What is an ACL Sprain?

An ACL sprain is a tear of the anterior cruciate ligament in the knee. This may occur from ‘jarring’ or ‘twisting’ your knee, either in a contact or non-contact situation, and either a sporting or non-sporting environment.

The anterior cruciate ligament (ACL) runs diagonally across the knee originating at the back of the femur (thigh bone) and attaches to the front of the tibia (shin bone). It provides support and stability to the knee joint, in particular, it prevents the tibia (shin bone) displacing forwards.

What causes an ACL sprain?

It is often injured during an uncontrolled or unsuspected/unpredictable fast and twisting movement when the foot is in contact with the floor. The foot is planted on the ground, whilst the knee twists inwards.

Such as in ‘missing a step’ or landing heavily. This is more common in the many ‘twisting and turning’ or ‘jumping and landing’ movements in sport. There is often a higher risk of injury in contact but many occur under bodyweight activity only.

What are the symptoms?

• Sudden pain in the knee.
• In most cases, you will be aware that something serious has happened.
• Some patients describe an audible pop or crack at the time of injury.
• Swelling in the knee usually develops rapidly, but not in all cases.
• Your knee may also feel very unstable.
• Your knee will also become warm to touch.

The rapid swelling is caused by bleeding within the joint. This is due to ruptured blood vessels around and within the ACL which causes bleeding into the joint cavity. Learn more about ACL sprains.