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
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:
Trying something different:
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 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.
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.
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.
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.
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:
Integrated Shock Absorbers
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.