Amputee Socks: How Does Sock Ply Relate to Sock Thickness?

Background: The term 'sock ply' may be a source of confusion in prosthetics practice because there may not be a consistent relationship between sock ply and sock thickness.

Objectives: The purpose of this study was to characterize how sock ply related to sock thickness for different sock materials commonly used in limb prosthetics. We also evaluated how sock thickness changed under loading conditions experienced while wearing a lower limb prosthesis compared with unstressed conditions.

Study Design: Experimental. Mechanical assessment.

Methods: Seven sock materials of varying ply and sheaths were tested using a custom instrument. Sock thickness under eight different compressive stress conditions and two different biaxial in-plane tensile strain conditions were measured.

Results: For socks woven from a single material, thickness under walking stance phase conditions averaged 0.7, 1.2 and 1.5 mm for 1, 3 and 5-ply, respectively. For socks woven from several materials, the corresponding results were 0.4, 0.7 and 0.8 mm, respectively. Sock ply did not sum, e.g. a 3-ply sock was not three times the thickness of a 1-ply sock.

Conclusions: Sock thickness and compressive stiffness are strongly dependent upon sock material, interface pressure, and in-plane biaxial strain.