Thorn injury mimicking a septic arthritis of the knee

Description: The possibility of penetrating injury with plant material should always be considered and excluded in cases of an apparent rapidly developing effusion, and careful attention to the history of injury must be made. A normal CRP result, as in this case, adds further weight to this differential diagnosis, as it would be considerably elevated in true infection. If an injury of this type is present then the treatment must be formal arthrotomy or arthroscopy and washout since simply washing out the joint but leaving plant material behind may lead to recurrent problems.
Jon D Clarke and Daniel D McCaffrey Ulster Med J. 2007 September; 76(3): 164–165.