The Knee-ds of the K2 Transfemoral Amputee

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As part of a recent joint educational effort by the American Academy of Orthotists and Prosthetists (the Academy) and the American Physical Therapy Association (APTA), a few colleagues and I were asked to address our processes and preferences in recommending the most appropriate prosthetic knee options for geriatric individuals with K2-level transfemoral amputations. As part of our presentation, my colleagues and I were asked to reference the published literature relative to this clinical question. In preparing that material, several key questions emerged: Are microprocessor-controlled knees (MPKs) appropriate? Can they facilitate a transition to a K3 functional level? Should the consistency and simplicity of a locking knee be considered? Not only does the current body of published evidence provide insight into these important questions, it also does a great deal to inform the prosthetic knee decision-making process.

MPKs for Everyone?

The MPK represents a substantial advance in our ability to improve the safety and consistency of prosthetic knee function. This technology can lead to considerable improvements in dayto- day function. But is it appropriate for all K2-level walkers?

Medipro Versatile OFM2HD knee with locking feature for K1 and K2 ambulators. Photograph courtesy of medi USA.

This question was addressed in a recent randomized crossover trial.1 The investigators began with 31 K2-level, unilateral, transfemoral amputees. After a week of activity monitoring and a baseline testing session, subjects were randomly assigned to one of two MPKs: the stance control Ottobock C-Leg Compact, or the swing and stance control Ottobock C-Leg. Following a week of acclimation to the new knees, the testing was repeated. Subjects were then assigned to the other MPK option, followed by an identical acclimation and testing process. They were then returned to their legacy non-MPKs.

The testing measurement used in this investigation was the recently developed Assessment of Daily Activity Performance in Transfemoral amputees (ADAPT), in which subjects with transfemoral amputations are asked to perform a series of simulated activities of daily living (ADL) that amputees have identified as being somewhat difficult to do. These include such tasks as reaching for items on a grocery store shelf, negotiating the aisles of a movie theater, getting and in and out of a car, and a number of other, similar activities. Performances are timed, and participants are asked to rate the difficulty level of each task.

Ottobock Compact MPK with remote. Photograph courtesy of Ottobock.

Given the heterogeneous nature of the population of people who function at the K2 level, it is not surprising to note a tremendous variability in performance on the ADAPT test. To reduce this variation and gain a better understanding of who might benefit from the MPK interventions, subjects were grouped into three functional sublevels: low, intermediate, and high. These divisions were primarily based on recorded daily activity levels and self-selected walking speeds.

With these divisions in place, some general observations could be made. For example, subjects in the intermediate and high subgroups tended to reduce their performance times on the simulated ADLs and recorded lower levels of perceived difficulty when performing the simulated ADLs with the MPKs. Those in the low subgroup tended not to reduce their performance times—sometimes scoring worse in the MPK conditions. While the individuals in all three subgroups were similar in terms of years post amputation, there were some important differences between them. Half of the subjects in the low subgroup had an amputation that was vascular in etiology, while individuals in the intermediate and high subgroups had amputations that were predominantly traumatic in etiology. Further, subjects in the low subgroup had a higher mean body weight and a higher mean age than subjects in the intermediate or high subgroups.

Ottobock C-Leg MPK. Photograph courtesy of Ottobock.

Additional insights can be gained from the attrition that occurred throughout the study. Of the 41 subjects who began the protocols, only 28 completed them. The most common reasons subjects dropped out of the study were technical problems, failure to obtain an appropriate fit and alignment of the various knee joints, and inability to ambulate safely with an MPK. The majority of those subjects who dropped out of the study had amputations that were vascular in etiology. In fact, there were more vascular subjects who dropped out of the study than those who completed it.

Taken together, the observations from the study suggest that while younger, more active patients with transfemoral amputations that are traumatic in etiology appear to benefit from the MPK interventions, older, more sedentary patients with amputations that are vascular in origin either did not benefit from, or simply could not walk safely with, the MPKs.1

To Bend or Not to Bend?

What then is the most appropriate prosthetic knee solution for the lower-functioning K2-level transfemoral walker? The answer suggested by a second crossover investigation may surprise many clinicians. In this study, a convenience sample of 14 geriatric subjects with unilateral transfemoral amputations of dysvascular etiologies was assembled. The subjects in the crossover study were community walkers, wore free-swinging prosthetic knees, and were at least three months removed from their hospital discharge.

Prior to the crossover to the experimental knee conditions, patients completed the Houghton Scale to gauge their current prosthetic usage. In addition, they performed a weekly two-minute walk test (2MWT) to assess their self-selected walking speed. The 2MWT was repeated every week until the patient plateaued in his or her performance, meaning the patient’s 2MWT scores were within 6 percent of the prior week’s performance. At this point, each subject was fitted with a manual locking knee. Following a week’s worth of community ambulation, the subjects repeated the 2MWT and provided additional subjective data.

Surprisingly, 11 of the 14 subjects expressed a preference toward the locked-knee condition. Additionally, across all subjects, gait speed during the 2MWT increased 21 percent in the locked-knee condition. Finally, scores on the Houghton Scale indicated that the locked-knee prostheses were worn and used more often than their free-swinging predecessors.


The Houghton Scale measures function of lower-limb amputees fitted with a prosthesis in terms of wear and prosthesis use. It consists of four items: the amount of time the prosthesis is used, the manner in which it is used, whether an assistive device is used outside, and the individual’s perception of stability while walking outside on a variety of terrain. Each item is scored on a four-point ordinal scale. The perception of stability questions are yes or no. The responses are summed to give a score from one to 12.

Lower Limb Prosthetic Outcome Measures: A Review of the Literature 1995–2005,” Journal of Prosthetics & Orthotics, 2006, vol. 18, no. 1S, pg. 13–45.

The differences in knee preferences can be partially explained by the subjects’ performances with the free-swinging knee. The average 2MWT distance with the free-swinging knee among those subjects who would ultimately prefer the locked-knee condition was 34 meters. By contrast, those who ultimately preferred the free-swinging knee walked more than twice as fast as their peers in this condition, with a mean 2MWT distance of 85 meters. Interestingly, however, even those subjects who said they would prefer the free-swinging knee condition walked substantially faster (26 percent) in the locked-knee condition.

The data from the subjective evaluations confirm what might be expected. Those who preferred the locked knee said the reasons for their choice included feeling more secure, not having to concentrate as much, and improved ability to climb stairs and walk outdoors. Those who preferred the free-swinging prosthesis were comparatively younger and more active patients and reported that they preferred the free-swinging condition because the knee felt more “natural.”

The study findings suggest that older subjects with transfemoral amputations who are at the lower end of the K2 functional level may benefit from the simplicity and consistency of a manual locking knee. However, even though the higher functioning subjects may walk more quickly with a locked knee, they may still prefer the more natural gait afforded by a free-swinging prosthetic knee.

Bridging the K3 Divide

The traits that differentiate the K3 ambulator from the K2 walker are the ability and/or potential to walk at variable cadences and negotiate most environmental barriers. For those patients who function at the upper end of the K2 level, the question becomes whether advanced knee technologies such as MPKs can facilitate a transition into variable-speed ambulation and the negotiation of environmental barriers. Recent research strongly suggests that they can.

In one of the earliest studies to evaluate the effect of MPKs outside of the regulated gait laboratory environment, Hafner and Smith conducted an informative clinical trial.3 Using both K2- and K3-level walkers, after a series of defined, short-duration crossovers between their existing conventional knees and newly fitted MPKs, patients entered into an extended 12-month evaluation period with their MPKs. The outcomes used in the trial spanned a broad range of functional measurements and considerations, and several improvements were observed. For the K2-level walkers, these included the quality and speed of gait during hill ascent, gait quality during stair ascent, walking speed across a defined outdoor obstacle course, and improved gait speed and accuracy during an attentional demand activity. Indeed, the relative increases in functional performance for the K2-level walkers were higher than those observed within the K3-level cohort.

Consistent with these improvements, of the eight subjects who began the study at a K2 functional level, half concluded the study at a K3 functional level. This observation is all the more striking when we clarify that the subjects in question presented with a mean time since amputation of 17 years. In other words, the observed improvements came after prolonged periods of prosthetic use and experience and can be reasonably attributed to the functional possibilities afforded by the advanced knee technologies. Almost by definition, a knee joint that facilitates a 10 percent increase in gait speed across an outdoor obstacle course with environmental barriers will redefine a K2-level ambulator as a K3-level walker.

These findings were further reinforced by the observations from another crossover study. In this investigation, ten K2-level subjects with unilateral transfemoral amputations were tested with their legacy conventional prosthetic knees followed by Ottobock Compact C-Leg MPKs.4 The study cohort presented with a mean age of 62 years and an average of nine years since the date of amputation. Each subject underwent a physical therapy training session in his or her existing prosthesis prior to a baseline data collection. These assessments included evaluations of ramp ascent and descent, the Timed Up and Go (TUG), and several questionnaires including both the Houghton and Activity-Specific Balance Confidence (ABC) Scales. Following these initial observations, subjects were provided with Compact C-Leg MPKs and attended several additional physical therapy sessions over the course of a three-month acclimation period, followed by a second, similar round of data collection.


The Timed Up and Go (TUG) test assesses a person’s mobility, balance, walking ability, and fall risk, and requires both static and dynamic balance. The TUG measures the time it takes for a person to rise from a chair, walk three meters at a comfortable and safe pace, turn around, walk back to his or her chair, and sit down. During the test, the person is expected to wear regular footwear and use any mobility aids that he or she would normally require. Scores range from one to five based on the observer’s perception of the patient’s risk of falling.

Rehabilitation Measures Database (


The Activity-Specific Balance Confidence (ABC) Scale is a 16-item self-report measure in which patients rate their balance confidence in performing several activities. Items are rated on a scale of zero to 100. A score of zero represents no confidence, while a score of 100 represents complete confidence. The patient’s overall score is calculated by adding item scores and then dividing by the total number of items performed.

Rehabilitation Measures Database (

Recalling that the distinguishing characteristics of a K3-level walker are the ability or potential to walk at variable cadences and negotiate environmental barriers, the observations of this trial further underscore the potential of MPKs to facilitate the transition from the K2 to the K3 functional level. On average, subjects increased their ramp ascent and descent by 28 percent and 36 percent respectively. This was accomplished through a combination of increased cadences and step lengths during both walking activities. These observations alone would seem to validate K3-level performance as they represent the abilities to ambulate at variable speeds and negotiate ramps, one of the most formidable environmental obstacles for an individual with a transfemoral amputation.

Augmented gait speeds were further validated by the improvements in TUG scores, which decreased from 24.5 seconds on average to just less than 18 seconds. The improved ability to negotiate environmental objects was further evidenced by the scores on both the Houghton and ABC Scales. A number of tasks on the ABC, such as walking up or down stairs, sweeping the floor, walking outside the house to a car parked in the driveway, and negotiating an escalator, include environmental barriers. Patient scores with the MPKs indicated improvements across both scales.

Delving a little further, the original ABC scores obtained with the legacy knee systems averaged a confidence level of 60 percent, consistent with the values reported by Miller et al. for community-dwelling adults with transfemoral amputations.5 By contrast, the average scores while using the C-Leg Compact were reported at just below 76 percent, a confidence that approaches that of normal, community-dwelling older adults.

Summing It Up

The K2 population is a very heterogeneous one, with patients who are challenged by outdoor environmental barriers, and patients who appear ready to accept the challenges of variable cadence and outdoor community walking. Despite these differences, they tend to share underlying concerns of safety and stability. Accordingly, at the transfemoral amputation level, the choice of prosthetic knee is an important one. This choice is made more difficult by the realization that appropriate solutions can range from the simplicity of a conventional manual locking knee to the technical elegance of today’s MPKs. The challenge for rehabilitation professionals is to match the appropriate knee option with the performance potential of each individual. Fortunately, a number of crossover trials provide insights to guide the decision-making process.

Phil Stevens, MEd, CPO, FAAOP, is in clinical practice with Hanger Clinic, Salt Lake City, Utah. He can be reached at


  1. Theeven, P., B. Hemmen, F. Rings, G. Meys, P. Brink, R. Smeets, and H. Seelen. 2011. Functional added value of microprocessor-controlled prosthetic knee joints in daily life performance of Medicare Functional Classification Level-2 amputees. Journal of Rehabilitation Medicine 43(10):906–15.
  2. Devlin, M., L. B. Sinclair, D. Colman, J. Parsons, H. Nizio, and J. E. Campbell. 2002. Patient preference and gait efficiency in a geriatric population with transfemoral amputation using a free-swinging versus a locked prosthetic knee joint. Archives of Physical Medicine and Rehabilitation 83(2):246–9.
  3. Hafner, B. J. and D. G. Smith. 2009. Differences in function and safety between Medicare Functional Classification Level-2 and -3 transfemoral amputees and influence of prosthetic knee joint control. Journal of Rehabilitation Research & Development 46(3):417–34.
  4. Burnfield, J. M., V. J. Eberly, J. K Gronely, J. Perry, W. J. Yule, and S. J. Mulroy. 2012. Impact of stance phase microprocessor-controlled knee prosthesis on ramp negotiation and community walking function in K2 level transfemoral amputees. Prosthetics & Orthotics International 36(1):95–104.
  5. Miller, W. C., A. B. Deathe, M. Speechley, and J. Koval. 2001. The influence of falling, fear of falling, and balance confidence on prosthetic mobility and social activity among individuals with lower extremity amputation. Archives of Physical Medicine and Rehabilitation 82:1238–44.

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