Mr. Uellendahl voices a number of concerns regarding the study methodology and the conclusions presented in our article. He correctly emphasizes the value of conceptualizing three phases of alignment: bench, static, and dynamic. Each step is an evolution toward the ultimate goal of a proper-fitting and well-aligned prosthesis, as agreed upon by a clinician and a patient. Manufacturers often offer recommendations for bench and static alignment based upon their extensive experience with specific components. Although these recommendations are not designed as a substitute for an iterative process of dynamic alignment, they do suggest settings that may help many patients benefit from a particular component. In this particular study, the Otto Bock C-Leg® was the component of interest.
We acknowledge that C-Leg knee center recommendations specified by Mr. Uellendahl are applicable to the bench alignment stage and that they may not be the same as the ultimate result of the entire alignment process. However, the C-Leg training course manual (dated May 6, 2001) also recommends placement of the knee axis of 0 to 5 mm anterior to the load line (Figure 1).
The “load line” is defined by Breakey1 as the vertical laser line projected from the center of pressure while the subject stands with the prosthetic limb on the Otto Bock LASAR Posture device and the sound limb on the compensation plate. Our original assessments of alignment and resultant conclusions were based upon this interpretation of the C-Leg alignment recommendations. Unfortunately, “load line” is a poor choice of terminology in that it may be confused with the same term as defined by Radcliffe.2 Radcliffe defines the load line as “the line along which the force between the foot and the floor acts. In general, it is not perpendicular to the floor surface since this force has two effects. First, it supports the body weight in a vertical direction, and second, it provides the horizontal forces necessary to cause motion of the body in the forward and medial directions” (pp 37–38). As mentioned in the original article, the line produced by the LASAR is the vertical component of the line projected from the center of pressure measured by the force plate.
Mr. Uellendahl correctly points out that the C-Leg alignment recommendation has since been revised by Dr. Blumentritt to be 30 mm posterior to the LASAR line. This revised recommendation was announced at the International Society of Prosthetists and Orthotists (ISPO) meeting held in Hong Kong in August 2004. This announcement occurred 1 month after our manuscript was published and, to our knowledge, the revised manufacturer’s recommendations have not yet been published.
Based on the current recommendation of the C-Leg knee center position 30 mm posterior to the LASAR line, re-evaluation of the data collected in this study shows that 11 of the 21 patients are aligned with a knee center 45 mm or more posterior to the load line. This is 15 mm (50%) more than Otto Bock's current recommendation of 30 mm (Table 1).
Even taking the revised alignment recommendations into account, the data presented still show that the majority of C-Leg users measured in this sample are statically aligned (with respect to the LASAR line) in a position substantially more stable than the manufacturer's recommendation. This information, coupled with the gait deviations documented by our trained physical therapist in the original population, suggests that these patients may indeed be aligned in a position more stable than is necessary for this knee.
Mr. Uellendahl has provided an excellent clinical example of a patient having an alignment more stable (in static alignment) than the manufacturer's recommendation but who is able to demonstrate stance flexion. The individual illustrated is a young amputee with active musculature and a knee center alignment 42 mm posterior to the LASAR line. This raises an important question about the range of acceptable deviation from the manufacturer's recommended alignment but does not preclude the possibility that this patient would experience this benefit of the C-Leg using less muscular activity and metabolic energy if the knee axis were aligned closer to the LASAR line. Previous work by Blumentritt et al.3 has suggested that there is an optimal final alignment for active amputees that minimizes the effort required to either stabilize the knee during stance or to flex it during swing phase and that marked deviation from this optimal position results in measurable increases in the hip extension or hip flexion moment generated by the amputee to control the prosthesis.
Positioning the knee center closer to the LASAR line may allow the user to experience stance flexion more readily than when the knee is in a more stable position. Clearly, it is important to a clinician that a patient maintains safety and control of the knee throughout gait, and some amputees may not be comfortable using the stance flexion feature, particularly if they are experienced traditional knee users. However, we are confident that with the proper experience and training, many users can experience and benefit from stance flexion of the C-Leg.
We acknowledge that additional data beyond that collected in our study may be needed to definitively answer the question of whether or not a C-Leg is suboptimally aligned. We submit that there is evidence to suggest that many of the patients studied in the published population are aligned in a position more stable than necessary. This position of knee alignment, unlike a position of knee instability, is not dangerous to an amputee. However, it may cause increased fatigue, difficulty initiating swing phase, or even prevent an amputee from utilizing the stance flexion feature of the knee. Our research and clinical experience with more than 20 C-Leg users supports this opinion, but we acknowledge that the manufacturer's recommendations may not be suitable for all patients.
We agree with Mr. Uellendahl that the position of the C-Leg knee center should be posterior to the LASAR line and closer to the current recommendation of 30 mm posterior to the LASAR line. The discrepancies between published recommendations clearly illustrate the need to more clearly define and document the range of accepted final alignment, particularly with respect to advanced prosthetic components such as the C-Leg. We kindly thank Mr. Uellendahl for his comments, criticisms, and insight into this very important aspect of clinical care. PRS intends further investigations of transfemoral alignment in an effort to study, document, and more completely understand this complex and critical issue.
Douglas G. Smith, MD
Brian J. Hafner, PhD
Laura L. Willingham
Kate J. Allyn, LCPO
Prosthetics Research Study, Seattle, WA
1. Breakey JW. Theory of integrated balance: the lower limb amputee. J Prosthet Orthot
2. Radcliffe CW. Functional considerations in the fitting of above-knee prostheses. Artif Limbs
3. Blumentritt S, Scherer HW, Michael JW, Schmalz T. Transfemoral amputees walking on a rotary hydraulic prosthetic knee mechanism: a preliminary report. J Prosthet Orthot