Chronic foot ulcers are one of the most serious complications for people with diabetes.1,2 A history of a previous lower-limb amputation increases the risk for further ulceration, infection, and subsequent amputation because of abnormal distribution of plantar pressures and altered bone structure.3,4
Effective distribution of pressure, with reduced peak plantar pressure during walking, can help to prevent injury.5–7 The ideal partial foot prosthesis for individuals with diabetes should therefore be able to distribute the loads generated during ambulation over the largest possible area of the residual limb, thereby preventing the creation of localized pressure points.8 Partial foot amputation prostheses can be divided into two categories: high-profile designs that extend onto the tibia up and low-profile devices that terminate inferior to the malleolus.9 It seems that many individuals with partial foot amputations prefer to use the low-profile prostheses, specifically, flexible and semiflexible designs.
Silicone prostheses that are flexible devices have a full-contact fitting on the residual limb. Silicone is an elastic material that can be used effectively within flexible foot prosthetic devices. Silicone gel has many positive properties, including proper distribution of stress with nonstress concentration, and it maintains original structure during the continued usage.
A review of relevant literature has suggested that, up to this point, silicone partial foot prostheses have predominantly used a consistent and equal level of material stiffness throughout their design and structure.10–12 Studies also suggest that there is a significant increase in acute levels of plantar pressure in the residual limb after partial foot amputations during ambulation.7,13,14 Changing the levels of pressure and redistributing them more effectively to reduce high concentrations of pressure would therefore seem to be beneficial to prosthesis users and could help to prevent repeat amputation. Given the current state of knowledge, the aim of this study was to manufacture an original prosthetic design for partial foot amputation using a silicone-based material with various levels of stiffness matched accordingly to each respective area of the residual limb. This could help to reduce pressures in potentially high-pressure areas and help to provide more pressure in appropriate tolerant areas that could maintain and enhance safe, normal usage, and function.
The patient was a 56-year-old woman who had diabetic transmetatarsal amputation 4 years ago. She was referred to the Department of Orthotics and Prosthetics in the University of Social Welfare and Rehabilitation Sciences, Tehran, Iran, for the provision of a suitable partial foot prosthesis. She was provided with a standard silicone partial foot prosthesis with equal stiffness across all areas of the interface. She used this prosthesis for approximately 1 year. She was not satisfied with her prosthesis and felt severe pain in some parts of the residual limb, leading to her having to use soft cotton in those parts of her prosthesis that were particularly uncomfortable. The patient gave informed written consent to participate in this study, and appropriate approval was also obtained from the ethics committee of the University of Social Welfare and Rehabilitation Sciences.
Residual limb pain was measured using visual analog scale (VAS) (0–100 mm). A VAS was used within a range of 0 mm (“no pain”) to 100 mm (“worst pain imaginable”) for evaluating the pain. The patient indicated her level of pain based on this score.
The 10-m walk test was used to evaluate the speed of walking in this study. The subject was asked to walk at a self-selected speed and to repeat three times, with times recorded by a chronometer. To calculate the speed (m/seconds), walking distance (10 m) was divided by elapsed time.15
No ulcer or bleeding was observed in either foot, and there was no apparent wound or scar on the residual limb. However, in some areas, the skin was red and sensitive to the touch (see Figure 1).
DETECTION OF HIGH-PRESSURE POINTS
The plantar pressure distribution on the foot was evaluated in barefoot walking using the Force-Sensing Resistor™ (Interlink Electronics FSR 402), and the pressure was evaluated at different areas of the residual limb, namely, the rear foot, midfoot, medial forefoot, and lateral forefoot, especially on the red and touch-sensitive points.
We fixed Force Sensitive Resistors on the residual limb using a piece of thin binder. The patient wore her older conventional silicone prosthesis and was asked to walk a distance of 10 m three times. Data related to the maximum plantar pressure were collected and calculated as an average for the three trials. Areas where the plantar pressure was over 70 N/cm2 were considered as the high-pressure point.16 They commonly matched with the red and touch-sensitive points. Based on the locations of high-pressure points, the new silicone prosthesis was designed and constructed as follows:
1—RESIDUAL LIMB IMPRESSION
Initially, high-pressure points were marked on the residual limb surface with a pencil fresco. The negative cast was taken from the healthy and the partially amputated foot by alginate (Tropicalgin; Zhermack, Italy). The impression mold was reinforced externally with three layers of plaster of Paris bandage to ensure structural integrity. Subsequently, the patient was asked to put her foot on the floor, which was examined as neutral, and to bear some weight on her residual limb as the cast dried. Once this was removed, a positive cast of two sides was prepared by filling the negative cast with liquid plaster of Paris (Figure 2).
2—COLOR MATCHING (INTERNAL COLOR)
With reference to the color of the patient's skin, silicone was mixed and colored to meet the best possible match with a suitable range of colors. The original silicone mix had a higher level of stiffness, similar to that employed within a standard prosthesis. The areas designated as suitable for this level of stiffness, such as the outer layer, of the prosthesis were fabricated using this type of silicone. A silicone material with lower stiffness was used in the high-pressure points as designated from the previous examination and assessment; these were mixed with distinct colors and correctly positioned be within the prosthesis.
A relative/family member with a healthy foot that closely matched with amputated foot of the patient (donor) was selected. An alginate impression was taken of the donor and filled with wax (Covex modeling wax, the Netherlands) to provide a positive mold.
In this stage, high-pressure areas were increased in size by the addition of wax to a height of 3 mm to provide enough space for silicone injection after the final molding (Figure 3). To obtain a suitable suction suspension, the cast was circumferentially reduced by 2 mm, with the silicone interface being effectively slightly smaller than the residual limb size.17 After this, a wax pattern was taken with the donor's foot being placed on the positive cast of the amputated foot. Two sheets of wax were put in the plantar surface area of the plaster mold (2 mm thick) to provide enough space for placement of the soft silicone after the final molding. The sculpting was modified to reconstruct the residual limb of the amputated foot to appear as similar as possible as the natural foot (Figure 4).
5—MOLDING FOR SOFT SILICONE INJECTION
The circular wax pattern was made with a thickness of 3 mm and matched accordingly to the high-pressure points in the residual limb with moldings made for them individually. Then, wax was melted into the boiling water and replaced with soft silicone (SILBIONE® RTV 4407 A/B; Bluestar Silicones, France), which was injected into the mold. The silicone material is delivered as two low viscous liquid components, which once mixed and cured, transform into an elastic and resistant material. Polymerization occurs without formation of heat. This type of silicone offers an outstanding balance between low viscosity and high mechanical properties, as well as an excellent compromise between damping and rebound.18 The typical properties are presented in Table 1. The silicone pieces were removed from the mold after setting and were fixed with silicone adhesive to the high-pressure points on the plaster mold of the residual limb. The mold was covered with two layers of thin socks (Figure 5).
A two-piece negative mold matched to the affected residual limb was constructed using epoxy (Mega tool, EPG 404; Ghaffari Chemical Industries, Tehran, Iran) from the reconstructed “natural” foot made from wax. The two pieces of the resulting former were kept together with manually applied screws and placed in some boiling water for 20 minutes, to enable the wax to melt, leaving the shell or negative cast of epoxy. This negative cast was then ready for silicone injection (Figure 6).
7—HARD SILICONE INJECTION
After 24 hours, hard silicone (RTV 3040; Shimie Afsoon Company, Tehran, Iran) was injected into the multipiece formats and closed tightly (Table 1). Layers of socks were laminated in the mold for silicone prosthesis reinforcement. After 24 hours, the silicone had set and the multipiece formats were separated. The positive silicone prosthesis was then removed from the multipiece formats.19,20
The last step of the prosthetic manufacture involved putting the new silicone prosthesis on the partial foot after secondary painting to the skin cover was matched to the healthy foot (Figure 7).
PATIENT EVALUATION BASED ON THE NEW PROSTHESIS
The patient was asked to use the new silicone prosthesis for 2 months. Then we fixed the Force-Sensing Resistor on the high-pressure points using a binder. The patient wore her prosthesis and, as before, was asked to walk a distance of 10 m three times. Data related to the maximum plantar pressure and mean walking speed were measured over 10 m (with the new silicone prosthesis) and were collected and calculated as an average for the three trials. Pain was measured using the VAS (0–100 mm) again. The data on plantar pressure, pain, and walking speed are presented in Table 2.
The result of this study showed that the maximum plantar pressure decreased on the high-pressure points and walking speed improved for the original new silicone prosthesis in comparison with the conventional silicone prosthesis. In the rear foot and lateral forefoot, the peak plantar pressure was decreased about 45%. The maximum pressure transmitted through the midfoot and medial forefoot was reduced by 35%. Self-reported pain with VAS also decreased during walking with the new silicone prosthesis.
Currently, the incidence of diabetes-related partial-foot amputations is increasing.21,22 The daily usage of a partial foot prosthesis with an acceptable appearance that is able to promote comfortable walking, with a user-friendly structure and good relief in high-pressure points, can restore confidence and psychological improvement for patients with partial foot amputation.
To reduce the side effects of the partial foot prosthesis with an equal level of material stiffness throughout the design and structure, it is important to use the most suitable stiffness of silicone for achieving best function and satisfaction.10 In this study, the use of different levels of stiffness in silicone used in the prosthesis caused increased pressure redistribution and the pressure has decreased in the sensitive areas of the residual limb. Consequently, long-term use may be possible without risk of skin irritation and ulcer formation.
A significant advantage of this new silicone prosthesis is providing patient satisfaction and preventing ulcers. The easy-to-use soft socket wall in sensitive areas of the prosthesis increased the useful time of prosthesis use23 and may contribute to decreased skin ulceration risk. To optimize the functional outcome of walking without increased risk of ulcers for individuals with foot amputation due to diabetes, maintenance of essential walking along with appropriate modifications to the prosthesis such as we used here, alongside an effective gait pattern and adequate foot care, is recommended.24,25
In this study, residual limb pain during walking with the new silicone prosthesis was considerably reduced and the patient could walk more rapidly. A decrease in the VAS scores indicates that the cushioning properties of the soft silicone are able to absorb more load, decreasing pain and increasing patient velocity of walking.
Accomplishing a well-designed partial foot prosthesis with different levels of stiffness in the high-pressure areas is the final goal of prosthetic rehabilitation. The possibility of achieving this goal could be increased with the use of a partial foot prosthesis for longer periods especially in complex forms of painful residual limb with different sensitive areas. Based on our present analysis, we observed better comfort and functional outcomes with the use of a partial foot prosthesis manufactured with this new technique. It should be emphasized that because of the limitations of this present study, the validity of this intervention should be reconfirmed by a large sample size and randomized clinical trials.
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