Position Of The Double Throw And Knot Slippage Across Throw Counts: A Benchtop Study Of 960 Knots In Four 3-0 Suture Materials

Introduction: Surgical knots fail in one of two ways: the knot slips, or the suture ruptures at the knot.Only the second is captured by the knotpull tensile strength manufacturers report.Yet slippage determines how many throws a surgeon must tie, and the knot tied may itself affect the load at which the suture ruptures.The surgeon's knot places a double throw first, to hold tissue apposition while the second throw is placed.Whether it also contributes to final security, and more so when placed second, has not been examined across throw counts with failure mode as the outcome.Materials and methods: Four 3-0 sutures were tested: Dafilon and Monosof (polyamide monofilament, non-absorbable), Monosyn (glyconate monofilament, absorbable) and Novosyn (polyglactin 910 braided, absorbable).Four configurations were tested at throw counts from two to five: square (1=1), surgeon's (2=1), reversed surgeon's (1=2) and double-double (2=2), with 15 knots at every combination.Knotted loops were tied wet by hand over a mandrel, tails cut to 3 mm, and pulled to failure at 150 mm/min.The primary outcome was failure mode, slippage or rupture.The secondary outcome was the load carried by the loop at rupture, analysed only where at least 14 of 15 specimens ruptured.Analysis used Firth-penalised logistic regression and rank-based comparisons.Results: 960 knots were analysed.Throw count dominated slippage across the whole range tested, from two to five (adjusted odds ratio (OR) 0.017 per additional throw, 95% confidence interval (CI) 0.009 to 0.030, p<0.001); slippage fell from 222 of 240 knots at two throws to 161 of 240 at three, 46 of 240 at four and 1 of 240 at five, each step significant (p<0.001).Configuration mattered almost entirely at three and four throws.The reversed surgeon's knot, which carries the same turns as the surgeon's knot and is therefore the comparison that isolates the position of the double throw, slipped less (OR 0.25, 95% CI 0.12 to 0.52, p<0.001).The square knot and the double-double differ from the surgeon's knot in bulk as well as arrangement, and their estimates reversed according to whether the model adjusted for throws or turns, so both are reported.The configuration effect was material-specific: present in both polyamides, present for the double-double in the braided material, and absent in the glyconate monofilament, where the surgeon's knot was as secure as any configuration tested.Throws needed for at most one slip in 15 ranged from three (Novosyn) to five (Monosyn).Loop rupture load ranged from 32.2 N (Dafilon) to 57.2 N (Monosyn); per strand, all exceeded the United States Pharmacopeia knot-pull minimum by 1.4 to 2.2 times.Conclusions: In this benchtop model a double throw placed second reduced slippage at the throw counts where knots are otherwise marginal, in both polyamides at matched knot bulk and in the braided material for the double-double.Five throws gave no slippage in any configuration or material except one knot in 15 in the Monosyn surgeon's knot, while three throws sufficed in the braided material and four in the polyamides.Because tightening force was not instrumented, and the surgeon's knot's first throw serves to hold apposition under tension, which a mandrel does not test, these are mechanical findings that require confirmation in tissue with instrumented tension before they should change practice.