[Ultrasonographic imaging of pneumarthrosis of the knee joint--case report and experimental cadaver study]

In case of penetrating injuries near to the knee joint showing only a small skin lesion and no damage to tendons and neuro-vascular structures, opening of the joint cavity has to be taken into consideration. Besides surgical revision of the penetration canal, radiological depiction of free intraarticular air may be useful. We report the cases of two patients, who suffered from intraarticular effusion and pain two, respectively three days after a penetrating injury. In both of them, free intraarticular air in the suprapatellar region could be depicted by ultrasonographic assessment. Subsequent evaluation of radiographs showed intraarticular air already being there at the day of admittance. Performing an experimental study in fifteen cadaver knee joint specimens, the reliability of this technique was evaluated and documented. Using an injection needle, air was insufflated into the knee joints with or without previous instillation of fluid. Afterwards sonographic assessment was performed using 7,5 - 13 MHz linear probes and Siemens (Elegra) equipment. Ultrasonographic detection of pneumarthrosis of the knee joint was possible early and reliably in both of the patients reported. With sufficient experience of the clinician and adequate equipment, even singular intraarticular air blisters are detectable, especially in cases of concomitant intraarticular effusion. By ultrasound it was possible to detect pneumarthrosis with at least 0,5 ml, by X-ray with at least 2 ml. Ultrasonography proved to be a reliable diagnostic tool for the detection of free intraarticular air within the knee joint. This fact should be considered in the differential diagnosis of unclear knee joint disorders after penetrating injuries.