Advantages and Disadvantages of Microprocessor-Controlled Prosthetic Ankles
By Brian Kaluf, CP, FAAOP, and Cody Smith, CO
One component that is a common denominator for all lower-limb prosthesis users is the ankle-foot component, which interacts with the ground-reaction force and affects the fit and function of the rest of the prosthesis. To replicate the function of the normal human ankle, prosthetic ankle-foot designs have been developed throughout the years to provide articulation in the sagittal plane and allow more natural, efficient, and stable gait for persons with lower-limb loss.
Prosthetists and patients have access to a large variety of prosthetic ankle-foot components; however, they must select one component that is indicated for most of the patient’s functional needs. This often involves a compromise between two competing characteristics, such as flexibility and stability.
The vast majority of available ankle-foot components are passive mechanisms with fixed ankles, such as energy-storing and -returning (ESAR) feet. ESAR feet are extensions of the solid-ankle cushion heel (SACH) foot design, originally developed to provide shock absorption, tibial progression, and support in late stance, but without the noise and durability issues of previous single-axis ankles. Limitations of SACH, single-axis, and ESAR designs have been described1 with recommendations to develop feet that allow foot flat earlier in the gait cycle to reduce heel-only loading while simultaneously preserving limb stability and allowing tibial progression. ESAR feet can store energy more efficiently compared to SACH feet, but fixed-ankle ankle-foot components cannot adapt with changes in terrain. Hydraulic ankles do realign to changes in terrain through a viscoelastic response. However, the hydraulic resistance settings remain constant throughout the ankle range of motion and do not adapt with variations in terrain or walking speed.
Certified prosthetists understand limitations of passive ankle-foot systems and balance trade-offs as they make recommendations to patients regarding the one component that supports the most desired activities and causes the fewest adverse reactions. In reality, no passive ankle-foot component can replicate all of the functions of the human foot. For this reason, there has been much recent development in adaptable ankle-foot technology.
Microprocessor-Controlled Prosthetic Ankles
Microprocessor-controlled prosthetic ankles (MPAs) provide distinct functional differences compared to passive ankle-foot mechanisms commonly used today. MPAs employ two distinct strategies to control ankle articulation. The first strategy adapts the ankle angle to match the slope of the terrain during swing phase but maintains a solid ankle during stance phase. With this approach, several steps are required to recognize the slope and adapt the ankle for subsequent steps (inter-step ankle adaptation). The second strategy approaches ankle articulation through a hydraulic cylinder during stance phase and adapts to a slope on each step-through (within-step ankle adaptation). Some MPAs provide within-step hydraulic ankle adaptation and inter-step terrain recognition with ankle adaptation for subsequent steps.
The first commercial release of an MPA component was the Proprio Foot (Össur, Reykjavik, Iceland) in 2006. Comparatively, microprocessor-controlled prosthetic knee (MPK) technology has been available for persons with transfemoral amputation much longer, with widespread adoption following the commercial release of the C-Leg MPK (Ottobock, Duderstadt, Germany) in 1997. Currently, MPKs are available from seven different manufacturers, and evidence of patient benefits from MPK technology has been appraised in recent systematic literature reviews.2,3
Since the initial release of the first MPA, more components have become available, with each system having different functions and specifications. Currently available MPA components include Elan (Elan, Chas. A. Blatchford & Sons Ltd., Basingstoke, United Kingdom), Kinnex (Freedom Innovations, Irvine, California, United States), Meridium and Triton smart ankle (Ottobock, Duderstadt, Germany), Proprio (Össur, Reykjavik, Iceland), and Raize (Fillauer, Chattanooga, Tennessee, United States). Component specifications are detailed in Table 1, including foot size range, body weight limit, build height, component weight, battery life, ankle range of motion (ROM), waterproof characteristics, within-step versus inter-step accommodation, heel height adjustability, smartphone versus button control, and warranty length.
MPA Research Evidence
To date, no systematic review of literature regarding MPA technology has been completed, although more than 13 publications on the topic exist. Research publications regarding MPA technology consist of laboratory kinematic and kinetic analyses, inter-socket pressure measurement, energetic evaluation, patient-reported outcome measures, and performance-based outcome measures.
Agrawal et al. compared the symmetry in external work (SEW) over level ground and found improved SEW with the MPA (Proprio Foot) over some feet, but not when compared to an ESAR foot.4 Alimusaj et al. investigated gait during stair ascent and descent and found an improvement in knee kinetics and kinematics with the MPA (Proprio Foot) set in a 4-degree dorsiflexed position.5 Wolf et al. compared residual limb socket pressure and performed gait kinematic and kinetic analysis on level ground, slopes, and stairs.6 This study found that with the adaptive mode, the MPA (Proprio Foot) could accommodate the slope and reduce socket pressures to be comparable with level ground walking. Fradet et al. performed gait kinematic and kinetic analysis during ramp ascent and descent and found that the MPA (Proprio Foot) adaptive mode helped reduce the prosthetic side knee extension moment during ramp ascent, but it caused gait adaptations during ramp descent despite the patients reporting “feeling safer.”7 Gailey et al. compared the patient-reported and performance-based outcome measures and found no significant differences between three fixed-ankle prosthetic feet and one MPA (Proprio Foot).8 Agrawal et al. compared SEW while walking on stairs and found an improvement in SEW on stairs with the MPA (Proprio Foot) over some feet, but not when compared to an ESAR foot.9 Delussu et al. compared the functional mobility, self-reported mobility, and energy cost of walking (ECW) over level ground and on a treadmill set at three slopes (-5 degrees, 0 degrees, and 12 degrees).10 This study found a significant improvement in ECW over level ground using the MPA (Proprio Foot) after a 90-day accommodation period. Darter et al. compared metabolic energy expenditure and ECW while walking on a treadmill set at three slopes
(-5 degrees, 0 degrees, and 5 degrees).11 The authors only found significant decreases in energy expenditure or ECW on ramp descent with the MPA (Proprio Foot) regardless if the MPA adaptive mode was turned on or off. Rosenblatt et al. compared minimum toe clearance (MTC) and found a significant increase with the MPA (Proprio Foot) on level surface and on an incline, although the results could be attributed to increased hip flexion instead of MPA function alone. Agrawal et al. compared SEW while walking on level ground with four prosthetic ankle-foot systems but did not find the MPA (Proprio Foot) to increase SEW.12 Agrawal et al. later compared SEW while walking on a ramp with the same prosthetic feet.13 The authors only found a significant difference in the Medicare Functional Classification System (MFCL) K-2 subgroup on ramp descent, with both ESAR and MPA (Proprio Foot) providing greater SEW than the other prosthetic feet. Struchkov et al. performed kinematic and kinetic gait analysis during ramp descent, which showed the MPA (Elan) performed more negative work, arrived at foot flat quicker, slowed the rotational velocity of the tibia, and allowed less negative work done by the residual knee.14 Ko et al. performed kinematic and kinetic gait analysis and found differences in joint moments, power, and stiffness in most joints in both legs and braking impulse over level ground with prosthetic feet with articulating ankles (Proprio Foot, Elan, Echelon).15
This body of evidence highlights benefits from MPA technology, but research with other MPA components and larger sample sizes is needed. Most of these investigations compared an MPA component (Proprio Foot) that only provides inter-step accommodation to terrain during swing phase. More studies of MPAs that employ a within-step accommodation strategy or both strategies are needed. The largest sample size included in these studies was 16 participants,5 while most of the studies enrolled 10 or less. The limited sample size and the multiple comparisons made with most kinematic and kinetic gait analyses greatly reduce the statistical power of these studies. Recent efforts to investigate more recently developed MPA components (Kinnex)16 have enrolled larger sample sizes.
Advantages of MPAs
MPAs provide advantages that can be experienced during level ground walking, while the greatest benefits are experienced on uneven and sloped terrain. Over level ground, MPAs aim to mimic the three rockers of stance phase, which require rapid foot flat while simultaneously maintaining stability for weight acceptance.1 Following foot flat, MPAs provide a true articulation about the ankle and tibial progression through the second rocker of stance phase. The resistance can be controlled by the microprocessor to adapt ankle stiffness to changes in walking speed. Patients describe the benefits of ankle articulation during the second rocker of stance phase as having no “dead spot,” a phenomenon experienced with fixed-ankle feet that inhibit tibial progression. During the third rocker of stance phase, MPAs can modulate the ankle stiffness to provide a gradual transition from ankle dorsiflexion to deflection of the carbon-fiber keel element during tibial progression. Some MPAs provide an additional advantage during swing phase due to relative ankle dorsiflexion. This increases the MTC and reduces the risk of stumbles and falls.17 MPAs that only provide inter-step ankle adaptation and present a fixed-ankle during stance may not exhibit these advantages over level ground.
These advantages over level ground are experienced to a greater extent over uneven and sloped terrain. The lack of articulation of fixed-ankle feet induces greater ankle reaction torque and pressure on the residual limb. This causes patients to adopt accommodation strategies or limits their mobility on these terrains as the residual limb and proximal joints are unable to tolerate the increased loads. MPAs have been shown to reduce the peak pressures experienced on ramp ambulation by accommodating the slope of a ramp.6 Additionally, by achieving foot flat walking on slopes to maximize contact area and base of support (BOS), MPAs have been shown to improve gait14 and make patients feel safer.7 MPAs provide an advantage over solid ankle feet on slopes for this reason.
Other advantages to MPAs are less obvious yet still important to the patient. Ankle articulation allows MPAs to be used with various heel height shoes. Another advantage is the reduced stress on proximal joints during prolonged seating.
These general advantages of MPA technology are not universally experienced with all MPA components as differences in specifications (Table 1), as well as accommodation strategy (inter-step versus within-step), affect the types of benefits experienced. While some research evidence supports these advantages, many of these advantages are anecdotes reported by patients or inherent design differences that have not been investigated in research studies.
Disadvantages of MPAs
General disadvantages of MPA technology can be characterized by additional weight, patient self-management requirements, and susceptibility to damage. MPAs weigh between 749 and 1,488 grams, which is nearly twice the weight of common ESAR feet. This increased weight can pose increased requirements of the suspension mechanism employed, and patients may experience greater motion (pistoning) of the residual limb within the socket. Another disadvantage is the battery charging requirement. The additional bulk of MPA components also can lead to difficulty in fabricating a custom-shaped cover that mirrors the anatomical shape of the contralateral limb.
MPAs employ a hydraulic cylinder that allows ankle articulation. Because a hydraulic cylinder absorbs and dissipates energy during the gait cycle, as opposed to storing and returning energy in an elastic element, some MPAs may reduce energy efficiency. There has been limited evidence regarding energetics with MPA technology, yet studies have not found significant increases in energy consumption between MPAs and ESAR feet.10,11 For some patients, the ankle articulation of some MPAs in stance phase may destabilize static standing balance. If the patient relies on the passive ankle reaction torque from solid ankle designs to maintain balance when his or her center of pressure nears the anterior or posterior border of the BOS, MPAs that allow ankle articulation in stance phase may present a disadvantage.
MPAs have more moving mechanical parts and electronic components that are more susceptible to general wear and accidental damage. This presents a durability issue, which requires routine maintenance for MPAs. MPA components are provided with product warranties of varying length, but a repair under warranty still requires additional clinical visits, which may provide an inconvenience for some patients. MPAs have varying levels of waterproof protection, which limits the environments in which they can be worn (see Table 1).
One final related disadvantage is that MPA technology is not covered universally across different health insurance plans. The increased cost of MPA components leads to increased scrutiny by third-party payors. Due to this uncertainty, patients may experience delays in receiving a new prosthesis with MPA technology. Therefore, the cost of MPA technology can present a limitation to some patients.
In recommending MPA technologies for individual patients, it is important to consider the advantages and disadvantages of MPA components. A thorough assessment of specific limitations experienced by patients using solid ankle feet and their functional goals is necessary for making decisions regarding MPA technology. Combined with knowledge of the benefits supported by research, familiarity with the unique differentiating functions of the variety of MPA components will assist practitioners in deciding when patients would benefit most from this technology and which MPA components are indicated for individual patients.
Brian Kaluf, CP, FAAOP, is clinical outcome and research director and Cody Smith, CO, is a certified orthotist and board-eligible prosthetist at Ability Prosthetics & Orthotics.
- Perry J, Boyd LA, Rao SS, Mulroy SJ. Prosthetic Weight Acceptance Mechanics in Transtibial Amputees Wearing the Single Axis, Seattle Lite, and Flex Foot. IEEE Transactions on Rehabilitation Engineering. 1997; 5(4):283-289.
- Kannenberg A, Zacharias B, Probsting
E. Benefits of Microprocessor-Controlled Prosthetic Knees to Limited Community Ambulators: Systematic Review. Journal of Rehabilitation Research & Development. 2014; 51(10):1469-1496.
- Sawers AB, Hafner BJ. Outcomes Associated With the Use of
Microprocessor-Controlled Prosthetic Knees Among Individuals With Unilateral Transfemoral Limb Loss: A Systematic Review. JPO: Journal of Prosthetics and Orthotics. 2013; 25(4S):P4-P40.
- Agrawal V, Gailey R, O’Toole C, Gaunaurd I, Dowell T. Symmetry in External Work (SEW): A Novel Method of Quantifying Gait Differences Between Prosthetic Feet. Prosthetics and Orthotics International. 2009; 33(2):148-156.
- Alimusaj M, Fradet L, Braatz F, Gerner HJ, Wolf SI. Kinematics and Kinetics With an Adaptive Ankle-Foot System During Stair Ambulation of Transtibial Amputees. Gait & Posture. 2009; 30(3):356-363.
- Wolf SI, Alimusaj M, Fradet L, Siegel J, Braatz F. Pressure Characteristics at the Stump/Socket Interface in Transtibial Amputees Using an Adaptive Prosthetic Foot. Clinical Biomechanics. 2009; 24(10):860-865.
- Fradet L, Alimusaj M, Braatz F, Wolf SI. Biomechanical Analysis of Ramp Ambulation of Transtibial Amputees With an Adaptive Ankle-Foot System. Gait & Posture. 2010; 32(2):191-198.
- Gailey RS, Gaunaurd I, Agrawal V, Finnieston A, O’Toole C, Tolchin R. Application of Self-Report and Performance-Based Outcome Measures To Determine Functional Differences Between Four Categories of Prosthetic Feet. Journal of Rehabilitation Research and Development. 2012; 49(4):597.
- Agrawal V, Gailey RS, Gaunaurd I, O’Toole C, Finnieston A. Comparison Between Microprocessor-Controlled Ankle/Foot and Conventional Prosthetic Feet During Stair Negotiation in People With Unilateral Transtibial Amputation. Journal of Rehabilitation Research and Development. 2013; 50(7):941.
- Delussu AS, Brunelli S, Paradisi F, et al. Assessment of the Effects of Carbon Fiber and Bionic Foot During Overground and Treadmill Walking in Transtibial Amputees. Gait & Posture. 2013; 38(4):876-882.
- Darter BJ, Wilken JM. Energetic Consequences of Using a Prosthesis With Adaptive Ankle Motion During Slope Walking in Persons With a Transtibial Amputation. Prosthetics and Orthotics International. 2014; 38(1):5-11.
- Agrawal V, Gailey R, O’Toole C, Gaunaurd I, Finnieston A. Influence of Gait Training and Prosthetic Foot Category on External Work Symmetry During Unilateral Transtibial Amputee Gait. Prosthetics and Orthotics International. 2013; 37(5):396-403.
- Agrawal V, Gailey RS, Gaunaurd IA, O’Toole C, Finnieston A, Tolchin R. Comparison of Four Different Categories of Prosthetic Feet During Ramp Ambulation in Unilateral Transtibial Amputees. Prosthetics and Orthotics International. 2015; 39(5):380-389.
- Struchkov V, Buckley JG. Biomechanics of Ramp Descent in Unilateral Trans-Tibial Amputees: Comparison of a Microprocessor Controlled Foot With Conventional Ankle-Foot Mechanisms. Clinical Biomechanics. 2016; 32:164-170.
- Ko C-Y, Kim S-B, Kim JK, et al. Biomechanical Features of Level Walking by Transtibial Amputees Wearing Prosthetic Feet With and Without Adaptive Ankles. Journal of Mechanical Science and Technology. 2016; 30(6):2907-2914.
- Kaluf B. Poster 7: Comparative Effectiveness of Microprocessor Controlled and Carbon Fiber Energy Storing and Returning Prosthetic Feet in Persons With Unilateral Transtibial Amputation: Pilot Study. PM&R. 2017; 9(9):S144.
- Rosenblatt NJ, Bauer A, Rotter D, Grabiner MD. Active Dorsiflexing Prostheses May Reduce Trip-Related Fall Risk in People With Transtibial Amputation. J Rehabil Res Dev. 2014; 51(8):1229-1242.
Table 1: Microprocessor Ankle Component Specifications
|Microprocessor Ankle||Kinnex||Elan||Proprio||Triton Smart Ankle||Meridium||Raize|
|Foot Size Range (cm)||24-30||22-30||22-30||24-30||24-29||24-30|
|Body Weight Limit (kg)||125||125||125||100||100||100|
|Build Height (mm)||184||175||159||150||175||89|
|Component Weight (g)||1,488||1,200||1,424||1,470||1,500||749|
|Battery Life (hrs)||24||27||24-48||96||24||18|
|Ankle ROM (deg)||30||9||20||34||36.5||18|
|N/A||IP54 splash||IP54 splash||Splash|
|Within-Step Versus Inter-Step Ankle Accommodation||Within-step||Both||Inter-step||Inter-step||Within-step||Within-step|
|Heel Height Adjustability (in)||2||Unspecified||2||2||2||2|
|Smart Phone App vs