March 2012

Ply trap: Prosthetic sock thickness varies

In the moment: O&P #13292734

By Emily Delzell

Recent research from the University of Washington in Seattle confirms what many practitioners have already learned from treating patients with lower limb amputat­ions: Sock thickness varies among dif­ferent sock products of the same ply and is also affected by biomechanical con­ditions.

“Manufacturers typically characterize amputee socks by ply, and sock ply is how practitioners communicate with patients as to how loose or tight the prosthetic socket feels,” said Joan E. Sanders, PhD, lead study author and professor in the Department of Bioengineering at the University of Washington, Seattle. “Because sock thickness for the same ply varies, practitioners and patients must adjust their interpretation of thickness depending on which sock is used; this can make communication confusing.”

In her study, published this month in Prosthetics & Orthotics International, Sanders and colleagues characterized how sock and sheath ply relates to thickness for seven sock materials, including an acrylic/Lycra blend, 100% cotton, a polyester blend, and 100% virgin wool, and a 100% polyamide sheath. They evaluated thickness under eight different compressive stress conditions and two biaxial in-plane tensile strength conditions.

The investigators performed mechanical assessments using a custom instrument as well as an electronic height gauge, testing socks straight from the package and then under different load conditions.

Socks of the same ply tested straight from the package had comparable thicknesses, except for those made of 100% virgin wool, which was thicker than average at medium to high ply (≥ 3 ply) and 100% cotton, which tended to be thinner than average at low to medium ply (≤ 3 ply).

Pressure and stretching substantially changed sock thick­ness and compressive stiffness compared with these measures straight from the package. The change in thickness for uncompressed to maximally compressed ranged from 30% to 36% for 3- and 5-ply socks; and 31% to 47% for 1-ply socks, depending on the material. In general, Sanders said, compression reduced thickness by about a third.

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 socks, respectively. For socks made from a blend, corresponding results were 0.4, 0.7, and 0.8 mm, respectively. Sock ply did not correlate with thickness as practitioners and patients might assume, Sanders noted; for example, a 3-ply sock was not three times the thickness of a 1-ply sock.

The discrepancies between labeled ply and actual sock thickness under different conditions and with different materials led Sanders and colleagues to propose an alternative nomenclature for prosthetic sock thickness based on sheath equivalence, or the number of polyamide sheaths that equal a sock’s thickness. (Sanders’ testing showed that one sheath is 0.21 mm thick.)

“The alternative nomenclature adds consistency to the field,” Sanders said. “It provides a term, sheath equivalence, that means the same thickness no matter what kind of sock is used. Further, our nomenclature reflects sock thickness under pressures. Thus our new term should be more tangible to practitioners and patients and provide clearer meaning.”

Jack E. Ullendahl, CPO, a prosthetist at Hanger Prosthetics and Orthotics in Phoenix, AZ, reviewed Sanders’ study for LER. He noted that the results confirm his clinical experience that many variables affect sock thickness, but questioned how readily practitioners might embrace the proposed nomenclature.

“While changing the no­men­clature sounds like a good idea, I don’t see the long-held convention [of using ply to describe thickness] changing any time soon,” he said. “The bottom line is that the patients need to be educated to control the fit of their prosthesis by how they put on socks, and which arrangement of socks and combination of sock ply feel right to them.”


Sanders JE, Cagle JC, Harrison DS, Karchin A. Amputee socks: how does sock ply relate to sock thickness? Prosthet Orthot Int 2012;36(1):77-86.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.