Exploring the Applications of Semi Custom Silicone Partial Foot Prostheses in a Pediatric Amputee Population

R. Dwight Putnam


Abstract

A well crafted custom silicone prosthesis can not only aid in scar tissue management and residuum protection but it can also restore the patients' self confidence and psychosocial well being (1). While enhanced cosmesis and durability are definitely desirable, outsourcing a custom or semi custom silicone prosthesis can be cost prohibitive. Due to the high cost of the labor intensive fabrication process, custom silicone prosthetics are not an option for all amputees. In the pediatric amputee population, where prosthesis's average life expectancy is 12 to 15 months, funding is definitely a challenge (2). During a year long Prosthetics residency at Texas Scottish Rite Hospital for Children, the author had access to a very diverse group of amputees. This experience offered a unique opportunity to fabricate semi custom silicone prostheses for a wide variety of patients, without burdening them with a price tag. Partial foot amputees, who would otherwise be candidates for traditionally fabricated prostheses, were given an opportunity to try a semi custom silicone prosthesis. In the course of identifying the benefits of silicone devices for this population, this pilot study emphasized cost effectiveness and functionality over cosmesis. This study attempted to establish and document an inexpensive and reliable semi custom silicone partial foot prosthesis fabrication system using the different techniques and materials available.

Patients, for whom a flexible "slipper" style prosthesis would be indicated, had an opportunity to participate in this pilot evaluation. These devices were fit as a usual and customary treatment option, along with other styles, such as thermoplastic AFO's and foam inserts. It is not uncommon for many of these patients to have several styles of partial foot prostheses.

Alginate impressions of the patient's amputated side were made and filled with a lab stone. The patient's sound side was matched to an "off the shelf" foot shell to determine the size and shape of the prosthesis. A wax sculpture of the prosthesis was constructed on the stone positive, which had been modified to achieve tension. A three part mold was fabricated and the lost wax method was used to replace the wax with tinted silicone. The end result was a semi custom silicone prosthesis that intimately surrounds the residuum providing protection, cosmesis and excellent suspension. These devices were delivered then monitored through follow up visits. The patient's acceptance or rejection of the prosthesis was documented. Any patient comments regarding fit, cosmesis, or comparisons to past prostheses were noted.

Introduction

According to a 2005 study, toe and foot amputations account for 43 percent of all lower limb amputations in the United States (3). Doug Smith, MD, writes that this increase is due mainly to the fact that "We understand the surgery and healing process better..." (4). in the past, orthopedic surgeons focused "too closely on preserving as much bone length as possible" which in some cases led to skin breakdown and ultimately to a higher level of amputation (4). Surgical techniques involving partial foot amputations no longer emphasize preserving "a little extra bone simply for the sake of saving it..."(4). These surgeons have begun to realize that "if there is inadequate padding and the end of the residual limb is not durable enough ...to withstand the forces of walking, even with the best prosthesis, pain, ulceration and infection can result" (4). While preserving as much toe lever as possible eases the patient's return to normal gait, it can also have negative results if there is not ample soft tissue coverage. Smith believes that the focus "on preserving as much bone as possible" can be lead to detrimental outcomes(4). He suggests "striking a balance between bone, muscle, skin and nerves" in order to achieve "a well padded shorter residual limb" (4). The benefits of this level of amputation include":1) the preservation of end weight bearing, 2) better proprioception, 3) reduced rehabilitation time 4) a more efficient gait with less energy expenditure and 5) distinct psychological advantages resulting from a less severe change in body image and less conspicuous prosthetic gait."(5).

With successful partial foot amputations being performed more frequently, it makes sense for prosthetists to explore and refine the prosthetic devices available for partial foot replacement. According to Condie and Bowers, partial foot prostheses fall into one of two categories, either "perimalleolar" or "high profile" (6). A basic foot orthosis with a foam toe filler, a slipper style flexible prosthesis, a Lange Partial silicone partial foot prosthesis, and a custom suction socket silicone partial foot prosthesis are all considered to be "perimalleolar" designs. The trim lines of these prostheses are either just proximal (supra) or just distal (sub) to the malleoli, depending on the style of prosthesis and the length of the residuum. The longer, more stable the residuum, the lower the trim lines. Transmetatarsal and tarsometatarsal (Lisfranc) amputees are usually candidates for "perimalleolar" prostheses.

The most common "perimalleolar"partial foot prosthesis is the slipper style flexible prosthesis (fig.1 ). As the name suggests, this prosthesis is worn as a slipper inside of the patient's shoe. The socket, laminated with a semi flexible resin, suspends on either side of the calcaneous with a foot plate that extends beyond the distal amputation. The dorsal aspect of the prosthesis, which is built up with foam and covered with leather, is hinged with the semi flexible socket for ease of donning and doffing. This prosthesis is able to provide a fairly intimate fit with a rigid forefoot to aid in the restoration of efficient ambulation, provided that the patient can "tolerate full plantar surface weight bearing"( 6).

In 1984, Lawrence Lange developed the Lange silicone partial foot prosthesis (fig.2 ). as an alternative to laminated, semi rigid prostheses and orthotic foot plates with toe fillers. According to Lange, the latter have design problems that include: 1) no means of securing the residual limb to the prosthesis and requires shoes with lace closures to supplement this feature. 2) Supramalleolar varieties are usually made from semi rigid materials and are difficult to don and doff, are uncosmetic and do not allow ankle motion (7).

This failure to accommodate anatomical ankle motion translates into "wasted motion" and "skin breakdown" (7). To address these failures, Lange created a system to fabricate an intimately fitting, aesthetically appealing silicone partial foot prosthesis with an "elastic, resistive toe lever" (7). Using prosthetic socks as an interface, this design incorporates a silicone elastomer socket laminated into a pre existing foot shell (filled with RTV foam) with a zipper on the posterior aspect, for added ease of donning and doffing. With the intimate fit and the functional toe lever, "...more natural ankle motion is achieved," and "there is resistance to drop off past the midstance part of gait, allowing for more normal ambulation" (7). The trim lines of the silicone socket can be adjusted "to enhance suspension and control of the device for different levels of amputation" (7).

A custom silicone suction socket partial foot prosthesis is an aesthetic, yet functional option for the distal amputee (fig.3 ). This style of prosthesis combines a total contact, suction suspension socket with a foot that is sculpted and painted to match the patient's sound side. A wax model is sculpted around the modified positive to represent the mirror image of the sound side foot. The lost wax method is used to replace the wax with silicone. The socket and the foot are fabricated as a single piece and then painted to match the patient's own skin. This system inherently possesses all of the qualities Lange designed into his prosthesis, without having to use a zipper, PVC foot shell, or prosthetic socks. The intimate fit of the suction suspension reduces the amount of "wasted motion" and promotes a "more natural ankle motion". The silicone total contact suction socket protects the residuum, provides excellent suspension and enhances proprioception. The durometer of the keel can be adjusted as well, increasing the efficiency of the patient's gait, resisting dorsiflexion and reducing the amount of distortion through the various phases of gait. The natural appearance of the foot, out side of the shoe, placates the psycho social anxiety common in most amputations. This style of prosthesis addresses both the psychological and functional needs of the patient.

The more proximal amputations, such as Chopart and occasionally Lisfranc, generally require prostheses with higher trim lines, in order to accommodate a more drastic loss of biomechanical function and surface area upon which to suspend the prosthesis and redistribute ground reaction forces. These "high profile" designs tend to lock out any residual ankle motion. The higher trim lines help to suspend the prosthesis, redistribute ground reaction forces and to resist dorsiflexion during gait. A modified AFO with foam toe filler and a PTB style hard socket laminated directly to a carbon footplate are two examples of "high profile" partial foot prostheses (fig.4 , fig.5 , fig.6 ). Regardless of the design, the partial foot prosthesis must resist an external dorsiflexion moment during "prosthetic forefoot loading". This moment causes the prosthesis to rotate in the direction of dorsiflexion. Each design of prosthesis resists this moment differently. Modifying the plaster model to load certain areas creates an intimately fitting, comfortable, and efficient prosthesis which helps restore toe lever and redistribute the forces that would otherwise cause the prosthesis to rotate. The perimalleolar designs use mediolateral calcaneal suspension and the plantar loading surface of the residuum to counteract the dorsiflexion rotation. In high profile designs, " the forefoot dorsiflexion moment is resisted by a force couple created by socket interface forces located anteriorly at the socket brim and posteriorly at heel level"(6).

Selecting the right device is crucial. Each patient presents a unique set of circumstances which need to be incorporated into the design of the prosthesis. Selection depends not only on the anatomy of the residuum but also the desires and abilities of the patient. A transmetatarsal or Lisfranc amputee with limited weight bearing and potentially troublesome skin who is concerned with only household transfers might select the simple foot orthosis with a toefiller and a custom shoe. A patient, with a similar amputation but with a healthy residuum and full weight bearing, who requires more "push off", might be a candidate for laminated slipper style prosthesis, a Lange prosthesis or even a Silicone prosthesis. The "high impact" partial foot amputee may require a hybrid combination of a prosthesis and an orthosis, which offers both limb replacement and limb support. These hybrids designs are available in various combinations. One such device would be the supramalleolar thermoformed prosthesis that includes both a carbon foot plate and a custom molded arch support inside a custom shoe. For the more active yet cosmetically concerned patient, a custom silicone foot with a carbon fiber ankle foot orthosis would be an option (fig.7 ). This combination of components helps to restore stability and toe off during ambulation and appeals to the aesthetic concerns of the patient.

The clinical goals of a partial foot prosthesis are basically to provide a comfortable fitting, aesthetically pleasing device that protects the residuum and enables the patient to ambulate using the most efficient gait possible. Recent studies by Michael Dillon suggest that regardless of the design of the partial foot prosthesis, the device only marginally restores the effective foot lever lost to amputation (8). However, this inability to restore the anterior lever arm does not decrease the actual functionality of the prosthesis. It merely re-emphasizes the importance of the other clinical objectives when designing a partial foot prosthesis. The fit of the socket, the protection of the residual limb, the appearance of the prosthesis and even the perceived improvement of gait all contribute equally to the function of a partial foot prosthesis.

The pediatric population of partial foot amputees presents with a unique set of potential obstacles, with regards to achieving a functional well fitting prosthesis. The primary obstacle is rapid growth. A partial foot prosthesis must some how accommodate a rapidly growing patient. A typical pediatric prosthesis only lasts 12 to 18 months. Children with partial foot amputations have been observed to experience both internal rotation of the ankle and eversion of the foot. Both of these factors need to be dealt with when aligning the prosthesis and establishing toe out. The durability of a pediatric prosthesis is also important. It must be able to handle the wear and tear of a highly active patient's daily routine. The design of a pediatric partial foot prosthesis must be flexible enough to accommodate all of the unique factors involved and be cost effective, due to the short life span of the device.

Methods

For the purpose of this study, focus was given to pediatric patients with any level of partial foot amputation. Both traumatic and congenital patients were observed. Once evaluated by the Clinic Team and given the physician's approval, a perimalleolar, semi custom silicone prosthesis was designed and fabricated according to individual needs of each patient. The patients were evaluated and measured in the standard manner. Tracings of patients' feet were made, in a standing position, and toe out established. The author took alginate impressions of the patients' residuums. Dental stone was poured into the alginate. The stone positive of the residuum was modified to achieve suction suspension. This positive was dipped into wax until a layer 2mm thick surrounded the residuum. This wax represents the socket of the prosthesis. Any potential bony prominences, eversion of the foot, or potentially sensitive areas were addressed by adding wax or plaster to the appropriate areas of the model. The author measured the patient's sound side and selected an appropriately sized foot shell. An alginate impression was taken of toe portion the foot shell. This impression was filled with hot wax. After approximately 30 minutes, the alginate can be removed from the wax copy of the foot shell. Using the tracings and toe out lines as reference, the author sculpted a wax model of the final prosthesis by joining the wax dipped stone positive to the wax toe section. A three part stone mold was fabricated of the wax model. The lost wax method was used to replace the wax with medical grade silicone. Once the silicone prosthesis cured and had been removed from the mold, it was prepped and painted to match the patient's own skin tone.

During the delivery of each prosthesis, the patients' comments regarding fit and cosmesis were recorded. The patients were observed ambulating first without the prosthesis and then with the prosthesis. The author documented any observed differences in gait. Six out of nine patients were seen for follow visits approximately three weeks after the delivery of the prosthesis. Two were contacted by phone. One failed to return phone calls. All were asked a series of questions regarding their satisfaction with the silicone prosthesis. The answers were recorded and the results were tabulated.

Results

A total of nine partial foot patients were seen. All received a semi custom silicone partial foot prosthesis. Seven were delivered in person. The other two were mailed after a successful test fitting. Six of the nine patients had congenital amputations; three were secondary to disease or trauma. They ranged in age from eleven months to sixteen years old. Four patients were male and five were female. Seven had worn some type of prosthesis previously, ranging from a laminated slipper style prosthesis to a thermoplastic AFO with a foam toe filler. Two had no experience wearing a prosthesis.

All patients/parents reported initial satisfaction with the cosmesis of the prosthesis. However, all of those patients who had previously worn a different style of prosthesis commented on a "weird feeling" once they had donned the prosthesis. These patients had worn socks as an interface and were not accustomed to the "skin fit "of the silicone device. One of the two new wearers reported a comfortable, snug fit. The other was not yet communicating verbally, however, it was observed that she "smiled and immediately began to ambulate and inspect her new foot in the mirror".

Seven patients were observed walking prior to donning their prostheses, both with and without a shoe. These patients were also observed ambulating with their prostheses, both with and without shoes. The author noted five of these patients had a markedly improved rollover while wearing their prostheses, without the shoe. Once the shoe was added to the equation, their rollover seemed to improve even more. One of the remaining two patients wore an AK prosthesis on the contra lateral side. Both of this patient's amputations, coupled with severe scarring on 80% of his body, made any gait evaluations extremely difficult. The last patient, who was only eleven months old, had only begun to ambulate shortly before receiving her new prosthesis. She walked with an extremely wide base of gait and no changes could be noted.

Six of the original nine were interviewed during follow up visits to the hospital. Two were interviewed over the phone. One could not be reached. Four reported wearing their prostheses full time, or all day for all activities. Two reported part time usage, four hours or less a day. Two had completely rejected their prostheses after approximately one week. One of the "rejection" patients, who has a Chopart amputation, reported that the silicone prosthesis didn't give her the same amount of support as her previous prosthesis, which was a copolymer AFO with a foam fore foot. The other "rejection" offered no verbal explanation, only that the patient had "lost interest in the prosthesis". Full time wearers reported using their prostheses for any thing from community ambulation to more vigorous activities such as soccer, basketball, and baseball.

Six "yes, no, or neutral" questions were asked during each follow up interview (fig.8 ). "Yes" answers indicated satisfaction with the prosthesis. "No" answers indicated dissatisfaction. "Neutral" answers indicated neither satisfaction nor dissatisfaction. Out of a total of fortyeight possible total responses; twenty-eight were positive, nine were neutral and eleven were negative. An overall success rate of 58.3% was recorded. All patients, even those who ultimately rejected the prosthesis, were initially happy with the cosmesis and the idea behind the device. Three of the four fulltime wearers reported 100% "Yes". A patient who ultimately accepted her prosthesis, remained "neutral "about gait improvement while wearing the prosthesis. The two part time wearers, while liking the look of the prosthesis, remained neutral on the majority of the other questions asked. The two rejections were recorded as liking the cosmesis, but as a "No" on all remaining questions.

Through out the course of this study, the fabrication process was tweaked and refined making it more cost effective and flexible in its applications. A three part mold/lost wax method of fabricating a hollow silicone foot shell and a separate silicone socket was introduced. This method will allow TSRHC to create a permanent library of molds which can be used multiple times on multiple patients, thus cutting down on man hours and materials. The use of a hollow silicone foot shell allows for an easily adjusted fore foot durometer and potentially even the addition of a carbon foot plate. The wax dipped, modified residuum is molded and a custom silicone socket is fabricated separately from the foot shell. This socket can be used both as a test socket and a definitive socket, provided the initial fitting reveals no necessary changes. Allowing the practioner to test fit the socket and to determine socket flexion and toe out prior to final fabrication, could potentially cut down on refabrication due to poor fit, insufficient toe out or not enough/too much socket flexion. During the test fitting session, the patient dons the silicone socket. The prosthetist can evaluate suspension, eversion and inversion. Wearing the socket, the patient dons the hollow foot shell. The practioner can then adjust socket flexion, toe out, inversion or eversion. All of these adjustments should be noted to allow for proper alignment transfer. Once all fit and alignment goals have been met and recorded, the merging process takes place. The final, cured prosthesis can be shaped and smoothed on a Trautman grinder, and painted to match the patient's own skin tone.

The flexibility of this fabrication system is such that growth, inversion/eversion and toe out/in can all be accommodated. The materials were chosen due to their durability and low cost. This method of fabrication provided reliable and cost efficient semi custom silicone partial foot prostheses.

Discussion

While the numbers involved in this study represent only a fraction of the total number of partial foot patients in the TSRHC population, they do represent a wide spectrum of potential wearers (fig.9 ). Four patients, from four years to sixteen years, are now wearing their semi custom silicone prosthesis full time (fig.10-13 ). These four patients represent both congenital and acquired amputations. Two are male, two are female. Three of these four have worn some type of prosthesis previously. This group, while small, represents the potential of the semi custom silicone prosthesis to appeal to a wide variety of patients, both with lower and upper extremity amputations. It is the author's intention to develop a more objective means of data collection. Hopefully through continued follow up visits, the questionnaire will be refined and the information collected will be less subjective.

Conclusion

Having established a reliable silicone fabrication system, TSRHC has expanded its' arsenal of partial foot prosthetic solutions. Even though Dillon et al's most recent study suggests that perimalleolar prosthesis designs do not restore the biomechanical function lost to partial foot amputation, it is important for every Prosthetist to develop his/her own practice based opinion on the effectiveness of such prostheses. It is also important to remember that the restoration of biomechanical function is not the only criteria to be considered when designing a partial foot prosthesis. The patient's own functional and cosmetic expectations, along with the practioner's clinical experience, are key ingredients in the recipe for successful perimalleolar partial foot prostheses.

Bibliography

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  7. Lange, L.R., "The Lange Silicone Partial Foot Prosthesis". JPO 1988: 12:14-18.
  8. Dillon MP, Fatone S., Hodge M.C., "Biomechanics of Ambulation After Foot Amputation: A Systematic Literature Review". JPO 2007; 19(8 Proceedings) p2-p61.
  9. Condie, D.N., Stills, M.L. "Partial Foot Amputations: Prosthetic and Orthotic Management", Atlas of Limb Prosthetics: Surgical, Prosthetic and Rehabilitation Principles, 2nd Ed, Rosemont, IL: American Academy of Orthopedic Surgeons 1992.
  10. Berk, G.M(CP), Rheinstein, J.(CP), Michael, J.W. (CPO), Stark, G.E.(CP). "Biomechanics of Ambulation Following Partial Foot Amputation: A Prosthetic Perspective". JPO 2007:Vol.19, Num.3s, pp.85-88.
  11. Figures 2,3, and 5 courtesy of www.oandplibrary.org/alp/chap16-02.asp
  12. Figure 6 courtesy of www.cc-mfg.com

R. Dwight Putnam; Texas Scottish Rite Hospital for Children, Dallas, Texas