Current Prosthetic Foot Design Subsets

Gerald Stark


New Challenges in Prosthetic Foot Selection

With the increasing number of prosthetic foot designs promising an ever widening spectrum of functions, the prosthetist is faced with new patient recommendation decisions. Although the basic families of feet still exist, such as Single Axis, SACH, Multi-Axis, Dynamic Response, new subsets are emerging that blur the lines between these accepted classifications. Also as foot designs incorporate new technology other terminology becomes necessary to differentiate the different classes of feet.

Physiologic vs. Prosthetic Foot Function

Jaquelin Perry, M.D. mentions three main functions of the physiologic foot as shock absorption, weight bearing stability, and progression.1 Valmassy further describes five functions of the foot being load bearing, leverage, shock absorption, balance, and protection.3 The success of any prosthetic foot design could be assessed by the number and degree to which it emulates these physiologic functions.

Shock absorption
Shock absorption is a primary function of closed chain pronation and tarsal mobility as the prosthetic foot makes initial contact, adapts the ground contours, and accepts weight in loading response to early midstance.1 Prosthetic shock absorption emulates the function of the physiologic foot, but usually has far fewer mechanisms to address it, in that there is no triplanar rotary motion or variable stability. The transverse rotary is approximated with the 5% of external rotation and motion allowed by the prosthetic hindfoot.9 The heel lever or posterior cushion plays a major part in the first rocker arc. It has been expressed by Dr. Perry that this first rocker function has been neglected when compared to the variety of forefoot keel designs.2

Weight Bearing Stability
Weight bearing stability refers to the advancement of the load vector from midstance to terminal stance loading the metatarsal heads which places increasingly greater demand on intertarsal stability to provide a relatively stiff lever arm for late stance.2 This load becomes greatest at heel rise as the foot rises up on the metatarsal heads. The increasing stiffness physiologically is a result of closed chain supination with subtalar inversion providing the necessary support of the calcaneus under the talus with subsequent external rotation of the tibia. Prosthetic weight bearing stability does not enjoy the variable stiffness offered by the physiologic foot, so its design is a compromise between the softness at the second rocker and the stiffness required at terminal stance. Usually designers try to error to the side of the stiffness since drop off at terminal stance would be a potentially dangerous presentation, especially for the transfemoral user. With the exception of the single axis foot, prosthetic feet usually have no mortise joint allowing easy tibial progression. Combined with a set heel height, this results in the tibial progression being slowed to one half the usual rate.2

Progression
During progression, the foot moves into the third rocker from terminal stance to preswing. This induces controlled dorsiflexion of the MP joint which is flexed to provide a broad and stable area of support to the toes. This in turn reduces the overall pressure on the metatarsals, as the load transfers distally and controls the shape and stability of the forefoot rocker and permits a longer forefoot roll.2 Prosthetic foot progression attempts to do the same primarily with dynamic response feet which flex with the load and carry the arc of motion to the toes whereas the SACH foot's arc of motion terminates before reaching the toes. Progression stiffness is directly influenced by the composition and geometry of the forefoot keel which may consist of multi-carbon plates, urethane sandwich, or a bottom carbon foot plate. The keel geometry also influences stiffness with crosssectional taper and angle or curve of the mid place providing spring stiffness. Observation of the ankle moment does show twice the plantarflexion moment for the Flex Foot acting to return energy2 desirable for faster walking speeds. Along with being symmetric for cost issues, a wide blade width accommodates a wide variety of COP pathways, but may decrease efficiency overall.

Foot Classifications

The original foot design classifications that have been used for since the 1980's listed the single axis, SACH, multi-axis, and dynamic response12. Jacquelin Perry refers to the anatomic or single axis, biomechanic or SACH, and dynamic or Seattle Foot and Flex Foot.2

The single axis foot as mentioned previously mimics anatomic ankle hinge movement but cannot rotate into position for load acceptance or transfer. Although it does emulate normal plantarflexion motion with a minimized arc of motion the rate exceeds normal foot drop by 50%.2 A soft bumper would present with less controlled plantarflexion and premature foot flat resulting in the reaction line being thrown anterior and the knee into extension. This induced knee extension is one reason why more active amputees find this foot "slow" or overly stable. A harder bumper used to counteract this would defeat the advantage of the single axis foot by making the arc of motion and recovery to a neutral position exaggerated. It must be remembered historically that single axis feet were the first feet that were laboriously custom made to the patient with a toe break placed 6mm posterior to the metatarsal heads. The foot rocker could be placed to augment the patient's movement whereas today's designs must depend on the alignment capability of the prosthetist to optimize the rollover characteristics.

The SACH foot was developed in the late 1950's to address some of the obvious maintenance and availability issues of the single axis foot by incorporating the functional needs of the foot into a integrated design. The heel cushion compressed to simulate the eccentric lengthening of the dorsiflexors and the keel the stiffening effect of the plantarflexors.8 It has been said that the SACH foot represented the first rollover shape, a concept mentioned in today's literature by Hansen et al.4

The multiaxial foot remains popular especially for activities on uneven terrain and can be divided into forefoot and hindfoot designs with integrated or multi-part configurations. The mechanisms can be a simple split toe variety, a carbon plate urethane overmolded sandwich, or a multipart design. The forefoot multiaxial designs are primarily the simple split toe varieties which help provide third rocker stability. With increased stiffness this can also simulate the plantarflexion of the first ray and push the path of the COP slightly laterally. Hindfoot plantarflexion provides an amount of inversion/eversion response good for the first and second rocker as the foot adapts to the ground during loading.

Dynamic Response feet emerged in the 1980's with the objective of providing better loading response the toes. At first Delrin™, a nylon which is easily machined, has a consistent spring constant, and excellent toughness, was utilized to create the anterior keel or spring board. Initially these designs were felt to be an improvement with prolonged foot flat stability, better tibial progression, and more support distally. Many users reported that the feet simply felt more "lifelike" although researchers could not verify these observations with quantifiable data. Other designs used phenolic and Fiberglas™ materials, but the use of high strength carbon struts greatly extended the amount of flexibility and energy return. The Flex Foot dorsiflexes to 20%, twice much as the normal 10% of dorsiflexion, which remedies the shortened stride length for all transtibial amputees. It was also shown to reduce vertical loading on the opposite side loading as the support was carried more distally and a plantarflexion moment was applied. It is interesting to note that this return is not fully realized until the foot is partially unweighted, leading some to question its value to stance phase. They suggest that the main benefit is to function to help initiate swing phase. The need for push off has been questioned by some as non-physiologic since the gastroc-soleus complex EMG signal dissipates at double support.

Functional Subsets

Recently many foot designs have blurred the lines of these classifications by hybridizing the properties of different classes, primarily dynamic response with multi-axis attributes. This leads to a classification system that focuses on the subsets of individual functional attributes that may be present on any foot. These subsets include Forefoot Keel, Heel Lever, Hindfoot Roller, Flexing Strut, Forefoot Inversion/Eversion, Multiaxis Hindfoot, and Integrated Shock.

The forefoot keel represents the most basic dynamic response foot which aids in general balance and load progression with any number of materials and configurations. The blade can be a single bladed member or consist of multiple separate members to approximate the medial and lateral columns of the anatomic foot. The 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 affect on the load progression.

The heel lever emulates the first rocker which defines the load acceptance and plantarflexion arc characteristics. Many systems simply use a cushion heel which simulates plantarflexion by compressing, but may delay foot flat stability. A true heel lever extends posteriorly from the forefoot keel or midfoot attachment and often provides stiffer support which reduces plantarflexion and may induce a greater knee flexion moment. A softer and/or shorter heel lever would have the affect of decreasing plantarflexion and the knee flexion moment. Recent designs have utilized multiple levers, linkages, or urethane bumpers or a sandwich to simulate the initial softness and progressive stiffness of the anatomic foot.

A hindfoot roller mechanism employed by many feet uses a rocker on a foot plate that approximates the second rocker from loading response to midstance. This mechanism does not necessarily lower the arc of plantarflexion, but emphasizes the rotary motion of the second rocker. Some mechanisms are simply rockers on a lower foot plate to ease the transition during the second rocker. With a complete circular mechanism that wraps superiorly, the hind foot roller can also function indirectly in shock absorption by emulating midstarsal dorsiflexion. Excessive rocker in late midstance would be non-physiologic and approximate a pathologic foot with excessive fallen arches and mid-foot motion such as the pathologic Charcot foot.

A flexing strut that extends to the proximal attachment usually incorporates the forefoot keel, but can be discontinuous to the attachment of the foot. The strut primarily comes as a wide rectangular cross section, but can be available in any number of Ushaped, circular, or multiple rod geometries. Using continuous fibers in the composite of the strut insures maximum flexibility and strength. All these designs function to provide the greatest amount of return providing twice as much of a late stance plantarflexion moment and a power component five times higher than the SACH foot.2 The longer the continuous fibers are in the lay-up of the composite, the greater the amount of flexion can occur. Unfortunately this increases the height requirement for the foot. These designs have the ability to dorsiflex more than anatomic at 23% as opposed to normal functional dorsiflexion 10%.2 This compensates for the relative step length shortening evident with tibial progression at 67% of normal (SACH foot at 33% of normal). Also the vertical shock to the contralateral limb is lowered since there is greater support in late stance. Most feet average 130% of body weight where as the flexing strut exhibits normal loading of 110% body weight.2 Some designs, created for athletic racing only, use the flexing strut for toe only gait, but not for general walking, since it obviously does not provide adequate weight bearing stability.

Forefoot inversion-eversion is primarily available as a split toe design, but can be available as a multimember forefoot approximating the medial and lateral column of the forefoot. Some systems are more integrated; molding different durometer materials or members together within the foot so there are not necessarily articulating parts. Newer designs create a forefoot composite urethane sandwich. The disadvantage of many of these systems is that they are non-adjustable that depend on the material stiffness of the design. It is important to note that the damping characteristics of the forefoot may dampen the desired energy return, or in a more favorable light, smooth late stance forces.

A multiaxis hind foot has existed historically as an articulating component with urethane rubber bumpers, bushings, spherical snubbers, or large rings. This component can also be separated into modular ankle units that can be used with a variety of foot designs. These articulating designs often require regular maintenance and servicing. Designs using the urethane sandwich can extend from the forefoot to the hindfoot also providing hindfoot motion and allowing midfoot torsion if required. Because of this increase translational motion and shear at the ankle, softer designs benefit with more rigid articulating members directing the force at the ankle which can cause excessive motion or breakage.

Along with multiaxis units, many feet 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.6 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.6 In parallel 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".6 The telescoping nature many shock absorbers may be non-physiologic since the overall length shortens. Some designs use a rotary motion linkage at the ankle or posterior to the forefoot keel to minimize this affect and imitate the shock attenuation mechanisms of the physiologic mechanism.

Future components are sure to continue this blending of componentry to incorporate greater foot function and movement. In the near future advances in composite geometry and material design will be able to provide more proportional 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 their 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.

Gerald Stark, BSME, CP, FAAOP; The Fillauer Companies; E-mail: gstark@fillauer.com

References:

  1. Perry, J., Gait Analysis: Normal and Pathological Function, SLACK, Inc., Thorofare, New Jersey, 1992. pp. 73-85.
  2. Perry, J., Amputee Gait: Chapter 30 Atlas of Amputations and Limb Deficiencies, 3rd Edition, Smith, D., Michael, J., Bowker, J., American Academy of Orthopedic Surgeons, Rosemont, IL, 2004. pp.367-384.
  3. Valmassy,R., Clinical Biomechanics of the Lower Extremities. Mosby-Year Book, Inc., St. Louis, MO, 1996, pp. 1-85.
  4. Hansen, Childress, Miff, Gard, Mesplay, The human ankle during walking: Implications for design of Biomimetic Ankle Prostheses., Journal of Biomechanics, No. 37, 2004, pp. 1467-1474.
  5. Hansen, A., Childress, D., Miff, S., Roll-over Characteristics of Human Walking on inclined Surfaces
  6. SA Gard, Ph.D., RJ Konz, M.S., The Influence of Prosthetic Shock Absorbing Pylons on Transtibial Amputee, Journal of Proceedings, AAOP, 2001.
  7. Northwestern University Transtibial Instruction Manual, Northwestern University Prosthetic Orthotic Center, Fall 1991.
  8. Radcliffe, C., Functional Considerations in the Fitting of Above Knee Prostheses, Artificial Limbs, 1955 2:35-60.
  9. Nielsen, D., Shurr, D., Golden, J., Meier, K., Comparison of Energy Cost and Gait Efficiency During Ambulation in Below-Knee Amputees Using Different Prosthetic Feet-A Preliminary Report. Journal of Prosthetics and Orthotics, Vol. 1, No. 1, pp. 24-31.
  10. Michael, J., Energy Storing Feet: A Clinical Comparison., Clinical Prosthetics and Orthotics, Vol. 11, No. 3, pp 154-168.