The Effect of Matching Leg Masses and Moments Of Inertia on Gait using Vacuum Suspension

Abstract

INTRODUCTION: The masses of trans-tibial prosthetic legs are typically half that of a normal unaffected leg. Manufacturers produce these lightweight prostheses to reduce sliding and levering of the residual limb in the socket. This movement is a result of the use of the traditional modes of suspension (straps, wedges, pins, sleeves, suction). With traditional suspensions, when prosthesis mass is increased to match the unaffected limb (Mattes et al., 2000) trans-tibial amputees became less symmetrical in their gait. PURPOSE: With the recent introduction of vacuum suspension, where there is no sliding or levering, we posed the following question: Would gait symmetry remain unchanged or improve when the mass and inertial properties of the two legs are matched? METHODS: A male trans-tibial amputee using vacuum (-78 kPa) suspension walked on a treadmill at 1.67 m/s and 0% grade under two conditions: 1) unmatched and 2) matched masses and moments of inertia. The values of his unaffected leg, as estimated from cadaver data (Winter, 2005), were 5.13 kg and 0.489 kg m2. The combined mass and moment of inertia of the unmatched condition were 2.49 kg and 0.254 kg m2, and 5.09 kg and.489 kg m2 for the matched condition. The combined masses and moments of inertia were estimated using an oscillation technique, parallel axis theorem, geometric modeling and cadaver data. Both trials were filmed and analyzed for step (half a stride) length and single leg stance duration. A total of 10 steps were analyzed and averaged for each condition. RESULTS: The average step lengths for the unmatched condition were 0.81 ±.02m for the unaffected leg, and 0.72 ±.02m for the affected leg. The average step lengths for the matched condition were 0.81 ±.02m for the unaffected leg, and 0.75 ±.02m for the affected leg. The average stance durations for the unmatched condition were 0.61 ±.02s for the unaffected leg, and 0.56 ±.01s for the affected leg. For the matched condition the average stance durations were 0.61 ±.01s for the unaffected leg, and 0.57 ±.02s for the affected leg. CONCLUSION: These results suggest vacuum suspension may prevent the loss of gait symmetry typically seen when using heavier prostheses; matched to the unaffected leg's mass and moment of inertia. Further research is needed to confirm this case study is representative of all trans-tibial amputees.