Successful prosthetic management after amputation of the lower extremity requires knowledge of the mechanics of the normal foot and ankle, as well as the biomechanical consequences of each amputation level.¹¹ Since the 1950s, the changes in prosthetic care have been in the area of materials and techniques, not with the mechanics of using such a device. These material and technique changes have had a significant impact on the overall function and comfort of an amputee using a prosthetic device. Materials such as silicone and polyurethane have become mainstream, helping to decrease the mechanical forces applied to areas of skin of a remnant limb. Carbon fiber and other materials have allowed prostheses to become lighter and stronger, decreasing the energy expenditure of the amputee during gait. All this is allowing the amputee to claim successes not previously achievable.

The latest evolution in prosthetic design is now beginning to emerge. Since the wars in Iraq and Afghanistan, large numbers of soldiers are returning with limb amputations. In addition, significant dollars are being applied to research in amputation care.

The role of the prosthetist is to provide an artificial replacement to compensate for the biomechanical loss of function caused by amputation. The obstacles in designing prostheses are material acceptance and comfort, compensation for lost function without restriction of the remaining joints, cosmetic appeal, and the often continuous changes in the size and or shape of the residual limb. The ultimate success of treatment, however, may be judged only by, and remains the responsibility of, the patient.

Partial Foot Amputation

Selecting the Level of Amputation

Determining the level of amputation is often controversial and regionally specific. Traditionally, maintaining length when performing partial foot amputation was standard. Today, quality is more critical than quantity. Current analysis has shown that the former goal of maintaining a longer lever might not affect function or quality of gait and that generation of power across the ankle is significantly decreased irrespective of partial foot length, once the foot is amputated proximal to the metatarsal heads.¹³

Contouring of the cut bones to provide a smooth surface for the tissues to travel over and to provide adequate soft tissue without excessive bulk are essential to fitting of the prosthesis and to comfort during ambulation. Myodesis or myoplasty decreases the possibility of adherent scarring and firms up the residual limb. Muscle transfer and tendon transfer balance the musculature of the foot for improved function and prevention of contractures.

In addition to surgical concerns, the patient’s age, gender, and previous (and future) activity levels play a role in determining the most beneficial level of amputation. The specific advantages of one amputation level over another are discussed in detail later, but several general principles should be considered when determining the appropriate amputation level. Skin or soft tissue adhesions, scars or skin grafts, and the distance from the distal end of the residual limb to the floor greatly affect the outcome of rehabilitation.

Skin and soft tissue adhesions should be avoided or removed (if present) at the initial surgery because the movement of the residual limb within a prosthesis or shoe is often significant enough to cause an ulceration of the skin at the point of adhesion. Revision of an amputation for adhesions should only be considered after appropriate prosthetic fittings have been attempted and have failed.

Adding to the controversy over selecting the amputation level is the presence of scar tissue and skin grafts (Fig. 29-1). In cases of trauma, sometimes a decision must be made between saving a partial foot amputation with the addition of a skin graft or performing a Syme or below-knee amputation without additional surgical intervention. In these cases, grafts and scars should be avoided if possible. However, if eliminating grafts and scars requires the next higher level of amputation, the patient’s sensation, age, activities, and future goals must be carefully considered before selecting the level of amputation.

Figure 29-1 Plantar scar tissue after burn and partial foot amputation, making prosthetic fitting and function difficult. Decreased sensation over scar tissue and lack of compliance led to significant ulceration. Conversion to higher level of amputation should be considered if skin is intolerant of normal ambulation.

The final general consideration when deciding between a partial foot amputation or a Syme level (or higher) amputation is the distance from the end of the residual limb to the floor. If an adult patient is to undergo a partial foot amputation, the distance from the plantar surface of the foot to the floor will be less than 1 cm. If prosthetic restoration is required for an athlete or active person, there is little room for the components required to replace the missing function during gait, and the level selection is critical. After an ankle disarticulation, however, at least 3.5 cm of space remains between the distal end of the residual limb and the floor, allowing the integration of various components. It is therefore necessary to understand the goal and activities of the patient before commencing with surgical ablation of all or part of the foot.

Postoperative care should focus on wound healing, contracture prevention, and pain management. Ambulation on the contralateral limb, if possible, should be encouraged early to aid in the rehabilitation. An early physical therapy and prosthetic consultation greatly assists with a successful outcome and improved patient expectations because often the expectation of function with a prosthetic device is different than what the device can provide.

The following sections discuss the traditional levels of foot amputation. Each section outlines the advantages and disadvantages of each amputation level from a functional point of view, with and without prosthetic intervention. The goal is to assist in determining the appropriate level for amputation surgery. There has been a recent increase in work being done on partial foot amputation that will be addressed as well.

Phalangeal Amputation

Surgical Considerations

When confronted with a phalangeal amputation, complete disarticulation is preferred to partial toe amputation (Fig. 29-2). A five-toe metatarsophalangeal (MTP) disarticulation may be indicated in patients with gross deformity when the toes are rigid, functionless, and painful and, occasionally, in patients with multiple painful MTP dislocations or significant painful clawing of the toes in the vascular-compromised foot. However, in a majority of cases, disarticulation of all five phalanges should be advanced to transmetatarsal amputation because of the potential for skin breakdown under the metatarsal heads. This is especially true in patients with vascular disease and associated neuropathy, who are at high risk for injuring an unprotected foot. The great toe should not be left if the rest of the toes are to be amputated because of potential hallux deformity and inability to wear standard shoes (Fig. 29-3).³⁶

Figure 29-2 Necrosis of toes resulting from blood clot. Ultimate amputation will be determined by circulation status. Metatarsophalangeal disarticulation should be avoided in this case because of difficulty with future ambulation and potential for skin breakdown at the metatarsal heads. This patient ultimately went on to transmetatarsal amputation and returned to full ambulation.

Figure 29-3 Temptation to salvage a remaining single digit in a traumatic amputation, such as this, should be avoided because the remnant toe and scar tissue would lead to increased long-term problems with skin breakdown and footwear compared with a transmetatarsal amputation.

Prosthetic and Biomechanical Considerations

Amputation of a single phalanx usually has minimal effect on gait or comfort during ambulation. Ankle moments, power, and propulsion are minimally affected with this level of amputation. However, if the toe was painful before ablation, some alterations to the gait pattern may be observed. Mann et al ²² evaluated patients with surgical ablation of the great toe with no other lower extremity disease. During gait, these patients merely lateralized the center of pressure at the later portions of the stance phase on the amputated side, compensating for the missing toe. However, the reduction of load-bearing areas can result in increased pressure over the metatarsal heads. In the dysvascular population, if adequate protection of the remaining foot is not provided, subsequent ulceration can occur around the lateral portion of the first metatarsal head, extending to the second or third metatarsal. There may also be some loss of push-off at late stance phase, retraction of the sesamoids, and metatarsalgia. A custom-made shoe or custom accommodative insert (or both) is indicated in these patients to protect the remaining foot.¹⁴ This may or may not include a filler for the ablated toe. Judicious use of the toe filler is warranted because often they can irritate the adjacent toes and create new problems for the user. Further, there appears to be little evidence of the efficacy of toe fillers in preventing adjacent toe angulation.

Ablation of the second toe (alone or in conjunction with other central phalanges) often requires an artificial toe filler to prevent a hallux valgus deformity. Removal of the entire second ray might be a more viable option because the foot will naturally narrow and allow use of normal footwear without the possibility of deformity of the hallux.

For other central or multiple phalangeal amputations, the MTP joints of the adjacent toes can deform unless some type of semirigid toe filler is provided. Again, there is currently little evidence demonstrating that toe drift can be prevented with the use of an orthosis, and compliance is often a concern. A custom-molded orthosis or shoe insert should be provided for the dysvascular or diabetic patient for protection of the remaining foot from further insult or amputation (Fig. 29-4). Amputation of two or more phalanges can cause some gait deviation, depending on which toes have been amputated. A carbon-fiber plate might be required in the shoe to offset any shoe deformation at late stance, but care must be taken not to make the shank too stiff so that it adversely affects late stance phase during gait.^11,40

Figure 29-4 Custom-molded insert for shoe after triple phalangeal amputation.

Cosmetic toe prostheses may also be custom fabricated and may be used in conjunction with the existing arch support or independently. Using the contralateral toe as a model, the cosmetic toe filler may contain layers of color, and even hair, to enhance the appearance. Such a device might have minimal or no functional quality but can appear quite lifelike (Fig. 29-5).

Figure 29-5 Custom cosmetic prosthesis colored and matched to the contralateral foot.

With MTP joint disarticulation of all toes, a prosthesis or orthosis must be provided to unweight the metatarsal heads and provide support to the arch, which is usually elevated after this amputation. The shoe shank must also be somewhat rigid to resist the dorsiflexion ground reaction force late in the stance phase. A custom-made shoe may be indicated for protection of the metatarsal heads, especially in the diabetic and dysvascular population, and a rocker-bottom shoe may assist with normal gait.

Ray Amputation

Surgical Considerations

Single-ray resections at the second, third, fourth, or fifth metatarsal have been quite successful and leave a functional partial foot that may not need prosthetic intervention.⁴³ Long-term observation of these patients is needed because the increased pressure under the remaining toes can lead to ulceration.^10,39 Loss of the first ray leaves the foot somewhat less effective than with removal of the lateral rays because the foot does not balance well with the loss of the windlass mechanism, and the increased pressure under the remaining metatarsals can lead to further ulceration (Fig. 29-6).^7,9,35

Figure 29-6 Removal of medial two rays unbalances foot and subjects it to further mechanical problems. Removal of an additional medial ray would subject the foot to unacceptable forces and ultimate failure.

When removing two or more rays, especially on the medial side of the foot, prosthetic or orthotic intervention for gait, comfort, and protection of the remnant foot and appropriate footwear become essential. Removal of two central rays also narrows the foot significantly, makes wearing shoes difficult, and can increase the potential for skin breakdown (Fig. 29-7). Studies have shown an increase in callosities, ulceration, balance problems, and pain with multiple ray amputation.^10,17,38 When the option is to remove several central or lateral rays or move to a higher amputation level (a transmetatarsal or Lisfranc amputation), the ray resection provides a more functional foot than a higher amputation does. When the option is to remove several medial rays or proceed with a transmetatarsal or Lisfranc amputation, the patient’s general health and activity level must be carefully considered before determining the appropriate level. (See the discussion of transmetatarsal amputation.)

Figure 29-7 Removal of central rays in a young patient can leave a functional foot but one that is difficult to fit into traditional footwear.

Prosthetic and Biomechanical Considerations

Custom-molded insoles may be used to distribute pressure evenly over the remainder of the foot after ray amputation to stabilize and protect the remaining foot. Softer, more conforming foam generally is used against the skin, whereas a firmer material is used for the base.25,26,37 Custom-made shoes may be required if significant function is lost after amputation or if the foot is severely misshapen and unable to fit into a standard shoe. Rocker-bottom soles are easily applied to shoes if ambulation becomes difficult or painful. Although rocker-bottom shoes have been advocated in many conditions such as this, their use is somewhat limited by the appropriate shoes, cosmesis, patient compliance (or lack thereof), and reimbursement issues.

Overall, the foot after ray resection is highly functional and might not require prosthetic intervention, especially if the medial column remains unaffected. Protection of the remaining partial foot and the contralateral limb should be the primary concern when treating patients with vascular disease and neuropathy or pain.

Transmetatarsal Amputation

Surgical Considerations

Transmetatarsal amputation was first described in 1855, but success at that time was limited. After the advent of antibiotics, this level of amputation was performed routinely in patients with limited gangrene, deformities of the toes, trauma to the forefoot, rheumatoid arthritis, or controlled infection.²³ The residual limb is acceptable both in function and in cosmesis, despite the splaying of the metatarsals often present after surgery.

Anecdotally, the length of the residual limb should be maintained as much as possible; however, a shorter residual limb that heals well, is better than a longer one that requires multiple procedures or open amputation. In addition, research is showing little functional difference between a long transmetatarsal amputation and a short transmetatarsal amputation. The base of the metatarsals must be preserved because of muscle insertions needed to maintain a balanced and functional foot. Surgical reduction and prevention of any potential equinus contracture must occur if the outcome is to be successful.

Biomechanical evaluation of transmetatarsal amputations shows significantly greater plantar pressure when compared with intact feet, most likely because of the smaller surface area. In addition, increased forces have been noted at the distal end of the limb, and a significant lack of dynamic dorsiflexion during late stance has been recorded. This loss of stance-phase dorsiflexion may be associated with the missing ground reaction force at the distal end of the foot at late stance, leaving a functional equinus contracture ¹⁷ with the potential ramifications of ulceration and distal limb pain. Therefore some form of protective footwear should be used in this population.¹⁷ Further, an intact or replanted tibialis anterior tendon is crucial to ensure proper support of the medial column. Achilles tendon lengthening should be considered for this population because current research has shown limited functional use of the gastrocnemius-soleus complex in patients with transmetatarsal amputation, regardless of the type of prosthesis they use.^5,14,15

Scar and graft tissue should be reduced, especially at the weight-bearing surfaces of the plantar and distal aspects of the residual limb, because these areas are often exposed to high stress during ambulation. All bony prominences should be smoothed and bone ends beveled to decrease the potential for injury from within the soft tissue envelope. Early mobility of the ankle can have positive implications for functional outcomes. Pain is often a concern at this level of amputation, and early pain intervention may be needed to enhance long-term outcomes.

Prosthetic and Biomechanical Considerations

Any device designed for a transmetatarsal amputation must compensate for the loss of the later portions of stance phase and the loss of cosmesis, and it must allow appropriate footwear and a comfortable gait pattern. With the removal of the metatarsal heads, distal weight bearing is no longer practical. Therefore any prosthetic device not only must protect the cut ends of the bones, both distally and on the plantar surface, but also must resist the natural dorsiflexion moment of the prosthesis on the residual limb at late stance phase.15,19 The only devices shown to be effective in improving late stance-phase kinematics (kinetics and temporal spatial factors) are designs that control dorsiflexion and have a relatively stiff forefoot that will permit the amputee to walk such that the center of pressure extends beyond the end of the residual limb under loading. It appears that low-profile prostheses and orthoses have no impact on the movement of the center of pressure in late stance phase and thus can be considered to be biomechanically accommodative interventions that will not have the potential to affect step length (Fig. 29-8).⁵

Figure 29-8 A carbon-fiber ankle–foot orthosis in conjunction with a custom, protective, partial foot prosthesis offers late stance-phase control because of the proximal-anterior–directed forces, with protection of the foot and a filler for the shoe. The configuration provides some cosmetic appeal, lightweight dynamic response, and moderate improvement in footwear options over previous designs.

A resistive force must be placed on the more proximal portion of the dorsum of the foot (through the shoe or prosthesis) or via an anteriorly directed force proximal to the calcaneus (Fig. 29-9). The late-stance dorsiflexion force can be overcome with a device that has an anterior strap proximal to the malleoli, as in an ankle–foot orthosis (AFO) design (Fig. 29-10). It is important that the foot plate be stiff enough to overcome the force needed to support the body in late stance but not so stiff that the third rocker is adversely affected and the patient is unable to roll over the foot. A rocker-bottom sole could be added to any shoe to allow a reduction of distal metatarsal pressure, if the patient is willing to use such a device (Fig. 29-11).