Suspension

This is the method through which the socket is secured to the residual limb. A strong connection reduces relative movement of the limb inside the socket and thereby improves prosthetic attachment. The user’s proprioception or awareness of where their prosthesis is will be improved, reducing the chance of tripping. Many factors can affect suspension and socket fit.

Relative Movement

The axial movement of the residual limb relative to the socket is known as pistoning. A prosthetist will aim to minimise this movement because it can lead to high pressures and loading rates at the distal end of the residuum and shear forces along its length. Additionally, pistoning can affect prosthetic control. A poor connection affects the user’s proprioception, leading to inconsistent foot placement, ground clearance and even tripping, which can, in turn, severely affect confidence. It is well known that lower limb amputees have a high risk of falling, with 58% falling at least once every 12 months17. Many also report a loss of prosthetic confidence and greater fear of falling17.

The connection between the residual limb and the socket can be improved by creating a vacuum between them. One way of achieving this is to wear a suspension sleeve over the top of the socket, in order to create an air tight seal around the residual limb. A one-way valve is fitted at the distal end of the socket and as the amputee puts weight on their prosthesis, air is expelled from the valve. Next, when the limb is lifted, the valve doesn’t allow air to pass back into the socket, creating negative pressure around the interface and a strong connection over the whole residuum surface. This method of suspension is sometimes called ‘suction suspension’.

This method of suspension can be further enhanced by drawing more air from the interface, increasing the level of vacuum generated with the use of a mechanical or electrical pump. This is known as Elevated Vacuum (EV). EV has been shown to be very effective at minimising pistoning, with reductions of over 69% and 83%, compared to suction18,19 and pin-lock20 suspensions, respectively. Many researchers and practitioners have reported similar observations21–24. One study found that none of its transtibial EV users reported multiple falls, while 75% of the non-EV users did25.

Other research used functional clinical tests to determine the effect of EV on the capabilities of elderly, dysvascular amputees26 – a group that is particularly susceptible to falls27,28. This study showed significantly improved results in balance tests for both K3 and K2 mobility walkers.

Improvements at the residuum-socket interface can influence the whole body. Better foot clearance reduces the need for gait compensations, like vaulting – many studies have described improvements in the symmetry of amputees’ gait patterns when using EV, compared to other suspension methods19,21,29,30.

Residuum Volume

The volume of the residual limb is another factor that influences socket fit and comfort. While a socket might be a perfect fit for the residual limb when cast, a loss of fluid over the course of the day affects limb volume and socket fit. A loose fitting socket will not only be less comfortable but it will also allow greater relative movement, hindering control and leading to chafing. This is a common problem; and while the magnitude and rate of daily changes are dependent on activity31,32, a residual limb can change in volume by 12.6% over a two week period33. One of the first scientific publications on EV investigated the effect it had on residual limb volume19. Eleven amputees had their residuum volume measured before and after walking for 30 minutes, with both suction and EV suspension. While with suction there was a mean 6.5% loss in volume, for EV there was a mean 3.7% increase in volume. Other studies have since confirmed the observation that residuum volume loss is prevented by EV21,34–36, which implies a better socket fit is maintained.

The vacuum created in the socket encourages blood flow into the residual limb, increasing the delivery of oxygen and the removal of waste product. This is especially beneficial for dysvascular amputees, who often have restricted circulation.

Consistency

When sensory control of the lower limb joints is lost, it is essential that the replacement behaves in a predictable way. Consistency of performance is vital for providing prosthetic confidence. In terms of socket suspension method, this means providing the same strong 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 those generated when using EV, can be demonstrated by using a pressure gauge37. 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 (becomes more negative), holding the socket to the residual limb. Comparatively, EV 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 graph above37, EV gave an approximate 85% increase in peak vacuum magnitude and an approximate 67% reduction in the ‘amplitude’ of the vacuum measurement signal.

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