A Calf Corset Weightbearing Ankle-Foot
Orthosis Design
J. Martin Carlson, CPO
Fran Hollerbach, CO
Bruce Day
Introduction
Designing and providing an ankle-foot orthosis (AFO) that allows the client to walk
comfortably and safely without bearing
weight through the skeletal elements of the
lower leg and foot has proven to be a daunting challenge. The patella tendon weightbearing (PTB) ankle-foot orthosis is often
prescribed for unweighting the lower tibia,
ankle and foot despite fairly wide recognition that it achieves only partial unweighting. This orthosis design dates back at least
to the early 1960s (1, 2, 3, 4, 5). Some articles
report very good clinical results but note that
extraordinary care in fabrication and follow-up is necessary (6). The orthosis presents
such a challenge due to the anterior-posterior (A-P) pressures and circumferential constraint in the proximal brim area.
It is worthwhile to note that a PTB AFO
presents significantly different challenges
than a PTB prosthesis. First, the "pre-flexion commonly fabricated into a BK prosthesis increases the projected undercut of
the patellar tendon and tibial flare areas.
Pre-flexed alignment is a much less practical
option in an orthosis and requires a
tighter A-P dimension. The second, and perhaps the more important, difference is the
pumping action derived from every step taken with a BK socket. That aid to circulation
does not exist in the AFO counterpart of the
PTB prosthesis. Finally, tightness of the
PTB AFO brim while sitting cannot be relieved by a slight distal displacement as occurs with a BK socket.
Design
Our earliest design efforts involved trying to
augment patella tendon weightbearing
(which we knew to be marginal) with calf
weightbearing. As time went on we found
calf weightbearing so effective, and patella
tendon weightbearing so problematic, we
dropped the latter and concentrated on optimizing the former. We created a design that
uses a calf corset to transfer weight to the
AFO. The corset circumferentially encompasses the inverted cone-shaped lower leg up
to the level of the apex of the gastrocnemius/
soleus muscle belly (see Figures 1
and Figure 2
). At
that point the posterior coverage ends, but
anteriorly, the orthosis extends up to or near
the patellar tendon. In virtually all nonparalytic cases, the calf muscles (especially the
plantar flexor muscle group) present sufficient projected undercut area to achieve 100
percent unweighting. To achieve this, however, requires certain design features, which
will be noted shortly.
We first used this design for people with
fractures of the distal tibia, talus or calcaneous. Those fractures were often transcondylar; some were surgically fixed internally.
The design also is useful for treating talar
Osteochondritis Dissecans. More recently
we have used it to follow reconstructive orthopedic surgery after crushing injuries to
the mid- and hindfoot. Some of our colleagues have used this design to unweight
neuropathic (Charcot) ankle and foot joints
and heel ulcers (7, 8).
Let us back up a bit and consider some of
the treatment factors and variables so we
may best understand the orthotic design requirements and rationale. First, we must appreciate that since the taper of the inverted,
cone shape (and the corresponding projected undercut area) is not large, we must
achieve an almost perfect match between the
contours and alignment of the corset and the
anatomy it is to fit against. Next, we must be
aware that edema, atrophy and other factors
can and do cause variations, over time, in the
size and shape of the lower leg. Changes in
size and shape usually are not symmetrical so
they contribute small but important alignment variations.
We have found the following design features essential for true weight transfer for a
period of months when the size and shape of
the lower leg has varied.
- The calf corset must be of a moldable
and self-reconforming material.
- The calf corset must be suspended within the orthosis frame in a way that allows the
medial-lateral (M-L) diameter, as well as the
A-P diameter, to reduce or pull-in as the cuff
is laced tight.
- The calf corset must be suspended in
such a way to allow some self-aligning within
the frame of the orthosis.
The reconforming requirements lead us to
use a full-leather, lacer corset. Leather has
been neglected by some practitioners, but it
offers good contourability, permeability and
tensile strength, making it a perfect material
for leg corsetry. It can accommodate complex and time-variable contours without
compromising structural stability or fit as
would fabric or plastic.
The calf corset is suspended within the
AFO structure at four points. The proximal
suspension points are simply riveted to the
plastic shell structure near the top of the
polypropylene shell's medial and lateral extensions. The plastic shell at those points
may spring inward or outward as needed to
follow size changes. The other suspension
points are attached at about mid-corset with
metal "leaf" springs. The leaf springs let the
corset pull away from the structure and/or
move anteriorly and posteriorly as necessary
to correct any alignment imperfections. You
can see how this suspension scheme transfers
loads from the conformable corset to a rather rigid weight-bearing structure without letting the latter's rigidity impose constraints
that hurt the corset's fit.
The corset and shell of the orthosis extend
to the proximal tibia anteriorly but only to
the crest of the calf muscle belly posteriorly.
The anterior trimline is proximal to minimize forces and pressures generated against
the tibia by floor reaction forces between
foot-flat and toe-off. Those forces can be
quite large in cases where orthotic ankle motion is totally or partially blocked. Modifying
the client's shoe to give a "rocker" or rollover effect is an option for reducing the magnitude of those anterior floor reaction forces
and the high stresses they induce across the
ankle joint and stirrup. The posterior trimline is as shown (see Figure 3
) to allow the
client to loosen the corset by merely pushing
the orthosis a centimeter distal on his or her
leg when seated. This very low posterior
trimline is an important measure of comfort.
The low posterior trimline in no way detracts
from the function of the orthosis.
When we first began to see how effective
the calf corset was, we decided to measure
and record the amount of weight transferred. That was soon abandoned as we discovered that by simple adjustment of the calf
corset height, we could eliminate weightbearing contact between foot and foot plate.
A calf corset weightbearing AFO fabricated
for the primary author confirmed that total
weight transfer could be accomplished and
maintained comfortably throughout a daylong wearing period.
Conclusion
Decisions about limiting ankle motion are
very important. For fractures we recommend that the ankle joint be fixed. Those
clients should be taught to walk without any
forceful calf muscle activity because the calf
muscles themselves can exert very large
forces on the bony elements-even in the
absence of weightbearing. We recommend
the same for clients with Osteochondritis
Dissecans, neuropathic (Charcot) joints or
metatarsal ulcers. Ankle joint range-of-motion may, in most cases, be increased by increments before discontinuing the orthosis
(i.e., fracture treatment).
When calf muscles are severely atrophied,
such as in spina bifida, they do not present
enough inverted cone taper for this type of
suspension. We have used a similar approach in one such case by attempting to
achieve suspension by using the tibial flare
area just distal to the knee joint. Although
we were successful, it was so challenging we
would list severe calf atrophy as a contraindication to the use of the calf corset weightbearing design. We would also urge caution
in using this design for patients with significant peripheral-vascular disease and for patients with significant sensation loss in the
calf area.
It seems very unlikely that the concept of
calf-corset weightbearing is new. It is such a
direct, simple and intuitive approach, many
others must have fabricated very similar designs in the recent and distant past.
Acknowledgements
The authors wish to acknowledge the late Dr.
Thomas Comfort for his support for this design
development.
J. Martin Carlson, CPO, is president of Tamarack
Habilitation Technologies Inc., 1471 Energy Park
Drive, St. Paul, MN 55108-5204.
Fran Hollerbach, CO, is retired from Northeast
Metro Technical College, St. Paul, MN.
Bruce Day is an orthotist at Gillette Children's
Hospital, 200 B. University Ave., St. Paul, MN
55101.
References:
- McIlmurray WJ, Greenbaum W. Patellar
tendon-bearing socket for weightbearing braces.
Veterans Administration Prosthetics Center,
New York Jan. 3, 1961 (unpublished).
- Nitschke, RO, Marschall K. The PTB knee
brace. Orthotics and Prosthetics, September
1968 ;22 :46-51.
- A manual for fabrication and fitting of the
below-knee weightbearing brace, VA Prosthetics
Center, New York April 3, 1967, "draft" copy
without list of authors.
- Lehmann J, Warren CG, Pemberton DR,
Simons BC, DeLateur, BJ. Load-bearing function of patellar tendon-bearing braces of various
designs. Archives of Physical Medicine and Rehabilitation August 1971;52:128-133.
- Warren CG, Lehmann JF. Effect of training
on the use of weightbearing orthoses. Physical
Therapy May 1975;55:5:119-124.
- Gristina AG, Nicastro IF, Clippinger F, Rovere GD. Neuropathic foot and ankle patellartendon-bearing orthosis as an adjunct to patient
management. Orthopaedic Review May
1977 ;6:5 :53-59.
- Doucette M. Advances in wound healing. A
verbal presentation at AAOP continuing education seminar, White Bear Lake, Minn., April 6,
1991.
- Hines K. Dysvascular orthotic applications.
A verbal presentation at AAOP continuing education seminar. White Bear Lake, Minn., April 6,
1991.
|
|
