When treating a lateral epicondylitis of the humerus with diclofenac patches, what is the recommended surface of the patch?

Let's assume that the entire patch (10x14cm) contains 140 mg of diclofenac (as in this one). I understand that reducing its size results in reducing the amount of diclofenac. However, I believe (and may be wrong) one should take into account the distance between the patch and the epicondyle. E.g. if the patch's size is reduced by half, the remaining 70 mg are "closer" to the epicondyle than the removed 70 mg (provided that the patch is always placed in such a way that the distance between the patch and the epicondyle is minimized).

So this question could be broken down into two main ones:

  • To what extent does the distance between the patch and the epicondyle matter for the treatment?
  • To what extent does the diclofenac dose matter for the treatment?

I'm ok to post them as two separate question if needs be. I don't have any preference.

  • I feel as though I commented somewhere else that I would come back and answer a similar question to this, but then I couldn't find it. Were there related questions to this that you also need answered (that aren't covered in my answer below)?
    – Atl LED
    Apr 13 '16 at 17:13
  • Were you also interested in the injury in the context of arthritis? Because that can change the reasoning somewhat substantially, though there's still not going to be lateral diffusion.
    – Atl LED
    Apr 14 '16 at 20:40
  • @AtlLED I don't remember, but there are so much content deletion on this website that it's difficult to follow. I'm interested in the context of tendon overuse. Jul 4 '16 at 3:43

The skin permeation profile (particularly the lateral diffusion coefficient) of a diclofenac (DK) transdermal patch, and percutaneous absorption in general, are in question here. I include the medical terms, mostly because it may help finding further reading beyond what I suggest.

An approachable introduction to percutaneous absorption is a review written by Dr. Paul Brisson (it was in fact, used a source text in my schooling).

Lateral diffusion, how any drug spreads out from the application site as opposed to down through the skin, varies on how the patch is actually made. This is often measured as cm2/h, as the diffusion happens over time. It should be noted that even with the constant application of the patch, you are going to get reduced diffusion based on concentrations gradients which roughly follow Flick's laws of diffusion.

A standard salt gel formulation of DK (which is a good baseline, as any modifications will be at best 100x change in ether direction) has a lateral diffusion coefficient of 9.65x10-9 cm2/h, and a rough saturation time of about 3 hours. Even if we assume 10 hours of diffusion, and a 100X increase in the coefficient (9.65x10-7), that still only gives us 9.65x10-7 cm2 lateral diffusion. Let's round that off to an even 97 square nanometers.

That's smaller than the cross sectional area of many viral particles. So you are quite right, a patch is most directly treating the area that it is directly on top of. But this ignores another factor in patch dosing, which is that the drug is transferred to the plasma (blood), and then distributed systemically throughout the body. Thus a significant portion of the dose that a prescribing physician is intending you to recieve might be through delivery from the blood.

So yes, you can sometimes cut a patch to deliver the drug better to a local area, but you should first check with your medical professional to be sure that the type of patch you are using can in fact be cut and still effective (some the nanogel suspensions can't be).

  • "supplying anti-inflamatory drugs to the entire joint" what does this mean? Apr 14 '16 at 2:58
  • @GrahamChiu I will certainly give deference to a rheumatologist or others with more relevant experience (I'm in ID), but my understanding is that supplying an NSAID to the surrounding tissues (ligaments, tendons, muscle, etc) that might not be inflamed results in better outcome than only applying to the inflamed/insulted tissue. Am I wrong in my understanding?
    – Atl LED
    Apr 14 '16 at 14:03
  • 1
    I'm not aware of this concept. Do you have an actual reference that is not behind a firewall? To me the entire joint also includes synovium, cartilage, bone etc, so treating those seems unnecessary. Apr 14 '16 at 19:20
  • @GrahamChiu I will dig for one. I agree the entire joint is poor wording. I'm editing to change it to "surrounding muscle and tendons."
    – Atl LED
    Apr 14 '16 at 19:24
  • @GrahamChiu It may be better if we hash out the details in chat, but I'm finding a lot of the sources behind pay walls saying things like NSAIDs for "surrounding supportive adipose, connective, and muscle tissue." I'm trying to find a primary study looking at drug levels in the tissue for lateral epicondylitis
    – Atl LED
    Apr 14 '16 at 19:55

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