ACL injury can cause knee pain because of the tearing of the anterior cruciate ligament connecting shinbone to thighbone(ACL).
Tough, rubbery cartilage absorbs shock between your shinbone and thighbone. Torn meniscus can cause severe knee pain.
Inflammation in the bursae, the small sacs of fluid, can cause knee pain.
Irritation and inflammation of one or more tendons.
Wear-and-tear condition of the cartilage.
An autoimmune condition affecting knee joints.
Uric acid crystals build up in the joint
Development of calcium-containing crystals in the joint fluid
Physical injury/trauma can also cause knee pain.