Technical Note: Simplified Alignment of
Articulations to Positive Models
John W. Michael, MEd, CPO
Carson D. Perry, CPed, CO
M. Louis Whitfield, CPed, RT(OP)
Richard P. Ferment, RT(O)
Introduction
The importance of precisely aligning
mechanical articulations with one another to reduce friction and joint wear
is well known (1). Many external alignment fixtures have been developed to
facilitate this process (see Figure 1
).
Recent literature shows an increasing interest in placing the external ankle joint in greater congruence with the
anatomic axis (2). But, particularly for
pediatric clients whose ankle mortises
are anomalous secondary to neuromuscular deficits, it can be difficult to
maintain both triplanar anatomic congruency and articulation alignment simultaneously using existing external
fixtures.
Although the technique of placing
an internal fixture within the negative
impression before filling it with plaster
is well established, no pediatric sizes
are available (see Figure 2
) (3,4). It is
sometimes difficult to determine precisely the proper anatomic location
deep within small pediatric impressions, and the extra time necessary to
place and later clean internal fixtures
adds cost to the procedure.
A simplified alignment procedure
that avoids these problems has been
developed and was received enthusiastically during a workshop for practitioners from Central and South America (5). Since the necessary fixtures can
be fabricated easily and only a drill
press is required for implementation,
this technique has proven to be practical and inexpensive in a variety of practice settings.
Technique
Placing a threaded rod in the negative
impression, which enters and exits at
precisely determined anatomic locations, can readily establish both anatomic congruency and articulation
alignment in all planes (see Figure 3
).
However, this approach is often impractical since the protruding rod interferes with placing the reinforcement
mandrel within the cast, particularly
pediatric cases.
It is easier to simulate the threaded
rod by boring two perfectly aligned
holes from opposite sides of the finished positive model, using a modified
drill press. The drill fixture consists of
five-inch square of 3/4-inch plywood
(13x13x2 cm) with a protruding steel
pin perpendicular to its surface. A broken drill bit, pressed into a slightly undersized hole, makes a durable alignment pin. This assembly is clamped to
the drill press so the tip of the drill bit
and the steel pin align precisely (see
Figure 4
).
The practitioner marks the desired
anatomic joint centers with an "X" or
both sides of the positive model. The
technician then drills a shallow two mm (1/8-inch) hole to identify the medial joint location. The model is rotated
so the two-mm hole keys onto the steel
pin and the center of the lateral articulation is directly under the drill bit (see
Figure 5
). The technician then bores
into the model to create the lateral
alignment channel, stopping just short
of the reinforcement mandrel. The
model is then rotated and placed with
the steel pin inside the lateral alignment hole. The medial side is drilled
similarly.
Fixtures to form a flat articular surface are created by brazing steel washers onto number 26 copper rivets as
shown in Figure 6
. Varying sizes of
washers allow technicians to select the
optimal configuration for pediatric and
adult orthoses. Pushing the rivet post
snugly into the alignment channels
completes the alignment process. If desired, the area beneath the washer can
be faired into the model with plaster of
Paris or polyethylene foam pads (see
Figure 7
).
Conventional vacuum-assisted thernoplastic molding over the model and
fixtures creates a plastic orthosis with
perfectly congruent and aligned circular areas ready for joint placement or a
second pull of plastic to form an overlap joint (see Figure 8
).
It is also possible to modify any of
the commercially available articulations to permit alignment with this
technique by replacing the articulation
axis with a longer machine screw.
Pushing the protruding machine screw
snugly into an appropriately sized
alignment channel maintains congruency so the thermoplastic can be molded directly over the commercial joints
(see Figure 9
).
Conclusion
This technique has been used for the
past six years on hundreds of articulated designs and proven to be simple,
reliable and replicable. It can be applied to any articulation, including finger, wrist, elbow, hip, knee and ankle
joints. Although more commonly used
for orthotic design, the same technique
can be used to align lower-limb and
upper-limb prosthetic joints as well.
John W. Michael, MEd, CPO, is director of professional and technical services for Otto Bock Orthopedic Industries, Inc., 3000 Xenium Lane North, Minneapolis, NM 55441; (612) 533-9464/(800) 328-4058.
Carson D. Perry, CPed, CO, is a certified orthotist at Duke University Medical Center, MO4, Davison Building, Box 3885, Durham, NC 27710; (919) 684-2474.
M. Louis Whitfield, CPed., RT(OP), is a registered technician at Duke University Medical Center, MO4, Davison Building, Box 3885, Durham, NC 27710; (919) 684-2474.
Rick P. Ferment, RT(O), is a registered technician at Duke University Medical Center, MO4, Davison Building, Box 3885, Durham, NC 27710; (919) 684-2474.
References:
- American Academy of Orthopedic Surgeons. Orthopedic Appliances Atlas, Volume One. Ann Arbor, Mich.: J.W. Edwards, 1952:378-9.
- American Academy of Orthopedic Surgeons. Atlas of Orthotics, Second Edition.
St. Louis, Mo.: CV. Mosby, 1985:30.
- Isman RE, Inman VT. Anthropometric
studies of the human foot and ankle. Bulletin of Prosthetics Research 1969; 10:11:97129.
- American Academy of Orthopedic Surgeons. Atlas of Orthotics, Second Edition.
St. Louis, Mo.: C.V. Moshy, 1985:363-4.
- Michael JW. Articulated Plastic AFO
Workshop. ACOPPRA II Conference;
Panama City, Panama, January 1992.
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