Common, and mostly minorStrong evidence
In US college basketball, injuries that kept players out for less than a day made up over half of all injuries, and across 11 studies the knee was 17.8% of 12,960 basketball injuries, behind the ankle (21.9%). In 100 US high schools the ankle and foot (39.7%) and the knee (14.7%) were the commonest places hurt, at 1.94 injuries per 1,000 player sessions, and across 22 studies ankle and knee injuries together were 41.3% of basketball injuries.
The knee costs the most time when it is a structural injury: in US college men, injuries inside the knee caused the most absences of more than 10 days, and in women, preseason practice injury rates were more than twice those of regular-season practice. In the NBA, "Patellofemoral inflammation is the most significant problem in terms of days lost in competition, whereas ankle sprains are the most common injury", and "True ligamentous injuries of the knee were surprisingly rare"; ACL tears were 0.8% of injuries across the NBA and the Women's NBA (WNBA) over 6 seasons.
Sources 1, 2, 3, 4, 5, 6, 7, 8
Sprints, stops, cuts and jumpsEmerging evidence
Basketball changes movement constantly: 8 elite Australian men changed what they were doing about every 2 seconds, players cover about 5 to 6 km in a game's live time, and US college men averaged about 55 to 70 jumps of more than 175 mm a session, in practice and games alike. Each landing brakes the body through the thigh muscles, the kneecap and the patellar tendon, which is why the front of the knee is where it usually hurts.
Sources 9, 10, 11
The kneecap: the commonest front-of-knee acheEmerging evidence
Kneecap pain (patellofemoral pain) is a diffuse ache around or behind the kneecap, worse on stairs, squats and after sitting. In 242 Canadian college players, 43% reported knee pain, and the authors found a diffuse ache around the front of the knee common and pain at the lower tip of the kneecap uncommon: most of it was a kneecap ache, not the tendon. At preseason screening of 810 US middle and high school players, front-of-knee pain was found in 25.3% of knees, more in girls than boys, and over one season new kneecap pain developed in 9.66 per 100 girls, all of them in middle school.
Sources 12, 13, 14
Jumper's knee: the tendon just below the kneecapStrong evidence
Jumper's knee (patellar tendinopathy) is pain at one point at the lower tip of the kneecap, sore at the start of a session, easing as you warm up and worse afterwards. In Norwegian elite athletes, current symptoms were found in 31.9% of basketball players (all men) and 44.6% of volleyball players; across 891 Dutch non-elite athletes in 7 sports it was 8.5%, and almost twice as common in men as in women. In 515 Canadian youth players over a season, 19.0% had patellar tendon pain.
In Dutch basketball and volleyball players, 13% developed it over about 3 years; men had twice the odds, and the authors advise taking the total tendon load into account. These are surveys and cohorts, so they show who gets it, not what causes it.
Sources 15, 16, 17, 18
Growing knees: Osgood-Schlatter and Sinding-Larsen-JohanssonEmerging evidence
In a growing child or teenager, a tender bump just below the knee is Osgood-Schlatter disease, and pain at the lower tip of the kneecap is Sinding-Larsen-Johansson syndrome. At preseason screening of school players, Osgood-Schlatter was found in 2.5% and Sinding-Larsen-Johansson in 4.8%. In older Finnish data, 21.2% of teenagers active in sport at 13 had had Osgood-Schlatter against 4.5% of those who weren't, and in 956 Brazilian 12 to 15 year olds it was 9.8%, linked with regular sport. NHS inform calls these knee growth area problems a common cause of knee pain in children and young people. They aren't growing pains, which the NHS describes as an ache in both legs in the evening or at night that has gone by the morning.
Girls who played only one sport had about 1.5 times the risk of kneecap pain diagnoses and about 4 times the risk of Osgood-Schlatter or Sinding-Larsen-Johansson, an association, not proof that specialising caused it.
Sources 13, 19, 20, 21, 22, 23
Twists and landings: the ACL, the meniscus and the kneecapStrong evidence
In US college basketball, the ACL injury rate was 0.29 per 1,000 sessions in women against 0.08 in men, most of them without contact, and the authors write that "the actual rate of injury remains low and should not be a deterrent to participation in sports". A later study found women still tear an ACL at higher rates than men in comparable sports. In 39 basketball ACL injuries on video, women landed with more bend at the knee and hip but had 5.3 times the relative risk of the knee caving inwards, and movements were often disturbed by opponents.
In 1,145 knees that swelled with blood after an injury, magnetic resonance imaging (MRI) found an ACL tear in 52%, a meniscus tear in 41% and a dislocated kneecap in 17%, and in those aged 16 and under the dislocated kneecap was the commonest. In the NBA, isolated meniscus tears cost about 41 to 44 days, and 19.4% of players didn't return to play.
Sources 24, 25, 26, 27, 28
When a scan or test mattersEmerging evidence
Jumper's knee and kneecap pain are diagnosed by examination. Tendon changes on ultrasound in pain-free tendons went with at least four times the risk of later tendon pain, but other factors beyond the tendon's structure are involved, and meniscus findings on MRI are common in people without knee pain. After a fall or a twist, the Ottawa knee rule decides who needs an X-ray (see the warning signs). An MRI matters after a pop with fast swelling, a knee that locks or gives way, or a kneecap that slipped out.
In a child, a limp or hip pain needs the hip examined, and the National Institute for Health and Care Excellence (NICE) asks clinicians to consider a very urgent X-ray for children and young people with unexplained bone swelling or pain, to look for a bone tumour, which is rare.
Sources 29, 30, 31, 27, 32
How it usually goesEmerging evidence
Aches settle within days. Jumper's knee is often slow: symptoms had lasted a mean of 32 months in elite athletes and 18.9 months in club athletes. In the largest trial's staged programme, 43% were back in their sport at their previous level at 24 weeks (against 27% with decline squats, a difference that could be chance), and the fastest return was after 4 weeks; 5 years on, 76% felt recovered and 71% were back in their chosen sport, though almost a quarter didn't feel recovered. In the NBA, all 46 players diagnosed with patellar tendinopathy in one study returned.
For Osgood-Schlatter, older Finnish data recorded a mean of 3.2 months off training; in a Danish programme, 80% reported success at 12 weeks and 90% at 12 months, and 69% were back in sport at 12 months. After ACL reconstruction, 86% of NBA players returned, at a mean of 11.6 months, 69.5% of WNBA players and 87% of high school players in one surgeon's accelerated programme, where 7.0% of girls later tore the graft and 16.1% the other knee; across sports, 65% returned to their pre-injury level. No one can promise a date.
Sources 15, 16, 33, 34, 35, 19, 36, 37, 38, 39, 40
When it isn't basketball: the causes the warning signs cover
Some knee pain has causes basketball didn't create, or needs more than a plan, and the warning signs follow the pages behind them. A knee that changed shape, swelled fast or went with a pop needs A&E, in the words of the NHS dislocated kneecap page, and the Ottawa knee rule, tested in emergency departments, picks out who needs an X-ray. A pop at the front of the knee with a knee you can't straighten can be a torn quadriceps or patellar tendon, as the American Academy of Orthopaedic Surgeons (AAOS) describes; steroids and some antibiotics can weaken these tendons. In children and teenagers, a pop below the kneecap can pull off a piece of bone or a sleeve of cartilage instead, as a case series of 90 young people describes. A repeat dislocation of the kneecap is when surgery is usually discussed, as nidirect explains. A meniscus whose symptoms haven't settled after three months of non-surgical treatment may need an MRI, the Evidence-Based Interventions programme says, and NHS inform says to talk to a professional if kneecap pain hasn't improved within 6 weeks.
A limp with knee or thigh pain in a child or teenager can come from the hip, as the NHS limping and irritable hip pages say, and a slipped growth plate needs checking the same day: NHS inform's page on a slipped upper femoral epiphysis (SUFE) describes it, and in a UK national study of 486 children the slip worsened the longer diagnosis took. NHS inform lists the signs in a child's knee that need a GP, and tender bumps below the kneecap are usually Osgood-Schlatter or Sinding-Larsen-Johansson, as the AAOS and two NHS trusts describe. A hot, swollen knee can be a joint infection, which the NHS septic arthritis page says is urgent; a swollen, throbbing calf a blood clot or a burst Baker's cyst, and sudden breathlessness a clot in the lungs, as the NHS DVT, Baker's cyst and pulmonary embolism pages explain. Someone who faints while exercising needs a 999 call, as the NHS fainting page says. Numbness around the genitals or both legs weak needs a 999 call, as the NHS sciatica page says. One spot on the shin bone that hurts with landings can be a stress fracture, as Health New Zealand describes, and bone pain that is constant or worse at night needs checking, the NHS bone cancer page says, though that is rare.
Sources 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91
