An Endoskeletal Hip Disarticulation
Prosthesis for the Toddler
Claude Lévesque, C.M., C.P (c)
Christine Gauthier-Gagnon, M.Sc. (a)
Mario Beauregard, B.Sc.
Introduction
Normal children undergo an orderly motor maturation during the first years of their
lives. They learn to roll over, to crawl, to sit
and to stand independently and finally progress to supported locomotion (6-12 months)
and independent locomotion (10-18 months)
(1). The supported and independent locomotion pattern of a normal child differs from
that of an adult. The young toddler walks on
the toes or flat-footed, with flexed hips and
knees, external hip rotation and a wide base
of support (1). As the babies rapidly acquire
skills and are able to walk without support,
they develop a more mature gait pattern.
Heel strike appears at the beginning of
stance, followed by toe contact and subsequent heel lift during maintained toe contact
at the end of stance. The hips and knees
extend during stance and the width of the
base of support narrows (2-4). Hence, when
fitting a unilateral child amputee, the prosthesis must be shorter than the intact limb to
compensate for the flexed hip and knee and
thereafter lengthened as the hip and knee
extend and the base of support narrows.
Children with a unilateral lower extremity
amputation are ready for a first fitting when
they pull up to stand. This usually happens
when the toddler is between nine and 12
months of age. Generally, toddlers are fitted
with wooden exoskeletal prostheses which
are more or less a miniaturized version of
adult artificial limbs. This approach may be
inappropriate because of the child's continual development and growth (5,6). Once fitted, the child needs to see the prosthetist
every three months to correct the overall
length of the prosthesis and adapt the alignment to his or her rapidly changing needs.
The exoskeletal prosthesis needs to be cut
for lengthening and realignment, and laminated again. This operation is time-consuming.
Since children have a greater need for adjustable devices than fully grown adults, the
concept of a modular endoskeletal prosthesis meets some of their requirements. It is
light-weight, adjustable and the cosmetic
cover is compressible. Unfortunately, the
components of the modular prosthesis such
as the pylon-hip connector, the pylon and
the pylon-ankle connector are not commercially available for children under the age of
three. Furthermore, the conventional cosmetic covers break easily and can be damaged by fluids, especially urine.
This paper will describe a modular hip disarticulation prosthesis designed for an 11-month-old toddler (Figure 1)
. It comprises a
conventional hip disarticulation socket with
a lateral window for ventilation and to reduce weight, a standard child hip joint and a
SACH foot. The pylon system consists of a
plastic tube that can be heated for alignment
corrections. The tube incorporates an endless screw proximally to accommodate the
child's rapid skeletal growth. The pylon is
covered with foam and a sturdy cosmetic
cover.
The prosthesis is light-weight and can be
easily lengthened or aligned. It provides aeration for the diaper wearer and is washable.
Furthermore, the parents are pleased with
its softness which makes hugging more natural.
Fabrication
The prototype developed combines a
modified conventional hip disarticulation
socket, a fabricated pylon-hip and pylon-ankle connector, a plastic shank tube and a
SACH foot. For the purpose of stability, no
knee component is incorporated in the prosthesis.
Socket
A conventional polyester resin hip disarticulation socket is moulded on the positive
mould of the child's pelvis. An opening is cut
in the amputated side of the socket (Figure
2)
. The socket is lined with flexible Aliplast
(3 mm (1/8 inch) thick). Aliplast is washable,
resistant to wear and does not absorb
odours.
Hip Joint
A standard Canadian type child hip joint
(7) was selected, fitted to the socket and
attached to a fabricated pylon-hip connector. The connector is an aluminum scaled-down counterpart of the adult pylon-hip connector. The dimensions of the connector
block are illustrated in Figure 3
. It is made to
enclose a 4.4 cm (1 3/4-inch) hip joint bolt
(Figure 3.1)
and is fixed to the hip joint with
four 1.2 cm (1/2-inch) long screws. The connector's backstop is at a 45 degree angle and
lined with leather (Figure 3.2)
. A 4.2 cm
(1 5/8-inch) long endless screw (1.2 cm in
diameter) is inserted in the bottom part of
the connector block (Figure 3.3)
and screwed
in at least 2.5 cm (1 inch). It is secured by a
fabricated steel screw collar (internal diameter: 1.2 cm (1/2-inch) (Figure 3.4)
). The non-threaded lower half of the endless screw is
glued in the plastic pylon (Figure 3.5)
and
secured with a screw. This endless screw is
flattened in one direction to facilitate tightening with a wrench (Figure 3.6)
. The endless screw allows up to 1.9 cm (3/4-inch)
lengthening of the prosthetic limb. Hence,
the prosthesis may grow with the child. For
lengthening beyond this range, the plastic
shank tube must be replaced.
To ensure hip extension during stance, a
1.9 cm (3/4-inch) wide elastic band is attached with a speedy rivet to the postero-superior half of the socket, in line with the
mid-point of the hip joint (Figure 4)
. The
band runs under the socket and is fixed to the
mid-line of the fabricated hip connector
block (Figure 3.7)
. The band is stabilized at
the lower third of the socket by a leather
strap anchored by two speedy rivets. A slight
tension exists in the elastic band when the
hip is in full extension.
Shank
The shank consists of a polyvinylchloride
(PVC tube, 2.2 cm (7/8-inch) in diameter
(internal diameter: 1.3 cm (1/2 inch). The
advantage of this PVC pylon is that alignment may be readily altered simply by heating the PVC tube with a heat gun.
Ankle-Foot Assembly
The PVC tube is inserted and glued (with
a screw inserted laterally) into a 2.2 cm (7/8inch) long connector cylinder (Figure 2)
distally. The lower third of the cylinder is rectangular to avoid axial rotation between the
shank and the foot. The inner hole of this
lower third is drilled and tapped (3/8-16 inch)
to receive the SACH foot bolt. The superior
part of the SACH foot (8) is carved to incorporate the connector. The aluminum connector is moulded in a fiberglass coating and
secured to the keel with two screws. The
SACH foot is lined with an anti-skid sole for
bare-foot walking.
Alignment
In the sagittal plane, for stability, the
weight line must be posterior to the hip joint
and fall in front of a point located at mid-length of the foot. The PVC tube is heated
and curved with a posterior convexity to accomplish this.
In the frontal plane, a large base of support is sought. Therefore the hip joint is
shifted laterally. In order that the foot swings
forward in the plane of progression as the
pelvis rotates internally, the hip joint is rotated externally 3 to 4 degrees. This external
rotation combined with a 10 degree toe-out
further increases stability.
Finally, the prosthesis is constructed at
least 1.3cm (1/2-inch) shorter than the opposite limb to compensate for the flexed position of the leg.
Cosmetic Covering
The cosmetic cover consists of a block (6.5
x 6.5 cm or 2 1/2 x 2 1/2-inch) of Super Constructa foam (9) whose length will comprise
the pylon from the hip joint to the ankle-foot
assembly. A 2.2 cm (7/8-inch) hole is bored
throughout the length of the foam block.
This hole is widened to enclose the hip joint
connector. Once the block is pulled over the
shank piece, it is sanded slightly smaller than
the dimensions of the other leg. A 2.2 cm
(1/8-inch) thick layer Forma Foam (9) is then
glued over it. The beveled edges of the Forma Foam are brought together at the back of
the leg. It is heated and moulded over the
Super Constructa foam. Since children
spend most of their time barefoot, for cosmesis, an extra piece of Forma Foam covers
the ankle-foot assembly. The leg and foot
are covered in two operations, leaving a demarcation line at the ankle joint, to allow
easy removal of the foot for servicing. Thus,
when lengthening or realignment are necessary, the foot is removed and the cosmetic
covering slid off. Finally, Newskin (10) is
sprayed over the Forma Foam.
Conclusion
Thus far, one pediatric patient has been
fitted with the prototype at the Montreal Rehabilitation Institute. This young hip disarticulation amputee is 11 months old. The
prosthesis meets the basic requirements of
prosthetic fitting. It was found to be comfortable and stable. It is functional in that it may
be lengthened and aligned without time consuming and costly manipulations. The cosmetic covering is water (and urine) resistant,
washable and, more important, resistant to
wear. Moreover, this endoskeletal prosthesis being pleasant to the eye and compressible, is well received by the parents.
At the Montreal Rehabilitation Institute,
PVC tube pylons and sturdy cosmetic coverings have been adapted to the needs of the
above- and below-knee child amputee.
Eventually, a knee unit will be developed for
this type of endoskeletal prosthesis and the
foam covering improved to allow free knee
motion during the swing phase of gait.
Claude Lévesque, C.M., C.P.(c), is with the Institut de réadaptation de Montreal, 6300, avenue
Darlington, Montreal, Quebec H35 2J4.
Christiane Gauthier-Gagnon, M.Sc.(a), is with
the Ecole de réadaptation, Universit~ de Montreal,
C.P. 6128, Succ. "A", Montreal, Quebec H3C 3J7.
Mario Beauregard, B.Sc., is with the Institut de
réadaptation de Montreal, 6300, avenue Darlington, Montreal, Quebec H35 2J4.
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