Abstract

Prosthetic knees are state-of-the-art medical devices that use mechanical mechanisms and components to simulate the normal biological knee function for individuals with transfemoral amputation. A large variety of complicated mechanical mechanisms and components have been employed; however, they lack clear relevance to the walking biomechanics of users in the design process. This article aims to bridge this knowledge gap by providing a review of prosthetic knees from a biomechanical perspective and includes stance stability, early-stance flexion and swing resistance, which directly relate the mechanical mechanisms to the perceived walking performance, i.e., fall avoidance, shock absorption, and gait symmetry. The prescription criteria and selection of prosthetic knees depend on the interaction between the user and prosthesis, which includes five functional levels from K0 to K4. Misunderstood functions and the improper adjustment of knee prostheses may lead to reduced stability, restricted stance flexion, and unnatural gait for users. Our review identifies current commercial and recent studied prosthetic knees to provide a new paradigm for prosthetic knee analysis and facilitates the standardization and optimization of prosthetic knee design. This may also enable the design of functional mechanisms and components tailored to regaining lost functions of a specific person, hence providing individualized product design.

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Keywords: prosthetic knee, transfemoral prosthesis, knee mechanisms, passive knee, above-knee prosthesis

2 Methods

2.1 Search strategy

A literature search was conducted until 1 June in eight English databases following PRISMA method. The used databases are Web of Science, Springer, Wiley, Science direct, IEEExplore, ASME, PubMed and Google Scholar. In addition, patents of passive prosthetic knee were explored via Google Patents. Eight English keywords, including &#;above-knee prosthesis,&#; &#;transfemoral knee prosthesis,&#; &#;prosthetic knee mechanism,&#; &#;passive prosthetic knee,&#; &#;brake prosthetic knee,&#; &#;polycentric prosthetic knee,&#; &#;mechanical knee,&#; or &#;transfemoral amputation,&#; are used during database retrieves. The beginning date and end date of these database searches were set from January 1, to the latest date provided by the databases.

Furthermore, a manual search was performed on three types of publications from the screened results of the database searches. The first type of publication was review articles, the second type was research articles of functional structure in mechanical knees, and the other type of publication was clinical studies of transfemoral amputation. Finally, 140 results of the manual searches were screened, including 113 journal articles and 27 patents.

2.2 Exclusion criteria

Any records that met the following four levels of criteria were deleted: 1) with irrelevant title or irrelevant keywords; 2) with irrelevant abstract or no relevant illustrations of passive prosthetic knees; 3) without the walking biomechanics related to prosthetic knees; and 4) without descriptions of functional structures or functional elements in passive transfemoral prostheses. The database and manual search and screening procedures are illustrated in the flowchart in Figure 1.

FIGURE 1.

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Flowchart of database search and manual search based on PRISMA.

2.3 Classification criteria

Passive knee prostheses from the screened publications and online information were classified based on the biomechanical challenges of persons with transfemoral amputation, namely, falls, osteoarthritis, and gait asymmetry.

A fall is mainly related to stance stability, which is the basic requirement of safety for all individuals in the K0&#;K4 levels. Stance stability is realized by functional structures such as four-bar linkages or by functional components, such as hydraulic units.

Osteoarthritis corresponds to stance flexion, which is desired by active users in the K3 and K4 levels. Stance flexion can reduce the impact from the ground and improve the comfortability of the residual limb. It depends on the functional structures of the prosthetic knee and allows for a limited flexion angle at the early-stance phase without losing stability.

Gait asymmetry is associated with swing resistance, where this essential function controls the maximum flexion angle and determines the timing of full extension. Swing resistance is regulated by the functional components that act on knee axis.

Mechanisms and components in knee prostheses are closely related to basic walking functions. Therefore, the biomechanical challenges and required functions of the knee joint are proposed first (Figure 2). Then, as the key solutions to those health problems, the functional structures and components of current passive prosthetic knees are illustrated. We wish to provide a better understanding of the basic functional principles of knee prostheses based on this framework.

FIGURE 2.

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Framework based on the required functions of prosthetic knee on the aspect of required functions during level walking.

7 Discussion

In this review, representative passive knee mechanisms are reviewed according to biomechanical requirements. Furthermore, we designed Table 2, which includes current commercial and recently studied passive knees, for users or developers to understand and analyze the functional mechanisms and components from the perspective of stability, ESF, and swing resistance. Accordingly, we present ideas about three general trends in the current and future development of prosthetic knees.

TABLE 2.

Passive prosthetic knees based on walking functions.

Functional mechanisms and components of passive knees Swing resistance Frictional Pneumatic Hydraulic Rotary Linear Stance Stance Stability Monocentric Knees Manual-Lock OT-3R95, OT-3WR95 Weight-Brake OT-3R49 OT-3R92, OS-OP4, NA-NK1 Hyperextension-Controlled OS-Mauch, BL-Mercury Polycentric Knees Elevated or OT-3R30, OS-Balance OT-3R106, OT-3R78, OS-Paso, OS-OHP3, BL-S500, ST-3A OS-Cheetah OT-3R67, OT-3R55, OS-OH5, OS-OH7, TL-X6, ST-3A Hyper-stabilized or Voluntary Controlled Four-Bar Linkage + Weight-Brake DAW-Sure Stance, Four-Bar Linkage + Hyperextension-Controlled BL-KX06 Knees + Lock SA (Knee) + SA (Lock) LCKnee (Andrysek et al., ) SA (Knee) + 4-Bar Linkage (Lock) NA-Hybrid Linkage Mechanisms NA-NK6 Stance, Stability and ESF Knees + ESF SA (Knee) + SA (ESF) BL-ESK Linkage Mechanisms OT-3R62, OS-Total TL-5PSOH OS-Total OT-3R60 Knees + Lock + ESF SA (Knee) + SA (Lock) +SA (ESF) MIT-Knee-1 (Murthy Arelekatti and Winter, ) OT-3R80 SA (Knee) + Linkage mechanisms (Lock) +SA (ESF) MIT-Knee-2 (Berringer et al., ) Open in a new tab

7.1 Adaptivity

Passive knee research mainly concentrates on the biomechanics of level walking. However, passive knees cannot meet the needs of the users&#; daily activities. The adaptivity of knee prostheses should be improved from two aspects.

7.1.1 Adaptivity to the environment.

Prosthetic knees are expected to deal with environmental elements including irregular terrain, ramps and stairs. Microcontrollers have been introduced to MPKs and allow automatic variation in the damping, which can accommodate a wider range of environmental factors. However, most MPK solutions are monocentric and are typically based on a single knee-axis structure. The knee-axis-based hydraulic unit of the MPK is required to provide adequate damping for stance flexion and stance stability, simultaneously. Thus, compared to a healthy knee, asymmetric gait with a smaller stance-flexion angle arises in MPKs (Thiele et al., ). The adaptivity can be improved by combining microprocessor-controlled units and passive mechanisms; for example, stance stability and ESF can be controlled by automatically adjusting the structures of knee axis and ESF mechanism, and the swing resistance can be regulated by microprocessor-controlled units. The functional components acting on different phases can be automatically adjusted to the optimized state according to the environmental factors without interfering with each other. In addition to the MPK solutions, the adaptivity can be enhanced only by passive mechanisms. A passive mechanism that acts as a lock axis has been added to a knee device; it locks the knee and generates an extension moment around the knee axis during the stance phase without using any actuators (Inoue et al., ). This mechanism enables the knee to adapt to stair ascent, which is based on the knowledge that GRF translates and increases when stance flexion occurs. Other mechanisms or intelligent units may be integrated with current passive knee, which can be further developed and optimized.

7.1.2 Adaptivity to users

Passive knees are not capable of recognizing an individual&#;s intent and can only use pneumatic and hydraulic units to change the damping force in a limited range with changing walking speeds. The estimation or recognition of a user&#;s locomotive intent is more important in state-of-the-art prosthetic knees, which can directly adapt for different speeds, terrain, and obstacles. Biomechanical instrumentation comprising angles, loads, and inertial sensors is commonly used in MPKs and APKs, which collect kinematics and force signals to match the predefined locomotive states. These signals are stable and highly repetitive, which makes the finite state machine (FSM) control strategy capable of commanding the knee to a robust and well-defined state. However, there is hysteresis in the FSM strategy (Martin et al., ). The locomotive state knowledge with sensor-based information comes from previous steps, and the angle and damping of the joint may not be best suited for the immediate current step. Furthermore, the sensor signals only reflect the movement of the prosthesis, not the intentions of users. It is still a limited framework that cannot adapt to arbitrary motions of the user. Non-invasive electromyography (EMG) is another method that is used as volitional control, but the weak signal amplitude, noise during acquisition, and muscle deficiency of the residual limb all restrict the quality and robustness of EMG. Thus, it appears to be less appropriate and far from being a stand-alone technology for dynamic locomotion. On the other hand, the EMG-based approach combining the embedded sensors exhibits higher adaptivity and stability (Peeraer et al., ; Au et al., ). In the authors&#; opinion, functional mechanisms and components are closely associated with walking biomechanics, and variation in locomotive states can be straightforwardly mapped to the functional axis in real time. For instance, a mechanical sensor mounted on a lock-axis structure can perceive the transition from stance to swing immediately. Feedforward or feedback can be achieved by adjusting the position of the virtual lock axis. The mechanical intelligence used for the adaptive prosthesis&#;user interaction remains a possibility in the future.

7.2 Controlled energy flow

Daily activities, such as running, jumping, or stair climbing, require significant amounts of energy input, thus leading to the need for APKs (Jacobs et al., ; Riener et al., ). Some of the latest prototypes have already improved kinematics for normal gait, which have even approached biological levels (Lawson et al., ; Zhao et al., ). However, an active prosthesis is normally heavier than a passive prosthesis, which leads to the primary drawback of a higher metabolic cost for the users (Pfeifer et al., ).

Passive knees are lightweight and energy-efficient because the mechanisms and components are highly matched to walking biomechanics. Therefore, one of the challenges in the future is how lightweight and effective functional mechanisms can be integrated into actuators to minimize user metabolic costs. Some novel actuator designs have already demonstrated progress in achieving this objective and have high-efficiency and elastic-compliant actuators that reduce the overall weight of the prosthesis (Pieringer et al., ). In these knee designs, there are similar principles between elastic actuators and passive mechanisms. For instance, the weight acceptance (WA) actuator in the CYBERLEGS Beta-Prosthesis knee provides the same functions as the ESF-axis mechanism in the passive knee (Flynn et al., ). The WA system locks a high-stiffness spring via a nonbackdrivable screw during loading, allowing stance flexion, while it can be disengaged by a low-powered motor without interfering with swing locomotion of the knee. In addition, the electromagnetic clutch in the CESA knee can be engaged or disengaged for blocking or enabling the swing flexion of the knee and acts quite the same as the lock-axis mechanism (Rouse et al., ). Integrating energy-storage mechanisms into actuators can be a promising design solution, since they help to develop small but powerful prostheses that can offer more natural gait due to compliant behavior and decreased weight. Because the &#;negative&#; work at the knee is greater than the &#;positive&#; work, a whole energy regenerative solution is still a challenge (Laschowski et al., ). The mismatch between the input and output energies in current knee devices indicates the difficulty of achieving high efficiency in a simple mechanism. This confirms that as the magnitude of the positive energy demand increases, the supplementary mechanisms that control energy-storage elements become more important.

7.3 Knee design specification

Prosthetic knee specification is lacking, with only one international standard (ISO) available for structural fatigue testing (Lara-Barrios et al., ). Various structures and components with different functions have increased the complexity of knee prostheses. It also increases the difficulty for users, doctors, and prosthetists to find updated knowledge on the latest developed prosthetic knee technologies. Thus, it is difficult to understand the relationships between knee functions and mechanisms, resulting in barriers to appropriate adjustment and ideal states. In addition, according to the author&#;s experience, a knee prosthesis is a vulnerable product after 3&#;5 years of use. If one of the functional structures or components breaks down, the entire knee prosthesis is discarded. The level of maintenance and interchangeability of knee prostheses is far from that of in industrial parts and products. This greatly increases the economic burden on users, and it is essential to improve the service life of knee prostheses.

In this review, we proposed the concept of functional mechanisms and components, not only to determine the explicit relationship between knee functions and structures of prostheses, but also to promote the construction of specifications and standards for prosthetic knee design. We suggest that the design of functional mechanisms and components be tailored to the lost functions of users. The components acting on the same functional axis are supposed to be interchangeable and easily installed, even if these parts may be made by different manufacturers.

Furthermore, the concept of functional mechanisms and components is intended to facilitate the development of knee prostheses. Typically, an intelligent knee prosthesis requires the integration of multidisciplinary knowledge, including human neuroscience, biomechanics, mechanical design, electronic design, motion control, and signal processing. To remove the barrier and facilitate progress in knee prosthesis research, a commonly used platform is desired. Thanks to open-source models, such as the open-source leg developed by the University of Michigan, researchers can directly test their control algorithms (Azocar et al., ). From the perspective of widely used products, designing a prosthetic knee should start from the basic functions, and the knees should be designed with lightweight and compact functional mechanisms. We aim to construct a framework that provides a theoretical system for those who are less aware of the structures and biomechanics of prosthetic limbs, thus accelerating the development and clinical testing of prosthetic knees.

8 Conclusion

This review provides a new paradigm of prosthetic knee analysis, which clearly outlines the complex mechanisms of diverse knee prostheses and builds straightforward relationships between prosthetic knee structures and human walking biomechanics. First, the main function of prosthetic knees is to maintain stability during the stance phase. The monocentric mechanisms, polycentric mechanisms, and GRF-affected mechanisms in passive knees are introduced. These mechanisms can satisfy the requirement of stance stability and avoid buckling at an early stance or stumbling at a late stance. Second, ESF is desired for shock absorption and leg braking in active (K3&#;K4) users. There are ESF mechanisms in passive knees that allow a limited flexion angle at the heel-strike stage without losing stability. Third, knee prostheses need to regulate the maximum flexion angle and eliminate end impact during the swing phase, thus achieving an energy-saving natural gait. The frictional, pneumatic, and hydraulic components that control the motion during the swing phase are listed.

The passive mechanisms and components provide a new perspective based on the biomechanical functions, and the mechanical structures of passive knees can be used and controlled independently without interfering with each other. This new insight enables the interchangeability of prosthetic knee structures and components. By replacing an unsuitable part, the performance of the whole knee prosthesis can be improved. Furthermore, it is possible to consider the connections between passive mechanisms and walking biomechanics in the design of semiactive and active knee prostheses. The actuation, sensing, and control units can be simplified by mechanical parts that intrinsically match human knee biomechanics. The hardware of an intelligent prosthetic knee is supposed to be achieved by integrating the functional mechanical parts, low-powered actuation system, and precise sensor elements.

Acknowledgments

The authors are thankful to TehLin® Prosthetics, in Changchun City, Jilin Province, for providing information of on-the-shelf prosthetic knees.

Author contributions

WL and WC were involved in conceptualization; ZQ, HS, and YC were involved in methodology; WL, LR, KW, and GW were involved in writing&#;original draft preparation; LR, KW, and LR were involved in funding acquisition.

Funding

This research was supported by the National Key Research and Development Program of China (No. YFC) and the National Natural Science Foundation of China (No. , No. , No. , and No. ).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher&#;s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Nomenclature

M he

extension moment of the hip joint

F sb

shear force at the center of pressure (COP) of foot during heel contact

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L b

total length of the residual leg from hip joint to the prosthetic heel

M bk

braking moment exerted by prosthetic knee

F lb

load force carried along the hip-COP line

F rb

load force resulted from Flb and Fsb

X b

vertical distance from the knee joint to hip-COP line

Y b

vertical height of the knee instantaneous center rotation at heel-contact

M hf

flexion moment of hip joint

F ls

load force carried along the hip-COP line at leg stumble

F ss

shear force at the COP at toe-off

F rs

load force resulted from Fls and Fss

L s

total length of the residual leg from hip joint to the prosthetic toe

X s

vertical distance from hip-COP line to knee joint (inverse to Xb in direction)

Y s

vertical height of knee instantaneous center rotation at toe-off

What You Should Know Before Getting a Prosthetic Leg

Prosthetic legs, or prostheses, can help people with leg amputations get around more easily. They mimic the function and, sometimes, even the appearance of a real leg. Some people still need a cane, walker or crutches to walk with a prosthetic leg, while others can walk freely.

If you have a lower limb amputation, or you will soon, a prosthetic leg is probably an option you&#;re thinking about. There are a few considerations you should take into account first. 

Not Everyone Benefits from a Prosthetic Leg

While many people with limb loss do well with their prosthetic legs, not everyone is a good candidate for a leg prosthesis. A few questions you may want to discuss with your doctor before opting for a prosthetic leg include:

• Is there enough soft tissue to cushion the remaining bone?
• How much pain are you in?
• What is the condition of the skin on the limb?
• How much range of motion does the residual limb have?
• Is the other leg healthy?
• What was your activity level before the amputation?
• What are your mobility goals?

The type of amputation (above or below the knee) can also affect your decision. It&#;s generally easier to use a below-the-knee prosthetic leg than an above-the-knee prosthesis. If the knee joint is intact, the prosthetic leg takes much less effort to move and allows for more mobility.

The reason behind the amputation is also a factor, as it may impact the health of the residual limb. Your physical health and lifestyle are also important to consider. If you were not very active and lost your leg due to peripheral vascular disease or diabetes, for example, you will struggle more with a prosthesis than someone who was extremely active but lost a limb in a car accident.

When it comes to amputation, each person is unique. The decision to move forward with a prosthesis should be a collaborative one between you and your doctor.

Prosthetic Legs Are Not One Size Fits All

If your doctor prescribes a prosthetic leg, you might not know where to begin. It helps to understand how different parts of a prosthesis work together:

• The prosthetic leg itself is made of lightweight yet durable materials. Depending on the location of the amputation, the leg may or may not feature functional knee and ankle joints.
• The socket is a precise mold of your residual limb that fits snugly over the limb. It helps attach the prosthetic leg to your body.
• The suspension system is how the prosthesis stays attached, whether through sleeve suction, vacuum suspension/suction or distal locking through pin or lanyard.

There are numerous options for each of the above components, each with their own pros and cons. &#;To get the right type and fit, it&#;s important to work closely with your prosthetist &#; a relationship you might have for life.

A prosthetist is a health care professional who specializes in prosthetic limbs and can help you select the right components. You&#;ll have frequent appointments, especially in the beginning, so it&#;s important to feel comfortable with the prosthetist you choose.

Rehabilitation Is an Ongoing, Collaborative Process

Once you&#;ve selected your prosthetic leg components, you will need rehabilitation to strengthen your legs, arms and cardiovascular system, as you learn to walk with your new limb. You&#;ll work closely with rehabilitation physicians, physical therapists and occupational therapists to develop a rehabilitation plan based on your mobility goals. A big part of this plan is to keep your healthy leg in good shape: while prosthetic technology is always advancing, nothing can replicate a healthy leg. 

Getting Used to a Prosthetic Leg Isn&#;t Easy

Learning to get around with a prosthetic leg can be a challenge. Even after initial rehabilitation is over, you might experience some issues that your prosthetist and rehabilitation team can help you manage. Common obstacles include:

• Excessive sweating (hyperhidrosis), which can affect the fit of the prosthesis and lead to skin issues.
• Changing residual limb shape. This usually occurs in the first year after an amputation as the tissue settles into its more permanent shape, and may affect the fit of the socket.
• Weakness in the residual limb, which may make it difficult to use the prosthesis for long periods of time.
• Phantom limb pain could be intense enough to impact your ability to use the prosthesis.

A Note on Phantom Limb Pain

Phantom limb pain, or pain that seems to come from the amputated limb, is a very real problem that you may face after an amputation. About 80% of people with amputations experience phantom limb pain that has no clear cause, although pain in the limb before amputation may be a risk factor.

Mirror therapy, where you perform exercises with a mirror, may help with certain types of phantom limb pain. Looking at yourself in the mirror simulates the presence of the amputated leg, which can trick the brain into thinking it&#;s still there and stop the pain.

In other cases, phantom limb pain might stem from another condition affecting the residual limb, such as sciatica or neuroma. Addressing these root causes can help eliminate the phantom pain.

Your Leg Prosthesis Needs May Change

At some point, you may notice that you aren&#;t as functional as you&#;d like to be with your current leg prosthesis. Maybe your residual limb has stabilized and you&#;re ready to transition from a temporary prosthesis that lasts a few months to one that can last three to five years. Or maybe you&#;ve &#;outwalked&#; your prosthesis by moving more or differently than the prosthesis is designed for. New pain, discomfort and lack of stability are some of the signs that it may be time to check in with your prosthetist to reevaluate your needs.

Your prosthetist might recommend adjusting your current equipment or replacing one of the components. Or you might get a prescription for a new prosthetic leg, which happens on average every three to five years. If you receive new components, it&#;s important to take the time to understand how they work. Physical therapy can help adjust to the new components or your new prosthetic leg.

Prosthetic Leg Technology Is Always Evolving

There are always new developments in prosthetic limb technology, such as microprocessor-driven and activity-specific components.

• Microprocessor joints feature computer chips and sensors to provide a more natural gait. They may even have different modes for walking on flat surfaces or up and down the stairs.
• There are also specialized prosthetic legs for different activities, such as running, showering or swimming, which you can switch to as needed. In some cases, your everyday prosthetic leg can be modified by your prosthetist to serve different purposes.
• Osseointegration surgery is another option. This procedure involves the insertion of a metal implant directly into the bone, so there is no need for a socket. The prosthetic leg then attaches directly to that implant. While this procedure is not right for everyone and is still under study, it can provide improved range of motion and sensory perception.

It&#;s important to remember that you&#;re not alone in navigating the many different prosthetic leg options. Your care team will help you weigh the pros and cons of each and decide on the ideal prosthetic leg that matches your lifestyle.

Johns Hopkins Comprehensive Amputee Rehabilitation Program

Having the support of a dedicated team of experts is essential when recovering from the amputation of a limb. At Johns Hopkins, our team of physiatrists, orthotists, prosthetists, physical and occupational therapists, rehabilitation psychologists and other specialists works together to create your custom rehabilitation plan.

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Learn more about our amputee rehabilitation program