Calculated functional joint center positions are highly variable in individuals with unilateral transtibial amputation walking with identical prosthetic ankle-foot devices.

BACKGROUND

Biomechanical modeling of the lower limbs, including prosthetic componentry, makes a number of assumptions that influence the data obtained and their subsequent interpretation. Calculated prosthetic ankle functional joint center (FJC) positions vary between devices and have been suggested as a possible method of comparing device function.

OBJECTIVES

The study aimed to assess the variability of joint center position estimates, calculated using an FJC methodology, in all three planes. This was assessed in participants with unilateral transtibial amputation using an identical prosthetic ankle-foot device during walking on a flat surface.

STUDY DESIGN

Case series.

TECHNIQUE

Three-dimensional motion capture recorded the position of markers placed on the shank and ankle-foot device of five individuals with unilateral transtibial amputation, as they completed 10 trials of level walking using the same ankle-foot device. The FJC between the prosthetic foot and shank segments were calculated for each trial.

RESULTS

The calculated FJC positions were highly variable across individual participants and within each individual. This variability was observed across all three planes of movement and resulted in calculated joint center positions created beyond the physical boundaries of the prosthetic device.

CONCLUSIONS

Biomechanical data are affected by lower limb and prosthetic device modeling assumptions. Definition of a prosthetic "ankle" joint using an FJC method results in highly variable "ankle" joint center positions when applied to a prosthetic ankle-foot device. Their use should be discouraged to avoid misleading interpretations of data.