Microprocessor-Controlled Prostheses for the Lower Limb
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Microprocessor-controlled prostheses use feedback from sensors to adjust joint movement on a real-time as-needed basis. Active joint control is intended to improve safety and function, particularly for patients who have the capability to maneuver on uneven terrain and with variable gait.
More than 100 different prosthetic ankle-foot and knee designs are currently available. The choice of the most appropriate design may depend on the patient’s underlying activity level. For example, the requirements of a prosthetic knee in an elderly, largely homebound individual will be quite different than a younger, active person. In general, key elements of a prosthetic knee design involve providing stability during both the stance and swing phase of the gait. Prosthetic knees also vary in their ability to alter the cadence of the gait, or the ability to walk on rough or uneven surfaces. In contrast to more simple prostheses, which are designed to function optimally at one walking cadence, fluid and hydrauliccontrolled devices are designed to allow amputees to vary their walking speed by matching the movement of the shin portion of the prosthesis to the movement the upper leg. For example, the rate at which the knee flexes after “toe-off” and then extends before heel strike depends in part on the mechanical characteristics of the prosthetic knee joint. If the resistance to flexion and extension of the joint does not vary with gait speed, the prosthetic knee extends too quickly or too slowly relative to the heel strike if the cadence is altered. When properly controlled, hydraulic or pneumatic swing-phase controls allow the prosthetist to set a pace that is adjusted to the individual amputee from very slow to a race-walking pace. Hydraulic prostheses are heavier than other options and require gait training; for these reasons, these prostheses are generally prescribed for athletic or fit individuals. Other design features include multiple centers of rotation, referred to as “polycentric knees.” The mechanical complexity of these devices allows engineers to optimize selected stance and swing-phase features.
Microprocessor-Controlled Prosthetic Knees
Microprocessor-controlled prosthetic knees have been developed, including the Intelligent Prosthesis (Blatchford, U.K.), the Adaptive (Endolite, England), the Rheo (Ossur, Iceland), the C-Leg and Genium Bionic Prosthetic System (Otto Bock Orthopedic Industry, Minneapolis, MN), and Seattle Power Knees (3 models include Single Axis, 4-bar and Fusion, from Seattle Systems). These devices are equipped with a sensor that detects when the knee is in full extension and adjusts the swing phase automatically, permitting a more natural walking pattern of varying speeds. For example, the prosthetist can specify several different optimal adjustments that the computer later selects and applies according to the pace of ambulation. In addition, these devices (with the exception of the Intelligent Prosthesis) use microprocessor control in both the swing and stance phases of gait. By improving stance control, they may provide increased safety, stability, and function; for example, the sensors are designed to recognize a stumble and stiffen the knee, thus avoiding a fall. Other potential benefits of microprocessor-controlled knee prostheses are improved ability to navigate stairs, slopes, and uneven terrain and reduction in energy expenditure and concentration required for ambulation. The C-Leg was cleared for marketing in 1999 through the 510(k) process of the U.S. Food and Drug Administration (FDA, K991590). Next generation devices such as the Genium Bionic Prosthetic system utilize additional environmental input (e.g., gyroscope and accelerometer) and more sophisticated processing that is intended to create more natural movement.
Microprocessor-Controlled Ankle-foot Prostheses
Microprocessor-controlled ankle-foot prostheses are being developed for transtibial amputees. These include the Proprio Foot® (Ossur) and the iPED (developed by Martin Bionics LLC and licensed to College Park Industries). With sensors in the feet that determine the direction and speed of the foot’s movement, a microprocessor controls the flexion angle of the ankle, allowing the foot to lift during the
swing phase and potentially adjust to changes in force, speed and terrain during the step phase. The intent of the technology is to make ambulation more efficient and prevent falls in patients ranging from the young active amputee to the elderly diabetic patient. The Proprio Foot™ is the only microprocessor controlled foot prosthesis that is commercially available at this time, and is a class I device that is exempt from 510(k) marketing clearance. The manufacturer must register the prosthesis with the restorative devices branch of the FDA and keep a record of any complaints, but does not have to undergo a full review. Information on the Ossur website indicates use of the Proprio Foot™ for low to moderate impact for transtibial amputees who are classified as level K3 (i.e., community ambulatory, with the ability or potential for ambulation with variable cadence).
In development are lower-limb prostheses that also replace muscle activity in order to bend and straighten the prosthetic joint. For example, the PowerFoot Biom (developed at the Massachusetts Institute of Technology and licensed to iWalk) is a myoelectric prosthesis for transtibial amputees that uses muscle activity from the remaining limb for the control of ankle movement (see policy No. 1.04.04 for a description of myoelectric technology). This prosthesis is designed to propel the foot forward as it pushes off the ground during the gait cycle, which in addition to improving efficiency, has the potential to reduce hip and back problems arising from an unnatural gait with use of a passive prosthesis. This technology is limited by the size and the weight required for a motor and batteries in the prosthesis. The Power Knee (Ossur), which is designed to replace muscle activity of the quadriceps, uses artificial proprioception with sensors similar to the Proprio Foot in order to anticipate and respond with the appropriate movement required for the next step. The Power Knee is currently in the initial launch phase in the United States.
Manufacturers must register prostheses with the restorative devices branch of the U.S. Food and Drug Administration (FDA) and keep record of any complaints, but do not have to undergo a full FDA review.
A microprocessor-controlled knee may be considered medically necessary in amputees who meet the following requirements:
- Demonstrated need for long distance ambulation at variable rates (use of the limb in the home or for basic community ambulation is not sufficient to justify provision of the computerized limb over standard limb applications) OR demonstrated patient need for regular ambulation on uneven terrain or for regular use on stairs (use of the limb for limited stair climbing in the home or employment environment is not sufficient evidence for prescription of this device over standard prosthetic application); AND
- physical ability, including adequate cardiovascular and pulmonary reserve, for ambulation at faster than normal walking speed; AND
- adequate cognitive ability to master use and care requirements for the technology.
A microprocessor-controlled knee is considered not medically necessary in individuals who do not meet these criteria.
A powered knee is considered investigational.
A microprocessor-controlled or powered foot is considered investigational.
Amputees should be evaluated by an independent qualified professional to determine the most appropriate prosthetic components and control mechanism. A trial period may be indicated to evaluate the tolerability and efficacy of the prosthesis in a real-life setting. Decisions about the potential benefits of microprocessor-knees involve multiple factors including activity levels, as well as the patient's physical and cognitive ability. A patient's need for daily ambulation of at least 400 continuous yards, daily and frequent ambulation at variable cadence or on uneven terrain (e.g., gravel, grass, curbs), and daily and frequent use of ramps and/or stairs (especially stair descent) should be considered as part of the decision. Typically, daily and frequent need of two or more of these activities would be needed to show benefit.
For patients in whom the potential benefits of the microprocessor knees are uncertain, patients may first be fitted with a standard prosthesis to determine their level of function with the standard device.
The following are guidelines from the Veterans Health Administration Prosthetic Clinical Management Program Clinical Practice Recommendations for Microprocessor Knees. (1)
PATIENT SELECTION AND IDENTIFICATION
- Contraindications for use of the microprocessor knee should include
- Any condition that prevents socket fitting, such as a complicated wound or intractable pain which precludes socket wear.
- Inability to tolerate the weight of the prosthesis.
- Medicare Level K 0—no ability or potential to ambulate or transfer.
- Medicare Level K 1—limited ability to transfer or ambulate on level ground at fixed cadence.
- Medicare Level K 2—limited community ambulator that does not have the cardiovascular reserve, strength, and balance to improve stability in stance to permit increased independence, less risk of falls, and potential to advance to a less-restrictive walking device.
- Inability to use swing and stance features of the knee unit.
- Poor balance or ataxia that limits ambulation.
- Significant hip flexion contracture (over 20 degrees).
- Significant deformity of remaining limb that would impair ability to stride.
- Limited cardiovascular and/or pulmonary reserve or profound weakness.
- Limited cognitive ability to understand gait sequencing or care requirements.
- Long distance or competitive running.
- Falls outside of recommended weight or height guidelines of manufacturer.
- Specific environmental factors—such as excessive moisture or dust, or inability to charge the prosthesis.
- Extremely rural conditions where maintenance ability is limited.
- Indications for use of the microprocessor knee should include:
- Adequate cardiovascular and pulmonary reserve to ambulate at variable cadence.
- Adequate strength and balance in stride to activate the knee unit.
- Should not exceed the weight or height restrictions of the device.
- Adequate cognitive ability to master technology and gait requirements of device.
- Hemi-pelvectomy through knee-disarticulation level of amputation, including bilateral; lower extremity amputees are candidates if they meet functional criteria as listed.
- Patient is an active walker and requires a device that reduces energy consumption to permit longer distances with less fatigue.
- Daily activities or job tasks that do not permit full focus of concentration on knee control and stability—such as uneven terrain, ramps, curbs, stairs, repetitive lifting, and/or carrying.
- Medicare Level K 2—limited community ambulator, but only if improved stability in stance permits increased independence, less risk of falls, and potential to advance to a less restrictive walking device, and patient has cardiovascular reserve, strength, and balance to use the prosthesis. The microprocessor enables fine-tuning and adjustment of the hydraulic mechanism to accommodate the unique motor skills and demands of the functional level K2 ambulator.
- Medicare Level K 3—unlimited community ambulator.
- Medicare Level K 4—active adult, athlete who has the need to function as a K 3 level in daily activities.
- Potential to lessen back pain by providing more secure stance control, using less muscle control to keep knee stable.
- Potential to unload and decrease stress on remaining limb.
- Potential to return to an active lifestyle.
- Physical and Functional Fitting Criteria for New Amputees:
- New amputees may be considered if they meet certain criteria as outlined above.
- Premorbid and current functional assessment important determinant.
- Requires stable wound and ability to fit socket.
- Immediate postoperative fit is possible.
- Must have potential to return to active lifestyle.
There are specific HCPCS codes that describe the microprocessor-controlled knee prosthesis:
L5856: Addition to lower extremity prosthesis, endoskeletal knee-shin system, microprocessor control feature, swing and stance phase, includes electronic sensor(s), any type
L5857: Addition to lower extremity prosthesis, endoskeletal knee-shin system, microprocessor control feature, swing phase only, includes electronic sensor(s), any type
L5858: Addition to lower extremity prosthesis, endoskeletal knee skin system, microprocessor control feature, stance phase only, includes electronic sensor(s), any type
There is a specific HCPCS code for ankle foot system with a microprocessor control feature:
L5973: Endoskeletal ankle foot system, microprocessor controlled feature, dorsiflexion and/or plantar flexion control, includes power source
BlueCard/National Account Issues
Contractual or benefit limitations on durable medical equipment or prostheses upgrades may be applicable.
New technologies that utilize microprocessor control are being developed. Based on currently available evidence, no microprocessor-controlled device has been shown to have better outcomes than other (e.g., earlier) models. If more costly, the prosthesis would be considered not medically necessary using the Medical Policy Reference Manual definition of medical necessity. Benefit or contract language describing the "least costly alternative" may also be applicable to prostheses.
This policy was created in 2003 and since then updated periodically using the MEDLINE database. The most recent update was performed through February 1, 2013.
Relevant outcomes for microprocessor-controlled lower limb prostheses may include the patient’s perceptions of subjective improvement attributable to the prosthesis and level of activity/function. In addition, the energy costs of walking or gait efficiency may be a more objective measure of the clinical benefit of the microprocessor-controlled prosthesis.
The literature primarily consists of small within-subject comparisons of microprocessor-controlled versus pneumatic prostheses, along with systematic reviews of these studies. Following is a summary of key studies to date.
In 2000, the Veterans Administration Technology Assessment Program issued a “short report” on computerized lower limb prosthesis. (2) This report offered the following observations and conclusions:
- Energy requirements of ambulation (compared to requirements with conventional prostheses) are decreased at walking speeds slower or faster than the amputee’s customary speed, but are not significantly different at customary speeds.
- Results on the potentially improved ability to negotiate uneven terrain, stairs, or inclines are mixed. Such benefits, however, could be particularly important to meeting existing deficit in the reintegration of amputees to normal living, particularly those related to decreased recreational opportunities.
- Users’ perceptions of the microprocessor-controlled prosthesis are favorable. Where such decisions are recorded or reported, the vast majority of study participants choose not to return to their conventional prosthesis or to keep these only as back-up to acute problems with the computerized one.
- Users’ perceptions may be particularly important for evaluating a lower limb prosthesis, given the magnitude of the loss involved, along with the associated difficulty of designing and collecting objective measures of recovery or rehabilitation. However resilient the human organism or psyche, loss of a limb is unlikely to be fully compensated. A difference between prostheses sufficient to be perceived as distinctly positive to the amputee may represent the difference between coping and a level of function recognizably closer to the preamputation level.
A 2010 systematic review evaluated safety and energy efficiency of the C-leg microprocessor-controlled prosthetic knee in transfemoral amputees. (3) Eighteen comparative studies were included that used objective/quantifiable outcome measures with the C-leg in one arm of the trial. Due to heterogeneity, meta-analyses were not performed. The 7 papers on safety had low methodologic quality and a moderate risk of bias, showing an improvement in some safety or surrogate safety measure. Effect sizes ranged from 0.2 (small) to 1.4 (large). Of the 8 papers identified on energy efficiency, 1 was considered to be of high methodologic quality, and 5 were considered to be of low quality. Two of the trials reported a statistical improvement in energy efficiency, and 4 reported some improvement in efficiency or speed that failed to reach statistical significance. There were no adverse events, safety concerns, or detriments to energy efficiency reported in association with use of the C-leg.
A number of lower-limb amputees returning from Operation Iraqi Freedom and Operation Enduring Freedom have received a microprocessor-controlled prosthesis from the Department of Veterans Affairs (VA); for example in 2005, 155 veterans were provided with a C-Leg. (4) A series of papers from the VA report results from a within-subject comparison of the C-Leg to a hydraulic Mauch SNS knee. (5-7) Eight (44%) of the 18 functional level 2–3 subjects recruited completed the study; most withdrew due to the time commitment of the study or other medical conditions. Of the 8 remaining subjects, half showed a substantial decrease in oxygen cost when using the C-Leg, resulting in a marginal improvement in gait efficiency for the group. (5) The improvement in gait efficiency was hypothesized to result in greater ambulation, but a 7-day activity monitoring period in the home/community showed no difference in the number of steps taken per day or the duration of activity. (6) Cognitive performance, assessed by standardized neuropsychological tests while walking a wide hallway in 5 of the subjects, was not different for semantic or phonemic verbal fluency and not significantly different for working memory when wearing the microprocessor-controlled prosthesis. (7) Although the study lacked sufficient power, results showed a 50% decrease in errors on the working memory task (1.63 vs. 0.88, respectively). Due to the lack of power, the effect of this device on objective measures of cognitive performance cannot be determined from this study. Subjective assessment revealed a perceived reduction in attention to walking while performing the cognitive test (effect size of 0.79) and a reduction in cognitive burden with the microprocessor-controlled prosthesis (effect size of 0.90). Seven of the 8 subjects preferred to keep the microprocessor-controlled prosthesis at the end of the study. (6) The authors noted that without any prompting, all of the subjects had mentioned that stumble recovery was their favorite feature of the C-Leg.
Kaufman and colleagues published 2 reports (2007 and 2008) describing a within-subject objective comparison of mechanical- and microprocessor-controlled knees in 15 transfemoral amputees (12 men and 3 women; mean age: 42 years) with a Medicare Classification Level 3 or 4. (8, 9) Following testing with the subject’s usual mechanical prosthesis, the amputees were given an acclimation period of 10 to 39 weeks (average of 18 weeks) with a microprocessor knee before repeat testing. Patients rated the microprocessor knee as better than the mechanical prosthesis in 8 of 9 categories of the prosthesis evaluation questionnaire. Objective gait measurement included knee flexion and the peak extensor moment during stance measured by a computerized video motion analysis system. Both the extensor moment and knee flexion were significantly different for the two prostheses, indicating a reduction in active contraction of the hip extensors to “pull back” and force the prosthetic knee into extension and resulting in a more natural gait with the microprocessor knee. Balance was improved by approximately 10%, as objectively determined with a computerized dynamic posturography platform. Total daily energy expenditure was assessed over 10 days in free-living conditions. Both daily energy expenditure and the proportion of energy expenditure attributed to physical activity increased. Although the subjects perceived that it was easier to walk with the microprocessor-controlled knee than the mechanical prosthesis, energy efficiency while walking on a treadmill was not significantly different (2.3% change). Taken together, the results indicated that amputees in this study spontaneously increased their daily physical activity outside of the laboratory setting when using a microprocessor knee.
Johansson et al. assessed energy efficiency in 8 amputees while using the C-Leg, Ossure Rheo, and hydraulic Mauch SNS knee. (10) The participants could ambulate at least at a functional classification K3 level and had approximately 10 hours of acclimatization with each prosthesis that was not his or her usual prosthesis (4 C-Leg, 1 Rheo, 1 Endolite, 1 Teh Lin, 1 Mauch). The order in which the knee systems were evaluated was randomized. Oxygen uptake was measured on a quarter mile indoor track and kinematic and kinetic data were collected in a motion analysis laboratory with subjects walking at self-selected speeds. Compared to the Mauch knee, oxygen consumption was significantly reduced for the Rheo (-5% reduction), but not for the C-leg (-2%). The Rheo and C-Leg were found to result in enhanced smoothness of gait, a decrease in hip work production, a lower peak hip flexion moment at terminal stance, and a reduction in peak hip power generation at toe-off.
In a manufacturer-sponsored study from 2007, Hafner et al. evaluated function, performance, and preference for the C-Leg in 21 unilateral transfemoral amputees using an A-B-A-B design. (11) Subjects were fully accustomed to a mechanical knee system (various types) and were required to show proficiency in ambulating on level ground, inclines, stairs, and uneven terrain prior to enrollment. Of the 17 subjects (81%) who completed the study, patient satisfaction was significantly better with the microprocessor-controlled prosthesis, as measured by the Prosthesis Evaluation Questionnaire (PEQ). Fourteen preferred the microprocessor-controlled prosthesis, 2 preferred the mechanical system, and 1 had no preference. Subjects reported fewer falls, lower frustration with falls, and an improvement in concentration. Objective measurements on the various terrains were less robust, showing improvements only for descent of stairs and hills. Unaffected were stair ascent, step frequency, step length, and walking speed. The subjective improvement in concentration was reflected by a small (nonsignificant) increase in walking speed while performing a complex cognitive task (reversing a series of numbers provided by cell phone while walking on a city sidewalk). A 2013 study by Highsmith et al. used a within-subjects pre- post- design, first evaluating outcomes with a nonmicroprocessor-controlled prosthesis followed by the same evaluation after receiving a microprocessor-controlled prosthesis. These researchers reported significantly improved descent times by 23% (6.0 vs. 7.7 seconds) and Hill Assessment Index scores (8.9 vs. 7.8) with a C-Leg compared to the subjects’ own non-microprocessor prosthetic knees. (12)
Hafner and Smith evaluated the impact of the microprocessor-controlled prosthesis on function and safety in level K2 and K3 amputees. (13) The K2 ambulators tended to be older (57 vs. 42 years), but this did not achieve statistical significance in this sample (p=0.05). In this per-protocol analysis, 8 level K2 and 9 level K3 amputees completed testing with their usual mechanical prosthesis, then with the microprocessor-controlled prosthesis, a second time with their passive prosthesis, and then at 4, 8, and 12 months with the prosthesis that they preferred/used most often. Only subjects who completed testing at least twice with each prosthesis were included in the analysis (4 additional subjects did not complete the study due to technical, medical, or personal reasons). Similar to the group’s 2007 report, performance was assessed by questionnaires and functional tasks including hill and stair descent, an attentional demand task, and an obstacle course. (11) Self-reported measures included concentration, multitasking ability, and numbers of stumbles and falls in the previous 4 weeks. Both level K2 and K3 amputees showed significant improvements in mobility and speed (ranging from 7% to 40%) but little difference in attention with the functional assessments. The self-reported numbers of stumbles and falls in the prior 4 weeks was found to be lower with the microprocessor-controlled prosthesis. For example, in the level K2 amputees, stumbles decreased from an average of 4.0 to 2.7 per month, semi-controlled falls from 1.6 to 0.6, and uncontrolled (i.e., complete) falls from 0.5 to 0 when using the microprocessor-controlled knee. Re-evaluation of each participant’s classification level at the conclusion of the study showed that 50% of the participants originally considered to be K2 ambulators were now functioning at level K3 (about as many K3 ambulators increased as decreased functional level). These results are consistent with the Veterans Health Administration Prosthetic Clinical Management Program clinical practice recommendations for microprocessor knees, which state that use of microprocessor knees may be indicated for Medicare Level K2 but only if improved stability in stance permits increased independence, less risk of falls, and potential to advance to a less restrictive walking device and if the patient has cardiovascular reserve, strength, and balance to use the prosthesis. (1)
Theeven and colleagues reported the effect of microprocessor-controlled prosthetic knee joints on functional performance in 28 Medicare Level K2 ambulators in 2011. (14) Functional performance was assessed with participants performing 17 simulated activities of daily living, first using their own mechanically controlled knee and then with 2 types of microprocessor-controlled knee joints (C-Leg and C-Leg Compact) in a randomized order with 1 week of acclimation. Performance times were significantly improved for the subset of activities that required balance while standing but not for other activities. Stratifying participants into low, intermediate, and high functional mobility level showed that the 2 higher functioning subgroups performed significantly faster using microprocessor controlled knee joints. Perceived performance was improved with the C-Leg for some subscales of the Prosthesis Evaluation Questionnaire (PEQ), but this did not translate to an increase in activity level. (15) With the C-Leg Compact, 2 of 8 subscales on the PEQ were improved, and only in the subgroup with high functional mobility. There was no change in activity level with the C-Leg or C-Leg Compact when compared to the mechanically controlled knee.
Another study compared the C-Leg Compact (stance phase only) with the participant’s usual mechanical prosthetic knee joint on ramp and community walking in 10 Medicare functional level K2 ambulators. (16) Seven of the 10 subjects used upper extremity assistive devices (e.g., a cane or walker) while ambulating. Gait analysis and functional assessments were conducted first with the participant’s mechanical knee and then, after training and a 3-month accommodation period, with the C-Leg Compact. Ramp ascent and descent were 28% and 36% faster, respectively, with the C-Leg Compact due to increases in stride length (17%) and cadence (16%). Participants also had significantly faster Timed Up and Go test (17.7 vs. 24.5 seconds) and higher functional scores on the PEQ. At the end of the study, the participants chose which prosthesis to keep; all 9 who were offered the opportunity selected the C-Leg Compact.
The Genium prosthesis was compared to the subject’s own C-Leg in a cross-over study with 11 transfemoral amputees. (17) This was a manufacturer-sponsored biomechanical study (e.g., comparison of ground reaction forces, flexion angles, load distribution) that did not evaluate clinical outcomes.
Early literature focused on the Intelligent Prosthesis (IP), which is similar to the C-Leg, but is not distributed in this country. Kirker and colleagues reported on the gait symmetry, energy expenditure, and subjective impression of the IP in 16 patients who had been using a pneumatic prosthesis and were offered a trial of an IP. (18) At the beginning of the study, the patients had been using the IP for between 1 and 9 months. Using a visual analogue scale (VAS), subjects reported that significantly less effort was required when using the IP prosthesis walking outdoors or at work at normal or high speeds, but there was no difference for a slow gait. Subjects reported a strong preference for the IP versus the standard pneumatic leg. Datta and Howitt reported on the results of a questionnaire survey of 22 amputees who were switched from pneumatic swing-phase control prostheses to an IP device. (19) All patients, who were otherwise fit and fairly active, reported that the IP was an improvement over the conventional prosthesis. The main subjective benefits were the ability to walk at various speeds, reduction of effort of walking, and patients' perception of improvement of walking pattern. Datta and colleagues also reported oxygen consumption at different walking speeds in 10 patients using an IP and a pneumatic swing gait prosthesis. (20) The IP was associated with less oxygen consumption at lower walking speeds only.
Conclusions: The literature consists of a number of small within-subject comparisons of microprocessor-controlled knees versus hydraulic knee joints. Studies on the C-Leg in Medicare level K3 and K4 amputees show objective improvements in function on some outcome measures and a strong patient preference for microprocessor-controlled prosthetic knees. Evidence on the C-Leg Compact in Medicare level K2 ambulators is limited and inconsistent. Only one biomechanical study of the next generation Genium prosthesis was identified.
Microprocessor-Controlled Ankle-Feet Prostheses
A 2004 Cochrane review of ankle-foot prostheses concluded that there was insufficient evidence from high-quality comparative studies for the overall superiority of any individual type of prosthetic ankle-foot mechanism. (21) In addition, the authors noted that the vast majority of clinical studies on human walking have used standardized gait assessment protocols (e.g., treadmills) with limited “ecological validity,” and recommended that for future research, functional outcomes should be assessed for various aspects of mobility such as making transfers, maintaining balance, level walking, stair climbing, negotiating ramps and obstacles, and changes in walking speed.
In 2009, Alimusaj et al. reported gait analysis with the Proprio Foot in 16 transtibial K3-K4 amputees during stair ascent and stair descent. (22) Results with the adaptive ankle (allowing 4 degrees of dorsiflexion) were compared with tests conducted with the same prosthesis but at a fixed neutral angle (similar to other prostheses) and with results from 16 healthy controls. Adaptive dorsiflexion was found to be increased in the gait analysis; however, this had a modest impact on other measures of gait for either the involved or uninvolved limb, with only a “tendency” to be closer to the controls, and the patient’s speed was not improved by the adapted ankle. The authors noted that an adaptation angle of 4 degrees in the stair mode is small compared to physiologic ankle angles, and the lack of power generation with this quasi-passive design may also limit its clinical benefit.
A 2012 randomized within-subject crossover study compared self-reported and objective performance outcomes for 4 types of prosthetic feet, including the Proprio Foot. (23) Ten patients with transtibial amputation were tested with their own prosthesis and then after training and a 2-week acclimation period with the SACH (solid ankle cushion heel), SAFE (stationary attachment flexible endoskeletal), Talux, and Proprio Foot in a randomized order. No differences between prostheses were detected by the self-reported PEQ and Locomotor Capabilities Index, or for the objective 6-minute walk test. Steps per day and hours of daily activity between testing sessions did not differ between the types of prostheses.
Fradet et al. reported gait analysis with the Proprio Foot in 16 transtibial amputees while walking up and down a ramp. (24) Results with the adaptive mode were compared with the Proprio Foot in a neutral angle and with results from 16 healthy controls. The adapted mode resulted in a more normal gait during ramp ascent, but not during ramp descent. However, some patients reported feeling safer with the plantar flexed ankle (adaptive mode) during ramp descent.
Au and colleagues reported the design and development of the powered ankle-foot prosthesis (PowerFoot Biom) in 2008; however, clinical evaluation of the prototype was performed in a single patient. (25)
In 2012, Ferris et al. reported a pre-post comparison of the PowerFoot Biom with the patient’s own energy-storing and -returning foot (ESR) in 11 patients with transtibial amputation. Results for both prostheses were also compared with 11 matched controls who had intact limbs. (26) In addition to altering biomechanical measures, the powered ankle-foot increased walking velocity compared to the ESR prosthesis and increased step length compared to the intact limb. There appeared to be an increase in compensatory strategies at proximal joints with the PowerFoot; the authors noted that normalization of gait kinematics and kinetics may not be possible with a uniarticular device. Physical performance measures were not significantly different between the 2 prostheses, and there were no significant differences between conditions on the PEQ. Seven patients preferred the PowerFoot and 4 preferred the ESR. Compared to controls with intact limbs, the PowerFoot had reduced range of motion, but greater ankle peak power.
Another similar, small pre- post- study from 2012 (7 amputees and 7 controls) found gross metabolic cost and preferred walking speed to be more similar to non-amputee controls with the PowerFoot Biom than with the patient’s own ESR. (27)
In a conference proceeding from 2011, Mancinelli et al. describe a comparison of a passive-elastic foot and the PowerFoot Biom in 5 transtibial amputees. (28) The study was supported by the U.S. Department of Defense, and, at the time of testing, the powered prosthesis was a prototype and subjects’ exposure to the prosthesis was limited to the laboratory. Laboratory assessment of gait biomechanics showed an average increase of 54% in the peak ankle power generation during late stance. Metabolic cost, measured by oxygen consumption while walking on an indoor track, was reduced by an average of 8.4% (p=0.06).
Conclusions: Several small studies have been reported with micro-processor controlled and powered ankle-foot prostheses for transtibial amputees. Evidence to date is insufficient to support an improvement in functional outcomes when compared to energy-storing and -returning prostheses. Larger, higher quality studies are needed to determine the impact of these devices on health outcomes with greater certainty.
Microprocessor-controlled prostheses use feedback from sensors to adjust joint movement on a real-time as-needed basis. The literature consists of a number of small within-subject comparisons of microprocessor-controlled knees versus hydraulic knee joints. For K3 and K4 level amputees, studies show an objective improvement in function on some outcome measures and a strong patient preference for microprocessor-controlled prosthetic knees. Benefits include a more normal gait, an increase in stability, a decrease in falls, and a decrease in the cognitive burden associated with monitoring the prosthesis. It is concluded that a microprocessor-controlled knee may provide incremental benefit for these individuals. Those considered most likely to benefit from these prostheses have both the potential and need for frequent ambulation at variable cadence, on uneven terrain, or on stairs. The potential to achieve a high functional level with a microprocessor-controlled knee includes having the appropriate physical and cognitive ability to be able to use the advanced technology.
Evidence is insufficient to permit conclusions regarding the effect of a microprocessor-controlled prosthesis on health outcomes in limited community ambulators. Evidence is also insufficient to permit conclusions regarding the effect of a next-generation microprocessor-controlled prosthesis on health outcomes. Therefore, these are considered investigational
The limited evidence available to date does not support an improvement in functional outcomes with a microprocessor-controlled or powered ankle-foot prostheses compared to standard prostheses. Therefore, microprocessor-controlled or powered ankle-foot prostheses are considered investigational.
Practice Guidelines and Position Statements
The VAs’ Prosthetic and Sensory Aids Strategic Healthcare Group was directed by the Under Secretary for Health to establish a Prosthetic Clinical Management Program to coordinate the development of clinical practice recommendations for prosthetic prescriptive practices. The New Technology Subgroup of the Pre-Post National Amputation Workgroup met in April 2004 to develop a proposal to define patient selection and identification criteria for microprocessor-prosthetic knees. Their proposal was based on recommendations arising from the May 2003 Microprocessor Prosthetic Knee Forum, hosted at Walter Reed Army Medical Center and sponsored and funded by the American Academy of Orthotists and Prosthetists. The resulting VA Clinical Practice Recommendations for microprocessor knees (1) are listed in Policy Guidelines.
Otto Bock, the manufacturer of the C-Leg has provided the following potential patient selection criteria (23, 29):
Amputees with mobility level “able to walk outdoors without limitations” and “able to walk outdoors without limitations plus engage in high performance activities” with at least one of the following findings:
- Diseases and/or complications due to an injury that increase the disability caused by the amputation
- Neuromuscular deficiencies of the extremities including deficiencies of the residual limb motor system
- Employees with professional activities requiring a high level of safety or long walking and standing
- People having parental authority for children up to an age of 6 years
- People with unilateral hip disarticulation amputation, and patients with hemipelvectomy amputation with good walking ability.
- People climbing stairs often (>100 per day), walking on slopes or uneven ground
- Active amputee able to walk fast (>5 kph or 3 mph) and/or walk long distances (5 km or 3 miles per day)
Medicare National Coverage
Durable medical equipment regional carriers (DMERC) are responsible for creating coverage policies for Medicare regarding durable medical equipment. There is no specific coverage policy on microprocessor-controlled knee prosthesis, in part because there is no specific HCPCS code describing this prosthesis. However, the DMERC document notes that a determination of medical necessity for certain components/additions to the prosthesis is based on the patient’s potential functional abilities. Potential functional ability is based on the reasonable expectations of the prosthetist and treating physician, considering factors including, but not limited to (30):
- the patient’s past history, AND
- the patient’s current condition including the status of the residual limb and the nature of other medical problems, AND
- the patient’s desire to ambulate
The document also provides the following classification of rehabilitation potential.
Level 0. Does not have the ability or potential to ambulate or transfer safely with or without assistance and a prosthesis does not enhance their quality of life or mobility.
Level 1. Has the ability or potential to use a prosthesis for transfers or ambulation on level surfaces at fixed cadence. Typical of the limited and unlimited household ambulatory.
Level 2. Has the ability or potential for ambulation with the ability to traverse low level environmental barriers such as curbs, stairs, or uneven surfaces. Typical of the limited community ambulatory.
Level 3. 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 utilization beyond simple locomotion.
Level 4. Has the ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact, stress, or energy levels. Typical of the prosthetic demand of the child, active adult, or athlete.
- VHA Prosthetic Clinical Management Program (PCMP). Clinical practice recommendations: microprocessor knees, 2004. See: Berry D. Microprocessor prosthetic knees. Phys Med Rehabil Clin N Am 2006; 17:91-113.
- U.S. Department of Veterans Affairs, Veterans Health Administration. Office of Research and Development, Health Service Research and Development Service, Management Decision and Research Center, Technology Assessment Program. Computerized lower limb prosthesis. VA Technology Assessment Program Short Report No. 2. 2000. Available online at: http://www.va.gov/VATAP/docs/ComputerizedLowerLimbProsthese2000tm.pdf. Last accessed January, 2012.
- Highsmith MJ, Kahle JT, Bongiorni DR et al. Safety, energy efficiency, and cost efficacy of the C-Leg for transfemoral amputees: a review of the literature. Prosthet Orthot Int 2010; 34(4):362-77.
- Department of Veterans Affairs Fact Sheet, VA’s Prosthetics and Sensory Aids. 2006. Available online at: http://www1.va.gov/OPA/fact/docs/pros-sensory.doc. Last accessed March, 2007.
- Orendurff MS, Segal AD, Klute GK et al. Gait efficiency using the C-Leg. J Rehabil Res Dev 2006; 43(2):239-46.
- Klute GK, Berge JS, Orendurff MS et al. Prosthetic intervention effects on activity of lower-extremity amputees. Arch Phys Med Rehabil 2006; 87(5):717-22.
- Williams RM, Turner AP, Orendurff M et al. Does having a computerized prosthetic knee influence cognitive performance during amputee walking? Arch Phys Med Rehabil 2006; 87(7):989-94.
- Kaufman KR, Levine JA, Brey RH et al. Gait and balance of transfemoral amputees using passive mechanical and microprocessor-controlled prosthetic knees. Gait Posture 2007; 26(4):489-93.
- Kaufman KR, Levine JA, Brey RH et al. Energy expenditure and activity of transfemoral amputees using mechanical and microprocessor-controlled prosthetic knees. Arch Phys Med Rehabil 2008; 89(7):1380-5.
- Johansson JL, Sherrill DM, Riley PO et al. A clinical comparison of variable-damping and mechanically passive prosthetic knee devices. Am J Phys Med Rehabil 2005; 84(8):563-75.
- Hafner BJ, Willingham LL, Buell NC et al. Evaluation of function, performance, and preference as transfemoral amputees transition from mechanical to microprocessor control of the prosthetic knee. Arch Phys Med Rehabil 2007; 88(2):207-17.
- Highsmith MJ, Kahle JT, Miro RM et al. Ramp descent performance with the C-Leg and interrater reliability of the Hill Assessment Index. Prosthet Orthot Int 2013 [Epub ahead of print].
- Hafner BJ, Smith DG. Differences in function and safety between Medicare Functional Classification Level-2 and -3 transfemoral amputees and influence of prosthetic knee joint control. J Rehabil Res Dev 2009; 46(3):417-33.
- Theeven P, Hemmen B, Rings F et al. Functional added value of microprocessor-controlled knee joints in daily life performance of Medicare Functional Classification Level-2 amputees. J Rehabil Med 2011; 43(10):906-15.
- Theeven PJ, Hemmen B, Geers RP et al. Influence of advanced prosthetic knee joints on perceived performance and everyday life activity level of low-functional persons with a transfemoral amputation or knee disarticulation. J Rehabil Med 2012; 44(5):454-61.
- Burnfield JM, Eberly VJ, Gronely JK et al. Impact of stance phase microprocessor-controlled knee prosthesis on ramp negotiation and community walking function in K2 level transfemoral amputees. Prosthet Orthot Int 2012; 36(1):95-104.
- Bellmann M, Schmalz T, Ludwigs E et al. Immediate effects of a new microprocessor-controlled prosthetic knee joint: a comparative biomechanical evaluation. Arch Phys Med Rehabil 2012; 93(3):541-9.
- Kirker S, Keymer S, Talbot J et al. An assessment of the intelligent knee prosthesis. Clin Rehabil 1996 1996; 10(3):267-73.
- Datta D, Howitt J. Conventional versus microchip controlled pneumatic swing phase control for trans-femoral amputees: user's verdict. Prosthet Orthot Int 1998; 22(2):129-35.
- Datta D, Heller B, Howitt J. A comparative evaluation of oxygen consumption and gait pattern in amputees using Intelligent Prostheses and conventionally damped knee swing-phase control. Clin Rehabil 2005; 19(4):398-403.
- Hofstad C, Linde H, Limbeek J et al. Prescription of prosthetic ankle-foot mechanisms after lower limb amputation. Cochrane Database Syst Rev 2004; (1):CD003978.
- Alimusaj M, Fradet L, Braatz F et al. Kinematics and kinetics with an adaptive ankle foot system during stair ambulation of transtibial amputees. Gait Posture 2009; 30(3):356-63.
- Gailey RS, Gaunaurd I, Agrawal V et al. Application of self-report and performance-based outcome measures to determine functional differences between four categories of prosthetic feet. J Rehabil Res Dev 2012; 49(4):597-612.
- Fradet L, Alimusaj M, Braatz F et al. Biomechanical analysis of ramp ambulation of transtibial amputees with an adaptive ankle foot system. Gait Posture 2010; 32(2):191-8.
- Au S, Berniker M, Herr H. Powered ankle-foot prosthesis to assist level-ground and stair-descent gaits. Neural Netw 2008; 21(4):654-66.
- Ferris AE, Aldridge JM, Rabago CA et al. Evaluation of a powered ankle-foot prosthetic system during walking. Arch Phys Med Rehabil 2012; 93(11):1911-8.
- Herr HM, Grabowski AM. Bionic ankle-foot prosthesis normalizes walking gait for persons with leg amputation. Proc Biol Sci 2012; 279(1728):457-64.
- Mancinelli C, Patritti BL, Tropea P et al. Comparing a passive-elastic and a powered prosthesis in transtibial amputees. Conf Proc IEEE Eng Med Biol Soc 2011; 2011:8255-8.
- Patient Indications: List of indications for the C-Leg and Compact. Otto Bock, U.S. Available online at: http://www.ottobockus.com/cps/rde/xchg/ob_us_en/hs.xsl/19348.html/. Last accessed January, 2009.
- LCD for Lower Limb Prostheses (L11464). Available online at: http://www.medicarenhic.com/dme/medical_review/mr_lcds/mr_lcd_current/L11464_2009-01-01_PA_2008-10.pdf. Last accessed January, 2009.
|ICD-9 Diagnosis||897.2 - 897.7||Traumatic amputation of leg; code range for above the knee amputation(s)|
|V43.65||Organ or tissue replaced by other means; knee|
|HCPCS||See Policy Guidelines|
|ICD-10-CM (effective 10/1/14)||S78.011-S78.929||Traumatic amputation of hip and thigh; code range|
|Z96.651-Z96.659||Presence of artificial knee joint; code range|
|ICD-10-PCS (Effective 10/1/14)||Not applicable. ICD-10-PCS codes are only used for inpatient services. There are no ICD procedure codes for devices.|
Adaptive, Knee Prosthesis
C-LEG, Knee Prosthesis
Intelligent Prosthesis, Knee
iPED, Foot Prosthesis
Knee Prosthesis; Microprocessor Controlled
|10/9/03||Add to Durable Medical Equipment section||New policy|
|4/1/05||Replace policy||Policy updated with literature review; policy statement unchanged. HCPCS coding updated|
|12/14/05||Replace policy – coding update only||HCPCS coding updated|
|04/25/06||Replace policy||Policy updated with literature review; policy statement unchanged. Reference number 5 added|
|06/14/07||Replace policy||Policy updated with literature review; reference numbers 1 and 8-15 added; policy statement revised, may be medically necessary for some patients; VA guidelines added.
|2/12/09||Replace policy||Policy updated with literature review through December 2008; reference numbers 16-19 added and reference 21 updated; policy statements added regarding ankle-foot and powered knee prostheses; these are investigational.Title changed to “prostheses for the lower limb” to include ankle-foot.|
|02/11/10||Replace policy||Policy updated with literature review through December 2009; references added; policy statements unchanged|
|3/10/11||Replace policy||Policy updated with literature review through January 2011; reference 21 added; policy number changed from 1.01.25 to 1.04.05 (prosthetics); policy statements unchanged|
|03/08/12||Replace policy||Policy updated with literature review through December 2011; Rationale revised; references 3, 16, 17, 22 added; some references removed; policy statements unchanged|
|03/14/13||Replace policy||Policy updated with literature review through February 1, 2013; Rationale revised; references 12, 15, 17, 23, 26-27 added and references reordered; policy statements unchanged|