Retracting Locking-Pin Mechanism That Allows Partial Prosthetic Socket Doffing during Sitting.

Residual limb volume loss over the day is a source of socket fit problems for people using transtibial prostheses.1. Research studies have shown that prosthesis users may gain fluid volume during walking, but lose fluid volume during sitting.2 Pin suspension systems may accentuate limb fluid volume loss during sitting. With the distal end of the residual limb held in place by the pin, the posterior proximal residual limb tissues in the popliteal area are stretched tightly over the posterior brim of the socket, focusing stress that may reduce limb fluid volume and cause soft-tissue damage. If the foot is flat on the floor, anterior distal pressures may increase. Intermittent doffing may reduce fluid volume loss and mitigate socket fit problems.3 In a study on 16 people with transtibial amputation, a 30-minute doff performed after activity significantly increased limb fluid volume compared with not doffing. While doffing a prosthesis with locking-pin suspension during sitting either partially or completely may reduce socket fit problems caused by fluid volume loss, re-donning the prosthesis can often be difficult. Typically, donning requires the user has access to the socket (i.e., the user must remove pants or other lower-limb garments). Further, the user may have difficulty aligning the locking pin with the shuttle lock since it is not visible during donning. A mechanism that conveniently allows partial doffing and facilitates easy re-donning may help prosthesis users to implement intermittent doffing practices and overcome discomfort and fluid volume loss during sitting. Several systems have been designed to facilitate re-donning, with most of them using a fabric pull cord that passes through a hole in the distal end of the socket.4 The user draws the cord to pull in and seat the liner. One system (Lanyard Plunger Pin, Bulldog Tools Incorporated, Lewisburg, Ohio) is simply a cord affixed to a locking pin. Other systems eliminate the locking pin and shuttle lock and instead attach the cord to the liner using a threaded fastener5 (Icelock 300, Ossur, Reykjavik, Iceland; Keep It Simple Suspension,6 KISS Technologies, Baltimore, Maryland; Hook and Loop System,7 HOLO). The cord is pulled proximally by the user and held against the socket to maintain tension in the cord. One system uses a ratcheting mechanism that allows tension to be incrementally applied to the cord.5 The primary limitations of all of these systems are that users must have direct access to the socket to operate them, and users must have sufficient dexterity, balance, and strength to pull the cord taut. Further, systems without locking mechanisms would be expected to have weaker socket connections and thus be less secure during active gait than systems that include them. The purpose of this research was to develop and test a suspension system that allowed partial or complete doffing but did not require access to the socket, nor require the user to pull a cord taught during donning. A retractable tethered pin system was developed to operate with a normal shuttle lock suspension. Initial tests with prosthesis users were conducted.