Chapter 16B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Partial-Foot Amputations: Prosthetic and Orthotic Management
David N. Condie, B.Sc, C. Eng.
Melvin L. Stills, CO.
NORMAL FOOT FUNCTION
The successful management of partial-foot amputation requires a clear understanding of the functions of the normal foot and the consequences of surgical ablation.
The normal foot is an extremely complex structure, the detailed function of which is still only partially understood. This discussion of the mechanics of normal foot function will be restricted to a brief consideration of load-bearing structure and the function of the foot joints during normal walking.
The foot is the means whereby the ground reaction forces generated during physical activities are transmitted to the body structure. During normal level walking these loads are directed initially onto the heel. The specially adapted fatty tissues of the heel pad are ideally suited to the absorption of the high forces generated at impact and during the subsequent loading of the limb. Once the foot is flat and until the heel leaves the ground as push-off is initiated, the supporting forces are shared between the heel and the ball of the foot, with only a small contribution from the lateral aspect of the midfoot.
This method of load transmission is commonly attributed to the "arch structure" of the foot, even though it is now clearly understood that its effectiveness is a function of a number of both structural and neuromuscular mechanisms.
Once the heel leaves the ground, the increased ground force associated with push-off must be transmitted through the area defined by the metatarsal heads and the pulps of the toes. As body weight is transferred to the contralateral limb, this load falls and localizes on the plantar surface of the hallux.
The functions of the joints of the foot have been the subject of endless investigation. Clearly the ability of the foot to alter its shape and alignment are of considerable importance in adapting to variations in the slope of the walking surface. A more subtle but equally important role concerns the absorption of the longitudinal rotations of the lower limbs that occur with each stride (Fig 16B-1.).
Internal rotation of the entire lower limb, which is initiated during the swing phase, continues after heel contact until the foot is flat. During this phase the foot pronates about the subtalar joint axis, thereby maintaining the normal toe-out position of the foot. Elevation of the lateral margin of the foot, which is a consequence of this movement, is counteracted by supination of the forefoot through a combined motion of the rays, thus ensuring that ground contact is achieved across the entire forefoot. After the foot is flat as the lower limb commences external rotation, the foot supi-nates about the subtalar joint axis to absorb this motion, thus avoiding slippage occurring between foot and ground. The associated depression of the lateral margin of the foot is in this instance counteracted by pronation of the forefoot, once again enabling the maintenance of full forefoot loading.
After the heel leaves the ground, external rotation of the limb continues; however, the subtalar joint now reverses its direction of motion to pronate in conjunction with the forefoot, hence transferring the area of support medially onto the first metatarsal head and finally the hallux as the foot loses contact with the ground.
A final word should be reserved for the role of the midtarsal joint. During the initial loading phase this joint acts in concert with the subtalar joint. Once subtalar supination commences, however, this joint locks and, by doing so, stiffens the long arch of the foot to prepare it for the higher dorsiflexion moment that it is subjected to after the heel leaves the ground.
THE DESIGN OF PARTIAL-FOOT PROSTHESES/ORTHOSES
Devices used in the management of partial-foot amputations may be called orthoses or prostheses. This ambiguity arises from the design of the various systems used. Traditional prosthetic solutions used for these patients were in general heavy and bulky, and this led to the widespread adoption of modified orthotic systems based on the ankle-foot orthosis commonly used to control ankle function (Muilenburg Prosthetics, Inc., Houston) (Fig 16B-2. and Fig 16B-3.).
Today the availability of moldable flexible materials permits the fabrication of partial-foot prostheses that are both functionally and cosmetically acceptable (Life-Like Laboratory, Dallas) (Fig 16B-4.).
There are many factors to take into consideration in the management of the partial-foot amputee, perhaps most importantly the condition of the soft tissue in the weight-bearing areas of the residuum. Can these tissues withstand both the direct and shear pressures that will occur during normal activity, or does the load need to be transferred to a more proximal normal tissue? What will be the functional consequences of the loss of the foot joints, and how can the prosthesis be constructed to provide some degree of compensation?
These issues and the associated biomechanical considerations will be discussed in the following description of the prostheses/orthoses currently in use for each amputation level.
AMPUTATION OF THE TOES
The functional requirement for this type of amputation is largely cosmetic; however, if the hallux is absent, some consideration should be given to providing resistance to hyperextension of the first metatarsophalangeal joint area both to reduce the effect of the loss of the final element of push-off and to prevent uncomfortable shoe deformation. In addition, it is desirable to resist deviation of the remaining toes toward the amputation site.
Patients may elect simply to use soft foam or cloth to fill voids left in the shoe. A preferable solution consists of a simple insole to which toe fillers on spacers formed from orthopedic felt or foam are bonded (Muilenburg Prosthetics) (Fig 16B-5.).
An alternative approach is the use of custom silicone rubber toes attached to the residuum with medical ad-hesives and held in place with a sheer nylon stocking; however, this technique is available at only a few specialized centers (Life-Like Laboratory) (Fig 16B-6. and Fig 16B-7.).
The biomechanical consequences of ray amputations will be largely dependent on the position and extent of the forefoot segments removed. In the case of diabetic patients, the resulting reduction in the area of the plantar surface available to transfer the forces encountered during physical activity may be significant. In those instances where the first or the fifth rays have been removed (with or without the intermediate rays), this effect will be aggravated by mediolateral instability and may result in more serious pressure problems, particularly during push-off (Fig 16B-8.).
Ray amputations will also reduce the effectiveness of the pronatory/supinatory movements of the forefoot by impairing both its interaction with the subtalar joint and its role in responding to irregularities and slopes in the walking surface. Custom insoles fabricated from pressure-sensitive materials may be used to distribute pressure evenly over the remainder of the foot. These insoles have a limited life expectancy since they are designed to gradually deform, thereby protecting the foot from excess pressures. Laminated foam insoles may be used to increase longevity. Generally a softer, more conforming foam is used against the skin, while a more durable, stiffer foam that will retain its shape longer is used for the base.
One of the principal problems encountered by the patient with a ray amputation is shoe fit. In more extensive amputations a foam insert may be used that will position the foot correctly in the shoe and avoid the necessity of purchasing split sizes of shoes (Life-Like Laboratory) (Fig 16B-9., Fig 16B-10., Fig 16B-11. to 16B-11).
All those considerations referred to in connection with amputation of the toes also apply to the treatment of trans metatarsal amputations; however, the more significant loss of the load-bearing surface under the metatarsal heads that is experienced by these patients must also be addressed, most commonly by utilizing a shoe insert molded accurately under the remaining area of the longitudinal arch (see Fig 16B-5.).
Since the subtalar joint remains free to function normally, this group of patients will experience some functional impairment due to the loss of normal forefoot mobility. Some flexibility in the construction of the forefoot filler to permit supination or pronation would be an advantage; however, this may be incompatible with the stiffening required to prevent shoe hyperex-tension during normal push-off (Life-Like Laboratory) (Fig 16B-12.).
TARSOMETATARSAL AND TRANSTARSAL AMPUTATIONS
With these more proximal amputations the prosthetic requirements become considerably more demanding. Basically, the requirement to replace the anterior support area of the foot remains the same; however, whereas for the more distal amputation levels the prostheses can be effectively interfaced with the stump by using suitable footwear, a more extensive socket is now indicated if relative motion between prosthesis and residuum is to be prevented when weight is applied to the forefoot.
Two basic biomechanical solutions are available. In the more traditional designs of prostheses (and some of the more recent ankle-foot orthotic solutions), the device is constructed to encompass the entire residuum and extend some distance above the ankle. In these designs the dorsiflexing moment created by forefoot loading is easily resisted by counterforces generated on the heel and at the anterior brim of the device (Fig 16B-13.).
This design may also be constructed so as to provide axial load relief in the event that full plantar weight bearing is contraindicated.
More modern designs of prostheses of the slipper type enclose only the residuum and terminate around the ankle joint. In these designs resistance to the dorsi-flexion moment is provided by the accurate fit of the socket on either side of the calcaneus (Fig 16B-14. and Fig 16B-15.). Obviously some means must be provided for permitting entry and removal of the residuum. For this, a variety of techniques are employed. (Jack Collins, C.P.O., Collins Orthopedic Service, Inc., Fayetteville, Ark).
Early prosthetic designs took a form similar to an ankle disarticulation (Syme) prosthesis; however, as has previously been mentioned, these have been found to be bulky and heavy (see Fig 16B-2.).
Alternative ankle-foot orthotic designs manufactured from thermoplastic materials are both lighter and more cosmetic; however, these are probably only indicated for those patients where it is necessary to transfer the weight-bearing forces above the ankle to unload fragile skin at the amputation site or to compensate for weakened ankle musculature (see Fig 16B-3.).
All above-ankle systems will inevitable restrict subtalar joint motion, thereby eliminating the normal mechanism for absorbing the longitudinal rotations of the limb. If slippage between the foot and the ground is to be avoided, the patient must adopt a modified pattern of hip motion.
Patients wearing above-ankle devices will have the further disadvantage of a reduced range of ankle motion. Since the extent of the residuum precludes the use of a normal prosthetic ankle mechanism, these patients will be required to adopt compensatory hip and knee joint movements to cope with this restriction. This problem may be best addressed by the use of a rocker sole and cushion heel adaptation to the amputee's shoe.
There appear to be four basic types of construction currently in use:
All of these systems are laminated or thermoformed about a positive model of the remaining foot. These models are carefully modified to decrease pressure where required and increase pressure where tolerated.
Rigid and semirigid systems incorporate a foam socket liner that acts as an interface between the walls of the socket and the surface of the skin. These liners may be of varying thickness and stiffness, depending on skin tolerance. They are also prone to deterioration and will require replacement in time due to decreasing thickness and softness of the material. The profile of the foot is restored by a soft or rigid buildup added to the socket. Rigid and semirigid partial-foot prostheses will generally require cushion heel and rocker sole modifications to the patient's shoes. The use of rigid and semirigid prostheses is today less common due to the availability of improved semiflexible and flexible designs.
Semiflexible designs utilize a combination of materials generally having urethane elastomer or a silicone base. These systems are fabricated over an exact model of the patients remaining foot. Care is taken to ensure a tolerable distribution of pressure. Reliefs are made for bone prominences, callosities, or sensitive areas. Material may be removed proximal to the calcaneus to improve the suspension of the prosthesis. A toe filler is attached to the socket either during the foaming processing or by gluing in place later.
These fillers may simply fill the shoe shape or be carved to simulate the contours of an actual foot and toes. Color is added during the foaming process or may be painted on to match skin tones at the time of fitting. Modification of the socket to relieve excessive pressure is generally achieved by modification to the outside surface of the shell, thereby maintaining the smooth integrity of the socket inner surface.
By reducing the socket thickness over the high-pressure area increased flexibility is achieved. The proximal edge of the socket opening is also thinned to avoid edge pressures. Some examples of semiflexible prostheses include the following:
Slipper-Type Elastomer Prosthesis
The slipper-type elastomer prosthesis (STEP) (Fig 16B-16.) manufacturing processing is somewhat complex. Permanent tooling is developed for each individual amputee and consists of a permanent polyester resin positive model and a negative mold of the finished artificial foot. The device is fabricated by using semi-flexible urethane elastomers.
The Collins Orthopaedic Service Partial-Foot Prosthesis
The socket for this prosthesis (see Fig 16B-15.) is fabricated over a modified positive plaster model of the stump. Silicone is laminated into a cloth material and reinforced with woven glass if needed for increased durability. A spring steel is attached to the plantar surface of the socket and extends to within 1 in. of the toe (distal end of the finished prosthesis). The contours of the foot are filled out by using prosthetic foam that is foamed in place by using a plaster toe mold.
The Imler Partial-Foot Orthosis-Chicago Boot
The socket for this prosthesis is vacuum formed over a modified plaster model in the manner of a University of California Berkeley shoe insert (see Fig 16B-17.). The resulting copolymer socket is inserted into a ure-thane elastomer (Lynadure, Medical Center Prostheses, Houston) cosmetic boot and is removable for adjustments. The finished prosthesis extends just above the ankle and is retained by lace-up closures anteriorly.
Lange Silicone Partial-Foot Prosthesis
The socket for this prosthesis (Lawrence R. Lange, C.P.O., Wheeling, WV) (Fig 16B-18.) is laminated over a modified plaster model in the usual manner by using a nylon-tricot cosmetic stockinette and Otto Bock silicone (Otto Bock Orthopedic Industry, Inc., Minneapolis). The socket is then bonded to a modified "Quantum" (Quantum Foot, Hossmer-Dorrance Corp., Campbell, Calif) or similar prosthetic foot shell. A zipper is added posteriorly, and a final silicone lamination is performed to finish the prosthesis. This prosthesis extends just above the ankle and uses a zipper closure for retention.
These four systems have all been used successfully in the management of the short partial-foot amputee. The degree of flexibility is determined by the amount of reinforcement utilized in the socket walls. Extensions above the malleoli are used to provide improved suspension of the prosthesis on the limb. Flexible (see Fig 16B-4., Fig 16B-7., Fig 16B-9., Fig 16B-11., and Fig 16B-12.) partial-foot prostheses constructed from reinforced silicone were originally introduced to provide cosmetic restoration only. Subsequent experience has demonstrated that this design of prosthesis, in addition, permits the successful restoration of balance and a more normal gait.
A lost wax method is used to create a negative impression of the foot to be formed. Pure reinforced silicone is used to form the socket and the foot simultaneously. Pigment is added to the silicone to closely match the basic tissue color of the individual. Detailed coloring is done at the time of fitting to match the natural skin tones.
These flexible partial-foot prostheses have worked particularly well on patients with adherent and fragile scar tissue, probably because silicone does not have the abrasive nature of the other materials traditionally used for socket construction (see Fig 16B-4., Fig 16B-8., and Fig 16B-12.).
Normal ankle and subtalar movements are theoretically possible for patients wearing below-ankle designs of prostheses. Since this is the case, then the provision of some alternative means of achieving forefoot rotation would appear to be indicated. In the absence of such a design a simple measure adopted by some prosthetists is to wedge the forefoot of the prosthesis laterally, thus ensuring that full forefoot contact is achieved when the foot is flat.
A comfortable socket and a balanced foot are the twin objectives of all partial-foot prostheses. The choice of design to be employed will depend on the level of amputation, the condition of the remaining soft tissues, and the status of the ankle.
The use of above-ankle designs should be limited to patients who require assisted ankle function, who experience difficulties with suspension, or who cannot tolerate full plantar weight bearing.
New materials and fabrication techniques have permitted the development of both cosmetically and functionally improved designs that may make partial-foot amputation a practical alternative to higher amputation where the pathology permits.
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- Condie D, Stills M: Biomechanics and prosthetic/orthotic solutions-Partial foot amputations, in Amputation Surgery & Lower Limb Prosthetics. Boston, Blackwell Scientific Publications Inc, 1988.
- Fillauer K: A prosthesis for foot amputation near the tarsal-metatarsal junction. Orthot Prosthet 1976; 30:9-11.
- Hayhurst DJ: Prosthetic management of partial foot amputee. Inter-Clin Info Bull 1978; 17:11-15.
- Imler CD: Imler partial foot prosthesis I.P.F.P.-"Chicago Boot." Clin Prosthet Orthot 1987; 12:24-28.
- Lunsford T: Partial foot amputations-Prosthetic and orthotic management, in Atlas of Limb Prosthetics. St Louis, Mosby-Year Book, 1981, pp 322-325.
- RECAL Literature Search, University of Strathclyde, National Centre for Training and Eduation in Prosthetics and Orthotics, Curran Bldg, 131 St. James Rd, Glasgow, 640LS Scotland.
- Stills ML: Partial foot prostheses/orthoses. Clin Prosthet Orthot 1987; 12:14-18.
- Wilson MT: Clinical application of RTV elastomers. Orthot Prosthet 1979; 33:23-29.
Chapter 16B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles