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Case Report

Using Clinically Relevant Outcome Measures to Assess the Ambulatory Efficiency, Balance Confidence, and Overall Function Associated With “Stubby” Prostheses and C-Leg Prostheses for a Patient With Bilateral Transfemoral Prostheses

Carson, Randy DPT; Stevens, Phillip M. MEd, CPO; Webster, Joseph B. MD; Foreman, K. Bo PT, PhD

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JPO Journal of Prosthetics and Orthotics: April 2010 - Volume 22 - Issue 2 - p 140-144
doi: 10.1097/JPO.0b013e3181d39a52
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The appropriate provision of rehabilitative care to patients with multiple limb loss is challenging. This case study presents the use of several clinically relevant outcome measures to assist in the determination of whether our case subject, who had undergone bilateral transfemoral amputations and amputation of the right upper limb at the transhumeral level, was better served by regular use of her bilateral “stubby” prostheses or bilateral C-leg prostheses. Specifically, the Physiologic Cost Index (PCI), Activities- specific Balance Confidence (ABC) Scale, and the Canadian Occupational Performance Measure (COPM) were used to quantify gait efficiency, balance confidence, and general functional abilities experienced in the two prosthetic systems.


Our case subject was a 46-year-old woman who underwent bilateral transfemoral and right transhumeral amputations as a consequence of necrotizing fasciitis and streptococcal toxic shock syndrome secondary to streptococcal infection. The subject received extensive and regular physical therapy from the time of her initial amputation through the data collection described, approximately 3 years later. Her initial prostheses were shortened prosthetics without knee joints, commonly known as “stubbies.”1,2 Later in her rehabilitation, the subject also received and excelled at gait training with the use of microprocessor-regulated C-legs (Otto Bock). At the time of data collection, the subject could safely and independently navigate variable terrains in both prosthetic systems but had a strong preference for her stubbies. The intent of this investigation was to better understand why our subject, who expressed a strong interest in increasing her use of the C-legs, maintained a preference toward the use of her stubbies.

The subject used the same sockets for all testings. The stubbies and C-legs could be alternately attached through the use of Ferrier couplers. Custom silicone cushion liners were used as an interface material against the subject's skin. Suspension was achieved through an elevated vacuum, double-wall prosthetic system in which a proximal sealing sleeve was attached to an inner socket, creating an air-tight suspension system. Vacuum was drawn within the inner socket system through a distal exit valve. The inner socket was connected, in turn, to a rigid outer socket through a distal lanyard attachment. The Ferrier couplers were connected to the distal aspect of the rigid out sockets.

Physiologic Cost Index

The PCI was used to quantify the differences in energy expenditure. The PCI has been shown to be an indicator of exercise load when compared with oxygen consumption while using a microprocessor knee.3 It also offers good clinical utility by allowing measurements to be taken on terrain typical of community walking. The PCI is calculated as follows:

All heart rate data were obtained via a Polar heart rate monitor. The subject's resting heart rate was established after the subject rested in supine for 10 minutes. The subject's walking heart rate was taken for every 50 feet. Distance was measured using a standard measuring wheel. This required one person for subject safety/guarding and heart rate monitoring, one person for measuring the distance, and one person for recording the data. The subject walked along a predetermined course that included smooth level surfaces, uneven concrete, grass, undulating smooth concrete, smooth descents, smooth off-camber descents, and smooth concrete ascents. The total distance of the trial was 3,000 feet. This allowed a collection of 60 heart rate measurements. The patient had ambulated across this course regularly in both prosthetic systems as a part of the physical therapy before any data collection. Data for the two conditions were collected on separate days.

Balance Confidence

The ABC scale is used to assess a subject's perception of their own balance confidence in the performance of defined daily activities.4 Within the amputee population, the ABC has been found to be both valid and reliable,5 and demonstrated a high correlation with mobility capability, mobility performance, and social activity.6 The ABC is a 16-item questionnaire in which the subject rates their confidence that they will not fall on a scale between 0% (no confidence) and 100% (total confidence) when performing a variety of upright activities with varying degrees of difficulty.

As with the PCI data, the ABC was administered to the subject on two occasions, once for each condition. In both cases, the ABC was administered after the successful negotiation of the walking course described above with the subject reporting confidence values for the prosthesis she was wearing at the time. The item of “... stand on your tiptoes and reach for something above your head?____%” was excluded because this activity is not possible in either of the prosthetic systems evaluated.


The COPM was used to help the subject prioritize and objectify the importance of her activities of daily living. A recent systematic review has supported the COPM as a valid, client-centered outcome measure.7

The tool is used to prioritize the most meaningful functional needs of an individual. It is divided into three main areas: self-care, productivity, and leisure. The patient identifies subcategories and functional activities across these three domains until a total of 27 functional activities have been recorded. After the functional activities have been identified, they are prioritized by the patient, allowing the evaluator to determine the five functional activities that are most important to the patient. The subject then rates those five most important functional activities on a 1 to 10 scale with regard to both performance and satisfaction with that performance.8

Our subject was asked to cite functional activities affected by her use of prosthetics that were important to her in each of the subcategories. The subject was given standard examples of activities in each subcategory. The subject was asked not to consider which prosthetic system she was using, but instead to identify the most important functional activities to her daily life with respect to prosthetic use. The subject prioritized the functional activities, and the five most important activities were identified. The subject was then asked to rate her perceived current performance in each of the five activities while wearing the stubbies and the C-legs. The subject was also asked to rate her perceived satisfaction with her current performance in each of the five activities while wearing the stubbies and the C-legs.

Statistical Analysis

Pre and post data were entered into Microsoft Excel 2008 for Macintosh (Microsoft; Redmond, WA). Next, data were imported into SPSS version 16 for Macintosh (SPSS Inc., Chicago, IL) for further analysis. Descriptive statistics were performed for the outcome variables of heart rate, PCI, COPM, and ABC. In addition, to evaluate the differences using the Stubbies versus the C-legs for the variables of heart rate and PCI, the paired t-test was used (two tailed, alpha level =0.05).



The subject was found to have a significantly lower heart rate when walking with stubbies (p < 0.01). Her average heart rate was 96.33 ± 6.48 bpm (peak: 110 bpm) with stubbies and 110 ± 13.34 bpm (peak:143 bpm) with C-legs (Figure 1). This translates to a PCI of 0.78 ±.18 and 1.06 ± 0.36, respectively (Figure 2). The subject completed the walk 3 min 40 sec faster with the C-legs (stubbies: 27:26 min; C-legs: 23:46 min). Despite this increase in speed while walking with the C-legs, the subject was found to have a significantly lower PCI when walking with the stubbies (p < 0.01; Figure 2).

Figure 1.:
Heart rate taken every 50 feet of a 3,000 foot, predetermined course that included smooth level surfaces, uneven concrete, grass, undulating smooth concrete, smooth descents, smooth off-camber descents, and smooth concrete ascents. Inlaid lower graph denotes the relative elevations at the respective distances.
Figure 2.:
PCI taken every 50 feet of a 3,000 foot, predetermined course that included smooth level surfaces, uneven concrete, grass, undulating smooth concrete, smooth descents, smooth off-camber descents, and smooth concrete ascents. PCI was calculated as (walking heart rate − resting heart rate)/meters per minute walking pace. Meters per minute walking pace was averaged over each 100-foot interval. Inlaid lower graph denotes the relative elevations at the respective distances.

Balance Confidence

The subject reported a higher level of balance confidence with stubbies versus C-legs. The subject averaged an overall confidence rating (range: 0–100) on the ABC of 84.33 ± 25.97 with stubbies and 43.33 ± 27.69 with C-legs. The subject rated her confidence in all categories higher with stubbies compared with C-legs with the exception of “Standing on a chair and reach for something,” which the subject graded her confidence at 0 with both prostheses.


The subject was shown to have a higher performance rating and satisfaction with performance rating with stubbies versus C-legs. The domains identified by the subject and the associated ratings for each prosthetic system are presented in Table 1. The subject averaged an overall performance rating (range: 1–10) on the COPM of 8.00 ± 1.73 with stubbies and 3.80 ± 1.10 with C-legs. The subject averaged an overall satisfaction with performance rating (range: 1–10) on the COPM of 7.20 ± 2.05 with stubbies and 4.60 ± 1.82 with C-legs.

Table 1:
Subject Canadian Occupational Performance Measure (COPM) data with stubbies and C-legs



For our subject, the stubbies were associated with a significantly lower physiologic cost than the C-legs. Even though the subject's velocity was much faster with the C-legs, the associated increase in heart rate seemed to compromise the efficiency of that system.

Balance Confidence

The subject perceived the stubbies to be considerably safer than the C-legs. Reporting across a cohort of 435 amputees in an outpatient setting, Miller et al.6 found a mean ABC score of 62 for this population. Although their cohort excluded bilateral cases such as our subject, it does provide a reference frame for our subject's reported scores of 84 and 43 for the stubbies and C-legs, respectively. Balance confidence has been shown to be directly related to mobility capability, mobility performance, and social interaction in amputees.6 This is consistent with our subject's usage patterns. She chooses to use her stubbies at home even though she is able to consistently walk on challenging terrain with C-legs during therapy sessions without loss of balance. When asked, the subject states that she feels safer with the stubbies. She has also identified the two primary factors that influence her preference for stubbies as 1) her overall increase in height while wearing C-legs and 2) the presence of articulating knees that may rarely buckle.


The subject consistently reported higher satisfaction of performance ratings with her stubbies with regard to her five most important functional activities while using prosthetics. The COPM is an excellent tool as it allows for assessment of client-centered goals. However, her goals were all balance related, and the results seem to be dependent on subject's confidence in the two prosthetic systems, suggesting the possibility of redundant information with his measure. There is no research that has attempted to correlate the findings of the COPM and the ABC. However, it seems to be a distinct possibility.


The purpose of this endeavor was to quantify some of the factors that influenced our patient's daily decisions regarding which prosthetic system to use. As indicated, our subject demonstrated the ability to ambulate safely and independently across variable terrains in both systems. However, factors outside of ability seem to influence the choice of which system to use. In questions of energy efficiency, balance confidence, and functional performance across daily activities identified by the subject, higher scores were consistently associated with the stubby prosthesis. These elements seemed to strongly influence our subject's preferences between the two prosthetic systems.


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6. Miller WC, Deathe AB, Speechley M, Koval J. The influence of falling, fear of falling, and balance confidence on prosthetic mobility and social activity among individuals with a lower extremity amputation. Arch Phys Med Rehabil 2001;82:1238–1244.
7. Dedding C, Cardol M, Eyssen IC, et al. Validity of the Canadian Occupational Performance Measure: a client-centred outcome measurement. Clin Rehabil 2004;18:660–667.
8. Law M, Baptiste S, McColl M, et al. The Canadian occupational performance measure: an outcome measure for occupational therapy. Can J Occup Ther 1990;57:82–87.

transfemoral amputation; stubbies; outcomes assessment; balance confidence; energy

© 2010 American Academy of Orthotists & Prosthetists