Knee injuries can sideline even the most active people, but few are as impactful as an anterior cruciate ligament (ACL) tear. Whether you’re an athlete, weekend warrior, or simply enjoy an active lifestyle, knowing what the ACL does, and what happens when it’s injured, is the first step toward protecting your knee.
This article kicks off our five-part series on ACL injuries, guiding you from understanding the ligament to diagnosis, treatment options, recovery, and long-term knee health.
What the ACL Is and Why It’s Important
The ACL is one of four main ligaments that keep the knee joint stable.
- Location: It runs diagonally through the center of the knee, connecting the femur (thighbone) to the tibia (shinbone).
- Function: It prevents the tibia from sliding too far forward and controls rotational stability – especially during cutting, pivoting, and landing.
When the ACL is intact, it acts like a strong, built-in seatbelt. Once it’s injured, the knee can feel loose or unstable, putting other structures (like the meniscus and cartilage) at risk.
How ACL Injuries Happen
Most ACL injuries occur during non-contact movements that put sudden stress on the knee:
- Pivoting or cutting to change direction
- Stopping abruptly after running or jumping
- Landing awkwardly from a jump
- Rapid deceleration when the foot is planted
- Direct impact to the knee (less common)
Sports like soccer, basketball, football, and skiing see high rates of ACL injuries, but they can happen during everyday activities too (1).
Early Signs of an ACL Tear
If you’ve injured your ACL, you might experience:
- A sudden “pop” or snap at the time of injury
- Rapid swelling within the first hours
- Pain and difficulty walking
- A feeling the knee is “giving out” or can’t be trusted
- Trouble fully straightening or bending the knee
Because these symptoms overlap with other knee problems, such as meniscus tears or collateral ligament injuries, proper evaluation is critical.
Why Diagnosis Matters
A missed or delayed ACL diagnosis can lead to worsening damage. Instability may cause additional meniscus tears, cartilage wear, and increase your risk for early arthritis.
A thorough assessment usually includes:
- History & physical exam: Specific tests like the Lachman or pivot-shift help assess ligament integrity.
- Imaging: MRI is the gold standard for confirming partial vs. complete tears and checking for related injuries. Ultrasound can sometimes help visualize dynamic instability.
- Treatment planning: Classifying the tear and identifying additional injury guides whether conservative care, orthobiologic therapy, or surgery is appropriate.
Early, accurate diagnosis sets you up for the best recovery and helps you avoid unnecessary complications down the road.
Emerging research continues to explore biologic options for ACL injuries. A recent, randomized controlled trial compared bone marrow concentrate with exercise therapy for certain ACL tears and showed that patients treated with bone marrow concentrate (BMC) combined with platelets showed meaningful gains in function and pain reduction, supporting alternatives to traditional ACL tear treatments in certain patients (2).
Coming Next: Diagnosis & First Steps After Injury
In Part 2, Dr. Papas will walk you through what to do right after a suspected ACL tear from initial rest and bracing to imaging, the role of the meniscus, and when to seek early specialist evaluation. We’ll also preview the range of treatment pathways, including new options beyond surgery.
Every step matters – don’t let an ACL injury hold you back. Our expert physicians can help pinpoint the cause and guide you toward the right plan, before more damage occurs.
Schedule a consult today and take the first step toward protecting your knee long-term.
References:
- Joseph, Allan M et al. “A multisport epidemiologic comparison of anterior cruciate ligament injuries in high school athletics.” Journal of athletic training vol. 48,6 (2013): 810-7. doi:10.4085/1062-6050-48.6.03
- Centeno, Christopher J et al. “Non-surgical treatment of anterior cruciate ligament tears with percutaneous bone marrow concentrate and platelet products versus exercise therapy: a randomized-controlled, crossover trial with 2-year follow-up.” BMC musculoskeletal disorders vol. 26,1 882. 30 Sep. 2025, doi:10.1186/s12891-025-09153-2
