide surgeon/designers in the development of the next generation of knee replacement implants.

Total knee arthroplasty is at the present time probably the most durable of all joint replacements that can be performed. 14 Lessons learned over past 20 years have refined surgical techniques and design. It is interesting to note the similarity between Charnley's early results with total hip replacement and those of Insall with the total condylar knee. In both instances the results with the pioneer design have not been exceeded by later designs. This is true because the early designs were based on the fundamental principles necessary for successful arthroplasty and because patient selection in early studies was limited to older people. Throughout the 1980s, surgical technique and design were modified in an attempt to improve on those already excellent results.

A number of technical issues now have general acceptance. Overall limb alignment must be in valgus so that medial compartment bone is not overloaded.22 Minimal thickness bone cuts on the tibia are desirable so that optimal bone stock remains to support the tibial component.11 Lastly, joint line height should be preserved within 1 cm for proper patellofemoral mechanics15 and ideally the patella should track centrally without tilting.
While technical points are agreed upon by most surgeons, questions of design and fixation remain. Recent designs have improved the patellofemoral articulation and tracking but the optimal kinematics of the tibial-femoral articulation are still debated. This article will outline successful and unsuccessful design features of implants that have been used. Design and fixation will also be discussed in relationship to cost, which is a significant factor in the decade of the nineties.


The design of the femur is significant for its role both in the femoral-tibial and patellofemoral articulations. At the femoral-tibial articulation, controversy remains as to whether or not to sacrifice the posterior cruciate ligament (PCL). Early designs were cruciate sacrificing. Conformity between the components provided anteroposterior (AP) stability and the femur rolled on the tibia during flexion. Maximum flexion achieved in this fashion averaged 90°, but this flexion was a function of both design and the use of a 90° AP bone cut.35 PCL-retaining designs were developed for increased stability but also to recreate femoral rollback, the natural posterior sliding of the femur on the tibia.18 The goal was to gain increased flexion (Fig 1). Without femoral rollback, the only mechanism for increasing flexion is for the anterior femur to lift off the tibia.16 Current PCL-sacrificing or substituting designs demonstrate increased flexion by promoting rollback within the design. Results of the two general designs appear to be comparable in the first decade of fup.6 Ewald summarized the projected long-term differences in 1990.13 The PCL-sacrificing knees would rely on conformity between the components for stability. This degree of constrain would lead to increased interface shear and eventual loosening. PCL-retaining knees relied on a sliding motion for flexion and used flat polyethylene surfaces. Both factors led to increased wear. Thus each design had weaknesses. One question must be whether these weaknesses can be compensated for by combining the best features of both designs into a common polyethylene articular surface.

The design of PCL-retaining prostheses has been shaped by the principle of 'kinematic conflict.'20 If the PCL is retained the articular surfaces must be flat so that the femur will slide more easily over the tibia. If the tibial surface is dished then the femur will impinge on the posterior lip, restricting the desired rollback (Fig 2). This impingement will also generate tensile and shear forces at the tibial component-bone interface. Therefore, most modern PCL-retaining devices have flat surfaces,32 or at least large radii of curvature, when viewed in the sagittal plane. Two major problems occur with flat surfaces. The contact area between the femur and plastic is smaller so that higher stresses are generated, resulting in accelerated wear.39 Second, at the extremes of motion the femoral component may be contacting only the edge of the tibia. This 'edge loading' concentrates high stress over a small area, which leads to wear and causes tension at the tibial interface of the opposite condyle. This will result in earlier loosening (Fig 3).

The authors believe that a compromise is possible. Conforming surfaces can be constructed so that posterior femoral displacement can occur with a dished tibia and minimal sliding. The center of rotation of the tibial dish is moved 3 mm posteriorly from the midline (Fig 4). As the femur rotates into flexion it is displaced posteriorly. This creates femoral rollback with minimal sliding in a prosthesis that is more conforming. Conformity leads to higher contact areas and makes edge loading less likely. Wear is therefore reduced.  Femoral design can also affect the contact area of articulation. In the total condylar knee the femoral condyles are round when viewed from the front and conform fairly closely with the tibial articular surface. The intercondylar eminence, which is concave on both sides, articulates with the central surfaces of both condyles, providing stability in the mediolateral plane and increasing contact areas (Fig 5).38 The tibial spines produce a similar effect in the normal knee. Once criticism of the total condylar knee was that with compressive loads the conformity of the components provided resistance to the tibial rotation that occurs during normal walking.34 Werner and colleagues were concerned that the conformity necessary to resist rotational forces would lead to the excellent long-term results of the total condylar knee suggest that this is not the case.35 The explanation for this difference between theory and clinical results is found in gait studies by us that demonstrate that normal tibial rotation does not occur during walking.11

In the coronal plane the authors feel that at least the central surface of the condyles should be rounded to articulate with a rounded surface of the tibial intercondylar eminence, increasing contact area and stability against mediolateral shear. Perhaps round surfaces both sagitally and coronally will ultimately return, but presently the femoral design must be rounded at least on the mediolateral surface to maximize contact areas and minimize wear. Conformity between the femur and tibia must be such that contact forces in both extension and flexion are less than 18 megpasq/mm.8 Buechel prefers contact forces of less than 10 megpasq/mm6 without question, the femoral component design is critically important in minimizing wear, which has become the primary cause of failure of total knee replacement. the total condylar knee has yet to show any reported failure rate from wear,35 whereas wear has been a serious problem for the Porous Coated Anatomic (PCA) knee, which is prototype of the flat femur/flat tibia design.



The design principles for the tibial articular surface have been partially described above. The articular surface is curved both in the anteroposterior and mediolateral planes to provide partial conformity and therefore increased contact area without being overly constrained. In the sagittal plane the center of rotation is moved 3 mm posterior to the midline to accommodate posterior femoral displacement in flexion (femoral rollback) with minimal sliding. In addition, the central notch is deepened to minimize impingement of the PCL in the notch. The eminence of the plastic provides a 5° hyperextension stop.

Cemented design
For cement fixation the authors feel that an all-polyethylene tibia is as good as, if not better than, a metal-reinforced tibial tray. Bartel suggests that a metal-backed tibial component would distribute forces more evenly throughout the tibial surface, especially with asymmetric loading, so that cancellous bone would be protected.2 However, in a properly aligned knee, when forces are distributed more equally, metal backing is not necessary. Clinical results confirm the suggestion that all polyethylene tibias may have equal or greater durability.1,21 L'Insalata, Stern and Insall reported equally good results in two groups of elderly patients, one with all polyethylene tibias and one with metal-backed components.21 Apel, Tozzi, and Dorr similarly found that there was no difference between metal-backed and all-poly components in their series of 131 knees.1 Interestingly, this clinical history parallels the evolution of cemented acetabular components. Charnley first popularized cemented acetabular components. Charnley first popularized cemented polyethylene acetabular prostheses. Metal-backed acetabuli were developed because of the theory that they would distribute stress better and reduce the incidence of loosening. However, clinical studies have clearly shown that a cemented all-polyethylene acetabulum is superior to a metal-backed one.28
Three reasons are apparent for the possible superiority of cemented all-poly tibial components. One is the thickness of the plastic, which should be 8 mm. Burstein has shown that the longevity of polyethylene doubles when the thickness is increased from 6 mm to 8 mm.8 This, therefore, could be the difference between a 10-year and 20-year total knee replacement result. Metal trays are about 3 mm thick. This means that, at a minimum, the thickest tibial component that should be used with a metal tray is 11 mm. This thickness requires excessive bone resection in a routine, primary knee.
A second reason to use an all-poly tibia is the teeter-totter effect of a metal tray (Fig 6). When one side of a metal tray begins to loosen, this teeter-totter effect with progress rapidly by creating tensile forces at the cement-bone interface on the opposite side and loosening the stem (The fulcrum). All-poly components do not teeter around the stem, and, in fact, if the plateau fixation loosens the stem may remain intact for a long period of time. A third reason to use an all-poly tibia is cost. A metal tray increases cost by a factor of 2, and a factor of 3 if the metal is porous coated. If all-polyethylene tibial components were to replace metal-reinforced ones for almost all cemented fixations, the saving would be $150 million.12 This is a significant figure in the face of fixed fees being paid to hospitals for patient care.  Metal-reinforced tibial components are still favored when a severe bony defect is present and the choice is made to use a metal wedge to manage a defect when the tibial component thickness is 15 mm or more, metal reinforcement is superior because of reduced stiffness of the implant. An all-poly implant of 15 mm is stiffer than a 15 mm metal-reinforced implant.

Another factor in tibial component design is symmetry. The Natural Knee (Intermedics Orthopedics, Austin, Texas) introduced an asymmetric tibial component to gain better coverage of the tibial plateau. Barter argued that success of metal-backed tibias depended on support by the cortical rim with the metal tray acting as a bridge over the weaker cancellous bone.2 Therefore, maximum coverage of the proximal tibia, which is only possible with asymmetric right- and left-sided components, would maximize support from the cortical shell. As asymmetric design provides its greatest benefits in noncemented fixation. For cemented fixation Walker has shown that 85% coverage is acceptable.37 This can easily be achieved with a symmetric component.

Cementless design
Bone ingrowth into the tibial component remains a challenge to surgeon/designers. For bone ingrowth a porous surface on metal is required. Optimal fixation by bony ingrowth requires early stabilization to prevent motion and adequate surface area for stable ingrowth fixation. Prostheses that fit into bone, as into the femur and acetabulum of a total hip prosthesis, have the best record of bone ingrowth on retrieval. With total knee replacement, the most consistent bone ingrowth has been on the PCA porous coated pegs that are driven into the proximal tibia.26 Current tibial designs rely on porous surfaces that sit on the bone and achieve initial stability and lift-off protection by screw fixation. Data from Volz36 and Whitesides41 support the use of screws for initial fixation. However, some micromotion is inevitable,4,9 which can lead to fretting at the screw plate junction, even if the component eventually becomes well fixed. Polishing at this junction has been observed by the authors in well-fixed components, which raises the possibility that particulate metal debris is being released in the joint. Peters, Engh, and Dwyer have observed osteolysis along the tracks of the screws.25
A recent study confirmed many surgeons' findings that it is nearly impossible to create a perfectly flat tibial bone cut. Thus, a rigid metal plate is placed on an uneven surface. Loading subjects it to rocking on 'peaks' of the proximal tibia, possible uneven settling, and uneven final surface contact. Sulberg and Manley have shown how fluid is pumped in and out from underneath the component with loading when any differential motion between metal and bone is present.30 The final mechanism for motion between the tibia and a metal tray is that the tibia spreads radially 100 microns when loaded.8 Therefore, motion at the interface results from applying a stiff flat metal plate to a more flexible, if somewhat uneven surface. Cement evens out the tibial surface and prevents motion. For noncemented fixation there must be a way to prevent this motion so that ingrowth can occur and later loosening is prevented.

One solution to lift-off of the tray has been the use of pegs and screws. To protect against breakage of the tray in case fixation is better on one condyle (causing bending of the tray), a stem is used. A long stem also gives lift-off protection. Another solution for the problem of a solid metal tray is to avoid a one-piece metal plate. This is the rationale behind the bicondylar tibia design, which is two condylar plates connected by a solid tibial polyethylene piece (Fig 7). Because flexible polyethylene is the bridge between the metal trays, compression on one plateau will not generate harmful tensile forces on the opposite one. A second feature of the bicondylar design is the 5 mm-deep boss, which is porous coated and press-fit into the bone. This box gives a large surface area of porous coating for fixation that is 'into bone.' It also gives lift-off protection to the condylar plate. The boss is press-fit into an undersized cylinder reamed into each tibial plateau. Each condyle has four smooth sharp pegs that provide rotational and shear stability. A single-piece plastic plateau snaps into both condyles. The single piece polyethylene connector gives the construct the advantages of a more flexible all-polyethylene implant, negating the teeter-totter effect while maintaining titanium surfaces for bone ingrowth. The condylar pieces are well fixed at surgery by the pegs and the press-fit of the box. The box provides a relatively large surface area of porous coating within the bone, greater than the porous coated pegs of the PCA, which have achieved good ingrowth.26 One potential drawback of the bosses is that they require reaming away good proximal tibial bone to implant them. When revision becomes necessary the tibial surface is usually recut at a depth of a few millimeters.34 This would negate the defect reamed away for the boss.



Patellar problems represent the greatest source of complication of modern total knee replacement.5,17,24,27 Modern design must attempt to improve the results with cemented patellae and especially with noncemented patellae, whicih generally have had poor results.

The authors' experience indicates that domed patellae, while producing good clinical results, have a high incidence of radiolucencies. In our series 21% of domed patellae demonstrate at least partial radiolucencies. In addition, the deformation and wear of all-polyethylene domed replacements is well documented.19 A conforming design provides larger contact areas with less wear and deformation. The additional constraint created by conformity of the patellofemoral joint has not been shown to lead to increased patellar loosening. Therefore, the recommended patella is a conforming all-polyethylene implant.

While a cementless patella would be a desirable option, the catastrophic failure of metal-backed patellae is well documented.3,31 Wear-through of the polyethylene at the periphery leading to metal-on-metal apposition, dissociation of the polyethylene from the metal backing, and fracture of the polyethylene at the edge of the metal skeleton have been reported. The problems of metal-backed patellae are yet to be solved. High forces are generated at the periphery where the polyethylene is thin and subject to compression and shear. In our series the percentage of patella replacements with tilting or slight subluxation is high, approaching 40%. Each of these tilted patellae have increased peripheral stresses. The combination of tilt and subluxation of the patella, even with successful, painless arthroplasties, does not bode well for the durability of currently used metal-backed patellae.

* * *

The last 20 years have provided valuable information as to what is and is not successful in the design of knee replacement. The superior results of early total knee designs have led us to conclude that a number of features, namely congruent articular geometry, thick tibial plastic, and all-plastic patellae represent the basis for superior designs available today. For cemented fixation a symmetric all-polyethylene tibia gives less expensive and equally good, if not superior, results. For cementless fixation a more flexible bicondylar design with into-bone fixation may represent the basic principles for future research to provide better bone ingrowth and reduce loosening. Finally, all-polyethylene and cemented patella is recommended.


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