When you have had a definite injury, the type of damage will depend on the nature of the injury:
Acceleration & deceleration injury
Acceleration injuries might include the lower leg slipping forwards when the body is stationary and unprepared.
Deceleration injuries include things like catching a foot on an obstacle or a pothole while running, when the lower leg stops suddenly, but the upper leg and body are still travelling forwards.
The structures most commonly damaged in these three mechanisms are the cruciate ligaments, sometimes both anterior and posterior if the injury was particularly forceful, but usually the anterior cruciate ligament (ACL) and less often the posterior cruciate ligament (PCL). These ligaments are right inside the knee joint and hold the two long bones (femur and tibia) together in the centre of their contact surface, so rupture means that the two bony surfaces simply slide over one another without any control.
Torque ('twisting') mechanisms may include jumping while twisting, when the foot lands and grips the ground surface, while the body continues the twisting motion with the full weight of the person behind it. Also kicking, again twisting the body while the susceptible foot is 'planted' on the ground.
It always seems to surprise people that it is the foot on the ground, and not the foot doing the kicking, which gets damaged.
Again the person hears a POP! and the same wobbly knee is the result.
If the ACL is gone as well as the meniscus, it is quite common for the meniscal problem to be missed, as the ACL symptoms completely dominate the picture and the victim is so intent on protecting the knee that the meniscus does not have an opportunity of revealing itself with its own set of symptoms.
Pure meniscal tears may allow the person to continue with their activity, although they will likely be in considerable pain and there is often some bleeding into the joint with consequent swelling.
The one time that a victim will definitely not be able to continue with the activity is when a largish piece of the torn meniscus gets stuck between the two long bones, causing locking of the joint so that is cannot bend or straighten. This may unlock by itself but often needs a doctor to manipulate the knee under anaesthetic.
This type of injury might occur if someone was running fast, and caught their leg in quite a deep pothole, stopping the leg completely while the body continues on at force.
It may also occur if someone is tackled badly in rugby, from an attacker diving down and grasping the person's leg from the front, stopping the leg while the body continues forward at speed.
Direct blow to the knee
A blow to the front of the knee can shatter the kneecap, damage softer the joint surfaces behind the kneecap or damage the nerves in front of the knee. The wrench of the femur bone away from the tibia can also damage the posterior cruciate ligament. Such injuries are often referred to as 'dashboard' injuries, as in the days before car seatbelts front passengers often propelled forwards, striking the front of the knee on the dashboard.
A blow to the side of the knee may damage the cruciate ligaments. The ligament on the side of the knee opposite to the blow may be damaged if the blow forces the knee space open.