What a twist does, and how common each cause isEmerging evidence
Twisting on a planted foot turns the shin under the thigh while the knee carries your weight. The knee is mainly a hinge with only a little rotation, so a twist loads the parts that limit turning and sideways movement: the two menisci (the cartilage cushions), the ligaments and the kneecap's restraints.
No study counts the causes of knee pain when twisting, so the best guide comes from people who were hurt. In 1,145 people in Sweden whose knee filled with blood after a twisting injury, MRI (magnetic resonance imaging) showed a torn ACL in 52%, a meniscus tear in 41% and a kneecap dislocation in 17%, often together; most happened in sport, and at 16 and under a kneecap dislocation was the commonest injury. In a German sports clinic, the ACL came first, then the inner meniscus and the MCL. Per game or training session, women tear the ACL about 1.7 times as often as men.
Pain on twisting without a big injury is a different picture: from mid-life a worn meniscus or arthritis is likelier, and pain at the front of the knee is usually the kneecap, which bending loads more than twisting (our reading of these sources).
Sources 1, 2, 17, 3, 5, 6
Telling the causes apartStrong evidence
ACL: most tears happen without contact, when changing direction, stopping or landing, often with a pop, a knee that gives way, and swelling within a day. Later the knee may give way when you twist or turn. In video of 10 non-contact tears in female handball and basketball players, the knee collapsed inwards and the shin turned within about 40 milliseconds of landing.
Meniscus: the NHS says a tear "usually happens when you twist your knee while playing sport", but can also come from "twisting when standing up". It hurts on the joint line on the side of the tear, may catch, lock or give way, and the swelling "may not start for a few hours or days". NHS trust leaflets list twisting, squatting, kneeling and stairs as triggers.
MCL: hurt by a force pushing the knee inwards or a quick twist; an NHS trust describes pain "especially on the inside, particularly with twisting movements". Kneecap dislocation: often a pivot or twist on a planted foot, with the kneecap going out to the side.
Sources 7, 8, 19, 9, 10, 20, 21, 11, 12, 13, 14
Inward, outward, and no swellingEmerging evidence
People search by the way the foot turns. The side of the pain is the better clue: an NHS trust says "The side of pain usually indicates if it is the medial (inside) or lateral (outside) meniscus that is affected". Clinicians turn the leg in their tests, but even they can't confirm a tear from a test alone: in a meta-analysis the tests were only moderately accurate, and in a UK study designed to mirror general practice each was about 53% to 55% accurate.
No swelling within hours makes a large ligament tear with bleeding into the joint less likely, but a meniscus tear can swell only over 2 to 3 days, and an MCL sprain may show mainly as inner-knee pain when you twist. Catching, locking, giving way or a knee that won't straighten matter more than swelling (our reading).
Sources 20, 22, 23, 1, 9, 11
What a scan shows, and what it doesn'tStrong evidence
Meniscus tears are common in knees that don't hurt: in 991 people aged 50 to 90, an MRI showed tears in 19% of women in their 50s up to 56% of men aged 70 to 90, and 61% of those people had no knee pain in the past month; in adults without pain or injury, 4% under 40 and 19% from 40. The National Institute for Health and Care Excellence (NICE) diagnoses knee arthritis from symptoms, without imaging, and an NHS trust says MRI scans "are often not necessary" for a meniscus tear.
A scan matters when it changes the plan. After an injury, clinicians use the Ottawa knee rule to decide on an X-ray (age 55 or over, tenderness at the bony point on the outer knee or only at the kneecap, unable to bend to 90 degrees, or unable to take four steps both straight after the injury and when examined); it picked up 98.5% of fractures across 6 studies. A child's fast-swelling knee injury needs an X-ray first, because a growth-plate injury can look like an ACL tear. An MRI matters when surgery is being considered: a locked knee, a tear that may be repairable, or an ACL injury with other damage.
Sources 3, 4, 5, 21, 49, 50, 47, 36, 24
Children and teenagersEmerging evidence
Kneecap dislocations peak at 14 to 18, in boys and girls about equally. Knee osteochondritis dissecans (OCD) peaks at 11 to 15 and brings pain and swelling with sport, and sometimes catching or locking. A discoid meniscus, a meniscus shaped like a half-moon or a full circle, can catch, pop or lock after a twist; many people with one never have a problem.
Two rules matter: a child's fast-swelling knee injury needs an X-ray first, and a hip problem can show up as knee pain, so a child with knee or thigh pain and a limp needs help the same day.
Sources 43, 1, 46, 15, 16, 47, 48
How it usually goesEmerging evidence
The NHS says a meniscus tear "can get better on its own without treatment, but it can take a while to heal"; NHS trust leaflets expect improvement over about 3 to 6 months. In the trials, both exercise and surgery groups improved, and 19% to 41% of the exercise groups later chose surgery. About half of a group of young adults who had rehab first for a torn ACL had a reconstruction within 5 years.
An MCL sprain takes "around 6 to 12 weeks" to heal, an NHS trust says, and discomfort for longer is normal. About 1 in 3 kneecap dislocations happen again. No one can promise a date.
Sources 26, 28, 29, 32, 35, 38, 21, 20, 11, 44, 10
