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Original Article

Prescribing Physician Perspective on Microprocessor-Controlled Prosthetic Knees

Morgenroth, David C. MD

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JPO Journal of Prosthetics and Orthotics: October 2013 - Volume 25 - Issue 4S - p P53-P55
doi: 10.1097/JPO.0b013e3182a88d02
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The goal of prosthetic prescription is to choose prosthetic components that will help to optimize functional quality of life while minimizing risk, according to an individual patient’s unique combination of abilities, goals, and lifestyle. Prescribing clinicians are faced with a lack of quality evidence, contradictory findings from research studies, various classification systems, industry claims of superior performance, and patient expectations, and all of this in an environment in which new prosthetic components continuously enter the commercial market.1 The decision can be especially challenging when the cost differential between components widens and a prescribing clinician considers the magnitude and type of perceived benefits that justify the significant increase in financial cost. One such notable challenge, and the focus of this article, is the clinical decision of when to prescribe a microprocessor knee (MPK) rather than a non-MPK (NMPK).


There are many elements of a thorough history and physical examination that should be factored into a prosthetic knee prescription, many of which are important when considering an MPK.


  • Medical comorbidities that may limit use of a prosthesis
  • In the case of recent amputation, did the patient walk significant distances and with variable cadence before amputation?
  • Does the patient use an assistive device? How often and far does he/she walk and what are the limiting factors (fatigue, shortness of breath, pain)? Does he/she regularly navigate stairs and hills? Does he/she have difficulties with any activities of daily living?
  • The patient’s goals and any perceived limitations of his/her current prosthesis that prevent achievement of these goals
  • Sufficient access to care providers for prosthetic adjustments and repairs, as well as access to medical care when necessary
  • Level of motivation: Potential to benefit from the use of certain types of prosthetic knees can be limited by a lack of motivation because optimal use requires training and practice
  • History of falls and/or fall anxiety: If so, an exploration of the circumstances of the falls or anxiety is indicated to determine whether component or alignment alterations may ameliorate them
  • Occupation, if still working: What are the physical demands of the job? Do these include multidirectional ambulation, navigating stairs, hills, or ladders regularly?
  • Avocational activities: Do these include golfing, biking, or other activities that would potentially benefit from modes or characteristics that are specific to certain MPKs, such as free swing mode or golf mode?
  • Aversion to technology/change or barriers to recharging prosthesis: For instance, there are some high-functioning, long-time transfemoral amputees who have grown accustomed to very basic prosthetic components and are not interested in newer components even under circumstances in which these offer potential functional benefit
  • Is the current socket fit and suspension system optimized? The best prosthetic knee will have little benefit in the case of a poorly fitting socket
  • Insurance coverage/financial situation


  • Length of residual limb: In the case of a through-knee amputation or a long transfemoral residual limb, some prosthetic knee units may extend the knee center of rotation asymmetrically compared with the contralateral side depending on the build height of the knee. This may be an aesthetic problem for some patients
  • Lower-limb strength and hip flexion or abduction contractures that may limit optimal use of a prosthesis
  • Gait and balance evaluation: Observe gait under multiple conditions if possible (flat ground, stairs, slopes). Does the patient become easily fatigued or unable to walk at variable cadences?
  • Body morphology (height and weight) of the patient may preclude the use of certain prosthetic components


Although there is an increasing focus on evidence-based practice in medicine, the number of studies and the quality of evidence vary considerably by field and disease process. The standard of evidence is generally considered to be a double-blinded placebo-controlled trial using validated outcome measures. However, this type of study can be logistically quite difficult when assessing prostheses. Studying outcomes related to prosthetic prescription carry methodological challenges, so it is no surprise that a recent review article summarizing safety, energy efficiency, and cost efficacy of a commonly prescribed MPK found generally low methodological quality with moderate risk for bias in a majority of studies reviewed.2 Although the methodological quality of evidence in the field of prosthetics will ideally increase in the future, intellectual rigor, clinical experience, and common sense are and will continue to be vital to the optimization of prosthetic prescription. A review article published in the British Medical Journal (BMJ) illustrates the danger of exclusive reliance on “gold standard” evidence by pointing out that although there are no existing double-blinded placebo-controlled trials demonstrating an association between the use of parachutes in preventing major trauma and sudden death, sky divers do not hesitate to wear parachutes.3 The distinction is clearly subtler when deciding to prescribe an MPK versus an NMPK, but the same principles apply.

Evidenced-based amputee rehabilitation practice and prosthetic prescription are characterized by the integration of clinical expertise and patient values with a critical read of the best available evidence from methodologically sound research, aimed at providing optimal clinical care.4–7 There is a clear need to expand the evidence base, as there is also a need to train more physicians in amputee rehabilitation and prosthetic prescription. With the multitude of prosthetic components on the market and no way for research to keep up and objectively test all of them, prosthetic prescription is inevitably affected by regional trends, advertising, and practitioner familiarity and experience. Even with a full cadre of evidence and a well-trained physician, it is still vital to read, interpret, and apply the body of evidence.


Given the generally substantial cost difference between MPKs and NMPKs, unbiased and high-quality evidence of the benefits of MPKs will likely become more vital to justify their prescription and use. A number of challenges exist to performing methodologically high-quality studies. Although MPKs are generally lumped into one category, they are a heterogeneous group. Some MPKs control only stance-phase resistance, some control only swing-phase resistance, and others control both stance- and swing-phase resistance. The default safety mode when the battery dies varies, as do the optional modes such as free-swing mode. Studying these knees as one group or attempting to apply the evidence from one MPK to another MPK has substantial limitations. Given the number of MPKs on the market, it is not feasible to study each individual MPK. It may be helpful to subcategorize MPKs by stance control, swing control, or stance and swing control. Similarly, research performed on one group of transfemoral amputee subjects may not be entirely applicable to an individual patient who does not have similar attributes. For instance, a study of gait efficiency in a group of transfemoral amputees of a traumatic etiology8 has substantially less predictive value when treating a dysvascular transfemoral amputee patient.

Another major challenge is overcoming expectation bias toward prosthetic components. Human behavior is influenced by expectations. There is a particular risk in research of expectations influencing findings, especially when there is an element of subjectivity in outcome measure or assessment. Blinding is used to try to eliminate such bias.9 Just as lofty advertising and a more expensive price tag can augment perception of sound quality in audiophile equipment, industry advertising and the expectation of newer, higher-tech, and more expensive prosthetic components such as MPKs can skew self-reported outcomes and functional performance. Although blinding of prosthetic components such as prosthetic knees is challenging, it is of paramount importance that prosthetics researchers consider creative solutions.

In addition, ensuring clinical significance and applicability of evidence is vital. This includes a thorough understanding of the functional challenges transfemoral amputees face. Statistical significance does not necessarily correlate with clinical significance. For instance, a majority of gait and metabolic outcomes in prosthetics research pertain to either flat-ground or treadmill walking although the ecologic validity of these situations can be limited. Research teams should ideally include a clinician to encourage the choice of clinically meaningful outcome measures. Clinician-researchers can be especially valuable members of the research team given their understanding of research design and clinically meaningful outcomes. Examples of clinically meaningful measures could include improved tools to objectively document falls and stumbles or measures related to low back pain and contralateral knee pain, which have both been shown to be extremely prevalent secondary disabilities in the transfemoral amputee population.10–12

Finally, a major limitation to current prosthetics research is the relatively small number of subjects in any given study, which limit subgroup analyses and the ability to use predictive tests to identify who will be successful users of devices. There is a need for multicenter trials with expanded numbers of subjects given the challenges of recruiting at any center.


Patients often go to a clinic asking for the “best” or “most advanced” prosthesis or prosthetic component. Although there are many prosthetic components that boast advantages, each comes with disadvantages as well. Therefore, the focus should be on how to optimize the prescription for each unique individual. This can be challenging and relies on an experienced clinician who can apply a critical read of available evidence and tailor it to specific patient goals and abilities. For instance, although an MPK may provide perceived benefits such as increased stability for some patients, others complain that certain MPKs “can’t keep up” and hold them back.

Although the Medicare Functional Classification Level (MFCL; more commonly referred to as “K-level”) is often used to determine insurance coverage eligibility for the prescription of MPKs, this is not a validated outcome scale and should not be used by researchers or clinicians. For instance, a recent study of K2 amputees using an MPK demonstrated a wide range of functional abilities, with the higher-functioning K2 individuals showing more benefit from the MPK.13 Given the generally substantial cost difference, to significantly benefit from the use of an MPK, a patient should:

  • Be able to transfer independently
  • Walk at variable cadences without an assistive device
  • Have sufficient endurance to walk greater than household distances
  • Have sufficient interest and motivation to work on gait training specific to certain MPKs

If prescribing clinicians are blind to differences in cost in their prosthetic prescription habits, payers will inevitably dictate prosthetic prescription with greater short-term cost scrutiny.


There are many MPKs on the market with considerably different functions, and individual patients have unique abilities and goals. The development of a purely algorithmic approach to determine prescription of an MPK versus an NMPK is unlikely to be effective at this point given the limitations of available evidence. The optimization of prosthetic prescription continues to rely on knowledgeable and thoughtful clinicians who understand unique patient characteristics and goals and understand prosthetic component capabilities and limitations. Although it may be tempting to hope for an algorithmic approach, given individual patient attributes, it is likely that encouraging the training of more physicians with a subspecialty interest in amputee rehabilitation will result in improved outcomes, especially when prescribing physicians, prosthetists, and physical therapists work as a patient-centered team.


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prosthetics; amputation; rehabilitation

© 2013 by the American Academy of Orthotists and Prosthetists.