Mechanisms and component design of prosthetic knees

Abstract

Prosthetic knees represent cutting-edge medical innovations that incorporate mechanical mechanisms and components to emulate the natural functionality of biological knees for individuals who have undergone transfemoral amputation. Although a broad spectrum of intricate mechanical systems and components is employed, their design often overlooks the walking biomechanics of the users. This article intends to address this issue by offering a comprehensive review of prosthetic knees through a biomechanical lens. We explore aspects such as stance stability, early-stance flexion (ESF), and swing resistance, elucidating how these mechanical mechanisms correlate with perceived walking performance, including fall avoidance, shock absorption, and gait symmetry. The criteria for prescribing and selecting prosthetic knees hinge on the interaction between the user and the prosthesis, encompassing five functional classifications labeled from K0 to K4. Misinterpreted functions and incorrect adjustments of knee prostheses can result in diminished stability, limited stance flexion, and an unnatural gait for users. Our review identifies both commercially available and recently studied prosthetic knees, contributing to a novel paradigm for analyzing prosthetic knees while facilitating the standardization and optimization of knee design. This approach may lead to the creation of functional mechanisms and components customized to restore the specific functions lost by the individual, ultimately advancing personalized 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 performed until June 1 across eight English-language databases employing the PRISMA method. The databases utilized include Web of Science, Springer, Wiley, Science Direct, IEEExplore, ASME, PubMed, and Google Scholar. Additionally, patents related to passive prosthetic knees were also explored via Google Patents. Eight English keywords were employed during database retrieval, covering terms such as "above-knee prosthesis," "transfemoral knee prosthesis," "prosthetic knee mechanism," "passive prosthetic knee," "brake prosthetic knee," "polycentric prosthetic knee," "mechanical knee," and "transfemoral amputation." The start and end dates for these database queries were established from January 1 to the most recent date available in the databases.

Moreover, a manual search was conducted on three categories of publications derived from the screened database results. The first category consisted of review articles; the second encompassed research articles focused on the functional structure of mechanical knees; while the third category included clinical studies pertaining to transfemoral amputations. Ultimately, 140 results from the manual searches were reviewed, comprising 113 journal articles and 27 patents.

2.2 Exclusion Criteria

Records that met any of the following four exclusion criteria were discarded: 1) titles or keywords that were irrelevant; 2) abstracts lacking relevance or no illustrations concerning passive prosthetic knees; 3) absence of walking biomechanics related to prosthetic knees; and 4) lack of thematic descriptions of functional structures or elements within passive transfemoral prostheses. The processes for database searches and manual review are depicted in a flowchart.

2.3 Classification Criteria

Passive knee prostheses were categorized based on biomechanical challenges associated with transfemoral amputation, including falls, osteoarthritis, and gait asymmetry.

Falls are primarily related to stance stability, which is fundamental to safety across all functional levels from K0 to K4. Stance stability is facilitated by functional structures like four-bar linkages and hydraulic units.

Osteoarthritis pertains to stance flexion, which is particularly desired by active users classified at K3 and K4 levels. Stance flexion can mitigate ground impact and enhance comfort for the residual limb. This function depends on the prosthetic knee’s structural integrity, permitting limited flexion during the early stance phase without jeopardizing stability.

Gait asymmetry is connected to swing resistance, a critical function that manages both the maximum flexion angle and the timing for full extension. Swing resistance is influenced by functional components impacting the knee’s axis.

The mechanisms and components within knee prostheses are closely tied to fundamental walking functionalities. The identification of biomechanical challenges and associated needs for knee joints are introduced first, followed by a depiction of the current functional structures and components of passive prosthetic knees as strategies aimed at overcoming these health concerns. We seek to impart a clearer understanding of knee prosthetic functions through this structured framework.

7 Discussion

This review highlights representative passive knee mechanisms based on biomechanical needs. We have developed a comprehensive table that encompasses both commercially available and recently studied passive knees, allowing users and developers to understand and assess the functional mechanisms and components concerning stability, early-stance flexion, and swing resistance. We propose three overarching trends in the ongoing and future evolution of prosthetic knees.

7.1 Adaptivity

Research into passive knees predominantly centers on the biomechanics of level walking. However, these knees often do not fulfill the diverse requirements of users' daily activities. Enhancements in the adaptivity of knee prostheses are essential on two fronts.

7.1.1 Adaptivity to the Environment

Prosthetic knees need to cope with various environmental challenges, including uneven terrains, ramps, and stairs. Microcontrollers are increasingly integrated into microprocessor-controlled knees (MPKs), allowing for adaptive damping to accommodate diverse environmental situations. However, most current MPK solutions are monocentric, relying on a single-knee axis structure. The hydraulic units associated with MPKs must provide adequate damping to balance stance flexion and stability. As a result, users often exhibit an asymmetric gait with a reduced stance-flexion angle when compared to natural knees (Thiele et al.). Enhancements to adaptivity may be achieved by merging microprocessor-controlled systems with passive mechanisms; for example, adjusting the structures of the knee axis and ESF mechanisms can optimize stability and ESF, while swing resistance can be fine-tuned through CPU-controlled units. Additionally, a passive locking mechanism added to a knee device can enhance adaptability during stair ascent, leveraging the knowledge that ground reaction forces increase with stance flexion (Inoue et al.). Further innovations could incorporate other mechanisms or intelligent units to augment current passive knee designs.

7.1.2 Adaptivity to Users

Current passive knees cannot discern individual intentions and can only utilize pneumatic and hydraulic units to adjust damping forces within limited parameters according to varying walking speeds. Recognizing or estimating a user's intention is increasingly vital in advanced prosthetic technologies, enabling adaptation to different speeds, terrains, and obstacles. Biomechanical sensors that monitor angles, loads, and inertial movements are common in MPKs and active prosthetic knees (APKs), collecting kinematic and force signals for predefined locomotion states. These signals exhibit stability and repeatability, making a finite state machine (FSM) control strategy effective for regulating the knee's positions. However, the FSM approach does face challenges (Martin et al.) as it primarily relies on prior movements, potentially misaligning the required joint positions and damping values with immediate active needs. Furthermore, the sensors reflect the prosthesis’s movement, not the user's intentions. Non-invasive electromyography (EMG) methods serve as an alternative for controlling intentions, but difficulties such as weak signal amplitude and noise during collection hinder robustness (Peeraer et al.; Au et al.). A more adaptive approach may arise from combining EMG signals with embedded sensors, thereby enhancing adaptability and stability. In our view, functional mechanisms and components are fundamentally linked to walking biomechanics, with user locomotion variations easily mapped to functional axes instantaneously. For example, using a mechanical sensor integrated within a lock-axis structure can identify transitions between stance and swing phases, allowing for real-time positional adjustments of the virtual lock mechanism. Hence, employing mechanical intelligence for the interaction between adaptive prosthetics and users remains a promising avenue moving forward.

7.2 Controlled Energy Flow

Activities such as running, jumping, and climbing stairs demand substantial energy input, establishing a necessity for active prostheses (Jacobs et al.; Riener et al.). Some recent prototypes have notably advanced kinematics for normal gait, approaching biological levels (Lawson et al.; Zhao et al.). Despite this, active prosthetic devices often weigh more than passive ones, incurring substantial metabolic costs for users (Pfeifer et al.).

Conversely, passive knees boast lightweight and energy-efficient characteristics, with their mechanisms and components closely aligned with the biomechanics of walking. A significant challenge moving forward is finding ways to integrate lightweight yet effective functional mechanisms into actuators to reduce user metabolic costs. Recent actuator innovations demonstrate progress in this area, offering high-efficiency elastic-compliant actuators that diminish the overall weight of prosthetic limbs (Pieringer et al.). In certain knee designs, principles shared between elastic actuators and passive mechanisms are evident; for instance, the weight-acceptance actuator in the CYBERLEGS Beta-Prosthesis mimics the ESF-axis mechanism found in passive knees (Flynn et al.). The weight acceptance system locks a high-stiffness spring via a non-backdrivable screw during loading, facilitating stance flexion, while disengagement occurs via a low-powered motor that does not impede swinging motion. Additionally, an electromagnetic clutch in the CESA knee is capable of engaging or disengaging swing flexion as required, mimicking the lock-axis mechanism (Rouse et al.). Striving for the integration of energy-storage mechanisms within actuators represents a beneficial design solution, as they contribute to developing smaller yet effective prostheses that emulate a more natural gait due to compliant behavior and minimized weight. Nevertheless, achieving a comprehensive energy regenerative solution remains a challenge (Laschowski et al.) due to the observed input-output energy mismatches in contemporary knee devices, highlighting the necessity for supplementary mechanisms that control energy storage elements as positive energy demands increase.

7.3 Knee Design Specification

The specifications for prosthetic knees remain inadequate, with only one international standard (ISO) made available concerning structural fatigue testing (Lara-Barrios et al.). The variety of structures and components that offer diverse functionalities has complicated knee prosthetics. This complexity has increased the learning curve for users, healthcare professionals, and prosthetists relative to the latest advancements in prosthetic knee technologies. Consequently, understanding the interplay between knee functions and mechanisms becomes challenging, leading to obstacles in achieving optimal adjustments. From personal observations, it's noted that a knee prosthesis may serve as a vulnerable product within a lifespan of 3 to 5 years. Breakdowns in any functional component typically necessitate discarding the entire prosthesis. The maintenance level and component interchangeability of knee prostheses are considerably inferior to standard industrial products, resulting in increased financial burdens for users. Improving the longevity of knee prostheses is thus pivotal.

This review offers the concept of functional mechanisms and components, which not only clarifies relationships between knee functions and prosthetic structures but also promotes the formulation of specifications and standards for knee prosthetic design. It is advocated that functional mechanisms and components should be specifically designed to address the unique functional deficits experienced by users. Components operating on shared functional axes ought to be interchangeable and installable with relative ease, even when sourced from different manufacturers.

Moreover, the concept of functional mechanisms and components is geared towards invigorating the development of knee prosthetics. Generally, intelligent knee prostheses require an amalgamation of multidisciplinary expertise encompassing human neuroscience, biomechanics, mechanical design, electronic design, motion control, and signal processing. Establishing a unified platform to facilitate progress in knee prosthetic research is crucial. Open-source models like the open-source leg from the University of Michigan allow researchers to test their control algorithms directly (Azocar et al.). From a design perspective, the creation of a prosthetic knee should prioritize fundamental functionalities, ensuring the knees embody lightweight and compact mechanisms. Our aim is to create a framework that delivers a systematic understanding for individuals less acquainted with prosthetic limb structures and biomechanics, thereby hastening the development and clinical implementation of prosthetic knees.

8 Conclusion

This review introduces a novel framework for analyzing prosthetic knees, clearly elucidating the intricate mechanisms of various knee prosthetics while establishing simple connections between their structures and human walking biomechanics. The foremost purpose of knee prosthetics is to sustain stability during the stance phase. Detailed explorations of monocentric and polycentric mechanisms, along with ground reaction force-influenced elements, reveal that passive knees can effectively enhance stance stability, preventing early-stage buckling or late-phase stumbles. Furthermore, ESF is essential for shock absorption and deceleration in active users (K3-K4). Passive knees exhibit mechanisms that permit a degree of flexion during the heel-strike phase without compromising stability. Lastly, prosthetic knees are expected to manage maximum flexion angles and mitigate end-impact during the swing phase, ensuring an energy-efficient and natural gait. Various frictional, pneumatic, and hydraulic components governing motion throughout the swing phase are outlined.

The insights surrounding passive mechanisms and components provide a new viewpoint centered on biomechanical functions, allowing for the independent use and regulation of mechanical structures without interference. This fresh perspective facilitates the interchangeability of prosthetic knee components, whereby replacing a misfitting part can enhance overall prosthetic performance. Furthermore, considerations about the connections between passive mechanisms and walking biomechanics may influence the designs of semiactive and active knee prostheses. By simplifying the actuation, sensing, and control units, one can achieve hardware appropriately suited to biomechanical parameters indicative of human knees. Ultimately, the integration of functional mechanical parts, low-powered actuators, and precise sensors is crucial for building a state-of-the-art intelligent prosthetic knee.

Acknowledgments

The authors gratefully acknowledge TehLin® Prosthetics, located in Changchun City, Jilin Province, for supplying product information on available prosthetic knees.

Author Contributions

WL and WC conceived the study; ZQ, HS, and YC carried out the methodology; WL, LR, KW, and GW were responsible for drafting the original manuscript; LR, KW, and LR contributed to funding acquisition.

Funding

This research received support from the National Key Research and Development Program of China (Grant No. YFC) and the National Natural Science Foundation of China (Grants No. \*, No. \*, No. \*, and No. \*).

Conflict of Interest

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

Publisher's Note

All claims made in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, editors, and reviewers. Mention of any product in this article does not guarantee endorsement by the publisher.

Nomenclature

M

extension moment of the hip joint

F sb

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

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