Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek emergency care if:
The kneecap is visibly out of place
The foot becomes cold, pale, blue, numb or unusually weak
There is an open wound over the knee or bone is visible
The injury involves major trauma or severe uncontrolled pain
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Get prompt medical assessment after a suspected dislocation even if the kneecap returned by itself.
An examination and imaging may be needed to check for a fracture or cartilage injury.
A knee that locks and cannot fully bend or straighten also needs prompt assessment because a loose fragment may be blocking movement.
3
Does any of this apply?
Book an appointment
Arrange routine follow-up if the knee repeatedly buckles, the kneecap keeps slipping, swelling continues, or rehabilitation is not restoring movement and function.
The article also says
Keep the knee still and supported while waiting for help.
Do not force the kneecap back into place.
Every sign above is quoted from the sofidex article on Unstable kneecap. If you are not sure, err upward — it is always reasonable to be checked.