ACL Knee Injury Symptoms

The shin bone, or tibia, is connected to the thigh bone, or femur, by a series of ligaments, including the anterior cruciate ligament (ACL). The ACL keeps the tibia from shifting ahead of the femur, maintains the correct angle between the tibia and femur, and prevents the tibia from rotating too much. ACL injuries are fairly common, especially for those who participate in high-impact sports, such as football. The ligament may overstretch or tear when you plant your foot and turn quickly or too sharply. However, ACL tears also occur outside sports; they can be work-related or due to a trauma, such as missing a step when descending stairs or a ladder. If you damage your ACL, you will notice some symptoms almost immediately.

First Signs of ACL Injury

Many people notice a popping sound or feeling in the knee at the moment they sustain an injury to the anterior cruciate ligament. At the same time, the knee may suddenly feel as though it is giving way. Tearing an ACL generally leads to immediate and significant pain, and the knee joint swells, usually within six hours. The American Academy of Orthopaedic Surgeons points out that the pain and swelling that accompany an ACL tear may subside, but the knee remains unstable.

Secondary ACL Injury Symptoms

An injury to the ACL may limit the range of motion in the damaged knee, according to the University of Michigan Health System. This means you may not be able to completely straighten your leg. Another symptom is persistent tenderness along the joint line.

Laxity in the ACL

When one of the knee ligaments sustains an injury, it becomes more lax, and the knee joint feels less stable. The online Sports Injury Clinic lists two common tests orthopedic doctors use to identify laxity in the ACL: the anterior drawer test and the Lachman’s test. These are done on both legs to help the doctor determine the degree of laxity in the injured knee, as compared to the uninjured knee. The anterior drawer test is positive if the ligament demonstrates laxity or too much movement between the tibia and the femur. The Lachman’s test is graded on a scale of 0 to 3, with 0 indicating no laxity at all and 3 indicating movement greater than 1 cm.

About this Author

Marcia Veach attended Mt. Hood Community College and the University of Oregon and holds degrees in both physical therapy and journalism. She has been an active health care professional for 30 years and a freelance writer for more than a dozen years. She has served as a writer and editor for business, nonprofit and health publications.