Restoring Ankle Power After Partial Foot Amputations
A partial foot amputation is the first consequence of failed treatments or complications in ulcer healing in people affected by vascular conditions. Among prosthetic and orthotic interventions most commonly used for partial foot amputations are basic rigid designs where the ankle motion, alignment, or a calculated propulsion are not possible or considered. A prosthetic approach embracing the biomechanical effects of these critical elements offers new possibilities in the treatment of these amputations.
Limb loss continues to grow in the United States and around the globe; 2.1 million people live with an amputation in the United States, and that number is expected to double by 2050.1 The link between diabetes and amputation is strong: 85 percent of lower-limb amputations are the result of foot ulcers.2 An ample number end up with a partial foot amputation, which is considered a minor amputation if the anatomical ankle joint remains intact. People who undergo partial foot amputations have likely altered their gait prior to amputation. Walking aid devices and therapeutic footwear designed to assist in healing stubborn ulcers affect ankle power generation and propulsion. After amputation, depending on its level, many patients have entirely lost propulsive power in the affected limb, and the foot's leverage has been seriously affected. While amputation helps resolve the ulcer caused by vascular insufficiency and infection, complications of new ulceration and reamputation are prevalent.
The long foot muscles affected by the amputation lose their attachment to the bone, resulting in a muscular imbalance. The triceps surae muscle exerts a strong pull on the residual limb via the Achilles tendon, which leads to the development of an equinus deformity. The more proximal the amputation, the more the dorsal flexors that control this pull lose their attachment. Due to this plantarflexed residual limb position, the range of motion of the upper ankle joint is severely limited, resulting in contractures.
The removal of the long and short foot muscles associated with the amputation results in reduced muscle strength of the dorsal flexors and plantar flexors.3 Both muscle groups are relevant to the physiological stance and gait. For example, the plantar flexors ensure the activation of the forefoot lever, while the dorsal flexors ensure the foot lifts during the swing phase. The extent of this reduction depends on how many muscles are still functional.
Biomechanical restrictions during stance and gait in patients with partial foot amputations are primarily due to the shortening of the forefoot lever. If there is no amputation, the forefoot lever is activated by the plantar flexors and allows for an energy-efficient stance and gait. During gait, a physiological heel lift, knee extension, and raise of the body's center of gravity in terminal stance take place. The goal of a prosthetic treatment is to compensate for the loss of bony and muscular structures caused by the amputation. The basis for this is the replacement of the removed bony structures with a mechanical forefoot lever. If the physiological activation by the plantar-flexors is no longer possible without restrictions, the forefoot lever must also be activated mechanically.
Prosthetic Treatment Requirements
Depending on the amputation level, there are different biomechanical requirements for the prosthetic treatment. The more proximal the amputation, the more the anatomical ankle joint must be stabilized, and the lost function must be compensated. A prosthesis for patients with partial foot amputations is designed to restore the function of the forefoot lever to replace the lost muscle function and establish a stable, dynamic balance.
This is relevant for a safe stance and high or prolonged loads, e.g., long walking distances. To come as close as possible to a physiological gait, the residual mobility in the anatomical ankle joint should only be minimally restricted. When walking with the prosthesis, shear forces on the residual limb should be avoided as much as possible, and a continuously increasing equinus position and supination are to be expected.
Limitations of Current Prosthetic Treatments
Several designs are used to manage partial foot amputations, including toe fillers, slipper sockets, AFOs, and clamshell sockets that all offer advantages and disadvantages. By general rule, the extension of the coverage directly relates to the length of the residual limb or amputation level.
A simple toe or forefoot replacement is used if one, several, or all toes are lost. If the focus is on cosmetic use, the replacement is usually made of silicone. Foam materials are used for simple volume compensations.4 However, a volume compensation placed loosely in the shoe causes irritation and pressure points at the distal end of the residual limb because the residual limb moves against the toe replacement when walking. If the big toe is lost, a functional compensation in the form of a carbon fiber sole is also necessary. If all toes are lost, a forefoot replacement with a shaft that extends over the midfoot can be used. This is usually made of silicone and enables a tight and optimal fit on the residual limb. Due to the amputation, the function of the short toe flexors to support the swing phase initiation is lost. This restriction cannot be compensated for by a toe prosthesis.
To compensate for the lack of function, especially in the case of short residual limbs, above-ankle foot prostheses are often combined with custom-made carbon fiber clamshell orthoses or prefabricated ankle-foot frame orthoses.5 A static carbon fiber orthosis does not allow any movement in the anatomical ankle joint. If the orthosis is equipped with a flexible foot piece, shear forces act on the end of the residual limb, causing
pressure points. Prefabricated AFOs without an ankle joint are not adjustable and do not provide adequate control over plantarflexion and dorsiflexion.5 The lack of a defined pivot point in this construction can shift the tibial shell on the leg. Rigid prefabricated AFOs may cause hyperextension of the knee joint (Figure 1).
Clamshell prostheses are custom made for the patient in different designs with closure or access flaps.5 All standard constructions allow for a good residual limb fit as well as a forefoot lever. The rigid connection of the lower leg and foot is used to reduce shear forces at the distal end of the residual limb. Depending on the remaining range of motion of the ankle joint, the prosthesis is either produced statically or with some range of motion. The construction with some range of motion does not provide the necessary stability. The static construction blocks the motion in the anatomical ankle joint, resulting in contractures and muscle atrophies.5
New Possibilities for a Partial Foot Prosthesis
To optimally adapt the prosthetic treatment to the patient, the individual condition of the muscles and foot bones must be considered. A methodology to structure the prosthetic treatment is to classify various amputation types into categories. The classification of partial foot amputations takes the length of the forefoot lever, the muscular balance between plantar flexors and dorsal flexors, and the muscle strength of the dorsal flexors into account (Table 1). For example, in type one, the attachments of the short and long toe flexors and extensors are no longer there, which is why the muscle strength of the dorsal flexors is restricted despite the muscular balance.
Since the development of an adjustable customized ankle joint system named Neuro Swing by the German company Fior & Gentz in 2011, the benefits of an adjustable and calculated component in treating patients affected by neurological conditions has been validated in published literature. Contrary to traditional generic rigid structures with unreliable flexibility, this mechanical ankle joint and its energy-storing precompressed disc force units result from an intelligent computer algorithm that calculates tolerable loading forces directly related to data obtained from the patient. The weight, height, activity level, foot length, manual muscle test, range of motion, pathological gait classification, fatigue, shoe conditions, and even waterproof conditions are some of the pieces of information processed to determine the ankle size; material selection between two kinds of titanium, aluminum or steel; design orientation between ventral or dorsal access; and footplate stiffness.
The unique characteristics of this technology and methodology offer new solutions in the treatment of partial foot amputations, customizing the range of motion and propulsion force according to available forefoot lever and restoring the ankle motion.
Managing the Forefoot Lever
The profession has been aware of the mechanical properties of carbon fiber and the advance in prosthetic devices since its introduction to the field. During gait, a thinner carbon fiber plate receives load at initial contact on a prosthetic foot, and the loading response releases energy immediately, simulating passive plantarflexion and supporting the trajectory to midstance. The user is stable at this single-limb stance gait phase, and the load is transferred to the anterior thicker carbon fiber plates; the rocker design injunction to the mechanical properties of this material receives the load, compressing the plates and simulating the dorsiflexion at the terminal stance. Finally, the plates release the stored energy defined as push-off or pre-swing and late plantarflexion.6,7,8 New hydraulic and intelligent ankle prosthetic systems offer alternatives where there is space for this and carbon fiber feet. The challenge becomes how to apply the mechanical properties of the carbon fiber when the ankle is still present.
One of the most common above-ankle interventions for partial foot amputees is a carbon fiber AFO in combination with partial foot fillers or silicone prosthesis. The ventral shell and extended footplate are intended to replace the missing lever arm to generate propulsion. Unfortunately, a monolithic solid carbon fiber design cannot independently select the resistance needed according to the gait phases.
The Neuro Swing ankle joint system uses exchangeable energy-storing precompressed disc force units. These cylindrical units are designed to contain the potential energy available on the compressed overlapping concave and convex discs without the necessary compression required for resistance as it occurs in a regular coil. These disc force units act like the carbon fiber plates in a prosthetic foot—the energy represented by the torque in Newtons per meter (Nm) is available immediately once a counterforce is applied. A less resistant unit allows passive plantarflexion independent from the higher resistant force needed at terminal stance (Figure 2, pg. 26). Like in a carbon fiber foot, this immediate resistance provides dynamic balance and stability except that the mechanical ankle motion keeps this support over any terrain.
A regular open-end coil, regardless of the size or diameter, requires compression to generate resistance. The lack of precompression, the nonexistent essential resistance, leads to a spring yield when loaded during stance and, due to the missing security, to an unstable stance and gait. The absence of immediate resistance forces lowers the body's center of gravity, which generates excessive knee flexion on the contralateral side, impedes the heel lift, and demands more energy from the user (Figure 3).
Functional Advantages of a Partial Foot Prosthesis With an Adjustable Customized Ankle Joint
Dynamic dorsiflexion stop
A dorsiflexion stop is necessary to activate the forefoot lever. The ventral energy-storing precompressed disc force unit results in a stable yet dynamic balance in a single-limb stance, a dynamic knee extension in late midstance, and a physiological heel lift in terminal stance. The dynamic dorsiflexion stop prevents knee hyperextension and shifting of the residual limb in the prosthesis.
Variable energy-storing force unit
The requirements for a prosthesis can change, sometimes severely, in therapy or due to a residual limb revision. Five exchangeable energy-storing force units offer independent torque resistance in the ventral and dorsal channels of the ankle joint, allowing the prosthetist to manipulate the dynamic response according to the needs and circumstances.
To achieve a physiological gait, the leverage ratios of the prosthesis must be adjusted to the patient. The partial foot prosthesis can also be easily adapted to different heel heights. This makes it easy to change footwear.
Defined pivot point and shear forces
A defined mechanical pivot point at ankle height plays an essential role for the dynamic dorsiflexion stop and thus the activation of the forefoot lever. The congruence between the anatomical and mechanical ankle joint and the centered rotation prevents the tibial shell from shifting on the leg or slipping on the residual limb during prolonged high loads. Reducing the shear forces on patients with sensory neuropathy is essential. A defined pivot point is also a prerequisite for passive plantar flexion.
Passive plantarflexion causes the foot to lower and is an important mechanism for shock absorption during load transfer.8 Thanks to the range of motion in plantarflexion, excessive torque in the knee can be prevented during loading response. This allows for physiological quadriceps loading and knee flexion. It also prevents muscle atrophy and contractures.
Passive plantarflexion is triggered by the heel lever, which runs from the heel strike to the ankle. The dorsal flexors control the heel rocker to prevent an uncontrolled landing of the foot.9 This muscular control is lost when the dorsal extensors are removed during amputation. The dynamic adjustable partial foot prosthesis enables the heel rocker against the resistance of the dorsal spring unit, as there is a defined pivot point and range of motion in plantar flexion. This can counteract the development of contractures and support the approximation to a physiological gait. The
resistance of the dorsal force unit can be precisely adjusted to the muscular control lost due to the amputation.
Adjustable range of motion
After surgery or residual limb revision, temporary immobilization of the anatomical ankle joint might be necessary. The range of motion can be blocked entirely and gradually
released. Thus, a precise adaptation to the range of motion of the anatomical ankle joint after the amputation is possible.
Restoration of the physiological gait of individuals after partial foot amputations and their dynamic balance and stability over any terrain directly influences the quality of life of this population. The lessons learned by applying adjustable customized
ankle joints in neurological conditions and the available evidence and research should be considered for this prosthetic treatment. We should question the continued prescription of the same devices for individuals without amputations and a pivot of motion congruent to the ankle to restore the push-off. Unnecessary complications can be avoided with a planned and adjustable device.
The use and application of adjustable customized ankle joints are relatively new in this part of the world. The Nuero Swing ankle joint system and its proposed methodology of ankle selection and calculated resistance force units are examples of how research and clinical outcome measurements can influence application from other fields to offer new solutions to our patients. O&P EDGE
Santiago J. Muñoz, CPO, FAAOP, is president of Equation Orthotic Technologies and clinical education specialist of Fior & Gentz. He can be contacted at firstname.lastname@example.org.
- Ziegler‐Graham, K., E. J. MacKenzie, and P. L. Ephraim, et al. 2008. Estimating the prevalence of limb loss in the United States: 2005 to 2050. Archives of Physical Medicine and Rehabilitation 89(3):422‐9.
- Owings, M., and L. J. Kozak, National Center for Health S. Ambulatory and Inpatient Procedures in the United States, 1996. Hyattsville, Md.: U.S. Dept. of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics; 1998.
- Dillon, M. P., and S. Fatone, et al. 2007. Biomechanics of ambulation after partial foot amputation: A systematic literature review. Journal of Prosthetics and Orthotics 19(8):2-62.
- Dillon, M. P., and T. M. Barker. 2008. Comparison of gait of persons with partial foot amputation wearing prosthesis to matched control group: Observational study. Journal of Rehabilitation Research and Development 45(9):1317-34.
- Kaib, T., and J. Block, et al. 2019. Prosthetic restoration of the forefoot lever after Chopart amputation and its consequences onto the limb during gait. Gait & Posture 73(1):1-7.
- Desloovere, K., and G. Molenaers, et al. 2006. How can push-off be preserved during use of ankle-foot orthosis in children with hemiplegia – A prospective controlled study. Gait & Posture 24(2):142-51.
- Owen, E. 2010. The importance of being earnest about shank and thigh kinematics especially when using ankle-foot orthoses. Prosthetics and Orthotics International 34(3):254-69.
- Perry, J., and J. M. Burnfield. 2010 Gait Analysis – Normal and Pathological Function, 2nd edition. Thorofare: Slack.
- Nolan, K. J., and M. Yarossi.2011. Preservation of the first rocker is related to increases in gait speed in individuals with hemiplegia and AFO. Clinical Biomechanics 26(6):655-60.