This is the second installment in a series of articles about partial foot amputations and prostheses. It builds on many of the concepts covered in the first article about Syme ankle disarticulation (see O&P Business News, Nov. 1, 2002). Clinically, hindfoot and midfoot amputations can be subdivided into two broad groups — those with ankle motion remaining and those lacking ankle movement.
Biomechanical Deficiencies
Numerous hindfoot and midfoot amputations are described in the literature and encountered in clinical practice, and each is somewhat unique. Space constraints limit this discussion to only the most common named entities.
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Figure 1. Hindfoot amputations that have no ankle motion are biomechanically similar to the Syme ankle disarticulation, except that the limited shortage makes it impossible to utilize most commercially available prosthetic feet. Amputation levels shown are Chopart, modified Chopart, Pirogoff and Syme (left to right). Reprinted with permission from Partial Foot Amputations. |
All hindfoot and forefoot amputations have similar biomechanical deficiencies related directly to the loss of the forefoot lever arm and its associated bony architecture and musculature. As a consequence, they all present with:
- Substantially reduced plantar weightbearing surface
- Loss of active pronation and supination during gait
- Absence of active push-off
These functional deficiencies result in a gait that is wide based, slower and less energy efficient than normal. More detailed discussion of the biomechanics of partial foot prostheses can be found in Partial Foot Amputations, second edition, by Soderberg et al.
Prosthetic Limitations
Hindfoot and midfoot amputees experience all of the prosthetic limitations common to the Syme level. In addition, because the residuum is now contained within the shoe, the use of standard footwear is not always possible. The bulk of the socket that is necessary to protect the plantar surface from excessive weightbearing forces also distorts the shoe and often requires the use of larger volume footwear than the normal foot needs.
Figure 2. In the 1800s when most hindfoot and midfoot amputations were first developed, the only option for many patients was a biomechanically unsophisticated elephant boot. This appliance is still common in many parts of the developing world when modern prosthetic care is unavailable. Reprinted with permission from Partial Foot Amputations. |
Due to the bulbous, irregular contours of hindfoot and midfoot amputations, a bivalve socket is generally necessary to permit donning. This is an inherently weaker structure than a one-piece socket, so even though the typical hindfoot prosthesis may be specially reinforced, it is much less durable than a Syme or transtibial prosthesis of equal weight and thickness.
Perhaps most importantly, the limited shortage that hindfoot and midfoot amputations present generally precludes the use of commercially available prosthetic feet. The prosthetist must typically custom make the foot for these levels, and if the leg length discrepancy is to be minimized, there is room for only a thin foot plate and toe filler. This gives rise to the paradoxical situation where the athletic transtibial amputee (despite having a much higher amputation level) can run with a vertical shock absorbing, carbon fiber dynamic response foot, but the hindfoot and midfoot amputee generally cannot run effectively or for any distance, having only a thin toe plate for propulsion.
Syme Analogues: Boyd and Pirogoff
The Boyd and Pirogoff hindfoot amputations are biomechanically and functionally identical to the Syme ankle disarticulation in that the ankle mortise is fused and all active motion is eliminated. The anatomical difference is that part of the calcaneus remains, and it is fused into the ankle mortise. Prosthetic management of these levels is also identical to that of the Syme procedure except that the minimal shortage makes it impossible to use most commercially available prosthetic ankle and foot mechanisms. Figure 1 illustrates the relative shortage of selected hindfoot amputations.
Nicolai Ivanovich Pirogoff was considered the greatest Russian surgeon of his time and remains one of the most renowned military physicians in history. He is believed to be the first surgeon to use plaster of Paris dressings to treat bone fractures. After being forced to amputate the feet of many soldiers during the Crimean conflict due to severe frostbite, he published his technique to address these thermal injuries in 1854. Pirogoff described his surgery as an “osteoplastic amputation” intended to create a residual limb that permitted distal end bearing.
Dr. Harold Boyd was a surgeon from Memphis, Tenn., who reported his method of fusing the calcaneous to the tibia in 1939 in an effort to create a “simpler Syme amputation” that could be fitted with a modified shoe. The slight shortage these amputations create is ideal to accommodate a device called an “elephant boot” — a simple leather pouch that encases the residual limb and fastens above the malleoli with a leather drawstring. These levels are sometimes selected in developing parts of the world when more sophisticated prosthetic care is not available locally. Figure 2 depicts a custom fabricated elephant boot made by a shoemaker.
When modern prostheses are available, these levels offer no pronounced advantages over the Syme ankle disarticulation. The exception is in the case of acquired amputation in the pediatric population, often due to lawn mower injuries or similar trauma. Children’s surgeons have advocated the Boyd level for these patients because it preserves more length than the Syme procedure, and it is not necessary to dissect the calcaneous from the heel pad. Although the space limitations are a problem at first, as the child grows and the involved leg lags behind the contralateral side, the shortage gradually increases to the point that these patients can use sophisticated prosthetic ankle-foot components as they approach skeletal maturity.
Absence of Ankle Motion
Partial foot amputations, by their nature, are both prosthetic and orthotic challenges. In fact, such devices are often referred to as “prosthoses” because they frequently contain elements of both devices. Specially modified or custom made shoes can suffice in certain cases, and shoe modifications are often helpful in improving the patient’s gait. Consequently, partial foot patients may be treated by pedorthists, orthotists or prosthetists. Ideally, the clinic team should consider all these potential options since an individualized combination of these modalities is often the most effective management for a specific amputee.
The array of possible devices for partial foot amputees can be classified on a continuum from the least to the most biomechanically sophisticated. The elephant boot is the least effective biomechanical solution, as it provides minimal protection to the residual limb, does nothing to reduce stride asymmetry and subjects the remaining plantar skin to full weightbearing forces. The custom made “short shoe” is more effective, since the interior includes a custom molded foot orthosis to reduce the peak loading on the foot remnant. The integral steel shank prevents the shoe from bending, protecting the end of the amputation from excessive pressures during rollover. The mini-rocker sole and the abbreviated forefoot encourage early unloading of the affected limb, providing further protection and increased comfort. The primary drawbacks of the “short shoe” are the unnatural appearance and short stride that result from this design.
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Some hindfoot and midfoot patients will be satisfied with a high top shoe, provided they have no plans to engage in vigorous activities or to walk significant distances. A steel shank is mandatory to protect the end of the residuum, and a rocker or roller sole is almost always helpful in reducing gait anomalies. In addition to a custom foot orthosis, they will need a toe filler of some type to prevent the forefoot of the shoe from collapsing. This approach is more acceptable to some individuals than the short shoe, because at least the footwear is normal in appearance.
Another option is to create a flexible supramalleolar prosthesis with a soft forefoot. Although many materials can be used, custom molded silicone devices have proven particularly successful in this configuration. It is important to note that all low profile devices discussed thus far should be considered limited activity designs, because they do not unload the foot remnant or provide significant biomechanical assistance during rollover.
In contrast, when it is necessary to unload the plantar skin, a bivalve socket is the preferred solution. This design can shift significant weight bearing forces away from the foot and onto the shin and calf tissues, although this is the heaviest and most bulky option. Figure 3 shows an adult with an acquired Boyd amputation, highlighting the limited shortage and the “inset” of the prosthetic foot due to the normal varus attitude of the tibia. Use of an outflare sole is often helpful to supplement mediolateral stability in such cases, even though many patients refuse to accept visibly modified footwear.
Ankle Motion
French surgeons developed the ankle-sparing hindfoot and midfoot amputations.
Jacques Lisfranc (1790-1847) was a field surgeon in Napoleon’s army serving on the Russian front who had studied under Dupuytren. He wrote about a new amputation technique, to treat forefoot gangrene, by disarticulating the forefoot from the midfoot. Soldiers who were dragged by their horse with the forefoot still trapped within the closed stirrup of that era often sustained metatarsal fractures, and the forefoot-midfoot junction eventually became known as the Lisfranc joint.
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Francois Chopart developed the amputation that bears his eponym in response to the horrifying mortality that followed metatarsal amputations of the day, which were usually performed to prevent the spread of forefoot gangrene. His disciple, Lafitteau, described the procedure in a 1792 article. Amputating through the hindfoot-midfoot junction decreased the number of post-operative deaths from infection by 50 percent in this pre-antisepsis era, as compared with amputating through the forefoot bones. Chopart succumbed to cholera in 1795.
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From a biomechanical standpoint, these amputations are similar. Although it seems intuitively desirable to preserve active ankle motion, this has proven to be of only limited practical value due to the short bony lever arm remaining. Because of this “leverage deficiency,” it is impossible for these patients to actively push off and raise their center of mass. Even with the best available prosthetic restoration, the residual limb cannot tolerate the forces necessary for active push off, although an artificial limb may restore some measure of passive rollover.
These amputation levels are inherently unbalanced, with all plantarflexors remaining but few, if any, dorsiflexors intact. This has two important consequences. First, the weak dorsiflexor control often results in “foot slap” since the rate of descent of the sole to the floor in early stance is largely uncontrolled. This is particularly true when the amputee tries to walk at a rapid pace or down inclines. Secondly, these levels of amputation have a well-documented tendency to develop an equinus contracture over time. Although an optimal prosthesis will try to resist this problem, the patient’s leverage deficiency makes it impossible for the artificial limb to prevent this deformity from developing.
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The only definitive method to avoid this complication is for the surgeon to balance the foot by carefully reattaching the severed dorsiflexor muscles. Ideally this is done at the time of initial surgery, although it can sometimes be done as revision surgery. Many surgeons advocate lengthening the Achilles’ tendon at the same time to weaken the plantarflexors and aid in achieving muscle balance. Although some concern has been expressed about weakening the gastroc soleus, since these patients are unable to use these muscles for effective push off, this is not a major loss.
Management
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Unfortunately, patients with Chopart or Lisfranc amputations have little biomechanical advantage over those with a Syme ankle disarticulation. Because these levels result in no length discrepancy, the prosthesis inevitably makes that leg somewhat too long; the use of a heel lift in the contralateral shoe is commonplace. The socket fits down inside the shoe and adds considerable bulk, making shoe fitting difficult.
Most prosthetists prefer to preserve active ankle motion, at least to some degree, when it is present. If nothing else, retaining plantarflexion motion in early stance simulates normal kinematics. However, an articulated appliance is only feasible when the foot remnant can tolerate substantial end weight bearing as well as the forces inherent in rollover.
Many variations based on thermoplastic ankle-foot-orthoses have been reported in the literature. Most prosthoses resist plantarflexion but permit passive dorsiflexion in hopes of delaying the onset of contracture. A custom molded AFO with toe filler is perhaps the most common device of this type, and it can be made from semiflexible or rigid plastics. Depending upon the trim lines provided, these devices may allow nearly full ankle motion, or only a jog of movement in late stance.
Some patients, particularly those with congenital absences, find that a low profile prosthesis can suffice. Figure 4 shows one type of silicone elastomer device for a congenital Lisfranc analogue. Many functionally equivalent alternatives using different materials have been described in the literature over the years. Although these are the most cosmetic restorations for this level of loss, some amputees cannot tolerate the skin-tight fit and limited weight bearing protection they offer.
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The recent availability of thin, lightweight carbon fiber AFOs has led to modification of these orthoses for the partial foot population. The anterior “shin guard” style shown in Figure 5 helps spread the loads generated during rollover thereby protecting the end of the residuum from excessive pressures. In this example, the patient uses the silicone restoration for light duty activities but adds the carbon fiber prepreg AFO over the prosthesis for activities where a stiffer anterior lever arm is desired.
Prosthoses based on articulated AFO concepts are another option that has proven successful when the patient can tolerate full weight bearing. One advantage of articulated designs is that they can enhance mediolateral ankle stability. Figure 6 shows a custom molded carbon fiber prosthosis that provides mediolateral stability and dorsiflexion control while allowing unrestricted plantarflexion motion.
When partial unweighting of the plantar surface will suffice, articulated AFO styles that fully enclose the shin may be effective, as illustrated in Figure 7. When it is necessary to significantly unload the plantar skin or to provide maximum rollover protection, a solid ankle bivalve prosthesis is almost always required. Although this eliminates active ankle motion, the increase in patient comfort and skin protection may justify this approach in specific cases.
Functional Levels
In recent decades, rehabilitation experts have increasingly acknowledged the impact of the patient’s functional level of independence on prosthetic design. This can be illustrated with reference to the Medicare K levels. Level 0 and level 1 amputees, because they are only physically capable of standing or indoor walking for short distances, may do well with a biomechanically simple prosthosis. A modified foot orthosis, minimally altered shoe or very low profile prosthesis might provide enough support and protection for their limited functional needs.
Many level 1 individuals who are also elderly, dysvascular and generally debilitated have cardiopulmonary limitations and problems with the contralateral limb that preclude more active walking. It is best to start these patients with the simplest alternative likely to fully meet their ambulation needs.
Level 2 individuals are independent indoors and can negotiate some outdoor situations. Their treatment must be individualized according to their aspirations and physical capability for reaching higher activity levels. Many do well with one of the modified AFO designs.
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Level 3 amputees are independent community ambulators, and their higher activity level subjects the residual limb to much greater forces than the level 2 patient experiences. Careful consideration must be given to protecting their amputations from excessive stress, and the more biomechanically elegant designs are commonplace for this group.
The partial foot amputee who aspires to level 4 function, which includes high impact activities, presents a difficult challenge. As already noted, it is not possible to restore active push off. In general, the solid ankle bivalve designs that emphasize unloading the plantar surface are recommended for this group. Even still, these amputees will have a noticeably abnormal gait and find it difficult or impossible to engage in distance running or jumping activities.
Special Case: Calcanectomy
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Improvements in surgical technique in recent decades have resulted in long-term survival of more of the foot, even in the face of significant trauma. One example is the increasing incidence of salvage of all of the bony architecture of the foot except the calcaneous. Despite the almost normal appearance of these feet, the biomechanical loss suffered when the calcaneous is removed is substantial. Initial contact and loading response are seriously compromised, with many patients adopting a gait pattern of forefoot loading throughout stance.
When the ankle mortise is also involved, motion restrictions and associated pain are common. This may require the use of a solid ankle bivalve prosthesis to effectively shunt hindfoot loading forces up onto the leg.
When the plantar skin is in good condition and the forefoot readily tolerates walking forces, a simple calcaneal prosthosis may suffice, as shown in Figure 8. Although sometimes made from elastomeric foams, most such devices are now custom molded from silicones. In the ideal case, they fit within the confines of a normal shoe, making the amputation and appliance undetectable to the casual observer.
Summary
Hindfoot and midfoot amputations were originally developed when sophisticated prosthetic care was unavailable. Biomechanically, they have no marked advantages over the Syme ankle disarticulation, despite preserving more of the foot. Without proper pedorthic, orthotic and prosthetic management, these patients can only walk or stand for limited periods of time because of the loss of more than half the weight-bearing surface of the foot. Even when residual ankle movement remains, the loss of the biological forefoot lever arm prevents the amputee from actively pushing off. This is true even with a prosthetic restoration.
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There are numerous prostheses, orthoses and shoe modifications that have been noted in the literature to be useful in these cases. As a rule of thumb, the lower the patient’s activity level, the more basic the restoration can be. When maximum unloading and protection during rollover are desired, a solid ankle bivalve socket is almost always required, even if ankle motion remains (Figure 9). For less demanding cases, modified AFOs often suffice, and these are probably one of the most commonly provided devices for this population.
Hindfoot and midfoot amputations are more challenging to fit than Syme ankle disarticulation in part because there is no space under the residual limb to place commercially designed ankle-foot mechanisms. Best practice requires careful assessment of the weight-bearing capacity of the residuum and consideration of each patient’s vocational and avocational aspirations to provide an individualized biomechanical solution combining principles from pedorthic, orthotic and prosthetic devices.
For more information:
- Soderberg B, Wykman A, Schaarschuch R, Persson BM. Partial foot amputations: Guidelines to prosthetic and surgical techniques. Second Edition. Publication 13 from the Centre for Partial Foot Amputees. 2001. Helsingborg, Sweden: Swedish Orthopaedic Association.
E-mail helsingborg@sol-ota.com.
Visit the Web site at www.sol-ota.com.
