Use of 3D engineering tool helps to model hip joint disease.

When 11-year-old Emma Thornton starts secondary school soon, her mother Theresa says it will be a fresh start for her because no-one will know what she has been through over the past five years. Problems with Emma's left hip joint have left her with difficulty walking and a painful, stiff hip which could lead to an early hip replacement when she is older. For an active, sporty child, Theresa knows it has been tough for her daughter to come to terms with her lack of mobility.

"She found it really hard not being able to take part in playground games with her friends. She couldn't do PE - and she just had to sit there watching everyone else doing it. As a result, she got angry and frustrated."

Emma, who lives in north London, has Perthes' disease, which affects the head of the femur - the ball part of the ball and socket joint of the hip - in children. It normally starts with groin, hip or knee pain and usually affects just one leg.

Scientists from the University of Hull, funded by Action Medical Research, have begun a research project to try to work out why some children develop it while others do not. With the help of a three-dimensional computer modelling technique, called finite element analysis, they are investigating how the shape and orientation of the hip joint influences the disease process. Such is the scope of the engineering tool that it has also been used in the design of cars and aircraft.

Prof Michael Fagan, who is leading the team carrying out the research, says the technique has distinct advantages for this kind of medical problem where modelling of bones is required. "It allows us to visualise the hip joint and pelvis in 3D, then vary the geometry in the model to look at the stresses and strains created on the bones."

Prof Fagan's theory is that Perthes' disease occurs because of a change in the biomechanics of the hip joint. "The thinking is that as the hip joint grows, loading is high on the joint and that can block blood supply to the femoral head causing the collapse of the bone."

His research team are using the same techniques to study whether certain activities and exercises, such as horse riding or swimming, might have the potential to stop the progression of the disease. At present it is not possible to predict which children will develop Perthes' disease. By the time they're diagnosed, their thigh bone can often be already damaged.

In Emma's case, there were crucial delays before and after diagnosis. She had been trampolining when her sister found her collapsed and crying on the floor, complaining that she couldn't stand up. "I thought she'd just pulled a muscle," her mum remembers. "I helped her up again and she was was limping for a few days. A few weeks later, she was still favouring her right leg and then we noticed she'd started turning her foot in."

X-rays showed that Emma had a larger hip joint gap on her left side, but they were assured it was unlikely to be Perthes' disease. But Theresa wasn't convinced. It was only after she researched Emma's symptoms on the internet and found out more about Perthes' that she demanded that Emma's X-rays be re-examined.

An MRI scan eventually confirmed a diagnosis of Perthes' in July 2008 - but then there were further delays. After surgery to release a tendon in her groin at a local hospital, Emma was advised to wear a hip brace for six weeks but when they finally decided to go and see a specialist at Royal National Orthopaedic Hospital, they were told she had missed the window for corrective surgery.

"He could see that she had a severe condition. He said all he could do was carry out a salvage procedure for now," Theresa says.

Emma has now had surgery to help correct the angle of her hip using temporary metal plates. Most children are lucky enough to recover from Perthes' disease naturally without any long-term disability, using a combination of physiotherapy, rest and plaster casts or braces. Emma's case may not be quite as simple but she is now coping well and, although she limps, she can run, jump and swim and play football - as long as she doesn't overdo it.

Starting a new school could be perfect timing and give her a whole new lease of life.