The advent and the refinement of microprocessor-regulated prosthetic knee joints (MPKs) have been the seminal advancements in the prosthetic management of patients with transfemoral amputations. Clinical observations and patient feedback have suggested that this technology has the potential to improve the health and the safety of thousands of patients. In the presence of this emerging paradigm change, the profession has a responsibility to address two fundamental questions. First, have we defined the clinical value of MPKs for ourselves, colleagues in rehabilitation, policy makers, and third-party payers? Second, have we defined those populations who will most likely benefit from the application of this technology? The remainder of this perspective piece will address several considerations that may ultimately inform these two fundamental questions.
For many years, there was a discrepancy between the focus of empirical research in this area and clinical observations. Perhaps because of the preceding work on first-generation MPKs that regulated only swing phase, early studies were focused on the laboratory-centric measures of energy expenditure and gait variables. Statements drawn from a 2005 review of the early MPK literature exemplify the misunderstanding that resulted from investigating these domains (emphasis added):
- “For level ground ambulation, the main advantage seems to be the ability of variable-dampening knees to adapt to different walking speeds.”1
- “These advantages substantially improve the mobility in individuals who live an active life … Generally prescribed to young and very active individuals.”1
- “Conversely, the prescription of variable dampening knees is often discouraged in the remaining amputee population.”1
- “… insufficient evidence to support the hypothesis that variable dampening knees provide advantages at SSWS [self-selected walking speed].”1
In contrast, patient-generated feedback tended to focus on the more practical considerations of stability, confidence, cognitive demands, and expanding activity levels, areas in which associated improvements were available to transfemoral patients beyond the “young and very active” who needed to “adapt to different walking speeds.”
Fortunately, recent years have seen the focus of many of the emerging studies shift over to those domains that seem to be more relevant to the health and well-being of the end users such as safety, balance, environmental object negotiation, and cognitive burden. Further, the relative improvements seen in these domains have tended to be more striking than those observed in the areas of energetics and gait metrics.
These observations are supported by the evidence statements made elsewhere in the body of these conference proceedings. Of the empirical evidence statements on the effect of MPKs versus non-microprocessor-regulated prosthetic knee joints (NMPKs), the only statement supported by a “moderate” level of evidence in the domain of “metabolic energy expenditure” was the statement that “the use of swing and stance MPKs results in an equivalent oxygen cost (at self-selected, slow, and fast speeds) compared to the use of NMPKs among individuals with unilateral transfemoral limb loss (emphasis added).”2 Further, despite the fact that various aspects of “gait mechanics” were included by 10 publications in the systematic review, none of the empirical evidence statements made in this domain were supported by a moderate level of evidence. To the extent the evidence statements could be made on “low” levels of evidence, these were largely statements of equivalence between MPKs and NMPKs.2 These observations confirm the commonly held clinical position that the true value of MPK technology will not likely be found in scrutinizing its effects on energetics and gait mechanics in level-ground walking.
Further insights from the systematic review are found in an examination of the four evidence statements that were supported by a moderate level of evidence and that identified a relative improvement associated with MPKs. These include statements that, compared with the use of NMPKs, MPKs result in the following:
- An increased self-reported mobility
- A decreased perception of the cognitive demand required for walking
- An increased subject-reported confidence while walking
- Increased self-reported well-being
These observations seem to explain one possible reason why an abundance of supportive literature for MPKs was not found in the early research on the technology. The value of the intervention was limited in the carefully controlled environment of the gait laboratory and its examination of unidirectional walking at a self-selected pace across level ground. Instead, the value of MPKs seems to be more evident in the challenges encountered during basic day-to-day environmental ambulation and function. These challenges include uneven terrain, stairs, slopes, and environmental perturbations. As the profession continues to define the value of MPKs to end users, these efforts should be focused on the overlapping domains of stability and confidence, cognitive burden during ambulation, activity and participation, and safety.
Early studies on MPKs created the impression that the benefits of the technology could be experienced only by high-functioning amputees, an unfortunate misunderstanding that continues to influence payers. By contrast, evaluations and observations made outside the traditional gait laboratory seem to strengthen the assertion that the benefits of MPKs are experienced in the daily negotiation of common environmental obstacles and tasks. Several established and emerging outcome measures provide some indication as to how the challenges to basic function regularly encountered by patients with transfemoral amputations might be evaluated.
ACTIVITIES-SPECIFIC BALANCE CONFIDENCE SCALE
The Activities-Specific Balance Confidence Scale (ABC), originally developed for the geriatric community3 but since validated within the amputee population4 represents one measure that can assess the impact of a treatment modality on activities that constitute the minimal functional demands of many within the amputee community. It requires subjects to rate their relative confidence in their ability to maintain their balance during basic functional tasks. These include the following:
- Walking around the house and up or down stairs
- Bending over to retrieve an item from the floor
- Reaching for an item at eye level
- Sweeping the floor
- Walking to a car parked in a driveway and walking across a parking lot
- Getting in and out of a car
- Walking up or down a ramp
- Walking in a crowded shopping center and being bumped into within this environment
- Stepping on or off an escalator, both with the use of the railing and while holding something that precludes the use of the railing
- Walking outside on icy pavements
Importantly, these activities are routinely encountered by individuals across a broad spectrum of functional abilities. To the extent that the ABC has been reported in the published literature, it has suggested that the benefits of MPKs can be experienced during these basic tasks that are foundational to independent living.5,6
ASSESSMENT OF DAILY ACTIVITY PERFORMANCE IN TRANSFEMORAL AMPUTEES
More recently, the Assessment of Daily Activity Performance in Transfemoral amputees (ADAPT) has been presented as another measure of basic functionality within the transfemoral patient population.7 It has demonstrated both a high test-retest reliability and the ability to differentiate the performances of two patient cohorts using MPK and non-MPK prosthetic knee joints.7 In contrast to the ABC, the ADAPT is a timed performance measure. Consistent with the ABC, the measure is based on activities germane to the basic objective of independent living. These include the following:
- Loading a shopping cart with objects of different size/weight from shelves at different heights, followed by bagging the groceries
- Unloading the shopping bags into shelves at varying heights
- Hanging towels to dry
- Getting up and retrieving the remote from the television
- Navigating a living room with obstacles on the ground
- Getting in and out of a car
- Walking sideways between two theater rows while holding a cup of water
- Navigating stairs with and without a cognitive dual task
- Walking up and down a sloped road, with and without a cognitive dual task
As with the ABC, the tasks that compose the ADAPT are very basic to daily function for even mildly active individuals. To the extent that the ADAPT has been used to evaluate MPKs, it has shown that, even with minimal accommodation periods, patients who have difficulty achieving variable speed ambulation seem to benefit from the use of MPK technologies for activities of living.8,9
COGNITIVE BURDEN OF AMBULATION
Our current understanding of the effect of MPKs on ambulation under cognitive load can be condensed into the two evidence statements from the systematic review, both of which were supported by a moderate level of evidence, summarized below (emphasis added):
- “The use of SNS MPKs results in equivalent ability to ambulate while performing a secondary cognitive task.”2
- “The use of SNS MPKs results in a decreased perception of cognitive demand required for walking.”2
Taken together, these evidence statements provide the impression that the basic need to walk safely will take precedence over thinking about less immediately relevant things. This is even more likely in most laboratory-performed cognitive loading tests in which the items demanding cognition (such as the performance of simple mathematic problems) are of little real concern to the subject. Alternately, it may be that the cognitive loads that have been placed upon study subjects have been inadequate to affect walking performance. In either event, the reported decreased perception of cognitive demand seems to indicate that the phenomenon of reduced cognitive load occurs when using an MPK even if the metrics used to date to record it have been unsuccessful. This hypothesis is bolstered by the anecdotal reports in the clinical environment that patients who transition from an NMPK to an MPK no longer need to “think about every step” or “think so much about walking.” For many transfemoral amputees, there is a very real anxiety that is experienced even during basic ambulation and obstacle negotiation that seems to be mitigated with the use of MPKs. The combination of negotiating the environments and tasks such as those identified in the ABC or the ADAPT while carrying a substantial, meaningful cognitive load may further clarify this anxiety and how it may be affected by prosthetic knee choice.
SAFETY: STUMBLES AND FALLS
To the extent that it has been reported, the use of MPKs has been consistently associated with a reduction of reported stumbles and falls.10,11 The metric is a challenging one for several reasons. First, it is most commonly assessed retrospectively as patients attempt to recall prevalence rates over a given period. As such, it may be prone to the biases of faulty recollection. Second, stumbles and falls can occur for a number of different reasons. Higher-functioning patients often experience falls as they push the bounds of their functionality and are less likely to adversely be affected by these events. By contrast, lower-functioning patients tend to fall because of their physical limitations and are often at a greater risk for fall-related injury.
Future evaluations on the effects of MPKs in mitigating stumbles and falls would benefit from a more objective method of capturing fall events and correlating these figures with activity levels. For example, the data of both Kahle et al.10 and Hafner and Smith11 suggest that, with the introduction of MPKs, the reduction in stumbles and falls among subjects at lower levels of functional ability is more pronounced than observed with higher-functioning subjects. These observations are consistent with the premise that falls among those with lower functional levels are often the results of their physical limitations and that these may potentially be reduced in this population with the application of MPK technologies.
The benefits experienced by individuals with transfemoral amputations using MPKs are suggested by patient comments volunteered in the clinical environment. These include such statements as, “I fall less,” “I feel more stable,” “I don’t have to think about every step any more,” “I feel more comfortable walking outside” and “I can do more.” Given these and the dozens of similar statements made by patients after transitioning to MPKs, it can be reasonably assumed that their value lies in their effects on the domains of safety, stability and confidence, activity and participation, and cognitive loading. Further, these benefits are more prevalent in activities outside level-ground walking such as the negotiation of daily environmental obstacles and tasks.
The microprocessor regulation of stance stability relies on hydraulic cylinders. Because such cylinders have long been used to regulate swing-phase kinematics across a range of gait speeds, this has led to the questionable impression that patients must be capable of variable speed ambulation above a baseline rate to benefit from an MPK. A related oversimplification is the premise that candidacy for an MPK is exclusively a product of elevated activity levels and abilities. These positions fail to consider the benefits of MPKs during the negotiation of environmental obstacles irrespective of velocity. Such characterizations are largely related to the assignment of patients according to the fairly subjectively defined Medicare Functional Classification Level (MFCL), with particular interest between the distinctions of levels 2 and 3 as defined below:
Medicare Functional Classification Level K2: Has the ability or potential for ambulation to traverse low-level environmental barriers such as curbs, stairs, or uneven surfaces. Typical of the limited community ambulator.
Medicare Functional Classification Level K3: Has the ability or potential for ambulation with variable cadence. Typical of the community ambulator who has the ability to traverse most environmental barriers and may have vocational, therapeutic, or exercise activity that demands prosthetic use beyond simple locomotion.
Although regulatory positions have largely restricted the use of MPKs to patients capable of variable cadence, clinical observation supports the position that candidacy is a more complex product of the requirements of an individual’s daily activities, the physical deficits that the individual must contend with, and the patient’s functional abilities.
DEFINING CANDIDACY: VELOCITY
Several publications have addressed the concept that the ability to ambulate at variable speeds is prerequisite to benefiting from the use of an MPK. Kahle et al.10 observed that among those patients who originally presented as K2 with an NMPK, 47% improved to a K3 with the use of an MPK. In addition, although they reported a mean reduction in fall rates of 64% among their entire study cohort of K2 and K3 patients, among those K2 subjects who did not improve to K3, an even more striking reduction in falls of 81% was observed.10
Similarly, Hafner and Smith11 observed that 50% of their K2 patients made the transition to K3 with the introduction of an MPK. Although the reported rates of uncontrolled falls among patients initially presenting as K3 decreased by 50% with the introduction of an MPK, the reduction11 among the K2 patients was more dramatic at 80%. Simulating environmental obstacles in the form of a 244-ft timed obstacle course that contained grass, wood chips, sand, ramps, and stairs, the K3 patients demonstrated a modest improvement of 7% with the introduction of an MPK. The improvement11 for the K2 patients was more evident at 12%. Finally, when examining the concept of cognitive loading while walking on an outdoor sidewalk, the timed performance of the K3 patients improved a modest 2% with the use of an MPK. By contrast, a much more appreciable improvement of 12% was observed among the K2 patients, for whom the divided-attention task was evidently more challenging.11
More recently, Burnfield et al.6 reported on a cohort of patients who had “… the ability to ambulate slowly with a prosthesis, but only limited capacity/potential to modify their walking speed or traverse low-level environmental barriers (e.g., curbs, stairs, or uneven surfaces)” (p. 96). Reporting on a crossover trial in which the subjects transitioned from an NMPK to an MPK with microprocessor control of stance only, the authors observed improved velocity of 28% in ascending and 36% in descending a sloped surface.6 Although the noted improvements were in the metric of velocity, it should be observed that these changes more likely reflect improvements in patient confidence than alterations in energy costs. These improvements were coupled with significant improvements in the timed up and go (TUG), a common performance measure of dynamic balance; the ABC, described earlier as a self-report measure of perceived balance confidence; and the Houghton score, a self-report measure of perceived activity, reliance on upper-limb assistive devices, and perceived stability.6 Improvements with the MPK for these K2 subjects were further supported by subjective reports of greater stability, greater confidence in new places, capacity to walk and think about other things or walk and talk on the telephone, and reduced fear and/or occurrence of falls.6
Also reporting on a cohort of patients classified at K2, Theeven et al.8 reported decreases in the mean times required to complete the common daily tasks that make up the ADAPT measure described earlier. These improvements were observed after a very limited acclimation period into each of two MPKs. Importantly, the authors found that not all patients benefited from the MPKs. Those functioning at the lower end of the classification level, characterized by lower self-selected walking speeds and activity levels, did not seem to benefit from the MPK intervention.8
Taken collectively, the literature cited above corroborates the clinical observation that patients do not need to be capable of variable speed ambulation above a baseline rate to benefit from the most compelling advantages associated with MPKs, namely, improved safety and reduced cognitive burdens. Similarly, the enhanced security associated with MPKs will, in many instances, allow patients who were previously limited to a single-speed walking ability to attain variable speed ambulation. Patients classified as K2 and K3 encounter environmental obstacles both at home and in the community. The difference between the two groups is in their ability to navigate those obstacles and the speed with which they do so. Given that the most compelling benefits associated with MPKs seem to be related to the metrics of balance, stability, and confidence, the decision to restrict their provision exclusively to patients classified as K3 or higher may deny these important benefits from the very population that would seem to have the greater need of assistance.
DEFINING CANDIDACY: EXPECTED ACTIVITY AND PHYSICAL DEFICITS
Patients’ ability to ambulate across a range of gait speeds is one consideration in the selection of appropriate prosthetic components. The functional requirements associated with their day-to-day life constitute a second. The deficits that affect their ability to navigate these daily challenges make up a third. The latter two considerations are important because these may identify a patient’s risk for injury during daily activity and generally tend to be inversely proportional to the consideration of functional ability, as suggested in Figure 1.
Speaking generally, patients with higher functional abilities tend to be at lower risk for fall-related injuries during daily activities. Although their aggressive lifestyles may retain some level of risk, they are generally safe because their abilities are consistent with the environments and obstacles that they face. The same cannot be said of those patients with lower functional abilities. Challenges such as additional comorbidities, age, compromised balance, reduced limb length, and weakness place them in a position where their daily environments and obstacles can be quite challenging, creating an elevated risk for injury during fundamental day-to-day activity. This is especially the case when subjects with reduced functional abilities and/or numerous physical deficits must ambulate long distances or navigate challenging functional tasks.
The commonly observed decision to base candidacy for an MPK arbitrarily on functional ability ignores the ability of MPKs to reduce the risk for injury for those patients who continue to contend with environmental obstacles in the face of physical deficits and despite limited functional ability. It is apparent from both clinical experience and emerging empirical evidence that the threshold for MPK candidacy has been set too high, potentially subjecting less functional transfemoral subjects to unnecessary risk for injury. Decisions to define candidacy should also consider the activity requirements faced by given subjects and the deficits they must overcome to meet these requirements.
With the clinical value of MPKs better defined and the indications of candidacy more broadly considered, the profession is better positioned to develop a valid, reliable, uniformly accepted assessment measure that might be used to evaluate prospective MPK users and document their candidacy. In doing so, the instrument will need to consider each of the areas described above including functional abilities, anticipated activities, and individual deficits.
Throughout the MPK literature, study cohorts often contain several subjects who fail to accommodate to the new technology and drop out of the study. Although these subjects always constitute a minority of the study population, their consistent presence is concerning to those who fear the possibility of such an expensive intervention being rejected by a patient. Little has been done to define the presentation characteristics of those subjects who ultimately prefer the function of their legacy NMPK or may otherwise reject the use of an MPK. The most comprehensive consideration is found in the work of Theeven et al.,8 who found that MPKs failed to improve ADAPT scores among K2 subjects with lower self-selected walking speeds and activity levels. Future research efforts are needed to further identify the threshold of functional abilities beneath which acceptance and benefit of an MPK are less likely. Although the current benchmark of variable speed ambulation seems to be too high, the profession has yet to identify an appropriate lower threshold. The development and broad acceptance of a standardized functional abilities assessment index would allow for a better determination of those subjects whose abilities qualify them for MPK consideration.
As suggested earlier, the value of MPKs seems to lie in their ability to affect considerations such as safety, confidence, and cognitive burden during the navigation of environmental obstacles outside level-ground walking. Consideration of the obstacles a patient will encounter during his/her current and anticipated lifestyle will inform how often and to what extent the safety benefits of an MPK will be used. This should be considered when attempting to develop an appropriate assessment measure.
Clinicians in amputee rehabilitation understand those deficits that may compromise a transfemoral patient’s ability to safely navigate day-to-day activities. These can include things such as obesity, poor balance, weakness, a shorter residual limb length, and injury to the contralateral limb. The difficulty in recruiting transfemoral study subjects and the overlapping complexities of each individual case make it unlikely that future research will document the effect of individual deficits on safe ambulation. However, such considerations can help predict a patient’s reliance on the safety parameters of his/her prosthesis and should be documented during the formulation of the treatment plan. Assuming an adequate level of functional ability, the presence of such physical deficits reinforces the potential benefit of an MPK in improving patient safety.
Microprocessor-regulated prosthetic knee joint technology has been available for almost 20 years. During this time, there has been an evolution in our profession’s understanding of the most compelling benefits offered by these modalities. The original emphases on energy costs and kinematic and kinetic gait variables have largely been replaced by our understanding of the abilities of MPKs to affect variables such as safety, confidence, and cognitive burdens, particularly during the navigation of environmental obstacles and tasks. With the refinement of this understanding, we are better able to speak of the indications for MPKs. In doing so, the original impression that MPKs were indicated for only highly active and able transfemoral amputees has been replaced by a recognition that less able subjects seem to benefit from the enhanced stability features offered by the technology and may ultimately benefit from them more than the early target populations. As these concepts of value and candidacy are better understood, the need for a comprehensive, broadly accepted assessment measure has become apparent. This measure should consider not only the physical abilities of potential users but also their anticipated activities and the physical deficits concurrent with their amputation.