Normal range of intraarticular fluid in the knee of healthy volunteers: easy evaluation with MRI

Abstract

In order to evaluate the normal amount of intra-articular fluid on knee MRI, we examined 36 knees of 18 normal volunteers on a 1.5-T MR system. Sagittal spin-echo T2-weighted images were obtained. We measured the maximum thickness and length of fluid seen in the suprapatellar pouch and in the posterior recesses of the joint. We also looked for fluid in other areas of the knee. The maximum thickness of fluid seen in the suprapatellar pouch was between 0 and 5 mm in all but three cases, where it reached 6, 8 and 9 mm. The maximal thickness of fluid seen in the posterior recesses of the joint was between 0 and 5 mm in all but two cases where it was 6 and 7 mm. The maximum lengths of fluid in the suprapatellar pouch and posterior recesses were much more variable than thickness and appear of less interest in establishing normal limits.

We conclude that the best and easiest way to quantify fluid in the knee joint is to measure its maximum thickness in the suprapatellar pouch and in the posterior recesses. Normal values in these two areas is generally between 0 and 5 mm; in rare cases, it can be between 6 and 9 mm. When the maximum thickness in one of these two areas reach 10 mm or more, a pathological joint effusion should be suspected.