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.
Keywords:
prosthetic knee, transfemoral prosthesis, knee mechanisms, passive knee, above-knee prosthesis
A literature search was conducted until 1 June 2022 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, 1950 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.
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 .
Open in a separate windowPassive 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 ( ). 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.
Open in a separate windowThere is a prosthetic knee with only a stability mechanism that allows for slight ESF; it utilizes the high resistance of a hydraulic system in the form of a rotary brake (Blumentritt et al., 1998; Lang, 2011) and SNS cylinder (Mauch, 1968). These mechanisms are directly connected to the knee axis; the leak rate of the hydraulic system must be finely tuned. However, this kind of knee still has some problems, such as flexing too slowly or being unable to resist body weight. A requirement that a structure independent of the knee axis should be designed in the prosthetic knee is presented. Therefore, ESF mechanisms are proposed for guaranteeing that a prosthetic knee is able to flex in a limited range without losing knee stability.
An ESF mechanism independent of the knee axis is proposed for achieving a more stable and natural gait. A knee can flex around the rotation axis of the ESF mechanism (ESF axis), while the knee axis is locked to ensure stability. Furthermore, it does not interfere with the motion of the swing phase.
The ESF axis can work with the knee axis, such as in a weight-activated knee ( ) (Blatchford and Tucker, 1980). The ESF axis is located anterior to the knee axis and benefits shock absorption and ESF. If GRF moves posterior to the knee axis, the body weight will activate the brake mechanism and block the rotation about the knee axis. Furthermore, rotation about the ESF axis is allowed, where stance flexion is restricted by hard rubber.
Open in a separate windowThe ESF axis can also cooperate with the lock axis, such as in the knee in (Arelekatti and Winter, 2015), where the ESF axis is located posterior to the lock axis. When GRF is posterior to the ESF axis, latching controlled by the lock-axis blocks the knee axis, while the residual thigh flexes relative to the shank about the ESF axis. The flexion of the thigh will be recovered by a spring if GRF translates anteriorly relative to the ESF axis (Arelekatti et al., 2019). This does not affect the releasing process of lock axis at preswing.
There are also ESF-axis mechanisms in polycentric knees. For example, a redundant five-bar linkage can form two different configurations of polycentric knees in stance and swing. As shown in , link 1 and link 2 are combined to form the thigh during stance, while link 4 and link 5 are combined to form shank during swing (Blumentritt et al., 1997; Grohs et al., 2019). ESF motion with high impedance does not interfere with the swing flexion with low impedance. Similarly, the same functionality is created by a seven-bar linkage mechanism, where the ESF motion is resisted by a bumper ( ) (Gramnas, 1998).
At heel-contact, the major function of the knee is impact absorption. During this process, GRF is posterior to the knee axis and causes a large flexion moment. Withstanding a great flexion moment, the thigh extensor muscles must perform negative work, making the knee joint flex at a limited angle within 20° (Murthy Arelekatti and Winter, 2018). The ESF motion allows a person to lower the center of mass of their body during stance, thus absorbing the striking impact force and ensuring the smooth transition from swing to stance. Flexion at early stance is not recommended for the low-function-level (K0∼K2) users, and rotation about the knee axis is accompanied by a high risk of buckling and falling. For knee prostheses without ESF mechanisms, users maintain an extended position during the whole stance phase and utilize the inertia of the trunk to move forward on the support limb. Therefore, it is recommended to use ESF mechanisms in knees of active users (K3–K4 level), which permits ESF in a limited range and simultaneously ensures the stability of the knee axis. The ESF axis controls flexion through resistant elements, including springs, elastic rubber, hard bumpers, and hydraulic absorbers (Arelekatti and Winter, 2015). These passive elements cannot adapt to body weight, speed, or terrain. Unlike a healthy knee, ESF mechanisms can only provide impedance for ESF but no power output for stance extension (Pfeifer et al., 2012). It still cannot replicate the extension torque profile of an anatomic knee due to the absence of adaptivity and energy injection.
In this review, representative passive knee mechanisms are reviewed according to biomechanical requirements. Furthermore, we designed , 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.
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.
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., 2019). 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., 2013). 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.
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., 2010). 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., 1990; Au et al., 2008). 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.
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., 1996; Riener et al., 2002). Some of the latest prototypes have already improved kinematics for normal gait, which have even approached biological levels (Lawson et al., 2014; Zhao et al., 2017). 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., 2015).
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., 2017). 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., 2018). 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., 2013). 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., 2019). 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.
Prosthetic knee specification is lacking, with only one international standard (ISO10328) available for structural fatigue testing (Lara-Barrios et al., 2018). 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., 2020). 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.
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.
The authors are thankful to TehLin® Prosthetics, in Changchun City, Jilin Province, for providing information of on-the-shelf prosthetic knees.
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.
This research was supported by the National Key Research and Development Program of China (No. 2018YFC2001300) and the National Natural Science Foundation of China (No. 91948302, No. 91848204, No. 52005209, and No. 52021003).
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.
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.
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