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Theory of Integrated Balance: The Lower Limb Amputee

James W. Breakey, PhD, CP

ABSTRACT

The author suggests that what he refers to as the "zone of integrated balance" is an important variable in determining the alignment of lower limb prostheses.

By using a force plate that objectively measures the center of pressure on the prosthetic foot, the weight and load lines of the patient can be determined. The distance between these lines is the "zone of integrated balance." The author suggests that the closer the lines approximate one another the more integrated is the balance of the prosthetic side in relation to the overall balance of the amputee.

In the author's clinical experience, a narrow zone of 0 to 1 cm has been found to be reasonable in providing an appropriate prosthetic alignment.

Introduction

Radcliffe presents the "zone of voluntary stability" as an essential component in the alignment of the transfemoral prosthesis. He explains that if the knee center is positioned within this zone, alignment stability in the sagittal plane will be achieved (1).

The author suggests another zone to be considered in achieving the appropriate alignment of a lower limb prosthesis. This is referred to by the author as the "zone of integrated balance" and defined as "a zone in which the center of balance line as determined on the prosthetic limb of the total subject is compared to the center of balance line on the prosthetic side only." The author suggests that the closer the approximation of these center of balance lines, the more integrated is the balance of the prosthetic side with respect to the overall balance of the amputee.

Prior to dynamically aligning a prosthesis in the sagittal plane, static alignment principles are followed. There are two accepted static alignment theories. One advocates the American Trochanter-Knee-Ankle (TKA) line, while the other refers to the European weight-bearing line. The author suggests that the amputee's center of balance is a more accurate reference in determining static alignment (2). Attempting to determine the individual's center of balance using a plumb bob would be futile. However, a device has been designed by Dr. Siegmar Blumentritt to assist prosthetists in determining the center of balance in their amputee patients.

Referred to as the LASAR (Laser-Assisted Static Alignment Reference) Posture (Figure 1) , it consists of a platform with pressure sensors in each of the four corners. These sensors measure the applied load while a microprocessor calculates the center of pressure in one plane. A laser is moved to that location by a stepper motor. When the amputee stands on this calibrated platform, a vertical laser line is projected on the subject from the center of pressure at the foot to the axilla level.

Center of Balance Reference Line

"The center of balance reference (CBR) line is a theoretical vertical line through the center of pressure that divides the individual in the sagittal plane into anterior and posterior parts" (2). There are actually two CBR lines to be considered. In the unilateral amputee, the CBR line of the total individual can be determined when the amputee stands with both feet on the platform. This line is referred to as the total weight line (Figure 2) . If the amputee stands with the prosthesis limb on the platform and the sound limb on a compensatory block, the CBR line for the single prosthetic limb can be determined. Referred to as the limb load line, at the initial static alignment stage this line normally differs in position from the weight line (Figure 3) . This difference is due to center of gravity variations for the remaining lower limb segments, both anatomical and prosthetic. In addition, compensations for differences in remaining muscular strength, joint position, prosthetic alignment, or socket fit can be influencing factors. The weight line takes into consideration the total individual and cannot by itself discriminate between compensated balance and optimal balance. In static alignment of a lower limb prosthesis, the load line is the preferred reference line. To determine optimal balance between the prosthetic and sound limb, however, both reference lines must be considered.

Clinical Considerations

The LASAR Posture can be used to assist in the static alignment of the prosthesis. Dynamic alignment is undertaken with the prosthetist relying upon clinical judgment and patient feedback. When the prosthetist and patient reach agreement on the dynamic alignment, the CBR lines are once again checked by using the LASAR Posture. Determining the patient's center of balance by locating the center of pressure on the prosthetic foot for both total weight and limb load line references offers alignment clues. The author suggests that the closer the two lines are in approximation, the more optimal the center of balance between the prosthetic and sound limb (Figure 4) . The same reasoning applies for the bilateral amputee.

Providing an integrated balance between these two lines will offer an appropriate prosthetic alignment for the amputee at that moment in the individual's life circumstance. As the patient's strength, range of motion, and proficiency increases or decreases, the zone width may change. Therefore, the prosthetic alignment will need to be rechecked and changed to accommodate the new situation.

Both preparatory and permanent lower limb prostheses have been aligned with the aid of the LASAR Posture, including 115 transtibial and 42 transfemoral amputees. In the majority of cases the criteria of a 1 cm zone width was met. Conversely, the ones that fell outside the zone had marked knee or hip flexion contractures-reinforcing the importance of maintaining full range of motion in remaining anatomical joints. Patients with small contractures, 0 to 10 degrees, the center of balance lines, could usually be integrated. In those patients whose more severe contractures could be reduced, when the prosthesis was realigned, the zone width narrowed, with many coming into the recommended range. In most cases, during the dynamic alignment process, if the desired zone width was not met, the alignment would be adjusted until achieved. In all situations the patients noticed an improvement during ambulation. In the alignment of transfemoral prostheses, the manufacturer's guide lines as well as Radcliffe's zone of voluntary stability were taken into consideration.

The author has noticed that in previously aligned prostheses where the zone of integrated balance was wider than the recommended 1 cm, the gait patterns were more asymmetrical between the sound and prosthetic sides. When these prostheses were realigned to position the weight and load lines in a close approximation, the patient noticed an improvement in the function and the prosthetist observed a visual improvement in the gait. A final note on this subject is that balance integration is independent of the position of the weight line and load line. That is, either can be anterior. What is important is the width of the zone. Nonetheless, in bilaterals, whichever line is anterior on one side should also be anterior on the other side.

The LASAR Posture has allowed the development of a thesis for a new alignment protocol for a lower limb prosthesis, and has offered the following basic advantages in the author's practice (2):

  1. By identifying subtle alignment discrepancies, enhanced clinical outcomes have been achieved.
  2. Reinforcement of clinical judgment and patient feedback that the alignment is satisfactory.
  3. Saves time in the alignment process.
  4. Use of an objective tool during the alignment process has increased patient confidence.
  5. Alignment parameters can be objectively documented.
  6. An objective method has provided reliable alignment and postural re-checks.

Discussion

When an individual loses a lower limb, compensations in the remaining functional anatomy can be expected. The degree of compensation will vary depending upon the range of motion in remaining joints, the muscle power moving these joints, and the efficiency of the neurological mechanism controlling these movements.

Compensations are acceptable. In an ideal world physical therapy treatment to strengthen remaining muscles and to gain full joint range of motion would not cease until the optimum had been achieved. Unfortunately, this is not the case. Only our most devoted, and usually athletic, amputees reach these heights. Prosthetists must be content with aligning a prosthesis to the compensatory functional anatomy and resultant movement patterns. Often this can be a challenge as alignment of a prosthesis is individualized.

Blumentritt reports clear differences between the posture of nonamputees and the prosthetic side of transtibial amputees. He also found that the posture on the amputee's sound limb approximated that of the nonamputee subjects, and that the position of the load line in transtibial amputees varies from 10 to 30 mms anterior to the knee joint center (3). The weight line is mentioned in the article but is not reported. He has also found that a load line approximately 15 mm anterior to the knee joint center appears to be the most efficient in transtibial gait (4). Similar findings have been observed at our clinic. In addition, in patients presenting the most aesthetic gait, the weight line approximated the load line in position from 0 to 10 mm.

Earlier, the author considered the load line to be the most important reference line in the static alignment of the prosthesis (2). As reported, the weight line may mask compensations being made by the sound side (2). The author has come to realize that compensation in a sound limb is a normal part of adaptation to wearing a prosthesis. Since an amputee must learn a new walking pattern to integrate the prosthesis into the body from a balance perspective, the total weight line and the limb load line should have as close an approximation as technically possible. This makes the most sense in integrating the prosthesis and the sound limb.

Many prosthetic feet are utilized today in the rehabilitation of lower limb amputees. The characteristics of different feet require gait patterns to be learned. Each newly-learned gait pattern will require different balance compensations through muscle activity and joint positioning. Many reference lines are presented by the manufacturers to aid prosthetists in static alignment. Without knowing the balance compensations made by the patient, these lines are hypothetical. The author suggests that the position of a hypothetical reference line on the foot is not as important as an equal relationship between the total weight and limb load lines on the center of pressure of the prosthetic foot.

Conclusion

A new protocol for aligning lower limb prostheses has been presented. In order for the prosthetist to be able to use this method a new, objective measurement tool is required.

Prosthetists must go beyond the plumb bob and straight edge methods if they are to move smoothly into the 21st century with its growing demands for objective outcomes measurements. The practice of prosthetics is changing, and prosthetists must welcome and embrace the objective technologies which are now available.

Acknowledgements

The author would like to express appreciation to Mike Gidding for his assistance and input in the evaluation of the LASAR Posture.


References:

  1. Radcliffe CW. The Knud Jansen lecture: above-knee prosthetics. Prosthet Orthot Int 1977;37:146-160.
  2. Breakey JW. Sagittal alignment of lower limb prostheses using the center of balance reference line. Proceedings AAOP 23rd Annual Meeting 1997;70-71.
  3. Blumentritt S. A new biomechanical method for determination of static prosthetic alignment. Prosthet Orthot Int 1997;21:107-113.
  4. Blumentritt S. 1997. Personal communication, March.