The knee injury that often has no pain is meniscal tears or ligament sprains.
Ligament sprains or meniscal tears can occur without any pain at the time of the injury although they may cause pain later on.
And in some cases the meniscal tears or ligament sprains may cause pain but not always.
Most often a cruciate ligament injury does not cause any pain.
Cruciate ligament injuries are often associated with a popping sound and severe pain.
However some people with cruciate ligament injuries may not have any pain or minimal pain at the time of the injury.
A person with a cruciate ligament injury may feel a popping sound, experience swelling, or leg buckling but pain may not be a primary symptom of the cruciate ligament injury.
Collateral ligament injuries which are similar to cruciate injuries or sprains, especially grade 1 sprains may only cause some mild swelling and tenderness with minimal pain.
And some people that have a torn meniscus may experience little to no pain, especially if the miniscus tear is small.
The severity of the knee injury will also impact the level of pain or no pain experienced with the knee injury and more severe knee injuries, such as a complete ligament tear, will often cause more pain.
Also every person's pain threshold and perception of pain are different, so some people may experience less pain than others do with the same knee injury.