Signs of an ACL Tear

How does an ACL tear happen?

An ACL tear most commonly occurs during a sudden change in direction, pivot, or deceleration. Importantly, many ACL tears are noncontact injuries—there does not have to be another player or object striking the knee.

Common mechanisms include:

  • Pivoting or cutting: The foot is planted while the body suddenly changes direction.

  • Sudden deceleration: Slowing down rapidly from running or sprinting.

  • Awkward landing: Landing from a jump with the knee in a vulnerable position.

  • Hyperextension: The knee is forced beyond its normal range of motion.

  • Contact injury: A blow to the outside of the knee or another force can cause the knee to buckle or shift.

A classic ACL injury may feel like the knee “gave way” or shifted, sometimes accompanied by a pop. However, not everyone experiences or remembers a pop.

What are the symptoms of an ACL Tear?

Diagnosis of an ACL Tear starts with the history. With an ACL injury, patients may experience:

  • A pop or popping sensation at the time of injury

  • Sudden knee pain

  • Rapid swelling, often developing within the first several hours

  • Difficulty continuing to play or bear weight

  • A sensation that the knee gave way, buckled, or shifted

  • Loss of normal knee motion, particularly difficulty fully straightening the knee

  • Knee instability with subsequent cutting, pivoting, or athletic activity

Rapid swelling is particularly important. A knee that becomes significantly swollen shortly after an injury may have bleeding inside the joint (hemarthrosis). An ACL tear is one of the important causes of an acute traumatic hemarthrosis.

Physical Examination

Your orthopedic surgeon will first examine the knee for swelling, range of motion, tenderness, and stability.

Joint Effusion

  • An effusion means there is excess fluid inside the knee joint. After an acute injury, a significant effusion can be an important clue that something substantial has happened inside the knee.

Lachman Test

  • With the knee slightly bent, the examiner stabilizes the thigh while gently pulling the shin forward. Increased forward movement of the tibia compared with the opposite knee, particularly when there is a soft or absent endpoint, suggests ACL insufficiency.

Anterior Drawer Test

  • The anterior drawer test evaluates forward movement of the tibia relative to the femur with the knee bent approximately 90 degrees.

  • Increased anterior translation compared with the opposite knee can indicate an ACL tear. However, hamstring activation and other factors can make this test less reliable in the acutely injured knee.

Pivot Shift Test

  • The pivot shift test evaluates the functional instability caused by an ACL-deficient knee.

  • The examiner applies specific rotational and valgus forces while moving the knee through a range of motion. In an ACL-deficient knee, the tibia can shift relative to the femur, producing a characteristic “pivot shift.”

  • The pivot shift can be particularly useful for demonstrating rotational instability, although it can be uncomfortable in an acutely injured knee and is often easier to perform once swelling and pain have improved or when a patient is under anesthesia such as during surgery.

Imaging

1. X-Rays

Although X-rays do not directly show the ACL, they are an important part of the evaluation of a suspected ACL injury.

X-rays can:

  • Identify fractures or other bony injuries

  • Evaluate the overall alignment of the knee

  • Identify associated injuries that may occur with an ACL tear

  • Look for a Segond fracture

  • Evaluate the tibial slope, which can be relevant when assessing ACL injury risk and knee stability

What is a Segond Fracture?

  • A Segond fracture is a small bony avulsion involving the outer (lateral) portion of the proximal tibia.

  • It is thought to represent portions of the anterolateral ligament complex as well as posterior insertional fibers of the Iliotibial band (IT Band).

  • Although small, a Segond fracture is strongly associated with ACL injury and should raise suspicion for an ACL tear.

Tibial Slope

  • The posterior tibial slope can also be assessed on a lateral X-ray. A steeper posterior tibial slope has been associated with increased stress on the ACL and may be relevant when evaluating patients with ACL injuries, particularly those with recurrent instability or subsequent ACL tears

2. MRI

MRI is the most useful imaging study for confirming an ACL tear and evaluating the rest of the knee.

MRI can show:

  • Whether the ACL is intact, partially torn, or completely torn

  • The location and pattern of the ACL injury

  • Bone bruising associated with the injury

  • Meniscus tears

  • Cartilage injuries

  • Collateral ligament injuries

  • Other ligament injuries

  • Osteochondral injuries and fractures

MRI is particularly valuable because an ACL tear rarely occurs in isolation. Identifying associated injuries can influence treatment decisions and help your surgeon develop the appropriate treatment plan.