It was an ordinary fall at school - the sort that happens in the rough and tumble of the playground a dozen times a week. For Joy Spence, then aged six, it was the start of five years of constant, racking pain. For weeks at a time she could only lie on her back in hospital. A precious year of education was missed and her childhood utterly disrupted.

'When Joy fell, she complained of backaches and I thought it was just a bruise,' said her mother, Carole. 'It went on for a few days and then one morning she screamed in agony and could not move.'

After several months, the pain was diagnosed as juvenile osteoporosis - brittle bone disease. Although rare in children, its effects can be devastating: now 11,

Joy spends much of her time in a wheelchair.

Mrs Spence had to suffer hearing her daughter's repeated screams of agony; she could neither hold nor hug her because it intensified the dreadful pain. As other children grew, she watched Joy shrink by 2in because of her crumbling spine. 'She had two little tyres that hung down around her waist, like an old lady,' she remembers.

Osteoporosis, or thinning of the bone, normally affects elderly women and men, and more recently it has been detected among over-enthusiastic slimmers and ballerinas. Older women are particularly vulnerable because they no longer make oestrogen, which helps to produce bone mass. In adults other causes include calcium deficiency, hormonal disorders and treatment with steroid drugs. Only 50 to 100 children in Britain suffer from the disease. About half will recover naturally; the others face the prospect of life in a wheelchair. Doctors admit they know neither cause nor cure for juvenile osteoporosis and are divided on how to treat it.

Dr Roger Francis, consultant physician at Newcastle General Hospital, a specialist in the disease, says little research has been carried out: 'It was described first about 30 years ago, but because no one was particularly interested, no one developed the expertise to handle it.'

In children it can occasionally be caused by steroid therapy or hormonal disturbances such as overactive thyroid, says Dr Francis. A further, rare, cause is coeliac disease, which results in poor absorption of nutrients from the intestine.

'But in children the majority of cases are what we call idiopathic, or of no known cause,' he says. As with adults, a calcium deficiency is usually implicated. Because the illness is so rare in children, even specialists may only see two or three with it at any one time.

Lack of research also means that for children most treatment is by trial and error. 'They are treated with mineral and vitamin supplements and sometimes given hormone treatment,' says Dr Francis. 'This tends to stop the bones getting any thinner. But it is all experimental at the moment.'

Before the playground fall which hurt her back, there had been only one indication that something might be wrong with Joy. She had broken her wrist in the sort of fall that should only have caused minor injuries. But there was no history of bone disease in the family and both of Joy's brothers, John, 15, and nine- year-old Paul, are fit and well.

After the fall, and an X-ray at the local hospital, the Spences' GP said that the base of Joy's spine was very soft and she should see a specialist urgently. 'We were sent to a paediatric consultant who said she had the spine of an old woman,' says Mrs Spence. At first doctors thought she had leukaemia; later they diagnosed a calcium deficiency but still did not know what was causing it. A bone scan showed her spine to be abnormally thin.

Three months later doctors finally made the correct diagnosis. By this time, recalls Mrs Spence, Joy's backaches were getting worse.

'One day she just screamed in agony. We couldn't move her and could not make her comfortable. She just had to sit there until the pain had got better.' Joy started to spend more and more time in hospital for 'bedrest' - which meant just lying on her back. 'It kept getting worse until it got to the point where she could not walk.'

At Great Ormond Street Hospital for Sick Children, where Joy spent five weeks, staff said they had never seen such severe osteoporosis in a child and could not understand how she coped with the pain. Joy's spine, said one consultant, was as thin as tissue paper and starting to curve like an old woman's.

She has been treated with hormones, steroids and painkillers; at one point she was put on a course of sodium clodronate, a drug which directs calcium from the blood to the bones. Nothing seemed to have any effect on her crumbling spine.

While some doctors advised hydrotherapy, swimming and physiotherapy, others were against it. 'You got to the point when you took in what people said and drew your own conclusions,' said Mrs Spence.

The strain on the family has been severe. 'We were like ships in the night with my husband, Andrew, working during the day and then going to see Joy at night while I would see her during the day,' says Mrs Spence. 'But you got used to traipsing from one hospital to another. And the boys were very tolerant.'

Friends and family, she says, have been supportive and sympathetic. 'But at the end of the day, it was our child and we had to cope with it. No one could really understand what we were going through. What we found hard was that the doctors said there was no cure for the disease. We thought we were going to have to live with the fact that Joy wouldn't get better.'

Eventually Joy was referred to Dr Roger Smith, a consultant at the Nuffield Orthopaedic Centre, Oxford, and one of Britain's few specialists in osteoporosis in children. Five years on and many treatments and hospitals later, her mother says there is a glimmer of hope.

Recent tests have revealed that Joy is developing 'masses of new bone'. Although she still spends much of her time in a wheelchair, it looks as though she is going to get better. Doctors still do not understand why this happens in some children and not in others. 'We don't have the faintest idea and there is no easy way of finding out,' says Dr Smith. 'This is a very rare disease, and looking at the cases of the last 20 years there is no way of predicting who will get better and who not.'

The National Osteoporosis Society has now launched an appeal for pounds 50,000 for research into the disease in children. The society is hoping to appoint a research fellow for two years to collect and collate information on all current cases and draw up a national register.

The research will look at whether there are any genetic factors involved in juvenile osteoporosis and will also investigate if adults who had the condition in childhood suffer again when they are older.

National Osteoporosis Society, PO Box 10, Radstock, Bath BA3 3YB (0761 432472).

(Photograph omitted)