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.
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