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Technical Note: Beyond the Four-Bar Knee

James W. Breakey, PhD, CP
Stuart H. Marquette, BS, CO

ABSTRACT

Programmable centrode knee units have been developed with the primary objective of adding a centrode that could be aligned taking into consideration the patient's residual limb length, strength, and gait determinants. This design allows the prosthetist to more accurately align a prosthesis to meet the amputee's individual characteristics. This article presents the basic function and benefits of the four-bar configuration, as well as the additional mechanical function of the five-bar knee unit. The benefits to the amputee wearing programmable centrode knee unit designs are discussed.

Introduction

The benefits of the four-bar linkage knee have long been known to the biomechanically inclined as the knee unit of choice for the transfemoral amputee. Green and Radcliffe have described the mechanical considerations of the four-bar knee and have discussed the advantages for the user.^1,2 The foundation of this article relies on their work.

The four-bar knee design provides the following benefits over the single axis knee unit designs:

1. Increased stability at heel strike and through early stance phase
2. Reduced energy dissipation by decreasing the magnitude of muscle activity
3. Shortening of the overall prosthesis length during swing phase
4. Enhanced voluntary control of knee flexion in terminal stance
5. Improved sitting cosmesis, provided by a more anatomical flexion pattern

Because of these attributes, four-bar knee units have gained wide acceptance in the prosthetic community. The characteristic that best describes the mechanical function of the knee unit is known as the instantaneous center of rotation (ICR) (Figure 1) . This center can be located by extending lines through both the anterior and posterior axes of the knee. The point at which the lines meet is the instant center. Mechanical control of the knee is determined at this center. This is the point at which the amputee feels control of the fully extended prosthesis. It is important to understand that as the knee joint flexes and extends, the position of the instant center changes. The path that the ICR follows is called the centrode (Figure 2) . A centrode defines the mechanical function of four-bar knee units.

Manufacturers provide guidelines to assist the prosthetist in the static alignment of four-bar knees in the sagittal plane. A reference weight or TKA line is usually provided, preferably intersecting the anterior superior pivot point of the four-bar knee unit. The Center of Balance reference (CBR) line is considered a more appropriate reference than an arbitrary line set by the manufacturer.³ This line takes into consideration a reference line through the center of pressure at the foot extending proximal through the center of mass of the individual.

There are three primary prosthetic functions that are determined by the pattern of the centrode:

1. The distance the ICR is posterior to the CBR line of the amputee determines knee stability. The further posterior this point, the more stable is the knee unit at heel strike (Figure 3) .
2. The better the ICR, the less musculature effort is required to control knee stability at heel strike. Since the control point of the knee unit is closer to the hip joint, the amputee has a mechanical advantage for the residual limb and therefore increases efficiency (Figure 4) .
3. An additional characteristic of controlled knee stability is the number of degrees a knee unit can flex before the ICR passes anterior to the CBR line. Once this occurs, voluntary control of the knee requires increased musculature force by the amputee (Figure 5) . In contrast, this anterior translation of the ICR enhances controlled flexion of the knee unit in terminal stance.

The understanding that each of these three factors can affect the amputee's control of the prosthesis raises the question of which centrode is more appropriate for a particular patient. The authors are unaware of any reported studies that might act as a guide in selecting a particular centrode for a particular amputee. Rather, practitioners depend on clinical experience and manufacturers' recommendations. In considering a knee unit, the prosthetist makes a choice among available knee centrodes and then applies alignment principles in an attempt to arrive at the most efficient and balanced gait for the amputee.

Current Technology

As the above principles became understood and gained acceptance throughout the prosthetic field, four-bar knee designs offering functional advantages to the amputee became available. Endoskeletal four-bars made their appearance in the United States in the 1970s. Although most were friction controlled at that time, they offered increased stability and control to the amputee over the single axis friction designs of the day. Otto Bock introduced their mechanical four-bar in 1979, followed in 1982 by Teh Lin, who provided the first reliable ball bearing designs. Shortly thereafter, the French company Proteor started manufacturing lightweight four-bar knees.

As the benefits of four-bar designs became more evident to a widening group of practitioners, other manufacturers began developing single centrode four-bar designs. Today, most prosthetic manufacturers produce one or more designs that they recommend be used for all classifications of patients, from the weak, short residual limb to the long and strong knee disarticulation. For example, Daw Industries offers 19 four-bar knees specifically designed for each amputee functional class.

A Step Beyond the Four-Bar

One of the most recent modifications of the four-bar linkage design is the addition of a fifth link. This addition offers a stance flexion feature for amputees in early stance and midstance phases of gait. When the amputee initially applies load to the prosthesis at heel strike, the fifth link effectively allows the knee to flex a few degrees. Otto Bock4 and Daw both offer a five-bar knee, although the mechanics of the movement are achieved differently. Whether the motion is controlled by hydraulic cylinders, urethane dampers, or adjustable spring pressure, the result is similar-the distal posterior axis of the posterior linkage moves anteriorly. This moves the ICR proximal and posterior, providing the patient with enhanced security and control for weight transfer onto the prosthesis (Figure 6) .

A further advantage is offered the prosthetist and patient by the five-bar knee from Daw. The prosthetist is offered the advantage of not only adjustable stance flexion but also a programmable centrode. The centrode can be matched to the amputee's individual functional requirements to meet stability and energy efficiency needs (Figure 7) . In other words, instead of aligning the socket and patient to the knee unit, the knee unit is aligned to the patient.

Discussion

Before the introduction of a stance flexion feature in a prosthetic knee, the transfemoral amputee had to keep the knee joint extended during early and midstance to prevent buckling. Descending inclines is a tenuous event for the patient with a conventional knee, especially since an important determinant in normal human locomotion is knee flexion during midstance.⁵ This determinant reduces the rise of the individual's center of gravity during single-limb support. Coordinating with two other determinants, pelvic rotation and tilt, the center of gravity is kept within an energy-efficient range.⁵ The transfemoral amputee had not had this benefit until the advent of the stance flexion feature, which has greatly facilitated going down inclines. The amputee's ability to use this feature during stance phase leads to a more aesthetic and energy-conserving gait.

The body is a sensory register, an instrument for action, and a stimulus to self and to others; therefore, patients attribute considerable significance to comparisons of their bodies to others, as well as perceptions of positive or negative responses from others.⁶ By teaching the amputee to utilize the stance flexion feature in early and midstance, the result could translate into a more positive body image. Anxiety about body image has been shown to have a negative impact on an amputee's psychosocial well-being.⁷ Obviously, then, enhancing the body image through improved functional designs that bring the amputee's gait pattern closer to normal would be a worthwhile goal.

Being able to match the appropriate centrode to the function and stability needs of the patient is another worthwhile goal. Such a match provides an advantage to the new amputee. As the patient progresses, the centrode can be adjusted to its new requirements. However, attempting to do this by eye is not as precise as using an objective tool to determine the center of balance reference line.⁸ It is important to integrate the amputee's center of balance between the prosthetic and sound limb to determine optimal balance and reduce compensated balance.⁹

Conclusion

The history of prosthetics has been to take available componentry and adapt it to the amputee. The prosthetist now has available knee components to match the centrode that is most beneficial to the amputee. Further advantages include the ability to adjust the centrode to meet progressions or regressions in the life circumstance of the patient. The ability to align the centrode to the patient's individual characteristics leads to an improved gait and enhanced control of the prosthesis. Using an objective tool to determine the CBR line will maximize this process. The prosthetist will then be able to focus on the finer points of gait and will be able to achieve optimal alignment, balance, and stability.

References:

1. Green MP. Four-bar linkage knee analysis. Orthot Prosthet 1983;37:15-24.
2. Radcliffe CW. The Knud Jansen lecture: above-knee prosthetics. Prosthet Orthot Int 1977;146-160.
3. Breakey JW. Sagittal alignment of lower limb prostheses using the center of balance reference line. Proceedings AAOP 23rd Annual Meeting 1997;70-71.
4. Blumentritt S, Scherer HW, Wellershaus U, Michael JW. Design principles, biomechanical data, and clinical experience with a polycentric knee offering controlled stance phase knee flexion: a preliminary report. JPO 1997;9:1:18-24.
5. Saunders JB, Inman VT, Eberhart HD. The major determinants in normal and pathological gait. JBJS 1953;35-A:543-548.
6. Shontz FC. Body image and its disorders. Intl J Psychiatry Med 1974;5:4:461-472.
7. Breakey JW. Body image: the lower limb amputee. JPO 1997;9:2:58-66.
8. Blumentritt S. A new biomechanical method for determination of static prosthetic alignment. Prosthet Orthot Int 1997;21:107-113.
9. Breakey JW. Theory of integrated balance: the lower limb amputee. JPO 1998;10:2:42-44.