48 The High-Performance Knee
M. D. Ries, J. Bellemans, J. Victor
Summary
Although total knee arthroplasty is effective in relieving knee pain and improving ambulatory function, the kine- matics after knee replacement are quite different from those of the normal knee. During flexion of the normal knee,the lateral femoral condyle moves posteriorly,caus- ing femoral external rotation. After knee replacement
“paradoxical” motion typically occurs. The femur is dis- placed posteriorly during extension and moves anterior- ly as the knee flexes. The cam-and-post mechanism of a posterior cruciate-substituting knee design limits anteri- or femoral translation so that the kinematics are less ab- normal than with a posterior cruciate-retaining design.
Additional modifications in the conformity of each tibial plateau and the cam-and-post geometry can further guide roll-back of the lateral more than the medial femoral condyle, causing femoral external rotation dur- ing knee flexion. Guided motion designs that reproduce more normal knee kinematics may result in improved range of motion and knee function following total knee arthroplasty.
Introduction
Total knee arthroplasty is a very successful treatment for arthrosis of the knee: Pain and function are improved, and most patients resume routine functional activities such as walking, transferring, and stair climbing, as well as low-impact recreational activities including golf, swimming, and use of a bicycle. However, the kinematics of the replaced knee are quite different from those of the normal knee.
The change in kinematics after knee replacement is not unexpected, since many of the anatomical structures present in the normal knee are not retained in the re- placed knee. During total knee arthroplasty, one or both cruciate ligaments are removed,both menisci are excised, the orientation of the joint line is changed from approxi- mately 3° of varus to one which is perpendicular to the mechanical axis, and the size and geometry of articulat- ing surfaces are altered. With appropriate surgical tech-
niques, implant design, and rehabilitation, satisfactory range of motion and stability are usually achieved to per- mit routine functional activities. However, patients who achieve greater range of motion may also have a compo- nent of flexion instability.Symptoms of instability include vague pain, intermittent effusions, and occasional giving way or “clunking” sensations. This is more often reported in posterior cruciate-retaining knee designs, particularly those with relatively non-conforming articular surfaces which permit more anteroposterior and rotational move- ments between the femur and tibia [1].
Kinematics of the Normal Knee
During flexion the lateral femoral condyle moves poste- riorly while the position of the medial femoral condyle is relatively stationary. This produces relative internal tibial rotation or external femoral rotation during knee flexion.
At high degrees of flexion the lateral femoral condyle may displace posteriorly to the point that it is partially sub- luxed from the tibial surface. The combined lateral femoral roll-back and femoral external rotation are es- sential to permit large degrees of knee flexion.
During early flexion (0°-30°) the patella is laterally displaced and tilted relative to the trochlear groove [2, 3].
As patellofemoral compressive forces increase during flexion, the patella becomes centralized in the trochlear groove. Also, during flexion, internal tibial rotation caus- es a relative medialization of the tibial tubercle which may facilitate patellar tracking.
Kinematics of the Replaced Knee
In vivo fluoroscopic studies have consistently demon- strated “paradoxical” motion in which the femur moves anteriorly during flexion and posteriorly during exten- sion [4].The lack of an anterior cruciate ligament permits relative anterior subluxation of the tibia,when the knee is in an extended position [5]. As the knee flexes, the femur moves “paradoxically”from a posterior to an anterior po- sition on the tibia. Anterior femoral translation may lim-
tion is variable after total knee arthroplasty, although ex- ternal tibial rotation is observed more frequently than in- ternal tibial rotation during knee flexion [4]. However, simultaneous MRI images of the tibiofemoral and patellofemoral joints of total knee arthroplasty patients indicate that external tibial rotation is associated with lat- eral patellar tilt and translation [6]. This suggests that the abnormal tibiofemoral kinematics after total knee arthroplasty may adversely affect patellar tracking,which can contribute to anterior knee pain, patellar subluxa- tion, and wear.
Tibial external rotation may also limit knee flexion, since excursion of the extensor mechanism may not be sufficient to permit full flexion as it does in the normal knee with tibial internal rotation and lateral femoral roll- back.Posterior-stabilized total knee arthroplasty patients appear to have less abnormal tibiofemoral kinematics than patients with posterior cruciate-retaining total knee arthroplasties [4]. However, the cam-and-post mecha- nism does not control rotation. As a result, knee flexion may still be limited and lateral patellar tracking can result if tibial external rotation occurs during knee flexion.
Relationship Between Kinematics and Range of Motion
Although total knee arthroplasty results in predictable pain relief and patient satisfaction, range of motion be- yond 120° is not consistently achieved and the kinematic behavior of the knee is markedly altered [4, 6, 7]. The nor- mal knee flexes to over 140°. If the knee functioned as a simple hinge mechanism, flexion to such a high degree would be limited by posterior impingement of the bony or soft-tissue structures (⊡ Fig. 48-1). Posterior impingement between the femur and the tibia during high flexion is lim- ited by the amount of posterior offset between the femoral condyles and posterior femoral cortex (⊡ Fig. 48-2) [8].
The medial condyle is larger than the lateral condyle.
Therefore, the posterior offset of the medial condyle is greater than that of the lateral condyle. During knee flex- ion the position of the medial condyle on the tibial plateau is relatively constant, similar to a ball-and-socket mech- anism. If the same mechanism occurred between the lat- eral condyle and tibial plateau, posterior impingement between the lateral femur and tibia would occur during knee flexion (⊡ Fig. 48-3). However, the lateral condyle moves posteriorly during knee flexion.At high degrees of flexion, the lateral condyle nearly subluxes off the poste- rior tibial plateau, and posterior impingement between the femur and tibia does not occur (⊡ Fig. 48-4). Greater posterior translation of the lateral femoral condyle com- pared with the medial femoral condyle is associated with
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⊡ Fig. 48-2. Posterior offset is the distance from posterior femoral condyle to femoral cortex (between arrows). With larger posterior offset, greater knee flexion can be achieved before posterior impingement oc- curs
⊡ Fig. 48-3a, b. a Sagittal view of the lateral femoral condyle articulat- ing with the lateral tibial plateau; b the medial femoral condyle articulat- ing with the medial tibial plateau. Since the medial femoral condyle is larg- er than the lateral condyle, posterior offset is greater medially than later- ally. If there is no posterior movement of the femur on the tibia during knee flexion, posterior impingement occurs between the lateral femoral cortex and tibial plateau, limiting knee flexion (arrow)
⊡ Fig. 48-1. Knee in high flexion. Flexion is limited by posterior bony or soft-tissue impingement (arrow)
a b
external rotation of the femur on the tibia. Femoral ex- ternal rotation during knee flexion, or relative internal tibial rotation, reduces the Q angle, which may also facil- itate patellar tracking in deep flexion.
Full flexion is rarely achieved after total knee arthro- plasty; this may be related to failure of the replaced knee to reproduce normal knee kinematics. The femoral condyles of the replaced knee are typically posteriorly translated on the tibia during knee extension as a result of absence of the anterior cruciate ligament (⊡ Fig. 48-5).Dur- ing flexion, the femur moves anteriorly on the tibia (para- doxical motion) which causes posterior impingement and limited knee flexion (⊡ Fig. 48-6). Knee kinematics are less abnormal with posterior cruciate-substituting than with posterior cruciate-retaining TKA since the cam-and-post mechanism of a posterior cruciate-substituting TKA lim- its anterior translation of the femur. However, few studies demonstrate significantly better flexion with posterior
cruciate-substituting compared with posterior cruciate- retaining knee designs despite the advantage of posterior roll-back caused by the cam-and-post mechanism [9].
This may occur because the cam-and-post mechanism does not control rotation between the femur and tibia.
Combined roll-back and external rotation of the fe- mur on the tibia or posterior translation of the lateral more than the medial femoral condyle, which occurs in the normal knee, may also improve quadriceps efficien- cy.The anterior translation of the tibia moves the tibial tu- bercle anteriorly,which increases the quadriceps moment arm in deep flexion. However, most patients with current total knee designs do not typically regain quadriceps strength adequate to get up from a seated position with- out use of their upper extremities.
Effect of Bearing Surface Geometry on Kinematics
Many tibial component surfaces are dished or concave in order to reduce UHMWPE stresses and provide addi- tional joint stability. However, posterior roll-back of the femoral condyle on a curved tibial surface is limited since the femoral condyle must push against the posterior lip of the tibial insert (⊡ Fig. 48-7a).With removal of the pos- terior lip,posterior roll-back can occur more easily (⊡ Fig.
48-7b). In order for the design of the bearing surface to cause guided joint motion, the joint compressive force should be directed to cause femoral roll-back. If the tibial surface is neutrally sloped,the joint compressive force oc- curs along the axis of the tibia (⊡ Fig. 48-7c). However, if the tibial plateau is sloped posteriorly, the joint compres- sive force is obliquely oriented, which includes a joint compressive force vector along the axis of the tibia as well as a force vector perpendicular to the axis of the tibia (⊡ Fig. 48-7d). The latter force vector can drive posterior movement of the femoral condyle.
⊡ Fig. 48-5. Following total knee arthroplasty, the tibia is subluxed anteriorly during knee extension as a result of absence of the anterior cruciate ligament
⊡ Fig. 48-6. During knee flexion, the femur moves anteriorly (para- doxical motion) which limits knee flexion as a result of posterior im- pingement (arrow)
⊡ Fig. 48-4a, b. a The lateral femoral condyle moves posteriorly relative to the tibia (arrows) so that posterior im- pingement does not occur. b As a re- sult of posterior movement of the lat- eral femoral condyle, flexion is in- creased until posterior impingement
occurs a b
Using differential geometries of the medial and lateral tibial plateau surfaces, selective guided posterior roll- back of the lateral plateau can be achieved (⊡ Fig. 48-7e).
If the medial plateau is dished or concave and the lateral plateau is posteriorly sloped and without a posterior lip, the medial femoral condyle will be relatively constrained in the tibial plateau while the lateral femoral condyle will move posteriorly during knee flexion (⊡ Fig. 48-7f).
Guided motion of a posterior-stabilized knee can also be influenced by the design of the cam-and-post mecha- nism.Conventional cam-and-post designs are symmetric
in cross-section and intended to allow unrestricted rota- tion between the femoral and tibial surfaces while still providing posterior roll-back of the femoral condyles (⊡ Fig. 48-8a). However, a conical cam or post can force more roll-back of one condyle than the other. For a coni- cal cam-and-post mechanism which is wider laterally than medially, more roll-back of the lateral than the me- dial plateau occurs (⊡ Fig. 48-8b).
In order to achieve the goal of relatively normal kine- matic function after total knee arthroplasty, designs and surgical techniques must produce greater posterior trans-
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medial plateau lateral plateau
lateral medial
⊡ Fig. 48-7a-f. a A concave tibial plateau. Posterior roll-back of the femoral condyle requires that the condyle roll uphill. b Without the posterior lip, posterior roll-back can occur more easily. c Neutrally sloped tibial plateau; joint compressive force is perpendicular to the tibial plateau. d Posteriorly sloped tibial plateau surface; joint compressive force is obliquely oriented, which provides a posteriorly directed force vector and drives the femoral condyle posteriorly. e Medial tibial plateau shown in a combined with lateral tibial plateau surface shown in d. f Guided motion caused by variable slope and conformity of articular surface shown in e. The lateral femoral condyle moves posteriorly as the knee flexes, while the medial femoral condyle po- sition remains relatively stationary
a
c
d
b
e
f
lation of the lateral than the medial femoral condyle dur- ing knee flexion in a controlled manner. This may be ac- complished by use of an obliquely oriented cam-and-post mechanism and a more posterior slope of the lateral than of the medial tibial plateau. The greater slope of the later- al plateau allows the femoral condyle to roll down the posterior lateral slope during knee flexion.
Design of a High-Performance Knee Using a Virtual Knee Computer Model
One great benefit of using an analytical model to evalu- ate the functional performance of a total knee design is that the test conditions do not change between experi- ments.Therefore,the functional behavior of a new design feature may be isolated by simulating subsequent design trials.
The method utilizes three-dimensional computer modeling of a lower limb mounted to a virtual Oxford knee testing rig in a physics-based dynamic software pro- gram. The Oxford rig is a modified slider crank mecha- nism where the hip is analogous to the slider, the femur is the connecting rod, and the tibia is the crank. The knee is then simulated by the joint between the crank and the connecting rod. This type of setup allows the experi-
menter to simulate different activities on the virtual Ox- ford rig - stair climbing, deep knee bend, gait, etc. – and to validate the results by comparison with empirical lab- oratory data from a physical Oxford rig.
In the virtual simulation, anatomically accurate fe- mur, tibia, and patella 3-D bone models with the desired anthropometrics are integrated with the model of the Ox- ford rig and serve as guides for joint line position, scale, and virtual ligament/muscle attachment points.The bone models also serve as geometric references for the virtual implantation of patellar, tibial, and femoral prosthesis 3- D models. Attached to the bone models are virtual mus- cles and ligaments that serve to drive and constrain the model during simulation.
The constraints imparted on the simulation include hip/ankle joint, ligament/capsule tissue, and the intrinsic constraints imparted by the geometric conformity of the TKA models and their respective stick/slip friction and stiffness characteristics. The hip joint is modeled as a rev- olute joint with 1° of rotational freedom that is parallel to the flexion axis of the knee. The ankle joint is modeled as a combination of several joints that combine to allow free translation in the M/L direction, as well as internal-exter- nal rotation (along axis of tibia), flexion, and varus/valgus rotations. All other degrees of freedom in the ankle are fixed. The ligament constraints are modeled as spring/damper elements and are attached to the virtual bones at their respective anatomical locations with the aid of the information derived from Blankevoort and Huskins [10]. The mechanical properties of the tissues were ob- tained from Woo et al.[11].The LCL is simulated with a sin- gle constraint element, and the MCL is modeled with two separate constraint elements to simulate the anterior and posterior fibers. For cruciate-retaining knees, a PCL ele- ment is added and attached at the respective anatomical locations. The final soft-tissue constraint is the general capsule force, which simulates the general soft-tissue re- actions of the knee capsule.This force is directed such that it draws the femur and tibia together in a similar manner to the capsular tissues in the actual knee.
The constraint characteristics imparted by the con- forming geometry of the TKA models vary with each de- sign. This is a variable that can be modified by the exper- imenter to assess the stability imparted by different geo- metric conformities. Other contact-oriented constraints include stick/slip friction and local deformation of the bearing surfaces. These friction and deformation con- straints are determined with an algorithm that models the polyethylene surfaces as a bed of springs with the stiffness characteristics of the polyethylene bearing ma- terial.The method allows for intermittent contact,contact pressure,and center of pressure determination.The algo- rithm is used to model the patellofemoral contact, the tibiofemoral contact, and quadriceps tendon femoral conformity.
⊡ Fig. 48-8a, b. a Rectangular cam and post causes similar medial and lateral femoral roll-back. b Conical cam causes more lateral than medial femoral roll-back
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b
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Flexion (deg) 0
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Arc Center Position (mm)
(-) posterior
Normal Knee PS Knee HP Knee
0 30 60 90 120 150
Flexion (deg) 5
0
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Arc Center Position (mm)
(+) anterior
(-) posterior
Normal Knee PS Knee HP Knee
0 30 60 90 120 150
Flexion (deg) 10
8 6 4 2 0 -2 -4 -6 -8
Axial Rotation (deg)
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(-) internal
Normal Knee PS Knee HP Knee
0 30 60 90 120 150
⊡ Fig. 48-9a-c. a Position of the medial femoral condyle arc center during knee flexion for the normal knee, conventional posterior-stabi- lized (PS) TKA, and high-performance (HP) TKA obtained from computer simulation. b Position of the lateral femoral condyle arc center during knee flexion for the normal knee, PS TKA, and HP TKA obtained from com- puter simulation. c Femoral rotation during knee flexion for the normal knee, PS TKA, and HP TKA obtained from computer simulation a
b
c
The driving elements in the model are the quadriceps and hamstrings muscle forces.The quadriceps muscle at- taches to the quadriceps tendon and is discretized into six components which conform to the distal head of the fe- mur or anterior flange of the TKA, permitting proper force transmission to the femur and patella. The patella is attached to the tibia tubercle through a patellar tendon force which conforms to the tubercle and TKA insert component. The simulation is driven by a controlled ac- tuator arrangement similar to the physical machine. A closed-loop controller is used to apply a tension to the quadriceps and hamstring muscles to match a prescribed knee flexion vs. time profile. No large antagonistic forces are modeled. Ground reaction forces are applied as varus/valgus forces and internal/external torques during the cycle using time history data derived from force plate experiments [12].
In the functional evaluation of a new implant design, two main activities are simulated: a complete cycle gait, and a 0° – 160° – 0° – 160° double deep knee bend cycle.
The double cycle is performed so as to capture the iner- tial loading conditions at full flexion (the second 0°).Data are reported via graphical animations and numerical re- sults. Usually, only the TKA components and soft tissues are displayed during the animations. Scaled force vectors for each tissue force are displayed on the model during animation, as are center-of-pressure locators (with scal- ing force vectors) for each tibiofemoral and patello- femoral contact zone. Eighty-four data time-histories are reported for each simulation. These include patello- femoral kinematics,tibiofemoral kinematics,tissue loads and locations, actuator loads and locations, contact forces, center-of-pressure locations and contact area, in- terface forces and locations, and all applied forces and lo- cations. Three different reference frames are utilized for the reporting of the kinematic and kinetic data. These reference frames are rigidly attached to the femur, tibia, and patella, respectively, at the interface where the im- plant models meet the bone models. This is done so that interface forces can be easily obtained by resolving all forces acting on an implant component to the corre- sponding reference frame located at the interface. In the simulation, kinematic and kinetic data are typically re- ported relative to the reference frame fixed in the tibia.
Kinematic and kinetic data for the patella can also be re- ported from the reference frame in the femur.Component orientation is reported using a three-cylindric model of knee motion similar to Hefzy and Grood [13] or a variety of other methods.
The computer model of the Oxford rig has been vali- dated in a variety of ways including mechanical tests, ca- daver tests, and live subject tests. Mechanical tests have been used to tune the performance of the contact force al- gorithm for both the shear component (friction/stiction) of the force and the normal component of the force. The
shear component of the contact force was tuned by com- paring the Instron test machine results for the ASTM 1223 component laxity test with a virtual model of the laxity test. The normal component of the forces was tuned by comparing the resulting contact area of virtual compres- sion tests with the contact areas reported from Instron- Fuji film tests.With the current set of contact parameters, the algorithm has consistently delivered results within 10% of mechanical tests [14].Cadaver tests have been used to tune the soft tissues (attachment locations and me- chanical properties) and the system controller function comparing the virtual Oxford rig results with the results of the physical machine.
Results obtained with the computer model are similar to the kinematic patterns observed after TKA with fluoro- scopic studies. For a posterior-stabilized knee, the femur moves anteriorly during knee flexion (paradoxical motion) until the cam-and-post mechanism engages. Following cam-and-post engagement, the femur moves posteriorly.
However, with the modifications introduced into the HP bearing surface and cam-and-post mechanism shown in Figs.48-7 and 48-8,a much more normal kinematic pattern occurs. The position of the medial femoral condyle is rela- tively stationary during knee flexion while the lateral femoral condyle moves posteriorly,very similar to the nor- mal knee (⊡ Fig. 48-9a and b). The differential positions of the femoral condyles are associated with femoral external rotation during knee flexion which follows the pattern of the normal knee (⊡ Fig. 48-9c). The computer simulation indicates that by modifications in the articular geometry and cam-post mechanism,guided motion can be achieved to more closely reproduce normal kinematics after TKA.
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