The BMJ’s columnist, Margaret McCartney, wrote that we need, “cast iron divisions between healthcare and industry” because their priorities are different. (Being The BMJ, there was no hyphen; but dictionaries give the substance cast iron as two words and the adjective cast-iron as hyphenated.) Margaret used the metaphorical cast iron instead of the literal clear, or distinct, because (I presume) she wanted to stress that these divisions should be immutable. The Royal College of Nursing asked for a “cast iron guarantee” that nurses will sit on the executive boards of NHS foundation trusts, presumably a guarantee that wouldn’t be broken. Not that guarantees or promises should be seen as sacrosanct: David Cameron gave a cast-iron guarantee that there would be a referendum on EU membership if he became Prime Minister, and look where that has got us.

The expression lends itself to diagnosis: “a cast-iron clinical diagnosis based on radiological findings, operative findings, and clinical follow-up.” And, with a clever allusion, there were “cast iron results” from a meta-analysis of giving iron along with agents to stimulate erythopoiesis in patients with cancer.

But it is a curious metaphor. I imagine that most people would interpret a cast iron guarantee as absolutely unbreakable: rock solid, to use another metaphor. But cast iron is not unbreakable. As I learned from my social history at school, the deficiencies of cast iron led to the invention of steel: to quote from a paper about the structural dynamics of actin, “actin becomes less like cast iron (strong but brittle) and more like steel (stronger and more resilient)”. Who wants a promise that is strong but brittle, ie, liable to shatter if hit hard? But steel is more bendy, and perhaps it’s better to have a guarantee that you know has shattered rather than one that has unsuspectingly mutated into something else.

Part of the reason that cast-iron has stuck may be cadence: the rhythmic flow of sounds or words. My examples are of the written word, but “steel divisions,” “steel guarantees,” “steel diagnoses” just sound odd: the single syllable is not enough and, however strong the material, that syllable peters out in a weak dribble. Carbon fibre is the strong material of the modern world, often lauded as stronger than steel. So should we try for carbon fibre guarantees? It’s got enough syllables but carbon fibre is not strong in all dimensions. Stronger than steel in its one intended dimension, it can, unlike steel, buckle in other dimensions. It is also, like cast iron, brittle.

I think Margaret is right, but the divisions are currently more like Plasticine™.

Neville Goodman is a retired consultant anaesthetist and a writer. He is co-author of a book on medical English.

Competing interests: I have read and understood BMJ policy on declaration of interests and declare that my only competing interest is my co-authorship of a book about medical English.