This is knot what you were expecting
Physicians who aren’t surgeons probably don’t realize it, but the big thing that’s been getting between the knot-tying specialists and perfect suturing technique all these years is a lack of physics. Don’t believe us? Well, maybe you’ll believe plastic surgeon Samia Guerid, MD, of Lausanne, Switzerland: “The lack of physics-based analysis has been a limitation.” Nuff said.
Alain Herzog / EPFL
That’s not enough for you, is it? Fine, we were warned.
Any surgical knot, Dr. Guerid and associates explained in a written statement, involves the “complex interplay” between six key factors: topology, geometry, elasticity, contact, friction, and polymer plasticity of the suturing filament. The strength of a suture “depends on the tension applied during the tying of the knot, [which] permanently deforms, or stretches the filament, creating a holding force.” Not enough tension and the knot comes undone, while too much snaps the filament.
For the experiment, Dr. Guerid tied a few dozen surgical knots, which were then scanned using x-ray micro–computed tomography to facilitate finite element modeling with a “3D continuum-level constitutive model for elastic-viscoplastic mechanical behavior” – no, we have no idea what that means, either – developed by the research team.
That model, and a great deal of math – so much math – allowed the researchers to define a threshold between loose and tight knots and uncover “relationships between knot strength and pretension, friction, and number of throws,” they said.
But what about the big question? The one about the ideal amount of tension? You may want to sit down. The answer to the ultimate question of the relationship between knot pretension and strength is … Did we mention that the team had its own mathematician? Their predictive model for safe knot-tying is … You’re not going to like this. The best way to teach safe knot-tying to both trainees and robots is … not ready yet.
The secret to targeting the knot tension sweet spot, for now, anyway, is still intuition gained from years of experience. Nobody ever said science was perfect … or easy … or quick.