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
Get emergency help if the foot becomes cold, pale, blue, numb, or unusually weak after a knee injury.
These symptoms may indicate a problem with blood flow or nerves.
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
Seek urgent medical care after an injury if:
The kneecap looks out of place or remains displaced
You cannot put weight on the leg
The knee becomes very swollen soon after the injury
The knee locks and cannot fully bend or straighten
You felt a pop followed by instability or major swelling
A hot, red, very swollen knee with fever also needs urgent assessment because infection is a possible cause.
These symptoms are not typical of a simple tracking problem.
3
Does any of this apply?
Book an appointment
Arrange a routine appointment if pain lasts for several weeks, regularly returns, or interferes with walking, stairs, sleep, work, or exercise.
You should also seek assessment if the kneecap feels unstable or has previously slipped out of place.
The article also says
Do not try to force a visibly displaced kneecap back into place.
Every sign above is quoted from the sofidex article on Kneecap tracking disorder. If you are not sure, err upward — it is always reasonable to be checked.