Objective: The aim of this study was to identify the detectable threshold of knee effusion by ultrasonography while infusing saline.
Design: Forty knee osteoarthritis patients were allocated randomly to either the midline or the lateral group. Intra-articular injection of 20 ml normal saline was performed under ultrasonographic guidance with the transducer fixated at the midline longitudinal or lateral longitudinal scan in the midline and lateral groups, respectively. We obtained ultrasonography images after infusing each milliliter and measured the maximum depth of effusion.
Results: The smallest amount of infusion detected by ultrasonography was 4.37 ± 2.11 ml in the midline group and 4.13 ± 1.71 ml in the lateral group. An effusion more than 2 mm deep was observed after infusing 7.84 ± 3.85 ml and 7.38 ± 3.01 ml in the midline and lateral groups, respectively. To obtain a 4-mm depth, infusions of 11.58 ± 5.68 ml and 13.13 ± 4.88 ml were needed in the midline and lateral groups, respectively.
Conclusions: To detect knee effusion by ultrasonography, infusion of 4.26 ml (SD, 1.92 ml) of solution is needed. We think that a depth of 2 mm is more appropriate than 4 mm as the definition of knee effusion using ultrasonography.