The Foundation of the Modern Prosthesis: The Liner – Part One
Odd that the squishy, soft pliable goo forms the critical interface between human and machine, but it does! Liners provide an essential protective barrier that dissipates pressure and is shear though it substrates, allowing the amputee wearing them to keep going when they would otherwise be sidelined to the bench to allow their limbs to recover. Runners go further and faster, grandpa walks more and sores don’t appear as frequently. Weak prosthetists appear good, strong prosthetist look like geniuses. I’ve not tallied up all the different liners, but I’d put the options north of one hundred. A brief look at Willow Wood’s website, show one manufacturer making more than 24 kinds of liners plus a custom version. A look at Ossur or Otto Bock would reveal similar numbers. Those are just the big boys in the liner market. Even Freedom Innovations has a liner offering. Endolite won’t be left out either, they have a liner that has laser etched sweat escape holes! Liners have a lot of different features that we must consider to make the right selection, which leads to the right foundation.
There are three materials that liners are made from, silicone, urethane, and thermoplastic elastomer (TPE). Urethane is known best for its flow property. It moves from high pressure to low better than any other liner material. Silicone, perhaps the most varied of the bunch, can be laminated, or injection molded. Additives can be placed in the material of a Silicon Liner like aloe. TPE is known for its ability to accommodate a large range of limb sizes with one liner size and often has mineral oil incorporated into it too. TPE liners excel for amputees with skin grafts, my patients with burns or split thickness grafts do very well with this type of liner.
The liners can be used to integrate the suspension of the prosthesis too. For years, more than 30, liners have used pins or lanyards to hold the prosthesis on. Now we can affix soft tabs with ski boot like locking mechanisms on to the liner and mount a ratchet lock on the socket and voila! the leg or arm is suspended, with no distal build up. It controls for rotation too. Ossur, more than a decade ago, brought Seal-In liners to the market initially as a below the knee liner. Interestingly, Seal-In liners are an ideal above the knee liner solution. With Seal-In technology, BK’s enjoy the lack of a sleeve and the inhibited knee flexion that entails while AK’s have a secure system that protects their limb. A Win-Win for everyone. In the next blog post we will continue to explain liners and the most important part of the liners!