Question
I read your answer to an earlier post about the Honey Locust not being poisoness, but I find it so hard to believe. I had a thorn go through the side of my tennis shoe  and lodge in the side of my ankle on the inside of my foot just under the ankle bone.  I had immediate pain. My husband pulled it broke off and I could see a portion of it sticking out side of my sock. My husband pulled it out and it appeared to all come out. I immediatly could not put weight on my foot, however thought I could work my way through it. I sat down for about 30 min and when I went to stand I could not apply any weight to my foot and could not walk. It felt almost like a sprain only the burning was almost unbearable. It swelled up and I started feeling ill. The next day I went to the Dr. and an x-ray showed nothing. I was admitted to the hospital, given a tetnus.. but on IV anti-biotics for 3 days after which I was released on oral antibiotics. It has been three weeks and I can wall but the pain is severe. It still burns like a fresh bee sting. Oh did I mention the did an MRI and could not find anything? No medical explaination for my continued pain after 3 weeks now. My ankle feels stiff almost arthritic.  Is there any formal research that has been done on this thorn?

Answer
Sorry for your problems --different folks are effected differently by getting stuck by these thorns. I could bot find any research on this  except for the Robert Nuss study.

There is not really a poison in the thorn but being stuck by any number of thorns (rose. locust etc.)they all seem to cause a medical problem. More then likely the dirt and dust gets into the wound and causes infection. Or when it is pulled out a small bit remains and festers.
The only site I found was from Penn State Robert Nuss, professor of ornamental horticulture.
http://www.aginfo.psu.edu/News/october99/toxic.html

"Thorny plants. The most common plant-related injuries are caused by thorns. "Most people don't seek medical help after getting stuck by a thorn," Nuss says. "But implanted thorns, needles or spines can cause infections or other medical problems."

Nuss says thorns embedded near joints can cause chronic arthritis. Thorns embedded near bones can mimic a bone tumor. Thorny plants that can cause painful injuries include roses, black locust and honey locust trees and blackberry bushes."  

we can rule out toxicology here, as Honey Locusts show no signs of toxicity, and only the seeds of the Black Locust are dangerous.
There is also the chance of an infection.

Now I guess that anything foreign to the body could be classified as a "poison" such as dirt or dust  on the thorn or the thorn itself but the plant itself is not toxic.