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The Wikipedia-article about Inflammation says

The five classical signs of inflammation are heat, pain, redness, swelling, and loss of function (Latin calor, dolor, rubor, tumor, and functio laesa).

What does 'loss of function' mean? And how does it occur?

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TL;DR: It originally referred to abnormality in "secretion" by the inflamed tissue and later became used to mean any abnormality at all.

This fifth cardinal symptom of inflammation seems to be rather mythological as laid out in detail by Rather (1971, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1749862/?page=3). This source explains that the term seems to have been invented at some point in the 19th century without proper definition. Later authors seem to have interpreted it as basically just meaning "not functioning properly" (although this is literally the definition of unhealthy as opposed to healthy, and not specific to inflammation).

According to Rather, the original source of the confusion may have been the phrase: "...heat, redness, tumour and pain; to which should be added an alteration or suspension of the natural secretions of the part." by Macartney in 1838 (page 8 of the same document). Various versions of mentioning a fifth cardinal symptom that involved altered function made their way through pieces of writing, until it received the Latin term "functio laesa" in the late 19th century (Rather cites a piece from 1882), lending it credibility as an established concept. The passage by Macartney was later cited by Marchand in 1924, who was trying to clear up the confusion around who established the four and potentialy fifth cardinal symptoms of inflammation. Marchand rephrased it into a "fifth cardinal symptom the alteration or disappearance of normal secretion (thus a functio laesa)", trying to explain what, according to Macartney, is the fifth symptom, and adding why it could be called a "loss of function".

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Usually, loss of function means some sort of limitation in physical performance. Either force, range of motion, or some other aspect of movement. In this context, loss of function can occur by at least two mechanisms:

  1. mechanical loss of function due to swelling of the tissue around the joint
  2. limitation due to pain associated with movement, part of movement, or too high load.
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