A research team studied the real-time effects of minor socket fit changes on a lower-limb prosthesis user’s biomechanics during standing and walking. They concluded that minor changes in socket fit may alter pressure and shear load transfer mechanisms at the interface.
The participant, who has a transtibial amputation, used his habitual total surface-bearing socket prescribed by a certified prosthetist and achieved a comfortable fit with two-sock fit and no liner. The researchers altered the socket fit with the addition or removal of sock layers to mimic relatively looser and tighter socket fits. Interface pressure and shear sensors were placed at load-bearing sites (the patellar tendon, popliteal fossa, and anterior-distal end) of the participant’s residual limb/socket interface to measure biomechanical interactions during standing and level walking.
The participant walked for five minutes following a prescribed route to ensure there was no discomfort. A self-selected walking cadence of 117 steps per minute and speed of approximately 1.35ms-1 were determined.
Level walking tests involved a five second standing phase followed by walking along an 8m level walkway at normal self-selected speed controlled with a digital metronome. A force plate was embedded halfway along the walkway to measure ground reaction forces.
The researchers found that although socket fit level was only slightly modified, changes in interface pressure and shear across the anatomical sites were still observed. Tighter fit corresponded to notable pressure reduction at anterior-distal during early stance and pressure increase at patellar tendon during terminal stance due to the residuum being pushed up.
Shear-to-pressure ratios were used to assess comfort, while pressure- and shear-time integrals assessed tissue health. The researchers observed more notable changes at tissue sites (e.g., the anterior-distal and popliteal fossa). They suggested that a combined evaluation of pressure and shear, including shear-to-pressure ratio and time integrals, may offer insight for residuum care.
They also found that during walking, a tighter socket fit resulted in greater circumferential and longitudinal shear stress, particularly at areas of high tissue concentration, such as the popliteal fossa that has high shear-to-pressure ratios as compared with other sites, which suggests increased axial and angular residuum movement in the socket, the authors wrote. On the other hand, looser fit resulted in increased movement within the socket leading to distally acting longitudinal shear at the patellar tendon, which indicates upward local residuum movement in the socket.
The open-access study, “Assessing Socket Fit Effects on Pressure and Shear at a Transtibial Residuum/Socket Interface¸” was published in Applied Bionics and Biomechanics.
