Researchers have unearthed a robust physics-based mechanism that dictates the sliding strength, or resistance to slippage, of the most common type of surgical knot. In particular, they have found that for a suturing filament with given elasto-plastic properties, the strength of the knot depends on the pre-tension applied to the knot during tying. This finding could be used to train surgeons to tie stronger, safer sutures – a skill that usually takes years to master. It could also help advance robotic surgery.
Surgeons routinely tie a series of square and “granny” knots in their daily work. These are flat knots tied in monofilaments that capsize into a sliding conformation, typically consisting of a series of half-hitches around a nearly straight, tense filament. Knots are used as ligatures during suturing and represent the weakest link in a stitch. If they fail, this can lead to wound dehiscence, in which a previously sutured incision re-opens and prevents wound healing.
In the new work, researchers led by Pedro Reis of the FLEXLAB at EPFL in Switzerland studied sutures made from commercial polypropylene filaments used in surgery. They analysed 50 to 100 sliding knots tied by Lausanne-based plastic surgeon Samia Guerid. They used a combination of mechanical testing, X-ray micro-computed tomography and computer simulations to mimic how these knots are tied, in a way that enabled them to measure the pre-tension that had been applied during the tying process.
This pre-tension, which permanently deforms or stretches the filament, cannot be quantitatively determined for surgeon-tied knots when tied with their hands, explains Reis. It is critical, however, since too little pre-tension causes the knot to come undone and too much snaps the filament.
A well-defined mechanism
“Our experimental system allowed us to systematically vary all of the important parameters in the knot-tying process,” Reis tells Physics World. “Our numerical model is complex because it needs to include knot topology, its nonlinear geometry, the elasticity of the filament, self-contact of the filament, frictional interactions and plastic deformation of the polymer (yet another nonlinearity). It is remarkable how, out of this ‘soup’ of ingredients, a robust and well-defined mechanism ‘pops’ out.”
What was also surprising, he adds, is that a well-trained surgeon, through much learned empirical experience, is able to target the “sweet-spot” of the ideal working regime of these knots, in between the limits of them being too loose or too tight.
The insight provided by the uncovered mechanism is fundamental, Reis says, since it could aid in surgical training and even advance the functionality of robotic-assisted surgical tools by enabling more effective knot-tying. “By incorporating our results into these devices, one could eventually aspire to a precision and efficiency level akin to an experienced surgeon.”
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In their study, which is detailed in Science Advances, the researchers tackled only the simplest of surgical knots. In the future, they plan to investigate a wider variety of surgical knots and develop more formal mathematical models to describe them.
“There are some specific aspects of our findings – for example, regarding the robust exponent of the power-law between knot strength and pre-tension – that we believe may be more general than the specifics of the two topologies (sliding granny knot and sliding square knot) we considered,” says Reis. “In other words, our study opens a very exciting door for future research on the physics of surgical knots. There is indeed a lot more about these structures that needs to be untangled…”
