Changed Prosthetic Mass Distribution Had No Effect on Transfemoral Amputee Gait

Short-term intervention with changed prosthetic mass distribution had no significant effect on the spatiotemporal and knee kinematical characteristics of the transfemoral amputee gait. However, it can affect the spatiotemporal symmetry of locomotion, according to recent study results.

“There has been a progressive trend in manufacturing of transtibial and transfemoral prostheses to reduce the prosthetic mass using lightweight but more expensive materials. It is claimed that such prostheses can improve the motor performance and reduce the energy cost of the amputee locomotion. However, the actual effect of the prosthetic mass properties on the amputee locomotion is still ambiguous,” Farzam Farahmand PhD,MSc, professor and head of biomechanics section of the School of Mechanical Engineering at Sharif University of Technology, Tehran, Iran, told O&P Business News. “The purpose of our study was to investigate the effect of the location of the center of mass of the prosthesis on the locomotion of the transfemoral amputees.”

Effects of changed mass distribution

Ten men with unilateral traumatic transfemoral amputation participated in gait analysis, a 2-minute walk and subject preference tests. Researchers added weights to the original prosthetic legs in four conditions: 600 g added to the ankle, 600 g added at 10 cm distal to the prosthetic knee, 300 g added to the ankle and 300g at 10 cm distal to the knee. Stride and step lengths, stepping speed, stance, swing and gait cycle duration, self-selected walking speed, cadence and knee flexion angle were measured for each prosthetic mass condition.

In both the prosthetic and intact limbs, researchers found no significant change between stride lengths, step lengths, stepping speeds, gait cycle duration and stance and swing phase durations with alteration of the prosthetic mass condition. However, when the center of mass location of the prosthetic leg was changed, the spatiotemporal symmetry of the gait was affected significantly.

Overall, six of 10 patients preferred the no-added mass prostheses, two preferred the prostheses with the mass added to the knee and the ankle, one preferred the mass added to the ankle and one preferred the mass added to the knee, according to study results. Step and stride lengths of the prosthetic limbs were significantly higher when mass was added to the prosthetic ankle vs. those of the intact limb. Except for the mass added to the prosthetic knee, the stepping speed of the prosthetic limb was higher vs. the intact limb for all prosthetic mass condition. Researchers found that swing durations of the prosthetic limb were significantly longer while stance durations were significantly shorter when compared with those of the intact limb in all prosthetic mass conditions examined. Study results showed alteration of the prosthetic mass condition had no effect on 2-minute walking distance, self-selected speed and cadence.

According to Farahmand, who also holds a joint appointment in the Research Center of Science and Technology in Medicine at the Tehran University of Medical Sciences in Tehran, Iran, these study results in combination with previous studies show that there is a tradeoff between the changes of kinematics and the kinetics caused by the alteration of the prosthetic inertial properties.

“In short term, amputees adapt to the prosthetic mass perturbations by employing primarily a kinematic invariance strategy, in which the kinematics remain fixed while the kinetics change. Thus a clinician might observe no changes in the kinematics of the amputee’s locomotion early after changing the prosthetic mass properties,” he concluded. “However, this behavior might have been achieved by paying a large energy cost. So, the appropriateness of the prosthesis shall be evaluated after a sufficient adaptation time using a 6-minute walking test or similar methods of energy expenditure assessment.”

Next step

Farahmand and colleagues continue to focus on transfemoral prosthetic design, as well as overall cost and gait.

“We are working on improved low-cost solutions for transfemoral prosthetics by developing new designs for their structure and controllers,” Farahmand said. “We have developed computer models for amputees gait to evaluate different designs and also perform experimental tests using motion analysis systems.”

For more information:
Hekmatfard M. Farahmand F, Ebrahmim I. Effects of prosthetic mass distribution on the spatiotemporal characteristics and knee kinematics of transfemoral amputee locomotion. Gait Posture. July 23, 2012. [Epub ahead of print]

Disclosure: Farahmand has no relevant financial disclosures.

Leave a Reply

Your email address will not be published.