An 8 year old boy was referred by his general practitioner to the paediatric outpatient department with pain and swelling of his left knee. This had been present for three months, during which the boy had attended the general practitioner on more than one occasion. A presumptive diagnosis of reactive arthritis had been made initially and he was treated with ibuprofen. His mother had a definitive diagnosis of rheumatoid arthritis and was concerned that he might be developing this.

When seen in outpatients, it was clear that the knee had been swollen for around six months. The swelling was more or less continuous. The pain was variable. The knee had been particularly painful at the onset. Intermittent stiffness of the knee was also present as was an intermittent limp.

On examination the child looked very well. His left knee had a small effusion and was slightly warmer to touch than the right but was not particularly tender. Movements were normal apart from minor limitation of extension.

When the child was asked if he had any idea why his knee was sore, he stated with some conviction that he knew what had caused it. He was in the school playground one day and knelt down to tie his shoelace, whereupon a sharp metal object, which he referred to as a staple, penetrated his knee. He had told this to his mother who dismissed it as a cause of his difficulties as she had had a look at his knee and could not see any obvious sign of penetrating injury.

x Ray examinations of the knee showed two radio-opaque foreign bodies. The boy was referred to the orthopaedic surgeons for arthroscopy and further management. Subsequently he had the staples removed.