Study Compares Safety of Prosthetic Knee Joints

A study led by Siegmar Blumentritt, PhD, professor at George August University, Goettingen and head of research for Otto Bock HealthCare, revealed evidence that suggests amputees greatly reduce their risk of falling in real world situations when using microprocessor knee joints as opposed to non-microprocessor knee joints. Blumentritt introduced his findings at the 2009 American Orthotic and Prosthetic Association (AOPA) National Assembly in Seattle. His pilot study was also published in the Journal of Prosthetics and Orthotics.

Safety concerns

Safety of the prosthesis is the number one concern while treating a transfemoral amputee. Falls in any setting may cause significant damage to the prosthesis, additional injury or hinder an amputee’s quality of life. Health care costs could increase considerably if a prosthesis is damaged due to a fall.

Recently, advances have been made in the development of functional knee joints. The knee joints closely simulate the kinematics of the biological knee.

“The potential for more normal kinematics will only be accepted and used by amputees if the design of the component provides a high degree of safety,” Blumentritt wrote in his study, The Safety of C-Leg: Biomechanical Tests.

As the latest technological advancement, many amputees simply assume the microprocessor knee joint to be the best in the market for superior balance. However, there was little data to support the subjective claim that the C-Leg combined increased function while also increasing safety.

“The purpose of this study was to determine the safety the prosthetic joint offers during ambulation,” Blumentritt said in his presentation.

The current study design allows for the comparison of the safety potential for different prosthetic designs, according to Blumentritt. The knee joints tested were the Active Line 3C1 (Mauch SNS hydraulic system), the Otto Bock 3R80, (rotary hydraulic system) and the C-Leg (electronically controlled hydraulic system).

The study tested five real-world situations on three experienced amputee patients wearing eight different knee joints. The situations included walking normally on even ground, abruptly stopping, side-stepping, stepping onto an obstacle and tripping the knee during the normal phase of gait swing. In order to properly trip the patient, the experimenter tugged on a string which was tied to the ankle of the amputee’s prosthesis during normal gait speed. The patient did not know when the experimenter would tug on the string. Amputees participating in the study wore safety harnesses that were attached to a track mounted on the ceiling.

The amputees in the study confirmed that the gait disruption during the test conditions were similar to real life circumstances that could potentially lead to falls.

One of the five real-world situations simulates walking on even ground The extention motion of the knee joint is disturbed by pulling a short cord
One of the five real-world situations simulates walking on even ground. The extention motion of the knee joint is disturbed by pulling a short cord.
Images: Siegmar Blumentritt.


After testing for each situation, Blumentritt acknowledged that amputees improved their balance when they were donning a microprocessor knee joint as opposed to a non-microprocessor knee joint. Only the C-Leg was considered safe for amputees under all five scenarios, according to Blumentritt.

The C-Leg offered the patients the ability to walk without thinking. Blumentritt ascribes this to the high degree of safety in the C-Leg that the patients perceived. The patients fitted with the C-Leg also reported that the knee joint allowed them to move more freely than their previous mechanical alternatives, according to the study.

“Microprocessor prosthetic knee joints significantly reduce amputees’ risk of falling compared to non-microprocessor knee joints,” Blumentritt said.

According to Blumentritt, when stepping onto an object, the stability and safety of the amputee patients differed significantly. Blumentritt stated that there was a high risk of falling when the patients donning simple mechanical mechanisms or Mauch hydraulic systems stepped onto the foreign object. According to Blumentritt, only the C-Leg flexed safely with the correct load dependent stance control required for safe weight acceptance.

During the simulated tripping scenario, the C-Leg reached full extension most quickly after disruption by the tug of the cord. The 3C1 also continued extending after the swing phase disruption, but did so at a slower rate of speed than the C-Leg.

Blumentritt noted in his presentation that the approved safety of the C-Leg under all five test conditions confirm objectively what has been subjectively assumed by O&P practitioners and patients – the C-Leg offered the greatest safety in real-world situations. He also noted in his study that the biomechanical findings suggest that the automatic stance control of the C-Leg design could provide amputees with a lower level of mobility, the opportunity to perform every day tasks and activities. — Anthony Calabro


I do agree that the microprocessor knees have the ability to prevent falls for the population it is designed for and I also think there is much more potential in future development of these kinds of knees. In the years since microprocessors knees have been on the market they are improving with each passing year and I am excited about the future for the knees.

The only concern that still exists in my mind is the durability factor on highly active individuals. I’m also not completely convinced that everyone fitting the microprocessor knees fully understands the adjustment capabilities or alignment principles behind the designs.

I can see a time in the future when the microprocessor knee will be everyday practice instead of ‘state of the art.’

—Thomas Karolewski, CP, FAAOP
Director of Prosthetics Education, Northwestern University Medical School, Prosthetic-Orthotic Center and
O&P Business News Practitioner Advisory Council member.

For more information:

  • Blumentriit S, Schmaltz T, Jarasch R. The safety of C-Leg: Biomechanical tests. Journal of Prosthetics and Orthotics. 2009; 21(1):2-15.

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