Monarthritis of the pediatric knee joint: differential diagnosis after a thorn injury.

Foreign-body injury of the juvenile knee is frequently seen in the outpatient department. Arthroscopy is the most important diagnostic and therapeutic procedure along with radiographic examination in a case of typical history and clinical symptoms. In this case, an intra-articular thorn in a juvenile knee was not diagnosed at arthroscopy despite typical injury. Five months later, the boy developed knee pain and joint swelling. Bacterial infection of the knee and rheumatoid causes were excluded by serologic blood analysis several times at the children's hospital. Magnetic resonance imaging of the knee revealed an intra-articular foreign body. At a second arthroscopy, the thorn was removed and a hypertrophic plica with dependent cartilage damage of the medial femoral condyle was resected. At follow-up after 4 weeks, the boy was free of pain and swelling. After the first arthroscopy, persistent knee swelling and pain had to be taken as a hint of an intra-articular irritation. All arthroscopic surgeons should do a complete diagnostic examination of the affected joint in case of a foreign-body injury.