What are the causes of the pain in an anterior cruciate ligament (acl) injury?

Swelling/bone bruise: Initial pain after tearing your acl is due to 3 main reasons. 1) bleeding from the tear fills the knee exerting pressure on the capsule which is very sensitive to pressure. 2) the forces that cause the tear cause the femur and tibia to smack together leaving characteristic bone bruises in the femur and tibia. 3) additional injuries, most commonly meniscus tears can add to the pain of acl injury.

Meniscal tears.: Often one or both of the menisci may be torn, as well as the ACL. The pain can come from a meniscal tear that tugs at the nerve endings, resulting in pain. swelling from the injury can cause pain as well.

Instability: In the acute phase, the rupture ligament will hurt. Additionally, the knee will fill with blood right after the injury. This hurts. Finally, there are often associated injuries, eg meniscus tears or bone bruises, that hurt. After the acute pain resolves, people with an acl deficient knee are limited by the subtle instability which can be felt as soreness or as instability.

Meniscal tears.: You may have associated meniscal tears, that can be responsible for the pain. Or, the swelling of the knee itself, can be causing the pain.