47 Bearing Surfaces for Motion Control in Total Knee Arthroplasty
P. S. Walker
Summary
A goal of knee replacement is to achieve normal function and kinematics. One solution is to preserve all of the lig- aments and use compartmental components. However, if complete resurfacing is indicated, one or both of the cru- ciates is usually resected, and dished bearing surfaces are used to replace their function. Intercondylar cams can be added to ensure posterior femoral contacts in high flex- ion. In this paper, an alternative scheme is presented where the bearing surfaces are not simple combinations of radii but are based on converging or diverging medial- lateral bearing spacing as flexion proceeds. This scheme produces natural roll-back with flexion, more pro- nounced on the lateral side, while preserving the laxity which is characteristic of the normal knee. It is proposed that this design can produce natural knee kinematics which may result in close to normal function.
Introduction
A frequently stated goal of knee replacement, especially now that increasing numbers of young and active patients are requiring such treatments, is to allow for the restora- tion of normal function. How to achieve this is a matter of controversy, however. A unicompartmental replace- ment, performed with a small incision and with the cru- ciate ligaments preserved, can produce kinematics simi- lar to that of a normal knee. However, when the joint de- struction is more generalized and when the cruciates are abnormal, it is not clear which type of total knee will pro- vide the most normal function.The choices of total knees are extensive. The type most commonly used today is the condylar replacement, subdivided into the cruciate-re- taining (CR) designs,which usually have bearing surfaces of moderate to low conformity and where the posterior cruciate is preserved, and the posterior-stabilized (PS) designs, which usually have moderate to high conformi- ty and where a cam-post is used to replace the function of the posterior cruciate [14]. There are significant varia- tions in each category, which has been demonstrated by simple laboratory measurements of the AP and rotary
laxity, while both symmetric and asymmetric surface geometries are represented [12]. A recent variant of the surface replacement is the medial pivot knee (Wright Mfg.Co.,Memphis,TN),which is an attempt to reproduce the relative stability of the lateral and medial sides.
Mobile-bearing knees of the rotating platform type are extensively used, with the rationale of reducing wear and providing additional freedom of motion,which can allow for some surgical error as well as more functional rotation [5]. Some rotating platform designs even provide a medi- al pivot point, which again is more analogous to normal kinematic behavior.
Determining the outcome of knee designs using the standard evaluation methods [10] has shown that many design types produce similar results in terms of basic func- tion and alignments. Clinical follow-ups have similarly shown that many designs have a similar durability [6].
However, recent evaluation methods which analyzed the expectations and results of different patient groups have shown that,in many cases,the results of total knees are not uniformly satisfactory from a functional point of view [32].
Activities involving the control of motion and position at high flexion angles have been particularly problematic.
What design criteria for a total knee can be formulat- ed which will result in the most successful functional out- come? One possibility is that,‘after implantation, the mo- tion of the knee will be indistinguishable from that of the natural knee in its healthy state’. Taken to its extreme, this criterion would require customizing each patient, but for off-the-shelf knee systems, average natural knee motion would be the criterion. Such motion can be described as follows: As flexion proceeds, there is a progressive poste- rior displacement of the lateral femoral condyle, the me- dial femoral condyle moving only a small amount [11, 13, 15, 20, 25]. In rigid body terms, with the origins of femoral and tibial axis systems in the center of the knee, the motion consists of posterior displacement of the femur combined with internal tibial rotation. It is emphasized that the above describes the neutral path of motion.
About this neutral path, there is significant anteroposte- rior and rotational laxity [4].
In this chapter, different design options are discussed with the purpose of identifying what types of design are
the bearing surfaces themselves are used to guide the motion as well as allowing for laxity and stability.
Designs for Guiding or Controlling the Motion
In a typical surface replacement knee, the femoral bear- ing surfaces are convex in the frontal and sagittal planes, while the tibial bearing surfaces are concave. With this geometry, if there is a compressive force acting down the tibial axis, the femorotibial contact point (or area) will remain at the ‘bottom of the dish’ or ‘dwell point’ of the tibial surface at all angles of flexion. The contact point locations will change,however,if shear forces and torques are applied similar to the way that gives the natural knee laxity about the neutral path. The contact point locations can also be modified by a cam-post mechanism located in the intercondylar area. Usually, the cam-post is used to produce posterior displacement of the contact points in high flexion. Use of a cam-post produces a defined mo- tion, with no laxity. This distinction of laxity or defined motion can be thought of as either guiding or controlling the motion. Different possibilities for producing these two types of motion will now be discussed.
Anatomical Approach
Some of the earlier designs reproduced the shapes of the distal femur and proximal tibia (⊡ Fig. 47-1).
The requirement for the Leeds knee of Seedhom [24]
was preservation of all of the ligaments and an anatomi- cal surface geometry in relation to the attachment points of the ligaments, so that normal motion and stability could be achieved. In addition, low surface conformity
in several sizes, molded from representative examples of knee specimens. The tibial bearing surface was made to be concave in both the ML and AP directions, thus de- parting somewhat from anatomy, at least on the lateral side. Seedhom also anticipated that laxity of the tibial bearing surfaces would reduce the shear stresses on the fixation, but he was also concerned to minimize the con- tact stresses on the plastic in order to minimize wear.This is a trade-off which is still a factor in today’s designs.
Ewald [9] took a similar approach of reproducing the anatomical bearing surfaces of the distal femur with a metallic shell, not unlike the shape of the MGH (Massa- chusetts General Hospital) hemiarthroplasty femoral condyle. However, there was one important difference compared with the Seedhom approach. Ewald proposed that because the cruciates and the menisci would not be viable in the arthritic knee, the combination of the femoral and tibial bearing surfaces would have to repro- duce their function in terms of providing the necessary laxity and stability. Furthermore, the tibial surfaces should be formed so that the rolling,sliding,and pivoting movements would actually be guided. Ewald proposed not rigid motion, but a guided neutral path with laxity about this path. To achieve this, one of the features of the tibial bearing surface was the divergence of the low points on the tibial surface from anterior to posterior, such that as flexion proceeded, the femoral condyles would locate more posteriorly and also the relative roll-backs of the lat- eral and medial sides would produce an axial rotation.
The only feature that differs from motion data available today was that the medial femoral condyle was said to pivot about the lateral, but that may have reflected early flexion when initial roll-back occurs on the medial side.
Geometrical Approach
In the early 1970s, there were a number of condylar de- signs which relied on geometrical shapes to control the motion. These approaches ranged from the shallow geometry of Townley to the cylinder-in-trough of Free- man and Swanson, to mention only two [14]. These op- posite approaches were intended, as above, to utilize the existing cruciate ligaments as an important part of pro- viding stability, or to eliminate these ligaments and rely on the reproducible geometrical surfaces of the implant for stability. In general, it was found that low inherent stability required accurate balancing of the ligaments;
otherwise an incidence of instability and failure would occur, whereas excessive constraint resulted in a high loosening rate, especially when the tibial fixation fea- tures were inadequate. An approach to solving these particular problems made use of partially conforming
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⊡ Fig. 47-1. Examples of total knees where the bearing surfaces of the femoral component are based on anatomical shapes. Left: the tibial sur- faces are designed for retention of the cruciates [24]; right: the tibial sur- faces replace the function of the cruciates and menisci [9]
bearing surfaces to provide stability but allow laxity [26].
This concept was incorporated into the original total condylar knee, which has been clinically successful over the long term, with few mechanical problems other than a limited range of flexion. The double-dishing of the bearing surfaces in the frontal and sagittal planes pro- vided the required stability and laxity without requiring the cruciate ligaments (⊡ Fig. 47-2).
It is emphasized that such bearing surfaces were in- tended to allow the multiplicity of joint motions which oc- cur in vivo, without providing a definite motion path oth- er than containing the femur in the tibial dish within rea- sonable bounds defined by the amount of tibial dishing.
Such bearing surfaces have been used in numerous de- signs for cruciate resection, for retention of the posterior cruciate,and even for retention of both cruciates.Many re- finements have been made to the radii of the bearing sur- faces to optimize the constraint characteristics and to minimize the surface and sub-surface contact stresses [23].
Intercondylar Cams and Other Mechanisms
The earliest examples of total knees which achieved guid- ed motion by intercondylar cams were the Kinematic/
Kinemax Stabilizer and the Insall-Burstein Posterior Sta- bilized, designed in the late 1970s [14]. In the former de- sign, the objective was to guide the posterior displace- ment of the contact points throughout the whole flexion range. This was achieved by a femoral cam whose center was offset from the center of the femoral condylar bear- ing surfaces.In the PS design,the objective was to produce posterior displacement and to prevent anterior subluxa- tion of the femur in late flexion. To achieve this, the femoral cam had a small radius, located posteriorly at a large eccentricity. Drawbacks of such designs were that the internal-external rotation was limited, and there was no mechanism for roll-forward as the knee was extend-
ed, although the femur would tend to return to the bot- tom of the tibial dish. Successful long-term results have been achieved with these designs. Recently, the design of intercondylar cams has been systematically analyzed to determine if improvements could be made to the cam- post shapes [29](⊡ Figs. 47-3 and 47-4).
Figure 47-3 shows a cam design where both roll-back and roll-forward are produced, using a convex tibial cam shape and a shallow femoral housing. Figure 47-4 shows a saddle arrangement on the tibia into which a shallow femoral convex surface is housed [28]. This produces controlled motion throughout the entire range of flexion, including roll-back in flexion and roll-forward in exten- sion. It is noted that the internal-external limitation can be addressed by using rounded cam shapes or by locating the plastic on a rotating platform.
frontally dished frontally shallow bottom of the dish
⊡ Fig. 47-2. A parametric description of contemporary total knees with partially conforming bearing surfaces. Varying the radii as shown pro- duces major differences in laxity and stability. BS bearing spacing; ROF ra- dius outer femur; RIF radius inner femur; ROT radius outer tibia; RIT radius inner tibia, RPSF radius posterior superior femur; RDF radius distal femur;
PDTA posterior-distal transition angle, i.e., angle between the large distal radius and the smaller posterior radius; RPT radius posterior tibia; RAT ra- dius anterior tibia
⊡ Fig. 47-3. A computer-generated intercondylar cam where the func- tions of the anterior cruciate and posterior cruciate are reproduced in early (right) and late (left) flexion [29]
⊡ Fig. 47-4. A computer-generated intercondylar cam which provides anterior-posterior motion control throughout the entire flexion range.
There is some posterior displacement of the femoral component with flex- ion [28]
and flexion have been achieved discretely.The TRAC knee (Biomet, Warsaw, IN), designed by Draganich and Pot- tenger [8], uses an inner pair of bearing surfaces which contact from 5° hyperextension to 8° of flexion with an anterior dwell point,and an outer pair of bearing surfaces which contact throughout the remaining flexion range, with a more posterior dwell point.The transition from the first pair to the second is obtained by an intercondylar cam. The plastic component itself is pivoted on a smooth metal tibial plate, resembling the LCS rotating platform design. The Kyocera Bi-Surface knee [1] addresses the goal of achieving a very high range of flexion with unre- stricted internal tibial rotation. For the major part of the flexion range,the knee behaves as a standard condylar re- placement with moderately conforming bearing surfaces.
Beyond that, the load is transferred to a spherical surface protruding posteriorly from the femoral intercondylar region, contacting within a spherical depression at the posterior of the plastic tibial component. There is some similarity to the Variable Axis total knee [19]. The Medial Pivot knee (Wright Mfg, Memphis TN) is a slightly differ- ent concept, in that there are no guide surfaces or cams per se. Instead, the tibial bearing surfaces are shaped so that normal knee motion is possible; the medial side re- mains in the same position during flexion but the lateral femoral condyle can displace posteriorly with flexion.
However,the design does not actively force this motion to occur, but allows each knee to move individually.
Guiding Surfaces
The concept of guiding surfaces is similar to that ad- vanced by Ewald [9], namely to guide the motion of the knee by interaction of the major bearing surfaces them- selves, but allowing for laxity to occur about the guided neutral path. It is possible that the natural knee uses such a strategy to enhance the posterior displacement of the lateral femoral contact area while maintaining an almost constant medial contact area.
Observation shows that the high points of the femoral condyles are almost parallel through most of the range of flexion [27]. However at higher flexion, where most of the internal tibial rotation occurs,the lateral femoral condyle appears to converge inwards (⊡ Fig. 47-5) This will assist an inward rotation of the lateral femoral condyle into a lower area of the lateral tibial plateau. Such geometry is not inconsistent with measurements made of the bear- ings surfaces by Ateshian et al. [2, 3] and by Cohen et al.
[7] using stereophotogrammetry. Such features could be reproduced in a total knee replacement.
In a standard condylar replacement,the bearing spac- ing (Fig. 47-2) of the femoral condyles is constant around
the sagittal contour, while the tibial surfaces are usually dished in the sagittal plane. In the absence of AP shear and axial torque, the femoral-tibial contact points will be located at the bottom of the tibial dish (dwell points) at all angles of flexion, so as to minimize the potential ener- gy of the system [26]. This is a very important principle of condylar replacement knees where the tibial surfaces are dished.However,if the bearing spacing of the femoral component converges with the flexion angle, a tibial sur- face can be generated where the low points change in an AP direction with flexion. Such a tibial surface was gen- erated in the computer so that the contact points on both the lateral and medial sides would displace posteriorly with flexion (⊡ Fig. 47-6).
The tibial surface was generated by embedding the femoral component into a tibial block in successive posi- tions with progressive roll-back.After successive Boolean operations (subtracting the imprint of the femoral condyles), the tibial surface was generated. Due to the convergence of the bearing spacing of the femoral component, as the femur was progressively flexed, the effective bottom of the tibial dish, or the position of least potential energy, also moved posteriorly with flexion.
To more closely reproduce natural knee motion where there is posterior displacement of the lateral femoral condyle with flexion, equivalent to a pivoting about the medial condyle, a different type of surface was formed [30]. The principle is illustrated in ⊡ Fig. 47-7.
The femoral condyles are represented by spherical sur- faces. On the left is shown a medial pivot knee where the femoral component has been rotated about the medial condyle and an imprint has been formed in the tibial
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150 deg flexion
75 deg flexion
⊡ Fig. 47-5. Mechanisms which may occur in the natural knee for pro- ducing internal tibial rotation with flexion, pivoting about the medial condyle, by convergence of the lateral femoral condyle in high flexion, and a decrease in radius of curvature
surface. Such surfaces will allow medial pivoting to occur but will not guide such pivoting. On the right is shown a femoral surface where the bearing spacing moves progres- sively inwards with flexion.Now,the tibial surface is formed by the femoral imprint with convergence. This means that at any angle of flexion there is a preferred position of the lateral femoral condyle based on the bottom of the dish.
In this sense, the motion is guided (⊡ Fig. 47-8 and 47-9).
Figure 47-8 shows a lateral femoral condyle where there is convergence of the bearing spacing from 75° to 150° of flexion. The imprint of the tibial surface clearly converges inwards with flexion. The medial tibial surface is merely the composite imprint formed from 0° to 150° of flexion,pivoting about the medial side.Figure 47-9 shows a knee replacement designed using this lateral converg-
ing geometry. As the femur is flexed, the lateral femoral condyle displaces posteriorly, along with an external rotation (equivalent to an internal tibial rotation).
One factor not taken into consideration in these con- verging models is the behavior of the contact areas if the femur was displaced from any neutral position. If the tib- ial surface was generated by the actual femoral compo- nent as described, on deviating from the neutral path in either AP displacement or internal-external rotation, edge loading of the plastic would occur. To account for this factor, a ‘generating femoral component’ needs to be defined which has larger radii of curvature in frontal and sagittal planes than the actual femoral component. This would allow clearance which would then provide laxity about any neutral position without edge loading.
⊡ Fig. 47-8. A particular example of a lateral femoral condyle where the bearing surface converges after 75° of flexion. This surface is used to gen- erate a tibial surface. The interaction guides the femur to pivot about the medial condyle as flexion proceeds from 75° to maximum
⊡ Fig. 47-9. Femoral and tibial surfaces generated using the principle of Fig. 8. The posterior displacement of the lateral femoral condyle and the medial pivoting are indicated by arrows
⊡ Fig. 47-7. Basic principle of guiding the motion during flexion by the shape of the femoral condyles and a generated tibial surface: Left:
there is no convergence and the tib- ial surface is generated by rotation about the medial condyle. There is no fixed neutral motion path. Right:
convergence occurs by a progres- sive reduction of the lateral bearing spacing with flexion. This results in a unique neutral motion path con-
sisting of posterior displacement of the lateral femoral condyle and pivoting about the medial side
⊡ Fig. 47-6. A femoral component with converging lateral and medial condyles has been used to generate a tibial surface which produces pos- terior femoral displacement with flexion
Once a knee replacement of any type has been formulat- ed, including a guided-motion knee as described above, the design needs to be evaluated prior to clinical applica- tion.What is required are laboratory tests which will pre- dict the behavior in a realistic functional situation. For evaluating a guided-motion knee, the paramount con- cern is the kinematics. The total knee has been designed to produce a particular type of motion, which will result in proposed functional advantages. An obvious example is a posterior-stabilized knee which is intended to cause the femoral contact points to displace posteriorly in high flexion.This is intended to provide a greater range of flex- ion by reducing posterior impingements, and by increas- ing the lever arm of the quadriceps for improved control at those high flexion angles. In general, the evidence is that the goals are realized, although this is not to say that other designs do not provide similar advantages.
However,if a knee replacement is to be designed to re- produce normal function, as far as allowed by the condi- tion of the patient, then a more general design criterion is required which can be tested mechanically. The pro- posal made in the Introduction is that “after implantation of the knee replacement,the motion of the knee in any ac- tivity is indistinguishable from the motion of the knee in its healthy intact state.” The motion can be defined as the relative motion of the femur and the tibia, measured with reference to Cartesian coordinate systems in each bone.
One inherent requirement is that the muscle activity after rehabilitation will resemble that of the knee in its healthy intact state, because knee motion will depend upon muscle action as well as upon external forces. Fur- thermore, the criterion would need to be tested in the patient, not in the laboratory.
To consider motion without the effect of muscles, a criterion can be proposed which takes account of only the inherent constraint characteristics of the passive ele- ments of the knee itself,including the implant.In this case the criterion would be that “after implantation of the knee
from that of the knee in its healthy intact state.” This means that if sets of external forces and moments are ap- plied to an intact knee, and then to the knee after im- plantation, at a range of flexion angles, the resulting dis- placements and rotations will be indistinguishable.In or- der to test such a criterion, one method would be to use a cadaver with all ligaments and menisci preserved, mea- sure the constraint characteristics, then implant the knee replacement and again measure the constraint. This test would use the cadaver as its own control, and would also reproduce the effect of the various passive structures.The test would be applicable to a design where one or both cruciates was preserved, or where both were resected.
Such a test was carried out on a plastic model of a guided-surface knee resembling the design shown in Fig.
47-9.Cadaveric legs were used,and the femur was mount- ed vertically, with loads applied to the quadriceps and hamstrings, twice the force in the former than in the lat- ter (⊡ Fig. 47-10). The leg was flexed from zero to maxi- mum and fluoroscopic images were taken. Markers in the femoral and tibial components allowed the contact points of the lateral and medial femoral condyles on the tibial plateau to be measured. The guided-surface knee was compared with a standard symmetric posterior cruciate- retaining knee. On average, from six specimens, it was found that the guided surfaces did produce more stabili- ty on the medial side and more posterior displacement on the lateral side. These tests were restricted to knee re- placements which preserved the posterior cruciate.
Other tests could be carried out which do not require the use of specimens. One of the simplest is constraint testing [31] where a compressive load is applied to the im- plant components and then a shear force is applied, the shear displacement being measured. Alternatively, a torque can be applied and the rotation measured. These tests are currently required by the FDA for new knee de- signs, as they provide a basic measure of the constraint [12]. However, the tests do not predict performance, in that the results are independent of whether the cruciates
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0 DEG Flexion 60 90 120 150
Guided Med Symm Med
Symm Lat Guided Lat 0
5
10
Posterior Displacement mm 15
Femur Clamped
⊡ Fig. 47-10. One of the methods that can be used to evaluate a guid- ed-motion knee. Compared with a standard symmetrical knee, the guided-motion knee with a con- verging lateral femoral condyle has produced increased lateral posterior displacement and reduced medial displacement. (Study coordinated by Mr. Scott Steffensmeier; data re- produced courtesy of Zimmer, Inc., Warsaw, IN, USA)
are preserved or resected,and there is no standard against which to compare the result. For designs such as rotating platforms, for instance, the rotational constraint is zero except for friction.Another test which could be applied is to use a modified Oxford rig, where the lower extremity is loaded with body weight and muscle forces are then used to balance the knee at a required flexion angle [18].
In testing a knee replacement, the criterion here could be to compare the three-dimensional motion of the intact with that of the replaced knee, with particular regard to posterior displacement and internal-external rotation. A robot can also be used with a similar criterion, with the added element of determining ligament forces [16].
From a practical point of view, guided-surface knees must be able to guide the motion in a way similar to that of standard knee replacements which have a distinct bot- tom of the dish for stability. For example, it would be un- satisfactory if a small amount of deformation or wear were to seriously disrupt the motion the surfaces were intended to provide. This consideration was tested using a computer model which approximated the inherent stability of surface designs by calculating the amount of material displaced from the neutral position, for defined amounts of anterior-posterior displacement or internal- external rotation [30].
Computer modeling of the entire knee assembly should be mentioned here, because it is a method of producing standard tests which are not subject to the variables of cadaveric tests, such as the dependence upon surgical placement and other factors. In principle, com- puter models can reproduce knees with different bone and ligament geometries and properties, as well as surgi- cal placements, thus providing a parametric evaluation under a range of test conditions. However, one weak link for the intact knee is that modeling the meniscus is prob- lematic.Nevertheless,a validated computer model should be regarded as ultimately the most satisfactory way of evaluating knee replacements for motion characteristics, as well as for other factors.
Finally, on a theoretical basis, can any implanted sur- faces satisfy either of the criteria stated above? It may seem self-evident that in a particular knee, if surfaces replicat- ing the natural knee were implanted,and the cruciates and menisci were preserved, the criteria would be satisfied.
However, it is not practical at present to retain the menis- ci and its attachments,in which case their stabilizing func- tion would have to be reproduced in a modification of the tibial bearing surfaces. Likewise, if one or both of the cru- ciates were resected,the tibial surface would have to be fur- ther modified. One factor which would make it difficult to precisely reproduce normal constraint is the friction be- tween the bearing surfaces.In the natural knee the friction is close to zero, whereas in a metal-polyethylene knee the friction is such as to produce considerable modification of contact point locations [17, 22].
In conclusion, while for the older, less active patient standard designs of today may be adequate from a func- tional point of view, for the more demanding patients, in future it may be shown that functional advantages will be obtained from an approach where normal constraint is reproduced, either by a guided-surface approach or by some other means.
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