The Below-Knee Bypass Prosthesis
Anthony J. Cappa, CPO, BS
Introduction
The below-knee bypass prosthesis (BKBP),
though seldom heard of, is a valuable tool in
rehabilitating complicated diabetic, dysvascular and delayed-healing below-knee amputees. Diabetic amputees with associated
vascular disease comprise the majority of
BKBP candidates. Of the five cases seen by
the author over a two-year period, three unilateral and two bilateral, all were diagnosed
as diabetes mellitus and three were complicated by peripheral vascular disease. A total
of six BKBPs were fabricated; one patient
wore them bilaterally.
While used primarily for delayed-healing
amputations, the BKBP can be of great value to patients suffering from persistent residual limb pain, dermatological pathology, recent skin grafting and bony overgrowth that
is sensitive or deemed too fragile to endure
sheer forces exerted by a conventional total
contact socket (see Figure 1
).
Treating a problematic or delayed-healing
BK amputation often includes ambulation
with a walker or crutches, or confinement to
a wheelchair until primary wounds close.
This treatment does little to aid in rehabilitation or to hasten the healing process. The
possibility of falling and injuring the already
precarious amputation site outweighs any
cardiovascular benefit an amputee would
gain by using a walker or crutches without a
prosthesis. A long period of confinement in
bilateral and delayed-healing cases only
serves to promote contractures of the hip
and atrophy of thigh musculature, and it
does little to boost amputees' morale or confidence.
A properly fitting BKBP coupled with
competent physical therapy can help speed a
patient's recovery and provide a smoother
transition from amputation to temporary
prosthesis. The patients previously cited
used the bypass prosthesis for an average of
two months (from date of delivery of bypass
prosthesis to date of delivery of temporary
prosthesis).
A poster exhibit of 23 patients with 24
amputations presented by Dr. Daniel Shapiro et al. in Washington, D.C., showed the
BKBP was used by patients an average of
four and one-half months. When wound
healing was complete, one bilateral and 17
unilateral patients were able to use a conventional patellar tendon-bearing (PTB) prosthesis. Of the 23 patients, only two failed to
ambulate independently with the bypass
prosthesis.
Measurements
If the clinic team foresees an extended healing
time or if the patient's limb does not lend itself
to an immediate PTB fitting, a bypass prosthesis should be ordered. The device should be
delivered as soon as possible, and the patient
should begin ambulating within one to two
days, thus minimizing the ill effects of wheelchair confinement or crutch walking.
The BKBP incorporates an adjustable
quadrilateral brim, modified Silesian suspension belt, knee joints with bail lock, a
SACH foot and wood ankle block (see Figure 2
).
Measurements should be taken with the
patient standing, if possible (see Figure 3
and Figure 4
).
Fabrication
After measurements are taken, the patient's
thigh is traced and then cast in plaster of
paris bandage with the wrap extending from
the ischial tuberosity to a point four inches
above knee center. A brim may be used to
aid shaping of the proximal section. The cast
is then poured, modified and made ready for
vacuum forming.
A length of 3/16-inch polyethylene is vacuum formed over the cast's anterior portion.
After the mold has cooled, the shell is removed. The proximal edges are smoothed;
the medial and lateral edges, reaching to the
distal end, should extend about 1/2-inch beyond the medial and lateral midlines to allow
for later trimming and subsequent tissue
compression.
The anterior shell is then placed on the
modified plaster model and prepared for
molding of the posterior shell. The posterior
shell is drape molded under vacuum using
3/16-inch high-density polypropylene (1/4-inch
may be used for heavier patients).
Once the anterior and posterior sections
are molded, a pair of 1/4-inch by 5/8-inch aluminum bail lock knee joints are aligned in
accordance with the measured knee's M-L
diameter, located on the model along the
midline of the thigh. The cast is then flexed 5
to 10 degrees, and the position of the uprights is marked. Two adjustable extension
bars measuring 1/8-inch by 5/8-inch by 7 inches
are drilled and tapped for 8-32 screws. The
extension bars are then contoured and
placed on the posterior shell in the previously marked positions.
The upper uprights are now contoured to
lie against the extension bars. Using a #19
drill bit, clearance holes are drilled in the
uprights for 8-32 screws. A minimum of
three screws must be in place in each upright
at all times.
Next, the SACH foot is mounted on a
wooden ankle block, a shoe is applied and
vertical lines are drawn along the block's medial and lateral midlines. A parallel line 1/4
inch anterior to the center line is scribed to
posteriorally offset the foot and facilitate
rollover during ambulation.
Dados are cut in the ankle block-centered on the previously scribed lines-to receive the lower uprights. At this point, refer
to the knee center to distal end of residual
limb measurement and to the knee center to
heel measurement. The top of the ankle
block must clear the distal end of the residual
limb. The ankle block may be trimmed if
necessary; however, there must be sufficient
length available to accept three #10 by 1 1/2.
inch wood screws to anchor the lower uprights to the block.
Refer to any tracing or measurements of
genu varum or valgum and adjust the contour accordingly. The distal ends of the uprights are contoured into the ankle block
channels.
Three holes are drilled in each upright to
accept the #10 wood screws, and the uprights are secured to the block. The ankle
block is shaped and laminated with carbon
fiber, nyglass and acrylic resin. A padded
calf band and strap may be fabricated from a
variety of materials, depending on individual
limb length, contractures or special skin conditions. This decision is left to the physician
and prosthetist.
A Silesian belt with a contralateral strap is
made and attached to the posterior shell.
Adjustable Velcro straps are attached to the
posterior shell to provide compression and
closure.
Fitting
A length of stockinette is drawn over the
entire limb, extending above the anticipated
end of the socket. With the patient in a supine position, the BKBP is slid under the
posterior aspect of the thigh and brought
into contact with the ischium. The anterior
shell is then fit, its proximal edges aligned
with the posterior shell and the straps tightened.
At this point the overall fit should be evaluated, paying particular attention to proper
knee M-L, upright clearance for genu varum
or valgum, and clearance between the distal
end of the residual limb and the top of the
ankle block. If the clearance between limb
and ankle block is less than two inches, apply
a force at the heel simulating weightbearing
and measure the amount of displacement.
Correct the length of the block as necessary
(see Figure 5
and Figure 5a
).
Connect the calf strap, but do not apply
pressure to the limb. The limb should be
free-floating to allow for piston action of the
limb when ambulating.
With the Silesian belt in place, the patient
is ready to stand. At this point static alignment can be done. Ensure that the proper
socket flexion and ground clearance for
swing through are present, and check the
overall comfort of the socket.
With delivery of the prosthesis, the patient
begins learning proper donning and doffing.
Practice the procedure for changing a sterile
dressing with the patient since the patient
will likely be discharged from the hospital
before wounds close completely. The patient
should practice sitting in the BKBP by unlocking the knee joint on the edge of a chair.
Correct entry and exit from an automobile
should be taught also, and the patient should
know whom to contact should any problems
or questions arise.
Conclusion
The below-knee bypass prosthesis is a valuable tool for the rehabilitation team. By hastening recovery, the patient's hospital stay is
lessened, financially benefiting the patient.
Patients learn the bilateral gait patterns
they will need when they receive their temporary prosthesis, thus reducing gait training. Should a patient's limb incur skin breakdown while wearing the temporary prosthesis, the BKBP can be used, allowing the
wound to close without disrupting the patient's daily routine.
Many options are available with a BKBP.
These include use of a rigid dressing with no
direct weightbearing, a stump shrinker sock,
and if a knee flexion contracture is present, a
splint and turnbuckle can be incorporated.
This article is meant to introduce a device
that has been under-utilized. With healthcare costs increasing in double-digit proportions and early mobilization of amputees being medically beneficial, this device should
find a permanent place in the rehabilitation
of those patients who, due to complications,
would otherwise remain sedentary.
ANTHONY J. CAPPA, CFO, BS, is a prosthetist/orthotist with the Hospital for Special Surgery,
510 E. 73rd St., New York, NY1002J. This article
is dedicated to his parents, Anthony and Elizabeth
Cappa.
References:
- Shapiro Dr. D, Cummings Dr. V, Lazzetti
Dr. J, Bodenstein N. Early Prosthetic Ambulation
in Delayed-Healing Below-Knee Amputees presented in poster form in Washington, D.C.
- Mensch G, Ellis PM. Physical Therapy Management of Lower Extremity Amputations. Aspen
Publishers, Maryland, 1986.
- Marquardt E, Correll J. Amputations and
prostheses for the lower limb. International Orthopedics 1984;8:139-146.
- Lange L. Prosthetic implications with the diabetic patient. Orthotics and Prosthetics Summer
1982;36:2:96-102.
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