What it isStrong evidence
The kneecap tendon attaches to a bony bump at the top of the shin, called the tibial tuberosity. In growing children that bump sits on a growth area of softer cartilage. Repeated pulling from running, jumping, kicking and kneeling irritates it, and the bump can swell and grow. It is usually in one knee, sometimes both.
It is a growth-area overuse problem, in the same family as heel pain in active children (Sever's disease). Despite the word disease, it isn't an infection and it isn't dangerous.
Sources 1, 2, 3
Who gets itStrong evidence
About 1 in 10 adolescents: 9.8% of 956 Brazilian 12 to 15 year olds had it. In Dutch general practice, 3.8 in every 1,000 young people aged 8 to 18 were newly diagnosed each year, more boys (4.9) than girls (2.7). It peaked at age 12 in boys and 11 in girls.
Sources 4, 5
Sport and growthEmerging evidence
Sport is the main link. In a Finnish study, 21% of students who were active in sport at 13 had had it, against 4.5% of inactive students. Girls who specialised in one sport had about 4 times the risk of girls who played several, and young athletes who did more than twice as many hours of organised sport as free play had more serious overuse injuries.
In a Danish study, teenagers with Osgood-Schlatter and pain-free teenagers were all highly active, yet those with it reported long-standing knee pain and poorer knee function that affected sport and quality of life. These are links, not proof of cause.
Sources 6, 4, 7, 8, 9
How it's recognisedStrong evidence
From the story and a tender bump: pain on the bump that is worse during and after sport and when kneeling, sometimes with swelling and a hard lump, and pain when the knee straightens against resistance. X-rays are not needed unless something doesn't fit, such as an injury, a hot swollen knee, night pain or a limp.
Pain at the lower tip of the kneecap in a growing child is a related problem (Sinding-Larsen-Johansson). Pain around or behind the kneecap is runner's knee; the same Danish group used the same cutting-back approach for it in 151 adolescents.
Sources 1, 2, 10, 11
How long it lastsEmerging evidence
Many guides say it settles within months, or once growth finishes (around 14 in girls and 16 in boys), and the bump may stay after the pain has gone. Most do improve, but not always quickly.
In a Danish group followed for 2 years, 37% still had pain from it, and more than 1 in 5 had stopped sport because of knee pain. In another hospital group, 60% of those who replied still had pain at a median of almost 4 years. In a survey of adults treated for it in hospital as teenagers, 85% still had a prominent bump and 73% still had some pain or problems there, and longer, more painful courses went with worse knee health. Only about a third replied, and all had been treated in hospital, so these figures over-represent the hard cases.
In an older follow-up of untreated knees, most people felt no limit on their activity, but 60% still could not kneel without discomfort.
Sources 12, 13, 14, 15, 1, 16
Why it's thought to happenMixed evidence
A common explanation is that bones grow faster than muscles in a growth spurt, so the tendon pulls harder. Tight front-thigh muscles were strongly linked with it in one population study, but a systematic review calls the causes controversial, and Cochrane notes the tight-muscle theory is proposed rather than proven.
Sources 16, 17, 4, 3
