Prosthetic Ankle Units Have Unanticipated Effects on Bilateral Amputees

Researchers from the Northwestern University Prosthetics-Orthotics
Center (NUPOC) have previously examined the ankle-foot relationship that occurs
during the stance phase of gait in able-bodied ambulators. They found that the
foot and ankle work together to create a smooth roll-over shape that improves
walking efficiency. They also observed that key characteristics of the
roll-over shape, the effective foot length ratio and the roll-over shape
radius, were invariant with speed and weight and remained constant with varying
types of walking surfaces.

“Knowing how the foot and ankle function in able-bodied gait
enables us to evaluate and better understand the role of prosthetic feet and
determine what they should be doing for prosthesis users,” Steven Gard,
PhD, executive director of NUPOC, told O&P Business News.
“Ideally, the
prosthetic foot should conform to some desired roll-over
shape that is proportional to that of an able-bodied ambulator, scaling with
the person’s height or his or her leg length.”

Using this idea, Gard and his colleagues set out to investigate how the
roll-over shape in lower-limb amputees is affected by combining prosthetic feet
and ankle units. They fitted 17 bilateral transtibial amputees with a mean age
of 53.6 years with Seattle Lightfoot II feet. Quantitative gait analyses were
performed as the subjects walked at slow, freely-selected and fast paces.
Subjects were then fitted with Endolite Multiflex Ankles and given a 2-week
adjustment period before being retested at the same three walking speeds. The
roll-over shape radii and effective foot length ratios were calculated and
compared for the two prosthetic configurations.

Roll-over shape

As the researchers examined their results, they realized that the ankle
units had a substantial effect on the ankle-foot roll-over shape. In their
retrospective data analysis, the researchers found that the ankle-foot
roll-over shape radii were significantly decreased when compared with the
baseline, which was expected. However, the researchers also discovered that
roll-over shape radii for the baseline condition were larger than the median
radii for the able-bodied controls. And when participants wore the ankle units,
the radii more closely resembled those of the able-bodied walkers.

“That result was surprising. Our concern initially was that by
fitting these ankle units and allowing increased dorsiflexion in stance phase,
we would decrease the radius of the rocker mechanism,” Gard said. “So
if we had the desired rocker characteristics with only the prosthetic feet,
then suddenly we would have a rocker radius that would be too small for

“But some of the subjects had too large of a rocker radius to begin
with,” he said. “Whenever we fitted them with the ankle units, as we
expected, the rocker radius was reduced. But it was a much better match to the
rocker radius that able-bodied individuals exhibit when they walk.”

Gait and stability

Along with the quantitative data analysis, the researchers also surveyed
the participants about their perceptions using the prosthetic configurations.
Gard said that most of the participants, many of whom had not been using ankle
units before, liked walking with them because they felt that their gait
performance improved. However, many participants also indicated that the ankle
units decreased their stability while standing. Gard said these types of
unanticipated effects should be more carefully considered when fitting
prosthesis users with ankle components.

“Prosthetists have to consider the overall function imparted by the
prostheses. There could be situations when the advantages of two different sets
of components are combined and create a negative consequence that the user will
have to contend with,” Gard said. “And after finding these results,
it occurred to us that prosthetists may need to perform several different
evaluations of function to detect and identify detrimental effects of combining
different components before the client leaves the clinic.”

Gard and his colleagues are planning to conduct a subsequent study to
further examine how standing and walking performance are affected as the
combined stiffness provided by a prosthetic foot and a
prosthetic ankle is varied and distributed between the two

“Hopefully, in a few years, we will be able to report results from
that study and be able to suggest ways to optimize the distribution of
stiffness between the components for improved balance and stability during
standing and walking,” Gard said. — by Megan Gilbride

For more information:

  • Gard SA, Su P, Lipschutz RD, Hansen AH. Effect of prosthetic ankle
    units on roll-over shape characteristics during walking in persons with
    bilateral transtibial amputations. J Rehabil Res Dev.


The investigation reported by Gard and colleagues is a good example of
clinically relevant research conducted by a multidisciplinary team of
investigators composed of biomechanists, engineers and a clinician that
provides targeted insights from the research laboratory that translate to the
clinic. Health care providers who treat persons with lower limb loss may find
the results of the study interesting.

Highlights of the findings are that a small cohort of adult persons
(n=17) with bilateral transtibial limb loss who wear lower limb prostheses
walked over ground in an instrumented gait lab with prosthetic foot and ankle
units (after 2 week habituation) appeared to demonstrate stance phase roll-over
behavior that is similar to non-disabled control subjects. In other words,
there may be value in adding a compliant ankle mechanism to a lower limb
prosthesis in persons with bilateral lower limb loss in order to preserve
roll-over behavior to the extent that is it nearly the same as persons with an
intact ankle/foot.

With any research investigation, there are limitations to study design,
and interpretation of results should be carefully weighed. The authors did cite
the limitations of their study and articulated their points well. However, the
limitations of the research design do not discount the findings which I believe
still have merit. Good research typically generates more questions and the
investigators have highlighted a “not so obvious” outcome that is
compelling. One feature I would have liked to have learned more about was the
performance of subjects’ right lower limb behavior since the investigators
only reported the movement performance of subjects’ left limbs under the
assumption that all subjects walked with “reasonably symmetric gait.”
Quantifying the behaviors of each lower limb may perhaps help explain how gait
was maintained as “reasonably symmetric” which would also be
interesting to explore.

— Christopher Hovorka, MS, CPO, LPO,

Co-director, MSPO Program, School of Applied Physiology, Georgia
Tech, Atlanta.

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