Operating room fatigue: is your twentieth surgical knot as strong as your first?

This study aimed to determine the tensile strength in a series of 20 consecutively tied knots. Knot tying is a universally used technique in surgical procedures, and as such, knot integrity and security are essential.

Twenty was the number of knots chosen as this is the average number of knots required for a vaginal hysterectomy. We used 0-0 gauge, nonexpired, polyglactin 910 to tie 20 knots in succession with less than 20 seconds rest between knots. The knots were tied without a surgeon's knot and 4 additional square knots (1 = 1 = 1 = 1 = 1). The knots were tied by 2 obstetrician/gynecologists investigators over the period of 2 weeks to minimize fatigue. The sutures were then soaked in 0.9% sodium chloride for 60 seconds and subsequently transferred to a Chatillon LTCM-100 tensiometer (Ametek, Largo, Florida) where the tails were cut to 3 mm length. The force required to break the knots was recorded. To detect a difference over time while maintaining power of 80% with a type I error rate of 5%, a minimum of 17 series of knots were needed (thus, 340 total knots after tying 20 knots per series). To buffer against unanticipated variability in the tensile strengths over time, we rounded the number of knot series up to 20, so a total of 400 knots were tied.

A total of 800 knots were tied. All the sutures broke at the knot and 36% untied. For analyses, the data for each series of knots were collapsed into quarters (ie, knots 1-5, 6-10, 11-15, and 16-20). A repeated-measures analysis of variance found that there were no statistically significant differences between the four quartiles (p = 0.87). A paired samples t-test comparing the first knots in each series with the last knots in each series showed no difference (p = 0.99). Similarly, a paired samples t-test comparing the first 10 knots to the last 10 knots showed no difference over time (p = 0.8). To determine whether there was a change in likelihood of knots coming untied, as more knots were tied, Cochran's Q was used to look across the entire series of 20 knots. This analysis of proportions coming untied revealed no differences over time (p = 0.61). To compare across quarters, a Friedman test was used and similarly showed no change over time (p = 0.92). The different investigators were controlled for in the analysis as a covariate, which turned out to be statistically significant, p = 0.003.

Under laboratory conditions, the order of knots tied does not change the tensile strength of the material. This would infer that fatigue does not influence the tensile strength for a series of 20 knots; however, additional studies with a larger number of knots series may be warranted.