Surgical knot tightening: how much pull is necessary?

Abstract

High-strength sutures allow tightening of a suture knot beyond the strength of the surgeon, possibly inflicting skin damage through the gloves. This study was undertaken to evaluate whether such effort is useful and how much tensioning on a surgical knot is necessary. Three different suture materials were tested: No. 2 Vicryl, FibreWire, and PDS. First, the force spontaneously applied on sutures during experimental knot tightening ("tying load") was measured in fifteen experienced surgeons. Second, with each suture material, surgical square knots were tied with increasing, standardized loads (range 0.5-50 N) using a custom-made apparatus. Thereby, knot seating after tying was evaluated, and by loading the knots to failure, evaluation for failure mode and failure load was performed. FibreWire 5-throw square knots always failed by complete slipping of all knots (resolving), independent on the tying load. A nonlinear decrease of knot slippage and increased failure load were seen with increasing tying loads for all sutures. Major differences were seen between 0.5 and 10 N for FibreWire (slippage: 25 mm) and PDS (99.6 mm), whereas Vicryl showed major differences (22.7 mm) between 0.5 and 2 N. Increasing the tying load from 10 to 50 N decreased the mean knot slippage from 12 (FibreWire, ±2.6 SD), 9 (PDS, ±1.8 SD) and 8 (Vicryl, ±1.3 SD) mm to 6 (±2.9 SD), 3 (±1.5 SD) and 4 mm (±0.9 SD), respectively. Slippage and self-seating of the knots under load is unavoidable even with highest tying loads. Relatively minor but possibly important differences can be seen for tying loads exceeding 2 N (Vicryl) and 10 N (PDS and FibreWire) for failure load and knot slippage. But also with a tying load of 50 N, a minimal slippage of approximately 3 mm seems unavoidable for all suture types. However, it is important to state that intense tightening does not prevent knot resolution and is only necessary in clinical situations that demand very tight sutures. Numbers and proper appliance of throws are more relevant than tying strength to reach the maximum failure load.