Where it hurts is the best clue, not a diagnosisEmerging evidence
Doctors start with where the pain is, how it started, whether the knee swelled and how quickly, and what brings it on, then examine the knee. Swelling within minutes to a few hours of an injury suggests a torn ligament, a break inside the joint or a dislocated kneecap; swelling over hours to days suggests a meniscus tear.
Examination tests are only moderately accurate on their own: for meniscus tears, tenderness on the joint line picked up 83% of tears and McMurray's test 61%, in studies of poor quality, and a 2026 review says the tests work best in combination. That's why this guide says a spot points to a cause rather than proving it.
Sources 1, 2, 3
Arthritis on the inner sideEmerging evidence
Across 16 studies of 3,786 knees with osteoarthritis, arthritis in the inner part of the main knee joint alone was found in 27%, and the inner part with the kneecap joint in 23%, while patterns involving the outer part made up 15%. Bow legs (varus) raised the odds of the inner side getting worse over 18 months about 4-fold in 237 people.
So from mid-life, arthritis is probably the commonest long-term cause of inner knee pain. That is our inference from where knee arthritis usually sits and how common it is; no study has counted the causes of inner knee pain as such. The National Institute for Health and Care Excellence (NICE) diagnoses osteoarthritis without a scan from 45, with pain on activity and morning stiffness of 30 minutes or less, and Arthritis UK says the pain can be felt all round the knee or "just in a certain place, such as the front or side".
Sources 4, 5, 6, 7
The inner meniscus: torn more often, and often without painStrong evidence
The inner (medial) meniscus is torn more often than the outer one: in 7,769 sports knee injuries at one centre, inner meniscus injuries made up 10.8% against 3.7% for the outer. It is attached along its edge and moves less, which makes it prone to tearing. From mid-life tears are often worn, and "an awkward twist when getting up from a chair" can be enough, the American Academy of Orthopaedic Surgeons (AAOS) says.
Tears are also common in knees that don't hurt. In 991 people aged 50 to 90, magnetic resonance imaging (MRI) showed tears in 19% to 56%, and 61% of those with a tear had had no knee pain, aching or stiffness in the past month; in people with knee arthritis, knees with a tear were no more painful than knees without. A tear on a scan has to match your symptoms.
Sources 8, 9, 10, 11, 12
An MCL sprain after a blow or a twistStrong evidence
The MCL runs down the inner side of the knee. It is hurt when the knee is forced inwards with the foot on the ground, such as slipping on ice, skiing, football or rugby, a fall in older people, or a blow to the outside of the knee. It is tender along the inner side, and twisting hurts most, an NHS trust leaflet says. MCL injuries made up 7.9% of 7,769 sports knee injuries at one centre.
Most MCL sprains heal without surgery, with early movement and gradual loading. Healing normally takes about 6 to 12 weeks, the leaflet says, and some discomfort and swelling for longer is normal. Swelling of the whole knee within a few hours, a pop or a knee that gives way suggests another injury as well.
Sources 13, 14, 8, 15, 1
Pes anserine pain: real, but scans rarely show itMixed evidence
Three tendons attach to the inner shin just below the knee, over a small fluid-filled sac (a bursa). Pain there, about 3 cm below the joint line and typically worse on stairs and getting up from a chair, is called pes anserine bursitis or pes anserine pain. It is diagnosed by examination.
Scans rarely show a swollen bursa, so they may miss it: it was found on 2.5% of 509 MRI scans of painful knees, where it most often looked like a meniscus tear, and ultrasound showed changes in only 3 of the painful knees of 37 women diagnosed with it in a clinic. In 170 knees with arthritis, ultrasound found the bursitis in 20%. A normal scan doesn't rule it out. In the only case-control study, of 22 women, knock knees (valgus) raised the odds about 5-fold.
Sources 1, 16, 17, 18, 19
Less common causes worth knowingEmerging evidence
A medial plica, a fold of the joint lining, is present in 18% to 60% of knees and usually causes no trouble; when it does, the pain is on the inner side of the kneecap above the joint line, and a review calls it a commonly missed diagnosis that only keyhole surgery can confirm. In 36 competitive breaststroke swimmers, 86% had had knee pain from the kick, most often on the inner side, and 47% of those with weekly pain had tender, thickened plicae; in 391 swimmers, 73% of breaststroke specialists reported knee pain.
The semimembranosus, a hamstring tendon at the back inner corner, can ache just below the joint line, often in runners; it's uncommon and little studied. The saphenous nerve can imitate a meniscus tear or arthritis: the skin along it is sore even to a light touch, and relief after a local anaesthetic injection confirms it. In 4 knees where a branch of the nerve was trapped by the sartorius muscle, surgery to free it relieved the pain.
Sources 20, 21, 22, 23, 24, 9, 25, 26
Two mid-life patterns that need a GPEmerging evidence
A single painful pop in mid-life can point to a tear at the root of the inner meniscus, where it attaches to the shin bone, as one study of people already having keyhole surgery for an inner meniscus tear found. In 234 people whose root tears were repaired, about 80% were women, the average age was 65, and about 80% remembered a painful pop.
Sudden, severe inner knee pain from about 60, with little or no injury, can be a small fracture under the joint surface (a subchondral insufficiency fracture, once called spontaneous osteonecrosis). The AAOS says it is more common over 60 and about three times more common in women, that early X-rays are usually normal and that MRI shows it early; reviews say it can progress if the fracture doesn't heal, which is why it is worth finding early.
Sources 27, 28, 29, 30, 31
When a scan or test helpsStrong evidence
Rarely at first. NICE diagnoses knee arthritis without imaging. After an injury, the Ottawa knee rule decides whether you need an X-ray: being 55 or over, tenderness on the bony bump below the outer knee or only on the kneecap, being unable to bend the knee to 90°, or being unable to take 4 steps. Across 6 studies of 4,249 adults, it picked up 98.5% of fractures.
An MRI helps with locking, painful catching, a knee that won't straighten, swelling that keeps coming back, or pain that persists after good non-surgical care, and UK guidance suggests one when meniscus symptoms haven't settled after 3 months of non-surgical treatment. A hot, swollen knee may need fluid drawn from the joint and blood tests to check for infection or gout.
Sources 6, 32, 1, 33, 3, 34, 35
How it usually goesEmerging evidence
New knee problems should begin to settle within 6 weeks, NHS inform says, and the Chartered Society of Physiotherapy says most muscle and joint problems get better within 6 to 8 weeks. An MCL sprain normally takes about 6 to 12 weeks to heal, an NHS trust leaflet says. With a worn meniscus tear, exercise did as well as surgery at 2 years, and bursitis usually goes away in a few weeks, the NHS says. No one can promise a date.
Sources 36, 37, 13, 38, 39
