Wikipedia:Reference desk/Archives/Science/2021 September 19

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September 19

Ligature in Surgery

Ligature (medicine) has been a stub for more than 15 years, and has only a few lines of content. Is there is corresponding article on surgery which covers this better and so this can be redirected or merged there? Jay ^(Talk) 10:10, 19 September 2021 (UTC)

The nearest I can see is surgical suture. You could discuss possible merger on the relevant talk pages. Each article does "see also" the other. Mike Turnbull (talk) 11:16, 21 September 2021 (UTC)

Yes, I had seen suture, but that article is broader than ligature which is about tying. So I guess, Ligature (medicine) is the only article on enwiki on the topic (which is surprising). Jay ^(Talk) 08:21, 22 September 2021 (UTC)

Note that the surgical procedure is commonly called ligation, on which we have no article, not even a stub, although it is mentioned several times in our Surgery article ("ligation – tying off blood vessels, ducts, or 'tubes'"; "efficient techniques for the effective ligation of the blood vessels during an amputation"; "a more effective method for applying ligature of the arteries in case of an aneurysm"; "first performed a successful ligation of the abdominal aorta"). We have a few specialized articles involving the procedure: DRIL, LIFT technique, Rubber band ligation, Trendelenburg operation and Tubal ligation.  --Lambiam 10:35, 25 September 2021 (UTC)

External v internal pressure

For simplicity, I'll say my lab works with worms. There is a cell underneath the hypodermis that (we think) becomes deformed by either an increase in hydrostatic pressure in the body cavity or by focal compression by objects passing through the body cavity. My coworker is convinced the deformation experienced by the cell can be recapitulated by just increasing the atmospheric (external) pressure. Intuitively I feel uniform external pressure through a homogeneous medium (air), transduced through first a solid but deformable barrier (worm skin) and then through an inhomogeneous fluid (worm guts) is a totally different regime from that of internal hydrostatic pressure or focal compression. My coworker "concedes" that different materials experience different compressibility, but somehow we diverge when it comes to the conclusion of this thought experiment I made up: Say we have a box floating in space (no gravity) with the "top" half filled with air and the "bottom" with water, with or without a solid barrier at the interface; and there is an over-easy egg on a wall in the air half and another on the floor of the water half; if we were to increase pressure by lowering the ceiling (air half) at epsilon increments, would both egg yolks burst at the same time? This is our last dance (talk) 20:57, 19 September 2021 (UTC)

The pressure in the air and water would be identical Greglocock (talk) 00:26, 20 September 2021 (UTC)

Without gravity, why would the water remain in the bottom half? Imagine Reason (talk) 00:54, 20 September 2021 (UTC)

Imagine Reason, I forgot that I needed the impermeable barrier in there. JoelleJay (talk) 04:27, 20 September 2021 (UTC)

If the barrier is flexible enough, its presence makes no difference. But if the moving ceiling reaches the top egg, that one may be first to burst. The forces acting on the worm cell may not be the same in all directions if the deformation is due to non-uniform compression forces, whereas with hydrostatic pressure it does not matter where the pressure comes from.  --Lambiam 05:17, 20 September 2021 (UTC)

The egg yolk is an incompressible liquid surrounded by a fragile membrane and in both cases has force applied equally in all directions ; for very much the same reasons that scuba and free divers eyes are unaffected at depth, neither would burst. 2A01:E34:EF5E:4640:DFC:5965:9101:1BA9 (talk) 16:19, 20 September 2021 (UTC)

Sure. That's why divers don't wear goggles or face masks. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:12, 20 September 2021 (UTC)

You are out of your depth. Yes 2A01, I agree. Thought for Bugs, what is the air pressure in a Scuba divers mask, compared with the surrounding water? Greglocock (talk) 22:22, 20 September 2021 (UTC)

I wouldn't know. But I would like to see a reference for the assertion the IP is making. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:25, 21 September 2021 (UTC)

Divers wear a dive mask because water has a different refractive index to air, not to compensate for hydrostatic pressure (which of course it doesn't; the pressure within the mask is maintained at ambient to prevent barotrauma such as petechial hemorrhage and ultimately exophthalmos due to mask squeeze). I'm really not sure what sort of mystical force you think might be operating unequally upon the yolk membrane to cause it to rupture, so I can't give you a reference for its inexistance (although I can cite the notorious non visual impairment of french scuba diver Jacques Cousteau despite the eleven Kilogram-force per square centimetre exerted on his eyes whilst at 100 metres), but since rupturing clearly involves one part of the surface moving in a different direction to another such that the membrane locally exceeds its elastic limit, you might like to ponder Newton's laws of motion and ask yourself where those opposing velocities come from in a homogeneous medium with only symmetrical pressure forces in play. If any of this is unclear to you, you might like to pose a new, specific, question on the science refdesk. 78.245.228.100 (talk) 05:22, 21 September 2021 (UTC)

Let me add an explicit conclusion to what I wrote above. There are three situations: (A) deformation of a cell due to focal compression; (B) deformation of a cell by hydrostatic pressure in water; (C) deformation of a cell by hydrostatic pressure in air. In (B) and (C) the pressure is isotropic; what matters is the pressure and not the medium through which it is delivered. Since the content of the cell is incompressible, no net force acts on its surface or content (ignoring gravity). So there will be no deformation. In situation (A) also shear stress is at work, giving a different, anisotropic force distribution. The original problem is to choose between (A) and (B). The (B) versus (C) thought experiment does not reflect this problem.  --Lambiam 10:50, 21 September 2021 (UTC)

Ahh I see where some of the confusion lies. By "hydrostatic pressure" in (B) I meant increases in the pressure within the body cavity, which is encased by an elastic membrane and expands into the surrounding tissues with mostly uniform compression (as in, the whole cell would be flattened uniformly). This is in comparison to (A), which involves objects impinging on small sections of the cell. In (C), the whole worm experiences isotropic volume decrease under external hydrostatic pressure, although the stress-strain curve is approximately linear at low stress/strain and then enters a nonlinear strain stiffening regime under larger pressure. So basically there's a limit to degree of compression. Applying negative external pressure produces a comparable stress-strain curve to the linear regime of positive pressure, but without the nonlinear response at increased negative pressures. You are correct that the main issue is the difference between (A) and (B), but I think it's also relevant to understand how the differing biomechanical properties of the internal vs external structures, and the barriers between them, affect their compressibility. In the thought experiment, there should have been a barrier at the air-water interface such that increasing the pressure on the air side by moving the ceiling down would deform the barrier down into the water, but this volume decrease in the water section would not be as much as the volume decrease in the air section. This is our last dance (talk) 20:20, 21 September 2021 (UTC)

I would need to know more about the meaning of "deformation" and what the reaction of this cell is. It seems to me that the answer depends on the way the cell measures this deformation. But it is possible that in the end the cell measures the difference between the internal and the external pressure, in which case just changing the external pressure (and consequently the internal pressure by the same quantity) will not recapitulate the normal behaviour of the cell.

By the way, your example with the chamber with water and air is rather not relevant, as 1) your cell doesn't sit at the air side of the skin but only at the water side of it 2) and of course by increasing pressure no egg will ever burst. You should rather decrase pressure, in which case it seems to me obvious that both cells will burst at the same time. The point is, as said, much more whether the skin is rather rigid or rather soft, that is whether it will maintain a pressure difference between inside and outside.

All togehther I think it will be necessary (and much more interesting) to find out experimentally whether the cell reacts the same way when your coworker increases only the internal pressure or only the external pressure. 2003:F5:6F1A:3900:9498:F95C:DD33:4EEF (talk) 18:31, 24 September 2021 (UTC) Marco PB