I stumbled upon some videos showing real brain surgeries and noticed how the surgeon often uses a hand-held drill to create a hole in the skull (for accessing the brain). This made me wonder:

How does the surgeon ensure that the drill doesn't drill deeper than they want, thus damaging the brain?

For example, just from drilling holes in wood, I know that at the moment the wood is completely punctured through, the drill jerks forward deeper into the hole (since my hand is still applying a force forward, and my reaction time isn't quick enough to stop my hand immediately after the wood is completely punctured). What prevents this from occurring during surgeries?


The drill is a specialised device and not a standard type of drill. It is called a trephine, which is an instrument used for cutting out a round piece of skull bone to relieve pressure beneath a surface in situations such as a subdural haematoma.

In the earlier days, the risk of damage to the brain was down to a combination of skill of the surgeon and luck. Since 1950, there have been trephines with motors or shafts which automatically stop when in contact with soft tissue.


Smith, G. W. (1950). An automatic drill for craniotomy. Journal of neurosurgery, 7(3), 285-286. https://doi.org/10.1080/00185868.1951.9951539

  • 1
    Even more fascinating follow-up question: How did ancient peoples -- even prehistoric peoples! -- manage to accomplish the same with crude tools? en.wikipedia.org/wiki/Trepanning That question won't fit here, but I'd love to see it.
    – Carey Gregory
    Apr 9 '21 at 1:12

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