Articles
Development, trial and comparative evaluation of a new below-knee prosthesis
From the Department for Orthopaedics and Accident Surgery (Senior Registrar: OTA Dr. Rauhut) of the German Bundeswehrkrankenhaus Berlin (Senior Registrar: OTA Priv. –Doz. Dr. Mager)
The orthopaedic department of the Bundeswehrkrankenhaus Berlin has been working towards improving existing prosthetic models and developing new manufacturing processes for a number of years. In the process in close cooperation with a Berlin-based orthopaedic workshop a method was developed which makes it possible to fit a below-knee prosthesis directly to the body of the patient which cuts out previously extensive intermediate steps (“direct-molding-method”).
This new method for manufacturing a below-knee prosthesis is to be introduced. The prosthesis is being tested on a standard patient group. After the conclusion of the test phase it will be implemented in a third-world country and follow-up examinations of the applied prosthetics will take place. The advantages and disadvantages of the prototype will be recorded. Despite simple manufacturing methods the prosthesis satisfies the requirements with regard to precise fit and comfort. The objectification of the fit of the prosthesis was made by measuring the pressure between the stump and the shaft of the prosthesis.
The following aims are pursued under consideration of possible implementation areas during the manufacture of the prosthesis:
- Manufacture independent of any infrastructure,
- Low costs,
- Short manufacturing times,
- Simple and easily learned manufacturing process,
- Good, comfortable fit,
- High durability
Material and methods
Manufacturing technique:
A technique has been developed for the manufacture of a full-contact shaft, which makes the previous intermediate steps unnecessary and allows the molding of the shaft material directly to the body of the patient by a vacuum method – the so-called “direct molding”.
Diagram. 1
The system consists of:
- Soft wall shaft (moldflex (Bauerfeind/Prothetik)
- Polyurethane resin glass fibre shaft (Firm Neuhof, Firm Össur)
- Atlas flexi foot with connecting and adjustment component (Firm Endolite, England)
Additionally a generator unit, a vacuum pump, a heating furnace and tools for the finish and fitting of the prosthesis are required.
After preparation of the stump the thermoplastic soft wall interior stabilizer, available in three basic sizes, is heated and with vacuum it is directly molded onto the stump of the patient. Subsequently the interior shaft is aligned and fixed to the stump. A bar is positioned in the distal stump to allow the prosthesis shaft to be adjusted to the foot component.
The material for the hard shaft is soaked in water and rolled over the stump. The molding and the hardening is also carried out with vacuum and is completed onto the patient with minutes.
Diagram. 2
After approx. 60 minutes the hard external shaft is ready for further development. The final hardening of the shaft will take approx. 24 hours depending on the humidity and heat in the environment. Via the Atlas substructure, depending on the patient, the prosthesis can be individually aligned. Thus after only a few hours the prosthesis is completed and first attempts to walk are possible.
Adjustments to the length of the prosthesis or position of the foot, which may be required during the first walking attempts are unproblematic and can be quickly carried out so that postural control can be optimized with no effort.
The costs for this prosthesis are approx. 400 € and are thus very inexpensive compared to other Central European models.
Pressure measuring system at the stump-prosthesis shaft interface:
As a parameter for the evaluation of the quality of the below-knee prosthesis, the pressure between stump and soft wall interior stabilizer, the so-called soft socket must be recorded. Therefore the pressure ratio whilst walking and standing still is examined.
The Pliance M Expert System (Firm Novel Munich) with the Pliance 16 M sensor is applied. Each sensor contains 16 individual sensors measuring 0.25 x 0.25 cm with a surface of 0.063 cm². A measurement is made every 0.02 seconds. 10 of these sensors are place at defined points on the below-knee stump.
Diagram. 3
Subsequently the below-knee prosthesis is placed over the same. Now the relevant pressure pattern on the stump-prosthesis interface can be determined “online” whilst walking and standing still. The following parameters are determined: Strength (N), Surface (cm2), Maximum pressure (N/cm2) and Average pressure (N/cm2).
The primary aim, to determine a “reference group” of normal values for a best fitting prosthesis by using pressure measurement on the prosthesis stump was discarded after an extensive group was measured. The recorded values fluctuated to the extent that the recording of an average value for one group was not possible due to the individual differences in quality. Thus the intra-individual comparison of the prosthesis model appeared much more important. Therefore six patients were selected from the aforesaid patient group and treated with the new model in accordance with the direct-molding method in addition to their previous below-knee prosthesis manufactured in accordance with Central European standards. All patients were routines prosthetic users and were below-knee amputees on one leg.
The implementation in the framework of a humanitarian mission:
After completion of the test phase the treatment of a patient group of below-knee amputees was carried out in November 2002 within a humanitarian aid deployment in Kunnamkulam, Kerala/India.
The team from the orthopaedic department of the Bundeswehrkrankenhaus and the orthopaedics workshop in Berlin brought the entire material and the necessary orthopaedic
technology of the direct-molding-system for 50 treatments with them to India; the local partner was the Tropical Health Foundation India (THFI). There was a local orthopaedic workshop. The technicians working there were to be trained to use the direct molding system.
The technical, temporal and comparable financial expenditure in relation to a “gold standard prosthesis treatment" was determined.
During the 11 day visit of the team to Kunnamkulam 85 amputees were examined, measured and statistically recorded. The causes for the amputations carried out were divided in:
- Traumatic cause of amputation in 58.9% of the cases,
- Related to blood vessels in 19.1%,
- Amputation as a result of tumours in 6.8% and
- Infection-related amputations in 15% of the examined leg amputees.
Due to a lack of medical conditions (open stump wounds, contractures, soft tissue problems, lack of coverage of soft tissue etc.) and/or incorrect indication (stump too short, amputation at thigh level, thigh amputation) 47 of the appeared Indian patients could not be treated.
Results
Intra-individual comparison of the prosthetic models:
The intra-individual comparison of the 6 test persons with the existing prosthesis and the new direct-molding-treatment of the below-knee stump results in the following values; for a better overview the measuring value is seen as a 100% reference, the deviation of the direct molding measuring value is given in percent.
Tab. 1
Despite a small group of test persons a trend is indicated with regard to the evaluation of the deviations of the measurement parameters in percent. Without exception the relevant parameters average pressure, maximum strength and average strength for the direct molding prosthesis are below the values of the previously existing prostheses.
The overall results show that although the reference parameters cannot be determined in comparison with groups, however this first data is able to prove a good quality of the fit to the prosthesis stump in an individual comparison. The implementation of the prosthesis model introduced here to a larger extent therefore seems to be justified.
The implementation in the framework of a humanitarian mission:
During the 11 day deployment to South India 38 patients were definitively treated with 41 prostheses. On average 3 working hours were necessary for the manufacture of each prosthesis.
During the manufacturing process it appeared that despite the first attempts to walk being possible after 1 hour, the final hardening of the plastic socket required 24 hours. Otherwise the individual plastic layers started to delaminate which would cause the prosthesis to break.
The original aim to treat the patients definitively on one day therefore had to be changed. Many patients had travelled several hundred kilometres for treatment. The return for a final fitting represented a great logistic and financial problem to these people (accommodation, food).
In comparison to the local systems however, the patients were not only being treated faster with the new system, but the quality had also improved. The pressure measurement at the stump of the prosthesis was carried out on 13 (30%) of the treated patients.
Technical difficulties such as the high level of humidity which the sensors were exposed to did not allow further measurements. The results of these measurements during the testing phase mainly showed balanced pressure patterns.
A successful comparative measurement on one test person who had previously been treated with a local prosthetic system, the so-called Jaipur Foot, with the direct molding prosthesis fitted directly to the body of the same test person, showed an impressively lower pressure on the prosthesis/ stump interface of the direct molding prosthesis.
This results from the application of pressure to the direct molding prosthesis over the entire stump in comparison to the pressure on certain points of the local prosthesis.
Subjectively the patients were very satisfied with the new model. The patients previously treated with a different system unanimously chose the prosthesis with the direct molding system. In comparison to Germany and/or Europe it is apparent that the patients have a natural affinity with movement and are able to use the prosthesis independently following a short induction.
Diagram. 4 and 5
The colour of the prosthesis plastic which is different to the skin colour of the treated Indians was considered a negative point. Also the required large plastic socket was considered aesthetically negative for short stumps.
The technical equipment for the fitting of the prosthesis worked without any problems, there were no technical malfunctions. The plastic material was malleable by more than 95%, the climate with temperatures of up to 34° however, also showed the limits of the material. The storage of the plastic hard socket on a long-term basis is only possible in air-conditioned rooms; otherwise it will spontaneously harden in the packaging and will become unusable.
The instruction of the local technicians was initially slow and tentative however improved continuously during the term of cooperation and finally led to the expected level of cooperation. The technical conditions of the local workshop were completely sufficient. We deemed further manufacture of the residual remaining materials to be guaranteed.
During the cooperation with the members of the local organisation the cost factor proved to be crucial. A local Jaipur-Foot prosthesis costs approx. 45 US$, the model introduced here approx. 400US$. Thus although the developed procedure is much less expensive than central European models (approx. 1,500 € - 7,000 €), according to Indian standards, these costs cannot be met without financial assistance.
Follow-up examination
After 15 months 30 of 41 (75%) of the manufactured prostheses were seen for a follow-up examination. Statements on patient satisfaction, objective fit of the prosthesis and the durability of the material should be made.
Tab. 2:
80% of the patients are satisfied to very satisfied with the comfort of the prosthesis and consider the comfort to be much better than that of the local Jaipur prosthesis (Table 2). The following findings can now be made in the follow-up examination in addition to the manufacturing phase:
Diagram. 6
- Over time the stump can recede (loss of muscle tissue etc.), the prosthesis loosens and the comfort is restricted. 5 prostheses (15%) therefore required the additional application of straps.
- The inlay (Moldflex.®) deteriorates under the burdens of a third-world-country. A regular renewal is not easily possible due to the costs.
- In individual cases the quiver and/or atlas connections could break during extreme physical pressure. The plastic connectors of the atlas system come loose and result in subjectively disruptive noises during movement.
- As already seen in the primary manufacture insufficient, short stumps can only be treated to a certain extent.
- Necessary adjustments in the alignment are possible without problems between the Atlas system and quiver.
- Pressure points or ulcers were practically unseen. The comfort is comparable to a high-tech prosthesis.
Discussion
Whilst in Central Europe the main cause of lower extremity amputations are impairments to the metabolic system and vascular problems, in crisis areas and in third-world countries a high number of amputations result from trauma. In comparison to Central European conditions these mainly young patients show better stump conditions, can be mobilised quickly and are in particular reliant on a replacement of the extremity to secure their living. To treat them with prosthetics however, is difficult for various reasons: The infrastructure is destroyed or is non-existent, the materials can only be procured with difficulty. Trained orthopaedic technicians are hard to find, so that experienced but nonetheless untrained personnel in the country is utilized. There is a lack of funds for the manufacture of the prostheses.
The manufacture of a conventional central European as well as the Indian Jaipur below-knee prosthesis requires at least two to three days for the basic construction and additionally one to two days for the finish depending on patients, consistency of the stump and therapy options. A plaster impression is made and the prosthesis shaft is formed according to the model of the cast. The attempts to walk can then commence; the independent use of the prosthesis can be learned in the subsequent period. In the event of the Jaipur prosthesis common in India, the patient receives a below-knee prosthesis made from the simplest materials with the final pressure being on certain points of the stump which can be uncomfortable. The patient improves the comfort of this prosthesis by wrapping cloth around the stump and the straps.
In third-world countries there are – if at all – only very few technical facilities. As a result of the infra- and social structure thus in many cases extensive travelling is required for an extended period of stay; in the end a treatment cannot be carried out due to the high costs involved.
The high-tech prosthesis models from Central Europe cannot be readily imported to third world countries, as there are no technical conditions in the event of repairs and the prosthesis will become useless.
Due to these basic conditions, which are found in current and future deployment areas of the Bundeswehr, there was a requirement to initially develop a manufacturing process for a below-knee prosthesis, which allowed a prosthesis to be manufactured by a fast and quick method independent of the existing technical facilities; it has to be cheap but comfortable and meet the physical requirements of a patient in a post-war country.
The "direct molding“ procedure introduced here, equates to the demands for a faster and simpler manufacturing process. The material and the required technology can be applied in a flexible manner in the sense of operational orthopaedics technology.
The fittings of 41 prosthesis had already taken approx. 14 – 16 weeks according to the local technology of India. With Direct Molding the same number of treatments was possible in 11 working days, thus 2.5 weeks. This also represents a significant financial and organisational relief for the patients, most of whom have travelled several hundreds of kilometres, due to the final adjustment being made in only one visit.
The local system as well as the Direct Molding System mostly uses the same means of manufacture. Additionally the Direct Molding procedure requires the prosthesis shaft to be applied to the stump via pressure. This is done with a simple, robust, mechanical system, which can be transported in a suitcase. Basically the manufacture is possible in third-world country circumstances with or without regional/technical conditions.
According to Central European standards this is a simple manufacturing process. In the first instance it is no more complicated than the technology for the manufacture of the Jaipur prosthesis. Although the instruction of the Indian personnel was successful in the manufacturing phase, the follow-up examination showed that the technology is not applied in routine circumstances. On the one hand the handling of modern material appears to be more difficult than expected, on the other the additional expenses do not allow it to be applied and practised on a regular basis.
The materials used have proven to be robust and reliable in the deployment area; material breakages were only recorded on rare occasions. The connection systems can break, however repairs and corrections are possible in this case. Only the soft interior wall inlay did not withhold extensive use.
However the cost factor is the crucial problem. The local prosthesis at 45€ is approx. ten times cheaper than the model introduced here, which costs 400€. Despite all qualitative and technical advantages over the local products this prosthesis will not be distributed in these countries without financial support.
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