What sort of problem is an anterior cruciate ligament (a.c.l.) injury?

Ligament sprain/tear: The acl is a ligament inside the knee that helps to keep the knee stable - keeps it from moving too far forward or backward. It works with the posterior cruciate ligament (pcl) to accomplish this. The acl is often injured when running or other fast movements when the lower leg is planted on the ground and the knee is bent and twisted. It can be a strain or sprain or a complete tear.

Knee instability: When you tear the acl, you lose one of the stabilizing ligaments in your knee. That can allow your knee to become unstable or shift with certain movements. A knee that keeps on shifting can wind up damaging other structures (the meniscus or articular cartilage). Some people can rehab their knee and may not need an acl reconstruction, while others will need a reconstruction even for daily activity.