Pressure dispersion in biological tissues

[okjhum]

Hi, being a skeptical MD (radiology), I'd like to have a physicist's view on a chiropractic mechanical device called the "Activator" (http://activator.com/, http://www.youtube.com/user/drcherok, http://www.youtube.com/watch?v=knM5dx2tFH0&feature=related), which in my view is a worthless and expensive scam. But the believers hold it in very high esteem. It's basic use is to find differences in leg length (really!) and where on the spine to correct that (indeed!) by adjusting which "misaligned" vertebra, with this gadget, that will deliver an exactly dosed mini-thrust by a little spring-loaded piston with a small amplitude. It is said to be "very gentle" (whereby they actually admit that ordinary chiropractic isn't so gentle, don't they?). If you look at the videos, the users invariably seem to place the Activator haphazardly even outside the patient's clothes, hand-held without support, so I can see no way how it could be exactly dosed at all. And even if it were, here is my question: How will the pressure wave from a couple of millimeters' thrust be propagated through the tissues towards the vertebrae? I guess it's a kind of cubic law of attenuation and scatter, as for sound.

Anyways, in chiro-world, this action realigns the vertebra and corrects the leg length difference (as measured with flexed knees and shoes on... but don't laugh yet). The thing is, when you palpate your spine or neck and "feel" the various bony prominences of the vertebrae, you are actually still quite far from the vertebra itself; it's only a what-would-you-call-it? A palpation by proxy? (please give me a better term.) The most popular vertebra in chiro-world is Atlas, the first cervical vertebra (C1). You can feel it just below your skull. However, on MRI and CT scans I measured the distance from the skin surface to the actual vertebra C1, and it is about 3 (three) cm from all directions to the most prominent parts of Atlas!

If, for the sake of reasoning, we suppose that it is possible to "adjust" a vertebra at all (which it isn't, but that's another discussion), how much remaining thrusting power of the delivered thrust would actually reach the bone at x cm from the body surface? /Thanks.

 

[Andy Resnick]

In all honesty, I used to think chiropractic techniques were wholesale garbage; now I am not so sure. People claim it reduces pain.

That particular device, it's doubtful it does anything meaningful- as you say, the idea is to apply a very precise load onto a joint, but the device does not appear to be placed very precisely, nor does there appear to be any control over the magnitude of applied load. Why apply the same magnitude stimulus to the neck as the lower back?

 

[Proton Soup]

IANAP, nor a fan of chiropractic in general, but it looks like a form of massage to me. I've personally had good experience with Clair Davies' self-treatment translation of the research of Drs. Travell and Simons on myofascial trigger points.

 

[Evo]

There was a program about the use of this device on 60 minutes, or a similar show, and they said that the device was useless.

Here is what Quackwatch has to say.

http://www.chirobase.org/06DD/activator.html

 

[okjhum]

Thank you all who replied. I already know the gadget is useless for medical purposes, as is chiroquackery as a whole, but I'm interested in the physics of Activator action. Anyone?

 

[Proton Soup]

Thank you all who replied. I already know the gadget is useless for medical purposes, as is chiroquackery as a whole, but I'm interested in the physics of Activator action. Anyone?

it just puts pressure in a small spot, like poking someone with your knuckle. more force in a small area allows you to massage a deeper spot. there's really nothing at all special about it.

 

[somasimple]

it just acts, as often, on nervous system.

 

[okjhum]

it just acts, as often, on nervous system.

Do you know this, or are you guessing? References?

 

[okjhum]

I'd like to see...
(1) ...a plausible formula for the pressure decay in human tissues, notably skin, subcutaneous fat, and muscle; and
(2) ...a map of typical pressure dispersion/diffraction in said tissues.

 

[okjhum]

According to the maker, http://activator.com/ActivatorInstrumentSalesVideo.html
the pressure is up to 28 lbs (as if that is a pressure?). Anyway, will not that mean that typically half is delivered to the patient (through the clothes!), and the other half as a reactive force in the opposite direction? Do we have also to account for the difference in mass between the practitioner's hand and the patient's body?
Sorry for these questions, but I predict it will be more fun to debunk the device with physics than just incredulousness.

 

[okjhum]

What I'm trying to say, and to get corroborated, is that there will be virtually no physical force at all delivered to the joints of the spine in one specific plane of the joint -- which typically is 2-3 cm or more in from the skin surface. So "adjusting" the spine or whatever with that device would be just magic.
Chers.

 

[Andy Resnick]

I'd like to see...
(1) ...a plausible formula for the pressure decay in human tissues, notably skin, subcutaneous fat, and muscle; and
(2) ...a map of typical pressure dispersion/diffraction in said tissues.

Here's a start:

http://linkinghub.elsevier.com/retrieve/pii/S0021929008000547

(muscle)

http://www.google.com/url?sa=t&sour...yt1fkO&usg=AFQjCNFtWjZEgexoc9npbLlPgtCMCFbS5g

(fat and skin)

http://silver.neep.wisc.edu/~lakes/Biom2.html

(bone)

 

[Moonbear]

Anyways, in chiro-world, this action realigns the vertebra and corrects the leg length difference (as measured with flexed knees and shoes on... but don't laugh yet). The thing is, when you palpate your spine or neck and "feel" the various bony prominences of the vertebrae, you are actually still quite far from the vertebra itself; it's only a what-would-you-call-it? A palpation by proxy? (please give me a better term.) The most popular vertebra in chiro-world is Atlas, the first cervical vertebra (C1). You can feel it just below your skull. However, on MRI and CT scans I measured the distance from the skin surface to the actual vertebra C1, and it is about 3 (three) cm from all directions to the most prominent parts of Atlas!

If, for the sake of reasoning, we suppose that it is possible to "adjust" a vertebra at all (which it isn't, but that's another discussion), how much remaining thrusting power of the delivered thrust would actually reach the bone at x cm from the body surface? /Thanks.

I call quack! (For those pitching the gizmo, not you.) You're not going to actually palpate C1 at all. It's buried quite deep under layers of muscles (trapezius, splenius capitis, semispinalis capitis, and the muscles of the suboccipital triangle). Not usually much skin and fat to get through there, at least. You really only start to palpate vertebrae around C6 and C7.

Looking at the little demo of the actuator thingy (basically just a spring-loaded contraption) in the second link you provided, it's not applying much force at all. It doesn't take much to make a ball jump the way it did in that little tube. About the only claim I'm going to believe from the first link is that it is probably good for testing reflexes. The size of the head on it is about the same as the head of a reflex hammer, and I expect the force was similar too.

What really tells me QUACK in all capitals were the rest of the information in that video. At the lumbar levels, which is the first "adjustment" in the video, there's a lot of tissue between the skin and vertebrae (especially in a person as overweight as the subject in the demo). If that "thump" was enough to move a vertebra, that subject would have been jumping from the pain of the whack to the skin and subsequent bruising.

But, even worse, the next "adjustment" was up in the cervical levels. In that one, the quack didn't state which level, but a rough guesstimate I'd say maybe C5 ish. She then follows that up by saying that level was the nerve to the sinuses while claiming it could treat allergies. Okay, first, no, the nerves innervating the sinuses are not coming from the cervical levels, they are cranial nerves (the maxillary nerve, which is a branch of the trigeminal nerve). Second, no, allergies are not in any way related to spinal nerve function, and are an immune disorder.

I stopped watching at that point.

Probably the most useful thing of the whole "treatment" was lying down and flexing and extending the knees several times to stretch out some tight muscles. If the pain is due to something like a muscle spasm or overused muscle (perhaps due to bad posture), some stretching and lying down flat could help relax the muscles.

 

[zoobyshoe]

What I'm trying to say, and to get corroborated, is that there will be virtually no physical force at all delivered to the joints of the spine in one specific plane of the joint -- which typically is 2-3 cm or more in from the skin surface. So "adjusting" the spine or whatever with that device would be just magic.
Chers.

To the extent you compress the tissue between skin and vertebra you can apply force to the vertebra. More force = more compression. Obviously a hard enough blow to someone's back could crack a vertebra. Simply palpating a vertebra also, does, indeed, apply force to it: it is the resistance against which the tissue is compressed, and must, therefore, experience a force. If your fingers perceive there is a hard shape against which you are compressing the soft tissue, then that hard shape is also experiencing a force, and exerting an equal and opposite force in return, as per Newton.

So, I don't believe you can debunk it based on the idea it delivers no force to the vertebrae. I think Moonbear's debunking is more to the point.

 

[okjhum]

Thank you, Andy Resnick (are you related to the legendary radiologist, Donald Resnick?), for the good links. Quite a bit above my level of comprehension, but I emailed the authors for more info.
Thank you, Moonbear, for your insights; you are good at putting words to my thoughts! :-)
Thank you too, Zoobyshoe, for your excellent summary. That, together with the reading provided by Andy, made me now even strongly suspect that *if* the Activator were set to sufficient velocity and amplitude to actually nudge a vertebra, it would first crush the interposed tissue. I overlooked that possibility before. Cheers!

 

[zoobyshoe]

...made me now even strongly suspect that *if* the Activator were set to sufficient velocity and amplitude to actually nudge a vertebra, it would first crush the interposed tissue.

None of the operative terminology is rigorous, so everything depends on what is meant by "nudge", "move" "adjust".

The vertebrae are held in position relative to each other by the disks, muscle and tendon. To the extent the device simply massages the muscles it might be claimed to "adjust" the position of vertebrae by altering the tension of the connective tissue. To the extent it actually applies force to a specific bone, which it well could without bruising or injury (as I mentioned simply palpating a bone applies force to that bone), that bone would just move, say .01 or .02 millimeters (a guesstimate) relative to its neighbors, and then resume its position when the force was removed. How much relative motion you get would be dependent on a specific individual's muscle and tendon tension. Someone with Ehlers-Danos Syndrome (which comes in varying degrees of severity) might demonstrate a more remarkable amount of vertebrae motion.

So, non-destructive force sufficient to temporarily alter its position relative to other vertebra can be applied to a vertebra, and repeatedly doing that with a reciprocating piston might accomplish a kind of massage of the connected muscles. "Nudging" a bone doesn't require bruising. In short, this is all about what you mean by "nudge", what they mean by "adjust", etc.

However, a sudden and permanent alteration of the relative position of these bones by direct force does not strike me as possible unless you permanently damage what holds them in position: disks, muscles, tendons.

 

[Proton Soup]

However, a sudden and permanent alteration of the relative position of these bones by direct force does not strike me as possible unless you permanently damage what holds them in position: disks, muscles, tendons.

the only thing permanent about chiropractic manipulation is your treatment schedule.