The most immediate symptom of a torn ACL is a sudden, sharp pain in the knee, often accompanied by a popping sound or sensation at the moment of injury. Within hours, the knee swells significantly, becomes difficult to straighten, and feels unstable when you try to stand or walk. These symptoms can range from mild to severe depending on whether the ligament is stretched, partially torn, or completely ruptured.
The Pop and Immediate Pain
Most people who tear their ACL describe hearing or feeling a distinct “pop” in the knee at the moment of injury. This is one of the most recognizable signs of an ACL tear and separates it from many other knee injuries. The pop is typically followed by immediate, sharp pain that makes it difficult or impossible to continue whatever activity caused the injury.
The pain is usually centered deep inside the knee rather than on one side or along the kneecap. It tends to be most intense in the first few minutes, then settles into a deep ache as swelling takes over. Some people with partial tears report less dramatic initial pain, which can lead them to underestimate the injury.
Rapid Swelling
Swelling after an ACL tear happens fast. The knee typically balloons within the first two to four hours, driven by bleeding inside the joint. This rapid onset distinguishes an ACL injury from conditions like tendinitis or minor sprains, where swelling builds gradually over a day or two.
The swelling makes the knee feel tight and warm to the touch. It also creates pressure inside the joint that contributes to stiffness and pain with movement. In the acute phase, this swelling alone can make it nearly impossible to fully bend or straighten the knee.
Instability and Giving Way
The ACL’s primary job is to keep your shinbone from sliding forward relative to your thighbone and to stabilize the knee during rotation. When it tears, the knee loses that internal check. You feel this as a sensation of the knee “giving way” or buckling, particularly when you put weight on it, try to pivot, or change direction.
This instability can be subtle with a partial tear, showing up mainly during athletic movements, cutting, or stepping on uneven ground. With a complete tear, even walking on flat surfaces can feel unreliable. Many people describe it as not being able to trust the knee, a feeling that something is fundamentally loose or disconnected inside the joint.
Loss of Range of Motion
In the days following an ACL injury, most people struggle to fully straighten (extend) the knee. Loss of extension is one of the most clinically significant issues because it affects your ability to walk normally and, if it persists, is associated with long-term problems including early-onset arthritis.
Bending the knee deeply is also limited, though this is often driven more by swelling and pain than by structural damage to the ligament itself. As swelling decreases over the first one to two weeks, flexion usually improves. Full extension, however, can remain limited and often requires deliberate rehabilitation to restore. Abnormal walking patterns tend to develop once crutches are no longer needed, including reduced hip and knee flexion on the injured side, which can create compensatory problems if not addressed.
How Severity Changes the Picture
ACL injuries are graded on a three-level scale based on how much of the ligament is damaged.
- Grade 1: The ligament is stretched but the fibers remain intact. The knee is still stable, and symptoms are relatively mild: moderate pain, some swelling, and tenderness without significant instability.
- Grade 2: The ligament is partially torn, with some fibers disrupted. This grade is actually rare. Symptoms include more noticeable looseness in the knee along with pain and swelling.
- Grade 3: The ligament is completely torn in two. This is the most common presentation when people talk about a “torn ACL.” The knee has no ligament-provided stability, swelling is significant, and the giving-way sensation is pronounced.
Signs of Additional Damage
The same force that tears an ACL frequently damages other structures in the knee. Meniscus tears are particularly common alongside ACL injuries. If you also tore your meniscus, you may notice symptoms that a standalone ACL tear doesn’t produce: a catching or locking sensation where the knee feels stuck in one position, sharp pain when twisting or squatting, and stiffness that doesn’t improve as swelling goes down.
Locking is a key differentiator. An ACL tear on its own does not cause the knee to lock. If your knee feels mechanically stuck, especially close to a straight position, that’s a strong indicator that the meniscus or another structure is involved. This matters because a concurrent meniscus tear can change the treatment approach and timeline.
How an ACL Tear Is Confirmed
A doctor will typically perform a physical test called the Lachman test, which involves stabilizing your thigh and gently pulling the lower leg forward to check for abnormal movement. Systematic reviews put the Lachman test’s sensitivity between 76% and 81%, with specificity between 85% and 89%, making it the most reliable hands-on exam for detecting ACL tears.
MRI is the standard imaging tool for confirming the diagnosis. It shows the ligament directly, reveals whether the tear is partial or complete, and identifies damage to surrounding structures like the meniscus or cartilage. MRI is particularly valuable because it catches injuries that a physical exam alone can miss, especially in a swollen, painful knee where the patient has difficulty relaxing enough for an accurate manual test.
What Happens if You Don’t Treat It
Without treatment or rehabilitation, the hallmark symptom of an ACL tear, instability, tends to persist and worsen over time. The knee continues to give way during activities, and each episode of buckling can cause additional damage to the meniscus and cartilage surfaces inside the joint.
Over months and years, this repeated low-grade trauma accelerates cartilage breakdown. People with untreated ACL tears have a significantly higher risk of developing osteoarthritis in the affected knee, and this can happen much earlier in life than typical age-related arthritis. Chronic symptoms often include persistent swelling after activity, a deep ache that worsens with use, and a progressive loss of the knee’s ability to handle any movement involving rotation or quick direction changes. Even daily activities like descending stairs or stepping off a curb can become unreliable.