Presumed intraarticular gas microbubbles resulting from a vacuum phenomenon: visualization with ultrasonography as hyperechoic microfoci.

OBJECTIVE: Hyperechoic microfoci are sometimes visualized in normal joints. We hypothesized that these microfoci may correspond to gas microbubbles produced by a vacuum phenomenon. The purpose of our study was to demonstrate the possibility of generating intraarticular hyperechoic microbubbles by creating a vacuum phenomenon through traction on a metacarpophalangeal joint.
MATERIALS AND METHODS: We applied manual traction to the second metacarpophalangeal (MCP) joint of 22 volunteer subjects to separate articular surfaces with the aim of producing a vacuum. For one subject, the production of a vacuum was verified on a radiograph performed during the traction maneuver. For all subjects, ultrasonographic examination of the MCP joints was performed before, during, and after traction maneuvers. Two radiologists evaluated the presence of intraarticular hyperechoic microfoci and measured the widening of the joint space during traction.
RESULTS: In the first subject, the widening of the joint space and the production of an intraarticular gas-like cavity by traction was confirmed on the radiograph. In 10 out of the 22 volunteers, the widening of the joint space was immediately followed by the appearance of a large intraarticular hyperechoic band, which disappeared when the traction was stopped, followed by the appearance of hyperechoic microfoci that persisted several minutes. The widening of the joint during the traction maneuver was greater in the group where hyperechoic foci were produced than in the group with no hyperechoic foci (mean 2.5 vs. 1.2 mm and 2.2 vs. 0.8 mm, respectively, for observers 1 and 2; P < 0.05, Mann-Whitney U test).
CONCLUSION: Intraarticular hyperechoic microfoci may be produced and persist in normal joints after a traction maneuver. They are presumed to correspond to microbubbles created by a transient vacuum phenomenon.