OTTO BOCK C-LEG ALIGNMENT
I would like to voice some concerns I have about the conclusions and the method of investigation reported in the article “Measurement of Knee Center Alignment Trends in a National Sample of Established Users of the Otto Bock C-Leg® Microprocessor Controlled Knee Unit” that appeared in the July 2004 issue of JPO. The article reports on the use of the Otto Bock LASAR posture device to evaluate alignment of the Otto Bock C-Leg. It concludes that based upon the measured alignment, “20 of the 21 participants were using C-Legs that were malaligned.” I strongly disagree with this conclusion.
The procedure for properly aligning a transfemoral prosthesis consists of three separate and distinct parts: bench, static, and dynamic alignment. These procedures should be followed for alignment of all transfemoral prostheses, including the C-Leg. The published Otto Bock alignment recommendations provided with the C-Leg refer to the horizontal arrangement of components in the sagittal plane during bench alignment. It is recommended that the knee center of the C-Leg be placed 0 to 5 mm anterior to the “reference alignment line,” and each foot approved for use with the C-Leg also has a recommended horizontal position relative to the reference line (Figure 1). The next step is static alignment of the prosthesis while the patient is standing. Typically, adjustments to alignment during static alignment are made to account for the changes resulting from the application of the body weight of the amputee. Compliance in the foot and ankle requires that the plantarflexion angle be increased to provide sufficient anterior lever in response to the patient's body weight. It is in static alignment that the LASAR posture device may be used to evaluate the position of a vertical line through the center of pressure, an effective indication of appropriate plantarflexion. In fact, the current recommendations for proper alignment of the C-Leg, as recommended by Siegmar Blumentritt1 of the Otto Bock Company in a presentation at the ISPO meeting in Hong Kong last August, are as follows:
Bench alignment is completed based on the published recommendations, and then static alignment is performed using the LASAR unit to adjust the plantarflexion so that the line falls 30 mm anterior to the knee center. Further subtle adjustments may be made during dynamic alignment.
During dynamic alignment, the plantarflexion attitude of the foot may require additional adjustment based on the observed gait and amputee feedback. Factors that affect the final plantarflexion attitude are: degree of dorsiflexion stiffness, length of the anterior lever, and inertia or forward momentum of the individual during ambulation.
The article states that 20 of the 21 devices were aligned posterior to the recommended alignment protocol. However, as discussed here, the recommended alignment protocol provided with the C-Leg refers to bench alignment, and it is inappropriate to apply that reference line to the vertical line through the center of pressure as produced by the LASAR posture device. Therefore, the conclusion that 20 of the 21 C-Legs were suboptimally aligned is inappropriate. Such conclusions cannot be reached given the data available. In addition, it is not possible to conclude from the LASAR alignment data collected that gait deviations, physical exhaustion, or discomfort are the result of suboptimal alignment because no such suboptimal alignment was revealed.
With regard to stance flexion, I point out that the position of the LASAR-produced line is of little use in determining knee stability at loading response, as suggested in the article. Finding the position of a vertical line through the center of pressure is not necessarily a predictor of knee moments during loading response. This point is illustrated in Figure 2, which shows a young traumatic amputee with a long residual limb standing in his C-Leg with 1C40 foot after dynamic alignment (the same prosthesis shown in Figure 1). The line produced by the LASAR posture is 42 mm anterior to the knee center. This patient demonstrated stance flexion, consistently lighting two of the knee flexion boxes and sometimes three in the Sliders program during loading response. Clearly, amputee input using hip musculature to produce moments around the hip joint, as well as foot-knee interaction, must be considered when addressing prosthesis stance-flexion.
Although I agree that alignment is critical to optimal function of any transfemoral prosthesis and that an improperly aligned prosthesis can cause the physical problems and gait deviations discussed in the article, the information obtained from the LASAR posture device is insufficient to form the conclusions made in the article.
Jack E. Uellendahl, CPO
Hanger Prosthetics & Orthotics
1. Blumentritt S. LASAR Posture Alignment System: Biomechanics and Practical Use
. Lecture at 11th World Congress of the ISPO, Hong Kong, China, August 3, 2004.