Factoid of the week

Why do shoelaces untie?

The scientists expected that the knots would come undone slowly. But their slow-motion footage — focused on the shoelaces of a runner on a treadmill — showed that the knots rapidly failed within one or two strides. To figure out why, O’Reilly and his colleagues used an accelerometer on the tongue of a shoe to measure the forces acting on a knot. They found that when walking, the combined impact and acceleration on a shoelace totals a whopping 7 gs — about as much as an Apollo spacecraft on reentry to Earth’s atmosphere.

this sounds more frivilous than it actually is: Because if an elderly person trips on an untied shoelace, they can break an arm or a hip.

This is also a problem in surgery: Braided silk usually doesn't untie easily, nor does Chromic catgut. We usually do three or four knots (square knots not grannie knots) and no problem.

But newer unifilament nylon does tend to untie itself.

And if it unties before the wound is healed, you end up with the wound falling apart.

Although I found that nylon skin sutures, even with six or eight knots usually don't start to unravel until the wound swelling goes down and the tension on the knot allows it to unravel.

but if a person is malnourished (i.e. low protein diet, diabetes, cancer) the wound could take longer than usual to heal.




when we were in Africa, we used monofilament fish line to sew the skin because the pre packed nylon or silk was too expensive and often poor quality.

The real problem was the catgut and newer absorb able suture: If the catgut (regular or chromic) was old, it could lead to internal bleeding. Not a big problem for under the skin, but for C Sections it could lead to bleeding or other complications (a uterus that would rupture in a later pregnancy, a dangerous complication). We would usually use the modern versions donated to us by German or American hospitals.

and no, catgut is not made from cats: It is made from sheep guts, or as Wikipedia explains:

Catgut suture is made by twisting together strands of purified collagen taken from the serosal or submucosal layer of the small intestine of healthy ruminants (cattle, sheep, goats) or from beef tendon.^[1] The natural plain thread is precision ground in order to achieve a monofilament character and treated with a glycerol-containing solution

but it is not used as much nowadays, especially in Europe, because of mad cow disease worries and because there is an alternative.