Accuracy and durability of Semmes-Weinstein monofilaments: what is the useful service life?

OBJECTIVE To evaluate the accuracy and effective service life of commercially available Semmes-Weinstein monofilaments with repetitive loading. METHODS We obtained 6 brands of Semmes-Weinstein monofilaments (SWM) from commercial vendors identified from the Internet and the federal registry. Five monofilaments of each brand were subjected to repeat loading cycles, allowed to rest over night and loaded again the next day. First, sets of five monofilaments were loaded 25 times for each of five days. Then sets of five monofilaments were subjected to 200 loading cycles a day for 15 days. A testing jig ensured each SWM was loaded perpendicular to a digital pressure plate. The buckling force was measured via the pressure plate to determine failure loads. Several statistical techniques were used to examine the behavior of the monofilaments over repeated loadings: time series analysis, ANOVA and nonparametric comparisons of load distributions. RESULTS The monofilaments tested were neither precise nor accurate. The plasticity of filaments increased with repeated loadings resulting in lower bending forces. Individual and average bending forces varied widely both within and between monofilament brands. All monofilaments showed a typical material failure pattern. Initially the bending force was high but rapidly decreased and then leveled out at levels 1-2g lower than the starting values. After resting over night, the initial bending force was again high but usually not as high as the previous day and bending forces decreased and then leveled out. Most monofilaments did not start at the accepted 10-g buckling force but varied by up to 30% (p<0.01). At best, monofilaments starting at the accepted 10±1g buckling force would remain within a usable range (9-11g) for 7-9 days or to evaluate 70-90 patients. CONCLUSION Commercially available SWM have significant variability within and between devices from different manufacturers. Their actual bending force varies widely from their designated 10g value. When used they have a short service life where the instrument is within 10% of their initial bending force which is not usually the stated 10g of force.