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The knee is the hardest-working joint in the body, and it is also the one I see injured most often. Nearly all common knee injuries fall into a few patterns: torn ligaments, torn cartilage, broken bones, and overloaded tendons.
I have spent more than twenty years in practice at the Bone and Joint Institute of Tennessee. My replacement practice shows me what an injured knee looks like twenty years later. That changes how seriously I take a “minor” injury today.
What Are the Most Common Knee Injuries?
The seven most common knee injuries are ACL tears, meniscus tears, MCL and LCL sprains, PCL injuries, fractures around the knee, kneecap dislocations, and tendon injuries such as patellar tendonitis. Pain and swelling are the usual first signs. Many injuries involve more than one structure at the same time.
How Is the Knee Built?
Three bones meet at the knee: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). Four ligaments act like strong ropes holding those bones together, two wedge-shaped menisci cushion them, and the quadriceps and patellar tendons power the joint.
Smooth articular cartilage caps the bone ends so everything glides. Because so many parts move under load, one bad twist can damage several of them at once. If you want the full tour, I break each structure down in my guide to knee ligament anatomy.
What Does a Torn ACL Feel Like?
A torn ACL usually announces itself with a pop deep in the knee, swelling within a few hours, and a feeling that the leg will not hold you. The anterior cruciate ligament (ACL) is the main rope that keeps the shinbone from sliding forward under the thighbone.
Roughly 200,000 Americans tear an ACL each year, most during sports that involve cutting, pivoting, or landing from a jump. Contact is not required; plenty of ACL tears happen with a simple plant-and-twist.
About half of ACL tears occur alongside damage to the meniscus, articular cartilage, or another ligament. That is one reason I take a swollen knee after a pop seriously. Complete tears in active patients are usually repaired with ACL reconstruction surgery.
How Do MCL and LCL Sprains Happen?
Collateral ligament sprains happen when force pushes the knee sideways. A blow to the outer knee strains the medial collateral ligament (MCL) on the inner side. A blow to the inner knee strains the lateral collateral ligament (LCL) on the outer side.
Doctors grade these sprains 1 through 3. Grade 1 is an overstretch, grade 2 is a partial tear, and grade 3 is a complete tear. Here is the good news I share in clinic: most MCL injuries heal well in a brace, without surgery.
What Is a PCL Injury?
The posterior cruciate ligament (PCL) tears when a bent knee takes a direct hit, which is why it is nicknamed the dashboard injury. PCL tears are often partial, and many heal on their own with bracing and structured therapy.
What Is a Meniscus Tear?
A meniscus tear is damage to one of the two rubbery shock absorbers that sit between the thighbone and shinbone. When patients tell me they “tore cartilage,” this is almost always what they mean.
Younger patients tear a meniscus by twisting, cutting, or pivoting. Older patients can tear a worn meniscus with something as small as rising from a chair; that pattern is called a degenerative tear, and it often travels together with early arthritis.
Catching, locking, and swelling that comes and goes are the classic signs. Many tears settle down without an operation, and I walk through that decision in my article on treating a meniscus tear without surgery.
Which Injuries Affect the Kneecap?
The kneecap is the most commonly fractured bone in the knee, and it can also dislocate, meaning it slides out of its groove. Both problems usually come from falls, direct impacts, or awkward twisting.
A patella fracture typically follows a direct blow: a fall onto a hard floor, a dashboard, or a sports collision. Some fractures heal in a brace that keeps the leg straight, while displaced fractures need surgery to restore a smooth joint surface.
A dislocated kneecap looks alarming but often slides back into place on its own. Once it has happened, the supporting tissues are stretched, so some knees stay loose and the kneecap slips again. I cover that problem in my guide to patellar instability.
What About Tendon and Overuse Injuries?
The most frequent tendon problem at the knee is patellar tendonitis, an overload injury of the tendon connecting the kneecap to the shinbone. Athletes know it as jumper’s knee, and it responds well to activity changes and therapy.
Complete tears of the patellar or quadriceps tendon are a different animal. They tend to strike middle-aged athletes who land hard from a jump, the leg suddenly cannot straighten, and prompt surgical repair gives the best result.
Overuse can also inflame the small fluid-filled cushions around the knee, called bursitis, or the band of tissue along the outer knee, called iliotibial band syndrome, a frequent complaint among long-distance runners.
When Should You See a Doctor for a Knee Injury?
See a doctor promptly if you heard a pop, the knee swelled within hours, you cannot bear weight, the joint locks, or it gives way when you try to trust it. Those signs point to a structural injury that home care will not fix.
Go to the emergency department, or call 911, for any of the following:
- The knee or leg looks deformed or out of place
- Numbness, tingling, or a pale, cold foot below the injury
- A fever with a hot, swollen knee
- Pain so severe you cannot move the leg at all
For a mild strain with none of those signs, the RICE method (rest, ice, compression, elevation) for the first 48 to 72 hours is a sensible start. If the knee is not clearly improving within a week, have it examined.
How Are Common Knee Injuries Treated?
Most common knee injuries are treated without surgery. Rest, bracing, anti-inflammatory medication, and structured physical therapy handle the majority of sprains, degenerative meniscus tears, tendonitis, and stable fractures.
Physical therapy deserves special mention. Strong muscles around the knee protect the healing structures and lower the odds of the next injury. In my practice, skipping rehab is the single most common mistake patients make.
Which Common Knee Injuries Need Surgery?
Surgery is generally reserved for complete ACL tears in active patients, locked or repairable meniscus tears, displaced fractures, complete tendon ruptures, and kneecaps that keep dislocating. Many of these repairs are done through knee arthroscopy. The surgeon works through small incisions with a tiny camera.
Can an Old Knee Injury Cause Arthritis Later?
Yes. A serious knee injury raises the risk of post-traumatic arthritis. Studies suggest roughly half of patients with an ACL or meniscus tear show arthritis on X-ray within 10 to 20 years.
This is where my world and the sports injury world meet. A fair share of the 700+ joint replacements I perform each year trace back to an injury from decades earlier. Treating the injury well now, and keeping the leg strong afterward, is the best insurance policy you can buy your knee.
When arthritis does show up after an old injury, there are effective nonsurgical treatment options long before anyone needs to talk about a replacement.
Knee Injury Care in Franklin and Middle Tennessee
If you have injured your knee and want a straight answer about what is damaged and what to do next, come see me. I see patients from Franklin, Nashville, Brentwood, Spring Hill, and Columbia at the Bone and Joint Institute of Tennessee, 3000 Edward Curd Lane, Franklin, TN 37067. No referral is required.
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References
- American Academy of Orthopaedic Surgeons. Common Knee Injuries. OrthoInfo. Link
- Musahl V, Karlsson J. Anterior Cruciate Ligament Tear. N Engl J Med. 2019;380(24):2341-2348. Link
- Lohmander LS, Englund PM, Dahl LL, Roos EM. The Long-term Consequence of Anterior Cruciate Ligament and Meniscus Injuries: Osteoarthritis. Am J Sports Med. 2007;35(10):1756-1769. Link
- Brown TD, Johnston RC, Saltzman CL, Marsh JL, Buckwalter JA. Posttraumatic Osteoarthritis: A First Estimate of Incidence, Prevalence, and Burden of Disease. J Orthop Trauma. 2006;20(10):739-744. Link
- National Institute of Arthritis and Musculoskeletal and Skin Diseases. Sports Injuries. NIH. Link
