Perthes disease: what it is, symptoms, treatment and how long it lasts

The hip is a ball-and-socket joint. The ball is the top of the thigh bone (the femoral head) and the socket is part of the pelvis. In Perthes disease the blood supply to the ball is disrupted for a time. Without a good blood supply the bone becomes soft and can lose its round shape.

Hull University Teaching Hospitals NHS Trust explains that the blood supply later recovers and new bone is made, but the ball does not always return to its original round shape. The condition is also called Legg-Calve-Perthes disease. It is rare, and it usually affects one hip; East Lancashire says both hips are affected in about 1 in 7 children, though not usually at the same time.

Hull explains that treatment cannot cure the condition, because the blood supply recovers through the body's own repair process. The main aim is to help the ball and socket stay round and to keep the hip moving. East Lancashire gives the same aim: to promote healing so that the hip joint regrows in the correct position.

The plan depends on the child's X-rays, symptoms and age and is set by an orthopaedic consultant. Hull notes that consultants within the same hospital and at different hospitals often treat Perthes differently, so it is reasonable to ask your own team why they recommend a particular approach.

Frimley Health NHS Foundation Trust lists physiotherapy, crutches, plasters or sometimes an operation to reshape the bone around the hip. Its leaflet says the aim of surgery is to keep the ball within the socket, and that the surgeon will discuss the options for your child.

• Rest, pain relief and modified activity (East Lancashire).

• Specific exercises to maintain strength and movement, with a home programme and sometimes hydrotherapy (Hull, Frimley).

• Walking aids such as crutches, and regular reviews and X-rays (East Lancashire).

• Surgery if movement is very poor and the ball and socket do not fit together properly (Hull); this may mean plaster casts for 6 weeks or using crutches without putting weight on the leg.

Hull says physiotherapy plays a large part in treatment, using exercises to maintain hip range of movement, improve muscle power and ease pain. Exercise and movement are good for children, so swimming and adapted PE are encouraged.

East Lancashire recommends swimming and cycling because they keep the joint mobile without pressure through it, and advises avoiding activities with direct impact through the joint, such as running and trampolines. Frimley says parents may be asked to limit activity, particularly contact sports, while the disease is active.

Practical points from the leaflets: sitting cross-legged on the floor can be painful, so a chair may help at school (Hull and Frimley); a wheelchair may help with long distances for some children, arranged through the therapist (Hull); and sharing the clinic letter with school can help teachers understand which sports to avoid (Frimley).

The cause is unknown. East Lancashire says it is not known why the blood supply is disrupted and that there is no known link between physical activity or other childhood diseases and the poor blood supply. Frimley Health likewise says there is very little understanding about why it happens.

In practical terms this means that nothing a parent did or did not do caused it, and it cannot be prevented. The leaflets agree that it is more common in boys than in girls.

East Lancashire says the symptoms usually develop gradually, and that the first sign is often a limp. The affected hip is often, but not always, painful. Hull adds that symptoms can vary from day to day and that pain in the hip, groin or knee may show up as fidgeting.

Pain from the hip is sometimes felt in the thigh or knee rather than the hip itself, so a child with a persistent limp or knee pain and a normal-looking knee may need the hip checked.

• A limp that may gradually worsen over a few weeks.

• Pain in the hip and groin, sometimes referred to the thigh or knee.

• Stiffness and reduced movement in the hip; bending the hip and moving the leg out to the side are most affected (East Lancashire).

• Wasting of the thigh muscles, so the affected leg may look thinner, and sometimes a shortened leg (East Lancashire).

It is a long process. Hull says that the whole process may last 3 to 4 years from initial symptoms to full regeneration of the bone. Frimley Health gives a usual range of 2 to 5 years, and East Lancashire says the thigh bone can take two or more years to regrow and remodel after the condition first starts.

During that time your child will usually have regular outpatient appointments until the bone has healed. The leaflets describe an outlook that is generally good: Hull says children usually recover function once new bone is formed and manage most sports and activities, and that the younger a child is when Perthes begins, the better the outcome is generally.

Both Hull and East Lancashire mention some possible longer-term effects, including some stiffness and a risk of arthritis in later life, depending on the final shape of the hip and the child's age at diagnosis.

Frimley says X-rays are taken to confirm the diagnosis and that blood tests are done to check that there is not an infection. Hull says that once established Perthes can be diagnosed on X-ray, but it is difficult to diagnose in the early stages because the first symptoms are due to inflammation of the joint, which does not show on an X-ray. For that reason, Hull says a child may have had problems for 6 months or longer before the diagnosis is made.

If an early X-ray is normal but the limp or pain continues, further assessment or repeat imaging is usually planned by the paediatric or orthopaedic team.

East Lancashire lists features that need urgent medical advice in a child with hip symptoms. These are not typical of Perthes disease itself and can point to another problem, such as infection.

• Increased temperature, or feeling hot and shivery.

• Redness and heat at the hip, knee or ankle.

• Inability to walk on the leg, or a sudden unexplained limp.

• Regular or constant night pain, or pain that does not settle with rest.

• Sudden deterioration of function or movement, swelling or bruising, or loss of sensation, numbness or persistent pins and needles in the leg.

On the NHS, a child with a persistent limp is usually seen by a GP and referred to a paediatric orthopaedic service, with physiotherapy involved alongside the consultant. Care is long-term, with repeat reviews and imaging, so continuity with one team is useful.

Some families also see a private paediatric orthopaedic surgeon for a second opinion. The medical evidence and treatment options are the same, and any plan should be shared with the child's NHS team and school.

This table is a simple guide to why a limp should be assessed rather than assumed to be a growing pain. Only a clinician can tell these apart.