Vacuum levels generated:
When sensory control of the lower limb joints is lost, it is essential that the replacement behaves predictably. Consistency of performance is vital in providing prosthetic confidence. In terms of socket suspension method, this means providing the same good connection throughout a gait cycle, from one step to the next, and day-to-day, over the lifetime of the socket.
The difference between the vacuum levels generated by suction suspension, and that generated when using EVS, can be demonstrated by using a negative pressure gauge30. Figure 1 illustrates these measurements. Commonly, when the user bears weight on their prosthesis during stance phase, with suction suspension, the magnitude of the vacuum is low. When the leg is lifted into swing phase, the vacuum increases in magnitude, holding the socket to the residual limb. Comparatively, EVS retains a high level during stance phase – higher, in fact, than the peak swing phase vacuum with suction. Additionally, the difference between stance and swing phase is less pronounced, so that the vacuum level is more consistent throughout the gait cycle. For the amputee illustrated in the graph30, EVS gave an approximate 85% increase in peak vacuum magnitude and an approximate 67% reduction in the ‘amplitude’ of the vacuum measurement signal.
Figure 1: Negative pressure within the socket when walking using a one-way valve suction suspension (grey) and an elevated vacuum (EV) suspension. N.B. Data recorded with Echelon Vac system.
The difference in vacuum generated by the AvalonVAC, compared to that generated by the Echelon Vac, is shown in Figure 2. Despite differences in the method used (keel vs springs, different socket, different pressure gauge), when the same patient was asked to walk at ‘K2 walking speed’ (~2km/h, short steps), the trend of vacuum level to number of steps taken was comparable to when measured at ‘K3 walking speed’ (4-5km/h) with Echelon Vac.
Figure 2: Comparison of the EchelonVAC and AvalonVAC vacuum generation by number of steps