Then Plaster-of-Paris (POP) bandages are applied to the stump to make a negative impression of the stump shape. In a PTB socket fabrication process, manual.
Development of an Alternative Socket Fabrication Method Using Hydrostatic Cast System: A Preliminary Study S.S. Syed Shikh, N.A. Abu Osman and L. Abdul Latif The proceedings from the International Seminar for Prosthetics and Orthotics ’98 in Japan estimated that 9 out of the 15 million people in the world that need prosthetic and orthotic services are located in South-East Asia. The seminar also recognized that a significant percent out of that 9 million people do not have access to the needed services. The disruption in the prosthetics and orthotics supplydemand chain in this region is largely contributed to the lack of expertise and high cost of these devices. Specifically, in Malaysia, the problem stems from the shortage of trained professionals in providing the services and non availability of facilities to train professionals. Fabrication of prostheses and orthoses is largely dependent on the skill and experience of a prosthetist. A trained prosthetist understands that there are known guidelines in producing prostheses but studies observing the artificial limb fitting process over the years have consented that this area is often still as much an art form as a science. [2] In the case of transtibial prostheses, the fit of the prosthetic socket is a major determinant of efficiency and comfort of the overall prosthesis. Owing to its function as a crucial component of the transtibial prosthesis, the socket has undergone tremendous development to improve its function and fit since the inaugural introduction of the patellar-tendonbearing (PTB) socket in the 1950s. [3] Since then, modifications to the transtibial socket have included adjustments to the shape, material, suspension and fabrications methods. Prosthetists now are able to choose from total surface bearing sockets, sockets made from thermoplastics or thermosetting resins, stereolithograph sockets and a myriad of others. [2] A particular debate when choosing the transtibial socket shape is that of either a PTB or non-PTB.
A PTB socket is fabricated under the principle that different soft tissues have different pain threshold values, thus pressure exerted in these areas should vary accordingly. The socket has pressure relief regions, in particular the patella tendon and the lower border of the tibial medial condyle. [4] Concentrated body weight at these limited regions contributes to the major drawbacks of a PTB socket; soft tissue stretch, skin abrasion and oedema. [5] The volume of the socket which does not match the volume of the stump due to the modifications for pressure relief areas add to the patient’s discomfort. In comparison, a non-PTB socket works on the grounds that pressure should be equally distributed throughout the entire stump. With this equal pressure distribution, the non-PTB socket overcomes the limited weight bearing area a PTB socket posses and thus provides a solution to the discomfort. In addition, since the volume of the socket is equal to the volume of the stump, a non-PTB socket helps in the stump’s blood circulation and provides better sensory feedback and proprioception. Traditionally, the fabrication of both PTB and non-PTB sockets are carried out in a three step process. The first step involves stump preparation. Here, with the patient seated and hips flexed to a 90 degree angle, the stump is wrapped in a stockinette sock. Then Plaster-of-Paris (POP) bandages are applied to the stump to make a negative impression of the stump shape. In a PTB socket fabrication process, manual pressure is applied to the patella tendon and popliteal space to produce the pressure relief areas. The negative cast is removed once the POP hardens. The fit of the socket produced from this casting process which is carried out while the patient is seated is questionable as the stump is under a no load situation. Since stump shape differs in both sitting and standing positions, the cast taken while a stump is under load would be a more accurate representation. [6] The second step involves the formation of a positive cast which is done by pouring POP into the negative mold. Further modifications to the pressure relief regions in a PTB
socket are carried out here. The final step in socket fabrication is the lamination of the positive mold to produce the final socket. Over the years, sockets have been laminated with different materials, including, epoxy glass with an extension of pure resin, polyethylene, thermoplastics and carbon fiber. The first step of the PTB socket fabrication process which is the manual pressure applications and rectifications is highly dependent on the skill of the prosthetist. Expertise and experience are the two factors that enable a prosthetist identify the bony prominences on a stump, the pressure relief areas and the correct amount of pressure to be applied when making the contours in a PTB socket. This dependency on skill contributes to the high cost of a prosthetic socket. As a result, numerous studies have been carried out to provide an alternative technique to this manual casting method. A popular approach introduced in the 1980s involved the utilization of Computer-Aided Design/Computer Aided Manufacture (CAD/CAM) system. The majority of the developed systems use scanning devices as stump measuring techniques, on screen rectifications and milling machines or rapid prototyping techniques to make a positive mold. Another technique involved the use of an outside pressure source for casting. Several innovations have been done in this area. Ossur® for instance, developed the Icecast® Anatomy system utilizing air-filled shaping chambers which provide the needed pressure to mold a socket relative to the patient’s anatomy. [8] A group from Northwestern University designed a CIR sand casting system for transtibial socket where pressurized tank filled with sand was used to obtain a negative cast of a transtibial stump. This method eliminated the use of POP in the whole socket casting process. [9] Another study also adopted the pressure casting method where instead of POP, carbon fibers were used as the material for socket fabrication. This allowed for a faster casting process as the carbon fiber socket can be used after the first cast.
The present study describes a hydrostatic tank casting system. The tank aims to be an alternative non-PTB casting method. Relying on hydrostatic pressure to produce the negative stump mold, this casting system will reduce the dependency on skill in prosthetic socket fabrication, apply uniformly distributed pressure during casting and produce a non-PTB socket that resembles the stump shape under load.
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