Design Principles, Biomechanical Data and Clinical Experience with a Polycentric Knee Offering Controlled Stance Phase Knee Flexion: A Preliminary Report
Siegmer Blumentritt, PhD
Hans Werner Scherer, OMM
Ulf Wellershaus, OMM
John W. Michael, MEd, CPO, FISPO
ABSTRACT
The OTTO BOCK 3R60 prosthetic knee joint allows controlled flexion of the prosthetic knee under weightbearing in early stance phase, more closely simulating this aspect of gait than was previously possible. As the amputee transfers weight onto the prosthesis in early stance phase, this knee gradually flexes up to 15 degrees, thereby cushioning the impact of weight acceptance.
In contrast to all previous polycentric knees, stability of this new design is biomechanically increased by stance flexion, rendering a locking feature unnecessary. Clinically, the amputee perceives this prosthesis as stable and comfortable. The biomechanical features of this design concept, initial clinical observations and preliminary conclusions from laboratory gait analysis are summarized.
Key words: Prostheses, Polycentric Knee, Stance Flexion.
Introduction
A significant number of new prosthetic concepts were introduced at the World Congress for Orthopaedic and Rehabilitation Technique in Essen, Germany, in 1994, including the OTTO BOCK 3R60 Ergonomically Balanced Stride (EBS) kneea reviewed in this article. This patented design (1,2) allows an EBS, including comfortable stance flexion in early stance, which simultaneously increases mechanical stability by moving the instantaneous center of rotation in a more proximal and posterior direction.
This new design, which actually increases stability as the knee is flexed in early stance, allows the amputee to walk with more normal gait kinematics than has been possible with previous prosthetic knee mechanisms. To help the clinician maximize the functional advantages offered by this new device, this article will summarize laboratory data and initial clinical experiences reported to date.
Biomechanical Considerations
The goal of all prosthetic knee mechanisms is to replicate, as much as possible, the kinematics and other functions of normal gait. Stance phase stability and swing phase response are both important aspects to consider.
Prior mechanical knee designs typically have failed to permit two key stance phase movements: 1) controlled knee flexion under weightbearing followed by 2) gradual extension under load. As a result, most amputees have been forced to walk on a knee that is fully extended throughout the entire stance phase. Figure 1
illustrates the abnormal knee movement that results with conventional prosthetic knees.
Normal stance phase knee flexion has several biomechanical advantages. In combination with plantarflexion at the ankle, knee flexion decreases the time interval between heel contact and foot flat. Stance flexion provides a cushioned transition to the full weightbearing of single support as weight gradually is transferred onto the forward leg. It also limits the rise of the center of gravity, which has been described as one of the key determinants of gait by Saunders et al. (3). It permits a smoother forward progression of the body than is possible with a stiff knee, thus increasing the efficiency of gait. Conventional prosthetic knees never have been able to restore these characteristics of normal gait.
The 3R60 knee allows up to 15 degrees of stance flexion, and the polymeric spring progressively cushions the increase in loading that occurs as weight is transferred onto the prosthesis. It should be noted that although this novel knee design allows a much greater range of stance flexion than was heretofore possible, it does not replicate the full range of normal stance phase knee flexion. This may be due, in part, to the loss of the distal insertion of the rectus femoris following transfemoral amputation. In the case of such an amputee, the rectus femoris no longer functions as a two-joint muscle and therefore can no longer contribute to knee control and mediolateral hip stability (4).
Mechanics of the 3R60 EBS Knee
In general, prosthetic knee stability occurs as a result of an extension moment generated whenever the ground reaction force (GRF) is anterior to the effective center of rotation of the device. For single-axis mechanisms, the line must pass anterior to the mechanical knee axis. For polycentric devices, the line must pass anterior to the instantaneous center of rotation (ICOR). So long as this extension torque is present, the prosthetic knee will be fully extended against its hyperextension stop as illustrated in Figure 2
.
With conventional prosthetic knee designs, as soon as the GRF falls posterior to the center of rotation the knee will flex abruptly. In essence, there are only two functional zones: one stable and the other unstable. Until recently, the only methods to allow safe weightbearing even if the GRF falls posterior to the knee were to add a weight-activated mechanical brake, of hydraulic flexion resistance or to manually lock the knee.
The 3R60 EBS knee offers a new solution to the challenge of increasing stance security and simultaneously overcomes many of the limitations of previous prosthetic designs. Figure 3
illustrates the unique pivoting posterodistal axis of the 3R60, which distinguishes this complex polycentric knee from the simpler four-bar linkages previously used.
During swing phase, the 3R60 EBS knee behaves as if it were a four-bar polycentric knee and offers the same range of clinical advantages, including increased toe clearance and a more controlled terminal deceleration. Figure 4
illustrates the swing phase centrode or path of the instantaneous center of rotation for the 3R60. The miniature hydraulic cylinder provides variable cadence damping. The independent swing phase flexion and extension adjustments are used to optimize the hydraulic resistance for each individual amputee's gait (5).
During stance phase, the patented posterodistal linkage moves through an arc, which causes the ICOR to move into a more stable position. Figure 5a
demonstrates how the combined movements of the distal articulations move the ICOR to a more proximal and posterior position than occurs at heel contact. This unique prosthetic knee actually becomes more stable mechanically as stance flexion occurs during the period following heel contact.
Figure 5b
graphically demonstrates how the centrode changes with increasing stance flexion. The prosthetic joint pivots about its anterodistal articulation up to 15 degrees. Even though the GRF then may fall posterior to the mechanical axes of the knee, as long as it falls anterior to the ICOR the prosthesis will be stable.
The rate of stance flexion is controlled by a composite spring and may be adjusted to each individual's requirements. An integrated stance phase hydraulic damper delays the release of this stored energy until slightly later in the gait cycle, rendering each stride more ergonomically correct.
In mechanical terms, the 3R60 may be considered as a five-bar polycentric knee with two degrees of freedom. Therefore, there are two mechanical centers of rotation. The ICOR is the center of rotation during swing phase while the anterodistal axis (labeled CR in Figure 6
) is the stance phase center of rotation.
The dynamic stance flexion feature of the 3R60 EBS knee can occur only when the GRF falls between the two centers of rotation of the joint as illustrated in Figure 6
. In this situation, a torque is generated, causing the upper-knee frame to pivot in a counterclockwise fashion about the anterodistal axis.
Later in the gait cycle, as soon as the GRF passes in front of the anterodistal axis, an extension moment results, and the upper-knee frame rotates back toward its original position.
Like most other polycentric mechanisms, the 3R60 can be voluntarily flexed under weightbearing whenever the amputee generates a hip flexion moment sufficient to place the GRF just posterior to the anteroproximal knee axis. During ambulation on level surfaces, the transition from stance flexion to extension of the upper-knee frame to preswing knee flexion occurs smoothly and almost effortlessly.
Equally importantly, the unique engineering of the 3R60 EBS knee permits the amputee to override the stability features and voluntarily bend the knee by exerting a stronger-than-normal hip flexion force. This is true even when stepping down from a curb or descending a ramp or decline. In contrast, other polycentric designs that use an automatic geometric locking feature for stability force the amputee to vault over the locked knee during descents, which can be disconcerting to some individuals.
In summary, the 3R60 EBS knee is unique in offering two degrees of mechanical freedom. Depending on the position of the GRF, the knee has three functional zones: 1) controlled stance flexion when the GRF falls between ICOR and the CR, 2) stability in extension when the GRF passes anterior to the CR and 3) a swing flexion mode whenever the GRF is posterior to the ICOR.
Gait Analysis Data
Gait data presented here were gathered from three amputees in an accepted fashion using instrumented gait analysis equipment as previously described by Blumentritt et al. in 1994 (6). All prostheses were fitted and dynamically aligned for each individual amputee by an experienced, trained prosthetist- orthotist, and the tested configuration was considered acceptable by both the prosthetist and the amputee.
Tables A
and B
summarize the basic patient data and the fundamental protocols for this preliminary study. Due to the limited number of subjects, statistical analyses were not performed, and therefore the information reported is not statistically significant. Several trends seemed clear, however, and will be presented for consideration.
Overview of Knee Angle
Figure 7
shows a typical movement curve for the 3R60 EBS knee throughout the gait cycle as measured in this study. Changes in the direction of knee motion, due to changes in the position of GRF line, are highlighted.
At heel contact, the GRF is anterior to the knee joint, and the knee is fully extended. As weight is transferred onto the prosthesis, the GRF moves slightly posterior to the anterodistal axis, and controlled stance flexion begins. The degree and rate of knee flexion are controlled by adjusting the preloading of the polymer spring. At the instant the forefoot contacts the ground, the GRF moves anteriorly and encourages knee extension. Swing phase knee flexion occurs when the amputee generates a hip flexion moment sufficient to move the GRF just posterior to the ICOR.
Stance Phase Knee Flexion Angle
Typical knee flexion angles for the gait cycle are illustrated in Figure 8a
. Figure 8b
depicts representative knee moments, which cause flexion when the sign is negative. When properly aligned, in clinical use the 3R60 EBS knee typically will exhibit between 8 and 12 degrees of stance flexion.
Alignment and Gait
Alignment of the knee (with reference to the socket and foot) significantly influences the stability and responsiveness of the prosthesis. Alignment of the 3R60 EBS knee also is important but differs somewhat from more familiar, conventional knees. Due to the complex polycentric mechanism, the 3R60 generally will have sufficient stability for the amputee to walk even if it is not optimally aligned. So long as the knee is within 30 mm (anterior or posterior) to the optimal alignment position, the prosthesis will be stable. However, the stance flexion feature will not be fully functional at the extremes of alignment.
If the knee is aligned posteriorly in an extremely stable position, the stance flexion feature cannot occur due to the constant hyperextension moment generated by the GRF. This extreme alignment also will make the transition from stance to swing phase difficult to initiate as is illustrated in the gait studies summarized in Figure 9
.
If the knee is aligned too far anteriorly, although stance flexion may occur, the automatic return to the neutral position will be delayed or absent, and the transition to swing phase flexion will be very abrupt. It is not possible to have a smooth, flowing gait with the knee aligned in this manner as Figure 9
shows.
Optimal dynamic alignment will result in a stable knee at heel contact, gradually increasing stance flexion, automatic return of the upper-knee frame toward a neutral position and a smooth transition to swing phase flexion. The manufacturer's static alignment recommendations should be followed closely, with the knee shifted in a linear fashion during gait trials to finalize the alignment. Tilting the knee itself in the sagittal plane has a minimal influence on knee function.
Effect of Foot Selection on Knee Function
To achieve maximum benefit from the biomechanical advantages of the 3R60 EBS knee joint, the correct foot must be combined with optimal alignment. For stance flexion to occur properly, the GRF must result in a knee flexion moment during the loading response portion of the gait cycle.
If a foot with insufficient plantarflexion resistance is selected, the GRF will cause a hyperextension moment throughout stance, and no stance flexion will occur.
On the other hand, a foot with excessive plantarflexion resistance will cause the GRF to move anteriorly too quickly, and the stance flexion period will be reduced. These effects were noted clinically and confirmed by objective gait analysis. Figure 10
presents typical results for one particular amputee and clearly illustrates the effect of specific feet on the amount of stance phase knee flexion that occurred for this individual.
For the case illustrated here, dynamic elastic response (DER) feet with a relatively firm heel wedge (such as the 1D10 Dynamic Footb or the 1D25 Dynamic Plusb) offered the most normal range of stance flexion.
Feet with very little plantarflexion resistance (such as the SACH and single-axis types tested) significantly decreased the amount of stance flexion measured. The multiaxial elastic keel foot tested (1M1)b offered good stance flexion, but the forefoot resistance was too soft for more active individuals such as this subject.
Clinical Recommendations
Clinical recommendations are based on the unique range of features (previously discussed) offered by the 3R60 EBS knee joint and the subjective feedback from more than 1,000 amputees presently using this knee worldwide. Functional advantages of this knee, as reported by amputee users, include:
- increased feeling of stability during level walking compared to less sophisticated designs;
- enhanced feeling of stability on irregular terrain since stance flexion increases inherent alignment stability;
- increased sense of toe clearance during swing phase, resulting in greater confidence in the prosthesis;
- more effortless perception of walking due to advanced stability combined with easy initiation of swing phase;
- improved feeling of stability and a sense of a more natural gait when negotiating inclines and declines;
- relief from having to concentrate on controlling the knee to simply being able to walk confidently; and
- increased participation in special activities such as gardening or recreational pursuits due to increased perception of stability and comfort when using the prosthesis
As noted previously, in addition to a well-fitted socket and proper alignment, an appropriate foot should be provided to complement the advanced knee functions. As a general guideline, a foot with a firm plantarflexion resistance and at least moderate dorsiflexion resistance is recommended. The range and rate of stance phase knee flexion then can be individualized by varying the preload on the composite spring. Swing phase damping and cadence response is provided by a miniature hydraulic cylinder with independently adjustable swing phase flexion and extension resistances. Figure 11
depicts these adjustment options.
Based on the authors' experience to date, it is possible to convert amputees who have previously used conventional knees to the 3R60, but they must be allowed a period of time to become accustomed to walking in a new manner. From using earlier knee designs, amputees will have developed the habit of using hip extensor musculature to keep the prosthetic knee fully extended throughout entire stance phase. This gait habit will initially preclude taking advantage of the available stance flexion feature of the 3R60.
Clear explanation and demonstration of the new capabilities offered by the 3R60 will help the amputee in making the transition to this new technology. Practice periods within parallel bars are recommended. Understandably, the amputee initially may feel insecure as soon as stance phase knee flexion begins and may therefore block further flexion by contracting the hip extensors more forcibly. Both the amputee and the clinical team must learn to "trust" the stance flexion feature and realize it is stable-rather than unstable-knee flexion. With encouragement, plus an opportunity to practice using the stance flexion feature, most amputees gradually will learn to accept the sensation of stance flexion and appreciate the benefits.
It should be noted amputees who receive the 3R60 as their first prosthetic knee adapt quickly and easily to all its functions since they walked with a similar range of stance phase flexion prior to losing their limb. Therefore, if the 3R60 is considered appropriate for a given individual, it should be provided with the initial prosthesis whenever possible.
The 3R60 is optimized for amputees with moderate functional capabilities who can be expected to walk on level indoor surfaces but may benefit from added stability, particularly when outdoors or on irregular surfaces. It is warranted for loading up to 100 kg or 220 lbs. The integrated miniature hydraulic cylinder provides cadence-responsive swing phase damping for walking at slow to moderate speeds. Amputees capable of walking at rapid cadences may prefer more powerful swing phase controls available in knees such as the 3R70 or 3R45.
Case Illustrations
The patient depicted in Figure 12
is typical of the type of person who will benefit from the 3R60 EBS knee. Now 52 years old, this patient has been unable to return to work following an industrial accident six years ago despite initial fitting with a friction brake stance control knee and later with a fluid-controlled hydraulic knee.
Because of the advanced stability of the 3R60, this patient now is able to walk freely and confidently across a variety of terrains, which was impossible for him to do with previous knee mechanisms. Subjectively, he considers the 3R60 to be a comfortable and stable prosthetic knee. He has used the 3R60 for more than three years, prefers its advanced functions and is unwilling to consider any other prosthetic alternatives.
It should be noted the amputee's functional needs and capabilities- rather than chronological age or other factors-determine whether the 3R60 is appropriate. Figure 13
depicts a 35-year-old individual with a brachial plexus paralysis as well as a transfemoral amputation, both secondary to a motorcycle accident.
The stability and comfort offered by the 3R60 EBS knee has permitted this young man to return to work and to enjoy walking out of doors, despite his multple disabilities. In addition, the unique stance flexion capability has enabled him to return to one of his favorite recreational sports activities: table tennis.
The unique stance flexion capabilities enhance amputee comfort and allow a more normal gait pattern than has previously been possible. New amputees adapt quickly to the 3R60 while more experienced ambulators may need time to learn to allow the prosthetic knee to flex during stance phase. The patented design features of the 3R60 EBK offer new possibilities in amputee rehabilitation.
Acknowledgements
This article is dedicated to Mr. Dr.-Ing.E.h. Max Nader in honor of this 80th birthday.
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