Perthes Disease: What Is It and Why Does Your Child Need Repeat Imaging?

Key takeaways

• Perthes disease most commonly affects children between the ages of 4 and 10.
• The femoral head (ball of the hip) loses its blood supply temporarily and then regrows.
• Recovery takes two to four years, requiring regular imaging at every stage.
• Cumulative radiation from repeat X-rays is a genuine concern, and low dose EOS imaging uses about 90% less radiation than a standard X-ray.
• Diagnosis and treatment decisions are made by a paediatric orthopaedic team, not an imaging clinic.

What are the symptoms of Perthes disease?

The earliest sign is usually a limp that comes and goes, sometimes with a mild ache in the hip, groin or knee. Because the pain can appear at the knee rather than the hip, Perthes disease is sometimes missed at first.

If your child develops a limp that does not clear up within a week or two, or complains of hip or knee pain with no obvious injury, see your GP. They can arrange the first assessment.

What causes Perthes disease?

The cause is a temporary interruption to the blood supply reaching the femoral head, the rounded top of the thigh bone that sits inside the hip socket. Without enough blood, the bone weakens (a process sometimes called avascular necrosis). Nobody knows exactly why this happens. It is not caused by injury, diet or anything a parent did or did not do.

What are the stages of Perthes disease?

Perthes disease moves through four recognised stages over roughly two to four years. Each stage looks different on imaging, which is why regular scans are needed to track progress and guide the orthopaedic team's decisions.

How is Perthes disease monitored with imaging?

Serial imaging, meaning repeat scans taken at intervals over months and years, is the backbone of Perthes management. Imaging shows the orthopaedic team how the femoral head is remodelling, whether it is staying well seated in the socket, and whether the treatment plan needs to change.

A child with Perthes disease may need many sets of hip images over the full course of the condition. That is a lot of radiation if each session uses a standard X-ray dose. This is where the cumulative dose argument matters most.

Your child's paediatric orthopaedic consultant decides when each scan is needed and what views are required. Paediatric imaging at ScanAlign simply provides the images at a much lower radiation dose, then sends the results to your child's clinical team.

Why does low dose imaging matter for children with Perthes disease?

Children are more sensitive to radiation than adults because their cells are still dividing rapidly as they grow. The more imaging sessions a child has, the more the doses add up. With Perthes disease requiring years of follow-up, keeping each individual dose as low as possible is a sensible precaution.

EOS captures the hip and pelvis in a single standing sweep using about 90% less radiation than a standard X-ray, around ten times less. Over many appointments, that reduction becomes meaningful. Standing imaging also shows how the hip behaves under the child's own body weight, which can be useful for assessing how the joint is functioning in real life.

ScanAlign, low dose standing imaging, is available at 19 Harley Street, London. You can book a paediatric monitoring scan once your child's consultant has requested follow-up imaging.

What is the long-term outlook for Perthes disease?

The prognosis depends mainly on two things: the age at which the condition starts, and how round the femoral head is when it finishes regrowing. Younger children (under six at onset) tend to do better because they have more growing time for the bone to remodel into a good shape.

Many children recover with a hip that works well and causes little or no trouble into adulthood. Some are left with a femoral head that is flatter or less regular, which can affect the range of movement and may increase the chance of hip problems later in life.

What helps during recovery?

The paediatric orthopaedic team may recommend physiotherapy to maintain movement, periods of rest or reduced activity, or in some cases a brace or surgery to keep the femoral head well contained in the socket. These are called containment treatments. ScanAlign does not provide treatment, but the imaging we produce supports the team making those decisions.

Frequently asked questions about Perthes disease