Socket design

Prosthetic sockets have been produced since antiquity in response to illness and injury, but remained simple in design and insufficient in quality until the second half of the 20th

Century (Al-Fakih et al. 2016). A group of clinicians in the USA created and described the patella-tendon bearing (PTB) socket, a revolutionary design which quickly became the clinical standard for below knee amputees (Radcliffe and Foort 1961; Foort 1965).

The philosophy behind the PTB design is that certain areas of the residual limb were more tolerant of load than others – broad, flat areas of thicker tissue could withstand higher greater magnitude and longer duration of load than those with thinner tissue covering, underlying ridges or points of bone or the stump end. A summary of load tolerant regions is shown in Figure 9.

Figure 9 - Key loading of a PTB socket. Green regions were considered load tolerant, red areas where bony protrusions limit acceptable load

The key feature of the PTB socket was the extensive load that was intended to be placed at the patella tendon. In some users this proved problematic – the excess pressure at this location caused pain or injury. Furthermore, the necessity of external suspension meant that pistoning (vertical motion of the limb relative the socket) with the effect that skin abrasions were common.

The next development in socket design was the introduction of total-surface bearing (TSB) sockets (Staats and Lundt 1987). These consist of a hard external socket and a silicone liner that forced the residuum into a conical shape, with distal suspension. A TSB socket with a

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silicone liner attempted to provide relatively equal loading across the residual limb – the high friction between the silicone and the socket reducing pistoning of the limb in the socket, and vacuum or a distal locking pin providing suspension (Kristinsson 1993; Hachisuka et al. 1998).

This has a different approach to producing a comfortable application of force: rather than applying considerable load to particular regions, the load was distributed, theoretically reducing the pressure gradients between regions and therefore reducing stresses within the tissue itself. The produced designs were typically less obtrusive than the previous PTB sockets, but came with other disadvantages. The use of a functional silicone liner layer is an additional expense, and is not easy to don or doff in people with limited mobility.

Furthermore, the use of close-fitting silicone liners was also associated with perspiration issues and resultant skin irritation (Hachisuka et al. 2001).

Another form of socket design explicitly used the hydrostatic principle of fluid mechanics to provide suitable loading. A variant of the TSB design, the socket shape is defined by

applying a consistent, equal load around the residual limb by means of hydraulics or pneumatics (Goh et al. 2004). The expectation is that tissue will ‘flow’ into a configuration that equalises load distribution, conforming to Pascal’s law of fluids. Such sockets also utilised the results of a study by Rogers and Wilson (1975) examining the response of tissue to extended loading – by keeping the pressure below the boundary of dangerous

combinations of load and duration, then applying external pressure at a ‘safe’ level during the casting process. Then, the produced socket theoretically maintains an even pressure distribution during day-to-day loading.

As noted by Silver-Thorn (1996), a further advantage of the hydrostatic technique is that the socket creation does not depend as much on the expertise of the prosthetist – rather than attempt to hand craft bespoke solutions, the socket shape is defined almost wholly by the equal pressure being applied to the limb. This makes hydrostatic sockets not only quicker to produce, but also simpler to reproduce, an aspect of PTB/TSB sockets that is almost impossible to achieve manually.

Computer-Aided-Design/Computer-Aided-Manufacturing (CAD-CAM) systems have also been used in the creation of below-knee prostheses (Saunders et al. 1985, 1989). Here, the shape of the residual limb is modelled, either by contact with a mechanical sensor, or by

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some imaging technique. Once the shape of the stump is obtained, the process of rectification, scaling or adjusting is completed digitally – in a sense mimicking the manual adjustments traditionally made on the plaster positive model in PTB socket manufacture.

Once a suitable shape has been produced, the shape of the corresponding socket is

defined, and supplied manufacturing system to either produce a positive cast that is draped with socket material, or, more recently, has the socket shape directly formed using additive manufacturing (Hsu et al. 2010). The latter – otherwise known as ‘3D Printing’ has seen only very limited practical use.

Such an approach has distinct advantages in terms of the reliability and repeatability of the socket creation process. The shape, structure and consistency of the residuum is tracked and recorded by the modelling system. Furthermore, the design of the socket can be recreated more easily, and can be finely adjusted if some elements prove unsuitable. Finally, the structure of the socket can be more creatively adjusted – one experimental system uses areas of variable compliance within the socket wall such that the socket can flex in preferential areas in response to load (Rogers et al. 2008). This was achieved by building in grooves into regions of the socket wall, techniques which cannot easily be achieved in traditional manufacturing. However, the utility of CAD-CAM systems is

restricted by the accuracy of the shape-sensing portion of the process: the method cannot easily account for the particulars of the interior anatomy of the residuum, and will

necessarily produce a model of the limb when it is unloaded. As the tissue flows during standing or walking, the shape of the ideal socket will also change.

Other amputation levels contain other approaches to socket manufacture. One example, developed for transhumeral amputees but recently applied in the lower limb is known as the high-fidelity socket. Rather than attempting to provide a socket that encompasses the residuum fully, the HiFi socket uses alternating longitudinal bands of high pressure and release (Williams and Altobelli 2011). This enables a firm grip of the femur, whilst also allowing tissue to flow into the voids in the socket wall.

A final note is made of the technique of osseointegration, which makes the socket component unnecessary in users who undergo this procedure (Brånemark et al. 2001). Predominantly tested in above-knee amputees (but also in transhumeral, transtibial and digit amputations), an implant is fixed within the long bone. A surgical process places a long

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metal stem along the bone axis, and bone is encouraged to infiltrate this device until it is rigidly fixed. The distal end of the stem protrudes through the stump and ends in an abutment to which the prosthesis is attached. The majority of walking forces are

transmitted through this linkage, facilitating a more natural connection to the prosthesis rather than through the soft tissue of the residuum. Users testing osseointegrated prostheses report encouraging recovery of function (Hagberg and Brånemark 2009).

The technique is not suitable for all amputees– surgical recovery and creating enough strength at the bone-metal interface is a long and challenging process, taking many months without full mobility. The inclusion of a metal stem also alters the mechanical performance of the bone around it, meaning that over time the distal edge of the femur becomes weaker. An issue is also found at the skin-stem interface – the tissue does not adhere effectively to the stem, meaning that wound drainage and infection remain common causes for complaint.