Lower Limb Prosthetics

Surgical Considerations

When confronted with a phalangeal amputation, complete disarticulation is preferred to partial toe amputation ( Fig. 35-2 ). A five-toe metatarsophalangeal (MTP) disarticulation may be indicated in patients with gross deformity when the toes are rigid, functionless, and painful. Occasionally, this procedure is also indicated in patients with multiple painful MTP dislocations or significant painful clawing of the toes in the vascular-compromised foot. However, in most cases, transmetatarsal amputation is preferred to disarticulation of all five phalanges 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 due to the high risk of hallux deformity and inability to wear standard shoes ( Fig. 35-3 ).

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 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 area 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. 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, decreasing 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 poor. A custom-molded orthosis or shoe insert should be provided for the dysvascular or diabetic patient for protection of the remaining foot from further injury or amputation ( Fig. 35-4 ). Amputation of two or more phalanges can create some gait deviation, depending on which toes have been amputated. Use of a high-quality over-the-counter shoe with a rocker forefoot is often helpful in these cases. A carbon-fiber plate in the shoe can be used to offset any shoe deformation at late stance, but care must be taken not to make the shank of the shoe too stiff so that it adversely affects late stance phase during gait.

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. 35-5 ).

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 often 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, provide extra depth especially in the diabetic and dysvascular population, and a rocker-bottom shoe may assist with return to normal gait. However, over-the-counter shoewear can be adequate if there is appropriate width and depth for a “misshapen” foot.

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. Long-term observation of these patients is needed because the increased pressure under the remaining toes can lead to pain, ulceration, and/or deformity. 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 or discomfort ( Fig. 35-6 ).

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 as well as 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. 35-7 ). Studies have shown an increase in callosities, ulceration, balance problems, and pain with multiple ray amputation. When the option is to remove several central or lateral rays versus moving to a higher amputation level (a transmetatarsal or Lisfranc amputation), the ray resection will provide 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. Ray resection is most successful if no more than two rays are resected.

Prosthetic and Biomechanical Considerations

Custom-molded insoles may be used to distribute pressure evenly over 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. 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. Over-the-counter shoewear may be available to meet these needs.

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.

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. The residual limb is acceptable both in function and in cosmesis, despite the splaying of the metatarsals and widening of the foot that is 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 muscle insertions are needed to maintain a balanced and functional foot. Surgical reduction and prevention of an equinus contracture is critical to a successful outcome.

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 as well as decreased late stance phase moments has been recorded. This loss of stance-phase dorsiflexion may be associated with the missing forefoot and decreased ground reaction force at the distal end of the foot at late stance, leaving a functional equinus contracture potentially leading to ulceration and distal metatarsal pain. Therefore, some form of protective footwear should be used in this population as a minimum treatment option. Further, an intact or replanted tibialis anterior tendon is crucial to ensure proper support of the medial column. Achilles tendon lengthening should be considered as well, since current research has shown limited functional use of the gastrocnemius-soleus complex in patients with transmetatarsal amputation, irrespective of the type of prosthesis they use, as well as to decrease the propensity to develop equinus contratures.

Scar and graft tissue should be reduced, especially at the weight-bearing surfaces of the plantar and distal aspects of the residual limb, as 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, the loss of cosmesis, and it also 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 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. 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. These characteristics will permit the amputee to walk such that the center of pressure extends beyond the end of the residual limb under loading.

Highly active patients with a trans-metatarsal amputation benefit from an anterior panel extending up the shin, such as an ankle–foot orthosis (AFO) with incorporated protective partial foot prosthesis, and a well-fitting shoe. This distributes the late-stance dorsiflexion force of toe-off proximally around the anterior tibia ( Fig. 35-8 ). It is important that the footplate of the AFO 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. Furthermore, the distal portion of the residual limb is well protected with this device. An AFO with an anterior panel appears to be the most effective design for return to higher levels of function after transmetatarsal amputation, and carbon-fiber AFO designs work quite well for this population. 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. 35-9 ).

Physical therapy may be required for gait abnormalities after the fitting of a prosthetic device in this population. Before a prosthetic fitting, many patients with transmetatarsal amputations ambulate with a flatfoot gait, often with the foot externally rotated, with little or no heel contact. Most patients do not require prosthetic intervention for walking short distances or for simple transfer because the length of the limb is preserved; however, this activity should be discouraged in the at-risk vascular population, as barefoot or unprotected ambulation can lead to further injury and limb loss.

Metal upright orthoses have little place in modern treatment of a partial foot amputation because of their excessive bulk and weight. They continue to be used for various applications when strong control of the ankle is necessary, where availability of materials is limited, or when fluctuating edema affects the ability to utilize a standard AFO design.

Lower demand patients may use lower profile prosthetics, but they should not expect them to improve their gait. It is evident that low-profile prostheses and orthoses have no impact on the movement of the center of pressure in late stance phase and thus are simply biomechanically accommodative interventions that will not affect step length or improve symmetry in gait. A resistive force 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. 35-10 ) is also ineffective in the restoration of normal gait.

In patients with limited ambulation skills a custom shoe insert with toe filler (without AFO or proximal support) can help to accomplish their goals if they ambulate with a flatfoot gait (i.e., no real heel strike or terminal stance). The insert must be supportive and well padded around the distal end. The shoe must also be comfortable, especially around the proximal-anterior edge of the remnant foot, since some pressure will be absorbed by this area during the latter portions of gait, even with a flat foot pattern. The sole of the shoe must have some rigidity as well because a shoe that is too flexible increases pressure over the distal end of the limb and at the anterior-proximal edge of the shoe ( Fig. 35-11 ). Patient education on the pitfalls of these devices is critical as they provide no protection, no improvement in function, and can be subject to premature failure. Options include the Imler partial foot prosthesis , Chicago boot, or the Lange partial foot prosthesis . The Lange partial foot prosthesis is somewhat more cosmetic, using prefabricated toes to enhance the appearance. However, these devices are limited in function, difficult to adjust, and have little long-term value to this population. The ultimate device for the achievement of function, comfort, and cosmetic appearance continues to be elusive.