Knee pain has dozens of possible causes, ranging from overuse injuries and cartilage wear to conditions that originate outside the knee entirely. The most likely explanation depends on your age, activity level, and whether the pain started suddenly or crept in over time. Here’s a breakdown of the most common causes and what each one feels like.
Osteoarthritis
Osteoarthritis is the single most common cause of chronic knee pain in adults over 50. The cartilage that cushions the ends of your bones gradually wears down, eventually allowing bone to grind against bone. Unlike injury-related knee pain, osteoarthritis develops without a specific event. The pain tends to be diffuse rather than pinpointed, worse at the end of the day, and aggravated by weight-bearing activities like walking or climbing stairs. Stiffness when you first stand up after sitting is a hallmark.
As the condition progresses, you may notice a grating or crunching sensation when you bend the knee, visible swelling, and gradual changes in leg alignment. On X-rays, doctors look for narrowing of the joint space, bone spurs, and changes in the bone just below the cartilage surface. Standing X-rays are more accurate than lying-down images because they show how the joint actually bears load. When the joint space disappears completely, creating a “bone-on-bone” appearance, that typically signals the point where joint replacement enters the conversation.
Ligament Injuries
Four major ligaments hold your knee stable, and each one tears through a different mechanism.
The ACL (anterior cruciate ligament) is the one most people have heard of. It typically tears during a sudden pivoting or cutting movement, often with an audible pop. Swelling develops within one to two hours, and the knee feels unstable, like it might buckle. ACL tears are extremely common in sports that involve quick direction changes: soccer, basketball, skiing.
The MCL (medial collateral ligament), on the inner side of the knee, tears when a force pushes the knee inward. Think of a football tackle hitting the outside of the leg. The PCL (posterior cruciate ligament) usually tears from a direct blow to the front of the shin, like hitting a dashboard in a car accident or falling hard on a bent knee.
The LCL (lateral collateral ligament), on the outer side, tears during hard contact, sudden direction changes, or twisting movements. Symptoms include pain, swelling, bruising, and a feeling that the knee might give out or lock up. LCL tears are less common than MCL tears but can be more complex to treat because of nearby structures involved.
Meniscus Tears
Your knee has two C-shaped pieces of cartilage (menisci) that act as shock absorbers between your thighbone and shinbone. Tears come in two very different varieties.
Traumatic tears happen in younger, active people (typically under 40) during a cutting or twisting motion while the foot is planted. Swelling develops over 24 to 48 hours, slower than with a ligament tear. The knee may lock, catch, or give way, and there’s usually tenderness right along the joint line. Men are more commonly affected.
Degenerative tears are a different story. They occur in middle-aged and older adults, often alongside osteoarthritis, with no single injury to point to. The pain builds gradually, can be hard to pinpoint, and flares with squatting or kneeling. Over time you may notice crepitus (that grating sensation), morning stiffness, or episodes of the knee catching. Rest usually helps, which distinguishes these tears from more advanced arthritis where stiffness dominates.
Patellofemoral Pain (Runner’s Knee)
Patellofemoral pain syndrome causes aching at the front of the knee, around or behind the kneecap. It’s one of the most common knee complaints in runners and cyclists. The core problem is believed to be excessive loading on the joint between the kneecap and the groove it slides through on the thighbone.
Biomechanical research points to a few key culprits. When the knee collapses inward during running (a movement called knee valgus), it increases the sideways force on the kneecap. This inward collapse appears to be one of the most consistent biomechanical factors linked to the condition in recreational runners. Increased knee bending angle during running is another factor, though this association appears stronger in men. Weakness in the inner quadriceps muscle and poor patellar tracking (the kneecap drifting or tilting as the knee bends) also contribute. The pain typically worsens going downstairs, sitting for long periods, or squatting.
Tendon and Overuse Problems
Patellar tendinopathy, often called jumper’s knee, causes pain just below the kneecap where the patellar tendon attaches. It’s most common in athletes who do repetitive jumping or sprinting. The tendon becomes irritated and sometimes thickened, and pain is worst during explosive movements or going downstairs.
Iliotibial band syndrome produces pain on the outer side of the knee. The iliotibial band is a thick strip of tissue running from the hip down to the outside of the knee, and it becomes irritated with repetitive bending, especially in runners and cyclists with tight hamstrings. The pain typically kicks in at a predictable point during a run and worsens until you stop.
Pes anserine bursitis causes pain on the inner side of the knee, just below the joint line. A fluid-filled sac becomes inflamed from overuse, and you may feel a tender nodule on the inner shin just below the knee. It’s common in runners and in people with osteoarthritis.
Gout and Pseudogout
Crystal deposits inside the joint can trigger sudden, intense knee pain that mimics an infection. Gout is caused by uric acid crystals, while pseudogout (formally called calcium pyrophosphate deposition disease, or CPPD) involves calcium-based crystals. Both cause rapid-onset swelling, warmth, and pain that can be severe enough to make the knee untouchable.
The knee is one of the most common joints affected by pseudogout. Episodes can last days or weeks. Gout more famously targets the big toe, but it hits the knee frequently as well. The two conditions look nearly identical from the outside, so distinguishing them requires analyzing fluid drawn from the joint. Treatment approaches differ, so getting the right diagnosis matters.
Referred Pain From the Hip or Back
Sometimes the knee itself is perfectly healthy, but it hurts anyway. This is referred pain, where the brain misinterprets signals traveling along shared nerve pathways.
The sciatic nerve forms from five nerve roots exiting the lower spine, travels through the buttock and down the back of the thigh, then branches near the knee. One branch, the peroneal nerve, wraps directly around the knee. When a herniated disc or bone spur in the lower back compresses one of those nerve roots, it creates inflammation that disrupts normal signaling. Your brain reads those disrupted signals as knee pain because the same nerve pathway serves both areas. This is why some people undergo knee evaluations only to discover the real problem is a lumbar disc issue.
Hip problems can do the same thing. Arthritis or impingement in the hip joint frequently sends pain to the front of the thigh and into the knee, especially in older adults. If knee imaging looks normal but the pain persists, the hip and lower back are worth investigating.
Growth-Related Causes in Young People
In adolescents between roughly 10 and 13, knee pain often traces to the growth plates. Sinding-Larsen-Johansson syndrome causes pain at the bottom of the kneecap in kids who do a lot of running, jumping, or squatting. The area becomes tender and swollen, and the quadriceps and hamstrings on that side lose flexibility. A similar condition, Osgood-Schlatter disease, affects the bony bump just below the knee. Patellar subluxation, where the kneecap partially slips out of its groove, is also more common in children and adolescents than in adults.
Septic Arthritis
A joint infection is the one cause of knee pain that qualifies as a true emergency. Bacteria enter the joint (sometimes after surgery, an injection, or through the bloodstream) and cause rapid swelling, warmth, skin color changes, and pain so severe that using the joint becomes nearly impossible. Fever is common. If you have a hot, swollen, extremely painful knee that developed quickly, especially with fever, this needs same-day medical evaluation. Untreated septic arthritis can destroy the joint within days.
First Steps After a Knee Injury
For acute soft-tissue injuries like sprains and mild strains, the current best-practice framework has moved beyond the old RICE protocol (rest, ice, compression, elevation). The updated approach is built around two phases.
In the first few days, the priorities are protection (avoiding movements that sharply increase pain while still allowing gentle movement), elevation above the heart, compression to manage swelling, and, notably, avoiding anti-inflammatory medications and ice. While ice numbs pain, it can slow healing by suppressing the body’s natural inflammatory response, which is an essential part of tissue repair. Complete rest is also discouraged because gentle movement promotes better outcomes than total immobilization.
In the days and weeks that follow, the focus shifts to gradually reloading the joint. That means resuming daily activities as tolerated, working on restoring mobility and strength through exercise, and staying optimistic about recovery. Pain is your guide: activity that doesn’t increase pain is generally safe to continue. A physical therapist can help structure this progression and ensure you’re rebuilding strength without re-injuring the area.