|
Hydrostatic
vs. Anatomic Contouring Methods
Gerald Stark, BSME, CP, FAAOP Director
of Product Development & Education
In recent years the focus of transtibial socket designs
has shifted from the PTB 2-Stage splinting technique to more rounded socket
shapes created with circumferential wraps and pressurized systems. This
has been primarily due to the wide practice of casting over suspension
and cushion gel liners, which create a more rounded shape due to their
applied circumferential tension. There have been published articles that
have attempted to compare the various methods and draw conclusions as
to which method is superior with regard to patient comfort and fit. The
same discussions arose with the advent of the PTB design (as described
by Carlton Fillauer, CPO in 1971) over simple circumferential wrapping.
These newer impression-taking techniques have merit. It is the feeling
of the authors that all methods can be applied successfully when properly
matched with the particular patient and componentry.
Old vs.
New Technology
Recently pressurized and vacuum molding methods have been received as
more “current” for the transtibial level than PTB methods that emphasize
direct musculoskeletal contouring. At the transfemoral level the opposite
is true. Rounded total hydrostatic designs have been supplanted by direct
musculoskeletal contouring present in ischial ramal-techniques that emphasize
the client’s anatomy to achieve a more biomechanic and intimate fit.
Description of Methods
Anatomic Splinting
Anatomic splinting techniques using an anterior panel that accentuated
the skeletal anatomy for the transtibial level were first described in
1971 by Carlton Fillauer. This grew out of modification techniques for
wooden sockets that first made block with an accurate impression of the
anterior surface, which was then glued, to a posterior block. Until then,
circumferential wraps had been used which tended round the triangular
anatomic cross-section of the transtibial limb. This method advocated
displacing tissue from load bearing areas and creating relief for prominent
areas. Once the splint was completely hardened the circumferential wrap
could not distort the anterior geometry. Also a narrow M-L dimension proximally
and distally was emphasized for biomechanic stability.
The PTB method has evolved over time. Carlton Fillauer,
CPO has said that many prosthetists in his opinion “over-modified” their
molds in the past with excessive buildups over pressure sensitive areas
and excavation in load bearing areas. He had always emphasized maximizing
surface bearing and mimicking anatomic shapes in the modifications with
as little distortion as possible. In his view the popliteal area was often
deepened excessively which caused discomfort in loading.
Prosthetists have added to this technique with modification
nuances to enhance patient comfort. If the patient is relatively fleshy
many prosthetists will let the A-P expand to attain a tight M-L dimension,
necessary for biomechanic support of the varus thrust at midstance. A
tight A-P in this instance decreases patient comfort especially in sitting
and may result in restricting venous return or “choking” of the limb.
The posterior trimline has also been and area of discomfort for many prosthetic
clients. Loading of the hamstring tendons with a medially angled popliteal
shelf had been employed, but many prosthetists now use a relieved “W”
shelf. This method increases comfort by blending the modifications with
the medial tibial flare ridge medially and the fibular head laterally.
The main advantage of this technique is the direct loading
and relief for the anterior surface of the residuum. Anatomic contouring
emphasizes loading and relief directly over the anatomic substructure
and so it greatly depends on the skill of the prosthetist. Conical limbs
or bony limbs that have individual characteristics work best with this
method. The main disadvantage is the consistency of the mold shape and
the incorrect placement of loading and relief areas.
Circumferential
Wrap
Circumferential wrapping is one of the original types of impression taking
methods. The limb is simply wrapped in an overlapping pattern. The tension
on the bandage is additive with each layer so it should be laid on the
limb with out tension. The advantages of this method are that it is relatively
quick, simple, and can be made over a liner. The disadvantage of this
method is that it is difficult to get a uniform thickness therefore a
uniform tension on the limb so roping may result in the mold. Additional
tension must be applied in modification with general removal of volume
to load the limb since tissue elongation or contouring methods is not
applied. There is usually no relief or tissue loading created during casting
and the shaped is quite rounded which gives little rotational control.
This method creates a proximal brim that may become quite bulky. Sensitive
or bony areas may require additional relief. This method works best with
fleshy clients that are well padded and not sensitive. This casting technique
is also applied when cushioned gel liners are used because the gel layer
acts as an additional fat layer creating a “fleshy” limb from a bony one.
The greatest advantage of this method is that the cushion liner durability
is greatly improved. This is because the liner is loaded uniformly with
no highly localized pressure areas to break down the cushioning material
and/or its outer textile matrix.
Pressure
Casting
Pressure casting involves simply wrapping the limb circumferentially and
placing it in a flexible tube or bladder and external air pressure is
applied. This technique is another way of achieving pressure loading and
tissue distraction other than manually pressure. The main advantage is
that the impressions are extremely consistent and do not rely heavily
on the skill of the prosthetist to create loading areas in modification.
An exact amount of pressure can be applied and used for soft, medium,
and firm skin. This method improves on the circumferential wrap because
some distraction is created if the limb is allowed to elongate and little
modification is needed to complete the limb. The disadvantage is that
it is difficult to control localized displacement of tissue in especially
bony or sensitive areas. Distal cups and relief pads, which may or may
not remain in the correct position become necessary. Also there is little
rotational control due to the rounded shape. This method works well on
well-padded limbs with cylindrical to slightly conical shapes. Often it
is employed over suspension sleeves or gel liners and does not require
the overall modification necessary with circumferential wrapping since
this has been achieved with external pressure.
Vacuum
Casting
Vacuum Casting also consists of a circumferential wrap that is placed
in a plastic sleeve from which air is removed. This forms the plaster
closely to the limb with a consistent overall pressure. But because the
pressure within the limb remains the same as the pressure exerted by the
external bag, no pressure or distraction is placed on the tissues. Direct
pressure can be applied through the bag in localized areas, but distraction
of the tissue is more difficult than standard 2-stage methods.
Mechanical
Brim
This German method uses a prefabricated or plaster cast proximal brim
suspended on a standing frame. The prefabricated proximal brims come ready
made to the size of the limb. Molding may create a more customized brim
over the limb. The lower half of the socket is formed by wrapping plaster
below the brim and attaching stockinette to the brim and pulling it to
the floor to impart transverse tension on the limb. The disadvantage of
this system is the transverse pressure is difficult to accurately reproduce
and the stockinette tends to bridge over any irregular geometry. This
method also seems to encourage the most amount of tissue elongation, which
has the benefit of creating distal padding and good distal end loading.
This appears to be a blending of manual forming proximally and transverse
loading distally. Because stockinette is used relief and loading areas
can still be created anywhere on the limb.
Elements
of a Good Transtibial Fit
There are certain elements that must be intrinsic to any transtibial method
chosen: 1) Soft Tissue Loading, 2) Relief for Sensitive Areas, 3) Provide
Total Contact, 4) Close Fitting M-L Dimension. As we have seen soft tissue
loading can be accomplished in a variety of ways. Direct pressure is more
accurate for specific patient needs and general pressure distributes load
over a more general area. The best methods seem to employ both to address
the overall loading and specific localized areas individual to the limb
shape. Relief for sensitive areas can be created in the mold process or
in modification, but over-modification should be avoided. Total contact
must be made for venous return and to maximize the support surface. This
must be made through encouraging elongation or during the modification
process. A close fitting M-L dimension is crucial in maintaining biomechanic
stability at midstance and comfortable loading. In some cases the A-P
may be expanded to allow a snug fit.
1) Gleave, J. A. E.: Moulds and casts for Orthopaedic
and Prosthetic Appliances. Charles C. Thomas, Springfield, Illinois, 1972.
2) Inman, V.T. & Eberhart: HD, Manual of Below Knee Prosthetics. Prosthetics
Research Group Biomechanics Laboratory, University of California, November
1959.
3) Fillauer, Carlton: “A Patellar-Tendon-Bearing Socket with a Detachable
Medial Brim”. Orthotics and Prosthetics, Volume 25, Number 4, pp.
28-34, December, 1971.
4) Quigley, M. & Wilson, A. B.: “An Evaluation of Three Casting Techniques
of Patellar-Tendon-Bearing Prostheses”. Orthotics and Prosthetics,
Volume 29, Number 2, pp.21-32, June 1975.
5) Kahle, J.: “Conventional and Hydrostatic Transtibial Interface
Comparison”. Journal of Prosthetics and Orthotics, Volume 11, Number
4, Fall, 1999.
|
2710
Amnicola Highway
