Squirt Shape Enters New Phase of Development

Joshua Rolock, PhD, Principal Investigator

Co-investigator:
Kerice Tucker

Funded by: Department of Veterans Affairs;
                 National Institute on Disability and Rehabilitation Research (NIDRR)

An increasing number of prosthetists are now using computer-based techniques in the planning and fabrication of prosthetic sockets. For these clinical professionals,computer-aided design and computer-aided manufacture [CAD/CAM] provides an alternative to hand fabrication for the production of artificial limbs, and results in greater control and productivity in the limb fitting process.

We have developed a device for the computer-aided manufacture of sockets which we call Squirt Shape. Squirt Shape is a fully-automated system for the production of sockets using computer-based shape information.

The Squirt Shape approach eliminates the need for the physical socket model that is required when molding sockets manually or when using commercial equipment for computer manufacturing of sockets. Consequently, socket production can be achieved more accurately and at lower cost than when using the other methods.

Briefly, the Squirt Shape technique uses a motion-guided extrusion device to deposit thin layers of plastic in precisely shaped cross-sectional profiles. The result is a three-dimensional object (the socket) that is composed of hundreds of plastic layers that are stacked upon one another and thermally bonded together (see figure 1). The socket is produced in about one hour by an unattended machine, using less than a dollar's worth of raw material. The sockets are typically fabricated with the inclusion of a standard shape at the end which facilitates assembly with the other limb components using off-the-shelf hardware.

In a new phase of development, we are investigating alternatives to traditional methods of assembling artificial limbs. Our approach will capitalize on the features of Squirt Shape to enable us to reduce or eliminate the need for fasteners such as screws, nuts and washers. Furthermore, we anticipate a reduction in the number of components needed to produce a limb, and consequently a reduction in the weight of the limb.

Our first effort in this new phase of development is the integration of the socket and pylon into a single-unit structure. Software was written to add the pylon to the socket shape data, and allows for alterations in the position and orientation of the "virtual" pylon, prior to fabrication, to enable alignment changes to be made. The resultant limb can then be fabricated in standard Squirt Shape fashion. One such integrated limb is shown in the accompanying figure (see figure 2) in comparison with a standard type of limb assembly.

Figure 2: The Integrated Limb. On the left, a prosthetic limb assembled in the conventional way (socket alone produced with Squirt Shape).  On the right, the integrated limb; connecting hardware and pylon are replaced by an integrated limb that is fabricated in a single piece using Squirt Shape.

In this instance, the composite weight was reduced from 1.44 lbs to 0.7 lbs. The strength of the integrated limb was tested in a materials test machine and had a strength of 308 ft-lbs at the ankle level. Fabrication time for the integrated limb was 1-3/4 hours which, while longer than the fabrication time for the socket alone, benefits from reduced assembly time, and reduced cost from the elimination of connector hardware.

Further goals of this project include the incorporation of the prosthetic foot, or portions of the foot, into the integrated limb, and the development of specialized attachment hardware for use with Squirt Shape. These improvements will increase the utility of CAD/CAM methods in prosthetics by further increasing productivity, accuracy, and reproducibility as compared to other production methods.