TECHNICAL FORUM---
Four-Bar Swing-Shell Closure for Weightbearong Orthoses and Prostheses
Tim Dennis, CO
Todd Anderson, CP
Richard Hall, RTPO
Timothy Ryan
ABSTRACT
The authors have developed a swing-shell closure with four bar linkage that combines the advantages of the familiar
bivalve with flexible overlap designs. Like the conventional
hinged bivalve or "alligator-mouth" design, the swing shell
has a rigid anterior panel that provides consistent weightbearing contours and positive vertical suspension. Like the
flexible overlap system, the swing shell also offers ease of
donning and accommodates volume changes. These advantages justify the added fabrication time, particularly when
atrophy or growth is anticipated. Encouraging results in 24
of 25 orthotic cases led to the successful incorporation of the
design into 15 prostheses for congenital anomalies.
Introduction
Circumferential containment orthoses, originally described as PTB braces, are commonly used in treating
diabetic neuropathies (1). To prevent plantar ulcers, the
orthoses may be used with stirrups, depth inlay shoes and
multidurometer custom-foot orthoses (2,3). When skin
breakdown is not an immediate concern, use of a molded
plastic footplate is an attractive option. Tibia/fibula fractures, non-union of ankle fractures, Charcot neuropathy
and severely unstable ankles secondary to arthritis also
may be treated successfully with these devices (4,5,6). Traditional bivalved designs often had to be refabricated during treatment to accommodate volume changes. The four bar swing-shell design offers an alternative to standard
bivalve or flexible overlap closures.
As Figure 1
illustrates, the swing-shell orthosis employs
an anterior panel connected to the posterior AFO by a
four-bar linkage that facilitates donning by offering a much
larger proximal opening than a hinged alligator-mouth closure would. Unlike the floating anterior panel of the familiar bivalved design, the swing-shell linkage maintains the
vertical alignment of the weightbearing anterior panel despite accommodating significant volume changes.
The swing-shell design also has been used on prosthetic
devices for patients with longitudinal fibular deficiencies
not treated by foot ablation (see Figure 2
) (7,8,9). These
pediatric patients often have difficulty consistently donning bivalved or anterior-opening prostheses and tend to
outgrow the hinged alligator-mouth designs quickly. The
swing-shell closure offers excellent weightbearing capabilities and consistent donning as well as a large opening to
allow passage of the foot.
Orthotic Fabrication
The posterior AFO segment is thermoformed in the conventional manner. The cast is draped with 5-mm (3/16-inch)
medium-density Pelitea then 5-mm polypropyleneb is thermoformed and trimmed so the medial and lateral trimlines
terminate at approximately the midline of the leg (see
Figure 3
). The Pelite is left long then skived very thin to
smooth the transition from anterior to posterior. In Figure
4
, a 5-mm piece of medium-density Pelite is wrapped
around the model, and the anterior plastic shell is thermo-molded over it. The anterior panel is trimmed to overlap
the posterior AFO by 2 cm.
Removing the Pelite from the overlap area creates sufficient space to accommodate the linkage bars that are cut
from 3-mm (1/8-inch) polypropylene. Figure 5
depicts a
typical pattern for the links suitable for an adult; the proximal bars should be about 2.5 cm (1-inch) shorter than the
distal pair. The links can be scaled up or down to accommodate pediatric or larger adult applications.
The proximal pivots are drilled into the AFO, and the
proximal links are mounted first, using Chicago screw
posts (also known as Gillette Bushingsc). Next, the distal
pivots are drilled into the anterior panel, and the distal bars
are mounted. Figure 6
details the attachment of the linkage
bars. After placing the anterior shell over the AFO, the
remaining pivots are marked, drilled and mounted. The
Pelite in the AFO is removed. Chafesd and two 3.8-cm
(1 1/2-inch) Velcroe straps are riveted on then new Pelite is
cut, skived and glued in (see Figure 7
).
Prosthetic Fabrication
The anterior panel of the laminated socket is removed and
discarded. Strips of Pelite are spot glued along the medial
and lateral trimlines to accommodate the swing hinges if an
unpadded anterior panel is desired. Next, 5-mm polypropylene or copolymer is thermoformed to create the anterior closure. The linkage is identical to the orthotic technique.
Clinical Experience
Northwestern Prosthetics & Orthotics Inc. has fitted 25
patients with the swing-shell AFO over two years. The
patients' ages ranged from 6 to 76, averaging 40.6 years.
Fifteen males and 10 females participated. The most common pathology was diabetes-related vascular disease (12
cases); six patients were being treated for fractures of the
tibia, calcaneus or talus. The other seven cases included
congenital deformity of the calcaneus, osteoarthritis, ossifying fibroma, and foot and heel ulcers of unspecified origin.
Only one patient has rejected the swing shell to date,
preferring his original alligator-mouth design due to the
increased leverage the pivot fulcrum offered, which he felt
allowed him to tighten the shell more easily. Perhaps an
improved closure method can be developed to overcome
this objection.
Two additional patients had large volume reductions
and required new swing-shell orthoses to compensate. The
remaining patients were generally pleased with the fit and
function of this design. Several who had previously worn
other containment orthosis designs commented on the
swing-shell closure's reduced bulk and greater ease of donning.
The Shriners Hospitals-Twin Cities Unit has fitted 15
swing-shell prostheses for fibular insufficiency over the
past two years. All patients were converted from anterior or posterior-opening laminated designs. None has expressed any interest in returning to his or her previous
closure style.
Conclusion
The four-bar swing-shell closure offers clinically significant
advantages over alternative designs in ease of donning,
accommodating volume changes, and maintaining weightbearing contours and alignment. Clinical experiences have
demonstrated its applicability for circumferential containment AFOs and prostheses for selected fibular deficiencies. This technique may be adaptable for other levels of
orthotic and prosthetic design such as posterior-opening
ankle disarticulation (Syme) prostheses and ischial containment KAFOs.
Tim Dennis, CO, is an orthotist at Northwestern Prosthetics & Orthotics Inc., 900 20th Ave., S., Minneapolis, MN 55404.
Todd Anderson, CP, is the prosthetic-orthotic manager at Shriners Hospitals for Crippled Children, Twin Ceties Unit, 2025 E. River Road, Minneapolis, MN 55414.
Richard Hall, RYPO, is the orthotics-prosthetics technician who participated in the development of the swing=shell PTB at Northwestern Prosthetics & Orthotics Inc. and is now at Tilges Certified Orthotic Prosthetic, Maplewood, Minn.
Timothy Ryan is a marketing consultant at Northwestern Prosthetics & Orthotics Inc. and a recent graduate in prosthetics from Northeast Metro Technical College in White Bear Lake, Minn.
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