Orthotic and prosthetic intervention for partial foot amputation is intended to restore the effective foot length and normalize gait and function. Research is only beginning to examine whether current interventions are up to the challenge.

By Michael P. Dillon, BP&O (Hons.), PhD

Partial foot amputation (PFA) is a common sequela to diabetes and vascular disease affecting more than 618,000 Americans. PFA is associated with a significant failure rate and numerous complications, including ulceration and secondary amputation.

Despite the prevalence of PFA and the numerous complications, formal study of the effect of prosthetic and orthotic intervention is a relatively recent phenomenon. Synthesis of the available literature shows that PFA influences the mechanics of gait – a glimpse of which highlights useful adaptations to spare the end of the residuum from extreme force and compensate for limited power generation at the ankle joint.

Although our understanding of prosthetic intervention is very much in its infancy, there are useful insights to help inform prescription, particularly when considered in the context of evidence based medicine.

Incidence and aetiology

Statistics reporting the incidence of PFA are staggering. Based on data from the Vital and Health Statistics, Ambulatory and Inpatient procedures¹ and accounting for the increase in the population of the United States, there are approximately 1.27 million Americans living with lower extremity limb loss, of whom about 618,000 have some form of PFA. Given the population of the US, the incidence of PFA is therefore 2 per 1000 individuals. PFA is twice as common as either transtibial (below-knee) or transfemoral (above-knee) amputation.^1,2 About three-quarters of PFAs affect the toes or disarticulate the metatarsophalangeal joint, with more proximal procedures less commonly performed.^1,3

While PFA affects a significant number of persons, these figures take on a whole new scale when one considers that the incidence of amputation more generally is two to three times higher in certain race- or ethnicity-based demographics^4,5 and the notion that the number of persons living with limb loss will double by 2050 as our population ages and the number of persons living with diabetes and vascular disease increases.^{6, 7}

It will come as no surprise to learn that most PFA are the end result of vascular disease, secondary to diabetes and its complications. The links between diabetes and amputation are strong. Diabetics are four times more likely to have an amputation than those without the disease.⁸ The rate of PFA increases rapidly after 40 years of age, in parallel with the increasing incidence of diabetes.⁹ However, diabetes is the primary cause of PFA, but not the only cause. The literature also highlights examples of PFA following trauma, limb deficiency or frostbite.^10-12

Complications

The vast majority of persons who undergo PFA will experience complications, particularly when concomitant health problems, such as diabetes, are present.¹³ Only about half of all PFAs heal,^13,14 and 30% to 50% of partial foot amputees will experience some form of skin breakdown,^13,15,16 skin grafts,^17,18 or wound failure.¹³ Studies estimate that 15% to 45% of persons with PFA experience some form or secondary amputation,^13,19,20 with two-thirds undergoing a higher level of amputation on the same limb.²⁰ Of those who have a PFA, between 15% and 30% will die within 12 months,^14,15,20 which is similar to the mortality rates cited by studies of lower extremity amputation in general.^{21, 22}

Prosthetic/orthotic interventions

A wide variety of devices have been used to manage the partially amputated foot, including: insoles, toe fillers, slipper sockets, ankle foot orthoses (AFO), and clamshell sockets.

As a generalization, the extensiveness of the intervention is proportional to the extent of limb loss. Persons with amputation of the toes or disarticulation of the metatarsophalangeal joint will likely use devices such as insoles or toe fillers (Figure 1). These devices tend to be made of foams with varying degrees of compliance to fill the shoe and redistribute pressure away from the sensitive end of the residuum, and are often collectively referred to as “below-ankle” interventions.

Persons with more proximal amputations, such as Lisfranc or Chopart amputations, would more likely use AFOs or prostheses incorporating an extensive socket that encompasses the leg and remnant foot (e.g., clamshell socket). These devices are often collectively referred to as “above ankle” interventions (Figure 2). The forefoot section of this type of device can be made of a variety of materials. In the case of the AFO, the polypropylene plate under the foot might extend the full length of the shoe and incorporate a foam toe filler. Sometimes a carbon-fiber AFO may be used. More substantive devices might include a laminated socket with the front half of a prosthetic forefoot or a carbon fiber foot plate bonded onto the socket. These types of devices may be designed with several goals in mind, such as protecting the residuum from high pressures, restoring the lost foot length, and moderating the external moments caused by loading the prosthetic forefoot.

The effect of prosthetic intervention on gait

Despite the prevalence of PFA and the seriousness of the complications, our understanding of the effect of prosthetic intervention is very much in its infancy. Formally evaluating the effect of prostheses using scientific methods is a relatively recent phenomenon.

Until fairly recently, our understanding of the PFA gait and the influence of prosthetic intervention was based on theoretical analyses.^17,23-25 These theoretical analyses stemmed from an understanding of gait in persons without amputation and were a reasonable first insight into the gait of persons with PFA, particularly given that no empirical research had been undertaken.

The number of publications on partial foot amputation has grown exponentially, with the majority of works being published in the last three decades.²⁶ The bulk of the literature focuses on surgical techniques or surgical outcomes such as mortality rates. The body of work examining gait and the influence of prosthetic intervention remains comparatively small, but it too is growing.

A systematic review, commissioned by the American Academy of Orthotists & Prosthetists (AAOP) as part of the 8^th State-of-the-Science Conference on the biomechanics of ambulation after partial foot amputation,²⁶ synthesised and appraised 28 publications from an unconstrained review of the literature through December 2006, including a dissertation²⁷ that has since been published.^28,29 A few other recent journal articles^30,31 extend our understanding beyond this systematic review, and an Evidence Note³² provides a condensed summary of the evidence.

Most of the literature examining the effect of prosthetic intervention on gait is observational in design – meaning that researchers have sought to measure various outcomes to observe and establish differences from a normal population without intervention. These sorts of investigations are consistent with the early works in any research area where investigators are working to establish a foundation of knowledge by defining the problem and establishing well rationalized questions or hypotheses, which can serve as a basis for higher level studies, such as experiments that compare interventions.

Given this understanding of our current position in the continuum of research, it will come as no surprise that the depth of our understanding is limited, particular when drawing comparisons between different prosthetic interventions.²⁶ Although there are ‘high’ levels of evidence to support a broad understanding that PFA generally affects temporospatial and ankle kinematic/kinetic parameters (e.g., PFA reduces power generation across the ankle joint),²⁶ there is only a ‘low-to-moderate’ level of evidence supporting our understanding of differences between levels of PFA (e.g. metatarsophalangeal amputation results in minor reductions in ankle power generation that are not significantly different from normal) and between interventions.²⁶

Although the evidence base is somewhat limited, a number of observations have been reported across investigations that provide insights into the effects of amputation and prosthetic fitting. More research is need to support the information presented here, it is hoped that the following section on ankle power generation and center of pressure (CoP) excursion will provide a glimpse that encourages wider reading and an exploration of more comprehensive descriptions.

Ankle power generation – Significant reductions in ankle power generation in partial-foot amputees have consistently been reported across investigations,^{10, 29,30, 33,34} leading to a high level of evidence.²⁶

Based on one person with metatarsophalanageal amputation analyzed as part of a larger study population, reductions in ankle power generation appear to be quite minor and at the lower end of that observed in a normal population.^29,34 Once the metatarsal heads have been compromised, through transmetatarsal, Lisfranc or Chopart amputation, power generation was virtually negligible irrespective of residual foot length or prosthetic intervention.^{10,29,30,33,34} Persons using ‘below ankle’ devices (e.g., insoles, toe fillers, slipper sockets) that allowed ankle range generated as much power as did persons using clamshell devices where ankle motion was eliminated.²⁹

Persons with PFA may compensate for reductions in power generation across the affected ankle using the hip joints bilaterally, but the evidence supporting this observation is limited^29,33 and contradictory.

On the basis of these consistent observations, authors have markedly different reflections. Some authors suggest that prosthetic intervention improves ankle power generation.^10,30 Others suggest that if the ankle doesn’t contribute to the work required to walk, then perhaps clinicians should consider ‘above ankle’ devices that eliminate ankle motion, particularly given the lower risk of ulceration associated with such interventions.³⁴

Center of pressure excursion –Several authors have noted that the distal excursion of the CoP is altered following PFA.^29,35-37 These alterations appear to be minor in persons with amputation at or distal to the metatarsophalangeal joint^35,37 and not meaningfully different from persons without PFA. In persons with Lisfranc and transmetatarsal amputation using insoles, toe fillers and slipper sockets, the CoP remains proximal to the end of the residuum until contralateral heel contact, when body weight could be shifted to the opposite limb. Only then is the CoP able to progress beyond the residuum, as the ground reaction force is rapidly declining. This would be an effective strategy to spare the end of the residuum from extreme force during late stance.

The available evidence suggests that the type of intervention plays an important role in restoring the normal excursion of the CoP. In persons with Chopart amputation using a clamshell prosthesis,^29,37 and a Lisfranc amputee using a Blue Rocker Toe-Off AFO,³⁶ the CoP was able to progress well beyond the residuum during late stance so that the peak ground reaction force was borne by the prosthetic forefoot.

It has been hypothesised that the ability of the prosthesis to restore the effective foot length relies on three prosthetic design features: a suitably stiff forefoot capable of supporting the amputee’s body mass; a socket capable of comfortably distributing to the leg and residuum pressures that result from loading the prosthetic forefoot; and a relatively stiff connection between the foot and leg segments to help control the moments caused by loading the prosthetic forefoot.^26,29,37

The insoles, toe fillers and slipper sockets fitted to the Lisfranc and transmetatarsal amputees use compliant foams to create the prosthetic forefoot, and even when these devices incorporate a socket, the surface area over which to distribute the interface pressures caused by loading the forefoot is relatively small.^29,37 By contrast, the Blue Rocker Toe Off AFO or clamshell prosthesis incorporate a relatively stiff prosthetic forefoot (e.g., a stiff carbon fiber plate or the front half of a prosthetic foot) as well as a substantial socket that covers most of the shin and leg, allowing the forces caused by loading at the toe level to be distributed over a wide area.^29,37 The decision to use any device needs to consider how it can manage the external moments, given the atrophy of the triceps surae typically observed in this population.^29,34

Clinical implications

While these insights into the gait of persons with PFA and the influence of prosthetic intervention may be helpful, they are but single pieces of a much larger and incomplete jig-saw puzzle. Much more work is needed in areas related to gait, but so too is work needed in areas of quality of life and reporting of adverse consequences, as illustrative examples. Only then can our clinical interventions be truly well informed by a comprehensive understanding.

Given our current point in the continuum of research, the practice of evidence-based medicine³⁸ is all the more important given that no intervention can, in isolation, be well informed by the available evidence. The provision of a prosthesis must continue to be based on the clinician’s experience in parallel with the available evidence and the patient’s values and wishes. For example, while some of the published literature argues that preserving ankle range of motion is not helpful in terms of ankle power generation, there are many circumstances related to this argument – such as descending a ramp vs walking on flat ground – that have yet to be investigated. The clinician and patient will always need to weigh various considerations to derive a reasonable prescription, but should do so from an informed basis. Prescribing a device for a patient with transmetatarsal amputation that preserves ankle range of motion to allow the “push-off” observed when the same device is used by persons without amputation is poorly informed by the available literature. If preserving ankle range of motion in the same patient is done for cosmetic reasons and because the risk of ulceration is minor, then this certainly provides a more compelling argument.

Conclusion

PFA affects about two of every 1000 people in the United States. Although amputation helps resolve some of the immediate concerns of vascular insufficiency and infection, complications such as ulceration and re-amputation are all too common.

Despite the prevalence of PFA and the numerous complications, formal study of the effect of prosthetic and orthotic intervention is a relatively recent occurrence that is revealing interesting insights into the influence of amputation and prosthetic intervention on gait.  Findings specific to the changes in ankle power generation and center of pressure excursion should serve as illustrative examples of the many interesting adaptations typical of this population. A comprehensive understanding of these adaptations and the influence of prosthetic intervention is a useful adjunct to prescription and management, particularly when considered in the context of evidence-based medicine.

Michael Dillon. BPO(Hons.), PhD, is a senior lecturer at the National Centre for Prosthetics and Orthotics and leads the Prosthetics and Orthotic Research Group within the Musculoskeletal Research Centre at La Trobe University in Melbourne, Australia.

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