From Longman Dictionary of Contemporary Englisha hairline crack/fracturea hairline crack/fractureMIa very thin crack a hairline fracture in a bone → hairline
Examples from the Corpus
a hairline crack/fracture• I was worried about a hairline fracture, but luckily it was just a bad lump.• The specialist he consulted on Monday diagnosed a hairline fracture of his right wrist and wants to take another look on Friday.• She had a hairline fracture in her leg. 