With the number of liners on the market today, the prosthetist is challenged to select the system which will work best for each client. Perhaps some examination of what it is that we want the liner to do will help to narrow our choice.

Lower extremity amputees will most often wear a liner next to the skin before donning a socket. In kinesiology, the scientific study of movement of body parts, we learn that walking occurs by pushing the foot into the ground. The best liner is designed to manage the forces applied to the prosthesis when the body moves. The forces which must be managed come in five forms. Contact with the ground by the prosthetic foot causes (#1) a high impact force, followed by (#2) shear (rubbing) and (#3) rotational (turning) forces. Repeating the cycle of contact with the ground causes (#4) repetitive force. The combination of all these forces is called (#5) the resultant force.

All liners are intended to manage these forces. Some do it better than others. Urethane liners have been designed to manage the forces better than most of the current popular silicone-type liners. Urethane has also tested to be hypoallergenic (causing the least allergic reaction). In my clinical experience, two liners-either a urethane liner or a recent platinum silicone liner-have exclusively and successfully been used with people having skin sensitivity issues.

The thickness of the urethane liner is determined so as to provide the least shear on the skin surface. This is done by designing the liner (custom, if necessary) to best simulate the hysteresis or recovery rate of the soft tissue, muscle and skin. The urethane material at 6.4 millimeters of thickness best matches the elastic property of the heel pad of the human foot. A small amount of barrier cream applied to the skin will help to minimize shear against the residuum.

Most liners are brought to the market as a good idea. For example, some have lubrication appearing from the liner material, which is considered a good thing. The liner must be porous for this to occur, however, which invites the possibility of keratin (the main constituent of skin and hair tissue) and perspiration to go into the pores of the liner, eventually setting up an untreatable bacterial surface. Other liners feel good to touch but do not necessarily stay resilient. One of the most popular thermoplastic elastomers in fact "packs out" and has no resiliency.

So why do not all prosthetists use the best scientifically based product when fitting amputees? The truth is the urethane liner must have a total surface-bearing environment to sustain itself and provide best comfort. Identifying the type of socket the prosthetist will be using will also then narrow his or her decision when contemplating liners.