When it becomes apparent that the current treatment is not proving effective for a debilitating disease or a trauma induced by accident or warfare, amputation is generally considered the medical intervention of last resort. Indeed, one would exhaustively consider any and all other medical alternatives to save a limb before deeming it necessary to amputate.
However, once the physician has made the decision to amputate, then one has to decide on the level of the amputation. Recent advances such as newer generation antibiotics and endovascular approaches to revascularization have aided in the ability to preserve as much of the affected limb as possible from the effects of disease or trauma.
The effort to preserve the foot, arguably, best illustrates that premise. Beginning from the most distal to the most proximal, amputation levels of the foot include: toe(s), ray and transmetatarsal (TMA) to preserve all or part of the forefoot and structures proximal to it; the Lisfranc procedure to preserve the midfoot and structures proximal to it; the Chopart to preserve the hindfoot; and the Symes amputation level that preserves the lower leg. The benefits of foot preservation include but are not limited to the ability to stand and to walk for short distances without any orthotic or prosthetic device and, for those who have already lost the contralateral lower limb, the ability to transfer with less effort from one place to another.
Let us take a closer look at the Chopart disarticulation at the midtarsal joint between the proximal talus and calcaneus, and the distal navicular and cuboid bones. Historically, research about this level of foot amputation and viable prosthetic devices for the partial foot amputee in general have proved to be indiscriminate and controversial. As in some areas of prosthetics, such as the effective suspension of below-knee prostheses to stay on the amputee’s limb throughout all phases of the gait cycle, overlaps and redundancies exist. At the Chopart amputation level, some fitting and fabrication strategies merged concepts of orthotic and prosthetic disciplines into a hybrid “prosthosis” device.
What Advantages Does The Chopart Amputation Offer?
The Chopart disarticulation was first described by Francois Chopart (1743-1795), a French surgeon. Although the procedure was initially successful, it fell into disrepute because of the equinovarus deformity that developed as a result of unopposed action of the soleus-gastrocnemius muscles and the Achilles tendon on the calcaneus and the anterior tibial tendon medially.
More recently, some have advocated the Chopart disarticulation with appropriate tendon balancing over that of more distal levels such as the Lisfranc or short transmetatarsal. The rationale is that surgeons may obviate the potential equinovarus deformity via the surgical removal of the cuboid bone during the procedure.1 Accordingly, one may entertain this level as a more definitive level of choice over questionable healing or success in comparison to a more distal level. Surgical interventions have included transferring the anterior tibial tendon to the talus, and ankle and subtalar fusion using an intramedullary nail. This method affords rigid control to the rearfoot to prohibit posterior calcaneal extension.2
Other surgeons have attempted to limit the risk of plantarflexion contractures and skin breakdown by contouring the anterior talus and calcaneus bones, resecting several tendons to the neck of the talus and sustentaculum tali bones, closing the wound with an anterior placement of the plantar flap, and performing lengthening of the Achilles tendon.3 These are just a few examples of how bone fusions, tendon transfers and tenotomies have progressed over the decades to preserve and balance all or part of a complex foot structure consisting of 28 bones (about 13 percent of the bones in the body), 33 joints, more than 100 muscles, ligaments and tendons, and a network of nerves, blood vessels, skin and soft tissue.
While each patient and circumstance for amputation is unique, the Chopart level amputation also has its advantages over more proximal levels. It has been well documented that cardiac demand is increased in the patient with any level lower extremity amputation. In fact, this is often directly related to increased morbidity and mortality after amputation. Accordingly, we recognize that the more distal level amputation may ultimately increase longevity. Additionally, as with any level foot or leg amputation, the psychological aspect of preserving as much foot as possible cannot be understated. The surgeon will generally know within a few weeks post-op if the surgical wound will heal successfully. However, the long-term success will clearly be dependent upon the postoperative prosthetic department fitting and patient compliance with the device.
Keys To Addressing Post-Op Complications
The difficulties with the longevity of a fully successful, rather than acceptable, outcome of the surgical procedure itself include the progressive development of equinovarus. We have seen that development despite the lengthening of the Achilles and routing the anterior tibial tendon through the talar neck. In an effort to counterbalance the varus pull of the transferred anterior tibial tendon, the podiatry department of the Miami Veterans Affairs Medical Center (VAMC) instructs its providers to transfer the peroneal tendons to the lateral wall of the calcaneus instead of just “letting them fly.”
When the patient presents at the amputee clinic, the talus and calcaneus remain after the Chopart amputation. The talus receives lateral and medial support from the fibula and tibia bones of the lower leg respectively, and allows dorsiflexion and plantarflexion movement as an ideal outcome. The subtalar joint joins the talus with the calcaneus permitting, ideally, medial and lateral movement of the lower leg over the heel and inversion and eversion moments of the residual foot. In short, the remaining hindfoot affords the amputee multiaxial movement and moments when it is fitted and aligned with a prosthetic, orthotic or hybrid “prosthosis” best suited to the need. That multiaxial movement is what separates the more sophisticated and costly prosthetic feet from those that are more suitable for lower activity level, below or above the knee amputees.
What You Should Know About Above Ankle Prostheses
The most important characteristics of the clinical evaluation are the activity, compliance and motivational level of the patient. It may or may not be related to the surgical outcome but the choice of the prosthetic device depends upon the patient’s ability to bear weight, ambulate and perhaps even to vary cadence. However, there are two basic types of Chopart prostheses: above ankle and below ankle.
Some of the above ankle designs will encase the affected limb and support the body’s weight through the patella tendon and the medial flare of the tibia. Another variation of that design includes the patella tendon bearing ankle-foot orthosis (PTB-AFO), which uses a patent bottom to substitute for the foot. The former Chopart prosthesis, weighing 5 pounds or more but rarely less, uses a very rigid footplate. Both may require a shoe lift on the contralateral side to keep the limbs equal in length, and both are indicated for level 0 to 1 ambulators who cannot bear weight on the affected foot.
Also keep in mind that normal gait may be compromised with either selection. Additional hip and knee flexion is not uncommon during the swing phase of the gait cycle to compensate for a partial foot restricted from total, if not partial, weightbearing and an ankle restricted or bypassed from its function in any and all spatial planes.
Some practitioners have used a solid ankle orthotic in the form of a Charcot Restraint Orthotic Walker (CROW) for the Chopart amputation level. It allows for a variable amount of partial foot weightbearing since one may fabricate this device to capture the load at the patella tendon or fabricate it to bear weight within the walls of the orthotic and throughout the affected limb by using a total contact socket compression technique or a combination of the two. A rocker bottom sole allows the device to further reduce pressure to the distal, anterior aspect of the foot. However, the ankle is restricted from its function in all spatial planes.
Another variation of the above ankle Chopart device is the use of a semi-rigid ankle AFO with a toe filler. The ability to bear weight completely and ambulate, as one may do with a semi-rigid AFO and toe filler, is a critical factor in selecting it over the Chopart prosthesis. One research study has shown there is an extra irregularity of weight progression during the gait cycle with the fixed ankle of the Chopart prosthesis over that of the AFO.4
Other research has shown that the Chopart clamshell prostheses with a rigid toe lever is able to restore the effective foot length and comfortably support the generation of substantial external moments during the terminal stance phase of the gait cycle to a greater degree than the below ankle, Chopart prosthetic device utilizing toe fillers and slipper sockets.5 The distinction between each study contrasts the rigid ankle, toe and foot of the Chopart clamshell prostheses with that of a semi-rigid AFO with toe filler and that of a below the ankle Chopart slipper socket with a toe filler.
Below Ankle Prostheses: Can They Have An Impact?
The second type of Chopart prostheses, orthoses or “prosthoses” involve the below ankle types. Some are characterized by a total contact, below ankle partial foot socket and a soft interface capable of providing total weightbearing. Other designs use silicone or urethane elastomer materials to combine both the socket and interface for the same purpose. Both are fabricated over a modified, positive plaster or CAD-CAM mold of the partial foot. The mold modification process accommodates pressure sensitive and loading areas of the foot. The “prosthosis” will usually include a toe filler and may include a footplate extending from heel to toe. While the effectiveness of the below ankle socket and its full length footplate is questionable at the more proximal, Chopart amputation level, they are of great benefit at more distal amputation levels (i.e. transmetatarsal).6
The below ankle prosthosis (with some variants known as slipper sockets with a toe filler) is more effective when one couples it with a running shoe or a modified shoe with a rocker bottom sole to allow easier rollover and shorter steps to effectively decrease pressures on the plantar and distal plantar surface of the foot when indicated.7 A high top shoe becomes a practical necessity for the more proximal, Chopart amputation level since it can provide very limited suspension when one solely uses it to keep the prosthosis on the partial foot of the higher activity level amputee.
The major flaws of the below ankle design is that it lacks: an effective lever arm that would help to lift the partial foot; all the components used in the fabrication of the below ankle device itself; and the shoe during the swing phase of the gait cycle for level 1 to level 2 ambulators. The lack of an effective lever arm on the below ankle prosthosis, especially the slipper socket with toe filler type, compromises any benefit an energy restoring, carbon fiber foot plate would provide at toe off stance phase. It places an additional burden on the higher activity, level 2 ambulator and the result is usually additional gait deviations.
A Closer Look At The Chopart AFO ‘Prosthosis’
The prosthetic department of the Miami Veterans Affairs Medical Center has designed a Chopart AFO prosthosis that combines the most functional aspects of above and below ankle Chopart devices. It is indicated for level 1 and level 2 ambulators. Research is planned to determine its suitability for level 3 ambulators who have the ability to vary their cadence during the gait cycle. Its chief distinction with other models is that it combines a posterior leaf spring AFO, an energy restoring carbon fiber footplate, which is utilized during the toe off segment of stance phase, and a custom fabricated, total contact, flexible (pelite) liner for the partial foot. The liner is contained within a partial contact carbon fiber socket. It weighs 16 oz.
This “prosthosis” offers a few key features. Between heel strike and mid-stance of the gait cycle, the ground reaction force permits a plantarflexion moment between the anatomic (partial) foot and ankle. That reaction force carries over to the posterior leaf spring, the total contact flexible (pelite) interface and the partial contact, carbon fiber socket of the prosthosis. All three components work in concert to allow a plantarflexion moment that smoothes the transition between heel strike and mid-stance.
The Miami VAMC Chopart-AFO prosthosis does not work against a viable, total weightbearing partial foot and ankle, and all its functions. A small amount of inversion and eversion is contained within the total contact flexible interface but that movement is mitigated with the use of a silicone, partial foot sock or similar accessory. Additionally, its design allows for axial rotation of the lower limb. However, the rigid, solid, above ankle Chopart model does not permit the patient’s use of the ankle during any phase of the gait cycle, stance or swing.
The posterior leaf spring of the Miami VAMC Chopart-AFO also provides the wearer with dorsiflexion assist from toe off at the end of stance phase through the swing phase of the gait cycle. The longer lever of the PLS is a great help in lifting and keeping the partial foot, the shoe and all the components of the 16-oz. prosthosis on the patient’s limb through swing phase. The attributes of the energy storing, carbon fiber foot of the prosthosis are much more effective when one combines them with the longer lever arm of the PLS during the toe off phase of the stance cycle as well. The below ankle Chopart variants do not have such capabilities.
The success of the Chopart amputation will vary widely. Orthotists and prosthetists are frequently challenged to provide an effective, functional and comfortable device to accommodate the patient’s need and activity level. That need is often determined by degrees (i.e. range of motion, weightbearing tolerance, the absence of complications such as equinovarus, plantarflexion contracture, ability to walk short distances on the residual limb, etc.).
A variety of orthotic, prosthetic and hybrid “prosthoses” have been designed, modified and improvised to meet that need. With so many options available and the number of partial foot amputations/ preservations rising, those in physical medicine, rehabilitation and podiatry continue their work to exploit any and every physical advantage the patient may present during the process.
Mr. Zernich is a certified prosthetist (CP) working at the Veterans Affairs Medical Center in Miami. He frequently writes on orthotic, prosthetic and rehabilitative medicine subjects.
Mr. Dowell is a certified, licensed orthotist-prosthetist (CPO/LPO) and laboratory supervisor at the Veterans Affairs Medical Center in Miami.
Dr. Rothenberg is a Fellow of the American College of Foot and Ankle Surgeons. He is the Assistant Chief of the Podiatry Section/Surgical Services at the Veterans Affairs Medical Center in Miami.
Dr. Cohen is a Fellow of the American College of Foot and Ankle Surgeons. He is the Chief of the Podiatry Section/Surgical Services and the Director of Podiatric Residency at the Veterans Affairs Medical Center in Miami.
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