Table of Contents


Section 3: Human Foot and Ankle Versus Prosthetic Foot/Ankle Mechanism Function

Prosthetic foot/ankle mechanisms are often studied and compared on the basis of how they substitute for human foot and ankle functions. Clinicians caring for persons with limb loss face no small task in deciding which foot/ankle components to prescribe for each individual.

Modern prosthetic practice includes a growing array of foot/ankle mechanism designs that offer a wide spectrum of functions, indications, and costs. New designs and materials have added properties and motions that blur the lines of the traditional prosthetic foot/ankle mechanism classification. This constant innovation requires a greater knowledge of human foot and ankle function and more descriptive terminology when recommending prosthetic foot/ankle mechanisms to patients.

Perry lists three main functions of the human foot and ankle as

  • shock absorption,
  • weightbearing stability, and
  • progression.

This section will review human foot and ankle function versus prosthetic foot/ankle mechanism function, traditional prosthetic foot classifications, and present information on functional subsets for prosthetic foot/ankle mechanisms.



THE HUMAN FOOT


HOW THE HUMAN FOOT AND ANKLE ACHIEVES SHOCK ABSORPTION, WEIGHTBEARING STABILITY AND PROGRESSION

SHOCK ABSORPTION is an important component of loading response, when the swinging foot rapidly decelerates from initial contact to foot-flat. While the foot plays an important role, the entire limb contributes to shock absorption. Stance phase knee flexion follows ankle plantarflexion. Eccentric dorsiflexor muscle action (primarily the tibialis anterior) provides muscular shock absorption during ankle motion. In addition, the anatomical structure of the foot contributes to shock absorption through tarsal mobility and various joint articulations. The foot is fairly rigid at the point of initial contact, and "unlocks" with a subtle degree of subtalar joint eversion, accompanied by internal rotation of the tibia and hip. The series of lower limb joint motions transforms the lower limb and specifically the foot and ankle complex into a loose packed structure that accepts weight bearing and provides shock absorption. As foot flat is achieved and the ankle rocker mechanism is engaged, body weight continues to advance over the lower limb, facilitated by motion of the ankle.

At this point, WEIGHTBEARING STABILITY is essential as the contralateral limb leaves the ground. Advancement of the loading force vector from the hindfoot to the forefoot places an increasingly greater demand on intertarsal and metatarsal alignment to alter the foot and ankle complex from a loose packed and flexible structure to close packed and rigid structure. Meanwhile, the medial longitudinal arch remains effective at absorbing energy and adapting to uneven surfaces and variable ground reaction forces.

After midstance, a series of alignment changes occur between the hip, knee and tibia that reverse from internal rotation to external rotation. External rotation at the hip and tibia is translated down the kinetic chain to again transform ankle and foot alignment. As such, the hindfoot and midfoot tarsals and forefoot metatarsals and phalanges gradually transform the foot to a rigid lever structure capable of transferring loading, providing stability and assisting in propulsion in late stance phase. This is achieved through the relationship between external rotation among the lower limb joints that is transferred to the hindfoot. This external rotation is transferred through the ankle mortise to affect alignment of the talus and produce subtalar inversion. Subtalar inversion transforms the hindfoot into a more rigid alignment which begins to "lock" the hindfoot.

As loading continues from midstance to late stance and upon heel rise, the metatarsalphalangeal joints undergo increasing dorsiflexion which creates tension upon the plantar fascia to effectively shorten its length. As the plantar fascia shortens, it produces the windlass effect which lifts the medial longitudinal arch and transforms the talonavicular and calcaneocuboid joints (collectively referred to as the transverse tarsal joint) into a close packed alignment where the navicular, cuneiforms and cuboid align similar to the trusses of a bridge. This series of events effectively completes the "locking" of the subtalar and transverse tarsal joints which produces a rigid foot and ankle complex to effectively act as a rigid lever system capable of transferring loading across the foot at the push off phase of gait.

During this period of propulsion or PROGRESSION, the foot moves into the forefoot rocker from terminal stance to pre-swing. Although the forefoot remains in contact with the ground, the body is progressing through contralateral swing in preparation for ipsilateral swing. At the end of stance phase, forefoot dorsiflexion reaches a peak, maximizing the windlass effect.



PROSTHETIC FOOT/ANKLE MECHANISMS


HOW FOOT/ANKLE MECHANISMS ACHIEVE SHOCK ABSORPTION

Prosthetic components often emulate shock absorption functions of the physiologic foot and ankle, but usually have far fewer mechanisms to do so. This is because they lack sufficient triplanar rotary motion or variable loading stability. The heel lever or posterior cushion plays a major part in the first rocker. In general, prosthetic foot/ankle mechanisms either attempt to emulate the shock absorbing lower limb motion during the first rocker, or attempt a completely different shock absorption approach.

Recreating the first rocker:
One approach to emulate the shock absorption that occurs during first rocker is to attempt to mimic human foot and ankle anatomy in the prosthesis. The single axis foot uses a hinge to recreate sagittal plantarflexion and dorsiflexion. Shock absorption occurs through the dissipation of energy in the plantarflexion bumper. Material properties of the bumper affect the amount of energy dissipation.

Trying something different:
Solid ankle designs arose in response to the need for a simpler, low-maintenance prosthetic foot/ankle mechanism than the single-axis design. However, the lack of motion in a solid ankle design necessitated a new approach to shock absorption during loading response. The cushion heel of the SACH design and several subsequent ESAR designs provides shock absorption through material compression. The magnitude of simulated ankle plantarflexion for the solid ankle cushioned heel (SACH) and similar "solid" ankle designs is roughly half that of the anatomical ankle. The Flex-Foot produces only one fourth of the magnitude of normal ankle plantarflexion motion. Deformation of the cushion heel of the SACH foot design and several other types of feet under loading allows shock absorption in the absence of simulated ankle plantarflexion.



HOW FOOT/ANKLE MECHANISMS ACHIEVE WEIGHTBEARING STABILITY

Prosthetic foot/ankle mechanisms do not yet offer the degree of variable flexibility and rigidity provided by the physiologic foot, ankle and lower limb kinetic chain. Prosthetic foot/ankle mechanism designs often compromise between the softness at initial stance and the stiffness required at terminal stance. Usually designers seem to err toward stiffness, because drop off at terminal stance (an effect of a prosthetic foot/ankle mechanism with insufficient rigidity) is undesirable, and even potentially hazardous. For example, a person with transfemoral limb loss utilizing a prosthetic foot/ankle mechanism with flexible keel and forefoot will encounter knee instability due to deformation of the prosthetic forefoot during terminal stance. With the exception of the single-axis foot, prosthetic foot/ankle mechanisms usually do not contain an ankle mortise joint which would facilitate tibial progression. Combined with a set heel height, this results in slowing of tibial progression to one half the usual rate. One disadvantage in foot/ankle mechanism designs with a cushion type heel is the prolonged "heel-only" contact. This produces an unstable external knee flexion moment until the forefoot makes contact. EMG studies reveal prolonged co-contraction of hamstrings and quadriceps muscle groups to maintain stability. This could be one reason that a soft heel bumper is typically recommended for a person with transfemoral limb loss level in order to prevent an excessive external knee flexion moment. Knee instability associated with prolonged loading upon the heel may also result in more falls when persons with limb loss walk on low-friction surfaces such as wet tile or ice.



HOW FOOT/ANKLE MECHANISMS MAINTAIN PROGRESSION

Progressive stiffness of the prosthetic foot/ankle mechanism is directly influenced by the composition and geometry of the forefoot keel. This may consist of multi-carbon plates, a urethane "sandwich," or a carbon footplate. The geometry of the keel also influences stiffness. The cross-sectional taper and angle or curve of the keel as well as the surrounding material provide spring stiffness. A problem often encountered with many keels integrated into foam has been that the distal end of the keel pushes through the foam foot. Although cloth reinforcement has reduced this tendency, it continues to be a problem with highly active patients. Observation of ankle moments reveals that the Flex-Foot keel acts to generate twice the degree of energy return compared to a SACH foot. Along with being symmetric and thereby reducing manufacturing costs, a wide blade width accommodates a wide variety of center of pressure (COP) pathways, but may decrease efficiency overall.



TRADITIONAL FOOT CLASSIFICATIONS

Foot classifications that have been used since the 1980s include:

  • Single axis
  • SACH
  • Multi-axis
  • Dynamic response

Single Axis Feet

Single-axis feet attempt to replace part of the anatomical ankle joint motion by incorporating a hinge at the approximate location of the transverse tarsal joint. The "single axis" of the foot/ankle mechanism mimics sagittal plane motion only. Passive control of plantarflexion and dorsiflexion is provided by variable-stiffness bumpers.

Single-axis feet differ from solid-ankle designs in several ways. During loading response, single-axis feet plantarflex. Range of motion and timing vary depending on bumper properties. A soft bumper may result in premature foot flat. By contrast, when bumpers are too firm the single axis foot may simply function as a solid-ankle foot.

Historically, single axis feet were the first feet that were laboriously made for the patient with a toe break placed 6 mm posterior to the metatarsal heads. The forefoot rocker was positioned so as to augment the patient's movement. Today's single-axis designs are prefabricated and generally depend on the alignment capability of the prosthetist to optimize their rollover characteristics.

SACH feet

As biomechanical understanding and manufacturing capabilities evolved after World War II, component designers moved toward better simulating functions of the human foot and ankle complex. In the late 1950s, studies on the biomechanics of walking resulted in the creation of the patellar tendon bearing (PTB) transtibial prosthesis including the concurrent development of the SACH foot. Functionally the SACH foot also helped to promote knee flexion that was important to PTB interface designs of the time.

The heel cushion compressed under loading to simulate ankle plantarflexion and the eccentric contraction of the ankle dorsiflexors during loading response. The rigid keel simulated the stiffening effect of the ankle plantarflexors and forefoot dynamics during late stance. The SACH foot also addressed some of the maintenance and availability issues of the single axis foot by incorporating the functions of the single axis foot into an integrated design.

Observational gait analysis typically reveals prolonged heel cushion compression of the prosthetic foot/ankle mechanism. Radcliffe advocated relative ankle plantarflexion alignment of the prosthetic foot/ankle mechanism in order to minimize heel cushion compression and to increase foot flat stability. This was particularly emphasized for stabilizing the prosthetic knee in the alignment of prostheses for persons with transfemoral limb loss.



Multi-Axial Feet

Watch the video clip of a human foot inversion/eversion detail.

Watch the video clip of the Pathfinder inversion/eversion toe detail.

The multi-axial foot/ankle mechanism was designed to provide the ability to accommodate uneven terrain beyond that of the single axis foot/ankle mechanism by allowing motion in all planes, not just plantar and dorsiflexion in the sagittal plane. These foot/ankle mechanisms can be a simple split-keel variety, a carbon plate urethane overmolded sandwich, a hindfoot articulation or a combination of these designs.

A split keel design allows for the forefoot of a foot/ankle mechanism to comply to the underlying surface as it is loaded. It behaves as two separate levers with a unified proximal junction.

The carbon plate urethane sandwich allows the lower plate to accommodate ground surface contours while the urethane exhibits elastic properties to allow for ground compliance and reduction of ground reaction forces transferred proximally to the residual limb.

Hindfoot articulations generally have elastic bumpers and bushings to allow for the plantar aspect of the foot/ankle mechanism to adapt to terrain through compression of these elastic members.


The Delrin® keel of the original Seattle Foot

Courtesy Seattle Systems

Dynamic Response Feet

Dynamic Response foot/ankle mechanisms emerged in the 1980s with the objective of providing improved response over existing designs by simulating passive subtalar joint motion within the prosthetic foot. The SAFE foot (one of the first flexible keel feet) was introduced followed by the Seattle Foot, The Carbon Copy II and the Flex-Foot. A plethora of feet falling into the classification of dynamic response have since been developed.

These designs employ a stiff anterior keel or leaf spring, made initially of Delrin® (a nylon that can be easily machined and offers consistent spring function and toughness) and subsequently of phenolic and Fiberglas materials and high strength carbon plates. This was done theoretically to store spring potential energy through deformation of the keel in mid to late stance and return a portion of this energy for propulsion in the absence of active ankle plantarflexors. These designs were thought to also offer prolonged foot flat stability, better tibial progression, and more support distally when compared with SACH feet.

Although this has not been verified with quantifiable data, many users report that dynamic response feet simply feel more lifelike when compared with other feet. Some report that although they like the springiness of the dynamic response feet, they may find themselves working against the action of the foot when walking at slower speeds, descending stairs, and even decelerating after running.



FUNCTIONAL SUBSETS

Recently many foot/ankle mechanism designs have blurred the lines of these classifications by hybridizing the properties of different classes, primarily by combining dynamic response feet with multi-axis attributes. As a result, the traditional classification system has become outdated with regard to ESAR foot/ankle mechanisms.

It may be useful to develop a classification system based on the subsets of individual functional attributes that may be present in any foot/ankle mechanism. Proposed subsets could include:

  • Forefoot Keel
  • Heel Lever
  • Hindfoot Roller
  • Flexing Strut
  • Forefoot Inversion/Eversion
  • Multiaxis Hindfoot
  • Integrated Shock


Copyright Otto Bock Healthcare

Forefoot Keel
The Forefoot Keel is characteristic of the most basic ESAR foot/ankle mechanism with any number of materials and configurations. The Forefoot Keel can be a single-bladed member or consist of multiple separate members to approximate the medial column and lateral column of the anatomical foot. Stiffness is directly dependent on the cross section, material, keel length, and geometry. Some designs use multiple layers that collapse progressively, and others use a urethane sandwich, which has a smoothing effect on the load progression.

Heel Lever
The Heel Lever emulates the heel rocker, which contributes to load acceptance and ankle plantarflexion characteristics. Many foot/ankle mechanisms simply use a cushion heel that simulates plantarflexion by compression, but this simple approach may delay the stability of foot flat. In other designs, such as the Flex-Foot Mod III, a heel lever projects posteriorly from the forefoot keel or midfoot attachment, and often provides stiffer support than a cushion heel. Recent designs have used multiple levers, linkages, urethane bumpers or a urethane sandwich to simulate the progressive stiffness of the anatomical foot and ankle.

Copyright Otto Bock Healthcare

Hindfoot Roller
A Hindfoot Roller mechanism used by many foot/ankle mechanisms uses a rocker element mounted on a footplate to approximate the ankle rocker from loading response to midstance. This mechanism emphasizes the rotary motion of the ankle rocker to ease the transition from loading response. When configured as a complete circular mechanism that wraps superiorly, the Hindfoot Roller can also function indirectly in shock absorption by emulating midtarsal dorsiflexion. Excessive rocker function in late midstance would be nonphysiologic, leading to a loss of support in late stance.



Copyright Otto Bock Healthcare

Flexing Strut
A Flexing Strut that extends to the proximal socket attachment originated with the Flex-Foot design. Contemporary designs usually incorporate the forefoot keel in one integrated structure, but the strut can be separate from the rest of the foot. The strut is usually a wide rectangular cross section, but can be produced in a number of U-shaped, circular, or multiple rod geometries. Using continuous fibers in the strut composition insures maximum flexibility and strength. All these Flexing Strut designs offer the greatest amount of energy return providing twice the degree of late stance ankle plantarflexion moment, and five times the power compared with the SACH foot. The longer the continuous fibers are in the lay-up of the composite, the greater the amount of bending flexion that can occur. Unfortunately, this design also increases the height of the foot. The longest Flexing Strut designs have been shown to dorsiflex to as much as 23 in late stance, significantly more than the normal functional dorsiflexion of 10.


Forefoot Inversion-Eversion
Forefoot Inversion-Eversion is commonly provided in strut systems by a split-toe design. Other designs are more integrated, molding different durometer materials or members together within the foot so there are not necessarily articulating parts. Some designs create a forefoot composite urethane sandwich. The disadvantage of many of these systems is that they are nonadjustable and depend on the material stiffness of the design. It is important to note that the damping characteristics of the forefoot may limit the desired energy return, or in a more favorable light, smooth late-stance forces.



Multiaxis Hindfoot
A Multiaxis Hindfoot has existed historically as an articulating component with urethane rubber bumpers, bushings, spherical snubbers, or large rings to dampen motion. This component can be a separate modular ankle unit that can be used with a variety of prosthetic feet, or it may be integrated into the foot/ankle mechanism itself. Multiaxis articulating designs often need regular maintenance and servicing. Some variants extend the urethane sandwich from the forefoot to the hindfoot, thereby providing some hindfoot motion in addition to allowing midfoot torsion.

Integrated Shock Absorbers

An absorber in series

Many prosthetic foot/ankle mechanisms also incorporate shock absorbers in a parallel or series configuration. A series configuration is usually found with a damper more proximal to the spring-like foot, whereas a parallel design has a damper and spring at the same level. It should be noted that a true shock absorber in engineering terms has a damper and spring in parallel where the spring functions to reset the damper once energy has been dissipated. The stiffness of the series configuration is limited by the softest component in the chain or the damper. The stiffness is truly a sum of the damper and spring together where the damper absorbs energy and prevents recoil of the spring and the spring resets the damper and prevents it from "bottoming out."

An absorber in parallel

The telescoping nature of many shock absorbers may be considered nonphysiologic, because the overall length shortens. Some designs use a rotary motion linkage at the ankle or behind the forefoot to minimize this effect, and attempt to imitate the shock attenuation mechanisms of the physiologic ankle/foot. Future components are sure to continue this blending of qualities to provide greater foot function and movement. In the near future, advances in composite geometry and material design will be able to provide more variable stiffness or flexibility characteristics for the forefoot and/or flexible strut. Longer-range expectations would be to create a foot in which the patient can actively vary the amount of shock absorption, stiffness, heel height, or third rocker support at the toes. With microprocessor and electronic control theories, this may be possible within the foot and ankle itself.

Watch the video clip of the Pathfinder heel spring/ankle motion.



HUMAN FOOT AND ANKLE VERSUS PROSTHETIC FOOT/ANKLE MECHANISM SUMMARY

This section has introduced the three main functions of the human foot and ankle-shock absorption, weightbearing and progression. We then elaborated on how the human foot and ankle achieves these functions and compared this to how prosthetic foot/ankle mechanisms are designed to simulate these same functions. Information about the traditional classifications of prosthetic foot/ankle mechanisms (Single Axis, SACH, Multi Axis and Dynamic Response) was presented in order to understand the history of these components relative to more novel designs. Finally a proposed group of functional subsets of existing ESAR prosthetic foot/ankle mechanisms was presented as a means to explore aspects of how foot/ankle mechanisms are designed to optimize simulation of the human foot and ankle in order to achieve shock absorption, weight bearing and progression.

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