Knee dislocations are a dangerous and easily missed diagnosis.  Morbidity is high as 20% have a serious vascular injury, many of which lead to amputation.  This can occur with even ground levels falls in elderly and obese patients, so a high index of suspicion is warranted.
What kind of dislocation?
These are tibiofemoral dislocations, not patellofemoral dislocations.  Keep in mind that 50% of these will reduce spontaneously, so checking for knee instability and inability to ambulate in a swollen knee is important.
If you find mobility or dislocation, what’s the next step?
Doppler/palpate the ankle for pulses.  If present, do an ABI.  >0.9 makes arterial injury unlikely.  if <.9, or pulses are absent, CTA of the limb is the next step.  This is a surgical emergency and emergent ortho/vascular evaluation and CTA is warranted ASAP!