Forefoot, Toe Amputations and Prostheses: The Quest to Restore Normal Ambulation

O&P Business News presents the third installment in a series of articles that examines lower level amputations.

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

  photo
  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.

drawing drawing
drawing
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.

photo  
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

 image
  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.
 photo
 photo
  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.

drawing “Bootie” style prostheses are sometimes necessary to provide supramalleolar suspension for transmetatarsal amputations and more proximal levels. The flexible material does not restrict active ankle motion, but provides no additional support or protection for the residual limb. Earlier designs were made from molded leather, but more perspiration-resistant flexible plastic and silicone laminates have displaced leather for most current applications.
Reprinted with permission from Partial Foot Amputations
 
 

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.

photo photo
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.

graphs
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

photo The cover of this Scandinavian 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.

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

  photo
  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.

drawing drawing
drawing
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.

photo  
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

 image
  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.
 photo
 photo
  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.

drawing “Bootie” style prostheses are sometimes necessary to provide supramalleolar suspension for transmetatarsal amputations and more proximal levels. The flexible material does not restrict active ankle motion, but provides no additional support or protection for the residual limb. Earlier designs were made from molded leather, but more perspiration-resistant flexible plastic and silicone laminates have displaced leather for most current applications.
Reprinted with permission from Partial Foot Amputations
 
 

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.

photo photo
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.

graphs
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

photo The cover of this Scandinavian 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.