The most important part of the prosthesis is the interface between the residual limb and the socket; this could also be considered the man-machine interface. This interface commonly includes a gel liner and is matched to the patient based on materials properties, size of residual limb, and suspension type. Liners are usually off-the-shelf products, but can be custom made for individuals with complicated fits and irregularly shaped residual limbs. The socket is custom made by a skilled prosthetist who creates a shape that ideally distributes pressure over a maximum surface area with reliefs provided over pressure-sensitive areas. Every residual limb is different, and that is why the device requires such customization. When the socket is fitting well, it’s like a good pair of glasses, you don’t notice them, your vision isn’t blurred, and you go about your day. When the socket is not fitting well, it increases pain/frustration and can lead to avoidance of an activity or cause skin breakdown with continued use. Although it’s important for the prosthesis to restore symmetry in movement and aesthetic form, if it’s not comfortable and won’t stay on, it’s much less likely to be worn.
Traditional methods of dealing with volume changes
What starts out as a great fitting socket may not stay that way because of many factors. For example, the reliefs made for boney prominences may not match up to the residual limb when volume changes. It’s normal for residual limb volume to be greater in the morning than in the afternoon and evening. It’s also normal for volume to decrease as activity increases. This is more substantial in the first year compared to subsequent years because of the maturation process of the residual limb healing and muscle changes. The use of a shrinker at night can lessen the impact of such changes. During the day, it is not uncommon to have to carry around socks of various thickness (ply) and tighten up the fit periodically. Learning which sock combinations to apply when is one of the most difficult things to teach and learn. Sometimes adding or subtracting socks is counter-intuitive and complicated with sensation problems, cognitive deficit, and the limitations in technology. Depending on the overall soft tissue volume and cardiovascular health of the individual the amount of volume fluctuation can vary greatly. Sensitivity to volume changes also varies greatly. For some individuals, comfort is achieved among a wide range. For others, it is not. As an example, my mother-in-law has always called my wife the princess and the pea when it comes to her socket comfort because she is bothered by seemingly undetectable changes. Sock thickness has traditionally been the only way to deal with volume management. This approach is not always effective for several reasons:
1) volume changes are not always uniform and socks only offer a way to deal with volume changes uniformly;
2) it is known that soft tissue is where volume changes occur but below the knee amputees, the socket encompasses the knee, which is mostly bone, and below the knee, which has more soft tissue. Use of full length socks creates excessive pressure on the sides of the knee when attempting to make up for overall loss of residual limb volume; and
3) excessive sock ply reduces the feeling of connectivity to the foot and ground and is not ideal.
That said, we as prosthetists have a number of tricks up our sleeves when it comes to volume management. Padding inside the socket is one option. With a flexible inner liner, padding can be placed between the carbon frame and inner socket. This is a good solution for weight loss or more permanent changes. Gel pads can be added in between the skin and gel liner in order to off-load pressure areas. A prosthetist is best skilled at determining the size and location of such padding as the biomechanics are somewhat complex. A key principle is not to pad an area of excessive pressure thinking the cushioning will be beneficial. In fact, the extra material will increase pressure in that area. A simple way to assess residual limb volume is to log a journal of measurements and to note which sock ply combinations were most comfortable. When doing this, make sure to use a cloth tape measure and measure in the same places over the liner each time. Drawing lines every inch or two inches on the liner helps keep these measurements consistent over time.
For extreme volume changes, such as the case when an individual is undergoing dialysis several times per week, having thicker and thinner liner options may be beneficial. On days when volume is low, a thicker liner may be worn, and on days when volume is high, a thinner liner may be worn. In even more extreme circumstances, the flexible inner socket may be removed on high volume days and placed back into the carbon socket on lower volume days.
Minute changes in volume during the day can affect certain individuals substantially, regardless of the amount of overall volume lost or gained. Half socks, covering only the bottom half or third of residual limb, are a good solution since much of the volume loss occurs in this area anyway. However, this doesn’t always work. A unique option that’s become available within the last couple of years is a user adjustable socket incorporating a Boa dial. Boa dials are common on certain snowboarding boots and hiking boots, among other things, and take the place of laces. Instead of pulling laces tight and tying them, the boa dial can be pressed in and turned clockwise to tighten and pulled out to release tension. There are now kits (Click Medical) which allow prosthetists to fabricate sockets with strategically located panels so that the user of the prosthesis can make micro adjustments to the fit of the socket periodically, throughout the day. This may not be the end all, be all solution, but so far the results have been pretty good for those individuals who struggle with comfort and do not do well with socks.
There is a lot of need for improvement when it comes to the socket and interface of prosthetic technology. An emerging alternative to sockets is osseointegration, a surgical procedure which allows the direct skeletal attachment of the prosthesis to the bone. This may be a good option for some people, especially those who've struggled with finding a comfortable fit; however, it isn't going to replace standard prosthetic options for everyone. There is still going to be a need for better socket technology. Recently, there are a new options with semi-custom sockets using straps and build-on-site technology. I am skeptical that these products would fit as well as a fully custom molded design. However, the user adjustability is intriguing and should not be dismissed so long as we are not neglecting sound biomechanical principles. There has been talk of sockets that self-adjust based on data acquired from imbedded sensors, but currently these are not commercially available. Developments in materials science and engineering will hopefully continue to take place in order to progress the activity level and the quality of life for prosthetic wearers.