The Design and Analysis of a Double Transtibial Composite Prosthesis and the Effect of
Lateral Movement Loads
By Max A. Willer
An Engineering Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the Requirements for the degree of Master of Engineering in Mechanical
Engineering
Introduction:
The use of prosthetic limbs as a medical solution for lost limb functionality has been in practice
for most of human history, from wooden crutches to wooden legs all the way up to the
articulating hands of the present and future. Those who use prosthetic limbs have long been
perceived as at a disadvantage in physical fitness and sport to those with full use of their limbs.
This perception however is on the
precipice of change as advanced
material technology and
fabrication techniques are applied
towards the design and
manufacturing of prosthetic
devices. In fact, some prosthestic
limbs have become so advanced,
that they have caused controversy
Figure 1:
http://www.eucomed.org/uploads/_key_themes/mdd/eucomed_incremental
_innovation.jpg
in the sporting world for giving an
unfair advantage over natural human limbs.
Oscar Pistorius is a South African sprint runner and a double transtibial amputee which means the
prosthetics he wears replace legs and feet that have been amputated below the knee. In 2007 he
gained attention by participating in able-bodied international springing competitions which
resulted in the International Association of Athletics Federations (IAAF) banning "any technical
device that incorporates springs, wheels or any other element that provides a user with an advantage
over
another
athlete
not
using
such
a
device."
(http://www.telegraph.co.uk/sport/othersports/athletics/2288489/IAAF-call-time-on-Oscar-
Pistorius-dream.html). This led to several studies as to whether there was indeed an advantage in
using the Flex Foot prostheses that Oscar Pistorius has become famous for using. One of the most
famous of theses is the Bruggemann study, a series of tests published in 2008 that reported that
Pistorius uses a quarter less energy expenditure than able-bodied runners with use of their lower
legs, ankles, and feet. (Biomechanics of double transtibial amputee sprinting using dedicated
sprinting prosthesesm Gert-Peter Bruggemann, Adamantios Arampatzis, Frank Emrich and
Wolfgang Potthast). A series of other studies followed showing that Pistorius used less metabolic
energy and foot to ground force than able-bodied runners, further pointing toward the mechanical
advantage of the Flex Foot prosthesis over conventional running (Weyand, et al. study, Grabowski,
et al. study).
Given that prosthetics are becoming so advanced that they give their wearers an advantage over
those with human limbs, it is natural to desire to expand the practical uses to other sports. The
Flex Foot prosthetics have been noted to almost force the wearer to bounce continuously from
foot to foot given their spring nature. In the same way, competitors in sports such as tennis
continually bounce from side to side while in play to prepare for quick reflex events like returing
a shot in the example below.
However, the Flex Foot Cheetah design is mainly catered to forward movements such as
sprinting. In the example above, the tennis players are moving omni-directionally, forwards,
backwards, and perhaps most stressfully to the ankle joint, from side to side.
It is in this way I propose to analyze the Flex Foot Cheetah general design and to modify it to a
design that accommodates the lateral forces seen by
athletes planting their feet and pushing off from side to
side.
Challenges of Study
The Flex Foot Cheetah by description is a laid up
composite laminate that is at least reinforced with
carbon fiber laminates. (Reference). Most challengingly
it is a proprietary design, which means to analyze it
will require reverse engineering it. This will mean
estimating the dimensions, materials, and composite layup
Figure 2: http://www.ossur.com/prostheticsolutions/products/feet/feet/cheetah
structure to arrive at appropriate baseline model from
which to design a lateral force transtibial prosthesis. To
accomplish this, I propose using the data collected from the
Bruggemann study, which illustrates the dynamic response of
the Flex Foot Cheetah. Getting within a reasonable degree of
accuracy to this model will help ensure a function lateral
movement prosthesis.
The next challenge of this study will be to accurately model the
Figure 3: Bruggemann Figrue 5
ground reaction forces seen in lateral movement of the
human body. Side to side movement sees greater forces in the human leg than running forward,
since the foot and leg has to support the weight of the body as well as stop its momentum in an
appropriate amount of time. Depending on the playing surface, different sports solve
withstanding lateral moving force with various shoe surfaces. Field sports like soccer and
American football give their athletes traction with cleats and ice sports give their athletes steel
blades. However, court sports like basketball, racquetball, and tennis require their athletes to
simply use the traction of a flat sole for movement. Luckily, studies have been done on shoe
materials and patterns best suited for the lateral movements of
tennis. These studies have detailed data on the force required of
the athletes interface with the ground as well as materials used.
Thus the next challenge will be to use these studies to determine
the contact surface area of the new lateral movement prosthetis
as well as the lateral force against the ground it can resist.
Figure 4: Clarke
After modelling the ground forces, the dynamic response of the baseline Flex Foot design will be
studied to examine the critical points to redesign to allow the maximum effectiveness for lateral
movement. The design will be iterated keeping in mind the following main requirement:

A large majority of the weight of the prosthetic design is in the connecting assembly to
the remaining lower leg. Thus the weight of the mechanical spring part of the prosthetic
should be kept at a minimum.
It is for this requirement that the Flex Foot original design utilizes a composite layup with
strengthening carbon fiber laminates, and this study will use this line of reasoning during
material selection.
With the main requirement in mind, the first iterations will likely expand the under-foot
contact area in order to meet the amount of friction force required by the lateral movement of
tennis or other court sports.
The next iterations will lead to fine tuning the ankle support axis
that contributes towards inversion and eversion of the foot as
related to the shin axis. This twisting action is important to get
tuned correctly. Too loose and the under-foot contact area will
overturn on its leading edge and too stiff will not allow the full
under-foot contact area to make contact with the ground and
provide full traction.
Figure 5:
http://www.courses.vcu.edu/
DANC291-003/unit_8.htm
The final iterations will fine tune the shin axis of the prosthetic device. The current Flex Foot
design only utilizes two bolts to sandwich the mechanical spring to the leg connecting section of
the prosthetic. These iterations will likely beef up the lateral
moment of inertia of the mechanical spring section and perhaps
iterate on the bolt connecting pattern.
Conclusion
The design of prosthetics are incredibly important to the
rehabilitation of those who have been born without full use of their
limbs and those who have lost it. In addition, the use of advanced
materials and engineering for sports has a long history of resulting
in advancements in areas that affect us all. The full redesign of the
Figure 6:
http://upload.wikimedia.org/wiki
pedia/commons/thumb/7/71/Osc
ar_Pistorius_at_International_Par
alympic_Day%2C_Trafalgar_Squar
e%2C_London__20110908.jpg/640pxOscar_Pistorius_at_International_
References:
Paralympic_Day%2C_Trafalgar_Sq
uare%2C_London_-_20110908.jpg
human ankle by man is still a long way away, but any step towards
that eventual goal is a worthwhile one.
Please see References folder of this directory and any hyperlinks provided in this report.