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Measurement of Knee Center Alignment Trends in a National Sample of Established Users of the Otto Bock C-Leg Microprocessor-Controlled Knee Unit

Willingham, Laura L. BS; Buell, Noelle C. MSPT; Allyn, Kate J. LCPO, CPed; Hafner, Brian J. PhD; Smith, Douglas G. MD

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JPO Journal of Prosthetics and Orthotics: July 2004 - Volume 16 - Issue 3 - p 72-75
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Alignment practices for transfemoral prosthetic devices are greatly influenced by traditional clinical training that attempts to establish a stable and safe prosthetic limb. Prosthetic knee components vary in design, resistance to buckling, and swing characteristics. Although different design characteristics lead to variations in the manufacturers’ recommended position of the knee center for optimal alignment of the prosthetic limb, the traditionally recommended alignment is posterior to a vertical reference line. Alignment practices for transfemoral prosthetics vary, depending on the design characteristics of each knee unit. However, it has become standard practice to align the knee center posterior to what is commonly called either the reference or “weight bearing” line to ensure safety and stability. This posterior placement creates an extensor moment at the knee, resulting in more stability for the amputee. The more posterior this knee center is placed, the greater the range of stability the amputee has before the knee will buckle. Early transfemoral publications 1 illustrate that alignment stability can be assessed in the sagittal plane with respect to three commonly used reference lines: the European line, the TKA line, and the MKA line. The European line projects vertically along the lateral aspect of the limb, passing through the center of the socket brim and bisecting the horizontal length of the foot. The TKA (trochanter, knee, and ankle) line is that which extends through the head of the greater trochanter, center of the knee, and the center of the ankle. Finally the MKA (medial, knee, and ankle) line is the line from the medial center of the socket to the center of the knee to the center of the ankle.

Most practitioners in the United States traditionally use a vertical reference line formed by dropping a plum bob from the trochanter, and then measure the position of the knee center and the ankle (or midfoot) from that line. New alignment tools, such as the Laser Assisted Static Alignment Reference (LASAR posture device; Otto Bock Health Care North America, Minneapolis, MN), can form a similar reference line by projecting a vertical laser line up from a force plate onto the patient. This projected laser line may indicate either the center of pressure (C of P) or be translated to a landmark on the patient, such as the trochanter, thus applying a visual demarcation similar to a plumb line. 2,3

The Otto Bock LASAR manual defines two reference lines: the center of gravity (C of G) line and the weight bearing line. 4 According to Otto Bock, the C of G line is the line reproduced on the body when both feet are placed on the force plate in the coronal or sagittal view. Similarly, the weight bearing line is defined as the line reproduced onto the body when the prosthetic side is placed on the force plate and the sound side is placed on a leveling plate in the coronal or sagittal view. However, we have found that the device is more accurately reproducing the C of P line, not the C of G line. 5

Winter 5 suggests that the terms C of G and C of P are often misrepresented and confused. The C of G is the net location of its center of mass in the vertical direction. It is the weight average of the C of G of each body segment. Winter further states that the C of G is a displacement measure and is totally independent of the velocity and accelerations of the total body or its individual segments. C of P is also a displacement measure, but it is the location of the vertical ground reaction vector from a force platform. It is equal and opposite to a weighted average of the location of all downward forces acting on the force plate. This second definition is more consistent with the data produced by the LASAR device.

Modern prosthetic technologies are far more complicated than traditional designs and offer a greater span of dynamic function to the user when aligned properly. Unlike traditional knee designs, the recommended alignment position for the knee center of the C-Leg microprocessor knee (Otto Bock Health Care) unit is located 0 to 5 mm anterior to the reference line. This alignment setting is recommended to optimize function and take advantage of the unique characteristics afforded by microprocessor and software control of swing and stance phase. The C-Leg is currently the only knee unit on the market recommending knee center placement anterior to the reference line. Other prosthetic knees, such as the Mauch knee (Össur, Reykjavík, Iceland) recommend a knee center of 0 as a maximum anterior position.

MATERIALS AND METHODS

A sample of 21 transfemoral C-Leg users attending a national amputee consumer conference was evaluated to measure the position of the knee center in both the coronal and sagittal planes. Each subject was invited to participate, and informed consent was obtained. Alignment was evaluated with respect to a reference line using the LASAR posture device as recommended by Otto Bock. Each participant was initially asked to step on the LASAR plate with both feet flat on the floor to assess possible excessive foot plantarflexion. It was hypothesized that excessive plantarflexion would simulate knee mal-alignment and adversely affect the position of the aligned knee center. Indications of excessive plantarflexion included an apparent gap between the heel of the prosthetic side and the LASAR plate or excessive hip flexion while standing, as well as significant, uneven weight distribution between limbs. Participants who presented with these indications were excluded from the study.

The accepted participants were then asked to stand for alignment measures with their prosthetic limb on the LASAR plate and their sound limb on an equal-height platform. The Otto Bock-recommended reference line used was the vertical line projected onto the patient from the force plate C of P. This reference line was chosen because it most closely represents the weight bearing line that influences knee stability via knee center location. Alignment was evaluated on two planes, sagittal and coronal, noting center of knee, and center of ankle in relation to the reference line.

Observational gait analysis was performed by a physical therapist with extensive experience in amputee rehabilitation. Amputations cause a loss of limb length, normal joint mobility, direct muscular control and local proprioception, particularly the precise awareness of foot contact on the floor. Modern prosthetic design has made major strides in replacing these deficits, yet the amputee gait is often limited compared with the able-bodied gait. 6 It was hypothesized that existing gait deviations derived from the amputation would be emphasized when walking with a malaligned prosthesis. When a transfemoral prosthesis is malaligned, the user or amputee will have to compensate biomechanically, increasing the appearance of existing gait deviations. When the C-Leg is ma-laligned, components of function such as stance flexion are neutralized, and the user then has to compensate biomechanically, as with any other malaligned prosthesis.

Each participant was asked to walk using his or her customary walking speed and then at a pace faster than customary walking speed. Customary walking speed is defined as the rate of walking that is voluntarily assumed. 6 Participants were then asked to walk at their fastest safest walking speed. At increased speed, gait deviations are usually magnified, and with the C-Leg, gait deviations such as waiting for the knee at the end of swing phase and excessive heel rise become more evident. Gait was observed and deviations noted by mild, moderate, or severe/significant deviations, as well as any use of an assistive device.

RESULTS

Of the approximately 40 C-Leg users attending the national consumer meeting, 21 volunteered to be evaluated under the presented protocol. Thirteen male and eight female participants between 24 and 73 years of age (mean, 44.5 years) were evaluated. The predominant amputation etiology was motor vehicle-related trauma. Table 1 lists participant demographics, amputation levels, and etiologies.

Table 1
Table 1:
Participant demographics

The evaluated participants were found to be aligned with the knee center posterior to the reference line by an average of 39 mm, with a range of 0 to 79 mm. Table 2 lists the distance from the reference line to knee center for all subjects. Since the recommendation by the manufacturer is to align the knee unit 0 to 5 mm anterior to the reference line, the prosthetic alignment of 20 of 21 of the microprocessor-controlled C-Leg knee units was suboptimal. The coronal view captured medial-to-lateral translations in alignment. Findings were documented (Table 2), but no significant correlation to alignment could be determined.

Table 2
Table 2:
Participant C-Leg data

Gait observations found gait deviations in 20 of the 21 participants. Major deviations noted in this sample were: decrease in weight shift, lateral trunk lean, pelvic obliquities, decrease in hip extension, uneven or absent arm swing, hip hiking, waiting for knee extension on terminal swing, uneven step length, abnormal swing phase, and uneven heel rise.

DISCUSSION

As with any lower limb prosthesis, the alignment of the C-Leg greatly influences its function, as well as how effectively the user is able to ambulate with the device. The issue of proper alignment is complicated in this particular device because computer software settings can be adjusted to address possible gait deviations. These deviations can be caused by a number of biomechanical reasons, but poor mechanical alignment has been noted to exacerbate gait problems. A trend in gait deviations caused by the C-Leg being aligned with the knee center too posterior or too stable was noted in this sample. To minimize such deviations, proper alignment of the device to the manufacturer’s specifications is required.

The primary physical advantage to having the knee center anterior to the reference line is that it increases the stance flexion. Stance flexion as witnessed on the C-Leg is a small, subtle “wobble” at loading response. During appropriate stance flexion, a 7° flexion angle can be witnessed before the knee goes back to full extension at terminal stance phase. This flexion helps with shock absorption by decreasing the impact at initial (heel) contact. This stance flexion capability also more closely mimics the natural pattern of normal human locomotion. In this sense, activities such as walking over uneven terrain, stairs, ramps, or hills that stress the sound limb may be relieved if the amputee could rely on the stance flexion.

Traditional alignment practices have been confined to mechanical alignment of the knee component itself. With the C-Leg, there exists an additional method of altering stability through software settings and programming the microprocessor controls. Poor mechanical alignment may be compensated for or masked by changes to the software programming parameters to portray normal human locomotion. Although the deviation may be decreased initially with these software changes, the user may be put in a potentially dangerous situation for long-term use. In clinical experience, the “toe load” breakpoint can be altered through programming parameters to be more or less sensitive. When set more sensitive, the knee is cued to “break” into swing phase earlier and under less pressure or ground reaction force. If attempting to correct a deviation and encourage knee swing, increasing toe load sensitivity would initially assist in ambulation. However, when at static stance or low level ambulation, this increase in toe load sensitivity may decrease stability and in fact unexpectedly initiate the knee free swing.

When altering the programming parameters, the physical malalignment may not truly become evident until the user has utilized the C-Leg for several weeks. Such signs of mal-alignment may include significant increase in existing gait deviations, muscle fatigue, and newly reported bodily pain or discomfort, especially in the back and sound side limb. The user may also report difficulty in swinging the prosthesis forward, bending the prosthesis at the knee, or difficulty with ambulation after several hours of wear. Although software adjustments may improve function, in reality these adjustments merely mask the issues and do not totally correct the problem. With time, the user may report significant physical exhaustion as a result of these programming changes when the physical mechanical alignment has not been addressed. Working with the physical mechanical alignment and reference lines produced by the LASAR posture device, the knee center can be more appropriately placed and accurately verified.

As is true with any amputee using any prosthetic limb technology, it is necessary to employ physical therapy to decrease, reduce, or prevent gait deviations from occurring with the C-Leg device.

CONCLUSION

The results showed that 20 of the 21 participants were using C-Legs that were malaligned. A trend in gait deviations caused by the C-Leg being aligned with the knee center too posterior or too stable was noted in this sample. The impact of aligning the C-Leg posterior to the reference line, as opposed to the manufacturer’s recommended position of 0 to 5 mm anterior to the reference line, is evidenced by increased gait deviations, physical exhaustion, and discomfort during normal use. It is clear that the traditional clinical principles of alignment coupled with the natural tendency to maintain prosthesis stability have led to alignment practices of placing the C-Leg knee center posterior to the reference (ie, C of P) line. Detailed practitioner education on the relationship between the C-Leg device and the influence of knee center alignment is needed. Additional study on the impact of knee center alignment location on clinical function also is needed if the ultimate effect of potential malalignments is to be understood.

ACKNOWLEDGMENTS

The team at PRS acknowledges the following for assisting the development of this research effort: volunteer amputees, the Amputee Coalition of America (ACA), and Otto Bock Health Care North America, specifically Greg Schneider, CP.

REFERENCES

1. Radcliff CW. The Knud Jansen lecture. Above knee prosthetics. Prosthet Orthot Int 1977;1:146–160.
2. Blumentritt SA. New biomechanical method for determination of static prosthetic alignment. Int Soc Prosthet Orthot 1997;21: 107–113.
3. Breakey J. Theory of integrated balance: the lower limb amputee. Prosthet Orthot Int 1998;10:42–46.
4. Otto Bock. LASAR Posture Manufacturer’s Protocols. LASAR Posture Device instruction guide. Minneapolis: Otto Bock Health Care; 2000:1–4.
5. Winter D. Kinetics: forces and moments of force. In: Winter D, ed. Biomechanics and Motor Control of Human Movement, 2nd ed. New York: Wiley & Sons Inc.; 1990:93–96.
6. Perry J. Gait deviations. Knee abnormal gait. In: Perry J, ed. Gait Analysis: Normal and Pathological Function. Thorofare, NJ: Slack Inc.; 1992:223–245.
Keywords:

alignment; C-Leg; gait; gait deviations; LASAR posture device; transfemoral amputees

© 2004 American Academy of Orthotists & Prosthetists