The current study data provide a transverse characterization of transfemoral prosthesis (TFP) users, their experience and satisfaction with their current prostheses, and their priorities for an ideal prosthesis, from which we can draw numerous insights pertinent to the user-centered design of technologically-advanced TFPs.
A. User Characteristics
The present study cohort presented a range of ages, lifestyles, limb loss etiologies, and social demographics, reflecting the diversity TFA population. A significant feature of the subject sample was the predominance (78.9%) of traumatic causes of amputation, differing markedly from the estimated 16.4% of total lower-limb amputations (4). In addition to potential differences in the prevalence of transfemoral vs. below-knee cases among traumatic vs. non-traumatic amputations, the predominance of traumatic and male amputees likely represents a recruitment bias at the primary survey administration site, the INAIL Prosthetic Center –a rehabilitation center for patients with work-related disabilities– an effect reported in previous studies with a predominance of TFAs (19).
Given the difference in amputation etiology distribution between the MPK and NMPK groups, the observed differences in age at amputation and time since amputation may be partially attributable to the contrasting clinical circumstances and pre-amputation lifestyles commonly associated with traumatic vs. non-traumatic (typically dysvascular) amputations. Whereas traumatic amputations are commonly associated with accidents during rigorous physical activity, non-traumatic amputations are overwhelmingly the result of vascular disease occurring secondary to many years of chronic metabolic diseases such as type II diabetes (29), which is strongly associated with sedentary lifestyles.
B. User Experience & Satisfaction
In aggregate, our survey data indicated consistently that subjects with MPKs use their prostheses more frequently, experience a greater sense of autonomy, and are overall more satisfied with their devices than those with NMPK-TFPs. Specific notable findings and implications are discussed below, by category.
Achieving independence in ADLs and a corresponding sense of personal autonomy is a primary clinical objective of prosthesis use, to which end regular, independent prosthesis usage in a range of activities and environments is instrumental (14,30–32). The positive correspondence between greater daily prosthesis utilization and greater autonomy among MPK vs. NMPK users in the current study (Table 4) affirms this picture. Moreover, the greater prosthesis usage by MPK users in home and recreational but not work/school environments implies both that MPKs effectively fulfill functional user needs over a wider range of activities than NMPKs and that they promote a greater overall level of activity.
Regarding the distribution of reported autonomy between groups, NMPK users exhibited greater variance relative to MPK users (Figure 1 – IQR), both within individual ADLs and across all ADLs. In addition to reflecting potentially greater diversity in the NMPK user population, the tighter and more favorable autonomy distribution among MPK users suggests that MPK use may have a positive influence in equalizing functional outcomes across a wide range of baseline user health characteristics. Moreover, the persistence of greater autonomy among MPK vs. NMPK users within both the demographically self-similar traumatic and non-traumatic sub-groups (Table 5) suggests that the prosthesis plays a significant role in facilitating autonomy, in concert with personal and clinical co-factors. In particular, the dramatic differences in median autonomy between MPK and NMPK users with non-traumatic amputations for all ADL suggests that patients with lower baseline health and/or functional capabilities may draw additional clinical benefit from the use of technologically advanced TFPs. By contrast, the positive differences in autonomy associated with MPK use among traumatic amputees were generally subtler (often evident in the variance rather than the median) and more task-specific, with the greatest differences observed for the more challenging tasks of stair descent, sit-stand transitions, and ramp walking, in addition to house work.
In sum, the observed differences in autonomy affirm a worrisome discrepancy in outcomes between TFA sub-populations that may be addressable with more advanced, more accessible, and/or more usable TFPs.
As a concrete clinical outcome complementary to subjective autonomy, the markedly higher incidence of falls among NMPK users in the present study must likewise be considered within a complex interplay of technical and clinical factors. In particular, fall risk among lower-limb amputees has been previously related to various biomechanical factors, including gait asymmetry, muscle weakness, and other neuro-musculoskeletal limitations (33,34), as well as to environmental factors such as irregular terrain, stairs, and slopes (35,36). The prosthesis plays a crucial role in safely negotiating these ‘challenge scenarios,’ with previous studies showing that the use of MPK prostheses can improve motor functions and reduce falls in amputees with lower mobility grades (37,38), in addition to promoting greater overall movement control, dynamic stability, and functional mobility (3,12,39). Significantly, these and other functional performance parameters such as walking speed various surfaces, incidence of stumbles and falls, and stair descent ability have all been linked with overall amputee satisfaction, wellbeing, and quality of life (12,37,38,40).
The current study findings thus reinforce an integrated clinical picture in which technologically-advanced TFPs can be powerful tools in promoting user mobility and autonomy, but one that must be employed as part of a comprehensive rehabilitation paradigm that emphasizes functional training in addition to proactive psychological support. Further evidence for such an integrated approach is found in this study’s observation that users of MPKs, via their common attribution of falls to personal and attentional rather than environmental or technical factors, exhibited a higher degree of control and ownership of their prostheses thus highlighting the symbiotic relationship between psychological factors and functional outcomes. Based on the preponderance of past and present evidence, it is reasonable to infer that more advanced TFPs can be more effective than traditional TFPs at realizing user potential that depends simultaneously on various health and dispositional factors, and that thoughtful prosthesis selection and configuration based on individual user needs will thus remain necessary to maximizing the benefit of such devices.
This study’s principal finding that strong majorities of TFP users in both groups regarded their prostheses either as useful tools for achieving personal autonomy or as extensions of their bodies is a positive indicator of successful functional rehabilitation and prosthesis acceptance in the study population. Moreover, the increased sense of anatomical ownership expressed by MPK users suggests that both naturalistic prosthesis function and the experience of using it in a synergistic manner may contribute strongly to the sense of body schema integration and corresponding acceptance. This implication echoes previous studies that have identified human interaction as a strong factor in improving the subjective sense of control of the artificial leg, thus enhancing the performance and management of ADLs (21,41).
This study’s observation of higher prosthesis satisfaction among MPK relative to NMPK users in all four categories of functionality, comfort, aesthetics, and general characteristics aligns with previous research that has found overall satisfaction to be influenced by several aspects of the prosthesis, including functionality, cosmetics, and usability (22,32). While it may be expected that traumatic amputees (accounting for 91% of the MPK group, vs. just 60% of NMPKs) tend to be more functionally capable than those with dysvascular amputations by virtue of better overall health, it may likewise be true that less healthy and less active individuals have lower mobility demands and/or expectations, thus making the net effect of prosthesis type on satisfaction unclear a priori. This complexity implies both that TFPs should be designed with more than bare-minimum functional autonomy requirements in mind (especially with respect to gait), and also that user satisfaction can be positively influenced by psychological counseling featuring proactive management of expectations and attitudes, regardless of prosthesis type. In sum, the positive association between functional, practical, and aesthetic dimensions of prosthesis satisfaction further reinforces that the benefits of higher-performing TFPs extend beyond purely functional outcomes and encompass overall user satisfaction and wellness.
C. Priorities for an Ideal Transfemoral Prosthesis (TFP)
Given the critical role of device design in prosthesis usability and user satisfaction, the evaluation of user priorities represents an indispensable foundation of the TFP design process. Design priorities reflect the user’s values, lifestyle, and goals for prosthesis use, which have been found to vary significantly by prosthesis type and user age in upper limb prostheses (32). While the median user priorities in the current study likewise varied between MPK and NMPK users, the magnitude and significance of these differences appear secondary to the high variation in priorities between individuals, reflecting the diversity in subjects’ demographic/clinical characteristics, functional ability, and autonomy evidenced by other survey sections. Despite the high variance in user priorities for specific features, common high-level groupings of priorities across all subjects remain pertinent and informative to TFP design.
Priorities for functional mobility (Pr-Fn). The prevailing functional priority of general stability across all TFP users agrees with previous findings among MPK users (38). Here, NMPK users were more consistent in ranking stability first, whereas MPK users expressed a comparably strong preference for stability and lifestyle adaptability. Given that MPK users reported significantly greater overall autonomy and satisfaction with functional mobility than NMPK users, this subtle difference suggests that design priorities are influenced both by user lifestyle (actual and desired), and also by user perceptions about the limitations of their current prostheses relative to their expectations of device capability. Moreover, we note that the highest-ranked functions were those applicable to a range of situations (overall stability, lifestyle-related functionality, adaptability of walking velocity), with more specific tasks of gait on uneven terrain, stair ascent, and ramp walking (up and down) falling in the second tier. Based on our analysis of free responses, “lifestyle functionality” was interpreted by subjects in a variety of ways (some more task-specific than others), thus rendering this priority interpretable in aggregate as a measure of the need for TFP versatility and adaptability to different tasks and environments.
Notably, “work-related functionality” represents an exception to the trend towards favoring versatility, ranking as a moderate-to-lower priority for both groups. The distinction in preference for lifestyle over work-related functionality is difficult to parse, given that the functional demands of lifestyle and work environments are not easily generalizable, nor discernible from survey data. What may be inferred regardless is that TFP users consider the ability to maintain their desired personal lifestyle a more important determinant of their satisfaction than their vocational ability.
Priorities for Active Assistance (Pr-AA). Overall, TFP user priorities for AA agree well with those for functional mobility, with the top functional priority of overall stability corresponding to the preference for AA during moments of instability. Likewise, the trend towards prioritization of ascent vs. descent functions corresponded to the preference for AA in those tasks. The prioritization of locomotion velocity provides a more nuanced picture: while the high prioritization of adaptability to walking velocity corresponds functionally to the preference for active assistance during fast versus natural versus slow speed walking, the highest speed form of locomotion – running – was among the least prioritized functionalities by both groups. This finding indicates that high speed walking differs significantly from running in terms of its personal value to users – perhaps owing to a difference in social utility. Given the unique biomechanical demands of running, the elimination of this function as a TFP design requirement would enable a valuable simplification in TFP design.
Regarding the future design and development of advanced TFPs, we note that for both gait speed and ascending vs. descending functions, user priorities for AA reflected the biomechanical demands of the highest-priority mobility functions, with preference to tasks demanding greater positive power output. By contrast, controlled descent and lower-speed gait are more easily achieved via the modulated resistance achievable by current MPKs. This high-level correspondence between priorities for AA and for overall prosthesis functionality suggests that these categories may be strategically merged into a single class of design requirements. In line with user-centered design recommendations from the fields of both lower-limb prosthesis design (21) and brain-computer-interface-based assistive technology (42), such a design process should focus first on defining user priorities regarding the desired tasks and activities to be performed with the prosthesis, based on user input.. Specific technical requirements such as active knee power should then be defined based on the biomechanical and ergonomic demands of those tasks, so as to enable users to perform their highest-priority activities in a safe, effective, and efficient manner.
Priorities for General and Socket Characteristics (Pr-GC; Pr-S). Current MPK and NMPK users expressed very similar priorities regarding both general device characteristics and socket design. Primary between-group differences in Pr-GC tended to concern more technical, higher-performance TFP traits such as battery life and water resistance, which likely reflects a difference in applicability of various traits to the user’s TFP rather than a fundamental difference in priority. Similarly, the only pronounced difference in Pr-S was the elevated preference for active-cooling by MPKs users, which is likely attributable to higher activity levels in this group, which would naturally result in more frequent sweating and residuum volume changes. The consistency of socket-related priorities across groups supports the modular design of high-performance sockets that are compatible with a wide range of prosthetic knee types, suitable to a range of activity levels, based on user lifestyle.
D. Application to User-Centered Design of TFPs
Following from the above discussion, this study’s findings may be synthesized into the following essential user-centered design principles for an ideal TFP:
- The safety and reliability of the TFP across a wide range of ADLs are fundamental design priorities for all TFA users, regardless of prosthesis type, demographic characteristics, and amputation etiology.
- Individual user needs and priorities vary significantly based on clinical characteristics, personal attitudes, and lifestyle, thus demanding modularity and/or customizability of various prosthesis components and characteristics, such as the dimensions, socket fit, and cosmetics.
- Versatility and adaptability are essential TFP user needs, which may be fulfilled by a combination of modular TFP designs and intelligent adaptive control algorithms, enabling device personalization based individual user capabilities and priorities.
- The potential value of active (e. positive power) assistance in a TFP is well recognized by all types of TFP users, with assistance desired preferentially during moments of instability, stair/incline ascent, and higher-velocity walking.
- To improve comfort and minimize skin problems, the ideal TFP socket design should enable routine modulation of shape and/or volume to accommodate changes in residual limb volume and tissue properties, in addition to regulating temperature and humidity.
- The minimization of prosthesis weight remains an important design objective.
- Cost remains a limiting factor in prosthesis selection for many TFAs, thus highlighting lower-cost approaches to improving TFP performance as an area of high clinical impact.
To fulfill these requirements in the development of future TFP systems, they should be used by TFP researchers and developers as design inputs to a rigorous user-centered design framework such as that proposed by Beckerle and colleagues (21), which posits a systematic process for merging human and technical factors in the design of advanced lower limb prostheses, with particular attention to TFP.
In addition to the above recommendations regarding TFP design characteristics, the present study reveals several valuable insights regarding the human-centered design process. First, the survey’s ranking of user design prioritizes without any corresponding measures of relative priority weighting favored the delineation between TFP features of similar priority, with the tradeoff of reducing the power to evaluate the absolute importance of specific design features. Second, survey questions regarding subjects’ functional capabilities were phrased in terms of subjective satisfaction and autonomy, making them imprecise as indicators of absolute functionality. Though this perspective is suitable for a user-centered design process that holds user satisfaction and wellbeing as its ultimate objectives, subsequent TFP research and development efforts should further investigate the relationship between specific design characteristics, objective functional performance, usability, and user satisfaction.
E. Study Limitations
While supportive of the current clinical understanding that more advanced TFPs promote higher levels of user mobility, function, and overall health, this study’s central findings of higher overall function and satisfaction among MPK relative to NMPK users must be interpreted within the context of two main study limitations. Most notably, owing to the survey’s limited characterization of numerous health-related co-factors such as participants’ overall health, condition of the intact limb, pre-amputation lifestyle, and baseline functional abilities (as quantified by standard clinical measures such as the K-level or functional assessment scales), the study was not able to conduct a thoroughly controlled test of the hypothesis that MPKs confer independent improvements in outcomes relative to NMPKs. Irrespective of these survey-specific constraints, it remains likely that the effects of MPK-TFPs on clinical and personal outcomes cannot be thoroughly isolated from general health and human factors even by controlling for primary indicators of such factors, due to the number and the limited quantifiability of personal factors that influence prosthesis selection.
Second, the aforementioned recruitment bias towards traumatic TFAs limits the generalizability of full-sample analyses to the general TFA population. Moreover, the primary study site (INAIL Centro Protesi) is the leading orthopedic rehabilitation center in Italy, where the intensive, integrative standard of care creates a patient population likely more functionally advanced and engaged with their prostheses than the global TFA population. This center’s standard care practice of prescribing MPKs following outfitting and initial rehabilitation with an NMPK-TFP MPKs may introduce an additional “healthy user bias” (43) among MPK users. While healthier users are presumably able to achieve the best functional outcomes from advanced TFPs, it remains possible that such devices could have the greatest marginal benefit for users with lower baseline abilities, as suggested by autonomy data among non-traumatic amputees (Table 5). Nonetheless, this study’s heavy representation of traumatic amputees and MPK users is well-suited to the study’s objective of evaluating user needs and priorities for the development of an advanced, actively powered TFP.
F. Conclusions
This study provides an extensive new characterization of transfemoral amputees (TFAs), encompassing their demographic and clinical characteristics, psychosocial attitudes and lifestyles, functionality and satisfaction with their current prostheses, and user needs and priorities for an ideal transfemoral prosthesis (TFP). Overall, users of TFPs with microprocessor-controlled knee units (MPKs) reported higher levels of activity, prosthesis use, and functional ability compared to those with NMPKs, corresponding to higher satisfaction and greater functional autonomy. While likely reflective of differences between MPK and NMPK users in terms of age and amputation etiology (and thus, overall health), these results reinforce the hypothesis that more advanced, actively controlled TFPs positively influence not only the safety and functional mobility of TFAs, but their overall sense of prosthesis acceptance and ownership, personal autonomy, and overall health and wellbeing. Based on past and present findings, it may be reasonably generalized that advanced TFP functionality, in conjunction with a variety of underlying clinical and personal factors, plays a significant role in enabling, maintaining, and/or reinforcing healthy mobility and lifestyle among TFAs.
Finally, a set of user-based design principles are synthesized based on survey data analysis. Future investigations should continue to develop, validate, and standardize measures of the functional abilities and personal priorities of TFAs, based on this study’s survey and findings. In this way, this study may serve as a foundation to build a clinical evaluation tool to optimally select and configure TFPs to fulfill the needs of individual users.