Introduction: Sub-acute septic arthritis of knee following a thorn prick injury to patella is an uncommon condition and seen in active playful children. The child may be afraid to narrate the incident to their parents or examining doctor, so a high index of suspicion is must. The delayed presentation is inevitable. Early synovitis due to the organic biological foreign body may progress to septic arthritis, if untreated may result in knee stiffness or chronic osteomyelitis of patella.

Case report: We report a case of thorn prick injury to the left patella in an 8-years-old boy who neglected the incident of thorn prick while playing with his peer group 10 days ago and presented with features of sub-acute septic arthritis. He was treated by arthrotomy, double inspection of the patella, removal of foreign body, synovectomy, lavage, antibiotics, and aggressive rehabilitation with an excellent outcome.

Conclusion: Sub-acute synovitis in active playful children should raise the suspicion of thorn prick even though no proper history is explained. Organic foreign body from plant materials is radiolucent and easily missed. High suspicion and timely surgical intervention, double inspection during arthrotomy identifies the foreign body. Once removed, with proper antibiotic and physiotherapy protocol followed has an excellent outcome.

Keywords: Organic thorn prick; double inspection; patella; sub-acute septic arthritis of knee.