Effective Prosthetic Foot Length Ratio
I have previously written about the "roll over shape" theory being developed by Andrew Hansen PhD and his collaborators at Northwestern University [ "Intriguing Article on Prosthetic Alignment" ], which offers a straightforward explanation for the underlying scientific basis for the complex, subjective, and often-subtle multidimensional linear and angular changes the prosthetist makes to prosthetic components during the dynamic alignment procedure. In his latest publication, Dr. Hansen has proposed that one potentially useful measurement that can be derived from the "roll over shape" would be the effective combined length of anterior and posterior lever arms within the prosthetic foot. [Hansen AH, Sam M, Childress DS, The Effective Foot Length Ratio: A Potential Tool for Characterization and Evaluation of Prosthetic Feet. Journal of Prosthetics & Orthotics 2004;16:41-45]
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This concept is based, in part, on prior research demonstrating that people with unilateral paralysis of the triceps surae muscles tend to walk with shorter sound step length and/or increase the loading on the non-affected limb. This pathological gait is characterized by decreased excursion of the center of pressure under the foot in the direction of forward progression. Dr. Hansen notes that many gait studies of amputees walking with a prosthesis show similar characteristics, although certain "extended keel" designs such as the Flex-Walk and Flex-Foot have been shown to reduce these gait asymmetries. His earlier work noted that the roll over shape of the Flex-Walk very closely approximated that of the physiological ankle-foot complex.
Hansen et al suggest that the total foot lever arm can be approximated by measuring the linear distance from the posterior heel to the most anterior aspect of the roll over shape, and term this the "effective foot length" or EFL. To facilitate comparisons regardless of foot size, they propose dividing the EFL by the total length of the prosthetic foot, which gives a ratio indicating the total effective lever arm. They call this the effective foot length ratio or EFLR.
Their paper also includes EFLR calculations for 15 commercially available prosthetic feet, with the Quantum and SACH having the smallest ratio and the Flex-Walk having the largest. Interestingly, none of the tested feet equaled the EFLR of the physiological ankle-foot complex.
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The discussion section of this paper is short, sweet, and to the point, noting that although this simplified measurement may have clinical value in discriminating between the multitude of commercially available prosthetic ankle-foot mechanisms, it depicts only one aspect of ankle-foot function and does not give a complete picture of the functional performance of these components. The authors also caution that their present measurements do not take into account the dynamic effect of different bumper durometers, keel spring configurations, or walking speeds.
This is the latest work in a long series of clinically relevant studies from Dudley Childress' group at the Prosthetics Research Laboratory and Rehabilitation Engineering Research Program. As the field evolves toward Evidence Based Practice, these insights and hypotheses may prove valuable in providing objective measurements to define and evaluate what currently must be subjective clinical judgments made by experienced practitioners.