Technical Note: Adding More Adjustability to the Hosmer VFJ-100 Vertical Fabricating Jig

Judd Lundt, B.S., A.E.

Successful and accurate alignment transfer and duplication using the Hosmer VFJ-100 Vertical Fabricating Jig relies upon a right angle relationship between the top of the foot and a pipe, fixed in some fashion in the socket. Thus the pipe, locked and keyed properly in the Mandrel Clamp, will always be vertical as long as the VFJ is mounted on a horizontal surface. If the Ankle Plate (which represents the junction between the top of the foot and the pylon clamp) is also properly locked and keyed, it will always be horizontal and at right angles to the pipe in the socket.

The VFJ-100 is constructed in such a way that once the pylon clamp is attached and the pipe affixed in the socket, all or part of the aligned prosthesis can be removed as the alignment will have been transferred to and "remembered" by the VFJ-100.

If the original model from which the socket was made is available and can be successfully fit back into the socket, the pipe can usually be expected to be off of vertical when the aligned prosthesis is attached to the VFJ100 at the ankle plate. This results from angulation changes made during the alignment procedure and will necessitate filling the socket with sand and/or plaster and the setting of a new, vertical pipe for alignment transfer. With the addition of an adapter developed from used Otto Bock endoskeletal components, this step can be eliminated.

The Otto Bock pyramid and setscrew system of alignment allows angulation changes that can also be "remembered" and returned to without concern for loss of alignment. By combining this system with the VFJ-100, non-vertical pipe can be easily accommodated. Figure 1 depicts two devices fabricated from Otto Bock 2R2 units that have been shortened beyond functional use as prosthetic components. They have been adapted for use with the VFJ-100.

The unit on the right in Figure 1 is a shortened 2R2 that includes two machine screws in holes drilled and tapped at right angles to each other in the remaining portion of the pylon tube. An index notch has been cut into the tubing parallel to one of the set screw axes. The tubing portion will accept a standard 3/4" pipe and the machine screws can be tightened to secure it in place. A line can be scribed around the installed pipe to include the notch. This will allow removal and replacement of the model pipe without loss of its orientation.

The other unit, shown in more detail in Figure 2 , is also a 2R2, but in this case the tubing end has been closed with a Kingsley Symes foot nut ground to fit and then secured with two copper rivets. The backside has been filled with a small amount of resin to stabilize the nut. This provides a standard 3/8" x 16 U.S. thread at the distal end. The unit is also notched parallel to the axis of one of the setscrews.

For use in conjunction with the unit in Figure 2 is an Otto Bock 2R8 ankle component, the hole of which has been bored to allow either a metric or U.S. foot bolt to easily slide through (left unit in Figure 3 ). The distal surface has been ground down to remove the three ridges that aid in securing the 2R8 to a foot. Metric and U.S. foot bolts can be used to attach this modified unit to whatever pylon component is used on the aligned prosthesis (Figure 4) ; however it will be necessary to remove excess material from the bolt head to prevent interference with its range of adjustment. This modified 2R8 may also be attached to a Mandrel Bushing through use of a short length of 3/4" pipe force-fit with a Kingsley Symes foot bolt from which the overhang has been ground off (Figure 5) .

With these two units, a non-perpendicular alignment can be easily accommodated in the VFJ-100. The unit with the set screws is attached to the pipe in the model and united with the Mandrel Clamp through the modified 2R8 and Mandrel Bushing. By adjusting the four set screws, the pylon attachment component of the prosthesis can be brought to horizontal and its hole aligned with the hole in the VFJ-100 ankle plate (Figure 6) . The two are then secured with a bolt in the usual manner. The pipe at the proximal end may be indexed by scribing a line that includes the notch. It may be removed if desired.

An alternative technique is to use the other unit with the 3/8" female threaded end, attached to the ankle plate with the modified 2R8 inverted as shown in Figure 7 . With this method, the pipe in the model is attached proximally to the VFJ-100 in the usual manner and left vertical. The pylon attachment of the aligned prosthesis will be non-horizontal. However, the entire system may be secured to the VFJ-100 Ankle plate through the 2R2 unit after it is brought to vertical by adjusting the set screws and then aligned with and secured through the hole in the ankle plate.

Through use of either the modified metric of U.S. foot bolts, the 2R8 may be attached to any available flat-bottom pylon attachment component. In addition to alignment transfer, the system may be used to bring an inverted model or socket into a desired flexion/extension and abduction/adduction attitude. This will allow addition of a horizontally oriented plaster buildup which is necessary for attaching some of the newer aboveknee components. Figure 8 shows a whimsical union of both of the modified units to show the significant range of angulation that can be achieved by combining the two.

The units depicted here were modified using simple techniques and parts and equipment found in most prosthetic fabrication facilities. The individual with access to machine tools could certainly achieve a more finished and permanent result. Furthermore, one need not be limited only to the several applications discussed nor the range of combination of parts. Conceptually, the idea is to bring the addition of the Otto Bock angulation mechanism to the VFJ-100. Specifically how this is implemented is limited only by the ingenuity of the person doing it.

Judd Lundt, B.S., AL., is with UCLA Rehabilitation Center's Prosthetics-Orthotics Program at 1000 Veteran Avenue, Los Angeles, California 90024; (213) 825-6341.

References:

Vertical Fabricating Jig, Parts Nomenclature Instructions for Use, Hosmer Dorrance Corporation, Campbell, California 95008.
Otto Bock Modular Leg Prostheses System Overview, Otto Bock Orthopedic Industry Inc., USA, Minneapolis, Minnesota 55422.