This is the final article in a series of articles about
amputations distal to the tibia. Although forefoot amputations result in a
longer residual limb than other partial foot ablations, objective gait analysis
data in recent years has made it increasingly clear that this does not
necessarily translate into more normal ambulation. With the exception of
amputation of the toes, loss of even a small portion of the forefoot results in
a significant physical disability. The degree of dynamic loss that the patient
will experience is not always directly proportional to the percentage of the
foot that has been removed, particularly once the metatarsal heads have been
lost. Gait studies suggest that no prosthesis, orthosis or shoe modification
presently known successfully restores active push-off for transmetatarsal
amputees.
Biomechanical Deficiencies
 |
|
| Pedobaragraph demonstrating the difference in lever arm and plantar weightbearing surface area between disarticulation at the metatarsal-phalangeal junction and transmetatarsal amputation. |
|
| Reprinted with permission from Partial Foot Amputations |
|
Forefoot amputations have similar biomechanical deficiencies due
to the loss of the anterior lever arm through disruption of its bony
architecture, ligamentous structures and musculature. The following problems
are present once the metatarsals have been affected:
- Reduced plantar weight bearing surface
- Impaired pronation and supination during gait
- Loss of active push-off
Weight bearing problems increase as more of the transverse and
longitudinal arch structures are disrupted, and the normal contours of the foot
are destroyed.
Forefoot amputees tend to adopt a gait pattern on the affected
side characterized by less active plantarflexion and delayed progression of the
center of pressure. Once the metatarsal heads have been removed, a shorter
stride length is common and the tendency to prematurely shift body weight to
the contralateral side becomes even more pronounced.
The primary advantage of forefoot amputations, compared with
higher levels of partial foot ablation, is that muscle balance is usually not
as seriously disrupted. As a consequence, it is believed that there will be
less risk of equinovarus deformities over time.
 |
 |
 |
|
| Figure illustrates osteology of the foot. Phalangeal amputation impairs the foot function somewhat but once the metatarsal heads are affected, the integrity of the arches is lost and active push-off becomes impossible. |
|
| Reprinted with permission from www.Bartleby.com |
|
Lever Arm Disruption
When considering the plethora of possible forefoot amputations,
it may be useful to classify them according to the degree of disruption of the
push-off structures of the foot that results. James Gage, MD, has written
extensively about the “lever arm disease” that children with cerebral
palsy experience when muscle imbalance or bony deformity rotates the foot out
of the line of progression, diminishing stability and active push-off. Forefoot
loss results in analogous problems.
 |
|
| Partial foot amputees who retain the metatarsal heads may find a modified custom foot orthosis with soft toe filler sufficient protection for limited ambulation. |
When all the metatarsals are spared (in the case of toe
amputations), presuming there is good plantar skin coverage that extends up
onto the dorsum of the foot remnant, the anterior level arm is only slightly
shortened. Static weight bearing is similar to the normal foot because the
longitudinal arch is still intact, along with the “tripod” formed by
the calcaneous and heads of the first and fifth metatarsal.
Because the major portion of the push-off mechanism of the foot
remains largely intact, the degree of biomechanical deficiency in toe
amputations is limited and a relatively simple appliance may be the only
treatment required to facilitate walking. Instrumented gait analysis has
demonstrated that, contrary to earlier assumptions, loss of the great toe does
not seriously disrupt walking on level surfaces.
Once the metatarsal heads have been transected, the longitudinal
arch is affected and the anterior lever arm is much more seriously compromised.
Even with optimal prosthetic management, active push-off is no longer possible
for these patients. Functionally, there seems to be little difference between
relatively longer or shorter transmetatarsal (TMT) amputations. Recent gait
studies suggest that TMT amputation results in similar functional impairments
to the Lisfranc and other more proximal levels.
Pedorthic Solutions
 |
|
| Cross section of “prosthosis” for toe amputation demonstrating extended steel shank, roller heal-and-toe configuration and custom molded multi-density foot orthosis with toe filler. This combination of orthotic, prosthetic and pedorthic principles provides more effective protection against excessive loading on the anterior residual limb than an inlay alone. |
|
 |
|
 |
|
| Inframalleolar “slipper type” prosthesis provides better protection for the anterior surface of the residual limb than a foot orthosis analogue, encouraging more active ambulation for moderately active individuals. |
As is the case with hindfoot and midfoot amputations, forefoot
loss can be managed with pedorthic solutions such as shoe modifications and
orthotic interventions such as modified foot orthoses, as well as with
prostheses. Although it might seem intuitive to choose a less complex device
for all cases where a longer foot remnant remains, the patient’s desired
activity level is a more clinically useful guideline — the higher the
desired activity level, the more biomechanically sophisticated the
“prosthosis” must be, particularly when metatarsal heads are not
intact.
Forefoot amputees who are limited household ambulators —
particularly if they are lightweight individuals — often walk slowly and
gently, therefore requiring only moderate protection against plantar forces on
the residual limb tissues. In such circumstances, a custom molded accommodative
foot orthosis with toe filler will frequently suffice. Footwear with a rocker
configuration, such as walking shoes, improves passive rollover for slow
walking. Unless the first ray is fully intact, the prosthosis or the shoe
itself should usually be stiffened with carbon fiber laminate or spring steel
to prevent excessive pressure on the anterior plantar surface of the residuum.
Such Medicare functional level 1 individuals usually require more complex
devices when the plantar skin or underlying boney architecture is less than
ideal, or when obesity results in substantially greater loading on the foot
remnant.
Some Medicare functional level 2 individuals with forefoot loss
will do well with a modified foot orthosis plus shoe alterations, but most
limited community ambulators do far better with a low profile slipper-type
prosthesis that fully encases the anterior surface of the residuum and better
protects the distal tissues. Many inframalleolar variations have been described
in the literature, with no specific prosthesis having been shown to be superior
to another. Laminated and thermoformed prostheses are the lightest available
alternatives, but many clinicians have reported good results with 100 percent
silicone prostheses, particularly when softer durometer material is
incorporated into selected areas within the prosthesis, to cushion and protect
the plantar skin and bony prominences.
Shoe modifications or rocker-type footwear are almost always
helpful in facilitating passive rollover, although the prosthesis itself can be
designed to preserve forward momentum even within unaltered shoes. Heavier
patients or those who walk on irregular surfaces may need additional shoe
modifications to increase stability or to protect the remnant foot.
Requires Protection
Medicare functional level 3 independent community ambulators
generally require the protection of a prosthesis that encases the foot, since
they walk longer distances over more difficult terrain than less active
individuals. Since almost all forefoot amputees retain active ankle motion,
inframalleolar prosthetic designs predominate although shorter transmetatarsal
ablations may require a supramalleolar “bootee” for adequate
suspension.
Walking shoes will augment the protection offered by the
prosthesis, but the use of modified footwear with extended forefoot
reinforcement and a custom roller sole is often more effective. As noted
before, heavier individuals or those who walk over rough terrain require more
extensive interventions. Independent community ambulators are capable of
walking briskly, and the faster cadence increases the destructive forces on the
residual limb, so a stiffer and more encasing prosthesis may be indicated for
this subset.
|
|||
Forefoot amputees who aspire to Medicare functional level 4
function, which includes participation in sports and recreational activities,
are generally quite disappointed with how little they can do safely despite
prosthetic fitting. Patients with forefoot amputations typically find it
impossible to engage in high impact activities without recurrent skin damage,
due to the loss of the dynamic architecture of the foot. Silicone prostheses
are believed to offer the greatest skin protection of the presently available
low profile alternatives, but this material is not always sufficient in
preventing breakdown for very active people.
As the patient’s activity level increases, the loss of
push-off becomes increasingly problematic. Unfortunately, active push-off
cannot be effectively restored by current prosthetic technology. Although this
was not widely recognized in the past, a number of instrumented gait studies in
recent years have concluded that TMT amputees are unable to generate anything
close to normal plantarflexion power during late stance. Results indicate
typical results showing that while neither the Syme amputee nor the partial
foot amputee demonstrate plantarflexion motion at the end of stance phase, the
partial foot amputee is also unable to generate a passive plantarflexion moment
in late stance despite having a mobile ankle and a much larger foot remnant
remaining. The precise cause for these findings has not yet been conclusively
established, but it appears that the partial foot lever arm is simply too short
to effectively create active plantarflexion once the forefoot is loaded by body
weight.
 |
 |
|
| Silicone prosthesis for first ray amputation. | ||
| Reprinted with permission from Partial Foot Amputations | ||
At present, the only way to offer maximum protection during
rollover would be to provide a bivalve prosthesis that completely immobilizes
the ankle. To fit a prosthesis that is more commonly used for Chopart and Syme
levels seems extreme for TMT amputees who retain a fully mobile ankle, but gait
studies and clinical experience both suggest that the forefoot amputee requires
maximum protection in late stance. There is some recent evidence suggesting
that ambulation after Syme amputation is more energy efficient than after
midfoot amputation, so it is possible that the more extensive prosthesis may
offer similar benefits to the partial foot amputee.
In some instances, a solid ankle AFO can be applied over a
silicone slipper prosthesis. The advantage to the AFO/slipper combination is
that the patient can wear the low profile prosthesis alone for less vigorous
tasks.
 |
|
| Gait analysis results showing ankle motion (top graphs) and ankle moments (bottom graphs), comparing Syme (graphs on right) to partial foot amputation (graphs on left). The vertical line at approximately 60 percent of the gait cycle indicates the stance-swing transition. Normal values are shown by the shaded gray curves; the dark black line depicts patient data for the amputated side. Results that show neither the Syme amputee (A) nor the PF amputee (D) demonstrates plantarflexion motion in late stance, indicating a lack of normal push-off. The stance phase plantarflexion moment is nearly normal with the Syme prosthesis (B), suggesting effective anterior support during roll-over. In contrast, there is virtually no plantarflexion moment throughout the entire gait cycle with the partial foot prosthesis. |
|
| Source: http://www.motionanalysis.com/pdf/gcmas2002/murphy.pdf. |
|
Little Advantage
 The cover of thisScandinavian text illustrates a silicone prosthesis that is worn full-time, along with a lightweight laminated carbon fiber AFO that can be applied over the prosthesis for more vigorous activities. The limited motion ankle joint in the AFO permits free plantarflexion for shock absorption during loading response but limits dorsiflexion in terminal stance to protect the anterior surface of the residual limb and provide more effective passive rollover. |
| Reprinted with permission from Partial Foot Amputations |
Forefoot ablations are usually considered relatively minor
losses compared to more proximal foot amputations, but recent gait studies
suggest that there is little advantage once the metatarsal heads have been
removed. However, the relatively balanced muscle picture that accompanies such
procedures does seem to minimize the risk of progressive equinovarus deformity
compared with more proximal foot amputations.
Higher activity levels will require more extensive protection of
the residual limb even though most of the foot remains intact. Modified foot
orthoses plus soft toe filler may suffice for limited ambulation, while
inframalleolar “slipper style” prostheses provide more forefoot
protection for moderately active people. A bivalve prosthesis, analogous to the
Syme type, offers maximum protection for high impact activities but
unfortunately also eliminates ankle motion. In all cases, specific footwear and
shoe modifications should be considered to augment the effectiveness of the
prosthosis.
For more
information:
- Boyd LA, Rao SS, Burnfield JM et al. Forefoot rocker
mechanics in individuals with partial foot amputation (abstract), Gait
& Posture, 1999; 9:144.- Dillon, MP. Biomechanical models for the analysis of
partial foot amputee gait. Doctoral thesis, Queensland University of
Technology, 2001 [http://adt.library.qut.edu.au/adt-qut/public
QUT20011008.094224/].- Gage JR. Gait analysis in cerebral palsy.
McKeith Press, London, 1991.- http://www.motionanalysis.com/pdf/gcmas2002/murphy.pdf
- Mann RA, Poppen NK, O’Konski M. Amputation of the
great toe: a clinical and biomechanical study. Clinical Orthopedics and
Related Research, 1988;226:192-205.
