What causes the pain with an anterior cruciate ligament (acl) injury?

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.