Sock Thickness, Compression Changes With Normal Use For Amputees

Study results recently published in Prosthetics and Orthotics International showed the thickness of socks changes non-linearly with time for amputees.

Joan E. Sanders, PhD, a professor in the Department of Bioengineering at the University of Washington in Seattle, and colleagues wanted to determine whether a correlation existed between the age of a prosthetic sock and the resulting change in thickness under standardized weight-bearing and non-weight-bearing activities.

“We thought that was important to convey to prosthetists and people using prosthetic limbs so they understand how sock accommodation works and how they can be affected by the change in thickness,” Sanders told O&P News.

Measurement of sock thickness

Sock thickness is important for amputees as the addition and removal of socks throughout the day is the most common method used to accommodate volume changes in the limb, but there is no standard sock recommendation for amputees, Sanders said.

Joan E. Sanders, PhD
Joan E. Sanders

“Most practitioners will ask, ‘How many ply are you using?’ Ply is definitely not a good standard. Ply is not the same as thickness. Different socks from different manufacturers, even though they are the same ply, are of a different thickness,” she said.

The researchers studied the use of both new and used socks among amputees to examine two factors: whether the thickness of a sock can be predicted by its age, and how resistance of a sock to compression changes with use. The researchers used a custom-designed instrument to measure sock thickness. The instrument applied compression through a pressure plate loaded with custom weights while measuring sock thickness with an inductive position sensor. The socks used included new and used socks from Knit-Rite, in Kansas City, Kan., and Royal Knit, in Lee’s Summit, Mo. Knit-Rite socks included thicknesses from seven different sock material models and six different sock ply, while Royal Knit socks included three different sock material models and five different sock ply. Additional used socks were donated from volunteer prosthesis users. The study only used prosthetic socks that were used by the donor to provide a comfortable socket fit or accommodate residual limb volume changes, and donors were required to provide the sock manufacturer, model, ply and date of first use for each donated sock. A total of 69 used socks from 20 donors were provided for testing, and a complete history was available for 42 socks. Socks with a complete history had a mean of 3.1+/-1.6 ply and a mean age of 8 months.

Variable changes

Sanders and colleagues hypothesized they would find a predictable change in sock thickness over time; however, that was not what they found.

“Socks do change in thickness, [but] there was no relationship between thickness and time,” Sanders said.

Some socks decreased in thickness by as much as 50% in a few weeks, but others took several months to reach that point. Change in sock thickness was defined as the used sock’s thickness expressed as a percentage of its new thickness.

The researchers also examined changes in stress-thickness response, or the percent thickness compressed between loaded and unloaded conditions at each tested pressure. They found the stress-thickness response decreased as sock ply increased; however, while sock thickness decreased with age, the stress-thickness responses remained consistent among used and new socks. At the same time, resistance to compression decreased as socks aged.

“The message is socks do change over time appreciably. The time course of the change is not predicable,” Sanders said.

She added, “We also found that socks get stiffer over time. They will feel stiffer to the patient as [the socks] age.”

Patient impact, future research

The results led the researchers to conclude that sock thickness cannot be determined by age alone. Because the changes vary, Sanders said, there is no standard time at which practitioners can recommend patients replace socks.

“Rather than using time, if one were to use the number of steps a person walks or how much the sock was worn — something more related to how the sock is being mechanically stressed — there might be a relationship there with thickness and age,” Sanders said.

Future research should focus on the relationship of amputee activity to changes in sock thickness, Sanders said. – by Amanda Alexander

Disclosure: Sanders reports no relevant financial disclosures.

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