What Happens to Your Knee When the ACL Tears? Understanding the Injury Before Surgery
Medicine Made Simple
The anterior cruciate ligament (ACL) is one of the most important stabilizers in your knee. When it tears, the knee becomes unstable and everyday activities like walking, running, or climbing stairs may suddenly feel difficult. Many people first experience swelling, pain, and a “popping” sound when the injury occurs. This article explains in simple terms what the ACL does, what happens to the knee when it tears, how doctors diagnose the problem, and why surgery may sometimes be needed to restore stability and function.
Getting to Know the Knee Joint
The knee is the largest joint in the body and is built to handle heavy loads and movement. It connects the thigh bone (femur) to the shin bone (tibia). The knee bends and straightens like a hinge, but it also allows slight rotation so you can pivot, twist, and turn.
Inside the knee are four main ligaments that act like strong ropes holding the bones together. Two are on the sides (the medial collateral ligament and the lateral collateral ligament). Two cross each other in the center, forming an “X.” These are the cruciate ligaments—the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL).
The ACL runs diagonally across the middle of the knee. It prevents the tibia from sliding too far forward and keeps the knee stable when turning or stopping suddenly. Without the ACL, the knee feels loose and unsafe, especially during quick changes of direction.
What Is an ACL Tear?
An ACL tear is an injury where the ligament fibers either stretch beyond their limit or snap completely. Partial tears are possible, but most injuries are complete ruptures. The tear can happen suddenly during sports, exercise, or even accidents like falling or slipping.
When the ACL tears, the knee often makes a popping sound. This is followed by sharp pain and swelling within a few hours. Many people describe a sensation that the knee “gave out.” After the initial injury, the pain may reduce, but the knee remains unstable and weak.
How Does an ACL Tear Happen?
ACL tears are very common in sports that involve sudden stops, jumps, or quick changes in direction, such as football, basketball, hockey, gymnastics, and cricket.
The injury usually happens when you land awkwardly after a jump, suddenly change direction while running, stop suddenly while sprinting, or twist your knee while the foot is planted. Direct collisions, like in contact sports, can also tear the ACL, though non-contact injuries are more common.
What Happens Inside the Knee When It Tears?
When the ACL tears, several things happen inside the knee: the tibia can slide forward too much, making the knee feel wobbly. Swelling occurs due to bleeding into the joint. Meniscus or cartilage damage may happen alongside the ACL tear. Finally, thigh muscles weaken due to pain and swelling. If untreated, repeated instability can cause arthritis.
Symptoms of an ACL Tear
The most common signs include a popping sound, sudden pain, rapid swelling, difficulty walking, and the feeling that the knee is unstable or gives way. Some people may walk normally after a few days, but instability returns when they attempt sports or sudden movements.
How Doctors Diagnose an ACL Tear
Diagnosis begins with a physical examination where doctors check for swelling, range of motion, and stability. Specific tests like the Lachman and anterior drawer test help identify ACL injuries. MRI scans confirm the diagnosis and show whether other structures are damaged.
Can the ACL Heal on Its Own?
Unlike muscles or skin, ligaments inside the knee do not heal well because of poor blood supply. A complete ACL tear usually does not grow back naturally. Some people manage with physiotherapy, but athletes or physically active people often require surgery.
Treatment Options After an ACL Tear
Not everyone needs surgery. Older or less active patients may do well with physiotherapy to strengthen surrounding muscles. For younger, active patients or athletes, ACL reconstruction surgery is common. Surgeons replace the torn ligament with a graft from the hamstring, patellar tendon, or donor tissue.
Why Surgery May Be Needed
The main reason for surgery is to restore stability and prevent further injury. Without surgery, an unstable knee can cause meniscus and cartilage damage, leading to arthritis. Surgery is usually planned after swelling subsides, and pre-surgery physiotherapy makes recovery smoother.
Recovery After ACL Tear or Surgery
Recovery depends on treatment. With physiotherapy, daily activities may resume within weeks but sports may remain limited. After surgery, rehabilitation is longer but more complete, often involving months of exercises and gradual return to sports after 6–9 months.
Risks of Ignoring an ACL Tear
Living with an untreated ACL tear may lead to repeated instability, meniscus injury, cartilage wear, and early arthritis. Addressing the injury early improves long-term knee health.
Conclusion
An ACL tear changes the stability of the knee and affects your ability to move confidently. It may also cause long-term damage if untreated. Understanding what happens inside the knee helps patients make informed decisions.
If you or someone you know has knee swelling, pain, or instability after injury, consult an orthopedic specialist early. Prompt diagnosis and treatment protect the knee for the future.
References and Sources:
American Academy of Orthopaedic Surgeons. ACL Injury
Mayo Clinic. ACL Injury Overview
Cleveland Clinic. ACL Tears
*Information contained in this article is not intended or designed to be a substitute for professional medical advice, diagnosis, or treatment. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other professional health care provider with any questions you may have regarding a medical condition or advice in relation thereto.
