？
Which joint did you dislocate?
Literal
Where [genitive-の] joint [subject-が] came-out [explanatory-question-の]?
A body-part question. はずれる ('to come off, come loose, be dislocated') is the intransitive pair of はずす ('to remove, detach'). Used for joints, はずれる means 'to become dislocated.' どこの in front of a noun is a genitive question — 'which/whose X.' Note the medical register uses intransitive verbs to describe what happened *to* the body, not what the body did.