Technical Note: Thermoplastic Use in the
Geriatric Knee-Disarticulation
Prosthesis
Joel J. Kempfer, C.P.
In the past, it has been necessary to fabricate a heavy cumbersome socket to accommodate the knee disarticulation residual
limb. What occurs when the geriatric knee
disarticulation patient with considerably less
strength, but the same limb volume, presents
him/herself for prosthetics fitting? One solution is to use a thermoplastic socket with
titanium endoskeletal components to fabricate a strong, yet extremely light-weight
prosthesis.
The subject of this technical note is a 65-year-old male with a right knee disarticulation amputated 35 years ago due to trauma.
He is unable to fully end bear due to distal
scar tissue. Pertinent history includes a brachio-plexus injury resulting in right upper
extremity paralysis. He experienced a left
hip fracture resulting in a total hip replacement, and has lost considerable weight and
limb volume during his convalescence.
When first seen, the patient was wearing a
five-year-old exoskeletal knee disarticulation prosthesis with medial door, polycentric
knee and SACH foot. This limb weighed 7
3/4 lbs. He was wheelchair bound, with minimal ambulation using a modified walker. He
desired a light-weight, stable prosthesis to
allow him transfer capabilities and limited
ambulation.
A prosthesis incorporating a quadrilateral, medial opening, thermoplastic socket
with distal end pad, and an endoskeletal system with a titanium, polycentric, manual
locking knee (Otto Bock 3R32) and a SACH
light foot was recommended.
The socket was fabricated with 1/4-inch
copolymer using the drape molding technique. A Pe-LiteŽ end pad was placed on
the posterior/distal cast before pulling. For
extra strength, an additional piece of 1/4 inch copolymer was placed in the joint attachment area at time of vacuum forming.
The medial door was subsequently formed
using the overlapping interlocking technique
(Figure 1)
, and the end cap formed during
this process was saved for cosmetic cover
attachment.
The socket was bench aligned and attached using five number 7 copper rivets
with burrs (Figure 2)
. This procedure is not
recommended by Otto Bock. Their representative states the attachment plate is designed for lamination only. Any other use is
at the prosthetist's risk.
It was felt that the limited use foreseen by
a geriatric patient precludes the risk of failure. This attachment has not failed on the
prosthesis fabricated.
The finished weight of this prosthesis is
4 1/2 lbs. Due to the considerable savings in
weight, the patient has progressed from the
wheelchair to ambulation with a cane and is
wearing the prosthesis all day. Because of
the inherent stability of the polycentric knee,
the patient requested removal of the manual
lock (Figure 3)
.
Since the fabrication of this limb, the author has become aware of an improved
method of socket attachment. By heating the
attachment plate with the plastic at time of
pulling, the plate can be sandwiched between the two layers of the thermoplastic
material, resulting in a much stronger knee/
socket attachment.
In conclusion, the author, as a prosthetist,
is very excited about the increased use of
thermoplastics in our discipline. Not only are
we able to save our patients considerable
energy expenditure by weight reduction, but
we are also limiting our health risks associated with laminating resins.
Joel J. Kempfer, C.P., is currently with Acme
Laboratories in Milwaukee, Wis.
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