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Clinical Evaluation of an Articulated, Dynamic-Response Prosthetic Foot in Teenage Transtibial and Syme-Level Amputees

Robin C. Crandall, MD
Todd F. Anderson, CP
Byron Backus, CP
Tony Frucci, CP

ABSTRACT

Nineteen highly active adolescent and teenage unilateral transtibial (TT) and syme (S) amputees were randomly selected for conversion to a prosthesis that incorporated a unique articulated foot design. The patients were analyzed early at the six-month conversion interval, and late at an average long-term followup of three years and five months. The modular, articulated foot, known as the "College Park Tru-Step," allowed for subtalar, ankle, and forefoot motion. All patients were carefully evaluated in a clinical setting; 16 (84%) completed questionnaires and returned for long-term follow-up (mean 3.4 years). All patients were active users of other varieties of dynamic response feet. In the early analysis period, 14 patients (87%) preferred this foot to their previous fitting, with seven patients (44%) rating it "much better." Two patients rejected the design early, and three additional patients rejected the design during long-term follow-up. The long-term success rate in 11 patients (69%) in the follow-up group was encouraging. Failure of component parts in the design was a particularly vexing problem for the prosthetic team, and is described. Modern lower extremity prosthetic design is also discussed.

Key Words: modular foot, articulated, transtibial, syme, amputee

Within the past 20 years, prosthetic limb design has advanced, specifically for both the deficient upper and lower extremity. With an emphasis on a very active lifestyle that includes sport activity as the norm, the lower extremity amputee requires as normal a gait as possible. It has recently been demonstrated that even when the most advanced dynamic response foot is incorporated into a modern lower-extremity prosthesis, gait velocity is still significantly less than normal.¹ To further muddle the issue, the transtibial (TY) amputee consumes about 35% more energy compared to the nonamputee.² It has become evident in our busy children's amputee clinic that foot selection is haphazard, and perhaps not based on sound scientific evidence about which component is critical to the performance of the individual. Many prosthetic options are available, including the Flex-Foot, the Seattle Lite Foot, elastic response feet of varied designs, and the College Park articulated foot.^13-15

Rao et al.¹ have recently shown that future prosthetic designs should "attempt to capture the dynamic characteristics of a normal articulation between the foot and shank segments during the early-stance weight-acceptance period." These authors feel that delays in the timing of early-stance events compromise shank and knee stability, and that both excessive and insufficient mobility of three popular prosthetic foot designs contribute to instability and decreased forward progression. Perry et al. have also recently shown that "prosthetic foot designs need to incorporate mechanisms that promote early foot flat while preserving limb stability."⁶ These authors studied primarily the single axis foot, the Seattle Lite Foot, and the Flex-Foot.

We report our initial pilot study experience with an articulated, dynamicresponse foot that is designed to promote early foot flat by allowing a modicum of ankle motion via an articulated viscoelastic "bushing" or biscuit-cushion effect. Stability of the foot can also be adjusted to some extent by using softer versus firmer components that are incorporated in the design. The engineers of the design have cleverly linked three carbon-composite "foot bones" with the four synthetic rubber pads to allow more natural foot and ankle motion (Figure 1 ). A similar concept in providing smooth load transfer, and ultimately, a soft landing, is incorporated in the landing gear of several commercially available high-performance aircraft, including all newly produced Mooneys. The current pilot study was designed to assess overall patient acceptance of the prosthetic design in adolescents with amputations at the transtibial and Syme level, with a follow-up of more than three years (mean, 3.4 years). Formal gait analysis is not available in our clinic, and was not done for the patients involved in this study.

Materials and Methods

Nineteen patients with unilateral below-knee amputees who were excellent ambulators were selected for conversion to the articulated foot design known as the College Park Tru-Step. The patients were randomly selected in consecutive order when their current prosthesis had worn out to the point of replacement, or when size adjustments were no longer possible by the prosthetic team. Within a few months in early 1995, the entire study group was selected and converted to the Tru-Step foot. No patients younger than 10 years of age were selected because of the size constraints of the Tru-Step foot. The range of foot size used in the study was 23 to 28 cm. All patients were supplied with a "spectra" sock, over which a cosmetic foot shell was applied. Stump length was not a specific criterion other than the stump having to be short enough to allow fitting of the device in "modular" fashion. In the original group, the age of the boys at the beginning of the study was 9 to 18 years, and the age of the girls was 14 to 18 years.

All patients were sent a questionnaire six months after the initial fitting. Sixteen patients responded to the questionnaire (84% ),and this group represented the final study group. The final study group was followed up for an average of three years and five months. The prosthetic team characterized 12 of the 16 patients as being in a "very high" activity level and involved in continuous or year-round sports. All 16 patients had been wearing dynamic response feet prior to the conversion to the Tru-Step (Table 1 ).

Ten of the patients in the final study group had some form of ankle disarticulation (syme), five patients had various-length transtibial or through-bone amputations, and one had a Boyd stump. Twelve of the 16 patients had congenital limb deficiencies (75%), a figure that correlates well with that in our general clinic population. The average patient age at the onset of the study was 14.6 years, which included 4 girls and 12 boys. The average age at final review was approximately 18 years, and the average follow-up time was three years and five months. The results of the study were grouped as early, representing the results from the original questionnaire, and as late, or the results at the clinical follow-up. Each patient had at least one new prosthesis at the final follow-up. All patients were examined by the authors at the final followup, and charts and records were available for all patients. Separate notes were available for review from the prosthetic team to document repairs and failures specific to the device tested. Photographic documentation of routine as well as catastrophic failures was obtained whenever possible. All patients were informed that they could return to their original dynamic response foot at any time.

Results

Early Results

In the initial six-month interval, 14 of the 16 patients (87.5%) preferred the Tru-Step foot to their previous fitting. Seven patients (43%) highly preferred the design. Two patients went back to their original foot design at the sixmonth interval analysis. Both of these patients (one boy, one girl) preferred the Seattle Lite Foot. One girl requested a return to the original foot at only three months, citing excessive foot shell noise and excessive foot shell wear as the primary problems. This patient was noted to have walked without a shoe over the foot shell on many occasions. Without a shoe over the prosthetic device, the two carbon-composite forefoot "bones" rapidly wear through the shell (Figure 2 ). In the early analysis, bushing and bumper failure was the rule rather the exception, with virtually all patients requiring some form of repair to the foot (Figure 3 ). Most of the patients liked the functionality of the device so much that they had no problem with the repair issue.

Early failure was often a result of our own poor selection of the appropriate bumper for the patient's weight and activity level (Figure 4 ). The manufacturer responded to our failures by supplying new parts, and at the end of the study the manufacturer developed stronger subassemblies. Bushing strength was improved, and patients were instructed on how to make minor field repairs themselves (Figure 5 ).

Survey Results

Twelve simple questions were asked of each patient, written in a vernacular that each individual had no difficulty understanding. The results for the first nine questions are shown in Table 2 . The final three questions were designed to elicit subjective responses in sentence form. The results of these questions are abbreviated for the purposes of this report.

Answers to question 10 (What do you like the most about your new foot?) included the following: flexibility in different directions, faster running (two patients), movement and comfort on flat floors, natural feel, natural flex, more spring, better jumping, more bounce, and easier running. For question 11 (What do you dislike most about your new foot?), three patients stated that they had no dislikes. All of the other negative responses were related to maintenance. Crushed bumpers and worn-out foot shells were the main complaints. Noise was important to several patients, with one patient going back to her Seattle Lite Foot as a result.

Interestingly, in response to question 12 (Would you like to continue with this foot, or go back to your previous foot?), all patients requested to continue with this foot, even though chart review indicates early return to the previous design in two patients. Some instances of a return to a previous design may have been orchestrated by the parents, because the requirement for repairs often meant travel of several hundred miles to our center.

Late Results

All patients were examined by the authors in long-term follow-up. It was apparent that three additional patients returned to their previous prosthetic device, or tried different devices. Two boys and one girl experienced longterm failures. One patient, a boy, continued with the College Park foot for nearly three years, preferring it to his previous design regardless of nearly constant repairs. The repair tissue was so overwhelming for this patient that the team recommended fabrication of the Flex-Foot prior to his graduation from our center at age 21 years. In another boy, the team recommended he return to a preexisting design because of excessive failure (six documented additional clinic visits), but the patient and family refused because of the greatly improved function. The girl who returned to her preexisting foot did so after using the College Park device for two years and four months. She indicated that cosmesis and noise were her primary reasons, although she preferred the functionality. Of the total failure rate of 5 of 16 patients (31%), 4 patients returned to the Seattle Lite Foot, and 1 patient began using a new design for the patient, a Flex-Foot. Four patients who used some form of the Flex-Foot design at the start of the study preferred the College Park foot at the conclusion.

All prosthetists on the team noted that the foot requires more maintenance. The repair issues were difficult initially because of lack of familiarity with the device. Early bumper and bushing failures became less of a problem in late review. Catastrophic failures generally occurred after bumper failure. Members of the prosthetic team indicated that the foot allows for precise "fine tuning" and alignment to meet the patient's needs. It is modular in that it allows for the exchange of components without foot replacement to accommodate body size fluctuations. It is now our routine to instruct patients and family in basic maintenance including foot shell replacement and limited bumper replacement.

Conclusion

We feel that the College Park Tru-Step device, by allowing a unique modicum of ankle mobility, represents an advance in lower-extremity modular prosthetic use. We have been encouraged by the results of this pilot study for our transtibial and Syme amputees. Some of the positive subject responses need to be objectified by formal gait analysis. As noted by Perry et al., "future prosthetic designs should provide transtibial amputees with improved ankle mobility that attempt to capture the dynamic characteristics of a normal articulation between the foot and shank segments during the early stance weight acceptance period."² We are encouraged by our 69% long-term success rate, but recognize that repair and maintenance issues, as well as noise, can result in patient failure in this more complex device. The patients in our study represent society's most active members, and thus provided a truly vigorous pilot trial of the Tru-Step foot. Contact with the manufacturer has helped in improving durability in the design. We will continue to offer this foot as an option in our active transtibial and Syme-level patients. We do not feel that there is any best device, but the Tru-Step is a viable alternative for our active teenage patient population.

Acknowledgements

The authors wish to express their appreciation to Ms. Deborah Finch for assisting in the preparation of this paper.

References:

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