6 Gait Analysis and Total Knee Replacement
T.P. Andriacchi, C.O. Dyrby
Summary
The relationship between ambulatory function and the biomechanics of the knee was examined during activities of daily living including walking, stair climbing, and squatting into deep flexion. Each activity was character- ized by a unique relationship between the primary mo- tion (flexion) and secondary movements (including in- ternal-external rotation, anterior-posterior displace- ment) that occur during the weight-bearing and non-weight-bearing phases of each activity. The results demonstrate that the secondary motion of the knee have an important influence on wear, stair climbing function, and the ability to achieve flexion during deep flexion.The short- and long-term outcomes of total knee arthroplas- ty require a better understanding of the relationship be- tween the primary and secondary motion of the knee during the most common activities of daily living.
Introduction
The primary goals of total knee replacement include restoring function and maintaining the long-term me- chanical integrity of the device.An understanding of knee kinematics during ambulatory activities is fundamental to meeting both of these goals. In particular, short-term outcome will be dependent on restoring ambulatory function during activities of daily living. Long-term fail- ure modes such as wear, fatigue failure, and loosening will be influenced by the kinematics of the joint, since the cyclic mechanical demands on the joint are dependent on ambulatory function. This chapter examines the rela- tionship between knee kinematics, patient function, and the mechanical factors that influence long-term failure modes of primary total knee replacement.
Defining Knee Motion (Kinematics)
As total knee arthroplasty (TKA) designs evolve there is a need to precisely define a method for describing knee motion. The motion of the knee is complex and involves
rotations and translations with six degree of freedom during most ambulatory activities. The material present- ed in this chapter is defined by the relative six degree of freedom motions between the femur and tibia.A joint co- ordinate system was defined on the basis of a coordinate system embedded in the femur and tibia [6]. The origin of the femoral coordinate system is located at the mid- point of the transepicondylar line (
⊡Fig. 6-1a). The origin of the tibial coordinate system is located at the midpoint of a line connecting the medial and lateral tibial plateaus.
Projection angles [1] were used to define relative rota- tions of the femur with respect to the tibia (
⊡Fig. 6-1b).An- gles were determined by projecting an axis from the femoral coordinate system onto a plane created by two axes in the tibial coordinate system. For example, project- ing the anterior-posterior (AP) axis of the femur onto a plane created by the AP and superior-inferior axis of the tibia was done to calculate the flexion-extension of the knee, femur relative to the tibia. This system allows for a consistent way to determine relative rotations at all flexion angles. Translation of the femur was determined by pro- jecting the femoral origin onto one of the tibial axes and determining the distance between that and the tibial ori- gin (
⊡Fig. 6-1a).For example,projecting the femoral origin on the AP axis of the tibia allows calculation of AP trans- lations, projection onto the tibial medial-lateral axis to determined medial-lateral translations, and the inferior- superior axis determined inferior-superior translations.
Primary and Secondary Motions of the Knee (Passive vs Active Function)
While the primary motion of the knee is flexion, the secondary motions, including AP translation, internal- external (IE) rotation, and abduction-adduction (AA), play an important role in the overall function of the knee joint [8]. During passive motion of the knee, the sec- ondary motions are coupled to knee flexion [16]. Certain passive motions of the knee (screw-home; external tibial rotation with extension [9] and femoral roll-back [3]
(
⊡Fig. 6-2); posterior movement of the femur with flexion)
have been characterized and regarded as fundamental to
normal knee function. The passive characteristics of the secondary motions of the knee have been related to the shape of the articular surfaces and ligament function [16].
The secondary motions are contained within an envelope of passive limits of the joint [4]. However, when extrinsic forces, such as muscle forces, are present the secondary motions are driven by the magnitude and direction of these forces, since secondary motions such as AP transla- tion or IE tibial rotation require relatively low forces to displace the joint from a neutral position [14, 15]. Thus, during weight-bearing activities the secondary motions of the knee are dependent on extrinsic forces acting dur- ing a particular activity [1].
The AP motion of the knee during walking (
⊡Fig.6-3
) provides an interesting contrast to the passive non-
weight-bearing motion of the knee described as roll-back (Fig.6-3).During walking the position of the femur at heel strike (HS) is posterior. This is consistent with the exten- sor mechanisms pulling the tibia forward during the final portion of swing phase. After HS, the femur trans- lates in an anterior direction through midstance to ter- minal extension. Similarly, the femur externally rotates while extending from HS to terminal extension (the re- verse of the passive screw-home movement). Again, this external rotation is caused by forces generated by muscle contraction and the inertia of the upper body rotating the femur while the foot is planted on the ground. Interest-
MLSI
AP Q
a
†
a b
⊡Fig. 6-1.The anatomical coordinate systems used to describe the motion of the femur with respect to the tibia. Flexion-extension (FE), abduc- tion-abduction (AA), and internal-external rotation (IE) were defined by the projection of the anatomical femoral or the axes onto planes fixed in the tibial coordinate system. The anterior-posterior (AP) displacement of the tibia was determined by the projection of the origin of the tibial coordinate system on the AP axis of the femur
Femoral Translation (cm) -Anterior/ +Posterior 2
1
0
0 20 40 60 80 100
Knee Flexion (Degrees)
Femoral Translation (cm) -Anterior/+Posterior 1.4
0.7
0
-0.7
-1.4
0 10 20 30 40 50 60 70
Knee Flexion Angle (Degrees)
⊡Fig. 6-2.Femoral roll-back during flexion to 90∞. The femur starts in the anterior position at full extension then moves posteriorly as the knee flexes
⊡Fig. 6-3.Averaged phase plots of secondary motions (femur relative to the tibia) versus knee flexion angle during walking for anterior-poste- rior translation. Arrows indicate direction of motion. Solid curve indicates stance phase while broken curve indicates swing phase. Shaded areas in- dicate the confidence interval. HS heel strike, MS midstance, TE terminal extension, TO toe off, MKF maximum knee flexion [6]
of the knee demonstrated that the femur and tibia are not guided solely by the bony and ligamentous structures during ambulation. The secondary motions during weight bearing cannot necessarily be predicted from pas- sive characteristics such as screw-home movement or femoral roll-back. The secondary motions occur within a range that depends on the angle of knee flexion, the ac- tivity performed,and muscle activation.Therefore,under weight-bearing conditions, secondary knee motions are dependent on the type of activity.
The secondary motions of the knee during activities of daily living are extremely important in restoring nor- mal function following TKA. The following provides spe- cific examples of the influence of knee kinematics during stair climbing, squatting, and walking on the outcome of TKA.
Activities of Daily Living and TKA Outcome
Stair Climbing
The ability to step up or down is required for restoring normal function following total knee replacement. The AP translation (secondary to flexion) of the knee has been shown to influence the ability of patients to ascend stairs in a normal manner [2]. Abnormal roll-back was one explanation given for the reduced quadriceps moment associated with cruciate sacrificing,because reduced roll- back would shorten the lever arm of the quadriceps mus- cle [2]. However, a recent study [1] demonstrated that the femur does not simply roll back with flexion during stair climbing. AP translation of the femur was dependent on the phase of the stair-climbing cycle. During the early swing phase, the femur moves forward with flexion as a result of the hamstring muscle producing knee flexion (
⊡Fig. 6-4). The femur begins moving posteriorly at approximately 45° of flexion, probably as tension in the posterior cruciate ligament (PCL) increases.
The importance of understanding the unique charac- teristics of AP translation during stair climbing was illus- trated in a study of patients with posterior stabilized (PS) TKA, cruciate retaining (CR) TKA, and aged-matched controls during stair climbing (
⊡Fig. 6-5). General pat- terns of AP translation for all three groups were similar but large differences were seen in the position of the femur at foot strike on the step. The PS design was more anterior relative to the CR design or to the control sub- jects. The PS design group reaches a maximum anterior position at approximately 70°, compared with 40° in the control group and 55° in the CR group. The cam-post mechanism for this particular PS design engaged at
approximately 70°. The results of this study suggest that restoring or replacing PCL function near 45° of flexion is an important consideration in total knee replacement, since PCL tension at 45° of flexion is needed to maintain the normal lever arm of the quadriceps during stair climbing.
A recent study [7] of 21 bilateral TKR with CR designs in one knee and PS designs in the contralateral knee sup- ported the conclusion regarding the function of the PCL during stair climbing. With the PS design, the maximum external knee flexion moment (sustained by net quadri- ceps contraction) was significantly reduced compared with the CR side and matched controls. There was a sig- nificant increase in hip flexion, with the PS design, which could be associated with a forward lean. Forward lean would allow the individual to move his or her center of
6
⊡Fig. 6-4.Averaged normal phase plot of femoral translation versus knee flexion during stair climbing. During early swing phase the femoral reference point (midpoint of transepicondylar axis) translates anterior with flexion. During late swing phase, the femoral reference point trans- lates posterior with flexion. During support phase, there is minimal trans- lation of the femoral reference point [1]
Femoral Translation (cm) -Posterior/+Anterior 3
2
1
0
0 20 40 60 80 100
Knee Flexion - Stair Climbing (Degrees) PS
Control CR
⊡Fig. 6-5.Comparison of averaged phase plots of femoral translation versus knee flexion during stair climbing for normal subjects, patients with a cruciate retaining design (CR) and patients with a cruciate sacrific- ing, i.e., posterior stabilizing (PS) design. The PS group maintained an anterior location of contact until approximately 70° of flexion where normally the cam engages. The CR group was also more anterior than the normal group. The swing phase anterior-posterior motion prior to stair climbing appears to be dependent on the function of the PCL [1]
mass in front of the knee, suggesting a compensation for reduced quadriceps efficiency (reduced lever arm). With the CR design, there could possibly be more normal femoral roll-back that would increase the lever arm, and therefore the mechanical advantage of the quadriceps.
This would allow a greater moment to be produced for the same amount of quadriceps activation.
Squatting into Deep Flexion
The capacity for deep flexion is essential for activities of daily living, especially for Indian, Middle Eastern, and Japanese cultures.However,even in Western cultures there are a wide range of activities (recreational and occupa- tional) that require deep flexion. For example, recent stud- ies [1,5,11] of deep flexion indicate the importance of IE ro- tation during squatting into deep flexion.Squatting from a standing position requires approximately 150º of flexion to a resting squat. Flexion between 0º and 120º is accompa- nied by approximately 10º of external rotation of the femur.
However, between 120º and 150º flexion, the femur exter- nally rotates an additional 20º.Therefore,beyond 120º flex- ion, the knee requires substantial external rotation to achieve deep flexion [5]. Currently, most designs of total knee arthroplasty can achieve only 120º flexion. However, patients requiring deeper flexion will need the capacity for substantial tibial rotation beyond 120º flexion.
Walking Kinematics and Wear
Implant wear is the primary mechanical factor limiting the long-term outcome of total knee replacement. The kinematics of the knee are a critical factor influencing wear at the joint [17, 18].Again, the secondary motions are an important consideration in the outcome of total knee replacement since these motions will have a substantial influence on wear. For example, subtle variations in rolling, tractive rolling, and sliding motion and the direc- tion of the pathway of motion can have substantial effects on the production of wear debris or cyclic fatigue of the ultra-high-molecular-weight polyethylene [18]. The de- gree of rolling and sliding can be quantified by the slip ve- locity. The magnitude of the interfacial slip velocity pro- vides quantification of the rolling versus sliding behavior of the tibiofemoral joint when relative motion occurs.For pure rolling, the interfacial slip velocity will approach zero [12, 13]. The absolute maximum slip velocities occur during swing phase just before heel strike. A previous knee simulator study [12] showed that the maximum wear rate was significantly greater when these slip velocities were incorporated as input to the simulator relative to studies where the slip velocities were not applied. There- fore, the high slip velocities during heel strike and during
swing phase indicate the potential for sliding motion that can produce a greater volume of abrasive wear debris.
The considerable differences in the wear scar forma- tion between retrieved and simulator tested implants [10]
can be explained by differences between in vivo kine- matics and the type of kinematics used in wear simula- tors. In addition, the variability of in vivo wear scar for- mation has been related to the variability of human gait following TKR [19]. Most of the variability in worn con- tact area may be explained by gait abnormalities of TKR patients. These abnormalities cause larger wear areas contributing to possibly higher wear rates. Since most TKR patients walk with an abnormal gait pattern, knee simulator input parameters should be reconsidered.
Conclusion
Motion of the knee is very complex and cannot be de- scribed by a single motion path. Typical activities of dai- ly living: walking, stair climbing, and increasingly deep knee flexion, show that knee motion is activity depen- dent. There is also evidence of different motion patterns in a single activity due to muscle activity or knee re- placement designs.In order for advancements to be made in the design of total knee replacement, one must under- stand not only the forces and moments, but also the six degree of freedom of motion of the knee.Internal-external rotations and anterior-posterior translations play an im- portant role in determining the longevity of knee re- placements.The successful outcome of TKA is dependent on the kinematics of the knee during activities of daily living.
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