Pain on the side of the knee usually comes from a soft tissue problem: an irritated ligament, a torn piece of cartilage, or an inflamed cushioning sac called a bursa. The specific cause depends on which side hurts, how the pain started, and what makes it worse. Outside (lateral) knee pain and inside (medial) knee pain have distinct sets of common causes, though a few conditions can affect either side.
Pain on the Outside of the Knee
The outside edge of your knee is supported by the lateral collateral ligament (LCL) and crossed by a long, thick band of tissue called the iliotibial band that runs from your hip down to your shinbone. Problems with either structure are the most frequent reasons for lateral knee pain.
Iliotibial Band Syndrome
Iliotibial band syndrome (ITBS) is the classic overuse injury of the outer knee. The band gets irritated and swollen when it’s stretched too tight and rubs against the bony knob on the outside of your knee. It typically causes a sharp, pinching pain that starts during a run or long walk and gets worse if you keep going. Runners, cyclists, and hikers are especially prone to it.
Common training errors that trigger ITBS include pushing intensity too quickly, skipping warm-ups or cooldowns, not stretching enough, running on tilted or curved surfaces, running downhill repeatedly, and wearing worn-out shoes. Rest, stretching, and correcting these habits usually resolve it over a few weeks.
LCL Sprains and Tears
The LCL stabilizes the outer edge of your knee joint. A direct blow to the inside of the knee, an awkward landing, or a sudden change of direction can stretch or tear it. You’ll typically feel sharp pain on the outside of the knee, and the joint may feel loose or unstable when you walk.
Ligament sprains are graded on a three-point scale. A grade 1 sprain means pain along the ligament but no instability, and recovery takes roughly 10 days. A grade 2 sprain involves a partial tear with some joint looseness, with an estimated return to activity around 20 days. A grade 3 sprain is a complete tear with wide joint gapping, and recovery averages about 60 days.
Pain on the Inside of the Knee
The inside (medial) compartment of the knee has more structures packed into a smaller space, so there are more potential pain generators. The medial collateral ligament (MCL), the medial meniscus, and several bursae all sit in this area.
MCL Sprains and Tears
The MCL is the most commonly injured knee ligament. It runs along the inner edge of the joint, and it usually gets hurt when a force pushes the knee inward, like a tackle from the side in football or soccer. Pain is felt directly along the inner knee, and swelling often follows quickly. The same three-grade system applies: grade 1 sprains heal in about 10 days, grade 2 in about 20, and grade 3 tears can take around 60 days.
Meniscus Tears
Your knee has two C-shaped pads of cartilage (menisci) that act as shock absorbers. The medial meniscus on the inner side tears more often than the lateral one, though either can be injured. Tears typically happen when you twist your leg suddenly, whether during a sport or just an everyday slip or trip.
The hallmark symptoms are distinct from ligament pain. People who tear a meniscus often feel something pop in the knee at the moment of injury. Afterward, the knee may lock up, catch during movement, or feel like it could give out underneath you. You might also find you can’t fully bend or straighten your leg. If the pain is right along the joint line (the crease where the thighbone meets the shinbone), a meniscus tear is high on the list of suspects.
Pes Anserine Bursitis
A bursa is a small fluid-filled sac that reduces friction between tendons and bone. The pes anserine bursa sits on the inner side of the shinbone, just below the knee joint, where three hamstring tendons converge. When this bursa gets inflamed, it causes a tender, swollen spot a couple of inches below the inner knee crease. The location is the key distinguishing feature: bursitis pain sits below the joint line, while meniscus and MCL pain are typically at or above it.
Pes anserine bursitis is common in runners, people with osteoarthritis, and those who are overweight. It tends to flare with stair climbing and when getting up from a seated position.
Conditions That Can Affect Either Side
Osteoarthritis
Arthritis can wear down cartilage in any compartment of the knee. When it affects the medial compartment, you get inner knee pain; when it affects the lateral compartment, it shows up on the outside. The pain is typically a deep ache that worsens with activity and improves with rest, and it tends to develop gradually over months or years rather than appearing suddenly. Morning stiffness that eases within 30 minutes is a classic pattern.
Patellofemoral Pain Syndrome
This condition usually centers around the kneecap, but the pain can spread to either side of the knee. It’s driven by irritation where the kneecap tracks against the thighbone, and it’s especially common in people who run, squat, or climb stairs frequently. If your side-of-knee pain gets worse going up or down stairs or after sitting for a long time with bent knees, this could be contributing.
Repetitive Strain
Using your knees for the same motion over and over, whether that’s kneeling at work, cycling long distances, or doing heavy squats, can irritate tendons, bursae, or ligaments on either side. The pain usually builds gradually and worsens with the aggravating activity. Rest and modifying the movement pattern are the first steps toward relief.
How Side Knee Pain Gets Diagnosed
A physical exam is usually enough to narrow down the cause. Your provider will press along the ligaments, joint line, and bursae to locate the exact point of tenderness. They’ll also perform specific stress tests: a valgus stress test pushes the knee inward to check the MCL, while a varus stress test pushes it outward to check the LCL. Separate maneuvers test for meniscus tears by rotating the shin while bending and straightening the knee, feeling for clicks or catching. If ITBS is suspected, there are specific tests that reproduce the pinching sensation on the outer knee.
Imaging isn’t always necessary. X-rays can show arthritis or fractures but won’t reveal soft tissue injuries. An MRI is the go-to when a ligament tear, meniscus tear, or other internal damage needs to be confirmed, especially if symptoms don’t improve with initial treatment or if the knee feels unstable.
Signs That Need Prompt Attention
Most side-of-knee pain improves with rest, ice, and activity modification. But certain symptoms warrant urgent evaluation. If your knee joint looks bent or deformed, can’t bear weight, swelled up suddenly, or you heard a popping sound at the time of injury, get to urgent care or an emergency room. A knee that’s badly swollen, red, warm, and tender, especially with a fever, also needs same-day medical attention. And if knee pain is interfering with your sleep or preventing you from doing daily tasks, it’s worth making an appointment even if it doesn’t seem like an emergency.