Comparative Efficacy of Microprocessor Knees: The OASIS 1 Findings

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By Phil Stevens, MEd, CPO, FAAOP

Microprocessor knees (MPKs) have been the most studied prosthetic technology since their introduction to the field several decades ago. Due in large part to these clinical studies and the positive patient outcomes generally associated with MPKs, they have also become widely utilized, with Ottobock reporting over 70,000 global fittings of its C-Leg. As a profession, there is a set of expectations we have come to associate with MPKs with respect to constructs such as stability, mobility, and quality of life. This article summarizes those expectations and their origins and reviews the recent findings of the Outcomes Assessment and Dissemination (OASIS 1) manuscript with regard to the comparative efficacy associated with different types of MPKs in these important areas of performance.



Establishing Expectations

Evidence-based medicine (EBM) is the conscientious use of the best available evidence in making decisions in the care of individual patients. Importantly, the best available research findings, often thought of as the core pillar of EBM, only constitute one of three types of evidence that are integrated in evidence-based practice. The other two types of evidence are the experiences of the treating clinician and the individual values of the patient being treated. These latter types of evidence are easily taken for granted, as they are immediately present in any clinical encounter. Clinicians have their experiences on hand during any clinical encounter. Similarly, patients carry their values with them into any clinical setting and, provided the clinical teams make appropriate inquiry, these values are readily integrated into clinical decision-making.

The best available research findings are often less immediately available for consideration. They may be tucked away in the publications of individual clinical trials in academic journals that are not readily available to practicing clinicians. This common deficit has been addressed across the healthcare spectrum with the creation of clinical practice guidelines (CPGs). These summary documents are intended to take the broad findings of clinical trials in a given area and synthesize them into actionable recommendations that can be readily integrated alongside clinician experience and patient expectation when confronting clinical decisions.

While CPGs were underrepresented in the prosthetics arena for some time, recent years have seen an increase in their availability. One such recent effort was focused on prosthetic knee selection and established the rationale behind the OASIS 1 analysis.1 While there are many described approaches to the development of CPGs, they are all based on a determination of the best available evidence for a given clinical question. Here we can defer to the readily recognized evidence pyramid, with systematic reviews and meta-analyses of clinical trials at its top, followed by individual clinical trials, observational studies of comparison groups, case series and case reports, and expert opinion. Because of the large body of evidence available with respect to MPKs and prosthetic knee selection, the CPG in question began with a survey of nine available systematic reviews and meta-analyses then available on this topic.1

Consistent with the CPG development recommendations of the American College of Physicians,2 once these source documents were identified, they were read with intent to harvest all available "evidence statements." Evidence statements are statements supported by clinical trials that address the potential benefits associated with a given course of care, the potential harms associated with a given course of care, and the comparative efficacy observed between two different courses of care. This effort led to a bank of some 39 such statements that were then organized into statements related to the benefits of hydraulic resistance, the comparative benefits observed with MPKs relative to non-MPKs, areas of observed equivalence between MPKs and non-MPKs, and the comparative benefits observed with MPKs for limited community ambulators.1

Each subset of the evidence statements was then synthesized into concise, actionable content designed to inform clinical decision-making. Recommendation 2 summarizes the observed benefits of MPKs compared to non-MPKs as follows:1

Microprocessor knee benefits compared with non-microprocessor knees:

  a)   With respect to self-report indices and measures, microprocessor knees are indicated to reduce stumbles, falls, and associated frustrations as well as the cognitive demands of ambulation.

  b)  With respect to self-report indices and measures, microprocessor knees are indicated to increase confidence while walking, self-reported mobility, satisfaction, well-being, and quality of life.

  c)   With respect to physical performance indices and measures, microprocessor knees are indicated to increase self-selected walking speed, walking speed on uneven terrain, and metabolic efficiency during gait.

Collectively, these guidelines, along with the evidence statements and individual clinical trials that support them, define a set of evidence-based expectations that are supported in the available literature. These include reductions in stumbles and falls, increased mobility, and improved quality of life. Individual clinicians can consider where these observations are consistent with their own clinical experience. 

How Well Do Individual MPKs Perform Against These Expectations?

Professionally, we recognize that while modern MPKs all pursue these common performance objectives, they each go about it a different way. Each of today's MPKs uses different mechanical approaches, regulated by different logic sets within their microprocessors, informed by different sets of mechanical sensors, and relying upon different mechanisms of actuation to alter their physical performance characteristics in response to environmental events. Researchers from Ottobock have published small case series in which these differences have been explored in highly controlled laboratory environments.3-4 In addition to explanations of the different MPK operational mechanisms, these efforts have provided initial insights into differences in self-selected walking velocities, stance flexion behavior, timing of swing phase release, and adaptations to stair and ramp decent in limited crossover experiments.3-4 These papers begin to address the question of how different MPKs adapt to various walking conditions and impediments.

This consideration of comparative efficacy is important because of the overwhelming predominance of the Ottobock C-Leg in the published literature. It is no overstatement to suggest that from the standpoint of the literature, our collective understanding of the value and benefit of MPKs is largely an understanding of the value and benefit of the C-Leg. To a great extent, the translation of those benefits to other MPKs has been a product of clinician experience acting within its important role in EBM.

Within the limited evidence available for other MPKs, there are some reports that begin to question the consistency of those expected benefits associated with the use of an MPK.5-6 These include data sets that suggest a potential increase in stumbles and falls for users of some MPKs.5-6 This data, combined with the comparative deficit in available literature for other commonly used MPKs, justifies the exercise of attempting to determine if there are differences in the outcomes experiences by users of the different types of MPKs.



Hanger Clinic, Austin, Texas, has established the collection of core outcomes at regular intervals across its national reach of patient care clinics. These outcomes include the considerations of mobility, collected in the form of the Prosthetic Limb Users Survey of Mobility (PLUS-M), satisfaction and quality of life using separate single item responses from the Well-Being subsection of the Prosthetic Evaluation Questionnaire (PEQ-WB), and reported incidence of injurious falls. This data has enabled a retrospective, cross-sectional analysis to compare
the outcomes reported by users of the different MPKs. This analysis was recently published as the OASIS 1 manuscript.7

The published analysis reports on the outcomes reported by 602 users of MPKs. More specifically, outcomes from 178 users of the Ottobock C-Leg, Blatchford Orion, and Freedom Innovation Plié were randomly selected along with the available 68 outcomes from users of the Össur Rheo knee. Demographics between the four cohorts were generally similar with only slight differences observed. Users of the C-Leg, for example, were older on average than their peers using other MPKs. A higher percentage of Plié users were female, and users of the Rheo were more likely to report employment.7

Scores for both quality of life and satisfaction were assessed across the four cohorts of MPK users. Median satisfaction scores were generally high, reported at approximately eight points on the ten-point scale for all but the Plié knee, which trailed by only a single point. Scores were highest among users of the Orion and the C-Leg. However, observed differences in this domain failed to reach statistical significance. Median scores for quality of life were also high, similarly reported at eight points on the ten-point scale for all but the Plié knee where the median score was seven points. Here statistical significance was reached for a single relationship with users of the C-Leg reporting significantly higher quality of life scores compared to users of the Plié.

In a separate analysis, these two considerations of well-being were analyzed across four different age ranges across all four cohorts to determine if any of the MPKs were more likely to be associated with stable scores across progressively aging populations. Here the study authors noted that both satisfaction and quality of life appear to be stable constructs, with no meaningful differences between the scores of different age groups across any of the four MPKs.7

Mobility was also assessed across the four cohorts. While the highest mean mobility scores were reported by users of the C-Leg, the differences were slight and failed to reach statistical significance. A similar analysis was performed across users of different ages to determine if any MPKs were more protective of the decline in mobility that is generally associated with the aging process. For users of the C-Leg and Plié, declines in mobility were observed in progressively older populations. This was not the case for either the Orion or the Rheo. However, this may have been due to the lower mobility values observed among younger Orion users and inadequate group sizes within several age rages among our limited sample of Rheo users.7

The injurious fall rate reported among MPK users was also considered. We elected to confine our standard outcomes collection to reports of injurious falls experienced over the past six months to mitigate the risk of recall bias when asking for the more generic events of stumbles and falls. More specifically, patients were asked, "Have you had a fall in the previous six months that resulted in a hospital or physician visit?" Because of the need to ensure that fall data was associated with the MPK in question, fall analysis was confined to those users who had been in possession of their MPKs for at least six months at the time of data collection. This reduced our sample size from 602 to 419 users. Across the entire MPK cohort, injurious falls were reported by 10 percent of the respondents. Within the specific cohorts, injurious falls were reported by 5.8, 9.2, 13.3, and 14 percent of the users of the C-Leg, Orion, Plié, and Rheo respectively. These differences failed to reach statistical significance relative to one another. However, when the fall rates of the differing MPK cohorts were compared against the fall rates of non-MPK users from a separate investigation, reported at 16.3 percent,8 only the C-Leg and the Orion were associated with a statistically significant reduction in injurious falls.7

Applications and Implications

As stated earlier, EBM is realized in the application of the best available evidence with respect to published literature, clinical experience, and patient values. Studies like OASIS 1 allow for a more nuanced view of the outcomes associated with different MPKs. The associated perspectives, when coupled with clinical experience and patient values, permit clinicians to recommend those MPKs with the best outcomes for the constructs of primary value and interest to the individual patient. Available data suggests that those patients who are indicated for an MPK to enhance their mobility or increase their quality of life may be well served by all of the MPKs analyzed within the OASIS effort, with clinical experience and patient values further informing that decision. However, if the primary indication for an MPK is to reduce an individual's risk of injurious falls relative to that associated with non-MPKs, a smaller number of MPKs appear more likely to result in the desired outcome.

As studies of comparative efficacy between similar components continue, there will be an added measure of accountability for manufacturers to expand their current focus on features and function to the more relevant considerations of patient experienced outcomes. In today's healthcare environment, value is determined by realized outcomes rather than proffered features. Studies like OASIS 1 will help clinicians make their differential component recommendations based on outcomes and evidence in addition to their observed clinical experiences and expressed patient values and preferences.

Phil Stevens, MEd, CPO, FAAOP, is a director with Hanger Clinic's Department of Clinical and Scientific Affairs. He can be contacted at



1.     Stevens, P., and S. Wurdeman. 2019. Prosthetic knee selection for individuals with unilateral transfemoral amputation: A clinical practice guideline. Journal of Prosthetics and Orthotics 31:2-8.

2.     Qaseem, A., V. Snow, and D. K. Owens, et al. 2010. The development of clinical practice guidelines and guidance statements of the American College of Physicians: Summary of methods. Annals of Internal Medicine 153:194-99.

3.       Thiele, J., C. Schollig, and M. Bellmann, et al. 2019. Designs and performance of three new microprocessor-controlled knee joints. Biomedizinische Technik/Biomedical Engineering 64:119-26.

4.       Bellmann, M., T. M. Kohler, and T. Schmalz. 2019. Comparative biomechanical evaluation of two technologically different microprocessor-controlled prosthetic knee joints in safety-relevant daily-life situations. Biomedizinische Technik/Biomedical Engineering 64:407-20.

5.       Prinsen, E. C., M. J. Nederhand, and J. Olsman, et al. 2015. Influence of a user-adaptive prosthetic knee on quality of life, balance confidence, and measures of mobility: A randomised cross-over trial. Clinical Rehabilitation29:581-91.

6.       Hafner, B. J., and R. L. Askew. 2015. Physical performance and self-report outcomes associated with use of passive, adaptive, and active prosthetic knees in persons with unilateral, transfemoral amputation: Randomized crossover trial. Journal of Rehabilitation Research and Development52:677-700

7.       Campbell, J. H., P. M. Stevens, and S. R. Wurdeman. 2020. OASIS I: Retrospective analysis of four different microprocessor knee types. Journal of Rehabilitation and Assistive Technologies Engineering 7:1-10.

8.       Wurdeman, S. R., T. A. Miller, and P. M. Stevens, et al. 2020. Microprocessor knee technology reduces odds of incurring an injurious fall for individuals with diabetic/dysvascular amputation. Journal of Prosthetics and Orthotics 32:64-9.