Variable stiffness prosthetic ankle

Abstract

An ankle joint comprises a frame, an ankle member, a lead screw, a lead nut housing, a multi-strut assembly, and a first spring. The ankle member is pivotally coupled to the sidewall of the frame about a pivot axis. The lead screw is mounted within the ankle member. The lead screw is rotatable about its longitudinal axis and pivotable about the pivot axis relative to the frame. The lead nut housing is mounted on the lead screw and movable along its length upon rotation of the lead screw. The multi-strut assembly is coupled between the lead nut housing and the arm of the ankle member. The first spring is coupled to the base of the frame and the multi-strut assembly. The multi-strut assembly defines a moment arm which causes the first spring to deflect when the ankle member rotates in response to an external force.

Description

Attorney Docket No.: NEX-19225VARIABLE STIFFNESS PROSTHETIC ANKLESTATEMENT OF GOVERNMENT SUPPORT

[0001] This invention was made with government support under Grant Number 1R21EB035665-01 awarded by the National Institutes of Health. The Government has certain rights in the invention.CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 687,879, filed August 28, 2024; the entire contents of which are hereby incorporated by reference.BACKGROUNDField of the Disclosed Subject Matter

[0003] The disclosed subject matter relates to ankle joint devices and, more particularly, to an ankle joint device with variable stiffness.Description of Related Art

[0003] Prosthetic ankle-foot devices play a crucial role in restoring mobility for individuals with lower limb amputations. However, current prescription practices face challenges due to the proprietary nature of commercial prosthesis technology, potentially leading to ill-suited devices that may discourage use or cause biomechanical harm. Existing variable stiffness ankles have attempted to address this issue, but many face limitations that hinder their adoption. Some designs suffer from backlash between components, making precise stiffness characterization difficult. Others lack the ability to maintain a consistent neutral ankle position across stiffness settings, further complicating the prescription process. Additionally, current variable stiffness prostheses can only vary dorsiflexion / forefoot stiffness or overall stiffness, reducing customizability in clinical settings. These challenges highlight the need for an improved variable stiffness ankle joint that enables precise stiffness characterization, maintains a consistent neutral position, and allows for efficient adjustment of both overall stiffness and the ratio of dorsiflexion to plantarflexion stiffness.- 1 -FH13002716.1Attorney Docket No.: NEX-19225SUMMARY

[0004] The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings.

[0005] To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter includes an ankle joint comprising a frame, an ankle member, a lead screw, a lead nut housing, a multi-strut assembly, and a first spring. The frame comprises a base and a sidewall extending from the base. The ankle member is pivotally coupled to the sidewall of the frame about a pivot axis. The ankle member includes a body and an arm extending from the body. The lead screw has a first end and a second end defining a longitudinal axis therebetween. The lead screw is mounted within the ankle member. The lead screw is rotatable about its longitudinal axis and pivotable about the pivot axis relative to the frame. The lead nut housing is mounted on the lead screw and movable along its length upon rotation of the lead screw. The multi-strut assembly is coupled between the lead nut housing and the arm. The first spring is coupled to the base of the frame and the multi-strut assembly. The multi-strut assembly defines a moment arm which causes the first spring to deflect when the ankle member rotates in response to an external force.

[0006] The multi-strut assembly may comprise: a first strut having a first end pivotally coupled to a first axle mounted to the lead nut housing and a second end pivotally coupled to a distal end of the arm via a second axle; a second strut having a first end pivotally coupled to the first axle; and a third strut having a first end pivotally coupled to the second strut and a second end coupled to the first spring. The first spring may have a first end fixed relative to the frame and a second end coupled to the third strut. The ankle joint may further comprise a bracket coupled to the sidewall. The first end of the first spring may be retained between the bracket and the base. The third strut may comprise a spring clamp. The spring clamp may be engaged with the first spring. The first strut may have a first length and the second strut may have a second length such that the first length is approximately the second length. The lead nut housing may enclose (e.g., enclose at least partially) a lead nut which is threadably engaged with the lead screw. The lead nut housing may comprise an axle which pivotally supports one or more struts of the multi-strut assembly. The ankle joint may further comprise- 2 -FH13002716.1Attorney Docket No.: NEX-19225 a pyramid adapter for attachment to a prosthetic pylon. The pyramid adapter may be coupled to the ankle member. The lead screw may be perpendicular to the axle. The ankle joint may further comprise an actuator housed within the body of the ankle member. The actuator may be operably coupled to the lead screw. The first spring may comprise a beam spring. The ankle joint may further comprise a second spring mounted in parallel with the first spring. The second spring may be deflected by the first spring when the ankle member is in a dorsiflexion configuration. The second spring may comprise a beam spring. The ankle joint may further comprise a fulcrum disposed between the first spring and the base. The fulcrum may be repositionable along a length of the base. The ankle joint may further comprise a fulcrum lead screw. The fulcrum may be threadably engaged with the fulcrum lead screw. The second end of the first spring may deflect in opposing directions relative to the base in response to movement of the ankle member between a dorsiflexion configuration and a plantar flexion configuration. The base and the sidewall may be integrally formed. The pivot axis may be defined by an axle comprising one or more shoulder screws supported by one or more ball bearings.

[0007] It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the disclosed subject matter claimed.

[0004] The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] A detailed description of various aspects, features, and implementations of the subject matter described herein is provided with reference to the accompanying drawings, which are briefly described below. The drawings are illustrative and are not necessarily drawn to scale, with some components and features being exaggerated for clarity. The drawings illustrate various aspects and features of the present subject matter and may illustrate one or more implementation(s) or example(s) of the present subject matter in whole or in part.- 3 -FH13002716.1Attorney Docket No.: NEX-19225

[0008] FIGS. 1A and IB are perspective views of an implementation of an ankle joint, in accordance with the present disclosure.

[0009] FIGS. 2 - 3 are schematic side views of the implementation of the ankle joint shown in FIGS. 1A and IB, in accordance with the present disclosure.

[0010] FIGS. 4A - 4C are schematic sides of the implementation of the ankle joint shown in FIGS. 1A and IB showing rotation of the ankle member, in accordance with the present disclosure.

[0011] FIG. 4D is a schematic side view of another implementation of an ankle joint, in accordance with the present disclosure.

[0012] FIGS. 5A and 5B are perspective views of an implementation of an ankle joint, in accordance with the present disclosure.

[0013] FIG. 6 is a schematic side view of an implementation of an ankle joint, in accordance with the present disclosure.

[0014] FIG. 7 is a rear perspective view of an implementation of an ankle joint, with a sidewall of the frame omitted for simplicity, in accordance with the present disclosure.

[0015] FIG. 8 is a schematic side view and inset of an implementation of an ankle joint depicting a fulcrum for adjusting pl antarfl exion stiffness, in accordance with the present disclosure.

[0016] FIG. 9 is a schematic side view of the ankle joint with annotations depicting the moment arm acting on the spring, in accordance with the present disclosure.

[0017] FIG. 10 is a graph of a simulated ankle torque versus deflection angle for various moment arm lengths for an implementation of an ankle joint, in accordance with the present disclosure.

[0018] FIG. 11 is a graph of a simulated stiffness versus deflection angle for various moment arm lengths for an implementation of an ankle joint, in accordance with the present disclosure.

[0019] FIGS. 12A and 12B are perspective views of an implementation of an ankle joint, in accordance with the present disclosure.

[0020] FIGS. 13 and 14 are schematic side views of an implementation of an ankle joint, in accordance with the present disclosure.

[0021] FIG. 15 is an isometric view of an implementation of an ankle joint, with a part of the sidewall of the frame omitted for simplicity in accordance with the present disclosure.- 4 -FH13002716.1Attorney Docket No.: NEX-19225DETAILED DESCRIPTION

[0022] The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

[0023] References herein to positions of elements (e.g., “top”, “bottom”) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary implementations, and that such variations are intended to be encompassed by the present disclosure.

[0024] The term "about" means a range of values inclusive of the specified value that a person of ordinary skill in the art would reasonably consider to be comparable to the specified value. In some implementations, "about" means within a standard deviation using measurements generally accepted by a person of ordinary skill in the art. In some implementations, "about" means ranging up to ±10% of the specified value. In some implementations, "about" means ranging up to ±5% of the specified value. In some implementations, "about" means the specified value.

[0025] Various implementations of an ankle joint are described herein. The ankle joint can be configured to operate as a quasi-passive system, in which a motor or manual mechanism alters the passive dynamics of the foot without adding energy to the gait cycle. The motor or manual mechanism can be used to reconfigure the internal geometry or mechanical boundary conditions of the system (e.g., position of lead nut along the lead screw or position of the fulcrum) to set a stiffness level. The stiffness remains constant within a given stance phase, and may only transition to a new state between stance phases. In various aspects, this quasi- passive architecture can enable active stiffness adjustment during the swing phase while functioning as a passive energy storage and return (ESR) foot during the stance phase.

[0026] In some implementations, the ankle joint is implemented as a prosthetic ankle. In some implementations, a legged robotic system, such as a bipedal humanoid robot or quadruped walking robot, includes the ankle joint. The ankle joint may be incorporated in a wearable exoskeleton or orthosis. The described ankle joint can be used in any walking or legged system.- 5 -FH13002716.1Attorney Docket No.: NEX-19225

[0027] The gait cycle refers to the sequence of motions occurring during walking (including walking on level ground, ascending stairs, descending stairs), beginning when one foot contacts the ground and ending when the same foot contacts the ground again. Each gait cycle includes two main phases: a stance phase, when the foot is in contact with the ground and bearing weight, and a swing phase, when the foot is in the air preparing for the next step. During the stance phase, the ankle undergoes a progression of motions. At initial contact, the ankle can experience plantarflexion (PF), a downward rotation of the foot, as the foot moves toward a flat position on the ground. During midstance, the ankle undergoes dorsiflexion (DF), an upward rotation of the foot, as the shank (lower leg) rotates forward over the foot. At push-off, the ankle undergoes PF to provide propulsion. During the swing phase, the ankle can undergo DF to provide toe clearance in preparation for the next step.

[0028] In different walking tasks, the ankle can benefit from different stiffness characteristics in these two directions. For example, walking on level ground may require a different stiffness in both PF and DF than for stair ascent or stair descent. Stair ascent may benefit from a higher DF stiffness, while stair descent may benefit from a lower PF stiffness. The ankle joint described herein enables stiffness tuning, allowing both DF and PF to be adjusted independently or together. In some implementations, a fulcrum disposed beneath a first spring of the ankle joint enables adjustment of PF stiffness (effectively changing the ratio of DF:PF stiffness), while a second spring can be used to bias DF stiffness to be greater than PF stiffness. In some implementations, a fulcrum disposed above a second spring (positioned in parallel with a first spring) provides adjustment of DF stiffness (effectively changing the ratio of DF: PF stiffness) instead.

[0029] Referring now to Figs. 1A and IB, a rear perspective view and a front perspective view, respectively, of an implementation of an ankle joint 100 are shown. The ankle joint 100 can include a frame 102, an ankle member 104, a lead screw 106, a lead nut housing 108 that at least partially encloses a lead nut threadably engaged with the lead screw 106, and a multi-strut assembly 118. The frame 102 can include a base 109 and a sidewall 110 extending upward from the base. The base 109 can function as a footplate and provide the structural foundation of the prosthesis. The base 109 can support a spring system and an adjustable fulcrum system (e.g., fulcrum and fulcrum lead screw), while the sidewall 110 can support the ankle member 102 and a bracket for spring retention.

[0030] In some implementations, the base 109 is elongated and contoured to conform to or interface with a prosthetic foot shell. In some implementations, the sidewalls 110- 6 -FH13002716.1Attorney Docket No.: NEX-19225 includes a pair of discrete vertical sidewalls, one on each side of the base 109, each with an aperture or mount for the pivot axle defining the ankle’s pivot axis 112. In some implementations, the sidewall 110 is formed as a partial or continuous enclosure, extending around a portion of the perimeter of the base 109. This enclosure may be U- shaped or C-shaped when viewed from above, creating a semi-enclosed housing around the ankle member.

[0031] In some implementations, the base 109 and the sidewall 110 are integrally formed as a single monolithic body. In some implementations, the base 109 and the sidewall 110 are separate components that are fixedly coupled together via mechanical fasteners. Materials for the base 109 and the sidewall 110 may include aluminum, stainless steel, or carbon-fiber reinforce nylon. The geometry of the base 109 and sidewall 110 may include ribs, cutouts, internal channels, or mounting recesses to reduce weight, manage loads, and facilitate integration of other components (e.g., fulcrum and lead screw).

[0032] The ankle member 104 can be pivotally coupled to the sidewall 110 about a pivot axis 112, allowing the ankle member to rotate relative to the frame 102. In some implementations, the pivot axis 112 is defined by an axle that includes one or more shoulder screws supported by one or more ball bearings.

[0033] The ankle member 104 can include a body 114 and at least one arm 116 extending therefrom. The body 114 can be configured to house (e.g., partially enclose) and support the lead screw 106, such that the lead screw extends through the body 114 along a longitudinal axis that is perpendicular to the pivot axis 112. In some implementations, the body 114 is a solid component with a bore or channel extending through its thickness, through which the lead screw 106 is received and supported. In some implementations, the body 114 includes a cavity or recess configured to receive an actuator (e.g., actuator). In some implementations, an actuator is disposed within the body 114 and is operably connected to the lead screw 106 to drive its rotation. For example, the actuator may be coupled to the lead screw 106 via a pulley and belt system, a gear train, or a direct-drive coupling.

[0034] In some implementations, the ankle joint includes a support structure (e.g., housing) configured to support both the lead screw 106 and a drive shaft of the actuator, maintaining their axes in a parallel orientation to ensure reliable transmission of motion. The support structure may include bushings, bearings, or alignment brackets to constrain motion and prevent misalignment during operation.- 7 -FH13002716.1Attorney Docket No.: NEX-19225

[0035] The lead screw 106 is mounted within the ankle member 104 such that it is both rotatable about its own longitudinal axis and pivotable about the pivot axis 112 as part of the ankle member’s motion. This enables the lead screw 106 to drive translation of the lead nut while moving in concert with the ankle member during ankle rotation. In operation, when the lead screw 106 rotates, whether manually or via an actuator, it drives the lead nut housed within the lead nut housing 108, causing the housing to translate along the screw’s length. The translation of the lead nut housing modifies the configuration of the multi-strut assembly 118.

[0036] The lead nut housing 108 can include a first axle 120, which pivotally supports one or more struts of the multi-strut assembly 118. In some implementations, the first axle 120 includes one or more axles. The multi-strut assembly 118 can include a first strut 118a, a second strut 118b, and a third strut 118c. In some implementations, the first strut 118a includes one or more first struts. The first strut 118a can have a first end pivotally coupled to the first axle 120 of the lead nut housing 108 and a second end pivotally coupled to a distal portion (free end) of the arm 116 of the ankle member 104 via a second axle 121. The second strut 118b can have a first end pivotally coupled to the first axle 120 and a second end pivotally coupled to a first end of the third strut 118c (via another axle). The first and second struts 118a, 118b can have about equal lengths. The third strut 118c can a second end coupled to the first spring 122a. The third strut 118c can have a length smaller than that of the first and second struts 118a, 118b. In some implementations, the third strut 118c includes a spring clamp that grips and transmits forces to the first spring 122a.

[0037] In operation, when the ankle member rotates, such as during dorsiflexion or pl antarfl exion, the arm 116 causes the multi-strut assembly 118 to articulate. This movement propagates through the struts to the third strut 118c, which in turn applies a bending force to the first spring 122a. The resistance experienced at the ankle is thus defined by the effective moment arm created by the geometrical relationship between the pivot axes, the lengths of the struts, and the position of lead nut housing 108.

[0038] Referring now to Fig. 2 and Fig. 3, schematic side views of an implementation of ankle joint 100 are shown with the near-side sidewall 110 omitted for simplicity. The ankle joint is shown at neutral position (Fig. 2) and at 20 degrees of PF (Fig. 3). The multi-strut assembly 118 can function as a modified four-bar linkage that allows stiffness adjustments by changing the effective moment arm without affecting the neutral angle of the ankle. By translating the lead nut housing (z.e., moving the first axle 120), the system dynamically- 8 -FH13002716.1Attorney Docket No.: NEX-19225 adjusts the geometry of the linkage, thereby changing the effective stiffness experienced during ankle rotation. As shown in Figs. 2 and 3, the linkage is defined using geometric references (annotated as “Point A”, “Point B”, “Point C”, “Point D”, and “Link 1”, “Link 2”, “Link 3”, and “Link 4”). These points correspond to pivot locations within the assembly, and the links represent linear connections between them. Point A may correspond to the pivot at the ankle member 104, point B to the pivot on the lead nut housing 108, point C to the joint between the first strut 118a and the arm 116. Link 1 may correspond to the virtual connection between points A and B, link 2 may correspond to the virtual connection between points B and C, link 3 may correspond to the virtual connection between points C and D, and link 4 may correspond to the virtual connection between points A and D. These labels are not indicative of physical parts but rather are used to illustrate the changing geometry of the linkage system as the lead nut housing translates and the ankle pivots. For example, adjusting the position of point B (by translating the lead nut housing 108) effectively changes the length of Link 1, thereby altering the mechanical moment arm between the ankle pivot and the spring. This change in geometry modulates the torque required to deflect the spring, enabling dynamic control of stiffness without altering the neutral ankle angle.

[0039] The lengths of the first strut 118a and the second strut 118b can be the same, so that when the ankle is in its neutral position, the two struts overlap from Point B to Point C. In addition, in the neutral position, the junctions between the ankle member 104 and first strut 118a and between the second strut 118b and the third strut 118c overlap at Point C. Because the lead screw 106 can pivot within the ankle member 104 about the pivot axis 112, changing the length of Link 1 (by translating the lead nut housing along the lead screw) does not cause the ankle to rotate or the spring to deflect (e.g., as shown in Fig. 4A). Instead, the first ends of the first and second struts 118a, 118b move together along a circular arc centered at Point C, with a radius equal to the length of Link 2. In some implementations, the length of Link 1 is about 20 mm to about 100 mm. Since the square of the moment arm is directly proportional to the stiffness, this provides a stiffness range over an order of magnitude.

[0040] In operation, when the ankle member 104 rotates in dorsiflexion or plantarflexion, the multi-strut assembly 118 causes the first spring 122a to deflect about a virtual rotation point, altering the torque response. The spring deflection opposes the rotation, providing a restoring force. The configuration enables bidirectional deflection such that the second end of the spring 122a moves in opposing directions during dorsiflexion and plantarflexion. When an external force is applied to the ankle e.g., during walking), the ankle member 104 rotates,- 9 -FH13002716.1Attorney Docket No.: NEX-19225 causing the first and second struts 118a, 118b to no longer overlap. The force is transmitted from the ankle member 104, to the first strut 118a, to the first axle 120 (lead nut housing axle), through the second strut 118b, through the third strut 118c, and into the spring. This motion deflects the spring, generating the restoring torque experienced at the ankle joint. This engaged state is illustrated in Figs. 4B and 4C, where the struts are no longer overlapping and the spring is deflected. Fig. 4B depicts a rotation of the ankle joint 25 degrees in DF, resulting in an upward deflection of the first spring 122a. In contrast, Fig. 4C depicts a rotation of the ankle joint 25 degrees in PF, with a corresponding downward deflection of the first spring 122a.

[0041] In some implementations, a slotted component can be used in place of the lead screw 106 and the lead nut housing 108 to define the motion path of the first axle 120. The slotted component may include an arc-shaped opening that constrains the motion of the first axle 120 along a predefined curved path. This path can correspond to a constant radius centered about a fixed reference point (e.g., about Point C). The slotted component may include a locking mechanism or non-backdrivable mechanism, allowing the prosthesis to maintain its stiffness setting under load without backlash.

[0042] In some implementations, a second spring 122b is positioned above or below the first spring 122a and arranged in parallel (with a vertical offset from one another), such that the spring are aligned in different parallel planes. For example, the first spring 122a may be disposed between the second spring 122b and the base 109, such that the second spring 122b is engaged only during dorsiflexion (when the first spring 122a deflects upward). In this configuration, the second spring 122b comes into contact with the first spring 122a when there is sufficient upward deflection. This arrangement effectively increases the resistance to DF without affecting the compliance of the prosthesis during PF, thus enabling the prosthesis to provide direction-dependent stiffness (e.g., bidirectional deflection of the first spring 122a). In this way, the two springs 122a, 122b together increase the DF stiffness relative to PF stiffness. The ankle joint can provide asymmetric tuning of DF and PF stiffness, which can expand the stiffness range available to the user. In some implementations, the first and second spring 122a, 122b are beam springs. In some implementations, the first and second spring 122a, 122b are coil springs. In some implementations, the first and second spring 122a, 122b are formed of carbon fiber. In some implementations, the first and second spring 122a, 122b are formed of fiberglass.- 10 -FH13002716.1Attorney Docket No.: NEX-19225

[0043] A bracket 124 can be used to support or retain ends of the springs by clamping the ends of the spring between the bracket 124 and the base 109. The bracket 124 can be coupled to the sidewall 110 and may include one or more mechanical fasteners, inserts, or guide channels to locate and constrain the spring.

[0044] In some implementations, the ankle joint 100 includes a pyramid adapter 123 affixed to the upper surface of the ankle member 104. The pyramid adapter 120 enables attachment to a prosthetic pylon, socket, or other upper limb interface.

[0045] Referring now to Fig. 4D, in some implementations, the ankle joint 100 can include a fulcrum 404 that determines the effective stiffness of the first spring 122a by altering its free length (z.e., the length of the spring that is allowed to deflect in response to loading). The fulcrum 404 is repositionable along the base and provides support at an intermediate point along the spring (between the first and second end of the first spring), such that only the segment of the spring beyond the fulcrum deflects during PF. This adjustment allows the prosthesis to change its PF stiffness without affecting DF behavior, thereby allowing adjustment of the DF to PF stiffness ratio. Because the spring is not deflected during neutral ankle position, frictional interactions between the spring and fulcrum are minimized during adjustment, thus avoiding backlash. Backlash is further reduced by overlapping the rotation arcs of the first strut and second strut 118a, 118b during stiffness changes. No spring deflection may occur until the ankle is actually rotated by the user.

[0046] The position of the fulcrum 404 along the length of the spring can be varied over a range defined by a distance d, from a more distal location (e.g., closer to the forefoot or toe region of the base 109) to a more proximal location (e.g., closer to the heel or pylon). When the fulcrum 404 is located distally, the first spring can deflect over a longer length, resulting in reduced stiffness. When the fulcrum 404 is located proximally, the effective free length is reduced, resulting in increased stiffness due to the shorter lever arm of the cantilever beam.

[0047] In some implementations, the fulcrum 404 is threadably engaged with a fulcrum lead screw 406, allowing adjustment of the position of the fulcrum 404 via screw rotation. In some implementations, the fulcrum 404 is mounted on a sliding track integrated into the base 109, allowing it be manually repositioned and locked into discrete positions (e.g., using detents, pins, or frictional elements). In some implementations, a rack-and-pinion system is used to drive the fulcrum forward or backward along the base. In some implementations, the fulcrum 404 is operably connected to an actuator (e.g., stepper motor) that enables electronic control of its position along the base 109. In operation, the actuator drives the fulcrum 404- 11 -FH13002716.1Attorney Docket No.: NEX-19225 forward or backward via, for example, a screw mechanism or a sliding carriage, thereby adjusting the contact point along the first spring and modulating PF stiffness. The actuator may be controlled by an onboard controller, a remote user interface, or an automated taskrecognition system.

[0048] Referring now to Figs. 5A - 5B and Fig. 6, views of an implementation of an ankle joint 500 are shown. Figs. 5A and 5B are a rear perspective view and front perspective view, respectively, of the ankle joint 500. Fig. 6 is a schematic side view of an implementation of ankle joint 500, with the near-side sidewall 110 omitted for simplicity. As shown in Figs. 5A - 6, the ankle joint 500 is similar to ankle joint 100. The ankle joint 500 can include the frame 102, the ankle member 104, the lead screw 106, the lead nut housing 108 that at least partially encloses a lead nut threadably engaged with the lead screw 106, the multi-strut assembly 118, and the first spring 122a. The ankle joint 500 may also include the second spring 122b.

[0049] In some implementations, the ankle joint 500 includes an actuator 506 configured to rotate the lead screw 106 and thereby translate the lead nut housing (with lead nut) to adjust the moment arm of the first spring 122a. The actuator 506 can be housed within a cavity of the ankle member 104 and supported at one end by a support structure 504. The support structure 504b can also support the first end of the lead screw 106 and maintain alignment between the actuator shaft and the screw.

[0050] In some implementations, a first pulley 502a is mounted on the output shaft of the actuator 506 and a second pulley 502b is mounted on the lead screw 106. A belt, cable, or other flexible transmission element may engage the pulleys to form a belt drive system, enabling rotation of the lead screw.

[0051] The actuator 506 may drive the lead screw 106 via intermeshing gears. For example, the actuator 506 may drive the lead screw 106 via a gear train, such as a spur gear or bevel gear arrangement, depending on the orientation. In a spur gear configuration, the output gear on the actuator shaft may directly engage a driven gear mounted to the lead screw 106, where the actuator shaft and lead screw 106 are parallel. Bevel gears may be used if the actuator shaft and lead screw 106 are oriented at an angle, such as perpendicular.

[0052] In some implementations, the ankle joint 500 includes the fulcrum 404 and fulcrum lead screw 406 (e.g., as shown in Figs. 6 - 8). The fulcrum assembly can be supported by a ledge 602, which is generally L-shaped in cross-section. The short, stepped portion of the ledge 602 can support one end of the first spring 122a, serving as a fixed spring mount- 12 -FH13002716.1Attorney Docket No.: NEX-19225 relative to the base 109. The longer horizontal surface of the ledge 602 extends beneath the first spring 122a and provides a platform along which the fulcrum 404 can be repositioned. The fulcrum lead screw 406 can be disposed within a recess or channel that extends along the longitudinal length of the ledge 602. This channel allows the lead screw to remain seated and laterally constrained. The fulcrum 404 can be operably coupled to the lead screw via a threaded interface such that rotation of the screw results in translation movement of the fulcrum along the ledge 602.

[0053] In some implementations, the fulcrum 404 has a semi-cylindrical geometry, with its curved surface positioned adjacent to the underside of the spring 122a and its flat surface resting on the flat upper surface of the ledge 602. This configuration maintains a vertical gap between a surface of the ledge 602 and the first spring 122a, such that the fulcrum only contacts the spring during PF deflection, when the spring bends downward into the fulcrum’s path.

[0054] Fig. 9 is a schematic side view of an ankle joint with illustrative annotations that depict the moment arm acting on the first spring 122a, along with reference lines identifying Link 1 and Link 2 of the multi-strut assembly 118.

[0055] Fig. 10 is a graph illustrating the simulated torque-deflection angle relationship for an ankle joint. The x-axis represents the ankle angle (in degrees), while the y-axis represents the ankle torque. The vertical dotted line represents the neutral ankle angle, labeled at 0 degrees, separating the PF (left) and DF (right) regions. As shown in Fig. 10, the slope of the torque-angle curve (stiffness) changes with changing moment arm. In Fig. 10, PF is indicated by negative deflection angles and DF is indicated by positive deflection angles. Each plotted line corresponds to a different effective moment arm length (ranging from 22 mm to 70 mm). The moment arm is defined by the perpendicular distance between the ankle’s pivot axis and the line of action of the spring force. The slope of each line indicates the angular stiffness of the ankle joint for a given moment arm configuration. As shown, stiffness increases approximately with moment arm length. That is, the greater the moment arm, the greater the torque generated for a given angular displacement.

[0056] Fig. 11 is a graph of a simulated ankle stiffness versus deflection angle for various moment arm lengths for an ankle joint. As shown in Fig. 11, stiffness trends non-linearly with deflection angle, however, this effect is diminished for plantarflexion stiffness and low dorsiflexion angles. In addition, the figure shows that there can be a maximum lOx stiffness range for both dorsiflexion and plantarflexion.- 13 -FH13002716.1Attorney Docket No.: NEX-19225

[0057] Referring now to Figs. 12A and 12B, views of an implementation of an ankle joint 1300 are shown. Figs. 12A and 12B are a front perspective view and a rear perspective view, respectively, of the ankle joint 1300. As shown in Figs. 12A and 12B, the ankle joint 1300 is similar to ankle joints 100, 500. The ankle joint 1300 can include the frame 102, the ankle member 104, the lead screw 106, the lead nut housing 108 that at least partially encloses a lead nut threadably engaged with the lead screw 106, the multi-strut assembly 118 (including the first strut 118a, second strut 118b, and third strut 118c), and the first spring 122a. The ankle joint 1300 may also include the second spring 122b.

[0058] The frame 102 can include a base 109 and a sidewall 110 extending upward from the base. The base 109 can have an elongate form and can be contoured with a convexly curved upper surface and a concavely curved lower surface. The sidewall 110 can extend along the base 109 and can be inwardly spaced from the perimeter edges of the base.

[0059] In some implementations, the ankle joint 1300 includes an actuator 506 configured to rotate the lead screw 106 and thereby translate the lead nut housing (with lead nut) to adjust the moment arm of the first spring 122a. The actuator 506 can be housed within a cavity of the ankle member 104 and supported at one end by a support structure 504. The support structure 504b can also support the first end of the lead screw 106 and maintain alignment between the actuator shaft and the screw.

[0060] The ankle joint 1300 can include the bracket 124 to support or retain ends of the springs by clamping the ends of the spring between the bracket 124 and the base 109. The bracket 124 can be coupled to the sidewall 110 and may include one or more mechanical fasteners, inserts, or guide channels to locate and constrain the spring.

[0061] In some implementations, a first pulley 502a is mounted on the output shaft of the actuator 506 and a second pulley 502b is mounted on the lead screw 106. A belt, cable, or other flexible transmission element may engage the pulleys to form a belt drive system, enabling rotation of the lead screw.

[0062] Referring now to Figs. 13-15, the ankle joint 1300 can include a fulcrum 1304 to adjust the PF or DF stiffness, thereby changing the ratio of DF:PF stiffness. In some implementations, the ankle joint 1300 includes a discretely adjustable fulcrum 1304. In some implementations, the ankle joint 1300 includes a slidably adjustable fulcrum (e.g., as shown in Fig. 8). In some implementations, the fulcrum 1304 has a generally cylindrical geometry, with its longitudinal axis oriented perpendicular to a longitudinal axis of the first spring 122a.- 14 -FH13002716.1Attorney Docket No.: NEX-19225

[0063] The ankle joint 1300 can include the ledge 602 which can support one end of the first spring 122a, thereby defining a gap between the first spring 122a and the base 109 along the length of the spring 122a. In some implementations, the ledge 602 is generally rectangular.

[0064] The ankle joint 1300 can include a series of indexed positions 1302 configured to receive or restrain the fulcrum 1304. In some implementations, a first set of indexed positions 1302 are disposed in a linear array between the base 109 and the first spring 122a. In some implementations, a second set of indexed positions 1302 are disposed in a linear array above the second spring 122b, such that the second spring 122b is disposed between the first spring 122a and the second set of openings. In some implementations, the indexed positions 1302 are defined by a clip-in mechanism (e.g., detents, recesses, latches) formed along the frame 102. In such implementations, the fulcrum 1304 can be engaged with a selected clip-in position. In some implementations, the indexed positions 1302 are provided as openings (e.g., circular openings) disposed along a portion of the frame 102 (e.g., formed in the sidewall 110 or in a separate plate coupled to the frame 102). The openings can be arranged in a generally linear array parallel to the longitudinal axis of the first spring 122a. In some implementations, a first set of openings are disposed in a linear array between the base 109 and the first spring 122a. In some implementations, a second set of openings are disposed in a linear array above the second spring 122b, such that the second spring 122b is disposed between the first spring 122a and the second set of openings. The fulcrum 1304 can be received in one of the openings. By fixing the fulcrum 1304 in a selected opening, the effective fulcrum location relative to the springs is discretely adjusted. For example, positioning the fulcrum 1304 in an opening located proximate the second spring 122b adjusts DF stiffness, while positioning the fulcrum 1304 in an opening located proximate the first spring 122a adjusts PF stiffness.

[0065] The ankle joint can be configured for manual and / or automated stiffness adjustment. In some implementations, the lead screw 106 can be manually rotated by a prosthetist or user, for example, by hand or using a tool, thereby changing the position of the lead nut housing 108 and modifying the effective moment arm. In motorized implementations, the ankle joint can include a first actuator that is operably coupled to the lead screw 106 and / or a second actuator that is operably coupled to the fulcrum 404. The first actuator and / or the second actuator can be operably connected to a control system (e.g., onboard microcontroller or external computing device). In some implementations, the lead screw 106 and / or the first- 15 -FH13002716.1Attorney Docket No.: NEX-19225 actuator are non-backdrivable, ensuring that the lead nut housing (with lead nut) remain in position under load.

[0066] In some implementations, the ankle joint is configured to detect user activity or gait phase. For example, sensors may detect when the user is walking, ascending or descending stairs, or standing, and may also determine whether the foot is in swing phase or stance phase. Upon recognizing a change in task, the controller may command the actuator to responsively adjust the stiffness while the foot is in the air. This ensures that by the time the user takes their next step, the ankle joint has already transitioned to a new stiffness setting tailored to the upcoming task. Such step-to-step stiffness adaptation can preserve biomechanical efficiency and preserved the burden on the user, since stiffness transitions are made outside of the load-bearing portion of the gait. In this manner, the ankle joint could be employed as a user’s task-adaptive, daily-use prosthesis rather than solely a prescription tool.

[0067] The ankle joint thus exhibits quasi -passive behavior: although stiffness settings can be changed between steps, the ankle maintains a fixed stiffness during each individual step. That is, stiffness does not vary continuously throughout the gait cycle, but rather can vary discretely in response to activity transitions. For example, the user may walk with a first stiffness setting suitable for level ground, then shift to a second stiffness setting when climbing stairs, without requiring manual intervention.

[0068] The ankle joint can serve as prescription tool, enabling rapid trail of different stiffness profiles to match a user’s preferences and biomechanics. For instance, the prosthetist may switch between two or more stiffness settings and instruct the patient to walk or perform a specific activity under each condition. Based on observed gait performance or user feedback, the prosthetist may finalize the stiffness setting for everyday use.

[0069] The lead screw 106 may be manually rotated by a prosthetist or user (e.g., by hand or using a tool). In some implementations, the ankle joint 100 includes an actuator (e.g., motor) housed within the ankle member 104 and operably coupled to the lead screw.

[0070] The ankle joint 100 may be operably coupled to a controller configured to manually or automatically adjust the stiffness settings of the ankle joint (e.g., overall stiffness and ratio of DF to PF stiffness).

[0071] In some implementations, the ankle joint (e.g., the actuator) is operably connected to a sensor system capable of detecting user activity (e.g., walking, stair ascent, stair descent, standing). Upon recognizing a change in task, the actuator may responsively adjust the stiffness while the foot is in the air (z.e., swing phase), such that by the time the user takes the- 16 -FH13002716.1Attorney Docket No.: NEX-19225 next step, the new stiffness setting is already applied. This can enable step-to-step adaptation without requiring the user to counteract changes mid-stance, preserving biomechanical efficiency.

[0072] The quasi -passive behavior of the ankle joint ensures that stiffness remains constant during each individual step, even if stiffness setting changes between steps. That is, the stiffness of the ankle joint may not vary continuously during gait, but rather the ankle joint may be configured such that it has responsive, discrete changes in stiffness over successive steps. For example, the user may walk with a first stiffness setting for level ground, and then a second stiffness (higher than the first stiffness) when ascending the stairs.

[0073] The ankle joint can serve as a prescription tool that allows prosthetists to trial multiple stiffness settings with a patient before setting a final stiffness setting for the device and purchasing an according commercial prosthesis. For example, the prosthetist may toggle between a first stiffness and a second stiffness setting and ask the patient to perform a task (e.g., walk) under each condition.

[0074] Overall, the disclosed ankle joint enables: a) variable transmission of torque via lead screw driven link length changes; b) passive or quasi passive operation depending on actuator inclusion; c) control and isolated adjustment of stiffness; d) constant ankle neutral position regardless of stiffness setting; e) enhanced stiffness ratio control via dual spring and fulcrum architecture; f) integration with commercial prosthetic systems via pyramid adapter; and g) low stiffness settings when link length approaches zero.

[0075] The disclosed system may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing implementations are therefore to be considered in all respects illustrative, rather than limiting of the invention. Having thus described several illustrative implementations, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to form a part of this disclosure, and they are intended to be within the spirit and scope of this disclosure. While some examples presented herein involve specific combinations of functions or structural elements, it should be understood that those functions and elements may be combined in other ways according to the present disclosure to accomplish the same or different objectives. In particular, acts, elements, and features discussed in connection with one implementation are not intended to be excluded from similar or other roles in other implementations. Additionally, elements and components described herein may be further divided into- 17 -FH13002716.1Attorney Docket No.: NEX-19225 additional components or joined together to form fewer components for performing the same functions. Accordingly, the foregoing description and attached drawings are by way of example only, and they are not intended to be limiting.- 18 -FH13002716.1

Claims

Attorney Docket No.: NEX-19225CLAIMS1. An ankle j oint comprising: a frame, the frame comprising a base and a sidewall extending from the base; an ankle member pivotally coupled to the sidewall of the frame about a pivot axis, the ankle member including a body and an arm extending from the body; a lead screw having a first end and a second end defining a longitudinal axis therebetween, the lead screw mounted within the ankle member, the lead screw being rotatable about its longitudinal axis and pivotable about the pivot axis relative to the frame; a lead nut housing mounted on the lead screw and movable along its length upon rotation of the lead screw; a multi-strut assembly coupled between the lead nut housing and the arm; and a first spring coupled to the base of the frame and the multi-strut assembly, the multi-strut assembly defining a moment arm which causes the first spring to deflect when the ankle member rotates in response to an external force.

2. The ankle joint of claim 1, the multi-strut assembly comprising: a first strut having a first end pivotally coupled to a first axle mounted to the lead nut housing and a second end pivotally coupled to a distal end of the arm via a second axle; a second strut having a first end pivotally coupled to the first axle; and a third strut having a first end pivotally coupled to the second strut and a second end coupled to the first spring.

3. The ankle joint of claim 2, the first spring having a first end fixed relative to the frame and a second end coupled to the third strut.- 19 -FH13002716.1Attorney Docket No.: NEX-192254. The ankle joint of claim 3, further comprising a bracket coupled to the sidewall, the first end of the first spring retained between the bracket and the base.

5. The ankle joint of claim 2, the third strut comprising a spring clamp, the spring clamp engaged with the first spring.

6. The ankle joint of claim 2, the first strut having a first length and the second strut having a second length, the first length approximately the second length.

7. The ankle joint of claim 1, the lead nut housing enclosing a lead nut threadably engaged with the lead screw.

8. The ankle joint of claim 1, the lead nut housing comprising an axle, the axle pivotally supporting one or more struts of the multi-strut assembly.

9. The ankle joint of claim 1, further comprising a pyramid adapter for attachment to a prosthetic pylon, the pyramid adapter coupled to the ankle member.

10. The ankle joint of claim 1, the lead screw perpendicular to the axle.

11. The ankle joint of claim 1, further comprising an actuator housed within the body of the ankle member.

12. The ankle joint of claim 10, the actuator operably coupled to the lead screw.

13. The ankle joint of claim 1, the first spring comprising a beam spring.

14. The ankle joint of claim 1, further comprising a second spring mounted in parallel with the first spring, the second spring disposed to be deflected by the first spring when the ankle member is in a dorsiflexion configuration.

15. The ankle joint of claim 13, the second spring comprising a beam spring.

16. The ankle joint of claim 1, further comprising a fulcrum disposed between the first spring and the base, the fulcrum repositionable along a length of the base.

17. The ankle joint of claim 15, further comprising a fulcrum lead screw, the fulcrum threadably engaged with the fulcrum lead screw.- 20 -FH13002716.1Attorney Docket No.: NEX-1922518. The ankle joint of claim 1, the second end of the first spring deflecting in opposing directions relative to the base in response to movement of the ankle member between a dorsiflexion configuration and a plantar flexion configuration.

19. The ankle joint of claim 1, the base and the sidewall integrally formed.

20. The ankle joint of claim 1, the pivot axis defined by an axle, the axle comprising one or more shoulder screws supported by one or more ball bearings.- 21 -FH13002716.1

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