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Apostrophe use and eponyms

Last revised by Daniel J Bell on 3 Apr 2023

Apostrophes are used according to standard English grammar rules. They are used for possessives and to indicate missing letters. 

When using a possessive it's usually by adding "'s" at the end, e.g. "the patient's disease". Apostrophes should not be used for pluralisation, e.g. "apostrophes" and not "apostrophe's".

The other main use is to indicate missing letters in so-called 'contractions', e.g. "it's", "don't" and "you're". Some of these forms have notorious counterparts without an apostrophe that have a different meaning, e.g. its (possessive of it), your (possessive of you). Although in written English, and hence Radiopaedia, generally the non-contracted form is preferred e.g. "cannot" instead of "can't", "we would" instead of "we'd" etc.

Although apostrophes were previously also used in the pluralisation of letters (B's), numbers and acronyms (ICD's) this use is becoming obsolete and we, therefore, advise against it and would write: Bs, ICDs, 5s, and Ss (for plural of 'S'). Read more about acronyms.

At we think that the use of apostrophes should be as limited as possible. In general, terms that often include apostrophes are mostly used in an adjectival sense and only in a very limited number of cases should the possessive nature of apostrophes be maintained for grammatical reasons.

A particular note should be made on eponyms as the use of apostrophes in eponyms is debated. Many diseases, anatomical structures and landmarks are named after a person, either a patient, a doctor or someone entirely different (also see Stigler's law of eponymy). There is variation in the spelling of such eponyms when it comes to the inclusion of an apostrophe, effectively alternating between using a possessive form (Down's syndrome) or a non-possessive form (Down syndrome). 

The non-possessive form (without an apostrophe) is becoming more common and widely advocated; National Institutes of Health (NIH) and World Health Organization (WHO) 1, AMA manual of style 2,  Stedman’s Medical Dictionary, and Dorland’s Illustrated Medical Dictionary 3 - yes the irony that both dictionaries use the possessive form is noted., therefore, aims for uniformity both regarding its own content and in the larger medical community and for this reason alone we prefer the non-possessive form.

Generally speaking, and mostly because of the plea above, is in favor of omitting apostrophes wherever possible. However, there are still some cases where it's necessary. These include:

  • anatomical structures that cannot be pluralised without becoming nonsense: Arnold's nerve, Waldeyer's ring, Bochdalek's flower basket, Lister's tubercle, Hoffa's fat pad

    • by using the rule of thumb with plurals other terms can do without an apostrophe: Morgagni hernia, Bochdalek hernia, Hoffa fracture, Meckel diverticulum

  • terms that include a fixed phrase that includes an apostrophe: shepherd's crook deformity, bull's eye sign

In all other cases, strongly encourages not using an apostrophe. This includes many terms for which many people actually include an apostrophe and versions with or without apostrophes can be observed throughout the literature, clinical practice or presentations.

A non-exhaustive list of terms that can do without apostrophes:

  • diseases: Parkinson disease, Huntington disease, De Quervain tenosynovitis

  • syndromes: Down syndrome, Chilaiditi syndrome, Löfgren syndrome

  • signs: Westermark sign, Rigler sign, Chilaiditi sign

  • lines: Shenton line, Perkin line, McGregor line

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