Poor knot security when closing running sutures with three threads.

BACKGROUND: Knot security varies with half-knot and half-hitch sequences, rotation sense, and the type and diameter of sutures. OBJECTIVES: To evaluate the security of knots with three threads, as used for tying off running sutures. METHODS: Three-way randomised experiments, with a factorial design and 10 knots/cell. The first experiment evaluated the knot security of a surgical knot (H2H1sH1s) for single stitches (2 threads) or running sutures (3 threads), using polyglactin 910 or polydioxanone, of 2-0 or 1 diameter. The second experiment evaluated the knot security of a running suture with 4 or 5 half-hitch sequences and 2 or 3 blocking knots, using polyglactin 910 or polydioxanone, and with diameters of 2-0 or 1. MAIN OUTCOME MEASURES: Knot security in Newton, measured with a digital dynamometer. With increasing forces, the knot either slipped open or blocked, causing the suture to break. RESULTS: In the first experiment, the knot security of surgical knots was less with monofilament (P=0.0004) and running sutures (P<0.0001), but not with suture diameter (three-way analysis of variance). Variability in outcome was greater with running sutures, especially with monofilament sutures of larger diameter. The second experiment demonstrated that half-hitch sequences occasionally opened with low forces, especially with larger monofilament sutures. CONCLUSIONS: Compared to knots tied with two threads, knots with three threads have a lower and more variable knot security, especially with larger monofilament sutures. WHAT IS NEW?: Knots with 3 threads should be avoided, as they may occasionally open with forces below 30 or 10 Newton.